Alexandre Disdier

Dimitri Masson

Thomas Brethomé

Marija Jankovic

Guy-André Boy

Toward a Human Systems Integration approach to the design and operation of a remote and virtual air traffic control center

Keywords: Human Systems Integration, User centred design, Remote tower operations, Simulation, Participatory design 1

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

Introduction

The increasing complexity of sociotechnical systems demands new approaches to Systems Engineering (SE) that no longer focus solely on technology but also consider people as a central element. Human Systems Integration (HSI) is one of these approaches. HSI is an interdisciplinary field that seeks to integrate technology, organizations and people throughout the entire system life cycle in order to achieve better overall system performance and minimize costs. The consequences of an improper integration of human and organizational elements into the design, development, deployment and operation of a complex system are twofold. On the one hand, it generates operational issues such as poor system performance, lack of trust from operators in the system and more generally system failure to meet its primary goals. On the other hand, it also affects designers and developers themselves since it may lead to improper design decisions, which in turn may cause tedious and costly redesigns of the system. Air Traffic Control (ATC) towers, from which controllers must inspect the tracks and their surroundings to give a comprehensive picture of the airspace situation, are complex sociotechnical systems. The successive European SESAR partnerships [START_REF] Bolic | SESAR: The past, present, and future of european ATM research[END_REF] seek to develop alternative control paradigms to reduce tower life cycle costs, ease traffic flow, and mitigate the environmental impacts of air traffic management. Remote and virtual air traffic control towers, which allow for air surveillance from a distant center located outside the airfield, are being studied today as a viable cost-reducing air traffic control model. Such systems are heavily human-oriented by nature. Consequently, their design, development and operation may benefit from an HSI approach which considers and integrates every stakeholder's concern. However, HSI methods and tools are still in heavy development, and there is a need to understand better how current SE processes can be adapted to integrate the human element into system design. In this paper, we derive a series of critical HSI research issues that should be addressed to help create and sustain complex sociotechnical systems. We conducted a series of semi-structured interviews with employees of an industrial company specialized in the design, integration and operation of critical systems primarily aimed at the defense and cybersecurity domains. Most interviewed participants are involved in developing air traffic control solutions in partnership with the French Airforce. We asked them a series of general questions about how they implement SE within the company and the amount of care given to human and organizational elements in the SE processes currently in place. We leverage the research issues that we drew from their answers to understand how we may define a more human-centered approach to creating and operating a virtual air traffic control system. The remainder of the paper is structured as follows. Section 2 gives a quick overview of the field of HSI and states the general problem of remote air traffic control. Section 3 describes the method we applied to identify the key characteristics of today's implementation of SE within the target industrial company. Section 4 presents the results we obtained and draws from them a list of key identified SE concepts that give room to a better human-centered approach. Section 5 presents the research gaps stemming from the derived concepts and discusses how we could apply an HSI approach to the design of a virtual air traffic control system. Section 6 concludes the paper.

2 Literature Review

Human Systems Integration (HSI)

HSI is an effort that strives to provide a set of methods, tools and processes as part of a wider SE approach to ensure that humans, organizations and technology are integrated in a cohesive manner into all stages of a system life cycle [START_REF] Boy | Human systems integration and design[END_REF]. In HSI, the human element is considered as being another component of a system along with traditional software and hardware components. The term "human" in HSI refers to all personnel involved with a given system, including not only end-users, but also owners, designers, test personnel, operators, maintenance personnel, support personnel, logistics suppliers and training personnel. Adopting an HSI approach involves defining the critical human-related areas that need to be correctly interrelated and integrated to achieve better system and operator performance. These areas, referred to as HSI domains, are project-specific and must be carefully defined at the very beginning of each project. For instance, NASA (2021) specifies six HSI domains: Human Factors Engineering (HFE), Operations, Maintainability and Supportability, Habitability and Environment, Safety, and Training. Other institutions, such as the Department of Defense and the US Air Force (US Air Force, 2009), may define slightly different HSI domains according to the purpose of their projects (e.g. Manpower, Hazard Management or Environment). Nevertheless, some common HSI domains such as Human Factors remain at the heart of the quality execution of an HSI effort. The key to implementing HSI is properly defining and analyzing the trade-offs between these domains at the beginning and during system development. HSI results from the combination of Human-Centered Design (HCD) and SE. It is particularly suited to the design and operation of complex systems, including systems of systems. Complexity in this case stems from the many interactions between the interconnected parts of a system, resulting in unanticipated emergent properties [START_REF] Booher | Handbook of human systems integration[END_REF]. This concept of emergence is key to understanding the intricate nature of human activity at operation time. HSI literature tends to promote Human-in-the-loop simulations (HiTLS) [START_REF] Rothrock | Human-in-the-loop simulations[END_REF] as a means to better understand this complexity of human behavior. Indeed, HiTLS may help identify early patterns and behaviors that were not anticipated at design time, capture emergent properties early in the system life cycle and adapt the design accordingly. When adopting a HiTLS approach, the system of interest should be designed as a virtual prototype with which end-users can interact. User activity is then observed and analyzed. The analysis results help the designers refine both requirements and prototype design. Working on a fully or partially virtual prototype makes this refinement phase more flexible and design modifications less costly. After several iterations and once the virtual prototype has been validated, a tangibilization phase should turn the virtual system components into physical components. There is still a lack of standard tools and frameworks to support such virtual HiTLS. We think there is a need to explore the use of non-conventional ways of doing Modeling and Simulation (M&S) [START_REF] Loper | Modeling and Simulation in the Systems Engineering Life Cycle: Core Concepts and Accompanying Lectures[END_REF] of complex sociotechnical systems. In fact, research regarding M&S has recently expanded to the use of products that were not initially intended for SE. Software like commercial game engines has already been assessed by actors such as the Department of Defense, NATO, Audi, and the maritime sector [START_REF] Hjelseth | Design and computer simulated user scenarios: Exploring real-time 3d game engines and simulation in the maritime sector[END_REF]. This use of entertainment technology has started to be studied as a support to SE processes under the name of gamification and serious games [START_REF] Uskov | Serious games, gamification and game engines to support framework activities in engineering: Case studies, analysis, classifications and outcomes[END_REF]. [START_REF] Madni | Expanding stakeholder participation in upfront system engineering through storytelling in virtual worlds[END_REF] states that game engine environments enable the creation of virtual worlds which aim to improve comprehensibility and transparency between system stakeholders. This is especially relevant to the goals of any HSI endeavor. Finally, it should be noted that there are commonalities between the HiTLS approach described above and research on Digital Twinss (DTs) design (VanDer-Horn andMahadevan, 2021). Camara Dit Pinto et al. (2021) defines a DT as "a dynamic representation of a physical system using interconnected data, models, and processes to enable access to knowledge of past, present, and future states to manage action on that system.". In fact, the virtual prototype being incrementally improved during HiTLS sessions is also a representation of its final physical complex system counterpart. In the remainder of this section, we introduce remote and virtual air traffic control towers as one example of complex sociotechnical systems that may benefit from virtual HiTLS.

Remote and Virtual Towers

A widely adopted air traffic control model today relies on a cab placed on top of a tower, from which a team of operators can visually scan an airfield and its tracks. These operators, referred to as Air Traffic Controllers (ATCos), establish a comprehensive air situation around the tower and interact with other controllers whose roles may differ. The role of a controller mainly determines their missions and the geographical area under their responsibility (Ground ATCos, Departing ATCos, Approach ATCos, Radar ATCos, En-route ATCos). Since the late 1990s, researchers and industrials have been working on conceptualising alternative air traffic control paradigms [START_REF] Fürstenau | Virtual and remote control tower -research, design, development and validation[END_REF]. The first studies primarily sought to augment the ATCo's view with optronic devices. For instance, [START_REF] Furstenau | On the use of transparent rear projection screens to reduce head-down time in the air-traffic control tower[END_REF] tried to superimpose the radar display with the out-ofwindow view in order to reduce the amount of time that the controllers' attention was not focused outside the tower. Research projects after that worked on a more disruptive air traffic control paradigm. [START_REF] Schaik | Detection and Recognition for Remote Tower Operations[END_REF] tried to remove the physical tower and replace it with a remote center, potentially located hundreds of kilometers away, in which a screen wall had been substituted for out-theview windows. The screens displayed a video signal from cameras near the target airfield. Separating the controlling center from the controlled airfield presents at least two advantages. First, the absence of a physical tower reduces construction and maintenance costs. Second, air traffic service providers may pool their human and technical resources from several airfields with little traffic into one remote center. The center is then referred to as a multiple remote center [START_REF] Papenfuss | Head up only -a design concept to enable multiple remote tower operations[END_REF]. Some air traffic control providers such as HungaroControl, Avinor and ENAC have already demonstrated the feasibility of remote tower operations in place of conventional towers [START_REF] Kearney | Multiple remote tower for single european sky: The evolution from initial operational concept to regulatory approved implementation[END_REF]. However, the camera-based remote control model introduces two significant concerns. On the one hand, technological constraints and costs limit its applicability, as transmitting a video signal from the airfield to the remote center requires highbandwidth communication facilities. Furthermore, such heavy equipment may make the deployment of a remote center lengthy and tedious. On the other hand, most camera-based remote tower prototypes today only shift the routine air traffic control issues to a remote location. These solutions do not reconsider the role of the human element within the system. Issues regarding the ATCos' trust in the system, focus, situation awareness, fatigue and comfort are not directly addressed. In particular, the workload is not improved since ATCos still need to continuously inspect the track and its surroundings. Even worse, stress may be increased because the sole restitution of the air situation around the airfield onto screens has proven insufficient, as a controller is also sensitive to non-visual cues such as sounds and vibrations [START_REF] Reynal | Audio focus: Interactive spatial sound coupled with haptics to improve sound source location in poor visibility[END_REF]. An air traffic control solution is not only a matter of technology and should take into account concerns from a broad spectrum of people. When the system is in operation, these people are primarily ATCos, technicians, maintenance personnel, ground support personnel (both on the remote center and on the airfield), and pilots. Should we now consider the entire system life cycle, the people involved extend to designers, developers, testers, engineers, managers, project supervisors, trainers, manufacturers, suppliers, qualifiers and regulators. External stakeholders may also be implicated, including weather forecast services, fire brigades and rescue teams. All the participants, whether humans, organizations or machines, must cooperate and coordinate efficiently to achieve the two main objectives of air traffic control as stated by the ICAO: prevent collisions and maintain an orderly flow of air traffic. Therefore, we consider air traffic control systems as complex, life-critical sociotechnical systems of systems. They are life-critical because an unanticipated or poorly managed event may result in severe injury or death. However, anticipating every potential event that may occur before a system is deployed is a difficult task. Since controllers are human, their behavior during operation is also highly unpredictable. As such, an air traffic control system, whether in situ or remote, is likely to show emergent properties that may not have been expected during design time. These concerns call for measures to add flexibility. This may refer to flexibility during operations, meaning that the whole system should be able to restructure itself to cope with incidents or unusual events. Nonetheless, this paper focuses on flexibility during system design, meaning that we want to detect these emergent properties as soon as possible, especially before any substantial financial commitment has been made. The subsequent sections provide guidelines for establishing an SE strategy that enables this design flexibility. It does so through the case study of the engineering team that develops the French Airforce's air traffic control systems. We asked the participants open questions about the nature of their work, the processes, methods and tools that are engaged in each phase of the system life cycle, the types of exchanges they have with the different stakeholders involved in the project, and the opportunities and limitations they see in the way that SE is currently being carried out within the company. We sought to understand through these questions the three broader interrogations that follow:

• Which SE processes, methods and tools, or variants, are currently used within the company to achieve program and project objectives?

