Marie Dupré
email: mariedupre92@gmail.com

Thierry Michels
email: thierry.michels@cirad.fr

Pierre-Yves Le Gal
email: pierre-yves.le_gal@cirad.fr

Crop drivers in the shift from synthetic inputs to alternative practices in diversified farming systems

Keywords: crop management, production case, farm level, horticulture, Réunion Island

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

Introduction

The growing use of synthetic inputs after the Second World War significantly contributed to increased agricultural productivity in both developed and developing countries. However, after several decades of intensive use, these inputs have been found to have negative effects on the natural environment and human health, raising questions regarding the sustainability of these production systems [START_REF] Tilman | The greening of the green revolution[END_REF]. Alternative systems have been proposed by some researchers, farmers and consumer movements, such as agroecology based on natural resources, biological processes and agrobiodiversity within farms and territories [START_REF] Gliessman | Agroecology: the ecology of sustainable food systems[END_REF][START_REF] Nicholls | Agroecological Principles for the Conversion of Farming Systems: Principles, Applications, and Making the Transition[END_REF]. Although politicians and citizens are increasingly sensitive to the damage caused by conventional agriculture, only a minority of farmers have adopted alternative practices up to now [START_REF] Geiger | Persistent negative effects of pesticides on biodiversity and biological control potential on European farmland[END_REF][START_REF] Nave | Why wheat farmers could reduce chemical inputs: evidence from social, economic, and agronomic analysis[END_REF].

Numerous studies have sought to understand what drive farmers to adopt certain alternative practices to synthetic inputs such as conservation agriculture [START_REF] Knowler | Farmers' adoption of conservation agriculture: A review and synthesis of recent research[END_REF] and cover crops [START_REF] Roesch-Mcnally | The trouble with cover crops: Farmers' experiences with overcoming barriers to adoption[END_REF]. These studies have shown that adoption is affected by diverse drivers related to the characteristics of the farm, farmer, and farming practices studied [START_REF] Pissonnier | Factors driving growers' selection and implementation of an apple crop protection strategy at the farm level[END_REF]. However, these studies, which focused on one type of practice, do not consider all of the decisions that a farmer must make at the level of his or her farm and their underlying rationales. Other studies, which have focused on all of the new practices adopted by farmers, have confirmed that multiple, interacting drivers influence changes in practices, including climate, economic, technological, social and political drivers [START_REF] Ouédraogo | Markets and climate are driving rapid change in farming practices in Savannah West Africa[END_REF][START_REF] Probst | Understanding change at farm level to facilitate innovation towards sustainable plant protection: a case study at cabbage production sites in urban West Africa[END_REF][START_REF] Padel | UK farmers' transition pathways towards agroecological farm redesign: evaluating explanatory models[END_REF]. [START_REF] Hill | Conceptual Framework for the Transition from Conventional to Sustainable Agriculture[END_REF] have proposed the three-step ESR framework to analyze these change processes, i.e., (i) Efficiency: improvement in the use of synthetic inputs; (ii) Substitution: when synthetic inputs are replaced by certified organic inputs that come from outside the farm, and (iii) Redesign: where the farmer rethinks the entire production system to use beneficial interactions between agroecosystem components, relying on resources from within the farm. Although the "E" step represents a way to reduce the use of synthetic inputs that farmers can justify based on economic reasons, the implementation of alternative practices starts with the "S" step, and the "R" step assumes a strategic change in the farmer's orientations [START_REF] Chantre | Trajectories of French Field Crop Farmers Moving Toward Sustainable Farming Practices: Change, Learning, and Links with the Advisory Services[END_REF].

These previous works focused on fairly specialized farms, where interactions between crop and eventually animal productions are minimized. In diversified systems, the implementation of alternative practices may be more complex, but the process also may be facilitated as farmers make changes based on the characteristics of the different productions on their farms [START_REF] Coquil | Transition to self-sufficient mixed crop-dairy farming systems[END_REF]. Although diversified systems are widespread around the world, there is a lack of information on how farmers implement alternative practices in such contexts.

This study is based on 28 comprehensive surveys conducted on diversified horticulture farms on the island of La Réunion (France). Farming systems are highly diverse on the island due to a large range of soil and climatic conditions, a wide diversity of farm resources (land, workforce), and the co-existence in the horticultural sector of formal marketing channels based on supermarkets and a tradition of informal short chains. In the 1980s, synthetic inputs were adopted on a massive scale in line with the European agricultural policy (CAP) at the time. However, CAP is currently encouraging farmers to reduce these synthetic inputs through bans on active substances and subsidies to adopt alternative agroecological practices. In this context, the study aims to understand the role of crop drivers in farmers' implementation of alternative practices to synthetic inputs involving weed control, fertilization and crop protection. After presenting the survey and analysis protocol, we show how the characteristics of each crop and the links between crops within a farm affect the implementation of alternative practices. These results are then discussed based on other cases and on their operational contributions to agroecological transition.

Methods

Context of the study and sample

La Réunion is a small French island located in the Indian Ocean (2512 km²; 21° 06′ 52″ South, 55° 31′ 57″ East) and integrated into the European Union. Diverse crops are cultivated including sugar cane, forage, fruits and vegetables (respectively 58, 26, 7 and 6% of the utilized agricultural area), and secondary crops such as spices. This diversity of output is related to the island's varied topography (altitudes of 0 to 3000 m), soils (27 types) and climate (from 500 mm/year and 24°C on average on the west coast to 8000 mm/year and 12°C at altitudes of 2000 m). Farms are small with an average size of 6 ha. Farms which do not grow sugar cane (61% of the total) are highly diversified.

The explanatory objective of this study, and its focus on farmers' decision-making processes, led to a research methodology based on case studies. Such a methodology allows an understanding of the processes studied by combining an in-depth investigation of each case and both comparative and inductive analyses of the information collected individually [START_REF] Eisenhardt | Theory building from cases: Opportunities and challenges[END_REF].

The value of this method depends on the diversity of the cases studied relative to the issue addressed in order to enrich the explanatory capacity of the research process. In that respect a limited number of farms (28) were studied in detail, taking into account their individual contexts.

The choice of these farms aimed to cover a wide range of situations rather than a statistically representative sample of the island's farming population. The selection was based on three criteria identified by past studies as being potentially related to the adoption of alternative practices [START_REF] Bellon | Characterisation of apple orchard management systems in a French Mediterranean Vulnerable Zone[END_REF][START_REF] Pissonnier | Factors driving growers' selection and implementation of an apple crop protection strategy at the farm level[END_REF] and for which data could be obtained easily. The criteria were: the main marketing channel as a proxy for consumer demand regarding the characteristics of agricultural products and, consequently, the kind of practices requested to produce them; organic certification as a proxy for strategic choices made by farmers that lead to the implementation of alternative practices; and farm size as a proxy for farm resources, especially labor and cash, which may hinder such implementation. The farms all cultivate citrus because the same sample was used to understand the diversity of agroecological practices in citrus orchards [START_REF] Dupré | Diverse dynamics in agroecological transitions on fruit tree farms[END_REF]. Due to the purpose of this study, four farms cultivating only citrus were removed from the original 32-farm sample, which ultimately included 6 holdings certified as organic farms and 22 non-certified farms (Table 1). Production was sold through cooperatives, direct sales, small resellers or directly to processors. Farmers' contact information was obtained from different sources: agricultural technicians, cooperatives, agricultural input dealers, certification bodies and consumer associations.

Surveys

The farmers were surveyed between December 2015 and September 2016. The surveys consisted of one or two semi-structured interviews lasting one to three hours with the farm head, combined with a visit of the farm. The discussion was organized in order to:

(i) identify the production cases implemented by each farm, a production case (PC) being defined as a combination of a crop, its production mode (e.g., field vs greenhouse; organic vs conventional) and its economic environment, which was defined as the marketing channel and technical support characterized by the absence or presence of contact between the farmer and an adviser for a given case;

(ii) describe the farming practices used on each PC that had been implemented on the farm for at least two years. Three sets of practices, common to all of the crops produced and which could involve the use of synthetic inputs, were investigated more precisely: crop protection, fertilization and weed control.

(iii) characterize the reasons behind farmers' choices in relation to both the specificities of each PC and the interactions between the various PCs within a given farm.

Based on this dataset, 93 PC were identified on the 28 farms surveyed. They included six perennial fruit crops, six semi-perennial crops (sugar cane, christophine and fruits) and seven short cycle crops (Table 1). Each crop has specific biological features that may affect the farmer's choices of farming practices regarding crop protection, fertilization and weed control (Table 2).

1 2 3 3 1;3;4 2 3 1 1;4 1 1 3 1 2 3 1 2 1 2 2;4 2 2;3 3 2 1 2 2 2;4 Perennial fruit crops Avocado 1 1 Citrus 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Lychee 1 1 1 Mango 1 1 1 1 1 Peach 1 1 1 1 1 1 Persimmon 1 Semi perennial crops Banana 1 1 1 1 Christophine 1 1 Papaya 1 1 1 1 1 Passion fruit 1 1 2 5 2 5 1 1 Pineapple 1 1 1 1 1 Sugarcane 1 1 Short cycle crops Chilli 1 1 Ginger 1 1 1 1 Maize 1 Pumpkin 1 Strawberry 1 1 Thyme 1 1 Tomato 1 1 2 5 1 2 5 2 5 1 Total PC 2 3 2 4 2 5 2 4 3 4 2 2 5 5 3 2 5 3 2 6 3 5 4 2 2 4 4 3 Livestock 1 1 1 1 1
1 1: South-west > 600m; 2: South-west 300-600m; 3: South > 500m; 4: West > 600m; 5: West < 600m; 6: South 300-500m; 7: South <300m; 8: East 125 (mountains). Each zone is characterized by specific temperatures and rainfall which potentially affect weed competition and pressure of pests and 126 diseases, and so the choice of farming practices. 127

2 As a proxy of the labor and cash resources available in the farm for implementing alternative practices 128 3 OF: Organic Farming. The six farms implement alternative practices by following OF specifications on all their crops. 129 4 1: Local retailers; 2: Cooperatives; 3: Direct selling; 4: Juice industry. Each market has specific requests regarding the products marketed, which 130 impact farming practices. 131 5 Open field crop for one case, greenhouse for the other one. 132

Choice of an ecologization indicator: the Technical Score

The ecologization of practices comprises both the reduction of synthetic inputs and the implementation of alternative practices to these inputs. The Technical Score (TS) was created to easily assess this dual process [START_REF] Dupré | Diverse dynamics in agroecological transitions on fruit tree farms[END_REF]. In this study, the only synthetic inputs considered are synthetic pesticides and fertilizers. All the other practices are considered as alternative. Some practices implemented by the farmers at the time of the survey were regrouped by production mode to avoid going into the details of each input. For example, treatments were divided into two modes: synthetic or certified organic.

