Influence of hydraulic retention time on indigenous microalgae and activated sludge process

Environmental impacts on algal-bacterial-based aquaponics system by different types of carbon source addition: water quality and greenhouse gas emission

Carbon source addition is an important way improving the carbon and nitrogen transformation in aquaculture system; however, its effectiveness of algal-bacterial-based aquaponics (AA) through carbon source addition is still vague. In this study, the influences of organic carbon (OC-AA system) and inorganic carbon (IC-AA system) addition and without carbon source addition (C-AA system) on the operational performance of AA system were investigated. Results showed that 10.1-19.5% increase of algal-bacterial biomass enhanced the purifying effect of ammonia nitrogen in OC-AA system and IC-AA system relative to C-AA system. Moreover, extra electron donor supply in the OC-AA system obtained the lowest NO3--N concentration. However, that was at the cost of aggravated N2O conversion ratio, which increased by more than 2.0-folds than other systems, attributing to 2.9-folds increase of nirS gene abundance. In addition, carbon source addition increased the pH and then decreased the fish biomass production of AA system. The results of this study would provide theoretical supports of carbon source addition on the performance of nutrient transformation and greenhouse gas effect in AA system.

Advances in responses of microalgal-bacterial symbiosis to emerging pollutants in wastewater

Nowadays, emerging pollutants are widely used and exist in wastewater, such as antibiotics, heavy metals, nanoparticle and microplastic. As a green alternative for wastewater treatment, microalgal-bacterial symbiosis has been aware of owning multiple merits of low energy consumption and little greenhouse gas emission. Thus, the responses of microalgal-bacterial symbiosis to emerging pollutants in wastewater treatment have become a hotspot in recent years. In this review paper, the removal performance of microalgal-bacterial symbiosis on organics, nitrogen and phosphorus in wastewater containing emerging pollutants has been summarized. The adaptation mechanisms of microalgal-bacterial symbiosis to emerging pollutants have been analyzed. It is found that antibiotics usually have hormesis effects on microalgal-bacterial symbiosis, and that microalgal-bacterial symbiosis appears to show more capacity to remove tetracycline and sulfamethoxazole, rather than oxytetracycline and enrofloxacin. Generally, microalgal-bacterial symbiosis can adapt to heavy metals at a concentration of less than 1 mg/L, but its capabilities to remove contaminants can be significantly affected at 10 mg/L heavy metals. Further research should focus on the influence of mixed emerging pollutants on microalgal-bacterial symbiosis, and the feasibility of using selected emerging pollutants (e.g., antibiotics) as a carbon source for microalgal-bacterial symbiosis should also be explored. This review is expected to deepen our understandings on emerging pollutants removal from wastewater by microalgal-bacterial symbiosis.

Achieving Discharge Limits in Single-Stage Domestic Wastewater Treatment by Combining Urban Waste Sources and Phototrophic Mixed Cultures

This work shows the potential of a new way of co-treatment of domestic wastewater (DWW) and a liquid stream coming from the thermal hydrolysis of the organic fraction of municipal solid waste (OFMSW) mediated by a mixed culture of purple phototrophic bacteria (PPB) capable of assimilating carbon and nutrients from the medium. The biological system is an open single-step process operated under microaerophilic conditions at an oxidative reduction potential (ORP) < 0 mV with a photoperiod of 12/24 h and fed during the light stage only so the results can be extrapolated to outdoor open pond operations by monitoring the ORP. The effluent mostly complies with the discharge values of the Spanish legislation in COD and p-values (<125 mg/L; <2 mg/L), respectively, and punctually on values in N (<15 mg/L). Applying an HRT of 3 d and a ratio of 100:7 (COD:N), the presence of PPB in the mixed culture surpassed 50% of 16S rRNA gene copies, removing 78% of COD, 53% of N, and 66% of P. Furthermore, by increasing the HRT to 5 d, removal efficiencies of 83% of COD, 65% of N, and 91% of P were achieved. In addition, the reactors were further operated in a membrane bioreactor, thus separating the HRT from the SRT to increase the specific loading rate. Very satisfactory removal efficiencies were achieved by applying an HRT and SRT of 2.3 and 3 d, respectively: 84% of COD, 49% of N, and 93% of P despite the low presence of PPB due to more oxidative conditions, which step-by-step re-colonized the mixed culture until reaching >20% of 16S rRNA gene copies after 49 d of operation. These results open the door to scaling up the process in open photobioreactors capable of treating urban wastewater and municipal solid waste in a single stage and under microaerophilic conditions by controlling the ORP of the system.

