Maud Robert

Axel Denneval
email: axel.denneval@gmail.com

Claire Chalumeau

Sylvain Iceta

Elise Pelascini

Emmanuel Disse

Revision of Mason's procedure (Vertical Banded Gastroplasty) to Roux-en-Y Gastric Bypass : role of an associated fundectomy in weight loss outcomes

Introduction

For the last number of decades, morbid obesity has been a public health issue in constant evolution [START_REF] Selassie | The epidemiology and aetiology of obesity: a global challenge[END_REF] . Since the beginning of the 21 st century, bariatric surgery has demonstrated superiority over medical treatment of obesity and remains the gold standard management of obesity and its comorbidities [START_REF] Buchwald | Bariatric surgery: a systematic review and metaanalysis[END_REF][START_REF] Sjöström | Effects of bariatric surgery on mortality in Swedish obese subjects[END_REF] . The surgical techniques have evolved greatly since the advent of bariatric surgery in the United States in the 1960's. The vertical banded gastroplasty (VBG) was one of the first restrictive techniques described by Mason, a pioneer of bariatric surgery, in 1982 [START_REF] Mason | Vertical banded gastroplasty for obesity[END_REF] . This procedure, widely performed by laparotomy in the 2000s [START_REF] Buchwald | Bariatric surgery: a systematic review and metaanalysis[END_REF][START_REF] Wang | Laparoscopic vertical banded gastroplasty: 5-year results[END_REF] , was gradually replaced by the adjustable gastric band, also favored by the advent of laparoscopy .

Mid to long term results of the VBG appeared to be less satisfactory and more heterogeneous, with complications reaching 45% in the literature [START_REF] Bekheit | Rejecting the demise of vertical-banded gastroplasty: a long-term single-institute experience[END_REF][START_REF] Scozzari | 10-year follow-up of laparoscopic vertical banded gastroplasty: good results in selected patients[END_REF] : gastro-esophageal reflux disease (GERD), dysphagia, weight regain due to the repermeabilization of the staple line and/or dilatation of the gastric pouch were reported to lead to a reoperation rate of 21.4% up to 56% (8,9) . As a result, revisional surgery for weight loss failure, weight regain or for complications of the VBG is often necessary. The conversion of VBG to RYGB remains the procedure of choice for many bariatric teams [START_REF] Vasas | Short-and long-term outcomes of vertical banded gastroplasty converted to Roux-en-Y gastric bypass[END_REF][START_REF] Suter | Laparoscopic Roux-en-Y Gastric bypass after failed vertical banded gastroplasty: a multicenter experience with 203 patients[END_REF][START_REF] Gagné | Laparoscopic revision of vertical banded gastroplasty to Roux-en-Y gastric bypass: outcomes of 105 patients[END_REF][START_REF] Mognol | Roux-en-Y gastric bypass after failed vertical banded gastroplasty[END_REF][START_REF] Gys | Conversion of Open Vertical Banded Gastroplasty to Roux-en-Y Gastric Bypass: a Single-Center, Single-Surgeon Experience with 6 Years of Follow-up[END_REF][START_REF] Alshehri | Safety and Efficacy Assessment of Revisional Bariatric procedures: A single Institute Experience[END_REF][START_REF] Iannelli | Laparoscopic conversion of vertical banded gastroplasty (Mason MacLean) into Roux-en-Y gastric bypass[END_REF] despite its technical difficulty and the significant risk of morbidity [START_REF] Roller | Revision of failed gastric restrictive operations to Roux-en-Y gastric bypass: impact of multiple prior bariatric operations on outcome[END_REF] .

Nevertheless, literature data is lacking regarding the outcomes of conversion of VBG to RYGB and its benefit to risk ratio.

We conducted a bicentric retrospective study of all the patients who underwent a conversion of VBG to RYGB over a period of 12 years. Our main objective was to analyze the indications, the weight loss and functional results of conversions of VBG to RYGB. The second objectives were to assess the safety and to identify the predictive factors of success or failure of the conversion.

Methods

Pre-Operative work-up

From February 2008 to January 2020, all the patients who underwent a conversion of VBG to RYGB in two specialized centers of bariatric surgery were included. This was a retrospective study of prospectively collected data. Demographic data (age, gender, weight, height, Body Mass Index-BMI), past medical history and especially weight loss history (initial weight at VBG, nadir weight and weight regain after VBG) were recorded. The indication for conversion was analyzed, whether for weight loss failure or for complications of the VBG (gastro-esophageal reflux, malnutrition, dysphagia, gastroparesis). A distinction was made between patients presenting a primary failure (having never satisfied the Reinhold's criteria (BMI ≤ 35 and/or EWL%≥50% [START_REF] Reinhold | Critical analysis of long term weight loss following gastric bypass[END_REF] )) and those presenting a secondary failure (initially satisfying the Reinhold's criteria but secondarily regaining weight, therefore not satisfying those criteria anymore). The pre-operative use of an upper gastro intestinal endoscopy (upper GI endoscopy), a gastrografin swallow and/or a gastric computed tomography with gas (gastric CT) was detailed: the presence of esophagitis, gastric pouch dilatation, staple line dehiscence and/or gastric stenosis at the calibration banding. Gastric pouch dilatation was considered significant when the volume exceeded 100cc on gastric CT and/or mentioned on the patient file through other exams (upper GI endoscopy, gastrografin swallow). All patients benefitted from a multidisciplinary re-evaluation (psychologic, dietitian and nutritional assessment) prior to validation of the indication to convert the VGB to RYGB by an accredited multi-disciplinary bariatric team.

Peri-and Post-Operative Data

associated procedures (fundectomy, cholecystectomy), peroperative and early complications (< 30 days) were collected. The surgical technique of conversion of the VBG to RYGB consisted of performing a new gastric pouch by stapling the pre-existing pouch using a linear stapler transversely above the calibration band and then vertically inside of the pre-existing staple line; the gastric pouch was calibrated by a 37French bougie. A fundectomy was performed if technically feasible without increasing the surgical risk from 2012: the aim was to resect the pre-existing staple lines and the calibration band to avoid possible leakage or other complications. The gastric bypass was then performed according to the Lonroth technique [START_REF] Lönroth | Laparoscopic gastric bypass[END_REF] , with a biliary limb of 50cm and an alimentary limb of 150cm. The alimentary limb was either antecolic in one center or retrocolic in the other. Early and late complications (> 30 days) were recorded according to the Clavien Dindo classification [START_REF] Clavien | The Clavien-Dindo classification of surgical complications: five-year experience[END_REF] . Postoperative visits were scheduled at 2, 6, 12, 18, 24 months and then annually. Weight loss was expressed in delta BMI (Initial BMI minus current BMI), in excess weight loss percentage (EWL% = ((initial weight -current weight) / (initial weight -ideal weight)) x 100) and in total weight loss percentage (TWL% = ((initial weight -current weight) / (initial weight)) x 100) at each time of the follow up. Ideal weight was calculated for a BMI of 25. The conversion of VBG to RYGB was considered a weight loss success if Reinhold's criteria were achieved at last follow up. Duration of the follow-up was recorded; patients were considered lost to follow up if no medical visit or news were found for more than one year.

Statistical analysis

Data were analyzed using medCalc® software v19.3 (MedCalc Software, Ostend, Belgium). Categorical variables were expressed as frequencies and proportions and were compared with X² or Fisher's test for the bivariate analysis. Continuous data were expressed as a mean or median with standard deviation (SD). A Student's t-test was performed for data with a normal distribution or a Mann-Whitney U test when appropriate. Data extraction was performed using Orange Datamining Software® v3.27.1 (University of Ljubljana, Slovenia) and its native ReliefF algorithm to determine predictive factors of weight loss success or failure according to Reinhold's criteria. A comparison of the two groups (weight loss success or failure at the end the follow-up) was performed using univariate analysis. A logistic regression analysis was then performed in order to define the independent factors involved in the weight loss success of the conversion of VBG to RYGB. A p-value <0.05 was considered statistically significant.

Results

Studied population and indications of the conversion of VBG to RYGB

Between 2008 and 2020, 85 patients were included: 51 in one center group and 34 in the other. There was 81.1% of women (n= 69) and 18.9% of men (n = 16). The mean age at conversion was 50±10.6 years (22 -73) and mean BMI prior to conversion was 40.6±8.9 kg/m² (21.5 -68.8). All VBG procedures were performed between 1990 and 2014, with a mean BMI of 46.64±7.04 kg/m² (33.98 -63.59) at the time of the VBG, reaching a mean BMI of 29.5±5.78 kg/m² (19.05 -47.46) at the nadir point, corresponding to a mean EWL of 82 ± 25.9% (26.6 -137.8). The indication of conversion to RYGB was mainly for weight loss failure of the VBG in 70 cases which were analyzed to find predictive factors of weight loss success or failure of the conversion: 27 of them (38.5%) also presented functional complications of the VBG (dysphagia +/-GERD +/-malnutrition +/-gastroparesis). The other 15 patients (17.6%) were converted for complications of the VBG but did not fail to lose weight (figure 1). The functional complications of the VBG were represented by dysphagia (n= 27/85, 31.7%), gastro-esophageal reflux (n = 22/85, 25.9%), malnutrition (n = 2/85, 2,3%), and in one case by a gastroparesis worsened by diabetes. Among the patients with weight loss failure of the VBG (n = 70), 9 were considered as a primary failure (12.8%) and 61 (87.2%) as a secondary failure (initial weight loss and weight regain). Five patients were excluded due to the technical failure of the conversion to RYGB (5/85= 5.9%) : the presence of scar tissues and severe adhesions did not make the dissection possible. In these 5 cases, the original indication for conversion was a weight loss failure of the VBG. Four patients underwent conversion to sleeve gastrectomy, and no intervention was finally performed for the fifth patient.

The pre-operative work-up included a gastroscopy for almost all cases (97.6%, n= 83/85), a gastric CT with gas in 47.1% of the cases (n= 40/85) and a barium swallow in 32 patients (36.7%): most common findings were a repermeabilization of the staple line (32.9%) and a dilation of the gastric pouch (35.3% ; Table 1).

Peri operative data and complications

The average operative time of the conversion was 214 ± 62 minutes (130 -460). Laparoscopy was possible in 84.7% of the cases. The most common associated operative procedure was the association of a fundectomy (58/85, 68,2%). A repair of incisional hernia was performed in seven cases, two of them being according to Goni-Moreno, a cholecystectomy in five cases, a sigmoidectomy and a right hemicolectomy in one case each. The median post-operative length of stay was 5 days (2 -124).

A per protocol analysis was conducted, with an exclusion of patients for whom the procedure was abandoned due to technical failure (n = 5): early post-operative complications were reported in 20 cases (25%), of which 13 (65%) required re operative procedures ( There was no post-operative mortality.