Calculated per PC, the TS enables all of the crops to be compared. It cannot, however, be applied directly to animal production because the three sets of practices considered, namely crop protection, fertilization and weed control, are specific to crop production. Animal production, present on 18% of the farms surveyed, was therefore excluded from the analysis of the TS.

The TS only considers the nature of the input used (synthetic or not) and not the quantity (Eq. 1).

It consequently is possible to conduct the analysis without quantitative data on the inputs. This degree of detail is thus adapted to the study of practice implementation that may not be accompanied by written records. For each set of practices (crop protection, fertilization and weed control), the score can be -1, 0 or 1. The accumulation of alternative practices in the same group is not counted. As a result, the sum of the scores of each group makes the TS vary from -3 (management based on synthetic inputs alone) to +3 (management with no synthetic inputs).

Values between -2 and +2 correspond to many combinations of synthetic inputs and alternative practices.

= ∑ ( -) (Eq. 1)

these diversified systems. The 45 qualitative occurrences collected from farmers' answers were then regrouped into three processes that were formalized qualitatively by comparing the farmers' decision-making processes driving them.

Results

A diversity of alternative practice implementation

The current systems were studied with regard to the implementation of alternative practices to synthetic inputs and the diversity of TS levels at the PC and farm scales. Twenty alternative practices were observed on the farms surveyed (Fig. 1). The most frequently implemented were mowing, weeding with tillage, spot application of herbicides, use of manure or compost, biopesticides and chemical traps. The rarest were agro-pastoralism, insect-proof nets and sprinkler irrigation for the control of certain pests. Plastic mulching, restitution of crop residues, release of natural enemies, cover crops and agro-pastoralism remained specific to certain PCs.

The fifteen other practices were most often partially implemented by farms, but sometimes were implemented on all of the PCs. The mean TS of the 93 PCs was 0.31. While positive, this average is far from the maximum score (+3), and shows that the reduction of synthetic inputs was ongoing for most of the PCs identified.

Effects of the characteristics of production cases on the implementation of alternative practices

First, the links between the diversity of TS levels and PC characteristics as defined above were explored. The relationships between the biological characteristics of the crops and practices are statistically significant except for the use of herbicide and synthetic fertilizers (Table 3).

Obviously crops that are not sensitive to fruit and vegetable flies do not need chemical traps or preventive actions involving the destruction of fruits damaged by flies. A high sensitivity to competition from weeds leads to a high implementation of mulching and a low implementation of a permanent cover-crop to mow. Natural mulching is mainly used for long cycle crops while plastic mulch, which degrades in a few months and has to be removed, is used on short cycle crops.

The PCs' specific economic contexts also impact the implementation of alternative practices when technical support is available. For example, the release of natural enemies is closely linked to the existence of strong technical support promoting this type of practice on certain crops (strawberries) and not on others (citrus fruits). The specific demands of buyers also influence farmers' choices of practices. Sales through channels with low visual quality requirements (direct sales or processing) lead to the absence of protective pesticides, whether or not organic certified.

However, by itself organic certification does not lead to a greater use of imported organic fertilizer or biopesticides, which are used in a similar proportion on non-certified PCs. With a much lower but still positive TS, the C2 cluster groups annual crops (strawberry, tomato)

and semi-perennial crops (passion fruit, christophine) mainly grown under shelters and accompanied by strong technical support recommending the release of natural enemies and the use of plastic or natural mulching rather than herbicide. However, the reliance on sales channels with highly demanding visual quality requirements limits the margins of reduction of the protective treatments.

The C3 cluster mainly contains perennial crops identical to C1, with a similar implementation of mowing, chemical traps and removal of fallen fruit. But the productions are marketed in sales channels with very demanding visual quality requirements which limit the reduction of the protective treatments and leads to a mean TS close to the C2 one. The C4 cluster includes annual or semi-perennial crops for which alternative practices are currently more risky or expensive than synthetic inputs, combined with the absence of specific technical support and very demanding sales channels with regard to visual quality. These combinations lead to a limited implementation of alternative practices and a mean TS similar to C3.

The last cluster (C5) has a very negative mean TS, reflecting a heavy reliance on synthetic inputs.

It mainly contains semi-perennial crops identical to C4, but the farmers cultivating them are facing constraints with regard to labor or input availability which hinder implementation of mowing and organic fertilization. The reduction of synthetic inputs is also limited by the requirements of the sales channels in terms of yield (sugar cane) or visual quality (pineapple and passion fruit for export).

Of the 19 crops identified in the sample, 10 belong to only one cluster, showing that biological features alone cannot explain the farmers' choices in terms of alternative practices. The crop environment in terms of technical support and marketing channels explains why the remaining nine crops are distributed between two, and more rarely three clusters. At the farm level, only seven farms have all of their PCs in one cluster, either because they are organic (C1), or at the other extreme because their labor availability limits the implementation of alternative practices (C5). For the 21 other farms, up to 3 clusters can be observed per farm regardless of their number of PCs (Fig. 2). This result demonstrates that the reduction of synthetic inputs is not a homogenous process on these diversified farms. This is due to the specificity of PCs, as shown in this section, but also to the variety of links between PCs within a farm, which will be shown in the following section.

Effects of links between production cases within farms on the implementation of alternative practices

The way a farmer chooses practices on one PC may depend on the other PCs cultivated within the farm. Three main types of links were identified between PCs: (i) ripple effect when the implementation of an alternative practice on one crop triggers its implementation on another crop; (ii) flow of goods between PCs that enables the implementation of an alternative practice;

(iii) conditionality when the implementation of an alternative practice on one PC is conditional on its non-implementation on other PCs, particularly in cases where limited labor or cash flows are constraints.

Observed 20 times, the ripple effect process covers two types of situations: (i) the farmer, having tested a new practice on one PC with satisfactory results, then implements the practice on another. This thus consists of a dissemination of skills acquired on an initial PC to other PCs on the farm; (ii) the farmer introduces alternative goods which are then pooled for use over several PCs to simplify management and reduce their costs per unit produced. The dissemination of skills is observed particularly in farms with at least one PC that has strong technical support (C2 cluster) which is the "gateway" to innovation. The practices disseminated are chemical traps, organic fertilization, natural mulching and mowing. For example, farmer F1 receives considerable technical support for his strawberry PC. In 2010, he is advised to stop using herbicides and to leave a permanent, mowed grass cover between rows. Satisfied with the effects of this cover on reducing mite pressure and the number of acaricide treatments, he began practicing mowing on citrus fruits in 2012 without having received technical advice for this PC.

The pooling of alternative goods is observed between all clusters except for the C5 cluster, which has a low TS. Pooled inputs are organic fertilizers, chemical traps and biopesticides. Pooled equipment consists of rotary cutters for mowing and grinding branches to recycle crop residues and natural mulching. For example, farmer F9 only cultivates perennial fruits which are all sold through the same sales channel. In 2014, he replaced synthetic fertilizers with an organic fertilizer (feather meal and poultry blood) on all of his PCs so that he only had to buy and transport one type of fertilizer. Farmer F7 bought a piece of equipment he could use both for shredding citrus branches after pruning and for mulching with Ramial Chipped Wood on tomato.

The existence of flows of goods between productions was observed 13 times. Farmers put them in place to reduce their purchases of external inputs and manage their 'waste', i.e., crop residues or livestock manure. These links were observed on the farms with mean TSs that were slightly positive to high. Seven types of goods are involved in these flows: seeds of flower strips and natural enemies that are initially purchased, multiplied on one PC, and then transferred to other PCs; natural mulch produced on the farm and used on some PCs; the residues of soil-less PCs and their substrate that are transferred to fertilize soil-grown PCs; manure/slurry from animals on the farm that is transferred to fertilize PCs; and finally hay produced from spontaneous ground cover.

Observed 12 times, conditionality in the allocation of farm resources is linked to two constraints which are sometimes combined: (i) a limited labor force which forces the farmer to choose less time-consuming practices for certain PCs to prioritize the management of other PCs; (ii) a limited cash flow which forces the farmer to concentrate investments in goods on certain PCs to the detriment of others. PCs that are major contributors to farm income or have higher economic margins are considered to be the "priority" for labor and cash flow.

These conditionality mechanisms lead to the only partial implementation of alternative practices that are more time-consuming or expensive: organic fertilization compared to synthetic fertilizers, mowing with some mechanization compared to herbicides, natural cane-based mulching which is expensive (€1,600/ha to renew once a year), the preventive removal of fallen fruit and flower strips that require additional time. This partial implementation negatively affects the TS per farm, as shown by the 5 farms which implement only this kind of link between PCs (Table 5). For example, farmer F22 has practiced the preventive removal of fallen fruit for citrus since 2006. This takes time during the harvest, which is already a peak period of work. Despite the effectiveness of the practice, his labor force constraints led him to decide not to implement it on christophine because it is a secondary PC on the farm. Farmer F1 has been using organic fertilizers on his strawberries since 2006, but not on his citrus trees because he considers that only strawberries allow him to generate a profit while using this expensive fertilizer.

This crop-link process concerns the majority of the farms surveyed. Only eight farms cultivating from 2 to 5 PCs do not show any link between PCs (Table 5), six out of them showing null or negative TS. For the 20 other farms, the process concerns up to five links on the farm. The group of 10 farms that only have links stimulating change of practices have the highest mean TS per farm, while the combination of both stimulating and hindering links leads to a slightly positive mean TS. Although each group shows some TS diversity, this trend illustrates the contribution of existing links on the implementation of alternative practices.

