Granular indigenous microalgal-bacterial consortium for wastewater treatment: Establishment strategy, functional microorganism, nutrient removal, and influencing factor

Pathways, challenges, and strategies for enhancing anaerobic production of short-chain and medium-chain carboxylic acids from algal slurry derived from wastewater

Algal slurry (AS) generated from microalgae-based wastewater treatment processes holds significant potential for carboxylic acids production through anaerobic digestion (AD), which have emerged as promising products due to their high energy density, great economic value, and versatile applications. A comprehensive analysis of the pathways and optimization strategies for producing short-chain (SCCAs) and medium-chain (MCCAs) carboxylic acids using AS substrates is presented in this review. It begins by introducing and comparing two types of microalgae-based wastewater treatment processes: the microalgae process and the microalgal-bacterial consortia process. Afterwards, the review systematically examines the metabolic pathways involved in SCCAs and MCCAs production using AS substrates. Moreover, pretreatment strategies for enhancing the release of organic matter are critically discussed. Ultimately, specific emphasis is placed on addressing technical challenges and discussing future perspectives. This review provides a deeper understanding of the mechanisms and strategies involved in carboxylic acids production from wastewater-generated AS.

Enhancing the performance of microalgal-bacterial systems with sodium bicarbonate: A step forward to carbon neutrality of municipal wastewater treatment

The microalgal-bacterial granular sludge (MBGS) process, enhanced with sodium bicarbonate (NaHCO3), offers a sustainable alternative for wastewater treatment aiming for carbon neutrality. This study demonstrates that NaHCO3, which can be derived from the flue gases and alkaline textile wastewater, significantly enhances pollutant removal and biomass production. Optimal addition of NaHCO3 was found to achieve an inorganic-to-organic carbon ratio of 1.0 and a total carbon-to-nitrogen ratio of 5.0. Metagenomic analysis and structural equation modeling showed that NaHCO3 addition increased dissolved oxygen concentrations and pH levels, creating a more favorable environment for key microbial communities, including Proteobacteria, Chloroflexi, and Cyanobacteria. Confocal laser scanning microscopy further confirmed enhanced interactions between Cyanobacteria and Proteobacteria/Chloroflexi, facilitating the MBGS process. These microbes harbored functional genes (gap2, GLU, and ppk) critical for removing organics, nitrogen, and phosphorus. Carbon footprint analysis revealed significant reductions in CO2 emissions by the NaHCO3-added MBGS process in representative countries (China, Australia, Canada, Germany, and Morocco), compared to the conventional activated sludge process. These findings highlight the effectiveness of NaHCO3 in optimizing MBGS process, establishing it as a key strategy in achieving carbon-neutral wastewater treatment globally.

Response of nitrifiers to gradually increasing pH conditions in a membrane nitrification bioreactor: Microbial dynamics and alkali-resistant mechanism

Nitrification and nitrifiers are pH-sensitive especially under the alkaline environment in the activated sludge system. However, it is unclear how nitrifiers and nitrification respond to long-term alkaline environment. This study employed a continuous flow membrane nitrification bioreactor to investigate the dynamics of nitrification efficiency and microbial community adaptation under a 320-day alkaline operation. Results showed that activated sludge adapted remarkably to a progressive increase in pH from 7.5 to 10.0, achieving robust nitrification with average ammonia removal efficiencies of 96.6 ± 2.2%. Subsequently, an integrated alkali-resistant mechanism of nitrifiers was proposed. Specifically, under the long-term operation of pH 10.0, certain bacteria secreted enhanced extracellular acidic polysaccharides (i.e., up to 10.95 ± 0.27 mg·g-1 MLVSS in soluble extracellular polymeric substances (EPS)) and acidic organic compounds (e.g., humic acids increased by 1.47-fold in tightly bounded EPS) to neutralize external alkalinity. Moreover, significant enrichments in both the ammonia oxidizing bacteria Nitrosomonas (by 1.3%) and the nitrite oxidizing bacteria Nitrospira (by 5.4%) were observed in a 170-day operation of pH 10.0 condition. Meanwhile, norank_f__JG30-KF-CM45 (2.0%) and Rhodobacter (0.9%) also contributed to ammonia removal at pH 10.0. On the cellular-level, bacteria enabled to maintain intracellular pH stabilization primarily through cation/proton antiporters, evidenced by significant increases in NhaA, TrkA and KefB activities by 98.0%, 151.7% and 115.2%, respectively. A 43.1% increase in carbonic anhydrase activity also facilitated consumption of aqueous OH- ions through biomineralization, leading to CaCO3 deposition on microbial surface. These findings further enhanced understandings of physiological adaptation of nitrifiers in the long-term alkaline activated sludge system.