Weight loss and functional results after the conversion of VBG to RYGB

The mean post-operative follow-up was 35.5 ± 27.8 months. Twenty-eight patients (n = 28/85, 32.9%) were lost to follow-up. Regarding patients who had a conversion in the context of a weight loss failure (n = 65), weight loss evolution is represented in figure 2. The mean BMI at the end of the follow up was 33.6±6.6kg/m² (15.8 -49.7) with a mean reduction of 9.5±4.8 BMI points, a %TWL% of 21.6±10.1% and an average % EWL of 55.8±30.4% (1.6 -192.8). The weight loss success rate according to Reinhold's criteria was 63.1%; 49 patients out of 65 had a fundectomy (75.4%) (n=49/65).. For the patients who had a conversion of VBG for functional complications alone (n=15) and/or associated with weight loss failure (n=22), the global resolution rate of symptoms was 89.1% (table 3). Dysphagia and GERD which were the most common symptoms improved in 96.3% and 86.4% respectively. Weight loss changes over time function of the indication for conversion to RYGB are represented in figure 3.

Predictive factors of weight loss success or failure of the conversion of VBG to RYGB

Using datamining analysis, we found that the main factors associated with failure of the conversion to RYGB were the absence of associated fundectomy, as well as a high BMI at each stage of patient management. Univariate analysis showed that mean BMI pre VBG, nadir BMI achieved after VBG, and at conversion to RYGB were significantly higher in the weight loss failure group compared to the success group (50.4, 34.2 and 47.1 kg/m² versus 45.3, 28.0 and 40.9 kg/m² respectively; p<.05). Primary failure of the VBG was also significantly linked to weight loss failure after the conversion to RYGB (30.0% versus 4.8% in the success group; p=.02). A fundectomy was performed in 82.9% of successfully converted cases and in only 62.5% of unsuccessfully converted cases, close to statistical significance (p=.06). Postoperative complications did not seem to differ between the two groups.

Using logistic regression analysis, we found that the addition of a fundectomy to the conversion to RYGB significantly reduced the risk of weight loss failure (OR=. No difference was found on the VBG pouch dilatation rate. Gastric pouch dilatation did not impact success or failure of the conversion to RYGB (OR=1.69, 95%CI[.46-6.14] ; p=.42).

Discussion

Our bicentric and historic series of conversion of VBG to RYGB including 85 patients is one of the largest published in the current literature. Other studies already published on the topic are mostly retrospective and monocentric, including from 45 to 153 patients [START_REF] Vasas | Short-and long-term outcomes of vertical banded gastroplasty converted to Roux-en-Y gastric bypass[END_REF][START_REF] Gagné | Laparoscopic revision of vertical banded gastroplasty to Roux-en-Y gastric bypass: outcomes of 105 patients[END_REF][START_REF] Iannelli | Laparoscopic conversion of vertical banded gastroplasty (Mason MacLean) into Roux-en-Y gastric bypass[END_REF][START_REF] Schouten | Conversion of vertical banded gastroplasty to Roux-en-Y gastric bypass results in restoration of the positive effect on weight loss and co-morbidities: evaluation of 101 patients[END_REF] . The study of Suter al [START_REF] Suter | Laparoscopic Roux-en-Y Gastric bypass after failed vertical banded gastroplasty: a multicenter experience with 203 patients[END_REF] is the only multicentric one, with 203 patients across four centers. Other studies on revisional surgery were also published but include several kind of revisional procedures, were nonspecific of revision of VBG to RYGB and with small sample sizes [START_REF] Frantzides | Laparoscopic Revision of Failed Bariatric Procedures[END_REF] .

In the literature, the laparoscopic conversion of VBG to RYGB seems to be the gold standard treatment at the current time, as much for weight loss failure as for the management of functional complications of the VBG. In our study, the conversion to RYGB has demonstrated to be effective on complications of the VBG with a resolution of dysphagia and GERD in 96.3% and 86.4% of the cases, respectively. Our results are similar to those published in the literature: Vasas, Athanasiadis and Gagné found a 100% resolution rate of dysphagia [START_REF] Vasas | Short-and long-term outcomes of vertical banded gastroplasty converted to Roux-en-Y gastric bypass[END_REF][START_REF] Gagné | Laparoscopic revision of vertical banded gastroplasty to Roux-en-Y gastric bypass: outcomes of 105 patients[END_REF][START_REF] Athanasiadis | Vertical Banded Gastroplasty Revision to Gastric Bypass Leads to Effective Weight Loss and Comorbidity and Dysphagia Symptom Resolution[END_REF] whereas Gys [START_REF] Gys | Conversion of Open Vertical Banded Gastroplasty to Roux-en-Y Gastric Bypass: a Single-Center, Single-Surgeon Experience with 6 Years of Follow-up[END_REF] and Schouten [START_REF] Schouten | Conversion of vertical banded gastroplasty to Roux-en-Y gastric bypass results in restoration of the positive effect on weight loss and co-morbidities: evaluation of 101 patients[END_REF] reported 94% and 90% of improvement. GERD was improved in about 95% of the cases in most of the series [START_REF] Vasas | Short-and long-term outcomes of vertical banded gastroplasty converted to Roux-en-Y gastric bypass[END_REF][START_REF] Gagné | Laparoscopic revision of vertical banded gastroplasty to Roux-en-Y gastric bypass: outcomes of 105 patients[END_REF] .

When analyzing the results of the other available surgical options to manage failure of VBG, conversion to sleeve seems responsible for a high rate of complications: a 14% rate of leaks was reported due to the fragility of the new gastric stapling performed on a previously stapled zone at the time of VBG [START_REF] Brethauer | Systematic review on reoperative bariatric surgery: American Society for Metabolic and Bariatric Surgery Revision Task Force[END_REF] . Redo VBG by re-stapling or re-calibrating the gastric pouch resulted in the same levels of operative failure [START_REF] Kuzminov | Re-operations after Secondary Bariatric Surgery: a Systematic Review[END_REF] . It seems to us that in the case of failure of restrictive surgery such as VBG, the addition of a malabsorptive component is a supplementary argument to optimize weight loss. The risk of worsening GERD symptoms already quite common after VBG, goes against the conversion to sleeve gastrectomy. The improvement of GERD following a RYGB is another argument in favor of this technique which stands as the procedure of choice when converting a VBG.

Revisional bariatric surgery is often technically demanding, due to severe adhesions secondary to previous surgeries. In our series, we reported 5 operative failures out of 85 (5.9%). This could also be explained by the fact that VBG were mostly performed by laparotomy prior the years 2000 increasing the risk of adhesions. Several authors reported longer operative times, higher rates of complications, and lower weight loss after revisional bariatric surgery compared to first-line bariatric procedures [START_REF] Fulton | A comparison of revisional and primary bariatric surgery[END_REF] . Our 25% rate of short term complications is concordant with other findings in the literature [START_REF] Suter | Laparoscopic Roux-en-Y Gastric bypass after failed vertical banded gastroplasty: a multicenter experience with 203 patients[END_REF][START_REF] Gagné | Laparoscopic revision of vertical banded gastroplasty to Roux-en-Y gastric bypass: outcomes of 105 patients[END_REF][START_REF] Schouten | Conversion of vertical banded gastroplasty to Roux-en-Y gastric bypass results in restoration of the positive effect on weight loss and co-morbidities: evaluation of 101 patients[END_REF] . Nevertheless, we observed a 3.75% rate of abdominal wall complications that are explained by the association of incisional hernia repair in 7 cases including 2 giant incisional hernia: this complex abdominal wall surgery represents a bias in our analysis of surgical complications. Early and late severe complications (> Clavien III) occurred in 16.2% and 13.7% of the cases respectively and are concordant with published data, despite the heterogenous follow up and classification: complications ranged from 6.2% up to 50% in the literature [START_REF] Mognol | Roux-en-Y gastric bypass after failed vertical banded gastroplasty[END_REF][START_REF] Gys | Conversion of Open Vertical Banded Gastroplasty to Roux-en-Y Gastric Bypass: a Single-Center, Single-Surgeon Experience with 6 Years of Follow-up[END_REF][START_REF] Alshehri | Safety and Efficacy Assessment of Revisional Bariatric procedures: A single Institute Experience[END_REF][START_REF] Athanasiadis | Vertical Banded Gastroplasty Revision to Gastric Bypass Leads to Effective Weight Loss and Comorbidity and Dysphagia Symptom Resolution[END_REF] . The long operative time reported in our study (mean of 214 minutes) can partly be explained by associated procedures (repair of giant incisional hernias requiring laparotomy and right colectomy in super obese patients [START_REF] Benoit | Right Colectomy with Absorbable Mesh Repair as a Salvage Solution for the Management of Giant Incisional Hernia with Loss of Domain: Results of a Bicentric Study[END_REF] ) and illustrates the complexity of patients being reoperated for failed VBG. The mean % EWL obtained in our series was 55.8%, with an average follow up of 35.5 months, corresponding to a success rate of 64,7% according to Reinhold's criteria. These results are satisfactory when considering that 82.4% of our population (70/85) had weight loss failure after the VBG, and reflect those of the literature (11,12,14,23) . It is worth noting that the mean initial BMI of 44 kg/m2 in our study is higher than in most other series and can explain the moderate weight loss observed.

One strong point of our study is the demonstration that patient's weight loss history was significantly linked to conversion results. Indeed, we found that primary failure of the VBG was significantly linked to weight loss failure after conversion to RYGB, with 30.0% of primary failure versus 4.8% in the weight loss success group (p=.02). Thus, patients with a good initial weight loss after VBG but who regained weight later (secondary failure) had greater chance of weight loss success after the conversion to RYGB: conversion was more effective after a regain of weight than after an insufficient initial weight loss. Thus, after primary failure of a restrictive procedure, and a probably severe 'obesity illness', the addition of a malabsorptive component by conversion to RYGB makes perfect sense. Beside the fact that the greater the initial BMI, the greater the objective (BMI ≤ 35 and/or EWL > 50%) is difficult to achieve, our study confirmed that the weight loss trajectory of the patient is marked by his bariatric history; it also confirmed the difficulties of the most obese to maintain their results over time whatever the surgical technique [START_REF] Wang | Roux-en-Y Gastric Bypass Versus Sleeve Gastrectomy for Super Super Obese and Super Obese: Systematic Review and Meta-analysis of Weight Results, Comorbidity Resolution[END_REF] .

Another interesting finding is the positive impact of a concomitant fundectomy on weight loss outcomes of the conversion of VBG to RYGB. While initially not systematic, the fundectomy became more frequent in order to avoid the risks of gastric staple line leaks or fundus dilatation if left in situ. This procedure has not shown an increased morbidity and seems to have a major role in weight loss: the addition of a fundectomy reduced the risk of failure by 4 in our series (OR= 0.27) and was already found as an effective procedure in the literature [START_REF] Zappa | Vertical Gastric Bypass with Fundectomy: Feasibility and 2-Year Follow-Up in a Series of Morbidly Obese Patients[END_REF][START_REF] Lesti | Laparoscopic Gastric Bypass with Fundectomy and Gastric Remnant Exploration : Results at 5-Year Follow-up[END_REF] . This could also have a positive impact on Type 2 Diabetes by reducing ghrelin secretion, responsible for increased appetite and hyperglycemia, as demonstrated in previous studies (31) . Indeed, the physiology of this hormone is characterized by an increased secretion during fasting periods and a rapid postprandial decrease. In obese patients, the level of ghrelin is elevated during fasting periods but suffers from diminished post prandial inhibition (32) : the reduction of ghrelin secretion as well as a reduction in serum plasma levels has already been demonstrated after RYGB with fundectomy (32) . Even if the population size and its bi-centric nature are strengths of our study, it would be interesting to perform a randomized control trial to confirm the real impact of fundectomy on weight loss results: this could lead to add this procedure to standard surgical practice. The measure of serum ghrelin level would be of major interest to verify the impact of this hormone on weight loss and Type 2 Diabetes evolution.