Discussion

A combination of crop drivers

Although based on a small sample of diversified farms, this study has shown that the reduction of synthetic inputs through the implementation of alternative practices combines various drivers linked to the crop level, as previously highlighted by [START_REF] Brodt | Influence of farm management style on adoption of biologically integrated farming practices in California[END_REF] in a study conducted in California among almond and wine grape growers. These drivers can be classified into four categories pertaining to: (i) the biological characteristics of crops, integrated into the farm's soilclimate context, including sensitivity to pests, (ii) the crops' specific environment regarding the availability of inputs, sales channels and the type of technical advice provided, (iii) the existence and the specific characteristics of the alternative practices proposed, such as their cost, workload and the technical complexity of their implementation, (iv) some farm's specific characteristics, particularly its labor and capital resources. The various combinations of these different categories of drivers within a farm explains why a farmer may decide to change his practices on one crop, for example one that is not very sensitive to pests or sold directly to consumers, and not on another crop whose specifications seem too restrictive to do so.

This study also emphasizes the impact on the change of links between agricultural activities within the farm. The dissemination of new skills, required for the technical mastery of alternative practices, is a key driver in stimulating processes of change [START_REF] Merot | Converting to organic viticulture increases cropping system structure and management complexity[END_REF][START_REF] Toffolini | Indicators used by farmers to design agricultural systems: a survey[END_REF]. Similarly, the pooling of inputs and equipment to reduce costs, or to the contrary, the concentration of these on the most lucrative activity, enables certain economic barriers to be overcome. [START_REF] Herrero | Smart Investments in Sustainable Food Production: Revisiting Mixed Crop-Livestock Systems[END_REF] also show that biomass exchanges, especially on mixed croplivestock farms, are particularly favorable to the implementation of alternative fertilization practices. Changes in the farmer's environment, such as the banning of certain products, incentives through technical advice and public support, as well as new consumer demands, are also important triggers for a reduced use of synthetic inputs [START_REF] Chantre | Trajectories of French Field Crop Farmers Moving Toward Sustainable Farming Practices: Change, Learning, and Links with the Advisory Services[END_REF].

Although generic, this crop-based framework of analysis is expressed differently according to the context in which the farm evolves [START_REF] Grover | Slow to change': Farmers' perceptions of place-based barriers to sustainable agriculture[END_REF]. The isolation of La Réunion as an island complicates access to alternative inputs such as organic fertilizers and pest control products, both in terms of physical availability and higher costs. Promoting the reduction of synthetic inputs in a territory thus leads to identifying its specific regional features [START_REF] Fairweather | Environmental beliefs and farm practices of New Zealand farmers Contrasting pathways to sustainability[END_REF].

From Substitution to Redesign

By distinguishing efficiency, substitution and redesign as pathways of change towards more sustainable farming systems, [START_REF] Hill | Conceptual Framework for the Transition from Conventional to Sustainable Agriculture[END_REF] opened a debate about the way to reduce synthetic inputs that remains lively and timely. Redesign advocates argue that substitution has the potential, through a "lock-in" effect [START_REF] Wagner | Pesticide lock-in in small scale Peruvian agriculture[END_REF], to block farmers at a low level of change of practices that is insufficient to meet current social and environmental challenges [START_REF] Rosset | Agroecology versus input substitution: A fundamental contradiction of sustainable agriculture[END_REF]. But some actors, including farmers' unions, highlight the economic, organizational and technical difficulties posed by redesign on farms. These difficulties force farmers to take risks while facing markets, and therefore consumers, who may not be "ready" to accept agricultural products with visual defects that are sold at higher prices.

By focusing on the crop level, our study does not assess the capacity or willingness of farmers to redesign their systems at farm scale following the principles of agroecology. This would involve not only reducing their use of synthetic inputs at field scale, but also implementing agroecological principles as intercrops, crop rotations, agroforestry or diversified landscape mosaics [START_REF] Altieri | The ecological role of biodiversity in agroecosystems[END_REF][START_REF] Jackson | Utilizing and conserving agrobiodiversity in agricultural landscapes[END_REF][START_REF] Martin-Guay | The new Green Revolution: Sustainable intensification of agriculture by intercropping[END_REF][START_REF] Rosset | Agroecology versus input substitution: A fundamental contradiction of sustainable agriculture[END_REF].

However, our crop scale study nonetheless highlighted that farmers follow a diversity of pathways: from individual changes of practices on selected crops to the simultaneous implementation of several alternative practices on all of the farm crops. Progressive pathways are consistent with the anti-risk strategies that most farmers are keen to implement [START_REF] Dupré | Diverse dynamics in agroecological transitions on fruit tree farms[END_REF][START_REF] Ridier | The role of risk aversion and labor constraints in the adoption of low input practices supported by the CAP green payments in cash crop farms[END_REF]) Indeed, they experiment and learn about crops managed under a redesign strategy while securing their income on crops where they use substitution. In doing so, they adapt to markets by redesigning crops with less demanding niche markets and by using substitution on crops whose conventional markets are very demanding.

Supporting farmers towards the redesign of agroecological farming systems

Given the diversity of individual situations, the approach used here, one based on the TS indicator, makes it possible to diagnose each farm and compare farms with each other while revealing how farmers reduce the use of synthetic inputs. This step is important to consider with farmers possible changes in their production systems [START_REF] Gal | How does research address the design of innovative agricultural production systems at the farm level? A review[END_REF]. Through its flexibility, the TS indicator makes it possible to take into account all types of practices, conventional and agroecological, in plant production. Its adaptation to animal production seems conceivable, integrating the practices of feeding, sanitary management and effluent management, according to the agroecological principles described by [START_REF] Dumont | Prospects from agroecology and industrial ecology for animal production in the 21st century[END_REF]. The TS could then provide a tool to assess the ecologization of mixed crop-livestock farms. However, this indicator, which can be used for static and dynamic studies, remains purely descriptive. It should be coupled with approaches such as Life Cycle Assessment [START_REF] Hellweg | Emerging approaches, challenges and opportunities in life cycle assessment[END_REF] to go further in assessing the effects of changes in practices on the environmental, economic and social performance of farms.

These tools could be integrated into on-farm extension services which are still largely lacking in contexts such as La Réunion, where technical support remains very sector specific [START_REF] Rebuffel | Le difficile accès au conseil pour les petites exploitations agricoles : le cas de Mayotte[END_REF]. This support to small diversified farms could be based on discussion groups and experiments between farmers [START_REF] Warner | Extending agroecology: Grower participation in partnerships is key to social learning[END_REF] with contexts of action and convergent objectives for greater effectiveness [START_REF] Oerlemans | Enhancing farmers' networking strategies for sustainable development[END_REF]. The analytical framework formalized in this study provides a basis for forming such groups.

Conclusion

Following the public health, food and environment challenges posed by a production-focused agricultural model, agroecological transition is a concept that has moved from the scientific sphere to political and social spheres. Through their farming practices, farmers are at the forefront of the debate, yet the reasoning behind their choices are not always clearly laid out. This study focusing on the crop level provides new insights by showing that on diverse farms, biological and economic specificities of crops and links between crops at the farm level can explain how and why farmers implement alternative practices to the use of synthetic inputs.

These results highlight the real efforts made by farmers to reduce their use of synthetic inputs, but also the gap between the entrenched positions encountered far too often in political and social debates, and the reality they experienced. Furthermore, barriers to change also depend heavily on actors outside the agrifood sector, especially consumers, because their quality and price requirements may be incompatible with farmers' technical and economic capacities of change.

The study and accompaniment of agroecological transition must therefore go beyond these ideological debates to propose to farmers trajectories of change which are adapted to their individual context and involve all of society in this transition.

Figure 1 .

 1 Figure 1. Extent of implementation of 20 agroecological practices on the 28 farms surveyed.

Figure 2 .

 2 Figure 2. Distribution of the 28 farms according to their number of production cases and their

Table 1 .

 1 Farm characteristics and number of production cases (PC) per crop and per farm. 124

	Farm code	F1 F2 F3 F4	F5	F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26 F27 F28 Total
	Zone 1	4	3	7	2	7	1	6	5	6	5	7	7	5	6	7	7	7	5	7	7	7	8	8	7	5	6	8	6
	Farm area (ha) 2	3 3.5	1	1.5	6.5	12 3.5 8.8	4	5	6.5	2.4	5	7.9	8	12	6	7	1	22	5	5	4	1	2.5	14	9	15
	Certification 3			OF OF			OF					OF							OF								OF
	Markets 4

Table 2 .

 2 Biological features of the 19 crops studied. High sensitivity or needs are indicated with "+" and low sensitivity or needs with "-". Sensitivity to fruit and vegetable flies (the main pests in the study area) and sensitivity to weed competition are based on expert knowledge. Nutrient needs are based on data from Chambers of Agriculture [Ferti-RUN software (http://www.mvadreunion.org/spip.php?article107)].

	Sensitivity to (fruit and vegetable) flies	Nutrient needs	Sensitivity to weed competition
	Perennial crops
	Avocado	+	+	-
	Citrus	+	+	-
	Lychee	-	-	-
	Mango	+	-	-
	Peach	+	+	-
	Persimmon	+	-	-
	Semi perennial crops
	Banana	-	+	-
	Christophine	+	+	-
	Papaya	-	+	-
	Passion fruit	-	+	+
	Pineapple	-	+	+
	Sugarcane	-	+	-
	Short cycle crops
	Chilli	+	+	+
	Ginger	-	+	+
	Maize	-	+	+
	Pumpkin	+	-	+
	Strawberry	+	+	+
	Thyme	-	-	+
	Tomato	+	+	+

Table 3 .

 3 Relationship between biological and economic characteristics of the 93 production 238 cases (PC) and implementation of alternative practices.