Attached indigenous microalgal-bacterial consortium with greater stress-resistance facilitated recovery of integrated fixed-film system after experiencing short-term stagnation inhibition

Stability of integrated fixed-film indigenous microalgal-bacterial consortium (IF-IMBC) requires further investigation. This study focused on the influence of short-term stagnation (STS), caused by influent variations or equipment maintenance, on IF-IMBC. Results showed that the IF-IMBC system experienced initial inhibition followed by subsequent recovery during STS treatment. Enhanced organics utilization was believed to contribute to system recovery. It is proposed that the attached IMBC possessed greater stress resistance. On the one hand, a higher increase in bacteria potentially participating in organic degradation was observed. Moreover, the dominant eukaryotic species significantly decreased in suspended IMBC while its abundance remained stable in the attached state. On the other hand, increased abundance for most functional enzymes was primarily observed in the attached bacteria. This fundamental research aims to bridge the knowledge gap regarding the response of IMBC to variations in operational conditions.

Effects of microalgal (Tetradesmus obliquus MCX38) attachment on photobioreactor treatment efficiency of raw swine wastewater

Attachment of microalgae on the inner surfaces of photobioreactors impacts the efficiency of swine wastewater treatment by reducing the light intensity, which has been overlooked in previous studies. This study investigated the relationship between microalgal attachment biomass and light intensity in photobioreactors, determined the optimal attachment time for effective pollutant removal, and clarified the mechanisms of microalgal attachment in swine wastewater. After 9 days of treatment, the attached biomass in the photobioreactor increased from 0 to 6.4 g/m2, decreasing the light intensity from 2,000 to 936 lux. At the 24 h optimal attachment time, the concentrations of chemical oxygen demand, ammonia nitrogen, and total phosphorus decreased from 2725.1, 396.4, and 87.2 mg/L to 361.2, 4.9, and 0.8 mg/L, respectively. Polysaccharides in the extracellular polymeric substances released by microalgae play a significant role in facilitating microalgae attachment. Optimizing the microalgal attachment time within photobioreactors effectively mitigates pollutant concentrations in swine wastewater.

Sustainable treatment of aquaculture water employing fungi-microalgae consortium: Nutrients removal enhancement, bacterial communities optimization, emerging contaminants elimination, and mechanism analysis

Fungi-microalgae consortium (FMC) has emerged as a promising system for advanced wastewater treatment due to its high biomass yield and environmental sustainability. This study aimed to investigate the nutrients removal, bacterial community shift, emerging contaminants elimination, and treatment mechanism of a FMC composed of Cordyceps militaris and Navicula seminulum for aquaculture pond water treatment. The fungi and microalgae were cultured and employed either alone or in combination to evaluate the treatment performance. The results demonstrated that the FMC could improve water quality more significantly by reducing nutrient pollutants and optimizing the bacterial community structures. Furthermore, it exhibited stronger positive correlation between the enrichment of functional bacteria for water quality improvement and pollutants removal performance than the single-species treatments. Moreover, the FMC outperformed other groups in eliminating emerging contaminants such as heavy metals, antibiotics, and pathogenic Vibrios. Superiorly, the FMC also showed excellent symbiotic interactions and cooperative mechanisms for pollutants removal. The results collectively corroborated the feasibility and sustainability of using C. militaris and N. seminulum for treating aquaculture water, and the FMC would produce more mutualistic benefits and synergistic effects than single-species treatments.

Unraveling biological behavior and influence of magnetic iron-based nanoparticles in algal-bacterial systems: A comprehensive review

Magnetic iron-based nanoparticles have been found to stimulate algae growth and harvest, repair disintegrated particles and improve stability, and facilitate operation in extreme environments, which help improve the wide application of algal-bacterial technology. Nevertheless, up to now, no literature collected to systematically review the research progress of on the employment of magnetic iron-based nanoparticles in the algal-bacterial system. This review summarizes the special effects (e.g., size effect, surface effect and biological effect) and corresponding properties of magnetic iron-based nanoparticles (e.g., magnetism, adsorption, electricity, etc.), which is closely related to biological effects and algal-bacterial behaviors. Additionally, it was found that magnetic iron-based nanoparticles offer remarkable impacts on improving the growth and metabolism of algal-bacterial consortia and the mechanisms mainly include its possible iron uptake pathways in bacteria and/or algae cells, as well as the magnetic biological effect of magnetic iron-based nanoparticles on algae-bacteria growth. Furthermore, in terms of the mechanism for establishing the algae-bacteria symbiotic relationship, the most recent works reveal that the charge effect, material transfer and signal transmission of magnetic iron-based nanoparticles possess a large array of potential mechanisms by which it can affect the establishment of algal-bacterial symbiosis. This discussion is expected to promote the progress of magnetic iron-based nanoparticles, as an eco-friendly, convenient and cost-effective technology that can be applied in algal-bacterial wastewater treatment fields.