• Who are the stakeholders participating in these processes, and what are their relationships? • Is the human element integrated for each stage of a system life cycle, including concept, specification, design, development, deployment, operation, training, maintenance, support, and disposal? We taped the interview sessions and manually transcribed the audio records. We then applied a two-cycle coding analysis on the transcriptions, following a grounded theory-inspired approach. [START_REF] Strauss | Basics of qualitative research[END_REF]. First, we extracted verbatim excerpts from the transcripts that we deemed relevant for understanding one of the three interrogations listed above. Each excerpt could be words, groups of words or even entire paragraphs. For each excerpt, we assigned a code, phrased as a claim capturing either a feeling (e.g. "System specification documentation often contains too many details") or a fact (e.g. "Users never interact with the successive system prototypes"). Each of the collected feelings and claims expresses one aspect of how SE is done within the company, whether positive or negative. Multiple excerpts could be assigned the same code. At the end of this step, we obtained 113 codes for a total of 236 verbatim excerpts. A second pass occurred to gather related codes into several clusters. We found that these clusters of code were interrelated: we therefore grouped them into tighter and smaller categories. Two clusters were aggregated into the same category when they shared a common HSI goal or when one had assuredly some influence on the other. We discuss in detail this clustering process as well as the resulting categories in the next section.

4 Interview analysis We identified 35 clusters of codes from the second pass analysis. Each cluster is composed of up to 8 first-level codes. The clusters correspond to categories that help clarify the aspects of current SE processes that may be addressed in order to improve the integration of the human element. We then tried to understand the relationships between these 35 second-level clusters by tagging each of them according to four criteria: 1. Its relevance to HSI. Some clusters reflect issues that do not directly relate to a lack of human consideration. Some are a consequence of the poor application of traditional SE standards and processes. Some are not human-related per se, but a quality implementation of HSI practices can strongly influence the issues they convey. The remaining clusters are critically human-related as they underscore issues that directly involve users, clients, or the developing team. 2. Its nature. We have found that every cluster is a high-level objective, an enabler for achieving a high-level objective, or a challenge for it. 3. The system life cycle stage it refers to. We talk here about stages as defined by the ISO 15288 SE Process Model, namely: Concept, Development, Production, Utilization, Support, Retirement [START_REF] Iso | ISO/IEC/IEEE International Standard[END_REF]. 4. The HSI domains it relates to. HSI domains are those defined by the literature, namely: Human Factors Engineering, Operations, Maintainability and Supportability, Training, Safety and Occupational Health, Manpower and Personnel, Sustainability, Habitability, Usability, Comfort, and Survivability [START_REF] Booher | Handbook of human systems integration[END_REF]US Air Force, 2009;NASA, 2021). This tagging process enabled us to group and factor the 30 critically or strongly human-related clusters into four categories that we deem relevant to improve the consideration of the people involved in both the utilization and the acquisition phase (Figure 2). The remainder of this section describes each of these categories. Most participants pointed out a substantial rigidity in the system acquisition stage. Participants particularly questioned the V cycle model, which does not encompass the high variability of customer and user needs. One difficulty lies in the fact that the V cycle constrains the engineering team to specify the system deeply before the actual development can start. Since development is delayed, communication with external stakeholders (i.e. customers, qualifiers and end users) is reduced to basic requirement elicitation methods through working groups and email exchanges. Even once some system component has been produced, external stakeholders usually experience passive interaction with it in the sense that they only get a grasp of its features through static PowerPoint presentations and noninteractive mock-ups. We see two downsides to this approach. The first one has to do with active stakeholder engagement, as discussed in section 4.2. The second one is that static communication between stakeholders does not enable the engineering team to analyze user activity in order to refine the elicited requirements and to detect the emerging properties of the system. As noted in section 2, these emerging properties are inherent to complex sociotechnical systems. They may lead to disastrous impacts on the system, the people around it or its environment. In addition, most interviewed participants vented the frustration they feel when they strive to integrate a particular feature into the system, which eventually gets rejected by the clients even though this feature had been articulated as a requirement from the start. One participant gave the example of a former ATC program requirement that expressed the need for the digitalization of stripping procedures used by ATCos to track the flights under their control area. After 18 months of development, the clients declined the two proposed interfaces as they did not see how those would fit their operational needs. A more flexible approach to design should have enabled the team to detect and circumvent this change of requirement earlier.

Engage the right stakeholders

According to participants' comments, one reported challenge is that clients and system qualifiers impose standards and regulatory procedures on the engineering team. This is particularly true for life-critical systems like ATC solutions which can be operated only once all regulatory assessments have been conducted and the system has received approval for deployment. Several participants considered this issue as problematic: the engineering team activities are tied up to the program or project external stakeholders' will. In our case study, this gets especially concerning when the system qualifier does not contractually commit to early baselines. In other words, only when the system has been fully developed do external stakeholders formally evaluate and approve it. This has caused trouble for interviewed engineers who had to do late redesigns as the system architecture has not been endorsed yet by the clients. One of the interviewed participants, a former military air traffic controller, also explained that experts like him were encouraged not to work upstream tightly with system developers, since not committing too early on system proposals enabled them to contradict more easily design decisions later on. The other issue with external stakeholders is their interference to communications with end users. By end users, we mean not only controllers, but technicians, maintainers, pilots, and any person related to system operation. Participants bring this topic as one of the most troublesome issues. Their comments helped us identify and validate the most significant interactions between the high-level stakeholders involved in the Airforce's air traffic control program. Figure 3 shows this organizational model: end users of the air traffic control program are part of the Air Traffic Control Squadron from the Airforce Command. However, the engineering team that works for the company in charge of the implementation deals mainly with the Product Owner, which also funds and qualifies the Air Traffic Control System. The Product Owner does not have expertise in ATC, so they are assisted by the Airforce's Air Warfare Center, comprised of former controllers and air traffic management experts. Therefore, the engineering team does not interact directly with end users, but with their representatives. While his organization is understandable given the vast number of end-users involved, three difficulties emerge: first, the Air Warfare Center controllers no longer exercise, and their requirements may be outdated and out of scope. Secondly, the Air Warfare Center has teams that rotate, so people that expressed the need at the beginning of the system life cycle are rarely the ones who will participate in the validation process. One participant stressed that in this regard, working with institutions with no intermediary validation team has proven much more efficient as engineers worked directly with permanent personnel who still use the system. Thirdly, since the qualifier has the final word on system approval, it may be perceived as a burden to system development as it may contradict end users' real concerns. As we have just seen, actual system users are not always directly involved during acquisition. Furthermore, when they are, they may have trouble propelling themselves in the future since the system has yet to be developed and integrated, resulting in a lack of understanding between stakeholders. This last point raises the issue of user engagement. As noted earlier, interactions with user representatives are heavily passive. In addition to the lack of feedback on user activity, static exchanges do not allow users to project themselves into the future and adequately get a mental model of the system, its operational context and its environment. We discuss in section 5 how HiTLS can also improve stakeholder engagement.

Consider post-deployment early

The engineering team should consider how the system will be supported, managed, and eventually dismantled from the early stages of the system life cycle. In this regard, interviewed participants have mainly expressed concerns about maintenance and documentation. Documentation content too often lack relevant information or, on the contrary, is cluttered with unnecessary details. They think that it would be beneficial, particularly for the maintenance personnel, to have smaller documentation that goes straight to the point without losing the reader's focus and attention. They claim that maintenance personnel has no interest in general terms or trade-off analyses, even though such information can represent a large amount of the document. Finally, they advocate for a finer decomposition of documentation, each part being related to one topic only (interfaces, trade-offs, signatory list, architecture...). All the produced records should then be digitally centralized not to accumulate too many documents.

Grow an organization-wide culture

The HiTLS-based methodology we mentioned earlier is highly iterative and builds upon agile principles. However, some participants expressed their reservations about agile methods as they are traditionally applied and sometimes even imposed by customers. They deem agile methods like SCRUM too theoretical as they actually are intensive for developers and too punitive as the team feels like work is never finished between two successive sprints, making motivation decline. They also reported to us that some projects mix highly detailed initial requirements with agile system acquisition processes, which makes it hard to be clear about which baseline ultimately serves as the reference for system verification and validation. Finally, participants were concerned about which hierarchical management model was best suited to agile development, and how agile methods could apply to non software-intensive systems containing hardware components that are hard to redesign or rethink in an iterative manner. Therefore, there is a need to inspire an HSI mindset among the collaborators and to reassure them that alternative SE strategies are not only possible but almost surely needed. Some participants' answers have also highlighted more high-level views of what appears to them as the most critical challenges for future systems SE research. Among the outlined perspectives, one questions the in-house education opportunities regarding SE and HSI training of internal teams, including engineers and developers, but also management personnel, executives and presales teams. This helps confirm that promoting HSI is a long-term effort that should emphasize dialogue and cooperation between all stakeholders, including non-technical ones.