The main limits of our study are its retrospective nature and its heterogenicity due to the inclusion of patients who have had a VBG by different surgeons, in different centers, in a variable period of time.

Conclusion

This historic, bi-centric study confirms that the conversion of VBG to RYGB remains the procedure of choice to achieve satisfying weight loss and resolution of functional complications. The addition of a fundectomy appeared to have a significant positive impact on weight loss outcomes. Our study showed that the weight loss trajectory of the patient is marked by his bariatric history with a significant negative impact of primary failure of the VBG on weight loss results.

  31. Lee H-M, Wang G, Englander EW, Kojima M, Greeley GH. Ghrelin, a new gastrointestinal endocrine peptide that stimulates insulin secretion: enteric distribution, ontogeny, influence of endocrine, and dietary manipulations. Endocrinology. 2002 Jan;143(1):185-90. 32. Chronaiou A, Tsoli M, Kehagias I, Leotsinidis M, Kalfarentzos F, Alexandrides TK. Lower ghrelin levels and exaggerated postprandial peptide-YY, glucagon-like peptide-1, and insulin responses, after gastric fundus resection, in patients undergoing Roux-en-Y gastric bypass: a randomized clinical trial. Obes Surg. 2012 Nov;22(11):1761-70.

Figure 1

 1 Figure 1. Flowchart

Figure 2 :Figure 3 :

 23 Figure 2: Evolution of % EWL after conversion of the VBG to RYGB for weight loss failure

Table 2

 2 ).

	Late complications of conversion of VBG to RYGB appeared in 23 patients (28.75%) of

which 11

(

Clavien III; 47.8%) benefitted from re-operative procedures. Medical late complications (Clavien I -II; n= 12, 15%) are presented in table 2.

Table 1 :

 1 Pre-operative workup and results

Table 2 .

 2 Post operative complications according to Clavien-Dindo classification in the per protocol

	population (n=80)

Table 3 :

 3 Functional results of patients operated for complications of VBG (GERD: Gastro

	Esophageal Reflux Disease)

Table 4 .

 4 Factors involved in weight loss success or failure of the conversion of Vertical

	Banding Gastroplasty (VBG) to Roux-en-Y Gastric Bypass (RYGB) according to Reinhold's
	criteria (BMI = Body Mass Index)

Table 1 :

 1 Pre-operative workup and results

Table 2 .

 2 Post operative complications according to Clavien-Dindo classification in the per protocol population (n=80)

	Early Complications
	Complication	Chalon-sur-Saône, n=30 (%) Lyon, n=50 (%)	Total, n=80 (%)
	Clavien I	0	1 (1,9)	(1.25)
	Dysphagia	0	1 (1,9)	(1.25)
	Clavien II	3 (10)	3 (6)	(7.5)
	Sepsis	1 (3.3)	2 (3,9)	(3.75)
	Urinary tract infection	0	1 (1,9)	(1.25)
	Anastomotic ulcer	2 (6.6)	0	(2.5)
	Clavien III	5 (16.7)	8 (15,6)	(16.25)
	Stenosis of the biliary limb	1 (3.3)	2 (3,9)	(3.75)
	Biliary limb leak	1 (3.3)	0	(1.25)
	Gastro-jejunal stenosis	0	1 (1,9)	(1.25)
	Gastro-jejunal leak	1 (3.3)	1 (1,9)	(2.5)
	Staple line bleeding	0	1 (1,9)	(1.25)
	Parietal thickness dehiscence	0	2 (3,9)	(2.5)
	Strangulated incisional hernia	0	1 (1,9)	(1.25)
	Small bowel kinking	2 (6.6)	0	(2.5)
	Total	8 (26.6)	12 (22,6)	(25)
	Late Complications
	Complications	Chalon-sur-Saône, n=30 (%) Lyon, n=50 (%)	Total, n=80 (%)
	Clavien I	2 (6.6)	8 (15,6)	(12.5)
	Dumping syndrome	2 (6.6)	5 (9,8)	(8.75)
	Incisional Hernia	0	2 (3,9)	(2.5)
	Gastro-gastric fistula	0	1 (1,9)	(1.25)
	Clavien II	1 (3.3)	1 (1,9)	(2.5)
	Posterior cord syndrome	0	1 (1,9)	(1.25)
	Entero-cutaneous fistula	1 (3.3)	0	(1.25)

Clavien III 5 (16.7) 6 (11,7) (13.75)

	Recurrence of incisional hernia	0	2 (3,9)	(2.5)
	Incisional hernia	0	1 (1,9)	(1.25)
	Gastro-jejunal fistula	0	1 (1,9)	(1.25)
	Long alimentary stump limb	1 (3.3)	0	(1.25
	Internal Hernia	0	2 (1,9)	(2.5)
	Small bowel kinking	2 (6.7)	0	(2.5)
	Abdominal pain	2 (6.7)	0	(2.5)
	Total	8 (26.6)	15 (29)	(28.75)

Conflict of interest. The authors declare that they have no conflict of interest.

Pre-conversion n, (%)

Post conversion, n (%)

Rate of remission (%)


A Peultier

Thierry Pauss

Ribeiro

M A Hernandez-Shek

S Mottelet

P Peultier

A Pauss

Thierry Ribeiro
email: thierry.ribeiro@unilasalle.fr

rheological properties of coarse biomass treated by dry anaerobic digestion

Keywords: Complex fluid, Dry anaerobic digestion, Lignocellulosic biomass, Rheology, Yield stress, Flowability

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The lignocellulosic biomass is defined as the biodegradable fraction of products and residues of biological origin, the term includes all the agricultural by-products and farm waste. The possibility of transforming lignocellulosic wastes into valuable products has given rise to a large number of physicochemical and biological processes. Among them, the Anaerobic Digestion (AD) process is a microbial process in which organic matter is converted into biomethane by a bacterial consortium in anoxic conditions [START_REF] Karthikeyan | Bio-energy recovery from high-solid organic substrates by dry anaerobic bio-conversion processes: A review[END_REF][START_REF] Sawatdeenarunat | Anaerobic digestion of lignocellulosic biomass: Challenges and opportunities[END_REF]. Today, AD technologies have proved their versatility in producing renewable energy from biomass and from organic wastes, avoiding in this way, the harmful effects of greenhouse gas emissions from fossil energy production [START_REF] Cadavid-Rodríguez | Production of volatile fatty acids from wastewater screenings using a leach-bed reactor[END_REF]. The rising demand for renewable energy and the physicochemical and biological variability of biomasses and organic wastes have led to the appearance of several AD technologies. These could be classified according to the Totals Solids (TS) content inside the reactor, the temperature of the process and the reactor type: Complete Stirred Tank Reactor (CSTR), Batch Reactor and Plug Flow Reactor (PFR). Wet Anaerobic Digestion (W-AD) treating biomass with fewer than 15% TS is the most popular technology in Europe [START_REF] Kothari | Different aspects of dry anaerobic digestion for bio-energy: An overview[END_REF][START_REF] Rocamora | Dry anaerobic digestion of organic waste: A review of operational parameters and their impact on process performance[END_REF]. In these systems, the low concentration of TS and the low-medium viscosity allow the use of stirred rectors for continuous biogas production. Despite the efficiency of W-AD and CSTR in the methane recovery from biomasses, this is not always adapted to all biomass types, principally coarse biomass. High reactor volumes, physical biomass pretreatments, pumping and mixing requirements made W-AD technologies profitable for very large agricultural operations but less suitable for small and medium biogas producers with lower investment capacities [START_REF] André | Solid anaerobic digestion: State-of-art, scientific and technological hurdles[END_REF].

In contrast to W-AD, Dry Anaerobic Digestion (D-AD) refers to systems treating concentrated biomass with more than 15% of TS. The fact of producing biogas from high-solid content biomasses makes these technologies very attractive for the treatment of agricultural by-products, green waste, farm waste and Organic Fraction of Municipal Solid Waste (OFMSW) [START_REF] Khalid | The anaerobic digestion of solid organic waste[END_REF][START_REF] Rocamora | Dry anaerobic digestion of organic waste: A review of operational parameters and their impact on process performance[END_REF]. Considering the presence of coarse particles and high medium viscosity, mixed reactors are no longer a sustainable option, instead, Leach Bed Reactors (LBR) operated in batch are the most common D-AD technology for the treatment of straw cattle manure with commercial technologies such as Bekon, SEBAC and the Naskeo-Methajade, followed by continuous PFR technologies such as the DRACO, KOMPOGAS, VALORGA and the EASYMETHA Technologies [START_REF] André | Solid anaerobic digestion: State-of-art, scientific and technological hurdles[END_REF][START_REF] Peultier | Dispositif pour la méthanisation par voie sèche de matière comprenant du fumier[END_REF][START_REF] Rocamora | Dry anaerobic digestion of organic waste: A review of operational parameters and their impact on process performance[END_REF]. Given the possibility of feeding D-AD reactors with non-pretreated biomass, the lack of pumping and mixing systems and their reduced volume compared with W-AD technologies; D-AD seems to be a more attainable technology for small and medium producers treating farm and local biomasses.

Rheological issues have been highlighted as major technical and scientific limitations for both continuous and batch processes D-AD technologies [START_REF] André | Solid anaerobic digestion: State-of-art, scientific and technological hurdles[END_REF][START_REF] Brambilla | Rheological properties of manure/biomass mixtures and pumping strategies to improve ingestate formulation[END_REF][START_REF] Lague | Engineering of land application systems for livestock manure: A review[END_REF]. Rheology concerns the study of matter deformation and flowability; rheological behavior of biomass would depend on the composition, the particle size and the physical properties [START_REF] Dasari | The effect of particle size on hydrolysis reaction rates and rheological properties in cellulosic slurries[END_REF][START_REF] Lam | Bulk density of wet and dry wheat straw and switchgrass particles[END_REF][START_REF] Samaniuk | Rheology of concentrated biomass[END_REF]. Conventional rheological determinations have already been successfully performed on concentrated animal slurry without the presence of coarse particles and long fibers. Depending on the TS content, this would behave as a Newtonian fluid or as shear-tinning fluid with a yield stress value [START_REF] Harold | Rheology and Flowability Properties of Liquid Dairy and Swine Waste[END_REF][START_REF] Hreiz | Rheological characterization of raw and anaerobically digested cow slurry[END_REF][START_REF] Landry | Physical and rheological properties of manure products[END_REF][START_REF] Stickel | Rheology measurements of a biomass slurry: An inter-laboratory study[END_REF]. However, in practice, animal slurry may contain coarse particles from the materials used as litter, which is basically lignocellulosic materials (straw, wood shaving, sawdust…) used to offer animal comfort, to avoid animal injuries and to keep dried the resting surfaces in animal sheds. The mixture of water, animal slurry and the lignocellulosic material would result in a new biomass material known in literature as spent livestock bedding or straw-cattle manure, which is an agricultural waste with a valuable energy recovery potential using D-AD process [START_REF] Riggio | Mesophilic anaerobic digestion of several types of spent livestock bedding in a batch leach-bed reactor: substrate characterization and process performance[END_REF]). In France for example, 70% of the total amount of wheat straw (WS) produced is used as animal bedding material, principally by dairy and beef cattle. Meaning a straw-cattle manure production estimated at 68.7 Mt/year representing a theoretical methane potential of 2.9*10 9 Nm 3 CH4/year and 28.3 TWh of theoretical power generation [START_REF] Degueurce | La méthanisation par voie sèche agricole appliquée aux fumiers de bovins : optimisation de la recirculation des lixiviats[END_REF].

