Industries such as oil and gas extraction, desalination, textiles, food processing, and energy production generate substantial volumes of hypersaline effluent laden with toxic compounds. Conventional biological wastewater treatment, reliant on freshwater microorganisms, is often ineffective for this challenging wastewater. A promising alternative lies in innovative microalgae-bacteria consortia as a low-energy treatment system, specifically adapted to high salinity and specific toxins: microalgae photosynthetically provide oxygen for bacterial aerobic degradation of organic matter, while bacteria supply inorganic carbon for algal growth. Together, they efficiently remove target nutrients, including carbon, nitrogen, phosphorus, and sulphur [1] [2]. Salinity profoundly alters the system's biological and chemical dynamics by enhancing ion pairing, which influences pH, precipitation reactions, and the bioavailability of inorganic carbon for microalgae. Furthermore, while industrial toxins can have a lethal effect on microalgae [3] [4], studies demonstrate that certain bacterial strains can mitigate this inhibition. This critical, synergistic effect has yet to be incorporated into mathematical models, which are essential tools for understanding, optimizing, and predicting the behavior of such complex systems. This study addresses this gap by enhancing the ALBA (Algae-Bacteria) growth model [5]. The upgraded model incorporates a sophisticated physicochemical framework to simulate saline conditions, including pH dynamics, chemical speciation, and ion pairing of key species. It also integrates a representation of copper toxicity. The model was validated through laboratory-scale cocultures in synthetic seawater with copper and at pilot-scale in outdoor raceways using saline digestate of varying salinity. The new ALBA model, with its advanced pH and speciation submodel, accurately predicted biomass inhibition across both scales. The results underscore that ion pairing significantly affects pH and critically governs the availability of inorganic carbon. This work deepens our understanding of microalgae-bacteria consortia in saline industrial wastewater and paves the way for developing control strategies to mitigate toxicity inhibition.

In wastewater treatment systems using microalgae called High Rate Algal Ponds (HRAP), UV radiation induces photochemical modifications of DNA and RNA, leading to inactivation and removal of pathogens present in the wastewater. However, high turbidity due to microalgae and detritus from the wastewater reduces UV penetration in HRAP. Microalgae are expected to contribute significantly to turbidity in HRAP, however, they are also responsible for high oxygen concentration, high pH and, in the presence of UVA, production of reactive oxygen species (ROS) favoring disinfection, questioning the relative impact of microalgae on pathogen inactivation. The purpose of this study was to investigate, in a laboratory UVA cabinet, the impact of microalgae on indicator viruses' inactivation, in terms of UVA attenuation (inhibition of inactivation) and production of ROS (enhancement of inactivation). Tests were performed in presence or absence of ROS quenchers. The presence of algae or algal organic matter (AOM) increased indirect photo-oxidation of MS2. However, in optical conditions more relevant to HRAP wastewater, no impact of indirect photo-oxidation of MS2 was measured. This study highlighted a significant negative impact of microalgae due to UVA attenuation over 30 cm depth together with a strong inherent capacity to produce ROS for virus inactivation.

High rate algal ponds (HRAP) are lowenergy consuming wastewater treatment systems where microalgae provide the oxygen to bacteria for aerobic degradation of organic matter. This thesis propose an integrative approach involving different study systems to investigate interactions within the microalgae-bacteria consortium in high rate algal ponds. The assumption that organic carbon is consumed solely by bacteria in HRAP has been discussed in the literature, raising the hypothesis that heterotrophic growth of microalgae could be relevant and potentially impacting the interactions with bacteria. Considering the complexity of trophic interactions inside the microalgae-bacteria consortium, a study in laboratory photobioreactor in controlled conditions using acetate as the organic carbon source and coupled to a modeling approach was conducted. These tools turned out to be oversimplified, however this work revealed that interactions between microalgae and bacteria in high-rate algal ponds would be more complex than a simple synergy relying on O2 and CO2 ex- changes. Besides, microalgae played a major rôle not only in trophic interactions with bacteria but also in disinfection mechanisms through Reactive Oxygen Species production.

Using solar light as an energy source in microalgae-based wastewater treatment is crucial for maintaining sustainability. To maintain treatment efficiency, microalgae need to exhibit resilience, especially during periods of reduced sunlight. Organic carbon, often abundant in wastewater, has been observed to enhance algal productivity and limit biomass loss under extreme light conditions. Microalgae have demonstrated the ability to adapt to different light levels, and carbon sources may play a significant role in this adaptation. However, microalgal growth modes can be influenced by various factors, including self-shading, affinity for organic carbon molecules, production of inorganic carbon through the oxidation of organic carbon, and nutrient consumption. Notwithstanding a lack of complete understanding regarding the synergy between carbon sources and light across diurnal cycles, further research in natural settings could still yield useful information regarding microalgal adaptation to dynamic solar conditions. This review concludes that a combination of laboratory-controlled studies, evaluating the relevance of different factors within the system, and outdoor, large scale operational high rate algal ponds (HRAPs) should be conducted to comprehensively investigate the complex mechanisms involved in HRAPs. This combined approach would contribute to optimizing the efficiency and sustainability of microalgae-based wastewater treatment processes.

The Peterborough high rate algal pond (HRAP) is a large scale (5000 m2) wastewater treatment system using microalgae, located in South Australia. The purpose of the study was to assess the homogeneity of wastewater composition in this meandering 1 km-long, 30 cm-depth, 4 m-wide channel, mixed by paddlewheel. In situ measurements of flow velocity, dissolved oxygen, pH and contemporaneous wastewater sampling were conducted along the length, depth and width of HRAP. The mean flow velocity, 0.2 m.s−1, was conserved along the channel with deviations observed near the pond bottom and the 180° bends. The chlorophyll a was constant through the channel length. The HRAP wastewater was homogeneously over saturated with photosynthetically derived dissolved oxygen (DO >20 mg.L−1) at midday. However, solids sedimentation occurred near the bends where flow velocities decreased to <0.05 m.s−1 on the inner bank. Nitrification was occasionally incomplete and varied along the channel length and through depth. Given alkalinity and DO was sufficient, factors impacting nitrification included the relative proportion of ammonia and nitrite-oxidizing bacteria as well as the stability and/or presence of algal-bacterial flocs affecting substrate access. Nitrification showed less variability when these flocs were absent, and the algal population was unicellular/colonial. The wastewater composition within the 1 km-long HRAP was predominantly homogenous and efficiently mixed by the paddlewheel driven by an electric motor with an energy consumption of 0.85 kWh. Further investigations are needed to clarify associations amongst mixing, flocs size, nitrification and denitrification in micro anaerobic zones within flocs.