Coagulation pretreatment coupled with indigenous microalgal-bacterial consortium system for on-site treatment of rural black wastewater

Improper treatment of rural black wastewater (RBW) presents substantial challenges, including the wastage of resource, environmental contamination, and economic consequences. This study proposed an integrated process for RBW treatment, consisting of coagulation/flocculation (C/F) pretreatment and subsequent inoculation of indigenous microalgal-bacterial consortium (IMBC) for nitrogen recovery, namely C/F-IMBC process. Specifically, the optimal C/F conditions (polyaluminium chloride of 4 g/l, polyacrylamide of 50 mg/l, and pH of 6) were determined through a series of single-factor experiments, considering CN, turbidity, and dissolved organic matter (DOM) removal, economic cost, and potential influence on the water environment. Compared to the sole IMBC system for RBW treatment, the proposed C/F-IMBC process exhibited a remarkable 1.23-fold increase in microalgal growth and a substantial 17.6-22.6 % boost in nitrogen recovery. The altered RBW characteristic induced by C/F pretreatment was supposed to be responsible for the improved system performance. In particular, the abundance of DOM was decreased and its composition was simplified after C/F pretreatment, based on the analysis for excitation-emission matrices with parallel factor and gas chromatography-mass spectrometry, thus eliminating the potential impacts of toxic DOM components (e.g., Bis(2-ethylhexyl) phthalate) on IMBC activity. It should also be noted that C/F pretreatment modified microbial community structure as well, thereby regulating the expression of nitrogen-related genes and enhancing the system nitrogen recovery capacity. For instance, the functional Cyanobacteria responsible for nutrient recovery was enriched by 1.95-fold and genes involved in the assimilatory nitrate reduction to ammonia pathway were increased by 1.52-fold. These fundamental findings are expected to offer insights into the improvement of DOM removal and nitrogen recovery for IMBC-based wastewater treatment system, and provide valuable guidance for the development of sustainable on-site RBW treatment technologies.

Microalgal-bacterial biofilms for wastewater treatment: Operations, performances, mechanisms, and uncertainties

Microalgal-bacterial biofilms have been increasingly considered of great potential in wastewater treatment due to the advantages of microalgal-bacterial synergistic pollutants removal/recovery, CO2 sequestration, and cost-effective biomass-water separation. However, such advantages may vary widely among different types of microalgal-bacterial biofilms, as the biofilms could be formed on different shapes and structures of attachment substratum, generating "false hope" for certain systems in large-scale wastewater treatment if the operating conditions and pollutants removal properties are evaluated based on the general term "microalgal-bacterial biofilm". This study, therefore, classified microalgal-bacterial biofilms into biofilms formed on 2D substratum, biofilms formed on 3D substratum, and biofilms formed without substratum (i.e. microalgal-bacterial granular sludge, MBGS). Biofilms formed on 2D substratum display higher microalgae fractions and nutrients removal efficiencies, while the adopted long hydraulic retention times were unacceptable for large-scale wastewater treatment. MBGS are featured with much lower microalgae fractions, most efficient pollutants removal, and acceptable retention times for realistic application, yet the feasibility of using natural sunlight should be further explored. 3D substratum systems display wide variations in operating conditions and pollutants removal properties because of diversified substratum shapes and structures. 2D and 3D substratum biofilms share more common in eukaryotic and prokaryotic microbial community structures, while MGBS biofilms are more enriched with microorganisms favoring EPS production, biofilm formation, and denitrification. The specific roles of stratified extracellular polymeric substances (EPS) in nutrients adsorption and condensation still require in-depth exploration. Nutrients removal uncertainties caused by microalgal-bacterial synergy decoupling under insufficient illumination, limited microbial community control, and possible greenhouse gas emission exacerbation arising from microalgal N2O generation were also indicated. This review is helpful for revealing the true potential of applying various microalgal-bacterial biofilms in large-scale wastewater treatment, and will provoke some insights on the challenges to the ideal state of synergistic pollutants reclamation and carbon neutrality via microalgal-bacterial interactions.