5 Discussion and future research Some of the outlined challenges mentioned above do not depend solely on the engineering team: they may stem from organizational complexity, regulatory constraints, and even the nature of relationships between stakeholders that may be tainted by conflicts of interest for financial and political reasons. Nonetheless, our results provide scope for further development towards a more human-related SE effort, especially regarding gaining better system knowledge during the acquisition phase. We believe that a framework that combines scenario-based design [START_REF] Rosson | Scenario-based design[END_REF] with software-intensive HiTLS can improve the understanding of the human characteristics of the people involved. The following example gives some guidelines as to how such a framework could support the design of a virtual air traffic control center. The traditional ATC tower cab is typically defined by one or more ATCos with different roles (ground, approach, apron...), each in front of an operator station equipped with at least a radiocommunication system, a stripping management system and possibly a radar display depending on the role. All operators can see the airfield and its surroundings through the panoramic window. An HSI-driven virtualization effort should start with a comprehensive task analysis of today's procedures, methods and challenges of the controller's jobs given different operational contexts. Let us suppose that we decide to virtualize the landing gear state verification procedure. When an aircraft is about to land on the track, military ATCos must double-check that the gear is down. First, the pilot sends a radio or audible signal to the controller, which the aircraft radiocommunication system can physically send only if the gear is actually down. One of the ATCos must then visually confirm through their binoculars that the gear is down. It should be noted that this procedure was initially designed to be at the pilot's initiative, hence reducing its stress level by not imposing more pressures and time constraints. This simple process illustrates a specific allocation of functions. Some are under the responsibility of humans (e.g. the controller visually checks the gear through the window). Some have been transferred to machines (e.g. the aircraft sends the signal if and only if the gear is down and the pilot decides so). In a remote environment, the out-the-view window is no longer here, so the "visual check of the gear through the window" function becomes deprecated. We could imagine a number of alternative allocations, but system users would only be able to project themselves into this context of operations and give an opinion once the system has been physically developed, built and integrated. Moreover, designers could not tell which solution to adopt, nor could they analyze user activity when users get confronted with the adopted solution. An HiTLS-based framework can circumvent the limitations described above. At this stage, designers must decide on an allocation compatible with remote center conditions. They need an early tool which users can interact with. HiTLS are not new in the field of remote ATC centers [START_REF] Schier | Remote Tower Simulation Environment[END_REF], but they are often carried out within heavy camera-based simulation environments and with costly physical devices. This is where a more software-based simulation environment supported by tools like game engines could be a significant asset to a more flexible acquisition of system knowledge. In our landing gear check example, we could quite rapidly set up a remote environment in which visual verification of the landing gear state is carried out by a camera well positioned on the airfield. It may be hard to determine whether the state of the gear should eventually be determined by the ATCo seeing the video signal sent by the camera, or by an image processing algorithm that determines this state automatically. Maybe the ATCo wants to have the freedom to set the automation level at run-time by themselves. If the system carries out the check, how does it deliver the signal to the ATCo? Should the ATCo get a video signal in any case? What happens if the landing gear is still up? And who is in charge of warning the pilot? These questions should be carefully discussed with domain experts. Then, the established scenarios should feed the simulation software to enable designers to assess human and system performance through HiTLS. There are many benefits to using an entirely game-based virtual simulation framework. First, virtual worlds enable flexibility in scenario creation, as the simulated environment can provide a common unambiguous language to users, designers and even non expert stakeholders. They can exploit visual cues given by the simulated environment to communicate more efficiently and agree on scenario staging and conduct characteristics. The simulation configuration can also be easily changed after the HiTLS has been carried out. Since all simulation assets are virtual, no commitment to any particular device is necessary. As such, if the simulation shows that the camerabased landing gear check is unsuitable to user needs, the camera can be ditched immediately. Stakeholders can therefore explore and define many scenarios in one session. Game-based simulations are easy to set up, as off-the-shelf game engines tend to have many layers of abstraction, including graphical scripting languages that enhance their access to non-developers. They significantly foster creativity as virtual worlds may comprise any imaginable asset, making the designed proposals highly disruptive compared to already existing solutions. In addition, game engines do not only handle visual information but can manage anything from sound to realistic real-time rendering, head-up displays, artificially intelligent agents, physics-based motion, virtual reality, haptic controllers, and many innovative interaction paradigms. Most of all, going virtual enables designers to simulate aspects that are irrelevant to the experiment. For instance, our landing gear scenarios should only focus on user activity regarding awareness of the gear state from the ATCo. Any technical aspects that do not serve to analyze this issue (e.g. data fusion of radar tracks, system interfaces with tactical data links, stripping system, flight plan management module...) do not have to be implemented to conduct the experiment. The assets that matter are simulated as well: the landing gear simulation designer has to give the plane a pre-determined path, but there is no need to model the intricate details about the plane cockpit or its fuel reserves, unless we decide to implement an offnominal scenario with an out-of-fuel emergency landing. This kind of simulation that voluntarily omits irrelevant subsystems is sometimes referred to as part-task simulation [START_REF] Loper | Modeling and Simulation in the Systems Engineering Life Cycle: Core Concepts and Accompanying Lectures[END_REF].

Conclusion

The objective of this research was to identify the challenges of current SE practices in the industry regarding the integration of the human element in the design of complex systems. We analyzed the processes and the difficulties of engineers, designers and experts from a defense company and have identified four categories of critical issues that we may address to better integrate the human element during system acquisition. These categories directly relate to the concerns raised by HSI practitioners. For our virtual air traffic control center project, we think that a strong emphasis should be put on modeling and simulation, especially game-based virtual HiTLS to help conceptualize, design and develop such a disruptive system with many intricate relations between human and machine elements. We believe that a computerbased simulation environment, properly built around collaboratively defined user scenarios, can be a real communicating tool that brings stakeholders together and provide a common framework for the flexible creation of safe, effective and efficient complex sociotechnical systems.

Figure 1 .

 1 Figure 1. Profile of the interviewed participants

Figure 2 .

 2 Figure 2. The four categories of absolutely or strongly human-related identified clusters

Figure 3 .

 3 Figure 3. Stakeholder organizational chart of the French Airforce ATC tower program

Research methodWe conducted semi-structured interviews with a convenient sample of seven employees from an industrial company that develops and manufactures cyberprotected critical systems in multiple fields (Figure1).


Lucie Sanchez

Sergi Vinardell

Jules Charreton

Marc Heran

Geoffroy Lesage
email: geoffroy.lesage@umontpellier.fr

Assessing the impact of granular anaerobic membrane bioreactor intensification on

Keywords: filtration flux, membrane fouling, techno-economic evaluation, granular sludge, anaerobic digestion

Introduction

Anaerobic membrane bioreactors (AnMBRs) have gained interest over conventional aerobic biotechnologies by combining the advantages of anaerobic processes with membrane technology. Anaerobic digestion reduces energy demand, produces fewer bio-solids, reduces sludge disposal costs and generates bioenergy through the complete conversion of organic materials into methane [START_REF] Vinardell | Advances in anaerobic membrane bioreactor technology for municipal wastewater treatment: A 2020 updated review[END_REF]. Micro-or ultrafiltration membranes offer strong retention of particles and microorganisms, leading to higher biomass concentration and longer sludge retention time (SRT), which is beneficial for slow-growth anaerobic microorganisms and the quality of the permeate. Therefore, the AnMBR is a cost-effective alternative that allows conversion of wastewater organic material into renewable methane energy while achieving a high-quality effluent, free of suspended solids and pathogens, which could be easily reused for various applications [START_REF] Aslam | Anaerobic membrane bioreactors (AnMBRs) for municipal wastewater treatment-potential benefits, constraints, and future perspectives: An updated review[END_REF][START_REF] Vinardell | Advances in anaerobic membrane bioreactor technology for municipal wastewater treatment: A 2020 updated review[END_REF].

Nonetheless, the AnMBR still faces some issues that have hindered its large-scale development for domestic wastewater treatment. The main operational and technical challenges in its application to domestic wastewater have been linked to low psychrophilic temperatures, lowstrength wastewater, dissolved methane and membrane fouling, among others [START_REF] Aslam | Anaerobic membrane bioreactors (AnMBRs) for municipal wastewater treatment-potential benefits, constraints, and future perspectives: An updated review[END_REF][START_REF] Lei | Application of anaerobic membrane bioreactors to municipal wastewater treatment at ambient temperature: A review of achievements, challenges, and perspectives[END_REF][START_REF] Vinardell | Advances in anaerobic membrane bioreactor technology for municipal wastewater treatment: A 2020 updated review[END_REF]. The large amount of dilute domestic wastewater results in lower methane conversion rates (L-CH 4 .m -3 ) and higher quantities of methane lost within the effluent [START_REF] Maaz | Anaerobic membrane bioreactors for wastewater treatment: Novel configurations, fouling control and energy considerations[END_REF]. The loss of methane in the liquid phase diminishes the potential for energy recovery and produces direct greenhouse gas emissions [START_REF] Smith | Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: A critical review[END_REF].

Membrane fouling has a high impact on the capital and operational costs of AnMBR due to its effect on permeate flux, membrane lifespan and energy demand [START_REF] Maaz | Anaerobic membrane bioreactors for wastewater treatment: Novel configurations, fouling control and energy considerations[END_REF][START_REF] Smith | Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: A critical review[END_REF]. Accordingly, all these factors decrease energy recovery efficiency and increase energy demand (e.g., membrane fouling mitigation, dissolved methane recovery, etc.), which reduce the economic and energy advantages of AnMBRs.