In the literature, few data are available on the rheological behavior of coarse biomass. This is explained by the fact that conventional rotating rheometers are not well adapted to heterogenous biomass having coarse particles and long fibers. So far, designed rotational equipment allows determining the rheological properties of concentrated biomass containing particles of a maximal size of 2 mm to 3 cm. This is the case of the torque rheometer used to measure the rheological properties of industrially grinded corn stover and of the large size rheometer that has been used to measure the rheological characteristics of industrially shredded straw-cattle manure [START_REF] Burlawar | Effect of temperature on the rheology of concentrated fiber suspensions[END_REF][START_REF] Ruys | Rhéologie des résidus agricoles pour un procédé multi-étapes de méthanisation en voie sèche[END_REF][START_REF] Samaniuk | Rheological modification of corn stover biomass at high solids concentrations[END_REF][START_REF] Schneider | Rheological properties of digestate from agricultural biogas plants: An overview of measurement techniques and influencing factors[END_REF][START_REF] Viamajala | Rheology of corn stover slurries at high solids concentrations -Effects of saccharification and particle size[END_REF][START_REF] Zafari | Livestock manure management and pelleting[END_REF]. All these studies agreed that concentrated biomass belongs to the category of yielding fluids and has a rheofluidifying behavior characteristic of non-Newtonian fluids where viscosity decreases non-linearly under a shear stress. Two rheological parameters are usually estimated: the yield stress (τo) which represent the stress required to initiate the flow of the material [START_REF] Derakhshandeh | Rheology of pulp fibre suspensions: A critical review[END_REF], second is the apparent viscosity, which measures fluid's internal flow resistance. Unlike Newtonian fluids, this would depend on the shear rate for viscoplastic fluids. Despite its importance in rheological characterization of fluid, it appears to be poorly documented for coarse biomass in the literature.

Knowledge of flow behavior and rheological determinations are essential in the engineering design, modeling and operation of a process in which coarse biomass handling is involved [START_REF] Lague | Engineering of land application systems for livestock manure: A review[END_REF]. This especially limits the application of continuous D-AD. In PFR operation, matter is required to flow through the reactor volume to enhance biogas production. Therefore, rheological limitations must be measured and monitored to ensure accurate operation of the continuously fed anaerobic reactor and to measure the effect of pretreatment operations on the rheological behavior of the biomass. Fortunately, D-AD process is not the only field of knowledge concerned with the rheological determination of fluids incorporating coarse particles; the concrete industry, the food industry and the geosciences have also been concerned with this issue. In response, alternative rheological devices have been designed to achieve the rheological behavior of the studied matrices. Overall, the cement industry seems to have made the greatest advances in the development and international standardization of these devices; the slump test, the L-Box, Vfunnel, J-Ring and sieve segregation are the most popular devices for determining the flowability and workability of fresh concrete [START_REF] González-Taborda | Tools for the study of selfcompacting recycled concrete fresh behaviour: Workability and rheology[END_REF]. The use Bostwick and Adams consistometers by the food industry to measure the consistency of viscoplastic fluids is longstanding [START_REF] Perona | Bostwick Degree and Rheological Properties: an Up-to-date Viewpoint[END_REF]. On the other hand, the use of Casagrande box and the ring shear test have been proved to be a suitable method for the cohesion and friction angle determination of soil aggregates [START_REF] Bernard | L'essai de cisaillement à la grande boîte de Casagrande : un banc expérimental pour évaluer les propriétés des sols grossiers et pour d'autres applications en géomécanique[END_REF]. The simplicity, the low-cost compared with rheological laboratory equipment and the possibility of carrying out these tests in field offer an interesting approach for the rheological determination coarse biomass.

Attempts have been made in order to adapt some of these devices to the rheological characterization of biomass. [START_REF] Baudez | Rheological interpretation of the slump test[END_REF] have used a slump test for the characterization of pasty organic solids. Subsequently, Garcia-Bernet et al. (2011) used it for the yield stress determination of anaerobically digested biomass. The V-funnel and the Abrams cone from concrete industry have been used in the viscosity determination and yield stress of straw-cattle manure [START_REF] Fernandez | Methods for the evaluation of industrial mechanical pretreatments before anaerobic digesters[END_REF]. No scientific evidence of the use of consistometers for biomass rheological characterization has been found in literature. [START_REF] Landry | Physical and rheological properties of manure products[END_REF] have used a shear-box to determine the rheological properties of manure products, the limited sample volume of the shear box and the presence of coarse aggregates present in most of the samples may have been responsible for the inconclusive results. None of the previous studies has been able to determine rheological behavior on biomasses having long lignocellulosic fibers, which is the case for the usual substrates for industrial methane recovery by D-AD technologies.

The rheological behavior of coarse lignocellulosic biomass remains an open question. Therefore, the aim of this study was to adapt three alternative devices to the rheological characterization of coarse lignocellulosic biomass. For this purpose, prototypes of the slump test, the Botswick consistometer and the shear-box devices were volume scale-up for the rheological analysis of coarse biomass. Analyses of simulated samples of straw-cattle manure have allowed identifying the advantages and limitations of each device in characterizing the rheological behavior of biomass.

Effects on physical and rheological characteristics due to changes in fiber length, TS content and the ratio of bedding material to animal slurry are discussed.

Theory and scientific background of rheological devices

Slump Test

The slump test was initially conceived for the empirical determination of consistency and workability of numerous mineral suspensions in the concrete industry by Duff Abrams in the early 20s. Since then, this is a standard technique wildly used throughout Europe and Asia (ASTM, 2015).

The slump test consists of measuring the deformation of a material induced by gravity and its own mass. Given the simplicity of the procedure and use of low-cost materials, the this is one of the most popular methodologies to ensure different loads of concrete under field conditions [START_REF] Domone | The slump flow test for high-workability concrete[END_REF]. For its realization, a conical or cylindrical container is filled with the material to be tested.

The container is rapidly risen allowing the collapse and deformation of the material. According to the rheological properties of the sample, two types of analyses can be performed (Fig 1a); analysis of the slump height and analysis of the spreading diameter, this last is often referred as spread flow test (BSI, 2010a). Mathematical and physical interpretation of the slump test results has made possible the correlation of its results with rheological properties of the materials. Hence, this has gained a special attention among the scientific community to determine the yield stress and the viscosity when the use of conventional rheometers is not sustainable given the presence of coarse aggregates and the need of carry out these measures in the field.

Since Abrams cone was the first experimental set-up developed, the conical shape has been more thoroughly studied and used than cylindrical shape in a slump test. The theoretical cone analysis relating the slump height with the yield stress was initially performed by [START_REF] Murata | Flow and deformation of fresh concrete[END_REF]. [START_REF] Chandler | The stacking and solar drying process for disposal of bauxite tailings in Jamaica[END_REF] adapted the slump test to a cylindrical shape because of simplicity and easier filling of the container. Nevertheless, Chandler mathematical interpretation failed in the analytic correlation of the slump height with the yield stress. Successful outcomes in the experimental and analytical interpretation of cylindrical slump test were obtained by [START_REF] Pashias | A fifty cent rheometer for yield stress measurement[END_REF]. According to their results, the velocity at which the container is lifted and the surface on which the test is undertaken do not affect the final slump height. These considerations might be correct for "true slump" analysis, however, in collapse or flow slump test the lifting velocity and the surface friction may play an important role in the results interpretation [START_REF] Gao | Spread is better: An investigation of the mini-slump test[END_REF]. A valuable asset of cylindrical shape over conical shape is the possibility of assembling additional weight on the top of the sample to increase pressure and the height of the slump, which allows the determination of high yield stress materials [START_REF] Baudez | Rheological interpretation of the slump test[END_REF]. The cylindrical slump test has been used for the rheological characterization of concentrated biomass which is the case of dry or semi-dry anaerobic digested biomass, effects of higher solid concentration and particle size over the yield stress have been studied by Garcia-Bernet et al. (2011) for biomass smaller than 50 mm using a sample volume of 1 L and the following cylindrical slump test model equation developed by [START_REF] Baudez | Rheological interpretation of the slump test[END_REF]:

= - = + - 2 1 + ( + ) 2 (1) = (2)
Where the slump ( ) is the difference between the initial ( ) and final ( ) height, this parameter directly depends on the water content and the aggregate shape and size distribution. is the gravity, is the density of the material, is the yield stress and represents the equivalent height of the added mass. This last parameter is the relation between the added mass, the density and the surface area of the cylinder.

Regarding samples undergoing a collapse slump a second analysis can be performed; the slump flow test is performed in more "fluid" samples which would show the evolution of the initial diameter [START_REF] González-Taborda | Tools for the study of selfcompacting recycled concrete fresh behaviour: Workability and rheology[END_REF][START_REF] Saak | A generalized approach for the determination of yield stress by slump and slump flow[END_REF]. Slump flow test is also a standardized method normed (BSI, 2019). In recent studies, [START_REF] Bouvet | The mini-conical slump flow test: Analysis and numerical study[END_REF]; [START_REF] Liu | Axisymmetric viscoplastic dambreaks and the slump test[END_REF]; Sung [START_REF] Choi | Estimation of rheological properties of UHPC using mini slump test[END_REF] have performed and simulated the slump flow test considering the evolution of the spreading diameter as a function of time and its correlation with the rheological properties. The curve fitting on the spreading diameter-time evolution is executed by the equation:

( ) = + ( + ) (3)
Where, Dn refers to the diameter evolution as a function of time, α is a constant, β is the slope of an increase in diameter over logarithmic time scale and γ is delayed period or the time taken until the material prescribes a linear deformation or flow. The delayed period is related to the yield stress of the material [START_REF] Gao | Spread is better: An investigation of the mini-slump test[END_REF] which prevents the flow at the beginning of the experiment and the slope of the development period which is inversely influenced by the apparent viscosity [START_REF] Liu | Axisymmetric viscoplastic dambreaks and the slump test[END_REF].

Consistometer

Also called "Flum test or L-box" in the concrete industry or Botswick consistometer in the food industry, the consistometer is wildly used to describe and analyze the flow behavior of viscoplastic suspensions (Fig 1b). It consists of a rectangular device composed of two parts: the sample chamber equipped with a dame brake and the canal of flowing [START_REF] Perona | Bostwick Degree and Rheological Properties: an Up-to-date Viewpoint[END_REF]. Research works for concrete suspensions have been conducted using a standard L-box (BSI, 2010b). To calculate the yield stress as a function of the spread length [START_REF] Nguyen | Correlation between L-box test and rheological parameters of a homogeneous yield stress fluid[END_REF] have developed the following equation:

= ! 2 ℎ + ! 2 ! ! + 2ℎ (4)
Where τo is the yield stress, L is the spread length, ho is the thickness of the deposit at x=0, lo is the width of box, ρ is the density and g gravity. [START_REF] Shan | Experimental investigation of flow of fresh self-compacting concrete in improved Lbox[END_REF] have proposed the presence of different zones in the consistometer profile while the experiment takes place. The hydraulic jump zone which is the nearest to the dame brake, the stable flow zone where the stable flow zone and a laminar flow zone are coupled, the sedimentation zone and the hydraulic drop zone. The physical and rheological properties of materials tested would modify the zones repartition, thus the distance achieved by the hydraulic drop zone.