Advances, challenges, and prospects in microalgal-bacterial symbiosis system treating heavy metal wastewater

Heavy metal (HM) pollution, particularly in its ionic form in water bodies, is a chronic issue threatening environmental security and human health. The microalgal-bacterial symbiosis (MABS) system, as the basis of water ecosystems, has the potential to treat HM wastewater in a sustainable manner, with the advantages of environmental friendliness and carbon sequestration. However, the differences between laboratory studies and engineering practices, including the complexity of pollutant compositions and extreme environmental conditions, limit the applications of the MABS system. Additionally, the biomass from the MABS system containing HMs requires further disposal or recycling. This review summarized the recent advances of the MABS system treating HM wastewater, including key mechanisms, influence factors related to HM removal, and the tolerance threshold values of the MABS system to HM toxicity. Furthermore, the challenges and prospects of the MABS system in treating actual HM wastewater are analyzed and discussed, and suggestions for biochar preparation from the MABS biomass containing HMs are provided. This review provides a reference point for the MABS system treating HM wastewater and the corresponding challenges faced by future engineering practices.

Harnessing the potential of microalgae-bacteria interaction for eco-friendly wastewater treatment: A review on new strategies involving machine learning and artificial intelligence

In the pursuit of effective wastewater treatment and biomass generation, the symbiotic relationship between microalgae and bacteria emerges as a promising avenue. This analysis delves into recent advancements concerning the utilization of microalgae-bacteria consortia for wastewater treatment and biomass production. It examines multiple facets of this symbiosis, encompassing the judicious selection of suitable strains, optimal culture conditions, appropriate media, and operational parameters. Moreover, the exploration extends to contrasting closed and open bioreactor systems for fostering microalgae-bacteria consortia, elucidating the inherent merits and constraints of each methodology. Notably, the untapped potential of co-cultivation with diverse microorganisms, including yeast, fungi, and various microalgae species, to augment biomass output. In this context, artificial intelligence (AI) and machine learning (ML) stand out as transformative catalysts. By addressing intricate challenges in wastewater treatment and microalgae-bacteria symbiosis, AI and ML foster innovative technological solutions. These cutting-edge technologies play a pivotal role in optimizing wastewater treatment processes, enhancing biomass yield, and facilitating real-time monitoring. The synergistic integration of AI and ML instills a novel dimension, propelling the fields towards sustainable solutions. As AI and ML become integral tools in wastewater treatment and symbiotic microorganism cultivation, novel strategies emerge that harness their potential to overcome intricate challenges and revolutionize the domain.

Optimizing aeration intensity to enhance self-flocculation in algal-bacterial symbiosis systems

Effectuating optimal wastewater treatment via algae-bacterial symbiosis (ABS) systems necessitates the precise selection of aeration intensity. This study pioneers an in-depth investigation into the interplay of aeration intensity on the microalgal-bacterial consortia's self-flocculation efficacy and the overall treatment performance within ABS systems. The research provides evidence for a direct association between aeration intensity and biomass proliferation, indicating enhanced pollutant removal efficiency with escalated intensities (1.0 and 1.5 L min-1), though the variance lacks statistical significance. The peak self-flocculation efficacy of the microalgal-bacterial consortium (82.39% at 30 min) was manifested at an aeration intensity of 1.0 L min-1. The meticulous analysis of biomass properties showed the complexity of self-flocculation capacity in the consortium, which involves a dynamic interplay of several pivotal factors, including floc size, zeta potential, and EPS content. In situations where these factors pose conflicting influences, the determining factor emerges as the dominant influencer. In this study, the optimal aeration intensity was identified as 1 L min-1, shedding light on the critical threshold for ABS system operation. This study not only enriches the understanding of microalgal-bacterial wastewater treatment mechanisms but also fosters innovative strategies to enhance the performance of such systems.