In recent years, several studies have applied lab-and pilot-scale AnMBRs to low-strength wastewater treatment at psychrophilic temperatures. As described in Table 1, various configurations and fouling control strategies have been tested to achieve high efficiencies of organic matter removal and methane conversion while mitigating membrane fouling. The chemical oxygen demand (COD) removal efficiencies obtained through AnMBR processes have typically ranged between 85% and 95% at both lab-and pilot-scales under sub-optimal conditions (i.e., ambient temperature and low-strength wastewater) (Table 1), suggesting that efficient methane yield and conversion rates can be reached (Chen et al., 2017b;[START_REF] Nie | Effect of anionic surfactant inhibition on sewage treatment by a submerged anaerobic membrane bioreactor: Efficiency, sludge activity and methane recovery[END_REF][START_REF] Shin | Temperate climate energy-positive anaerobic secondary treatment of domestic wastewater at pilot-scale[END_REF]. Then, many investigations have focused on AnMBR configuration and membrane fouling (Table 1). The different system configurations have mainly differed in terms of (i) integration of the membrane (i.e., submerged, external submerged, side-stream), (ii) type of anaerobic bioreactor (i.e., continuous stirred tank reactor [CSTR], upflow anaerobic sludge blanket [UASB], etc.), (iii) membrane module type (i.e., flat sheet, hollow fiber, etc.), (iv) fouling control strategies and (v) operating conditions. Innovative strategies have been implemented to better control membrane fouling [START_REF] Aslam | Membrane bioreactors for wastewater treatment: A review of mechanical cleaning by scouring agents to control membrane fouling[END_REF], including rotating membrane (Ruigómez et al., 2016a(Ruigómez et al., , 2016b)), dynamic membrane [START_REF] Hu | Psychrophilic anaerobic dynamic membrane bioreactor for domestic wastewater treatment: Effects of organic loading and sludge recycling[END_REF][START_REF] Quek | Applicability of upflow anaerobic sludge blanket and dynamic membrane-coupled process for the treatment of municipal wastewater[END_REF][START_REF] Yang | Upflow anaerobic dynamic membrane bioreactor (AnDMBR) for wastewater treatment at room temperature and short HRTs: Process characteristics and practical applicability[END_REF], granular activated carbon media [START_REF] Evans | A comparative pilot-scale evaluation of gas-sparged and granular activated carbonfluidized anaerobic membrane bioreactors for domestic wastewater treatment[END_REF], sponge media (Chen et al., 2017b) and granular biomass (Chen et al., 2017a;[START_REF] Gouveia | A novel configuration for an anaerobic submerged membrane bioreactor (AnSMBR). Long-term treatment of municipal wastewater under psychrophilic conditions[END_REF]. However, gas sparging remains the most frequently used approach for long-term mitigation of membrane fouling despite its energy consumption, which amounts to up to 70% of the total AnMBR energy input [START_REF] Batstone | The role of anaerobic digestion in the emerging energy economy[END_REF][START_REF] Smith | Navigating Wastewater Energy Recovery Strategies: A Life Cycle Comparison of Anaerobic Membrane Bioreactor and Conventional Treatment Systems with Anaerobic Digestion[END_REF].

Among the innovative strategies that have been tested, the use of granular biomass is an attractive cost-effective solution, as it reduces energy consumption for the control of membrane fouling [START_REF] Martin-Garcia | Impact on reactor configuration on the performance of anaerobic MBRs: Treatment of settled sewage in temperate climates[END_REF] and does not involve additional material costs or technical complexity. Granulation is a process resulting in dense granule-shaped biomass which improves biological activity and strength [START_REF] Van Lier | Anaerobic Wastewater Treatment[END_REF]. Unlike the suspended sludge configuration, a granule-based AnMBR (G-AnMBR) helps to lower the concentration of suspended solids in contact with the membrane, improving the control of membrane fouling and decreasing cake-layer formation [START_REF] Vinardell | Impact of permeate flux and gas sparging rate on membrane performance and process economics of granular anaerobic membrane bioreactors[END_REF]. However, the G-AnMBR configuration does not fully overcome membrane fouling, as complete membrane retention leads to the accumulation of colloidal and fine particulate matter, either introduced from the influent or released from the granular sludge bed due to too strong hydrodynamic conditions [START_REF] Anjum | Trends and progress in AnMBR for domestic wastewater treatment and their impacts on process efficiency and membrane fouling[END_REF][START_REF] Vinardell | Impact of permeate flux and gas sparging rate on membrane performance and process economics of granular anaerobic membrane bioreactors[END_REF]. To mitigate membrane fouling, gas sparging is also used in this granule-based configuration. Kong et al. (2021b) reached the maximal TMP (235 mbar) in 8 days at a filtration flux of 17.8 L.m -2 .h -1 (LMH) and at a specific gas demand (SGD) of 0.75 m 3 .m -2 .h -1 . [START_REF] Wang | Identification of gas sparging regimes for granular anaerobic membrane bioreactor to enable energy neutral municipal wastewater treatment[END_REF] applied a SGD of 0.2 m 3 .m -2 .h -1 and achieved the maximal TMP (550 mbar) in less than one day when the flux was increased from 5 to 10 LMH.

Gas sparging is cost-intensive and damages granule structures which increases granules fines and dissolved and colloidal organic matter (DCOM) release in the bulk [START_REF] Martin-Garcia | Impact on reactor configuration on the performance of anaerobic MBRs: Treatment of settled sewage in temperate climates[END_REF]. For these reasons, avoiding the use of gas sparging may assist in maintaining the integrity and the particle size distribution of granules, with a substantial benefit in energy consumption [START_REF] Sanchez | Enhanced organic degradation and biogas production of domestic wastewater at psychrophilic temperature through submerged granular anaerobic membrane bioreactor for energy-positive treatment[END_REF]. In addition, it appears that a high permeate flux cannot be maintained, which reduces the competitiveness of G-AnMBR compared to conventional processes (AeMBR). Hence, it is necessary to explore the behavior of the process following the increase of permeate flux and, more specifically, to characterize membrane fouling and its economic and energetic implications during process intensification.

This study aimed to evaluate the impact of G-AnMBR process intensificationby increasing the organic loading rate and the permeate fluxon anaerobic digestion performance, membrane fouling and process economics. The G-AnMBR system was operated without gas sparging to limit the energy consumption during domestic wastewater treatment and ambient temperature.

To the best of our knowledge, the impact of process intensification on membrane fouling of a G-AnMBR in submerged mode and operated without gas sparging, has not yet been evaluated.

For this reason, a G-AnMBR without gas sparging for fouling control was operated under four permeate flux conditions of 2.8, 3.6, 4.1 and 6.0 LMH and OLR of 0.5, 0.8, 1.0, 1.6 kgCOD.m - 3 . d -1 . Anaerobic digestion performance and membrane fouling behavior were assessed. Based on the experimental results, an economic evaluation was conducted to elucidate how the intensification of the process (through OLR and permeate flux increase) might influence the operational and capital costs of a G-AnMBR operated under low filtration rates.

Materials and methods

G-AnMBR setup

The G-AnMBR system consisted of a parallelepipedic tank (266 × 68 × 523 mm) with a working volume of 6.2 L that was operated in continuous mode for 357 days. The membrane module was immersed in the middle of the liquor (at a height of 205 mm from the bottom). The ultrafiltration membrane was a polyethersulfone (PES) flat-sheet membrane with a pore size of 0.04 µm (Microdyn Nadir ® , Germany). The module was composed of six membrane sides, providing a total surface area of 0.34 m². The permeate was suctioned through a peristaltic pump (LeadFluid ® , China) following an operation cycle of 8 min 15 s of filtration, 30 s of relaxation, 45 s of backwash and 30 s of relaxation. No method for the mitigation of membrane fouling was used apart from the intermittent cycle. The operation of the reactor was automatically managed by automation software developed by AC2I Automation (France). The reactor was thermoregulated at 25°C. The influent was fed at the bottom of the reactor by a peristaltic pump and diffused over the entire length by a pierced hollow tube (ø 1 mm) to minimize dead zones. Continuous recirculation from the top to the bottom of the reactor was used to provide an upflow liquid velocity (ULV) of 2.6 m/h [START_REF] Boulenger | Traitements biologiques anaérobies des effluents industriels[END_REF] and no other mixing that this recirculation was operated. A schematic representation of the labscale reactor is shown in Fig. A1.

A complex synthetic influent was used to mimic domestic wastewater treatment, as described in previous studies [START_REF] Layer | Organic substrate diffusibility governs microbial communi ty composition, nutrient removal performance and kinetics of granulation of aerobic granular sludge[END_REF][START_REF] Sanchez | Enhanced organic degradation and biogas production of domestic wastewater at psychrophilic temperature through submerged granular anaerobic membrane bioreactor for energy-positive treatment[END_REF]. The feed solution was prepared every week with and stored at 4°C for one week. During the overall experiment, the average composition of the influent was approximately 290-350 mgCOD.L -1 for total COD (tCOD), 250-290 mgCOD.L -1 for soluble COD (sCOD), 40-60 mgCOD.L -1 for particulate COD (pCOD) and 160-210 mgCOD.L -1 for volatile fatty acids (VFA).

The G-AnMBR was inoculated with granular sludge from a mesophilic (35-38°C) UASB that treats wastewater from a recycled paper factory (Saica Paper Champblain-Laveyron, France) at a high organic loading rate (18 kgCOD.m -3 .d -1 ). The granular sludge was acclimatized stepwise to the low-strength (0.5 kgCOD.m -3 .d -1 ) and psychrophilic temperature (25°C) for 20 months before the beginning of this study. The total solids concentration in the G-AnMBR was 70-80 gTS.L -1 . No sludge was purged during the 357 days of the experiment except for sampling.

Experimental design and operating conditions

Four experimental periods were used to assess the impact of design parameters on G-AnMBR performance, membrane fouling behavior and economic assessment. The influent flow was progressively increased, with a direct impact on hydraulic retention time (HRT), organic loading rate (OLR) and permeate flux (J 20 ). The G-AnMBR was then operated according to the following conditions: instantaneous filtration fluxes (J 20,inst ) of 2.8, 3.6, 4.1 and 6.0 LMH, HRT from 14 to 5 h, and OLR from 0.5 to 1.6 kgCOD.m -3 .d -1 (Table 2), which are within the common range of AnMBR studies for domestic WW treatment [START_REF] Vinardell | Advances in anaerobic membrane bioreactor technology for municipal wastewater treatment: A 2020 updated review[END_REF]. The end of each period was reached when a steady efficiency of COD removal was achieved and the fouling rate was constant and considered to be sufficiently long to be representative of the period. The maximal TMP recommended for the membrane supplier was 400-500 mbar.

Membrane fouling indicators

The extent of membrane fouling was evaluated through the monitoring of transmembrane pressure (TMP). A pressure gauge was installed on the permeate line, and data were recorded every 15 seconds. Data processing allowed calculation of the average maximal TMP per day.

Filtration resistance analysis was used to understand fouling mechanisms. Fouling resistance was measured using Darcy's law (Eq. 1) and the resistance-in-series model (Eq. 2).