A review on algal-bacterial symbiosis system for aquaculture tail water treatment

Aquaculture is one of the fastest growing fields of global food production industry in recent years. To maintain the ecological health of aquaculture water body and the sustainable development of aquaculture industry, the treatment of aquaculture tail water (ATW) is becoming an indispensable task. This paper discussed the demand of environmentally friendly and cost-effective technologies for ATW treatment and the potential of algal-bacterial symbiosis system (ABSS) in ATW treatment. The characteristics of ABSS based technology for ATW treatment were analyzed, such as energy consumption, greenhouse gas emission, environmental adaptability and the possibility of removal or recovery of carbon, nitrogen and phosphorus as resource simultaneously. Based on the principle of ABSS, this paper introduced the key environmental factors that should be paid attention to in the establishment of ABSS, and then summarized the species of algae, bacteria and the proportion of algae and bacteria commonly used in the establishment of ABSS. Finally, the reactor technologies and the relevant research gaps in the establishment of ABSS were reviewed and discussed.

Effect of different mixed light-emitting diode light wavelengths on CO2 absorption from biogas and nutrient removal from biogas slurry by microalgae and fungi induced using strigolactone and endophytic bacteria

In this study, biogas and biogas slurry were simultaneously treated using two symbiotic systems: Chlorella vulgaris-Ganoderma lucidum-S395-2 (endophytic bacteria) and Scenedesmus obliquus-G. lucidum-S395-2. The influence of different mixed illumination (red and blue) intensity ratios on the algal symbionts' extracellular carbonic anhydrase activities was investigated, as well as the rates of microalgal growth and photosynthesis. The treatment performance was simultaneously assessed in terms of the efficiency of organic matter or nutrient removal and the level of CO2 absorption. The results indicated that red-blue light combinations with an intensity ratio of 5:5 were optimal. When comparing the performance of the two symbiotic systems, the C. vulgaris-G. lucidum-S395-2 symbiont co-culture system achieved significantly improved photosynthetic rates, biomass growth, and treatment effects. Under the optimal treatment conditions, the organic matter and nutrient removal rates were 81.06% ± 7.06% for chemical oxygen demand, 82.32% ± 7.18% for total nitrogen, and 82.98% ± 7.26% for total phosphorus. In addition, the rate of CO2 removal from biogas was 63.38% ± 5.35%. PRACTITIONER POINTS: The red and blue light intensity ratio of 5:5 showed the best removal performance. C. vulgaris-G. lucidum-S395-2 system obtained the best photosynthetic performance. The carbonic anhydrase activity had positive effects on CO2 removal performance.

Environmental impacts of antibiotics addition to algal-bacterial-based aquaponic system

Antibiotics usage is a double-edged sword among the production promotion and environmental aggravation of aquaculture system. In this study, the effects of sulfadiazine addition on algal-bacterial-based aquaponic (AA) system were thoroughly investigated. Results showed that sulfadiazine addition increased the nitrogen (N) and carbon (C) recovery of AA system by 1.3 times and 2.9 times, respectively. Meanwhile, the global warming potential was increased by 63% due to aggravated nitrous oxide (N₂O) emission. This was mainly because sulfadiazine increased the abundance of nirS genes and decreased the abundance of nosZ genes, which subsequently led to higher N₂O accumulation. Furthermore, resistance gene (sul-1, sul-2, and intI-1) abundance in the treatment group was an order higher than that of the control group, which would give rise to the environmental risk for agroecological system. KEY POINTS: • Sulfadiazine addition increased NUE at expense of aggravated GHG emissions. • Sulfadiazine disrupted the balance between the abundance of nirS and nosZ genes. • Sulfadiazine addition increased the resistance gene abundance of AA system.