Shear-box

Also named "Casagrande-box", the shear-box consists of two independent rectangular half-boxes on a horizontal plane which corresponds to the shear plane. For its realization, a sample is placed into the shear-box, then a normal effort is applied for compressing the sample. The upper box is tracked or pushed and the sample is sheared. Shear strength data is registered and analyzed to identify the maximal shear strength in which the sample is deformed. Finally, the sheared sample is taken out and the test will be performed with a new sample using different normal efforts. The results of the test are depicted using the Coulomb law expressing the rupture domain of a material (Fig 1c), the shear force (τ) directly depends on the applied normal effort (σ) and two intrinsic material characteristics: the internal friction angle (φ) and the cohesion (c) [START_REF] Labuz | Mohr-Coulomb Failure Criterion[END_REF].

τ = σ tan φ + * (5)
Despite the potential of the shear-box characterizing rheology of different solid materials, its use remains dedicated to research on fine soils because of the limited sample shear plane area (from 5*5 cm to 10*10 cm). However, new shear-box size (from 30*30 cm to 50*50cm) has recently been implemented to determine shear strength of coarse aggregates such as ballasts [START_REF] Bernard | L'essai de cisaillement à la grande boîte de Casagrande : un banc expérimental pour évaluer les propriétés des sols grossiers et pour d'autres applications en géomécanique[END_REF]. A large volume shear box has been also used to determine landfill stability using a large shear-box having a shear plane area of 1*1.8 m and a working volume of 2 m 3 . Biologically treated, screened waste has shown friction angles from 33° to 38° [START_REF] Kölsch | Shear strength of waste, in: Third Internantional Workshop "Hydro-Physico-Mechanis of Landfills[END_REF]. In an attempt for characterizing manure products, [START_REF] Landry | Physical and rheological properties of manure products[END_REF] have performed measures of cohesion and internal friction angle using a shear-box 1.1L with non-screened samples, cohesion pressure and friction angle for cattle manure were around 3 kPa and 35° respectively. However, the presence of large aggregates and the limited volume of the box have been found responsible for inconclusive results.

Materials and Methods

Experimental set-up

Slump Test

The experimental device built for this research work is depicted in Fig 2a . A cylindrical PVC tube diameter 20 cm, height 34 cm and total sample volume of approximately 10 L was used as a sample chamber. This sample volume was identified as representative based on the heterogeneity of the biomass, the presence of long lignocellulosic fibers and the manipulability of the test. The cylinder mass was increased by adding iron bars in order to avoid leaks of material before the test realization. All the experiments were conducted over a high-density polyethylene (HD-PE) surface of 1 m 2 . A disk (0.25 kg) of the same diameter of the cylinder was used to prevent the material from sticking to the inner perimeter of the cylinder. Weights (2 -10 kg) can be loaded on the disc plate in order to increase the zo in Eq. ( 2) and determine high yield stress matrices. At first, the cylinder was gently filled using the hands and trying to avoid air cavities in the sample. Once the disc plate and the weights were mounted and the cylinder chamber was removed allowing the collapse and flow of the material. A graduated vertical axis (1 cm diameter -0.4 m height) was used to measure the final slump height. The final slump was measured 30 min after the start of the test. This experimental time was chosen on the basis of previous experiments, the results suggesting that more than 95% of the slump height was reached in this time. Once the slump height was measured, Eq. ( 1) was used to calculate the yield stress of the material. In the samples in which a collapse slump was observed, a video recording of the diameter evolution was made by means of a cell phone (One plus 6.0) placed on a PVC structure above the vertical axis at a height of 1.32 m from the HD-PE surface (Fig 2a). Diameter changes were recorded at a rate of 30 frames per second. A video processing technique was performed to measure the evolution of the diameter over time of the selected photograms using AutoCAD to calculate the surface and perimeter of the spreading after the collapse of the material. The roundness factor (RF) was selected as the parameter that determines the circular shape of spreading flow and the satisfactory performance of the experiment, using the following equation:

+ = , -./ -0 (6)
Whereis the polygon surface and -0 is the circle surface having the same perimeter as the polygon; the value of RF would oscillate between 0 for a line and 1 for a perfect circular shape.

Experiments having at least 0.9 RF value were considered as satisfactory. Curve fitting was performed on the data expressing the diameter evolution with time using Eq. (3). Finally, the delayed period in the spreading of materials was estimated using the methodology developed by Sung [START_REF] Choi | Estimation of rheological properties of UHPC using mini slump test[END_REF].

Consistometer

Scale-up of Bostwick consistometer in PVC was built in our laboratory (Fig 2b). The rectangular chamber (14.7L) was filled gently with the material. The total height of the chamber is 0.5 m, in this work the chamber was filled at the height of the dam break (0.2 m) simulating the operational conditions of the Botswick consistometer. The width of the box was 0.25 m and the total spreading distance 0.75 m. The consistometer is equipped with a manual lifting jack allowing an inclination angle between 0° and 30°. For all the tests in the present work, the angle of inclination was 0°.

Once the sample is loaded, the dam break is promptly open allowing the sample to flow. Video analysis made it possible to study the flow. First, the evolution of the spreading distance was analyzed following the same steps performed equally in the slump test analysis (Sung [START_REF] Choi | Estimation of rheological properties of UHPC using mini slump test[END_REF]. The same analysis developed by [START_REF] Shan | Experimental investigation of flow of fresh self-compacting concrete in improved Lbox[END_REF] has been performed in this work by analyzing the flow statements in each experiment. The yield stress has been also calculated using

Eq. ( 4), values of yield stress using the slump test and the consistometer have been compared. As in slump test technique, a sample having a spread along the box length, analysis proposed by Sung [START_REF] Choi | Estimation of rheological properties of UHPC using mini slump test[END_REF] was developed.

Shear-box

A rectilinear shear-box with a shear plane area of 0.5*0.3 m has been constructed in our laboratory

Materials

Cow slurry and WS were sampled from farms in the northern of France. Cow slurry had a TS content of 12.8% and a measured density of 995 ± 0.4 kg m -3 . Physical aspect of WS samples has been given as supplementary materials. The WS particle density (1275 kg m 3 ) reported by [START_REF] Lam | Bulk density of wet and dry wheat straw and switchgrass particles[END_REF] were used to calculate the volume of particles and achieve porosity calculation. Regular mayonnaise used as control fluid was supplied by Promocash Ltd. Table 1 presents the physical characteristics of the composites prepared from biomass used in this work.

Experimental procedure

In order to evaluate the use of the proposed devices, the methodology has been divided in three phases as follows:

• Phase A: Selection of the control fluid and comparing with literature values for animal slurry

In literature, new rheological devices have always been first calibrated using a well-known rheological behavior fluid (i.e. clay, Carbopol gel) before determination of biomass rheology [START_REF] Baudez | Rheological interpretation of the slump test[END_REF][START_REF] Klingenberg | Rheometry of coarse biomass at high temperature and pressure[END_REF][START_REF] Ruys | Rhéologie des résidus agricoles pour un procédé multi-étapes de méthanisation en voie sèche[END_REF]. In order to identify the most convenient fluid to calibrate our rheological devices several alimentary plastics yield fluids have been considered and a check list was performed as follows: first of all, their rheological behavior must be similar to animal slurry. Agricultural wastes are usually heterogenous mixtures between animal slurries and bedding materials (straw, wood shaving, sawdust). Therefore, the standard fluid must emulate the presence of animal slurry. Secondly, control fluid had to be a food product to be used in available devices in rheological laboratories; the characterization of wastes is strictly forbidden in order to protect the health of laboratory users and avoid the chemical, physical or biological contamination of the equipment. In the third place, low rheological and biological evolution in environmental conditions of the control fluid were desired, the rheological behavior must be guaranteed by the commercial supplier. Finally, given the tested volumes of the constructed rheological devices, the control fluid should be commercially available in important volumes, regular in quality with time and also as cheap as possible. Considering the previous check list, the following fluids were considered: concentrated tomato puree, mayonnaise, prepared gels from starch, toothpaste and fruit jam.

• Phase B: Rheological characterization of control fluid

Using the previous check list, mayonnaise was chosen as control fluid, rheological characterization was performed and compared with rheological behavior of cow slurry. The rheological measurements of the control were performed in duplicate using a stress-controlled rheometer MCR92 (Anton-Paar) equipped with a parallel-plate ribbon (diameter 50 mm) and a gap of 1.5 mm.

To validate the use of mayonnaise at room temperature for the calibration section, several rheological measurements were performed at temperatures ranging from 10 to 25°C. The experimental procedure was conducted as follows: a sample of mayonnaise is spread on the bottom plate of the rheometer, then the parallel-plate ribbon is lowered pressing the sample at the desire gap. Any excess material was removed and the thermal chamber was closed. Once the sample reaches the desired temperature, the sample was rotationally sheared by an increasing ramp of the strain from 0 to 25 s -1 . In addition, the Bostwick degree was used to compare the consistency of the control and cow slurry at different TS content according to the protocol used by [START_REF] Perona | Bostwick Degree and Rheological Properties: an Up-to-date Viewpoint[END_REF].

• Phase C: Calibration of constructed devices and rheological characterization of mixtures of animal bedding (Wheat Straw) and the control fluid

In this phase, several mixtures of the control fluid with wheat straw were prepared in order to emulate the straw-cattle manure. Non-Shredded and Shredded WS were used in the calibration section work. Non-Shredded WS was sampled from a dairy farm in Beauvais (France). In this farm cylindrical bales of WS are unrolled on cow sheds as litter without mechanical fiber size reduction twice a day. According to fiber-length distribution analysis of non-shredded WS, 93.7% of the total mass have length sizes between 5 and 20 cm. Fiber's diameter was between 3 and 5 mm. By other hand, Shredded WS was collected from an automatic litter system (Stromatic-Germany) on a dairy farm located in Talmas (France). This system is equipped with a hammer mill which led to fiber breakdown (<5 cm in length). Once the wheat straw bales are shredded, an air distribution system spread the bedding material on the cowshed's floor. Cowshed's mass balances were performed to estimate the ratio between WS and feces. In order to identify the bedding material treatment and operations in each farm, a research survey has been performed, its results suggested that between 3 and 8 kg of WS day -1 cow -1 is spread in the stalls; in addition, an adult cow would produce between 30 and 50 kg of feces per day (Font-Palma, 2019). Using the previous values in mass balance, WS represents between 4 and 14% of the total mass of animal manure, whereas 86 to 96% of the total mass is given by the feces and urine. Total solids (TS) content in straw-cattle manure has been found between 14 and 27%. However, in systems non-equipped with separation between urine and feces and bedding material the TS content has been found around 10%. Based on the mass balances results, eight mixtures w/w of WS and the control fluid (mayonnaise) were prepared to calibrate each of the constructed rheological devices (Table 1). This calibration has allowed to identify the range of applications and limitations of each device for the analysis of real biomass samples.