𝑅 𝑡 = 𝑇𝑀𝑃 µ 20 . 𝐽 20

Eq. 1

𝑅 𝑡 = 𝑅 𝑚 + 𝑅 𝑓 = 𝑅 𝑚 + (𝑅 𝑟𝑒𝑣𝑒𝑟𝑠𝑖𝑏𝑙𝑒 + 𝑅 𝑖𝑟𝑟𝑒𝑣𝑒𝑟𝑠𝑖𝑏𝑙𝑒 + 𝑅 𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙 ) Eq. 2
where R t is the resistance (m -1 ), TMP is the transmembrane pressure (Pa), J 20 is the normalized flux at 20°C (L.m -2 .h -1 ), µ 20 is the viscosity of water at 20°C (Pa.s), R m is the membrane resistance (m -1 ), R f is the fouling resistance (m -1 ) and R reversible , R irreversible and R residual are the resistances caused by reversible fouling, irreversible fouling and residual fouling, respectively (m -1 ).

All resistances were measured by filtering deionized water through the membrane. R m was measured with a clean membrane, R t was measured at the end of experimental periods with a fouled membrane and R f was deduced. Subsequently, the fouled membrane was physically cleaned by water rinsing with 5 L of deionized water allowing for calculation of R reversible .

Finally, the membrane was chemically cleaned by soaking (2 h) in a 0.2% NaOCl solution, providing the resistance removed by chemical cleaning R irreversible and the residual resistance R residual .

Analytical procedures

Influent, supernatant and effluent were sampled twice per week to measure the tCOD and sCOD concentration using commercial kits (Hach,Germany,LCK 500,314,514). The supernatant was sampled on the liquid recirculation line. The sCOD concentration was measured after prefiltration at 0.45µm. sCDO removal was divided into 'biological removal' and 'membrane removal' (section 3.1.1). The sCOD removed between influent and supernatant was associated to 'biological removal' whereas the sCOD removal measured between supernatant and permeate was related to 'membrane removal'. It should be noticed that the 'membrane removal' takes into account the membrane barrier rejection and the potential biological removal by the biofilm developed on membrane surface. VFAs were analyzed regularly by ion-exclusion chromatography (ICS-900, Dionex, USA; column BP-OA_2000, Benson Polymeric Inc., USA) coupled with UV detector (210 nm). H 2 SO 4 (0.05N) was used as eluent at 0.4 mL.min -1 . Six

VFAs were quantified, namely acetic acid, propionic acid, butyric acid, iso-butyric acid, isovaleric acid and valeric acid. All samples were passed through a 0.22 µm filter prior to HPLC-UV analysis. Mixed liquor suspended solids were measured according to standard methods (APHA et al., 1998).

A three-dimensional fluorescence excitation emission matrix (3DEEM) was used to characterize the composition of the DCOM of the foulant. After physical cleaning, the collected foulant was mixed thoroughly and pre-filtered through a 1.2 µm filter to conserve the colloidal compounds. A fluorescence spectrophotometer (FL 6500, Perkin-Elmer, USA) was used with excitation and emission scan ranges of 200-500 nm and 280-600 nm, respectively. Data analysis was performed according the protocol of [START_REF] Jacquin | Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein-and humic-like substances in full-scale membrane bioreactor (MBR)[END_REF]. Three regions of fluorophores were distinguished, namely (a) region I + II, associated with protein-like molecules, (b) region IV, corresponding to soluble microbial product (SMP)-like molecules, and (c) region III + V, related to humic substances.

Quantification of proteins (PN) and polysaccharides (PS) was conducted following the Lowry and Dubois methods, respectively [START_REF] Dubois | A Colorimetric Method for the Determination of Sugars[END_REF][START_REF] Lowry | Protein measurement with the Folin phenol reagent[END_REF]. Bovine serum albumin (BSA) and glucose were used as standards. All samples were pre-filtered through a 0.45 µm filter before PN and PS quantification.

The extracellular polymeric substances (EPS) of the fouling layer were quantified because they play a key role in membrane fouling. The EPS basically can be divided into three fractions depending on their structure: soluble EPS, also known as soluble microbial products (SMPs), loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS). The heating extraction method described by [START_REF] Li | Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge[END_REF] was used. The concentration of each EPS fraction was quantified as the sum of PN and PS contents.

COD mass balance

The COD mass balance (gCOD.d -1 ) was determined according to Eq. 3. corresponds to the COD used for the reduction of sulfate by sulfate-reducing bacteria (0.67 gCOD.gSO 4 -1 ); and 𝛥𝐶𝑂𝐷 𝑏𝑖𝑜𝑚𝑎𝑠𝑠 is the COD used for biomass synthesis (0.1 gVSS.gCOD -1 ).

The dissolved methane (𝐶𝑂𝐷 𝐶𝐻4 𝐿 ) was quantified experimentally following the headspace method described by [START_REF] Sanchez | Enhanced organic degradation and biogas production of domestic wastewater at psychrophilic temperature through submerged granular anaerobic membrane bioreactor for energy-positive treatment[END_REF]. Total methane produced was calculated using the theoretical value of 0.38 L-CH 4 /gCOD removed at 25°C, allowing for the gaseous methane (𝐶𝑂𝐷 𝐶𝐻4 𝐺 ) calculation.

Preliminary economic evaluation

The economic assessment was conducted for four scenarios, corresponding to the four operating periods. The design parameters, process performance and membrane fouling rates of the four experimental periods were used for the calculations. The economic analysis was performed considering a wastewater flow rate of 20,000 m 3 .d -1 (i.e. 100,000 population equivalent).

Detailed information of the design criteria for each scenario can be found in Table A1 of the supplementary data.

The capital and operating costs influenced by permeate flux, membrane fouling rate and process performance were included in the economic evaluation. For this reason, the costs not influenced by the evaluated parameters have not been included in the economic evaluation because they are expected to be similar for the four scenarios. Capital expenditures (CAPEX) comprised the purchase of the membranes, the bioreactor construction and the combined heat and power (CHP) unit for biogas valorization. Operational expenditures (OPEX) comprised the chemical reagents needed for membrane chemical cleaning, energy consumption for CHP unit and the equipment replacement cost (i.e., membrane and CHP unit). Revenues corresponded to the energy recovered from the biogas. All design and cost parameters used to calculate costs and revenues are provided in Table A3 of the supplementary data.

The chemical cleaning protocol was adapted from [START_REF] Brepols | Strategies for chemical cleaning in large scale membrane bioreactors[END_REF] and used a 0.05% NaOCl solution (2 h) and a 2,000 mg.L -1 citric acid solution (2 h). A chemical cleaning was considered required when the maximal applicable TMP of 400 mbar was reached. It was assumed that the initial membrane permeability was recovered after each chemical cleaning.

Chemical cleanings have an impact on membrane lifetime because the chemicals damage the membrane materials and modify their properties and performance [START_REF] Chheang | Effects of Chemical Cleaning on the Ageing of Polyvinylidene Fluoride Microfiltration and Ultrafiltration Membranes Fouled with Organic and Inorganic Matter[END_REF]. The replacement of the membrane was presumed to be conducted when a maximum cumulative chlorine contact of 500,000 mg.L -1 .h -1 was reached [START_REF] Robles | Model-based automatic tuning of a filtration control system for submerged anaerobic membrane bioreactors (AnMBR)[END_REF][START_REF] Vinardell | Impact of permeate flux and gas sparging rate on membrane performance and process economics of granular anaerobic membrane bioreactors[END_REF].

The present value of the net cost (PV NC ) was calculated for the four scenarios as the difference between the PV of the gross cost (CAPEX + OPEX) and the PV of the electricity revenue considering three plant lifetimes (i.e., 20, 30 and 40 years) and a discount rate of 5%. Detailed information regarding the equations used for the economic evaluation can be found in Table A3 of the supplementary data.

Results and discussion

3.1 Overall treatment performance

Biological and membrane rejection performances

Table 3 summarizes the average tCOD, VFA and MLSS values for the four periods. The tCOD removal efficiency ranged between 82.5% and 92.6%, with average tCOD concentrations in the effluent varying from 22.5 to 52.9 mgCOD.L -1 , which was in compliance with European Union discharge standards (Directive 91/271/EEC). The MLSS in the permeate were below 5 mg.L -1 in every experimental period, which was considerably below the 35 mgSS.L -1 regulation discharge (Directive 91/271/EEC). The concentration of MLSS in the supernatant slightly increased with the decrease of HRT (Pearson's correlation coefficient r = -0.852). This result was expected because (i) ultrafiltration membranes retain particulate matter, which can accumulate over time and (ii) lower HRT induces higher OLR, increasing levels of particulate and colloidal materials and thus promoting biomass growth (R. Chen et al., 2017b;[START_REF] Huang | Submerged anaerobic membrane bioreactor for lowstrength wastewater treatment: Effect of HRT and SRT on treatment performance and membrane fouling[END_REF]. Solid concentration build-up near the membrane has been previously correlated with TMP increase [START_REF] Gouveia | A novel configuration for an anaerobic submerged membrane bioreactor (AnSMBR). Long-term treatment of municipal wastewater under psychrophilic conditions[END_REF]. Fig. 1a presents the distribution of sCOD removal during the different periods. Under all conditions, 10 ± 1.5% of the sCOD influent was removed by the physical membrane barrier and/or by the biomass attached to the membrane surface. This highlights that the effectiveness of membrane separation was not affected by the filtration flux. The retention of particulate matter and some DCOM by the membrane was observed to improve effluent quality further and enhance the removal rate of organic material [START_REF] Gouveia | A novel configuration for an anaerobic submerged membrane bioreactor (AnSMBR). Long-term treatment of municipal wastewater under psychrophilic conditions[END_REF][START_REF] Sanchez | Enhanced organic degradation and biogas production of domestic wastewater at psychrophilic temperature through submerged granular anaerobic membrane bioreactor for energy-positive treatment[END_REF][START_REF] Smith | Psychrophilic anaerobic membrane bioreactor treatment of domestic wastewater[END_REF]. No statistical difference was observed among the tCOD removal efficiencies achieved in all experimental periods (p > 0.05) with the exception of Period 4.0 (p < 0.05). In this latter case, when the OLR increased from 1.0 to 1.6 kgCOD.m -3

.d -1

, biological sCOD removal dropped to 57%. The G-AnMBR performances rose progressively for 45 days until reaching an average biological sCOD removal of 80.6 ± 5.5 and a tCOD removal rate of 92.7 ± 1.5%. Thus, the anaerobic microbial community eventually adapted to harsh operational conditions, probably due to membrane retention that uncoupled HRT and SRT and thus allowed for acclimation of anaerobic bacteria and archaea [START_REF] Stuckey | Recent developments in anaerobic membrane reactors[END_REF]. These results are consistent with those obtained by Ji et al. (2021), who operated a submerged AnMBR for domestic wastewater treatment at 25°C under different operating conditions. For an OLR between 1.5 and 0.7 kgCOD.m -3 .d -1 (HRT 6-12 h), a steady high COD removal efficiency above 89% was observed. However, the biological performance substantially decreased at an OLR of 2.1 kgCOD.m -3 .d -1 (HRT 4h), although the organic material retained by the membrane compensated for the decrease in bioactivity (Ji et al., 2021a).