Effect of operational parameters, environmental conditions, and biotic interactions on bacterial communities present in urban wastewater treatment photobioreactors

The effects of water depth, operational and environmental conditions on bacterial communities were analyzed in microalgal-bacterial outdoor photobioreactors treating urban wastewaters from March to August 2014. Three raceway photobioreactors inoculated with Scenedesmus sp. and with different water depths (20, 12, and 5 cm) were used at different dilution rates (0.15, 0.3, 0.4, and 0.5 d^-1). A thin-layer reactor with 2 cm water depth and operated at 0.3 d^-1 was used as a control. The results showed that biomass productivity increased as water depth decreased. The highest biomass productivity was 0.196 gL^-1d^-1, 0.245 gL^-1d^-1, and 0.457 gL^-1d^-1 for 20, 12, and 5 cm depth raceway photobioreactors, respectively. These values were lower than the maximum productivity registered in the control reactor (1.59 gL^-1d^-1). Bacterial communities, analyzed by high-throughput 16S rRNA sequencing, were not affected by water depth. A decrease in community evenness was related to a decrease in nutrient removal. Hetetrotrophs and phototrophs, mainly from the family Rhodobacteraceae, dominated bacterial diversity. The community changed due to increasing temperatures, irradiance, and organic carbon, ammonia, and phosphate contents in the photobioreactor-influent as well as, microalgae inhibition and higher organic carbon in the effluent. The photobioreactors shared a core-biome that contained five clusters of co-occurring microorganisms. The bacteria from the different clusters were taxonomically and ecologically different but functionally redundant. Overall, the drivers of the community changes could be related to abiotic variables and complex biological interactions, likely mediated by microalgae excretion of organic substances and the microorganisms' competence for substrates.

Study of enhanced nitrogen removal efficiency and microbial characteristics of an improved two-stage A/O process

During the cold winter in northern China, the temperature is generally below 8°C, and low water temperature significantly inhibits biological treatment processes, especially the biological denitrification process. To solve this problem, this study proposed an improved two-stage A/O process with built-in submerged biofilm modules. Experimental water was acquired from the Sanbaotun Wastewater Treatment Plant, which is situated in the city of Fushun, Liaoning Province. After one year of experimental research, the improved two-stage A/O process proved to be significantly better than the traditional two-stage A/O process, especially in winter. In the one-year experiment, the average removal rates of COD, TN, and NH₄⁺-N in the improved two-stage A/O process were 85.2%, 77.6%, and 96.9%, respectively. Microbial properties of the process were studied by means of high-throughput sequencing. High-throughput sequencing was conducted on the biofilm of the improved two-stage A/O terminal aerobic tank and the activated sludge of the conventional two-stage A/O aerobic tank. The result showed that the microbial diversity and abundance of the biofilms were considerably higher than those of the activated sludge during stable operation in winter. Under low-temperature conditions, the main denitrifying bacteria of the improved two-stage A/O process was Terrimonas, belonging to the sphingolipid class of Bacteroides, and the main genus of nitrifying bacteria was Nitrospira, belonging to the nitrite oxidizing bacteria.