Physical Measurements

Total solid content, bulk density and total porosity of biomass samples were measured. Biomass samples were oven dried at 105 °C for 24 h to determine the total solid content (APHA, 2017). The bulk density of samples was calculated using the mass-volume ratio measurement and porosity (1)

with the following equation, where 2 3 is the empty volume over the total volume (2 4 ). The total volume is the sum of the empty volume and the volume occupied by the particles. viscosity of mayonnaise were reported in literature [START_REF] Izidoro | Sensory evaluation and rheological behavior of commercial mayonnaise[END_REF][START_REF] Ma | Rheological characterization of mayonnaise. Part II: Flow and viscoelastic properties at different oil and xanthan gum concentrations[END_REF][START_REF] Maurer | Etude rhéologique et texturales de dispersions alimentaires[END_REF]. Odds-on viscosity values in literature are mainly explained by temperature effects and differences in the composition of commercial mayonnaise. Thus, in order to avoid experimental bias in the results in next sections of this work, rheological determinations with the slump test, the consistometer and the shear-box were performed at room temperature from 18.5 to 23.2 °C. Rheological analysis in literature for cow slurry has reported a shear-thinning Herschel-Bulkey behavior, similar behavior was observed for the mayonnaise, having a yield stress and the decrease of viscosity when the shear rate increases (Fig 3a). Moreover, experimental values of the rheological characterization for mayonnaise were close to the values for cow slurry presented in literature; 5 to 50 Pa for yield stress and 0.1 and 22.9 Pa.s -1 for apparent viscosity varying TS content and the sample temperature (Achkari-Begdouri and Goodrich, 1992;[START_REF] Chen | Rheological properties of sieved beef-cattle manure slurry: Rheological model and effects of temperature and solids concentration[END_REF][START_REF] El-Mashad | Rheological properties of dairy cattle manure[END_REF][START_REF] Hreiz | Rheological characterization of raw and anaerobically digested cow slurry[END_REF][START_REF] Landry | Physical and rheological properties of manure products[END_REF]. Botswick consistometer was used in another attempt to compare rheological properties of cow slurry and mayonnaise. The Bostwick test was suitable for samples of cow slurry having between 5 and 12.5% of TS (Fig 3b). Eq. ( 8)

describes the logarithmic decrease in the flow distance (L) at 10 seconds caused by the increase of TS.

= -30.82 (%;<) -64.39 = 0.97

Flow behavior of mayonnaise resulted similar to cow slurry flow having between 10.0 and 12.5 %TS at 20°C (Fig. 3b). Using Eq (8) and the mayonnaise Botswick distance, the equivalent %TS would be around 10.6 %TS. This value was used in Table 1 in order to estimate the possible TS content in real straw-cattle manure samples.

Although mayonnaise has a different chemical composition from cow slurry, similarities on rheological data and flow behavior of both fluids confirm the fact that mayonnaise was a sustainable control fluid to test and calibrate the developed rheological devices proposed in this work for the analysis of coarse biomass.

Effect of physical properties on rheological determinations

Bulk density and porosity play a key role in D-AD process. These would determine the physicochemical and biological interactions between the solid and the liquid phase in the D-AD digesters. Bulk density and porosity depend on material composition, moisture content, particle shape, size and orientation, specific density of individual particles, and applied axial pressure [START_REF] Lam | Bulk density of wet and dry wheat straw and switchgrass particles[END_REF][START_REF] Mani | Effects of compressive force, particle size and moisture content on mechanical properties of biomass pellets from grasses[END_REF][START_REF] Naimi | Data-based equation to predict power and energy input for grinding wheat straw, corn stover, switchgrass, miscanthus, and canola straw[END_REF]. Rheological and physical properties are intimately linked. The reliance of rheology on bulk density is easily seen in Eq (1) and Eq (4),

where yield stress is dependent on the bulk density and the deformation of materials. It is noteworthy to mention that despite the importance of bulk density, few studies have highlighted its relation with rheological behavior of biomass. However, given the fact that bulk density depends on several material parameters, rheology of biomass is also a function of all these parameters. The relation between TS content and bulk density has been studied in the past for animal manure; mathematical models have been proposed relating the TS content with the bulk density (Achkari-begdouri and Goodrich, 1992;[START_REF] Chen | Rheological properties of sieved beef-cattle manure slurry: Rheological model and effects of temperature and solids concentration[END_REF][START_REF] Houkom | Effect of Moisture Content on Thermal Diffusivity of Beef Manure[END_REF][START_REF] Wang | Physical properties of dairy manure preand post-anaerobic digestion[END_REF], Model of bulk density evolution with the TS content proposed by [START_REF] Landry | Physical and rheological properties of manure products[END_REF] showed in Eq. 9 were compared with the results of this work; the bulk density of the prepared mixtures of mayonnaise and WS is depicted in Fig 3c . Considering the types of WS used in this work, two models for bulk density evolution with TS content have been proposed in Eq. ( 10) and Eq (11).

Dairy cattle [START_REF] Landry | Physical and rheological properties of manure products[END_REF] = 0.0367 ;< A -0.873 ;< + 8.9 ;< + 1000

(9)
Non-shredded WS = 0.0493 ;< A -3.6305 ;< + 44.699 ;< + 758.24

( = 0.98) (10)
Shredded WS = 0.0827 ;< A -6.1497 ;< + 104.22 ;< + 365.87

( = 0.98) (11)
Porosity is a measure of the void spaces in a material. In Table 1 porosity values and TS content increase exponentially with the increase of WS content. In this work, prepared mixtures with shredded WS have shown lower porosity values than those prepared with non-shredded WS.

Porosity is an important parameter since liquid and solid animal dejection would be stocked in the void spaces of the litter material; micropores would absorb water and macropores would stock solid animal manure [START_REF] Hernandez-Shek | Quantifying porosity changes in solid biomass waste using a disruptive approach of water retention curves (WRC) for dry anaerobic digestion[END_REF]. Porosity of the bedding material would determine the holding capacity of animal dejection, consequently, in cow sheds using shredded WS the frequency of litter change is four times per day; in contrast, systems using non-shredded WS the litter is changed twice a day. This fact is explained since water absorption is higher in shredded WS and the hold capacity is rapidly reduced. Thus, in order to keep dried the sheds, it is necessary to raise the frequency of litter change. Porosity of the bedding material would determine the distance and the interaction between particles affecting the matter friction and finally the rheological properties of the biomass determined by using the proposed devices in the next sections.

Slump test results

Yield stress Determination Using the Slump Height

Yield stress values for mixtures analyzed using the slump test are depicted in Fig. 4a. Slump test of mayonnaise has indicated similar results from those determined in section 4.1 using the rheometer, 44 ± 2 Pa and 43 ± 3 Pa, respectively. The sample volume (10 L) for this version of the slump test was found as sustainable for the analysis of samples with long fibers until 26.5% TS, beyond this value the test could not be achieved due to the large porosity of the sample, instead of slump development, the only deformation observed was the compression of the sample and a reduction of the volume caused by the adding mass.

Yield stress increases exponentially with WS content; this effect is more pronounced in the case of non-shredded WS whose values of yield stress were always higher than the samples with shredded WS. Two exponential models were established for the yield stress evolution of samples with coarse biomass (Table 2) using Eq. (12) proposed by Garcia-Bernet et al. (2011b).

C = -(D (E * 4F) -1) ( 12
)
Where C is the yield stress, a and b are material dependent constants? The parameter a were between 10.02 and 15.65 for shredded and non-shredded WS respectively. The value of b obtained for this research was 0.158 for both WS types used, meaning that the particle size does not have a huge effect on this parameter. Similar values of a and b were obtained by Garcia-Bernet et al.

(2011b) analyzing the yield stress of organic solid waste grinded at two different sizes, 12 and 50 mm using a sample chamber of about 1 L (diameter 10 cm, height 18 cm).

Yield stress could be reduced twice by decreasing the size of the particle Table 2. Prepared mixtures at the same WS (or TS) content using shredded and non-shredded WS have shown different yield stress value. This indicates that TS content is not the only parameter determining rheological behavior, particle size also plays a key role in rheological behavior [START_REF] Ruys | Rhéologie des résidus agricoles pour un procédé multi-étapes de méthanisation en voie sèche[END_REF]. the flow of this biomass could not take place naturally, which implies the requirement to resort mechanical instruments to forward the matter inside the continuous D-AD reactors. Consequently, industrial size reduction may not be the optimal option to increase neither the flowability nor the methane potential of straw-cattle manure in continuous D-AD processes. On the contrary, energy requirements for size reduction could have an antagonistic role on the biomass energy recovery since it is a high-power consumption process.

Delayed Period Determination Using Spreading Diameter Evolution

In this work, analysis of the slump height has been coupled with analysis of spreading diameter in order to achieve the determination of rheological characterization. The spreading diameter as a function of time for the control and the prepared mixtures with non-shredded WS is depicted in Fig 4b . Samples from 10.6% to 18.5% TS content showed the evolution of the diameter over time.

Reduction in the spreading diameter inflicted by the increase of WS content was easily observable.

Moreover, samples with the same TS content but different size of fibers showed different spreading behavior (Table 2). Over 18.5% TS content, biomass was severely structured and entangled by the WS presence, thus no spreading diameter was observed in the whole test duration. The RF has been calculated using Eq. ( 6) and values over 90% were considered optimal to continue with rheological analysis. As can be seen in In the case of the control, the average delayed period was 0.0083 seconds; it means that the start of the flow does not take very long before of prescribing a slope in the spreading diameter evolution with the log time. Low R 2 values of curve fitting for the delayed period were obtained using the slump test. This is explained given the fact that video tracking struggled with interferences in the first moment of the test when the chamber is lifted, the flow starts and the measure of the first spreading diameters is difficult due to the presence of the chamber and the operator hands in video recording. Sung [START_REF] Choi | Estimation of rheological properties of UHPC using mini slump test[END_REF] developed minislump test (190 cm 3 ), using this volume, video tracking from the bottom of the test was feasible using a transparent surface. In this work, having a volume fifty times higher this condition could not be achieved because it was not possible to raise a transparent surface to carry out the slump test 2 meters over the soil. Thus, video tracking was made from the top of the experiment according to the experimental set-up depicted in Fig 2a . Better results of have been determined by using the consistometer. In this case, the listed interferences were not present in the video recording analysis (section 4.3.2).