	239
	PC features			Practices		Test p-value
	Biological			Traps/prophylaxis
				Yes	No
	Sensitivity to fruit flies	Low	7	27	4.00e-02 a *
			High	26	33
				Synthetic fertilizers
				No	Yes
		Nutrient needs	Low	5	7	3.51e-01 b
			High	48	33
				Mulching
				Yes	No
		Cycle duration	Short	15	7	5.05e-05 a *
			Medium	13	13
			Long	7	38
				Mulching
				Yes	No
	Sensitivity to weed competition	Low	12	46	3.77e-05 a *
			High	23	12
				Mowing
				Yes	No
			Low	46	12	3.01e-04 a *
			High	14	21
				Herbicide
				No	Yes
			Low	24	34	8.53e-01 a
			High	13	22
	Economic	Certification		Imported organic fertilizers
	context			Yes	No
			OF	5	9	1.52e-01 b
			None	14	65
				Biopesticides
				Yes	No
			OF	5	9	5.69e-01 b
			None	36	43
				Release of natural enemies
				Yes	No
	Technical support	Low	3	77	1.09e-03 b *
			High	5	8
				Organic or synthetic
				protection treatments
				No	Yes
	Marketing constraints on visual	Low	13	11	3.11e-04 a *

are inexpensive and (ii) do not require specific technical support, combined (iii) with marketing channels accepting low visual quality (direct sales, processing or sales under the AB label), which enables limited use of protective treatments. It includes mainly perennial crops that are either not very susceptible to competition from weeds, which favors the implementation of mechanical mowing, or highly susceptible to fruit flies, leading to the implementation of the preventive removal of fallen fruit and chemical traps.

Table 4 .

 4 Distribution of the 93 production cases between the five clusters according to their crops

	Cluster	C1	C2	C3	C4	C5	Total
	TS mean	1.62	0.33	0.17	0.10	-1.75	0.33
	Perennial crops
	Avocado	2					2
	Citrus	11		16		1	28
	Lychee				3		3
	Mango	1		4			5
	Peach	4		2			6
	Persimmon			1			1
	Semi-perennial crops
	Banana	1			3		4
	Christophine		2				2
	Papaya				5		5
	Passion fruit		3		4	1	8
	Pineapple				2	3	5
	Sugarcane					2	2

Table 5 .

 5 Distribution of farms and TS per farm according to the kind of links between production

	cases

Acknowledgments

We are very grateful to the farmers and all of the stakeholders we met for the valuable data they provided and the rich discussions we shared with them. We thank Grace Delobel for editing the paper in English, and the two anonymous reviewers whom comments helped us to improve the article. This work was supported by the French Agricultural Research Centre for International Development (CIRAD) and the European Agricultural Fund for Rural Development (EAFRD).

= Technical Score of crop j i = index of group of practices (Protection; Fertilization; Weed control) Ai = 1 if at least one alternative technique is implemented for group i; 0 otherwise Ci = 1 if at least one synthetic input is used for group i; 0 otherwise

Analysis of the implementation of alternative practices

The TS per PC was calculated based on the current practices for the 93 PCs identified on the 28 farms. First, the TS of PCs on the same farm were compared with each other. In order to then compare farms, a TS per farm was calculated by summing the TS of each PC on the farm divided by the number of PCs considered. Relationships between the biological, technical and economic characteristics of PCs and the implementation or non-implementation of alternative practices were statistically tested for all occurrences reported at least once by a farmer. The Fisher test was used for occurrences that were poorly represented (number <5) or had unbalanced marginal sums in the contingency table; the Chi-2 test was used for the other cases.

A multivariate statistical analysis combining a Multiple Correspondence Analysis (MCA) with an Ascending Hierarchical Classification (AHC) [START_REF] Cortez-Arriola | Leverages for on-farm innovation from farm typologies? An illustration for family-based dairy farms in north-west Michoacán, Mexico[END_REF][START_REF] Kuivanen | Characterising the diversity of smallholder farming systems and their constraints and opportunities for innovation: A case study from the Northern Region, Ghana[END_REF] was then performed to regroup the 93 PCs into homogeneous clusters with regard to their TS values divided into three classes: Low (<-1), Intermediate (-1 to 1) and High (>1). The analyses were performed on R (version 3.3.2) with the FactoMineR package (version 1.34) [START_REF] Josse | FactoMineR: An R Package for Multivariate Analysis[END_REF].

Based on the 28 farmers' interviews, this PC-focused analysis was then complemented on each farm by the identification of links between PCs leading to a shift of practices towards the reduction of synthetic inputs. Indeed, it was assumed that a change in the practices used for a PC could stimulate or hinder changes in the other PCs cultivated on the farm and impact their TS in


I Gonzalez-Salgado

L Cavaillé

S Dubos

E Mengelle

C Kim

M Bounouba

E Paul

S Pommier

Y Bessiere
email: yolaine.bessiere@insa-toulouse.fr

Combining Thermophilic Aerobic Reactor (TAR) with Mesophilic Anaerobic Digestion (MAD) improves the degradation of pharmaceutical compounds

Keywords: pharmaceutical compounds, Thermophilic Aerobic Reactor, Mesophilic Anaerobic Digestion, reduction of sludge production Nomenclature

The removal efficiency of nine pharmaceutical compounds from primary sludge was evaluated in two different operating conditions: (i) in conventional Mesophilic Anaerobic Digestion (MAD) alone and (ii) in a co-treatment process combining Mesophilic Anaerobic Digestion and a Thermophilic Aerobic Reactor (MAD-TAR). The pilot scale reactors were fed with primary sludge obtained after decantation of urban wastewater. Concerning the biodegradation of organic matter, thermophilic aeration increased solubilization and hydrolysis yields of digestion, resulting in a further 26% supplementary removal of chemical oxygen demand (COD) in MAD-TAR process compared to the conventional mesophilic anaerobic digestion. The highest removal rate of target micropollutants were observed for caffeine (CAF) and sulfamethoxazole (SMX) (>89%) with no substantial differences between both processes. Furthermore, MAD-TAR process showed a significant increase of removal efficiency for oxazepam (OXA) (73%), propranolol (PRO) (61%) and ofloxacine (OFL) (41%) and a slight increase for diclofenac (DIC) (4%) and 2 hydroxy-ibuprofen (2OH-IBP)

Introduction

Over the last twenty years, various studies have quantified concentrations and occurrences of pharmaceutical compounds in wastewater treatment plants (WWTPs). Data reported generally refers to the removal from aqueous phase without distinguishing between sorption of compounds onto sludge and/or biodegradation processes [START_REF] Verlicchi | Occurrence of Pharmaceutical Compounds in Urban Wastewater: Removal, Mass Load and Environmental Risk after a Secondary Treatment-A Review'. Science of The Total Environment, Special Section -Arsenic in Latin America, An Unrevealed Continent: Occurence[END_REF]). However, a significant fraction of pollutants can be sorbed onto the sludge, depending on the respective physicochemical properties of molecules and sludge as well as on operating conditions. The main mechanisms for the removal of organic compounds from wastewater are sorption and biodegradation. Volatilization mechanism could be neglected due to small Henry's law constants of most pharmaceutical compounds [START_REF] Joss | Biological Degradation of Pharmaceuticals in Municipal Wastewater Treatment: Proposing a Classification Scheme[END_REF].

Sorption onto sludge follows two mechanisms: absorption and adsorption. On the one hand, hydrophobic interactions (absorption) occurs between the aliphatic and aromatic groups of a compound with the lipophilic cell membrane of the microorganisms or the lipid fractions of the suspended solids. On the other hand, the electrostatic interactions (adsorption) take place because of interactions of positively charged groups of chemicals with the negatively charged surfaces of the microorganisms [START_REF] Verlicchi | Occurrence of Pharmaceutical Compounds in Urban Wastewater: Removal, Mass Load and Environmental Risk after a Secondary Treatment-A Review'. Science of The Total Environment, Special Section -Arsenic in Latin America, An Unrevealed Continent: Occurence[END_REF].

In this study, we considered nine molecules based on their variety of physicochemical properties, their prevalence in WWTP effluents, their uses for different pathologies and as a marker of human activity. These criteria led us to choose carbamazepine CBZ (anticonvulsant); diclofenac DIC, ibuprofen IBP and 2-hydroxyibuprofen 2OH-IBP (analgesics); sulfamethoxazole SMX and ofloxacin OFL (antibiotics); propranolol PRO (beta bloquer); oxazepam OXA (psycholeptic) and caffeine CAF (stimulant). Table S1 shows their therapeutic class, chemical structure, Henry's Law constant, pKa and LogKow values. It can be noticed that [START_REF] Besse | Exposure Assessment of Pharmaceuticals and Their Metabolites in the Aquatic Environment: Application to the French Situation and Preliminary Prioritization[END_REF] included ofloxacin, propranolol, carbamazepine, sulfamethoxazole and diclofenac in their preliminary risk assessment of pharmaceuticals showing high Risk Quotient ratios.

One extended approach to evaluate the sorption of hydrophobic substances is the octanolwater partition coefficient KOW that indicates the lipophilicity of the sorbed compound.

However, some studies revealed that sorption behavior present significant deviations from log KOW for pharmaceuticals [START_REF] Golet | Environmental Exposure Assessment of Fluoroquinolone Antibacterial Agents from Sewage to Soil[END_REF][START_REF] Urase | Separate Estimation of Adsorption and Degradation of Pharmaceutical Substances and Estrogens in the Activated Sludge Process[END_REF][START_REF] Carballa | Determination of the Solid-Water Distribution Coefficient (Kd) for Pharmaceuticals, Estrogens and Musk Fragrances in Digested Sludge[END_REF][START_REF] Kümmerer | The Presence of Pharmaceuticals in the Environment Due to Human Use--Present Knowledge and Future Challenges[END_REF] indicating that polar and ionic interactions contribute to sorption behavior.

Overall, phase distribution of a pollutant depends on the physicochemical properties of the molecule such as stereo chemical structure, pKa, solubility, polarity or log KOW and the external conditions, including pH, redox potential and temperature [START_REF] Kümmerer | The Presence of Pharmaceuticals in the Environment Due to Human Use--Present Knowledge and Future Challenges[END_REF]. Finally, solid-liquid partition coefficient Kd provides distribution behavior of the compounds between liquid and solid phases.

Few authors have studied solid-liquid coefficient during anaerobic digestion. Table 1 summarizes Kd coefficients found in literature for primary sludge. Ofloxacin and propranolol have the highest expected sorption potential while ibuprofen and caffeine have the lowest one.