Treatment of antibiotic-containing wastewater with self-suspended algae-bacteria symbiotic particles: Removal performance and reciprocal mechanism

Emerging contaminants such as antibiotics in wastewater have posed a challenge on conventional biological treatment processes. Algae-bacteria symbiotic mode could improve the performance of biological treatment processes. Self-suspended algae-bacteria symbiotic particles (ABSPs) were prepared with Chlorella vulgaris and Bacillus subtilis using the sol-gel method and hollow glass microspheres in this study. The removal effect of nitrogen and phosphorus as well as the feedback mechanism of ABSPs under tetracycline stress were investigated through three-cycles wastewater treatment experiments. The antioxidant enzyme activity and phycosphere extracellular polymeric substance (EPS) content were identified as well. The results indicated that the removal rates of NH₄⁺-N, TP, COD, and tetracycline in the ABSPs group finally reached 96.18%, 95.44%, 81.36%, and 74.20%, respectively, which were higher than the single algae group apparently. The phycosphere EPS content increased by 20.41% and algae cell structure maintained integrity in ABSPs group as compared with that in single algae group. This study demonstrates that the self-suspended ABSPs can improve contaminants removal performance and alleviate the antioxidant stress response of algae through algal-bacterial reciprocity mechanism.

Start-up and maintenance of indigenous microalgae-bacteria consortium treating toilet wastewater through partial nitrification and nitrite-type denitrification

Microalgae-bacteria consortium (MBC) provides an alternative to sustainable treatment of human toilet wastewater (TWW) and resource recovery. This study compared the conventional activated sludge system and wastewater indigenous MBC system (IMBC) for nitrogen removal in TWW through the coupled partial nitrification (PN) and nitrite-type denitrification process. PN was firstly established by alternating FA and FNA. Subsequently, the successful PN maintenance with the nitrite accumulation rate ranging between 90.1-95.3% was achieved using two strategies: light irradiation with the appropriate specific light energy density at 0.0188-0.0598 kJ/mg VSS and the timely nitrite-type denitrification with the algae-secreted organics as the carbon source, eventually resulting in the nitrite accumulation rate ranging between 90.1-95.3%. In the IMBC-PN system, bacterial metabolism contributed to 91.5% of nitrogen removal and the rest was through microalgal assimilation. This study offers a sustainable hybrid IMBC-PN process for high NH₄⁺-N strength wastewater treatment (e.g., TWW), which theoretically saves 23.5% aeration and 34.2% carbon source as well as reduces 17.0% sludge production.

Potential interactive effect on biomass and bio-polymeric substances of microalgal-bacterial aerobic granular sludge as a valuable resource for sustainable development

The algal/bacterial biomass and extracellular polymeric substances (EPSs) existing in microalgal-bacterial aerobic granular sludge (MB-AGS) offer a promising bioresource. The current review-based paper presents a systematic overview of the compositions and interactions (gene transfer, signal transduction, and nutrient exchange) of microalgal and bacteria consortia, the role of cooperative or competitive partnerships of MB-AGS in the treatment of wastewater and recovery of resource, and the environmental/operational factors affecting their interactions and EPS production. Moreover, a brief notes is given on the opportunities and major challenges of utilizing the microalgal-bacterial biomass and EPS for phosphorus and polysaccharides chemical recovery, renewable energy (i.e. biodiesel, hydrogen, electricity) production. Overall, this compact review will pave the way for developing MB-AGS future biotechnology.

Interaction of Scenedesmus quadricauda and native bacteria in marine biopharmaceutical wastewater for desirable lipid production and wastewater treatment

Improving knowledge of the alga-bacterium interaction can promote the wastewater treatment. The untreated marine biopharmaceutical wastewater (containing native bacteria) was used directly for culturing microalgae. Unlike previous studies on specific bacteria in algal-bacterial co-culture systems, the effect of native bacteria in wastewater on microalgae growth was investigated in this study. The results showed that the coexistence of native bacteria greatly promoted the microalgae growth, ultimately producing biomass of 0.64 g/L and biomass productivity of 56.18 mg/L·d. Moreover, the lipid accumulation in the algae + bacteria group was 1.31 and 1.13 times higher than those of BG11 and pure algae, respectively, mainly attributed to the fact that bacteria provided a good environment for microalgae growth by using extracellular substances released from microalgae for their own growth, and providing micromolecules of organic matter and other required elements to microalgae. This study would lay the theoretical foundation for improving biopharmaceutical wastewater treatment.