Sung [START_REF] Choi | Estimation of rheological properties of UHPC using mini slump test[END_REF] has found linear relationship between the delayed period and the viscosity for Bingham fluids. In this type of fluids, the shear stress and the shear rate have a linear relationship, thus viscosity value is constant at different shear stress. Studies on coarse biomass rheology suggest that above a low mass fraction of solids, biomass suspensions are viscoplastic, shear thinning materials with a yield stress. The steady-state shear stress as a function of shear rate has been represented by the Bingham, Casson and Herschel-Bulkley models [START_REF] Battistoni | Pre-treatment, measurement execution procedure and waste characteristics in the rheology of sewage sludges and the digested organic fraction of municipal solid wastes[END_REF][START_REF] Samaniuk | Rheology of concentrated biomass[END_REF][START_REF] Stickel | Rheology measurements of a biomass slurry: An inter-laboratory study[END_REF], being this last, the predominant model for coarse and concentrated biomass [START_REF] Klingenberg | Rheometry of coarse biomass at high temperature and pressure[END_REF][START_REF] Ruys | Rhéologie des résidus agricoles pour un procédé multi-étapes de méthanisation en voie sèche[END_REF][START_REF] Schneider | Rheological properties of digestate from agricultural biogas plants: An overview of measurement techniques and influencing factors[END_REF]. As consequence, in this work the linear relationship of viscosity with the delayed period cannot be considered as lineal. Therefore, the consistency coefficient (K) and the flow behavior index (n) need to be included into the mathematical model in order to achieve a more complete rheological characterization based on the Herschel -Bulkey model. Further investigation and discussion of these relations are the subject of future work.

Consistometer results

Surge profile analysis and yield stress determination

Flow behavior is a function of physical and rheological properties (density, yield stress and viscosity) [START_REF] Nguyen | Correlation between L-box test and rheological parameters of a homogeneous yield stress fluid[END_REF]. 3 shows the values of mathematical parameters describing the surge profiles. [START_REF] Shan | Experimental investigation of flow of fresh self-compacting concrete in improved Lbox[END_REF] have used an empirical fitting model (order 4) of depth profiles (y) with the variable of the distance (x). In this work polynomial fitting was performed, second order presented Eq. ( 13) was enough to have an optimal correlation with the experimental values (R 2 > 0.97):

G = ℎ C + H / I + H I (13)
Where, h0 denotes the thickness of the deposit at x=0, A1 and A2 are fitting parameters involves the shape of the profile, x denotes the distance.

Integration of the fitting equations was performed in order to achieve the sample displaced volume since the beginning of the experiment and also the sample volume stocked in the sample chamber which did not flow naturally. As can be seen in Table 3, for shredded WS at 14.6% TS the percentage of the sample volume that flowed was only the 9.7% of the total sample volume, the remaining would be stocked in the sample chamber. The increase TS content would prevent the flow of the sample until inhibit completely the flow of values near to 18.5% TS. Yield stress was determined using Eq. (4) which relates this rheological property with the distance of the flow and the height (ho) of the fluid at x = 0. This equation has been successfully used for the range yield stress from 14 to 75 Pa for self-compacting concretes [START_REF] Hosseinpoor | Numerical simulation of self-consolidating concrete flow as a heterogeneous material in L-Box set-up: Effect of rheological parameters on flow performance[END_REF][START_REF] Nguyen | Correlation between L-box test and rheological parameters of a homogeneous yield stress fluid[END_REF]. Comparing yield stress values obtained from the slump test (Table 2) with those obtained in the consistometer (Table 3), some similarities are observed for mixtures using shredded WS. In contrast, the results for non-shredded WS were irregular while comparing the yield stress obtained using both devices, this could be explained as the edge effects are more important as the particles size increases. Unfortunately, the use of Eq. ( 4) for determining the yield stress from consistometer was sustainable only for samples having up to 13% TS content of non-shredded WS and 14.6% TS of shredded WS. This means that slump test was more versatile and with a wider measurement range for determining the yield stress than the consistometer.

Delayed period determination using flow distance

Analysis of the spreading distance with time in consistometer has been performed in the same way as the analysis of the previous spreading diameter using slump test. Table 4 presents the constant values determined using Eq. (3) changing diameter for spreading distance in the consistometer. Fig 5b depicts the evolution of the delayed period with the WS content. As can be noticed, better (R2)

with exponential fitting has been observed using data from video tracking of consistometer than with the slump test. The estimation of delayed period was easier, since videos do not have interferences in the first moment when the gate is open, thus. The calculated value of delayed period for the control using both devices was the same 8.3*10 -3 s. Corroborating that delayed period calculated with these devices do not depend on their geometry and their volume. Samples with non-shredded WS exhibited slightly superior values of delayed period than shredded WS;

(except for 18.5% TS), meaning that inertia provided by non-shredded WS could be slightly higher in mixtures having Non-shredded WS than in those having shredded WS. Fib. 4c depicts the correlation between the spreading diameter and flow inhibitions determined using the slump test and the consistometer at 300 seconds respectively. Since both devices remains under the same physical principles, good results correlation was observed.

finally reducing the rheological properties. This hypothesis was demonstrated by the use of the shear box, as can be seen, mayonnaise would increase the pore suction differently in mixtures with the same amount of WS but different particle size. The cohesion of the biomass would rise, but once the mixture does not have enough mayonnaise to partially fill the pores, the cohesion would be reduced and similar phenomena observed in "sand castle" building would be observed.

Angle of internal friction was around 15° for non-shredded WS samples between 18.5 and 22.5%

TS. The presence of long fibers presented in samples of non-shredded WS explain higher angle of internal friction than shredded samples. As cohesion, angle of internal friction can be affected by the porosity of the biomass. In shredded samples, the control fluid would fill the pores reducing friction between particles thus shear strength is also reduced. In samples with shredded WS having more than 26.5% TS, higher values of internal friction angle were observed in contrast to samples with non-shredded WS. Quantifying changes in cohesion and friction angle of biomass could offer a valuable information about the rupture domain and the necessary strength to shear the matter as a function of material depth in continuous D-AD reactors.

Applications and limitations of the constructed devices according to the apparent rheological behavior of coarse biomass

Since standardized measurement systems are not feasible for coarse biomass, in this work different systems have been adapted allowing for the aspired rheological measurements. The use of the alternative devices developed in this work is based on the apparent rheology of the sample and the consistency limits between liquid, plastic and solid states [START_REF] Mousavi | Diffuse reflectance spectroscopy for rapid estimation of soil Atterberg limits[END_REF]; considering the state of each sample one device could achieve better results than others. A general overview of applicable volume ranges, particle sizes, TS and long fiber concentration, advantages, disadvantages and applications of measuring devices used for rheological characterization of coarse biomass is presented in Table 6. The biomass particle size and water content would be important considerations in order to choose the device to use. For agricultural biomass, the transition between these limits and the rheological behavior are not only function of the TS content but is also depending on the livestock species, the diet of the animals, the type, amount and change frequency of the bedding material, the presence of other residues and the conditions of waste storage before handling [START_REF] Lague | Engineering of land application systems for livestock manure: A review[END_REF][START_REF] Landry | Physical and rheological properties of manure products[END_REF].

Considering the application of each device, below from the liquid limit, the biomass would behave as a liquid, given the large amount of free water its shear resistance would be insignificant and it would easily spread when it is spilled, this is the case of cow slurry with TS content below 5% determined by the Botswick consistometer (Fig 3b). This biomass can be pumped and mixed effortlessly. Once the liquid limit is exceeded by an increase in TS content, the biomass would increase its rheological properties, the biomass water absorption would reduce the free water, increasing the amount of capillary and bound water (Garcia-Bernet et al., 2011a). Non-Newtonian behavior appears; shear-thinning behavior is one of the most frequently present in non-diluted livestock slurry, at this moment, slump test offers better results in rheological behavior than the consistometer and the shear-box. Once the TS content exceeds the plastic limit, the biomass should behave as a porous viscoelastic solid and would not flow under applied stress; instead, other deformations may be observed, for instance compression and torsion. For samples with this rheological behavior, the shear-box can be used to achieve cohesion and friction angle measurements.

Biomass heterogeneity implies different and particularly adapted handling processes. These processes might impact the physical and rheological properties of biomass. For instance, the untreated cow spend bedding is a porous elastic solid, its physical and rheological characteristics would change while AD is enhanced to a plastic material. The three devices proposed in this work could offer a sustainable range of options for understanding and measure the evolution of biomass rheological behavior treated by D-AD. Moreover, they are low-cost compared to conventional rheometers and could be operated in the field, avoiding the transport of samples and facilitating the decision making based on the rheological properties and the flowability of the biomasses treated in D-AD. Finally, procedures and protocols to implement and standardize the use of these devices in the rheological analysis of biomass could be easily modified from their original application from the concrete, food and soil analysis.

Conclusion

Three alternative devices were adapted and calibrated to the rheological analysis of coarse biomass. Flowability and rheological properties of biomass are function of fiber content and particle size. Yield stress and delayed period were measured using the slump test, the flow profile was measured using the consistometer and the variations on the cohesion and friction angle were determined using the shear-box. These devices are cheap, simple and their operation is easy to perform in the field. This work represents an advance in the setting-up of standardized methods to the rheological determination of coarse biomass.

Supplementary material

Visual aspect of raw and shredded wheat straw

Acknowledgments

The authors wish to thank the Association Nationale Recherche Technologie (ANRT) for the financial support of this work and for the Ph.D. grant of Manuel HERNANDEZ-SHEK (CIFRE n° 2017/0352). The authors want to thank Joseph Fayolle for his careful reading of this manuscript.

For their kindness and help, a special thanks to Mathieu Forster, Benoît Lerzy, Laura André, Jerôme Rançon, Luc Hanneuse and François Buche, members of different platforms from UniLaSalle (Beauvais).

Figure captions

(

  Fig 2c).The approximative cost of the materials for this device was 600 €. The shear-box contains a sample of approximately 60 L. The shear-box has been designed to be movable, since the interest of determining rheological properties of biomass on site. A load cell (H3C3-500 kg) provided by ZEMIC has been used connected to a RS232 receptor to register the changes in force during the shearing test. Once installed, the shear-box is gently full using hands; the desire normal stress is with disc weights up to the desired pressure (0.1, 2, 4 and 6 kPa). A hand winch has been used to track the upper box and sample shearing. A computer reordered the experimental shear force. The experimental data were fitted with Scilab 6.0 using an asymptotic curve (Gompertz equation) to determine the maximal shear strength. The test was carried out four times per sample with a different normal pressure, the measured maximal shear strengths were used to plot the Mohr-Coulomb break lines allowing the determination of the friction angle and the cohesion expressed in Eq (5) for each sample.

  slurry vs. mayonnaise: rheological comparison Mayonnaise rheograms were constructed for each temperature from 10 to 25°C considering the Herschel -Bulkey model (Fig. 3a). Values for yield stress were on average 42.4 ± 2.8 Pa for mayonnaise at the evaluated temperatures. Viscosity appears more sensitive to changes in temperature than yield stress. At 10°C the consistency coefficient (K) 40.45 Pa.s n , this value decreased until 36.84 Pa.s n with the increase of temperature until 25°C. The flow behavior index (n) barely variated, obtaining values on average 0.32 ± 0.01.Similar values for yield stress and

Fernandez

  et al. (2020) have achieved a reduction of yield stress of cattle manure from 755.3 to 281.0 Pa by the use of industrial hammer mills. Even if a reduction of yield stress has been observed by reducing the particles size, yield stress values for shredded WS are still very high, thus

  present in Eq. (3). The impact of rheological properties over the spreading diameter of the samples are observed as soon as the sample chamber is removed. Therefore, in order to analyze the first instants of the slump test, the time axis is presented in log scale (Fig 4c). In this representation, the existence of the delayed period ( ) indicated by Sung Choi et al. (2016) is easily observed graphically. The delayed period is the time undertaken for the fluid to overcome the yield stress and to reach a consistent slope in the spreading diameter vs log time representation. The yield stress is closely related to the delayed period; it would prevent the flow of the sample at the first instance of the slump test. Thus, the increase of WS has exponentially increased not only the values of yield stress but also of delayed period (Fig 4d).