The rest of the molecules would be distributed in relatively same percentages in liquid and solid phases. As we will discuss in 3.3 section, Kd is not enough to evaluate the distribution of the molecules, Total Suspended Solids (TSS) concentration should be added for a complete characterization. However, Kd can help to compare qualitatively the sorption of target molecules in sludge when determined in the same conditions. Anaerobic digestion process involves hydrolysis, acidogenesis, acetogenesis and methanogenesis phases. Generally, the hydrolysis of organic matter is considered as the limiting step in anaerobic digestion [START_REF] Nguyen | A Little Breath of Fresh Air into an Anaerobic System: How Microaeration Facilitates Anaerobic Digestion Process[END_REF][START_REF] Chen | Review on Microaeration-Based Anaerobic Digestion: State of the Art, Challenges, and Prospectives[END_REF].

Various thermal, chemical and enzymatic processes are known to improve the removal of recalcitrant organic matter [START_REF] Paul | Biological Sludge Minimization and Biomaterials/Bioenergy Recovery Technologies: Paul/Sludge Minimization[END_REF]). Among all these processes, micro-aeration has drawn attention in recent years because of its positive impacts as enhancing hydrolysis, dealing with overloads [START_REF] Ramos | The Potential of Oxygen to Improve the Stability of Anaerobic Reactors during Unbalanced Conditions: Results from a Pilot-Scale Digester Treating Sewage Sludge[END_REF] or enhancing biogas quality by reducing the concentrations of H2S [START_REF] Díaz | Performance Evaluation of Oxygen, Air and Nitrate for the Microaerobic Removal of Hydrogen Sulphide in Biogas from Sludge Digestion[END_REF][START_REF] Giordano | Long-Term Biogas Desulfurization under Different Microaerobic Conditions in Full-Scale Thermophilic Digesters Co-Digesting High-Solid Sewage Sludge[END_REF]. Aeration could be implemented intermittently or continuously in different stages of anaerobic digestion, as pre-treatment, co-treatment or post-treatment [START_REF] Nguyen | A Little Breath of Fresh Air into an Anaerobic System: How Microaeration Facilitates Anaerobic Digestion Process[END_REF].

As pre-treatment, [START_REF] Johansen | Enhancing Hydrolysis with Microaeration[END_REF] found a positive effect on the hydrolysis of proteins and carbohydrates during anaerobic digestion of primary sludge. [START_REF] Montalvo | Microaerobic Pretreatment of Sewage Sludge: Effect of Air Flow Rate, Pretreatment Time and Temperature on the Aerobic Process and Methane Generation[END_REF] found an increase of protein and total sugars solubilization, CODs and methane production associated to a 27% COD removal increase. [START_REF] Charles | Effect of Pre-Aeration and Inoculum on the Start-up of Batch Thermophilic Anaerobic Digestion of Municipal Solid Waste[END_REF] observed cellulase and protease activities after aeration pre-treatment of organic fraction of municipal solids waste, confirming the hydrolytic capacities of exo-enzymes excreted by aerobic bacteria. Three full-scale systems have implemented microaeration in anaerobic digestion with positive effects in H2S removal [START_REF] Kobayashi | Characterization of Sulfide-Oxidizing Microbial Mats Developed inside a Full-Scale Anaerobic Digester Employing Biological Desulfurization[END_REF][START_REF] Jeníček | Simple Biogas Desulfurization by Microaeration -Full Scale Experience[END_REF][START_REF] Giordano | Long-Term Biogas Desulfurization under Different Microaerobic Conditions in Full-Scale Thermophilic Digesters Co-Digesting High-Solid Sewage Sludge[END_REF].

As post-treatment, 47 % of COD removal in MAD and 28% in TAR was obtained in a system formed by two stages MAD/TAR (MAD followed of TAR, without recirculation) [START_REF] Park | Sequential Sludge Digestion after Diverse Pre-Treatment Conditions: Sludge Removal, Methane Production and Microbial Community Changes[END_REF].

As co-treatment, micro-aeration has also shown positive effects, [START_REF] Xu | Optimization of Micro-Aeration Intensity in Acidogenic Reactor of a Two-Phase Anaerobic Digester Treating Food Waste[END_REF] studied the effect of intermittent micro-aeration rates of anaerobic digestion of food waste. They concluded that adequate micro-aeration rate can enhanced the hydrolysis of solid organic waste and methane yield by 16.9% and 18%, respectively. Thermophilic aerobic treatment coupled to conventional mesophilic digestion, as a cotreatment, has not been yet extendedly studied. Compare to a single mesophilic digestion, coupled process can get an additional solid removals and COD solubilisation [START_REF] Dumas | Combined Thermophilic Aerobic Process and Conventional Anaerobic Digestion: Effect on Sludge Biodegradation and Methane Production[END_REF][START_REF] Park | Sequential Sludge Digestion after Diverse Pre-Treatment Conditions: Sludge Removal, Methane Production and Microbial Community Changes[END_REF][START_REF] Fu | The Thermophilic (55°C) Microaerobic Pretreatment of Corn Straw for Anaerobic Digestion[END_REF]. [START_REF] Fu | The Thermophilic (55°C) Microaerobic Pretreatment of Corn Straw for Anaerobic Digestion[END_REF] improved the methane yield and enhanced the hydrolysis step in the digestion of corn straw by microaeration pre-treatment at thermophilic temperature (55° C). With TAR as a co-treatment, [START_REF] Dumas | Combined Thermophilic Aerobic Process and Conventional Anaerobic Digestion: Effect on Sludge Biodegradation and Methane Production[END_REF] obtained a 30% supplementary increase of COD removal in lab-scale Mesophilic Anaerobic combined with a Thermophilic Aerobic Reactor (MAD-TAR) treating secondary sludge.

Anaerobic digestion is the most widely used method for treating (with valorisation) sludge produced in WWTPs [START_REF] Gherghel | A Review on Wastewater Sludge Valorisation and Its Challenges in the Context of Circular Economy[END_REF] and the evaluation of the degradability of pharmaceutical molecules during digestion may be a key point to evaluate the ecotoxicological risks of digestate disposal. Table S2 summarizes the concentration of target micropollutants found in the literature during mesophilic anaerobic digestion and thermophilic anaerobic digestion (TAD). Some of molecules as IBP and OFL still had a high concentration in sludge after MAD.

The additional reduction of organic matter expected during coupled MAD-TAR digestion could be critically important as treatment of sludge represents approximately 50% of the total running cost of WWTPs [START_REF] Gherghel | A Review on Wastewater Sludge Valorisation and Its Challenges in the Context of Circular Economy[END_REF]. Although various studies have investigated the fate of pharmaceutical compounds in the anaerobic digestion [START_REF] Carballa | Fate of Pharmaceutical and Personal Care Products (PPCPs) during Anaerobic Digestion of Sewage Sludge[END_REF]Narumiya et al. 2013;[START_REF] Samaras | Fate of Selected Pharmaceuticals and Synthetic Endocrine Disrupting Compounds during Wastewater Treatment and Sludge Anaerobic Digestion[END_REF][START_REF] Malmborg | Pharmaceutical Residues in Sewage Sludge: Effect of Sanitization and Anaerobic Digestion[END_REF][START_REF] Martín | Pharmaceutically Active Compounds in Sludge Stabilization Treatments: Anaerobic and Aerobic Digestion, Wastewater Stabilization Ponds and Composting[END_REF][START_REF] Gonzalez-Gil | Is Anaerobic Digestion Effective for the Removal of Organic Micropollutants and Biological Activities from Sewage Sludge?[END_REF]Gonzalez-Gil et al. 2017), removal of some persistent micropollutants remains contradictory, i.e. removal of CBZ was reported from 85% [START_REF] Malmborg | Pharmaceutical Residues in Sewage Sludge: Effect of Sanitization and Anaerobic Digestion[END_REF] to <0 % (M. Narumiya et al. 2013). In addition, their behaviour under thermophilic digestion has been little studied and never for a co-treatment such as MAD-TAR though it gives very promising results.

In this work, the degradation of nine selected compounds (caffeine, ofloxacin, sulfamethoxazole, propranolol, carbamazepine, oxazepam, diclofenac, ibuprofen and 2 hydroxy-ibuprofen) and their sorption behavior was evaluated in a pilot-scale system. A QuEChERS-based method was modified to analyse targeted molecules in aqueous and total daily-composite samples attempting a mass balance of the compounds in the system.

Conventional mesophilic anaerobic digestion (MAD) was compared to a system coupling a thermophilic aerobic reactor with MAD (MAD-TAR).

Material and Methods

Analytical standards and reagents

Analytical standards of ≥98% purity of nine selected molecules were purchased from Sigma-Aldrich (Saint Quentin Fallavier, France), with the exception of oxazepam and IS for ofloxacin, propranolol, sulfamethoxazole, oxazepam and 2OH-ibuprofen that were furnished by Alsachim (Illkirch Graffenstaden, France). All compounds were supplied in powder except IS for caffeine and carbamazepine were obtained as methanolic solution at a concentration of 1 mg/mL and 100 µg/mL, respectively.

Tri-sodium citrate dihydrate (Na3Cit, 2H2O) and disodium sulfate (Na2SO4) were obtained from Sigma-Aldrich. The LC-MS-grade solutions used, including methanol (MeOH), acetonitrile (ACN), formic acid and acetic acid (AA) were obtained from VWR Prolabo (Fontenay-sous-Bois, France) as well as the products ethylenediaminetetraacetic acid disodium salt dihydrate (Na2EDTA, 2H2O), citric acid monohydrate (Cit, 1H2O), primary secondary amine (PSA) and C18 adsorbents (SUPELCO).

Anaerobic digesters

Biodegradation of the nine molecules was evaluated in two different configurations: (i) a conventional mesophilic anaerobic digestion (MAD) and (i) Mesophilic Anaerobic Digestion combined with a thermophilic aerobic reactor as a co-treatment process (MAD-TAR). The MAD was a stirred tank reactor with an effective volume of 260 L equipped with an agitator and a double jacket. The TAR, with a liquid volume of 40 L, was also equipped with an agitator and a double jacket. The aeration was operated discontinuously in the TAR compartment to provide a dissolved oxygen concentration, measured by VisiFerm™ DO optical sensor (Hamilton) between 0.2 and 0.5 mg L -1 with an air flowrate of 300 L h -1 . MAD was connected to the TAR by means of an ALBIN peristaltic recirculation pump (type ALH15). Its flow rate was fixed at 100 L h -1 and 15 L were recirculated from one compartment to the other three times per day corresponding to a retention time of 0.9 days in the TAR. The two compartments were heated by two independent cryostats in order to maintain the temperatures suitable for aerobic thermophilic (60 °C) and anaerobic mesophilic (35 °C) processes. For the MAD campaign, the 40 L compartment was maintained at 35°C without aeration to be in fully MAD conditions while maintaining all other operating conditions similar. Biogas composition (CH4, CO2 and O2) in MAD outlet was measured online (X-Stream X2GP continuous gas analyzer).