Fig. 1b shows the VFA concentrations measured in the influent, supernatant and effluent of the G-AnMBR. The VFA concentration is expressed as the sum of all VFAs converted into COD equivalents. Among the VFAs analyzed, acetic acid and propionic acid were the predominant compounds (10-120 mg/L); butyric acid, iso-butyric acid and iso-valeric acid were near the limit of quantification (<2 mg.L -1 ); and valeric acid was below the level of detection (<0.5 mg.L -1 ). VFAs are key intermediate products of anaerobic digestion and their concentrations within the mixed liquor give an indication about the process stability and the proper functioning of methane-producing archaea [START_REF] Van Lier | Anaerobic Wastewater Treatment[END_REF]. The lower the amount of VFA, the more efficient the anaerobic reaction chain. Low amounts of VFAs were measured, and no VFA accumulation was observed in the G-AnMBR supernatant during the experimental periods. This analysis confirms that (i) a steady state was reached and (ii) the anaerobic bacteria and archaea were consistent with all the conditions evaluated. Furthermore, Fig. 1b reveals very low VFA concentration in the permeate (1.9 ± 1.3 mgCOD.L -1 ), revealing that these compounds were biologically degraded and additionally removed in the membrane separation step (Fig. 1b).

Theoretically, VFAs could pass through the membrane due to their low molecular weight but this phenomenon has been previously observed, demonstrating the positive impact of fouling layer biological activity (Chen et al., 2017a;[START_REF] Martinez-Sosa | Anaerobic submerged membrane bioreactor (AnSMBR) for municipal wastewater treatment under mesophilic and psychrophilic temperature conditions[END_REF]. Previous studies have hypothesized that the biomass attached to membrane surface is considerably active, even more active than suspended biomass, because of lower mass-transfer limitations [START_REF] Smith | Psychrophilic anaerobic membrane bioreactor treatment of domestic wastewater[END_REF]. kgCOD.m -3 .d -1 for periods 1, 2, 3 and 4, respectively).

COD mass balance

COD mass balance distribution for the four operational conditions is provided in Table 4. The COD contained in the effluent and the methane production were the two major factors affected by the changes in OLR. As expected, the quantity of COD in converted into methane increased considerably, from 2.7 to 7.4 gCOD.d -1 (2.7-fold), and the influent flux increased from 3.55 to 10.70 gCOD.d -1 (3-fold). However, the fraction of the tCOD converted into methane decreased from 77% to 70% as the OLR increased from 0.5 to 1.6 kg.m -3 .d -1 , respectively. At the same time, the proportion of COD in retrieved as dissolved methane (lost methane) decreased from 27% to 10% when the OLR increased (Table 4). These results indicate that a trade-off must be found in the applied operating conditions to enable both a high rate of conversion of organic matter to methane and a lower concentration of dissolved methane.

Dissolved methane measured during the periods was lower as OLR increased (r = -0.95).

Specifically, dissolved methane concentration was 21.2 ± 1.3 mg.L -1 for Period 1, 20.5 ± 2.4 mg.L -1 for Period 2, 14.7 ± 2.1 mg.L -1 for Period 3 and 9.3 ± 3.0 mg.L -1 for Period 4. Similarly to the present study, [START_REF] Yeo | Contribution of Liquid/Gas Mass-Transfer Limitations to Dissolved Methane Oversaturation in Anaerobic Treatment of Dilute Wastewater[END_REF] observed that the fraction of dissolved methane (relative to the total methane produced) decreased from 35 to 14% as the OLR increased from 0.4 to 1.1 kgCOD.m -3 .d -1 . The increase in gas production at higher OLR levels led to an increase in local turbulence and mass transfer, allowing more methane to escape the liquid phase and diminishing the level of supersaturation [START_REF] Yeo | Contribution of Liquid/Gas Mass-Transfer Limitations to Dissolved Methane Oversaturation in Anaerobic Treatment of Dilute Wastewater[END_REF].

Membrane fouling behavior

Membrane filtration performance

Fig. 2 shows the extent of membrane fouling through the TMP evolution along the operational periods. Table 5 provides the membrane fouling rates obtained from the TMP profiles.

Substantial differences in TMP profiles were observed between each period. During Period 1 (Jp 20,net = 2.8 LMH), TMP slowly increased to 46 mbar with a progressive rise of 0.03 kPa.d -1

(Table 5). The very low membrane fouling rate suggests that the membrane was operating at sub-critical filtration flux, as no severe fouling was observed. When the instantaneous filtration flux was increased to 3.6 LMH (Period 2), the TMP reached 305 mbar (ΔTMP = 236 mbar) after 65 days of filtration. The TMP profile was characterized by a slow upward trend (0.16 kPa.d -1 ), followed by a much faster fouling rate (0.87 kPa.d -1 ), resulting in an average fouling rate estimated at approximately 0.40 kPa.d -1 (Table 5) . In Period 3 (Jp 20,inst = 4.1 LMH), TMP rapidly and steadily increased, with an average daily rise of 0.65 kPa.d -1 , until a maximal TMP of 430 mbar was reached. During Period 4, the initial filtration flux targeted (Jp 20,inst = 7.5 LMH) led to very strong and fast membrane fouling; the average instantaneous flux in this period was of 6.0 ± 1.4 LMH. The TMP increased from 40 mbar to 530 mbar within 9 days (day 290 to 299), and constant instantaneous filtration flux was unable to be sustained, thus decreasing slowly to 5.7 LMH. On day 303, the membrane was removed from the reactor for physical and chemical cleaning. The filtration for Period 4 was restarted, and the same sharp upward trend in TMP was observed after the membrane cleaning. The membrane fouling rate was approximately 2.86 kPa.d -1 . On day 316, the maximal TMP was reached; then, the instantaneous permeate flux dropped progressively to 4.1 LMH until the end of the experiment.

The concept of critical flux states that under fixed operating conditions (e.g., MLSS, hydrodynamics conditions, membrane properties) there is a threshold flux above which a sustainable flux cannot be further maintained [START_REF] Bacchin | Critical and sustainable fluxes: Theory, experiments and applications[END_REF]. Fig. 2 clearly shows that apart from Period 1 (Jp 20,inst = 2.8 LMH), the critical flux was exceeded because apparent fouling was observed during the experimental periods. Conversely, a critical instantaneous flux of 5 LMH was obtained in a mesophilic (30°C) AnMBR at a TSS concentration of 50 g.L -1 when gas sparging (U g = 35 m/h) was applied [START_REF] Jeison | Cake layer formation in anaerobic submerged membrane bioreactors (AnSMBR) for wastewater treatment[END_REF]. The lower critical flux obtained in this study can be attributed to the very high concentration of solids and the poor shear rate due to the absence of gas sparging.

These results confirm that the TMP substantially increased with the permeate flux, which confirms the high impact of permeate flux on membrane fouling. The increase in filtration flux led to higher convective forces in such a way that foulants were pushed toward the membrane surface more harshly and quickly; thus, the lift and diffusion forces had less effect in carrying foulants away (Chen et al., 2017a).

Membrane fouling resistances

The extent of reversibility of the fouling layer was evaluated based on the distribution of the fouling resistances (Table 6). Because the fouling rate in Period 1 was very low (Table 5) and the TMP on day 134 was approximately 50 mbar, no membrane cleaning was performed between Period 1 and Period 2. Therefore, no membrane fouling resistance distribution is available for Period 1. Fouling resistance (R f ) corresponded to 15.0 ± 1.1 × 10 12 , 16.8 ± 0.6 × 10 12 and 25.7 ± 4.7 × 10 12 m -1 for periods 2, 3 and 4, respectively. These results demonstrate that R f was strongly correlated to permeate flux and OLR (r > 0.99, p < 0.05). R reversible accounted for 100%, 97.9% and 97.7% of the membrane fouling resistance, indicating that fouling was mostly reversible under all the operating conditions tested. External deposition, especially in the cake layer, was the main mechanism involved in G-AnMBR fouling, in agreement with previous studies [START_REF] Anjum | Trends and progress in AnMBR for domestic wastewater treatment and their impacts on process efficiency and membrane fouling[END_REF][START_REF] Kaya | Treatment of chemical synthesis-based pharmaceutical wastewater in an ozonationanaerobic membrane bioreactor (AnMBR) system[END_REF][START_REF] Lin | A review on anaerobic membrane bioreactors: Applications, membrane fouling and future perspectives[END_REF]. Only residual fouling of 0.1 ± 0.0 × 10 12 and 0.2 ± 0.1 × 10 12 m -1 were measured for periods 3 and 4. High filtration fluxes have been observed to promote the extent of intermediate pore blocking in the early filtration step, which may lead to more severe or resistant fouling [START_REF] Lin | A review on anaerobic membrane bioreactors: Applications, membrane fouling and future perspectives[END_REF][START_REF] Yao | Dual-objective for the mechanism of membrane fouling in the early stage of filtration and determination of cleaning frequency: A novel combined model[END_REF]. However, the contribution of residual fouling to the total resistance was minor in comparison to the clean membrane (R m = 0.8 ± 0.1 × 10 12 m -1 ).

These results are promising for mitigation of G-AnMBR fouling, suggesting that a welldesigned backwash (i.e., physical cleaning) could lead to longer filtration performance. Further research is needed to optimize the backwash parameters, such as intensity, frequency and duration, for the purpose of developing sustainable G-AnMBR domestic wastewater treatment.