  Fig 5a shows the surge profile and front position experimentally obtained for mixtures with non-shredded WS at 300 seconds. Agreeing with slump test previous results, flow distance achieved by samples with non-shredded WS was always lower than those using shredded WS. Table

Fig. 1 Fig. 2 •

 12 Fig. 1 Procedures and analytical methods

  analysis, possibility of including image analysis systems -Easily profile of flow acquisition at different times -Effect of slope -Low-cost (around 200 €) -Feedback and state of the art for food and concrete applications -Possibility of study the effect of an applied pressure at the sample chamber in the flow -Bulky set-up -Liquid samples with yield stress values lower than 100 Pa -Shear rate values still undefined, thus difficult to rely the delayed period with indicative values, the use of each device depends on the TS and the particle size distribution of samples Long fiber content (% w/w

Table 2 ,

 2 the circular shape of the spreading diameter was guaranteed with RF values over 91.2 %. Average and standard deviation of the diameter achieved at 300 seconds have been determined. Low values of SD of the duplicates indicate that the experiments were carried out in optimal conditions. Table2presents the constant fitting values for each mixture of WS and mayonnaise. As can be observed, log equation was well correlated with the experimental data allowing the estimation of changes on parameters α, β and

Table 1

 1 Physical properties of raw materials and prepared mixtures between WS and mayonnaise *TS values calculated by the results of phase A where cow feces at 10.6 % TS depicts the same flow behavior of the control

				Non-shredded WS	Shredded WS
	Sample	%WS	%TSeq	Bulk density (kg m -3 )	Porosity (%)	Bulk density (kg m -3 )	Porosity (%)
	Control	0	10.6*	896 ± 12	0.0 ± 0.0	894 ± 15	0.0 ± 0.0
	Mixture 1	1	11.4*	851 ± 21	5.2 ± 2.4	858 ± 5	4.4 ± 1.7
	Mixture 2	3	13.0*	841 ± 13	6.9 ± 1.5	870 ± 5	3.7 ± 0.5
	Mixture 3	5	14.6*	775 ± 6	19.1 ± 1.1	806 ± 13	11.2 ± 1.4
	Mixture 4	10	18.5*	706 ± 3	22.9 ± 0.6	771 ± 21	16.2 ± 2.3
	Mixture 5	15	22.5*	415 ± 9	55.5 ± 1.0	477 ± 18	48.9 ± 1.9
	Mixture 6	20	26.5*	356 ± 3	62.4 ± 0.3	380 ± 15	59.9 ± 1.6
	Mixture 7	25	30.5*	134 ± 2	86.0 ± 0.2	165 ± 15	82.9 ± 0.5
	Raw wheat straw	100	90.0 ± 0.1	33 ± 2	97.7 ± 1.6	46 ± 3	96.8 ± 0.8

Table

  Rheological properties estimated from the slump test results

	Material	%TS τo (Pa)	Round Factor (RF)	α x 10 -2	x 10 -3	x 10 -3	R 2	Diameter (cm) 300 s
	Control	10.6 43 ± 3	99.7 ± 0.2	46.8 ± 0.7	59 ± 2	8 ± 3	0.963	80.4 ± 0.6
		11.4 49 ± 1	95 ± 2	46 ± 1	54.1 ± 0.2	7 ± 1	0.942	76.7 ± 1.1
		13.0 171 ± 8	93.4 ± 0.4	38.6 ± 0.7	54 ± 2	5 ± 1	0.955	60.6 ± 0.7
	Non-Shredded	14.6 361 ± 2	96 ± 2	28.2 ± 0.6	22 ± 2	10.7 ± 0.2	0.956	40.5 ± 1.5
	WS	18.5 588 ±	99.3 ± 0.4	19.4 ± 0.2	2.35 ± 0.05	14 ± 2	0.851	20.9 ± 0.1
		22.5 673 ±		No spreading diameter observed
		26.5 882 ±		No spreading diameter observed
		11.4 47 ± 2	98.8 ± 0.1	45.3 ± 0.5	58.7 ± 0.1	10.3 ± 0.7	0.950	78.8 ± 0.5
		13.0 57 ± 1	97.2 ± 0.1	37 ± 1	43 ± 1	15 ± 6	0.907	62.0 ± 0.6
	Shredded WS	14.6 127 ±	97.4 ± 0.1	31.8 ± 0.1	27 ± 2	7 ± 3	0.967	47.1 ± 1.1
		18.5 393 ±	99.6 ± 0.2	18.7 ± 0.1	0.55 ± 0.05	35 ± 1	0.816	19.0 ± 0.1
		22.5 491 ±		No spreading diameter observed
		26.5 653 ± 5		No spreading diameter observed

Table 3

 3 Yield stress and surge profile for the determination from the consistometer results

	Material	%TS	τo (Pa)	h0 (cm)	A1	A2	Volume flow (%)
	Control	10.6	32 ± 1	8.4 ± 0.1	5 ± 1 • 10 -4	10.4 ± 0.1 • 10 -2	55.6 ± 0.1
		11.4	42 ± 1	8.5 ± 0.1	12 ± • 10 -4	8.6 ± 0.2 • 10 -2	54.7 ± 0.1
	Non-	13.0	111 ± 2	10.4 ± 0.4 53 ± 3 • 10 -4	10 ± 2 • 10 -2	45 ± 1
	Shredded WS	14.6	ND	12.9 ± 0.4 28 ± 9 • 10 -2	46 ± 10 • 10 -2	9.7 ± 0.8
		18.5	ND	18.8 ± 0.3 22 ± 7	6 ± 4	1.6 ± 0.8
		11.4	40 ± 1	8.4 ± 0.1	6.5 ± 0.5 • 10 -4	10.5 ± 0.2 • 10 -2	53.8 ± 0.3
	Shredded WS	13.0	60 ± 8	9.5 ± 0.5	44 ± 11 • 10 -4	5 ± 5 • 10 -2	48.8 ± 0.8
		14.6	151 ± 10	12.1 ± 0.1 2.5 ± 0.8• 10 -2	11 ± 6 • 10 -2	31 ± 2
		18.5	ND	19.1 ± 0.1 0.4 ± 0.3	6.0 ± 0.1	5.5 ± 0.4
		ND = non-determined , A1 and A2 fitting parameters Eq. 16

Table 4

 4 Spreading distance analysis from the consistometer results

	Material	%TS	α • 10 -2	• 10 -2	• 10 -2	R 2	Distance (cm) 300 s
	Control	10.6	27.1 ± 0.3	5.8 ± 0.1	0.8 ± 0.1	0.898	60.1 ± 0.1
		11.4	24.5 ± 0.6	5.3 ± 0.3	0.9 ± 0.1	0.914	54 ± 1
	Non-	13.0	14.9 ± 0.6	3.48 ± 0.05	1.3 ± 0.1	0.894	34.7 ± 0.9
	Shredded WS	14.6	2.1 ± 0.2	0.58 ± 0.06	2.6 ± 0.1	0.726	5.4 ± 0.5
		18.5	0.3 ± 0.1	0.09 ± 0.01	3.2 ± 0.3	0.593	0.8 ± 0.1
		11.4	25.1 ± 0.5	5.47 ± 0.04	0.9 ± 0.1	0.927	56.3 ± 0.3
	Shredded WS	13.0	17.6 ± 0.4	4.08 ± 0.04	1.2 ± 0.1	0.913	40.9 ± 0.6
		14.6	6.7 ± 0.1	1.79 ± 0.04	2.3 ± 0.3	0.829	16.9 ± 0.2
		18.5	0.3 ± 0.1	0.08 ± 0.01	3.7 ± 0.6	0.487	0.67 ± 0.03

Table 5

 5 Angle of internal friction and apparent cohesion for mixtures of non-shredded and shredded WS and mayonnaise

			Non-shredded WS			Shredded WS
	%TS	18.5	22.5	26.5	30.5	18.5	22.5	26.5	30.5
	Normal Effort (kPa)			Shear strength (kPa)
	0.1	1.86	3.57	2.60	1.54	1.48	1.93	2.06	1.93
	2	2.21	3.49	2.40	2.22	1.63	2.41	3.65	2.84
	4	2.92	4.49	3.33	3.13	2.29	2.92	4.42	3.71
	6	3.34	4.81	3.72	3.34	2.54	3.11	5.44	4.23
	Cohesion (kPa)	1.56	3.17	2.16	1.37	1.39	1.97	2.22	1.98
	Friction angle (°)	15.1	15.9	14.7	19.8	11.0	11.6	28.9	21.5
	R 2	0.90	0.91	0.89	0.90	0.94	0.96	0.97	0.98

Table 6

 6 Overview of applicable volume ranges, particle sizes, TS and long fiber concentration, advantages, disadvantages and applications of measuring devices used for rheological characterization of coarse biomass

	Measurement system	Analysis	Volume	Applicable particle size	Applicable TS and fiber concentration*	Advantages	Disadvantages	Application in field
					< 20 % w/w of coarse	-Simple set-up		-Solid and viscoplastic
	Slump Test	Yield stress determination using the slump height	1-10 L	< 15 cm fiber length	fiber concentration < 27 %TS	-Feedback and state of the art for concrete applications -Mobile use possible -Change of cylindrical volume to adapt to samples -Not rotating parts -Low-cost (around 200 €)	with rheological parameters difficult to rely the delayed period -Video recording of first instant of test limited by physical interferences -Shear rate values still undefined, thus	samples of feeding and digestates -Untreated digestate
						with higher coarse particles samples

Shear-box results

Experimental data of shear force was fitted using an asymptotic curve (Gompertz equation) in order to facilitated and to standardize the determination of the maximal shear strength. 5. Given the large sample volume of this device and the number of shear test performed for each sample (at least 4 by sample), random duplicates were carried out to estimate the measure experimental error; 9.9 % of error is assumed for all values in this section. Shear text was suitable method for samples having more than 14.6 %TS, under this value, sample settlement and liquid state did not allow the use of the constructed device.

Changes on the cohesion values could be seen with the WS content for both types of WS studied.

For non-shredded WS, the highest cohesion observed was 3.17 kPa for samples with 22.5% TS, after this value cohesion value decreased. In samples with shredded WS, similar observation was made, instead of having a maximal cohesion at 22.5% TS the maximal cohesion was observed at 26.5% TS. This behavior on cohesion evolution could be explained by the influence of the pore suction created by the mayonnaise stocked in pores of the WS matrix.

Reducing the particle size would reduce the macro porosity of the biomass (Table 1). Differences on cohesion values comparing both WS types could therefore be explained by differences in pore size and distribution in the biomass. In a hypothesis established by [START_REF] Viamajala | Heat and mass transport in processing of lignocellulosic biomass for fuels and chemicals[END_REF], size reduction would reduce the amount and size of macropores, so less liquid would be necessary to fill the pores raising the amount of free water and reducing the interaction between particles and