After inoculation by the secondary sludge from Cugnaux WWTP, the digester was fed with primary sludge from our pilot-scale settling tanks. Digester was operated semi-continuously by feeding the primary sludge automatically once a day. The duration of feeding, withdrawal and recirculation from MAD to TAR were adjusted to maintain a total sludge retention time (SRT) of around 20 days and a residence time in the TAR of 0.9 day. MAD and MAD-TAR campaigns were carried out in two consecutive years, between February and April, in order to have the same type of seasonality in medication intake. The Organic Loading Rate (OLR) was 1.2 kgCOD m -3 day -1 during MAD and 0.9 kgCOD m -3 day -1 during MAD-TAR campaign.

Sampling and analytical techniques

To evaluate the performances of the two studied processes, daily-composite samples of primary and digested sludge were collected regularly during steady-state periods. Sludge samples were stored at 4 °C and analyzed on the same day. In addition, aliquots of the total and dissolved fractions (obtained after 4500 g centrifugation for 15 minutes) were stored at -20 °C for micropollutants analysis. The sludge and the supernatants were characterized by Total Suspended solids (TSS), Volatile Suspended Solids (VSS) and Chemical Oxygen Demand (COD).

For quantification of targeted micropollutants, a QuEChERS-based modified method followed by UHPLC-MS-MS was performed both on total and aqueous samples. The protocol consisted in introducing 2.5 mL of real matrices in a 50 mL polypropylene (PP) Falcon tube (Falcon tubes from VWR Scientific Products). Quantification of the target analytes was performed by the internal standard approach, thus a volume of 0.2 mL of IS solution at 300 ng L -1 was spiked to samples. The PP tubes was left for 2 h on a shaking table to ensure the homogeneity of the sample, and more importantly the adsorption of the IS on the sludge.

Under slow stirring, 1.25 mL of EDTA, 1.25 mL of citrate buffer (232 g L -1 Na3Cit, 2H2O and 115.2 g L -1 Cit, 1H2O), 5 mL of acidified ACN and 4 g of Na2SO4 are added in tubes that are immediately vortexed (Heidolph™ Multi Reax Vortex Mixer from Fisher Scientific) at maximum speed for 1 min and then centrifuged at 7,100 g for 5 min. The entire organic phase was transferred to a 15 mL Falcon tube containing 0.075 g of sorbents (PSA and C18 in ratio 1/1) and immediately vortexed at maximum speed for 1 min and then centrifuged at 7,100 g for 5 min. The purified organic phase was then recovered in Pyrex tubes and evaporated under reduced pressure for 7 h at 45°C using the Low BP program of the EZ-2 Envi evaporator system (Genevac). A volume of 1 mL of a solution of ACN/water 95/5 (v/v) was added and pyrex tubes were vortexed at maximum speed for 1 min. The liquid was filtered through a 0.2 µm membrane (Minisart RC 15, Sartorius, France) and transferred in vials before analysis.

Liquid chromatography was carried out using an Ultimate 3000 UHPLC System from Dionex (France). Sample aliquots (10µL) were injected onto an ACQUITY UPLC HSS (High Strength Silica) T3 (100 mm x 2.1 mm, 1.8µm) column from Waters. Column oven temperature was maintained at 35°C and flow rate was 0.6 mL min -1 . For the positive ionization, the mobile phase was composed of solvent 1 (HCOOH 0.1% in pure water) and solvent 2 (HCOOH 0.1% in ACN) with the gradient as follows: 0-0.5 min 95/5, 0.5-7 min 95/5 to 0/100, 7-8 min 0/100, 8-8.2 min 0/100 to 95/5 and 8.2-9.5 min 95/5 (ratios expressed as 1/2 ratios). For the negative ionization, the solvents 1 and 2 are pure water and ACN, respectively. Detection was achieved with an Applied Biosystems Sciex QTRAP® hybrid linear ion-trap triple quadrupole mass spectrometer (Foster City, USA) equipped with a Turbolon-Spray Interface. The instrument was operated in ElectroSpray (ESI) positive (+) or negative (-) in Multiple Reaction Monitoring (MRM) mode (dwell time, 80 ms). Details of analytical conditions are presented in supplementary information (Table S3). Performance characteristics of the method were evaluated by accuracy and precision on total fraction of primary sludge and digestate. Method accuracy (estimated by means of relative recovery (RR) experiments) and precision (expressed as intra-day repeatability in terms of relative standard deviation (RSD)) were studied by spiking samples at different concentrations according to the concentration levels found in real samples. RR of the analytes ranged within 90%-120% and the intra-day RSD values were found to be lower than 20% indicating a satisfactory accuracy and precision. For all matrixes (primary sludge and digestate, total and liquid fractions), limit of detection (LD) was calculated according to the EPA's method by considering 3 times the standard deviation determined by analyzing 8 blank samples or spiked samples at low levels when it is necessary. Therefore, it is a robust and reliable method for complex and simple matrices which makes it possible to study the performances of the MAD and MAD-TAR processes. For each campaign, 13 samples were taken and analyzed in duplicate by adding internal standards for each targeted molecule targeted prior to extraction. For each samples and analytes, standard deviations of less than 10% were obtained.

Calculations

Mass balances and pharmaceuticals removal

Because of the discontinuous feed and withdrawal (one per day) and the variation of concentrations in primary sludge, all the data were cumulated for calculations. When a stationary state is reached, a straight line must be obtained when representing Σmoutlet = f (Σminlet). The obtained slope allowed to calculate the removal efficiency of the compound (ηremoval = (1-slope) x100). This method was applied for VSS, COD and micropollutants.

Statistical analysis was carried out following [START_REF] Sperling | Assessment of Treatment Plant Performance and Water Quality Data: A Guide for Students, Researchers and Practitioners[END_REF] recommendations.

Linearity of the regression was confirmed for all parameters by ensuring that the linear model was satisfying the assumptions of linearity, independence, normality of residuals and homogeneity of variances. Furthermore, calculations of the 95% confidence interval of the slope of linear model obtained for MAD and MAD-TAR campaign allowed evaluating if performances were significantly different (not overlap of the confidence interval). This analysis was performed with R software.

Distribution of target molecules between solid and liquid phases of sludge

Solid-liquid partition coefficient Kd (L kgTSS -1 ) of target molecules in sludge and supernatants of each sample was calculated as:

, = , , = , ,
• / ,

• eq. 1

Where Ci,sorbed and Ci,liquid are the solid (µg kgTSS -1 ) and liquid (µg L -1 ) concentrations of molecule i, respectively. Ci,total is the total concentration of molecule i in the sludge (µg•L -1 )

and Ci,supernatant is the supernatant concentration of molecule i (µg•L -1 ) in the liquid phase after centrifugation of the sludge. Vsupernatant / Vtotal is the ratio of the supernatant and total volumes after centrifugation.

The amount of compound i in the liquid phase in function of log Kd value for the range of TSS sludge concentrations was assessed as:

100 -, , = ! , • "#! , •
eq. 2

Oxygen consumption in TAR reactor.

In order to evaluate the fate of organic matter (i.e. removal in aerobic or anaerobic conditions), the oxygen uptake rate (OUR) in the TAR was calculated from the decrease of the dissolved oxygen concentration (CO2) during non-aerated periods.

$%& = -'( ) eq. 3

Results

Start-up and operation

Although the feeding showed fluctuations, as can be expected when working with real influent, the VSS and CODt concentrations after digestion remained constant with standard deviation below 4%. Considering comparable operating conditions (see Table 2), higher elimination of solid organic matter was achieved in the hybrid MAD-TAR system (around 76% for VSS) compared to the mesophilic one (around 47%) confirming lab results obtained a few years ago [START_REF] Dumas | Combined Thermophilic Aerobic Process and Conventional Anaerobic Digestion: Effect on Sludge Biodegradation and Methane Production[END_REF].

During MAD-TAR campaign, COD balances were performed to estimate the fate of transformed COD. Collected grab samples at the inlet and outlet of the TAR compartment allowed estimating that 26.6% of the total removed COD was eliminated in this thermophilic compartment. The additional COD removal compared to MAD campaign being eliminated in the aerated compartment, no additional methane recovery in the MAD compartment was allowed. Furthermore, the average OUR in the TAR was 7.5 ± 0.8 mgO2 L -1 h -1 that corresponds only to 3.9% of the total removed COD that was degraded in aerobic conditions.

Even if volatilization of certain substances such as volatile fatty acids could occur, this observation tends to show that the thermophilic condition could have more effect than the aeration. Two main mechanisms could be associated with this temperature effect. On the one hand, the microbial activity could be directly positively affected. Indeed, several authors reported a significant increase in hydrolysis rate in thermophilic compared to mesophilic conditions [START_REF] Siegrist | Mathematical Model for Meso-and Thermophilic Anaerobic Sewage Sludge Digestion[END_REF]Lynd et al. 2002). [START_REF] Ge | Relative Kinetics of Anaerobic Digestion under Thermophilic and Mesophilic Conditions[END_REF] even demonstrated that this rate strongly follow the Arrhenius relationship, with nearly a doubled value when increasing the temperature by 20°C. On the other hand, temperature may play a role in physical phenomena, in particular on mass transfer and solubility even if their effect on the rate of microbial utilization remains to confirm (Lynd et al. 2002).