Membrane foulants characteristization

The composition of the reversible fouling was characterized through 3DEEM analysis (Fig. 3a), while the 3DEEM content of mixed liquor and permeate have been analyzed and published in previously published study [START_REF] Sanchez | Enhanced organic degradation and biogas production of domestic wastewater at psychrophilic temperature through submerged granular anaerobic membrane bioreactor for energy-positive treatment[END_REF]. Based on previous works, three regions of fluorophores were distinguished, namely (a) region I + II, associated with protein-like molecules, (b) region IV, corresponding to soluble microbial product (SMP)-like molecules, and (c) region III + V, related to humic substances [START_REF] Chen | Fluorescence Excitation-Emission Matrix Regional Integration to Quantify Spectra for Dissolved Organic Matter[END_REF][START_REF] Jacquin | Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein-and humic-like substances in full-scale membrane bioreactor (MBR)[END_REF] (see Fig. A2). Since polysaccharides are not analyzed through fluorescence excitation emission, the periods, followed by the SMP content and LB-EPS content. Nonetheless, the TB-EPS content progressively decreased from 63% to 43% as the permeate flux increased from 1.8 to 3.6 LMH, respectively. Conversely, the fraction of SMPs extracted from the fouling layer increased from 9% to 31% with the increase of permeate flux. The increase in SMPs was probably caused by the OLR increase and HRT decrease, which stimulated production of SMPs and reduced their degradation [START_REF] Huang | Submerged anaerobic membrane bioreactor for lowstrength wastewater treatment: Effect of HRT and SRT on treatment performance and membrane fouling[END_REF].

On the one hand, it might be concluded that bound-EPS (TB-and LB-EPS) represented an important fraction of the foulants deposit because bound-EPS were mostly within the fouling layer, in agreement with previous studies (R. Chen et al., 2017a;[START_REF] Gao | Structure of cake layer in a submerged anaerobic membrane bioreactor[END_REF]. On the other hand, the SMP fraction increased with the increase of filtration flux and fouling resistance, which could suggest that SMPs play an important role in membrane fouling. [START_REF] Ding | A comprehensive study into fouling properties of extracellular polymeric substance (EPS) extracted from bulk sludge and cake sludge in a mesophilic anaerobic membrane bioreactor[END_REF] studied the extent of fouling caused by the three EPS fractions extracted from the cake sludge of a mesophilic AnMBR. It was found that at the same TOC concentration, SMPs caused the highest filtration resistance (50.2%) followed by LB-EPS (19.8%) and TB-EPS (30.0%). It was suggested that the SMP fraction exhibited a lower energy barrier than the bound-EPS fractions, so that SMPs needed to overcome weaker repulsive interaction energy to adhere to the membrane surface [START_REF] Ding | A comprehensive study into fouling properties of extracellular polymeric substance (EPS) extracted from bulk sludge and cake sludge in a mesophilic anaerobic membrane bioreactor[END_REF]. Accordingly, it is important to consider both the relative contribution and specific fouling resistance when evaluating the impact of EPS fractions (i.e., SMP, LB-EPS, TB-EPS) on membrane fouling. Although no clear conclusion can be provided regarding the major EPS contributor, the abundance of bound-EPS indicated the presence of biomass attachment on the membrane surface, while the SMP revealed an implication of dissolved compounds in global membrane fouling. Therefore, it is possible to conclude that the fouling was mainly due to the combination of organic and biological fouling.

To summarize, fouling layer analysis indicated that PN materials were the main foulant in the G-AnMBR system that was studied. PN content was highly influenced by the operating conditions. EPS clearly played a relevant role in membrane fouling. The role of EPS fractions in membrane fouling is still not clear; however, it is hypothesized that they can (i) participate in biomass-membrane bonding (bound-EPS), (ii) form a strong interaction with the membrane surface (SMPs) and (iii) create a wide EPS network, consolidating the cake layer.

Preliminary economic evaluation

Fig. 4a shows the gross cost, electricity revenue and net cost for the four scenarios evaluated for a plant lifetime of 30 years. Fig. A3 and A4 of the supplementary information illustrate the same results for a plant lifetime of 20 and 40 years, respectively. The results show that scenarios 3 and 4 featured the lowest net cost for the G-AnMBR system under study. The lower net cost of these scenarios compared with scenarios 1 and 2 can be mainly attributed to the lower membrane area required. This effect indicates that operating the membrane system at J 20,inst below 3.6 LMH is not economically competitive for a G-AnMBR system operated without gas sparging. Of note, the net cost of Scenario 3 (Jp 20,inst 4.1 LMH) was slightly lower than Scenario 4 (6.0 LMH) (Fig. 4a). In Scenario 4, high amounts of chemicals were needed for membrane cleaning (due to the high fouling rate) with a direct impact on membrane replacement frequency. In this regard, the extra costs regarding membrane cleaning and replacement in Scenario 4 did not offset the lower membrane purchase costs when compared with Scenario 3.

Accordingly, it is important to achieve a compromise solution between operating the membranes at relatively higher fluxes (CAPEX) while consuming a moderate amount of chemicals (OPEX). In this regard, further pilot-and demonstration-scale studies are necessary to determine, at larger scale, the optimum flux and chemical cleaning conditions to reduce the costs and improve the economic competitiveness of the G-AnMBR system. Besides cost considerations, the operation of the G-AnMBR operated without gas sparging has the potential to make the WWTP energy positive due to the biogas (60-70% CH 4 ) produced in the system (0.4-0.5 kWh•m -3 ).

Fig. 4b shows the gross cost distribution for the different scenarios. Detailed information regarding the distribution of the gross costs can be found in Table A4 of the supplementary information. Membrane purchase accounted for more than 49% of the gross cost for all the scenarios, although its contribution progressively decreased from 89% to 49% as the permeate flux increased from 2.8 to 6.0 LMH, respectively. The chemical cleaning also featured a relatively important impact on the gross cost in scenarios 2, 3 and 4. The contribution of chemical cleaning sharply increased from 0.5 to 26.0% as the permeate flux increased from 2.8 to 6.0 LMH, respectively, because the extent of membrane fouling was substantially higher at higher fluxes. This was particularly important in Scenario 4, in which the high fouling rate and chemical cleaning frequency made replacement of the membrane system necessary during the plant's lifetime, with a direct impact on the OPEX (see Table A4 of the supplementary information). This highlights that the membrane chemical cleaning strategy is a key economic driver, especially in those scenarios in which intensive chemical cleaning is needed to control long-term membrane fouling. Figure 4b also shows that the bioreactor construction contribution decreased from 7.1 to 3.8% as the OLR increased from 0.5 to 1.6 kgCOD m -3 d -1 , respectively.

This illustrates that increasing OLR has also an impact on the economic balance of the system.

Overall, these economic results further corroborate that achieving a trade-off between increasing the flux and OLR (system intensification) without requiring intensive physical and chemical cleaning strategies to control membrane fouling is important for the economics of a G-AnMBR system operated in submerged mode and without gas sparging.

Conclusions

The present work studied the impact of design parameters (i.e., OLR, permeate flux, HRT) on a G-AnMBR operated with a submerged UF membrane and without gas sparging as fouling control for mainstream domestic wastewater at ambient temperature. Four design OLRs were tested (between 0.5 and 1.6 kgCOD.m -3 .d -1 ) resulting in four HRT and permeate flux conditions.

 The G-AnMBR achieved a rapid steady-state and high COD removal efficiencies (>90%) for HRTs ranging between 14 and 7 h.

 The proportion of tCOD converted into methane was 77% at the lowest OLR and 70% at the highest OLR, confirming the decrease in biological activity with increasing OLR .

However, more methane was recovered in the gaseous phase at the highest OLR (86% of the total methane produced) in comparison to the lowest OLR (65% of the total methane produced).

Hence, at higher OLRs, the amount of bio-methane energy lost in the effluent and the greenhouse gas emissions were lowered.

 Membrane fouling increased with the permeate flux and OLR, highlighting the important role of those parameters in cake layer formation and build-up. Under all conditions, proteinaceous colloidal and macromolecules were the main components of the G-AnMBR cake layer. Membrane permeability was almost totally recovered after physical cleaning with water rinsing, demonstrating that fouling was mainly reversible.

 Finally, the economic evaluation showed that filtration flux, and the resulting membrane surface area, is a crucial parameter for G-AnMBR economics, with Scenario 3 (4.1 LMH) being the most economically favorable option.

(SAnMBR). Environ. Sci.

Technol. 50, 12750-12758. https://doi.org/10.1021/acs.est.6b03581

Fig. 1 -

 1 Fig. 1 -(a) Distribution of sCOD biological removal, membrane removal and effluent content; (b) VFA concentration and removal efficiency under all operating conditions (HRT 14, 10, 7, 5 h; OLR 0.5, 0.8, 1.0, 1.6

Fig. 2 -

 2 Fig. 2 -Transmembrane pressure and instantaneous filtration flux during the operational periods.

Fig. 3 -

 3 Fig. 3 -Composition of membrane foulants obtained through (a) 3DEEM (I + II: protein-like substances, III + V: humic-like substances, IV: SMP-like molecules); (b) PN and PS concentration; (c) EPS concentration.

Fig. 4 -

 4 Fig. 4 -(a) Present value (PV) of the net cost, gross cost and electricity revenue; (b) gross cost distribution for the four scenarios evaluated for a 30-year plant lifetime.

Table 1 -

 1 Reactor configurations, operational parameters, treatment performance and membrane fouling strategies presented in recent AnMBR studies for domestic wastewater treatment at ambient temperature.