The long overlooked microalgal nitrous oxide emission: Characteristics, mechanisms, and influencing factors in microalgae-based wastewater treatment scenarios

Microalgae-based wastewater treatment is particularly advantageous in simultaneous CO₂ sequestration and nutrients recovery, and has received increasing recognition and attention in the global context of synergistic pollutants and carbon reduction. However, the fact that microalgae themselves can generate the potent greenhouse gas nitrous oxide (N₂O) has been long overlooked, most previous research mainly regarded microalgae as labile organic carbon source or oxygenic approach that interfere bacterial nitrification-denitrification and the concomitant N₂O production. This study, therefore, summarized the amount and rate of N₂O emission in microalgae-based systems, interpreted in-depth the multiple pathways that lead to NO formation as the key precursor of N₂O, and the pathways that transform NO into N₂O. Reduction of nitrite could take place in either the cytoplasm or the mitochondria to form NO by a series of enzymes, while the NO could be enzymatically reduced to N₂O at the chloroplasts or the mitochondria respectively under light and dark conditions. The influences of abiotic factors on microalgal N₂O emission were analyzed, including nitrogen types and concentrations that directly affect the nitrogen transformation routes, illumination and oxygen conditions that regulate the enzymatic activities related to N₂O generation, and other factors that indirectly interfere N₂O emission via NO regulation. The uncertainty of microalgae-based N₂O emission in wastewater treatment scenarios were emphasized, which would be particularly impacted by the complex competition between microalgae and ammonia oxidizing bacteria or nitrite oxidizing bacteria over ammonium or inorganic carbon source. Future studies should put more efforts in improving the compatibility of N₂O emission results expressions, and adopting consistent NO detection methods for N₂O emission prediction. This review will provide much valuable information on the characteristics and mechanisms of microalgal N₂O emission, and arouse more attention to the non-negligible N₂O emission that may impair overall greenhouse gas reduction efficiency in microalgae-based wastewater treatment systems.

Nitrogen recovery from wastewater by microbial assimilation - A review

The increased nitrogen (N) input with low utilization rate in artificial N management has led to massive reactive N (Nr) flows, putting the Earth in a high-risk state. It is essential to recover and recycle Nr during or after Nr removal from wastewater to reduce N input while simultaneously mitigate Nr pollution in addressing the N stress. However, mechanisms for efficient Nr recovery during or after Nr removal remain unclear. Here, the occurrence of N risk and progress in wastewater treatment in recent years as well as challenges of the current technologies for N recovery from wastewater were reviewed. Through analyzing N conversion fluxes in biogeochemical N-cycling networks, microbial N assimilation through photosynthetic and heterotrophic microorganisms was highlighted as promising alternative for synergistic N removal and recovery in wastewater treatment. In addition, the prospects and gaps of Nr recovery from wastewater through microbial assimilation are discussed.

Understanding the influence of free nitrous acid on microalgal-bacterial consortium in wastewater treatment: A critical review

Microalgal-bacterial consortium (MBC) constitutes a sustainable and efficient alternative to the conventional activated sludge process for wastewater treatment (WWT). Recently, integrating the MBC process with nitritation (i.e., shortcut MBC) has been proposed to achieve added benefits of reduced carbon and aeration requirements. In the shortcut MBC system, nitrite or free nitrous acid (FNA) accumulation exerts antimicrobial influences that disrupt the stable process performance. In this review, the formation and interactions that influence the performance of the MBC were firstly summarized. Then the influence of FNA on microalgal and bacterial monocultures and related mechanisms together with the knowledge gaps of FNA influence on the shortcut MBC were highlighted. Other challenges and future perspectives that impact the scale-up of the shortcut MBC for WWT were illustrated. A potential roadmap is proposed on how to maximize the stable operation of the shortcut MBC system for sustainable WWT and high-value biomass production.

Insights into activated sludge/Chlorella consortia under dark condition compared with light condition

Bacteria-algae consortia in the light bring the benefit of O₂ production and CO₂ reduction for wastewater treatment, while the bottleneck for application is how it behaves in the dark. In this study, inoculum ratio and sludge retention time (SRT) affected nutrient removal rather than chemical oxygen demand (COD) removal. Dark conditions (with a sludge/Chlorella inoculum ratio of 1:2 at a SRT of 15 d) achieved comparable performance to those of light conditions, due to bacteria contribution and mechanical aeration. Compared with light conditions, the ratio of Chla/Chlb decreased and Caro/(Chla + Chlb) increased to response oxidative stress. In the dark, algae were associated with Nitrosomonas and Dechloromonas. Flavobacterium disassociated with Chlorella in the dark but associated with Chlorella in the light. Moreover, nitritation genes (amo and Hao) and denitrifying gene (narH) were up-regulated, while P metabolism genes (PPX and PPK) were down-regulated. It is proposed to enrich Nitrosomonas in the night and denitrify polyphosphate accumulating organisms (DPAO) in the daytime to establish short-cut nitrification and denitrifying phosphorus removal in practical applications.