Fate of pharmaceutical compounds during anaerobic digestion

The total concentration (µg L -1 ) of the nine selected compounds is illustrated in Figure 1 for both primary sludge and digestate from MAD (Figure 1a) and MAD-TAR (Figure 1b)

processes. All the target molecules were detected and quantified in the primary sludge (see Table S4 for limit of detection). Even if some variations in the concentrations have been observed between the two experimental campaigns (-23% in average during the MAD-TAR experiment) the order of magnitude were the same, allowing for a relevant comparison of performances between the two operating conditions. The highest concentrations of targeted compounds were observed for OFL and CAF (more than 40 µg L -1 ) while medium to low concentrations were reported for SMX, CBZ, PRO, DIC, OXA, IBP and 2OH-IBP, around 1.0-10.0 µg L -1 .

The comparison of removal efficiencies in MAD and MAD-TAR are reported in Figure 2.

Compounds placed above the dotted line show better removal efficiency by the innovative process (MAD-TAR) compared to conventional process (MAD).

The highest removal yields were obtained for CAF and SMX (>89%) with slight increase in MAD-TAR campaign for CAF, but with no significant differences between both processes for SMX. The removal of several other target compounds was significantly improved by MAD-TAR process. Moderate removal ranging between 54% and 75% were observed for OFL and OXA respectively in MAD-TAR compared to only 9% and 49% in MAD. Moreover, while PRO was not removed at all during MAD process, a rate of 69% was achieved in MAD-TAR.

DIC and 2OH-IBP presented low removal during MAD-TAR (6-10%) while they were accumulated in MAD. Finally, a negative removal rate was observed for IBP and CBZ in both processes, MAD (-30% and -71%, respectively) and MAD-TAR (-35% and -91%), underlining that concentrations were higher in the digestate than in primary sludge.

In MAD conditions, similar removal efficiencies were reported in the literature for OFL, PRO (Narumiya et al. 2013) 2013) showed a moderately-high removal, but recent studies found a recalcitrant behaviour after anaerobic processes [START_REF] Alvarino | Understanding the Removal Mechanisms of PPCPs and the Influence of Main Technological Parameters in Anaerobic UASB and Aerobic CAS Reactors[END_REF][START_REF] Phan | The Fate of Trace Organic Contaminants during Anaerobic Digestion of Primary Sludge: A Pilot Scale Study[END_REF] as underlined in our results.

DIC is generally persistent with low removal efficiencies [START_REF] Alvarino | Understanding the Removal Mechanisms of PPCPs and the Influence of Main Technological Parameters in Anaerobic UASB and Aerobic CAS Reactors[END_REF][START_REF] Ghattas | Anaerobic Biodegradation of (Emerging) Organic Contaminants in the Aquatic Environment[END_REF][START_REF] Phan | The Fate of Trace Organic Contaminants during Anaerobic Digestion of Primary Sludge: A Pilot Scale Study[END_REF][START_REF] Gonzalez-Gil | Biotransformation of Organic Micropollutants by Anaerobic Sludge Enzymes[END_REF]) whereas, moderate removal was showed in some lab-scale data [START_REF] Carballa | Fate of Pharmaceutical and Personal Care Products (PPCPs) during Anaerobic Digestion of Sewage Sludge[END_REF][START_REF] Malmborg | Pharmaceutical Residues in Sewage Sludge: Effect of Sanitization and Anaerobic Digestion[END_REF]. CBZ is also recalcitrant in wastewater treatments, low or negligible removal was reported in the studies of [START_REF] Ghattas | Anaerobic Biodegradation of (Emerging) Organic Contaminants in the Aquatic Environment[END_REF][START_REF] Alvarino | Understanding the Sorption and Biotransformation of Organic Micropollutants in Innovative Biological Wastewater Treatment Technologies[END_REF][START_REF] Lakshminarasimman | Biotransformation and Sorption of Trace Organic Compounds in Biological Nutrient Removal Treatment Systems[END_REF][START_REF] Phan | The Fate of Trace Organic Contaminants during Anaerobic Digestion of Primary Sludge: A Pilot Scale Study[END_REF][START_REF] Kent | Treatment of 17 Alpha-Ethinylestradiol, 4-Nonylphenol, and Carbamazepine in Wastewater Using an Aerobic Granular Sludge Sequencing Batch Reactor[END_REF].

Redox conditions played an important role conditioning the development of microbial populations and thus on biodegradation pathways thanks to the complementary hydrolytic capacities of external enzymes excreted by thermophilic aerobic bacteria. Some molecules are only degraded under specific aerobic, anaerobic or both conditions. [START_REF] Stadler | Effect of Redox Conditions on Pharmaceutical Loss during Biological Wastewater Treatment Using Sequencing Batch Reactors[END_REF] and [START_REF] Alvarino | Understanding the Sorption and Biotransformation of Organic Micropollutants in Innovative Biological Wastewater Treatment Technologies[END_REF] studied the influence of redox potential in the removal of organic compounds, showing that in the case of SMX the microaerobic conditions increased their removal. [START_REF] Phan | The Fate of Trace Organic Contaminants during Anaerobic Digestion of Primary Sludge: A Pilot Scale Study[END_REF] reported that molecules containing only electron donating functional groups (-NH2, -OH, -CH3, -OCH3…) in their structure are more susceptible to biodegradation, as is the case of SMX. In anaerobic digestion. [START_REF] Ghattas | Anaerobic Biodegradation of (Emerging) Organic Contaminants in the Aquatic Environment[END_REF] proposed a degradation pathway via reduction of the electron withdrawing sulfonyl group. In the case of IBP, this molecule contains three electron donating functional groups and it should be biotransformed but it remains recalcitrant after anaerobic digestion, maybe because their substitutions in their aromatic ring. In the case of DIC, a possible pathway could be the reductive dichlorination [START_REF] Phan | The Fate of Trace Organic Contaminants during Anaerobic Digestion of Primary Sludge: A Pilot Scale Study[END_REF] or a biodegradation catalysed by decarboxylases or acetate kinases [START_REF] Gonzalez-Gil | Biotransformation of Organic Micropollutants by Anaerobic Sludge Enzymes[END_REF]. Concerning SMX, they reported that most probably pathway was the abiotic reduction of the Ne-O bond mediated by microbial reaction. About DIC, they proposed that their biodegradation may be catalysed by decarboxylases or acetate kinases [START_REF] Gonzalez-Gil | Biotransformation of Organic Micropollutants by Anaerobic Sludge Enzymes[END_REF].

Microaeration on anaerobic digestion affects microbial communities by increasing the diversity and activity of hydrolytic and fermentative microorganisms [START_REF] Nguyen | A Little Breath of Fresh Air into an Anaerobic System: How Microaeration Facilitates Anaerobic Digestion Process[END_REF][START_REF] Chen | Review on Microaeration-Based Anaerobic Digestion: State of the Art, Challenges, and Prospectives[END_REF]. Comparing microaeration to strict anaerobic condition, an increase of phylum Firmicutes (class Clostridia), which is related to hydrolysis step was found. In addition, microaeration also affects the methane-producing archaea presented in anaerobic digestion: Methanosarcina, slightly aerotolerant, and Methanobacterium both doubled (Fu et al., 2016). Cometabolism is considered the main mechanism of pharmaceutical degradation during biological wastewater treatments (Gonzalez-Gil et al., 2017). Further research is necessary to understand the degradation pathway attributed to co-metabolism during hybrid digestion process and the additional degradation of some targeted micropollutants during MAD-TAR.

Therefore, MAD-TAR process allows the exposure of compounds to different redox and temperature conditions improving significantly the removal efficiency of OXA, PRO and OFL and slightly DIC and 2OH-IBP concomitantly with the improvement of overall organic matter degradation. These are very positive results considering the ecotoxicological risks of OXA, PRO and OFL as underlined by [START_REF] Besse | Exposure Assessment of Pharmaceuticals and Their Metabolites in the Aquatic Environment: Application to the French Situation and Preliminary Prioritization[END_REF]. However, IBP and CBZ even show more negative efficiencies and the mechanisms involved need further investigations. The negative removal or apparent production of some molecules have already been reported in the literature, explained by the transformation back of their metabolites to their parent compounds. Indeed, parent chemicals are often excreted from the human body with a number of associated metabolites. In the case of IBP, it is excreted as the unchanged drug in 1% and 76% as metabolites [START_REF] Petrie | A Review on Emerging Contaminants in Wastewaters and the Environment: Current Knowledge, Understudied Areas and Recommendations for Future Monitoring[END_REF]. There is a general lack of data about metabolite concentrations and biotransformation during wastewater treatments but some authors reported higher metabolites concentrations than parent compound in urban wastewater for IBP [START_REF] Brezinova | Occurrence and Removal of Ibuprofen and Its Metabolites in Full-Scale Constructed Wetlands Treating Municipal Wastewater[END_REF]). In the case of CBZ, the negative removal or apparent production was also reported (Narumiya et al. 2013;[START_REF] Falas | Tracing the Limits of Organic Micropollutant Removal in Biological Wastewater Treatment[END_REF]. [START_REF] He | Simultaneous Determination of Carbamazepine-N-Glucuronide and Carbamazepine Phase I Metabolites in the Wastewater by Liquid Chromatography-Tandem Mass Spectrometry[END_REF] reported the quantification of some phase I and phase II metabolites of CBZ showing that CBZ Nglucuronide (CBZ-Glu) concentration (59.4 ng L -1 ) was higher than CBZ (53.8 ng L -1 ) in wastewater influent from Kyoto WWTP (Japan). In their effluent, CBZ concentration increased by 15% and CBZ-Glu decreased by 19%. They attributed the increase of CBZ to the deconjugation of CBZ-Glu. Further research is needed to confirm this hypothesis in IBP and CBZ as some metabolites may be biologically active and have similar pharmacological activity than parent compound [START_REF] Besse | Exposure Assessment of Pharmaceuticals and Their Metabolites in the Aquatic Environment: Application to the French Situation and Preliminary Prioritization[END_REF].

Partitioning pharmaceutical compounds

Considering the possible relation between sorption on solids and inaccessibility for biodegradation, distribution of compounds was studied based on measured concentration in total and soluble phases, as described in section 2.4. From eq. 1, it is obvious that a single value of log Kd is not enough to evaluate the partition of the compounds between solid and liquid phases and that this partitioning constant should be always given together with the TSS concentration. Indeed, in the case of primary sludge of MAD campaign, which presented the highest TSS concentration, a log Kd value below 1.73 and 0.78 indicates that the percentage of compound in the liquid phase is above 50% and 90%, respectively (Figure 3a). At the opposite, when considering the lowest concentrated sludge, i.e. the digestate during MAD-TAR campaign log Kd values below 2.31 and 1.36 indicate a percentage above 50% and 90% respectively (Figure 3b).