	Reactor	Membrane setup	Fouling control strategy	Scale	Type of WW	CODin	T	HRT	OLR	COD removal Methane yield	Flux	dTMP/dt Reference
	-	-	-	(liters)	-	(mg.L -1 )	(°C)	(h)	(kg.m -3 d -1 )	(%)	(L-CH 4 /gCOD removed )	(LMH)	(kPa.d -1 )
	UAGB	HF Sub.	Sponge cubes + intermittent cycle	(L) 3	Synthetic 330-370	20	12	-	93.7	0.156 (G)	5.3	0.5	(C. Chen et al., 2017b)
	UAGB	HF Sub.	Granular sludge + Intermittent cycle	(L) 4	Synthetic 320-360	20	12	0.64-0.72 *	91.3	0.16 (G)	7	0.9	(C. Chen et al., 2017a)
	UAGB	FS Sub.	Granular sludge + Intermittent cycle	(L) 6.2	Synthetic 300-350	25	13.9-4.9	0.5-1.6	83-93	70-77% a (G + D)	2.8-6	0.03-2.9	This study
	UASB	FS sub.	Dynamic membrane	(L) 6.9	Raw	107-137	25-30	3-6	1.5-3.0	64-71	0.354 (-)	100	1.5-6.2	(Quek et al., 2017)
	UAGB	HF Ext. sub.	Granular sludge + Intermittent cycle + gas sparging	(P) 72.5	Raw	221	16.3	8	<1.0 *	83	-	5-15	0.2-960 *	(Wang et al., 2018)
	UAGB	HF Sub.	Granular sludge + Intermittent cycle + gas sparging	(P) 459	Raw	978	18	9.8-20.3	0.72-3.18	≈90	0.216-0.226 (G + D)	10-14	0.1-1.9	(Gouveia et al., 2015)
	CSTR	FS Sub.	Intermittent cycle + gas sparging	(L) 6	Synthetic 700	25	8-48	0.35-2.1	≈95	0.277-0.328 (G + D)	1.1-6.5	-	(R. Chen et al., 2017a)
	CSTR	FS Sub.	Intermittent cycle + gas sparging	(L) 6	Synthetic 700	25	8-48	0.35-2.1	>90	0.305-0.338 (G + D)	1.1-6.5	0.05-1.1 *	(R. Chen et al., 2017b)
	CSTR	-Sub.	Gas sparging	(L) 6	Synthetic 492	25	24-12	3-6	97-94	89-84.7% a (G + D)	-	0.08-0.3	(Nie et al., 2017)
	CSTR	FS Sub.	Intermittent cycle + gas sparging	(L) 40	Raw	428-477	14-26	12-48	0.23-0.9	69-89	0.28-0.35 * ,b (G + D)	1.6-6.6	<0.06 *	(Plevri et al., 2021)
	CSTR	HF Ext. sub. Gas sparging	(P) 630	Synthetic 304-388	23	8.5	0.85-1.1 *	88-92	0.076-0.115 (G + D)	17	0.02-0.61	(Dong et al., 2016a, 2016b)

Table 2 -

 2 G-AnMBR operating conditions during the different operating periods.

	Parameters	Period 1	Period 2	Period 3	Period 4
	Days of operation (d)	1-134	135-203	204-289	290-357
	Jp 20,inst (LMH)	2.8 ± 0.1	3.6 ± 0.1	4.1 ± 0.3	6.0 ± 1.4
	Jp 20,net (LMH)	1.3 ± 0.1	1.8 ± 0.1	2.5 ± 0.2	3.6 ± 1.2
	HRT (h)	13.9 ± 0,5	9.9 ± 0.3	7.0 ± 0.7	4.9 ± 1.4
	OLR (kgCOD.m -3 .d -1 )	0.5 ± 0.1	0.8 ± 0.2	1.0 ± 0.4	1.6 ± 0.7
	Temperature (°C)	25.0 ± 0.3	25.0 ± 0.8	24.9 ± 1.2	25.3 ± 2.0
	pH (-)	7.2 ± 0.6	7.0 ± 0.2	7.3 ± 0.2	7.4 ± 0.2
	Redox (mV)	-488 ± 22	-488 ± 17	-494 ± 14	-478 ± 23
	ULV (m.h -1 )	2.6 ± 0.03	2.6 ± 0.03	2.6 ± 0.03	2.6 ± 0.03

Table 3 -

 3 Influent and effluent compositions and removal efficiencies (mean values ± SD; n ≥ 10).

	Parameters	Units	Period 1	Period 2	Period 3	Period 4
	OLR	kgCOD.m -3 .d -1	0.5 ± 0.1	0.8 ± 0.2	1.0 ± 0.4	1.6 ± 0.7
	tCODin	mgCOD/L	311.4 ± 54.7 346.4 ± 63.8	303.0 ± 91.0	298.8 ± 74.5
	tCODeff = sCODeff mgCOD/L	22.5 ± 7.7	24.8 ± 7.4	32.7 ± 17.6	52.9 ± 28.4
	tCODremoval	%	92.4 ± 3.1	92.6 ± 2.2	89.1 ± 4.7	82.5 ± 7.7
	VFAinf	mgCOD/L	209.2 ± 43.2 159.2 ± 38.0	170.7 ± 10.0	190.3 ± 11.1
	VFAeff	mgCOD/L	0.0 ± 0.0	3.6 ± 4.6	2.3 ± 1.7	1.6 ± 2.2
	VFAremoval	%	100 ± 0	97 ± 4	96 ± 5	99 ± 1
	MLSSsupernatant	mg/L	88 ± 50	179 ± 108	209± 106	194 ± 99
	MLSSeff	mg/L	3.6 ± 3.9	1.3 ± 2.1	1.8 ± 0.7	3.8 ± 2.8

Table 4 -

 4 COD mass balance during each experimental period.

	Period	1		2		3		4
		gCOD.d -1	%	gCOD.d -1	%	gCOD.d -1	%	gCOD.d -1	%
	Influent COD	3.55	100	5.50	100	7.10	100	10.70	100
	Effluent COD	0.27	8	0.41	7	0.83	12	1.74	16
	Sludge growth	0.47	13	0.72	13	0.89	13	1.27	12
	Sulfate reduction	0.10	3	0.16	3	0.20	3	0.28	3
	Dissolved CH4	0.95	27	1.34	24	1.37	19	1.04	10
	Gaseous CH4	1.77	50	2.86	52	3.82	54	6.37	60
	Total CH4	2.72	77	4.20	76	5.19	73	7.41	70

Table 5 -

 5 Fouling rates at different filtration conditions.

	Period	Filtration conditions	Fouling rates
		Jp20,inst (LMH)	dTMP/dt (kPa.d -1 )	dR/dt (m -1 .d -1 )
	1	2.8 ± 0.1	0.03	0.9 × 10 11
	2	3.6 ± 0.1	0.40	9.3 × 10 11
	3	4.1 ± 0.3	0.65	10.3 × 10 11
	4	6.0 ± 1.4	2.86	34.0 × 10 11

Table 6 -

 6 Fouling resistance distribution for experimental periods 2, 3 and 4.

	Resistances	Units	Period 2	Period 3	Period 4
	Rfouling	×10 12 m -1	15.0 ± 1.1 (100%)	16.8 ± 0.6 (100%)	25.7 ± 4.7 (100%)
	Rreversible	×10 12 m -1	15.0 ± 1.1 (100%)	16.4 ± 0.7 (97.9%)	25.1 ± 4.6 (97.7%)
	Rirreversible	×10 12 m -1	-	0.3 ± 0.1 (1.9%)	0.4 ± 0.1 (1.6%)
	Rresidual	×10 12 m -1	-	0.1 ± 0.0 (0.2%)	0.2 ± 0.1 (0.8%)

Overall, these results have shown that achieving a compromise solution considering permeate flux, OLR and chemical cleaning for membrane fouling control is important to reduce the net cost of G-AnMBR systems operated without gas sparging.On this basis, the G-AnMBR could be a sustainable and efficient process for domestic wastewater treatment for local applications in which important financial, technical and energy resources cannot be deployed.

Acknowledgments

This work was supported by a grant overseen by the French National Research Agency as part of the "JCJC" Program BàMAn (ANR-18-CE04-0001-01). The authors wish to acknowledge Valérie Bonniol (IEM) for her assistance in the development of the VFA quantification.

additional foulant characterization were performed through PN and PS quantification (Fig. 3b).

Among the three fluorescent regions investigated, the major volume of fluorescence belonged to region I + II (72-85%), relating to colloidal and macromolecular protein-like compounds [START_REF] Jacquin | Three-dimensional excitation and emission matrix fluorescence (3DEEM) for quick and pseudo-quantitative determination of protein-and humic-like substances in full-scale membrane bioreactor (MBR)[END_REF]. In addition, between 11% and 20% of the total fluorescence volume was part of region III + V, corresponding to humic substances. Measurement of PN and PS contents, presented in Fig. 3b, corroborates the predominance of PN materials within the fouling layer, with proportions of PN approximately 55%, 77% and 84% for periods 2, 3 and 4, respectively.

It can therefore be assumed that PN in the form of colloidal materials and macromolecules prevailed in the G-AnMBR foulant, in accordance with previous observations (R. Chen et al., 2017a;[START_REF] Yao | Micro-particles-A Neglected but Critical Cause of Different Membrane Fouling between Aerobic and Anaerobic Membrane Bioreactors[END_REF][START_REF] Zhou | Characterization and Significance of Sub-Visible Particles and Colloids in a Submerged Anaerobic Membrane Bioreactor[END_REF]. The PN to PS ratio (PN/PS) was found to be strongly correlated with the decrease in HRT (r = -0.996, p < 0.05), demonstrating the rise in PN concentration in the cake layer during the experimental periods. Several factors, including the following, may be responsible for this phenomenon: (i) the increase of the permeate flux leads to higher OLRs, which promote the growth of biomass and biopolymers (mainly composed of PN and PS) [START_REF] Huang | Submerged anaerobic membrane bioreactor for lowstrength wastewater treatment: Effect of HRT and SRT on treatment performance and membrane fouling[END_REF]; (ii) the decrease of HRT reduces degradation of polymers and macromolecules because PN-based organic matter has been found to be more slowly and weakly biodegradable than that based on PS, thus leading to PN accumulation (R. Chen et al., 2017a;[START_REF] Yang | Degradation properties of protein and carbohydrate during sludge anaerobic digestion[END_REF]; and (iii) PN has been reported to have higher hydrophobic characteristics than PS. Accordingly, PN has more tendency to adhere to membrane surface [START_REF] Kaya | Investigation of membrane fouling in an anaerobic membrane bioreactor (AnMBR) treating pharmaceutical wastewater[END_REF].

Because EPS have been identified as a key factor in membrane fouling [START_REF] Anjum | Trends and progress in AnMBR for domestic wastewater treatment and their impacts on process efficiency and membrane fouling[END_REF]R. Chen et al., 2017a;[START_REF] Ding | A comprehensive study into fouling properties of extracellular polymeric substance (EPS) extracted from bulk sludge and cake sludge in a mesophilic anaerobic membrane bioreactor[END_REF], the fouling layer was further fractionated into SMP, LB-EPS and TB-EPS to identify the contribution of EPS fractions to total fouling. Fig. 3c shows the repartition of the three EPS fractions extracted from the cake layer in periods 2, 3 and 4. Of the three EPS fractions, the TB-EPS were in higher proportion within the fouling layer for all

Supplementary information

E-supplementary data for this work can be found in the e-version of this paper online.