Thus, even if the partition coefficients were not greatly modified during digestion, the significant decrease in TSS in the MAD-TAR campaign leads to higher fractions in the liquid (Figure 3a) compared to the MAD campaign (Figure 3b).

The distribution of targeted compounds between the particulate and liquid phases are presented in Figure 4. Primary sludge presented similar sorption behavior for the two campaigns, OFL and PRO showed the highest sorption (>90%) followed by OXA (71%).

IBP, SMX, DIC and CBZ range of 35-63%. Finally, a low sorption is found for 2OH-IBP As indicated in Table S5, for a same affinity constant, the fraction in liquid after digestion is significantly different in MAD and MAD-TAR due to drastic changes in TSS. Indeed, the suspended solid concentration dropped from 18.5±3.4 to 11.7±0.5 g L -1 during the MAD experiment and from 15.4 ± 2.1 to 4.8 ± 0.7 g L -1 during the MAD-TAR campaign.

The difference in phase distribution after digestion can partly be attributed to the different pH value of primary and digested sludge because of the presence of compounds containing functional groups which can be protonated and deprotonated [START_REF] Ternes | A Rapid Method to Measure the Solid-Water Distribution Coefficient (Kd) for Pharmaceuticals and Musk Fragrances in Sewage Sludge[END_REF]). In function of pH, the compound can be neutral, cationic, anionic or zwitterionic, consequently its physical, chemical and biological properties may be affected [START_REF] Verlicchi | Occurrence of Pharmaceutical Compounds in Urban Wastewater: Removal, Mass Load and Environmental Risk after a Secondary Treatment-A Review'. Science of The Total Environment, Special Section -Arsenic in Latin America, An Unrevealed Continent: Occurence[END_REF]) by inducing positively charged compounds, that are likely to interact with the negatively charged surface of sludge, or at the contrary anions inducing hydrophilic behaviour [START_REF] Alvarino | Understanding the Sorption and Biotransformation of Organic Micropollutants in Innovative Biological Wastewater Treatment Technologies[END_REF]).

Among the compounds mostly present in liquid phase after digestion (pH 7.5 in MAD -7.4 in MAD-TAR), IBP, 2OH-IBP and DIC have a carboxylic acid group (pKa = 4.15 -4.63). For a pH higher than pKa value, the fraction of anionic species increased. Initial pH of the primary sludge was 6.1-6.2, after digestion of the organic matter, the pH increased resulting in an increase of hydrophilicity of the compounds, in line with our results. For instance, in the case of IBP, the liquid fraction in primary sludge was 65% (MAD) and 59% (MAD-TAR) and it increased to 87% (MAD) and 92% (MAD-TAR) after digestion. Similar increase of liquid partition was found in DIC and 2OH-IBP. The same partitioning behaviour was also expected in SMX (pKa 1.8 and 5.6). At the range of pH in anaerobic digestion, the fraction of anionic species should also increase due to the dissociation pathway of sulfonamides and consequently an increase of hydrophilicity is likely to occur. [START_REF] Ma | Oxidation of Sulfonamides in Aqueous Solution by UV-TiO-Fe(VI)[END_REF] confirmed in their study of oxidation of SMX that the dissociation of SMX increases with increasing pH.

Consequently, deprotonated compounds were more readily to oxidation. In our study, partition behaviour of SMX remained almost constant before and after digestion (liquid fraction slightly increased 5-11%).

Among the compounds present mostly in solid phase, OFL has a zwitterion structure with a basic amino group (pKa 9.28) and an acid carboxylic group (pKa 5.97). Between pH 6.0 and 9.3, cationic (protonated amino group), anionic (deprotonated carboxyl group) and zwitterion species would be present. At pH of digestion (7.4-7.5), zwitterions should be dominant. In accordance with Narumiya et al. 2013, due to the increase of zwitterionic species, the hydrophobicity of OFL would also increase; this fact may explain the tendency to sorb. In our study, sorption behaviour of OFL did not significantly change and remained at high level with more than 99% of the molecule located in the particulate compartment. OXA is uncharged under normal pH conditions [START_REF] Svahn | Describing Sorption of Pharmaceuticals to Lake and River Sediments, and Sewage Sludge from UNESCO Biosphere Reserve Kristianstads Vattenrike by Chromatographic Asymmetry Factors and Recovery Measurements[END_REF], it has also a zwitterion structure (pKa 1.55, 10.9). Between pH 6 and 11 zwitterion and anionic species would be predominant. With increasing pH during digestion, anionic species would also increase, resulting in higher hydrophilicity, in agreement with our results: the liquid fraction of OXA in primary sludge was 28-29% (MAD and MAD-TAR) and it increased to 65-67% (MAD and MAD-TAR) after digestion.

CAF (pKa 10.40) and PRO (pKa 9.42) are cationic species when pH is lower than pKa value due to their protonated amino group. During the digestion, pH increased and the fraction of cationic species would decrease in favour to their neutral conjugates. The increase in the neutral fraction would decrease the hydrophilicity of the molecules and hence liquid fraction would decrease (Narumiya et al. 2013). We found this behaviour for CAF during MAD campaign, where the liquid fraction decreased from 79% in primary sludge to 70% after digestion. However, liquid fraction of CAF increased after digestion in MAD-TAR campaign.

In the case of PRO, there were not a significant increase of hydrophilicity in MAD campaign (liquid fraction only increased 3% after digestion) but the liquid fraction increased from 10% in primary sludge to 22% after digestion during MAD-TAR experiment. Consequently, a parameter other than pH influenced their solid-liquid partition during MAD-TAR campaign.

CBZ (pKa -3.8, 13.9) is a neutral compound and hence sorption should be governed by hydrophobic interactions [START_REF] Hai | Carbamazepine as a Possible Anthropogenic Marker in Water: Occurrences, Toxicological Effects, Regulations and Removal by Wastewater Treatment Technologies[END_REF] When considering the compounds that suffered a biodegradation during digestion, results tend to show an improvement of biodegradation in MAD-TAR that may be associated with the increase of the fraction in the liquid phase in the case of DIC, OXA and 2-OH IBP (Figure 5).

The increase of the removal of OFL and PRO may be linked to the increase of solid reduction in MAD-TAR because of the fact that these molecules are largely sorbed in solids. Indeed, the concentration of solids in MAD-TAR decrease down to 4.9 mg L -1 compared to 11.7 mg L -1 in MAD campaign resulting in an increased availability of compounds in the liquid phase.

SMX and CAF are widely removed in MAD and MAD-TAR campaigns, so a clear link between liquid fraction and removal cannot be established. Furthermore, liquid fraction of IBP decreased and CBZ increased during MAD-TAR campaign and both components showed more negative efficiencies during this campaign. Thus, bioavailability cannot be the only mechanism driving the biodegradation and reversibility of the biological reactions has to be considered.

Conclusions

The removal efficiency of nine pharmaceutical compounds in conventional Mesophilic Anaerobic Digestion (MAD) was compared with Mesophilic Anaerobic Digestion combined with a Thermophilic Aerobic Reactor (MAD-TAR). In addition to a significant improvement in the degradation of solid organic matter (76% vs 47% for MAD-TAR and MAD respectively), interesting results were also obtained on the targeted compounds. Moderate (DIC, 2OH-IBP) to high (OXA, PRO, OFL) improvement of removal was observed in MAD-TAR while removal higher than 90% was found for CAF and SMX whatever the process.

Even if the mechanisms implied in the fate of IBP and CBZ need further investigation, hybrid process MAD-TAR that combines two redox conditions (anaerobic and micro aeration) presents a feasible alternative to improve not only the removal a wide number of pollutants, but also overall organic matter degradation.

(

  20% and 24 %, MAD and MAD-TAR respectively %) and CAF (21 and 34 %, MAD and MAD-TAR respectively).The liquid fraction of SMX, 2OH-IBP and PRO increased slightly after digestion in MAD (3-5%) and moderately in MAD-TAR (11-19%); CBZ and OXA increased moderately in MAD (7%-13%) and significantly in MAD-TAR (27-39%); IBP and DIC increased significantly in MAD (23%-34%) and in MAD-TAR (22-28%). Only CAF presented a contradictory behaviour, liquid fraction increased in MAD-TAR (16%) and decreased in MAD (-8%). Only OFL showed no significant change in sorption after digestion, 0 and 2% in MAD and MAD-TAR, respectively.

Figure 1 .Figure 3 .Figure 4 .

 134 Figure 1. Concentration of targeted compounds in primary sludge and digestate in a) MAD (n=13); b) MAD-TAR (n=13)

Figure 5 .

 5 Figure 5. Removal efficiencies versus fraction in liquid phase for MAD ( ) and MAD-TAR ( ) for biodegradable compounds. The error bars represent the 95% confidence interval.

  without influence of pH[START_REF] Urase | Separate Estimation of Adsorption and Degradation of Pharmaceutical Substances and Estrogens in the Activated Sludge Process[END_REF] M.

	Carballa et al. 2008). Svahn et al. (2015) also suggested that hydrophobic adsorption would
	predominate because of the high apolar to polar surface ratio of the CBZ. According to our
	results, liquid fraction of CBZ increased after digestion in MAD and MAD-TAR process and
	hence a parameter other than pH should influence CBZ partitioning behaviour during
	anaerobic digestion

Table 1 Main characteristics of MAD and MADTAR digestion.

 1

	MAD	MAD-TAR

Wastewater Settling tank Primary sludge MAD TAR Digested sludge Biogas 9 pharmaceutical compounds studied COD removal: 73f Removal of pharmaceutical compounds: MAD vs MAD-TAR

	CAF	DIC
	IBP	CBZ
	SMX	2OH-IBP
	OFL	PRO
	OXA

Acknowledgement

This work is part of the SMS project "Micropollutant Source Separation for health risks control and preservation of the environment" funded by the Adour-Garonne Water Agency and the French Agency for Biodiversity and of the project ANR Design (ANR-17-CE22-0017). We acknowledge C. Claparols from ICT for the UHPLC-MS-MS analysis.