Convergent community structure of algal-bacterial consortia and its effects on advanced wastewater treatment and biomass production

Microalgal-bacterial consortium is an effective way to meet increasingly stringent standards in wastewater treatment. However, the mechanism of wastewater removal effect has not been properly explained in community structure by phycosphere. And little is known about that the concept of macroecology was introduced into phycosphere to explain the phenomenon. In the study, the algal-bacterial consortia with different ratios of algae and sludge were cultured in same aerobic wastewater within 48 h in photobioreactors (PSBRs). Community structure at start and end was texted by metagenomic analysis. Bray-Curtis similarities analysis based on microbial community showed that there was obvious convergent succession in all consortia, which is well known as "convergence" in macroecology. The result showed that Bray-Curtis similarities at End (overall above 0.88) were higher than these at Start (almost less than 0.66). In terms of community structure, the consortium with 5:1 ratio at Start are the more similar with the consortia at End by which the maximum removal of total dissolved nitrogen (TDN, 73.69%), total dissolved phosphorus (TDP, 94.40%) and NH3-N (93.26%) in wastewater treatment process and biomass production (98.2%) higher than other consortia, according with climax community in macroecology with the highest resource utilization than other communities. Therefore, the macroecology can be introduced into phycosphere to explain the consortium for advanced wastewater treatment and optimization community structure. And the study revealed a novel insight into treatment effect and community structure of algal-bacterial consortia for advanced wastewater treatment, a new idea for to shortening the culture time of consortium and optimize predicting their ecological community structure and predicting ecological community.

Solids Residence Time Impacts Carbon Dynamics and Bioenergy Feedstock Potential in Phototrophic Wastewater Treatment Systems

The use of wastewater-grown microalgae has the potential to reduce the cost of algae-derived biofuels while simultaneously advancing nutrient recovery at water resource recovery facilities (WRRFs). However, a significant barrier has been the low yield and high protein content of phototrophic biomass. Here, we examine the use of solids residence time (SRT) as a selective pressure in driving biochemical composition, yield, biofuel production, and WRRF nutrient management cost. We cultivated mixed phototrophic communities in controlled, laboratory-scale photobioreactors on the local WRRF secondary effluent to link SRT with biochemical composition and techno-economic analysis to yield insights into biomass composition and downstream processing effects on minimum fuel selling price. SRT significantly impacted biochemical composition, with total and dynamic carbohydrates the highest at low SRT (total carbohydrates being 0.60 and 0.32 mg-carbohydrate·mg-protein^-1 at SRT 5 and 15 days, respectively). However, there were distinct differences between extant, steady-state performance and intrinsic potential, and longer SRT communities were able to accumulate significant fractions (51% on an ash-free dry weight basis, AFDW %) of carbohydrate reserves under nutrient starvation. Overall, hydrothermal liquefaction (HTL) was found to be more suitable than lipid extraction for hydrotreating (LEH) and combined algal processing (CAP) for conversion of biomass to fuels, but LEH and CAP became more competitive when intrinsic carbon storage potential was realized. The results suggest that the use of algae for nutrient recovery could reduce the nutrient management cost at WRRFs through revenue from algal biofuels, with HTL resulting in a net revenue.

Bio-products from algae-based biorefinery on wastewater: A review

Increasing resource demand, predicted fossil resources shortage in the near future, and environmental concerns due to the production of greenhouse gas carbon dioxide have motivated the search for alternative 'circular' pathways. Among many options, microalgae have been recently 'revised' as one of the most promising due to their high growth rate (with low land use and without competing with food crops), high tolerance to nutrients and salts stresses and their variability in biochemical composition, in so allowing the supply of a plethora of possible bio-based products such as animal feeds, chemicals and biofuels. The recent raising popularity of Circular Bio-Economy (CBE) further prompted investment in microalgae, especially in combination with wastewater treatment, under the twofold aim of allowing the production of a wide range of bio-based products while bioremediating wastewater. With the aim of discussing the potential bio-products that may be gained from microalgae grown on urban wastewater, this paper presents an overview on microalgae production with particular emphasis on the main microalgae species suitable for growth on wastewater and the obtainable bio-based products from them. By selecting and reviewing 76 articles published in Scopus between 1992 and 2020, a number of interesting aspects, including the selection of algal species suitable for growing on urban wastewater, wastewater pretreatment and algal-bacterial cooperation, were carefully reviewed and discussed in this work. In this review, particular emphasis is placed on understanding of the main mechanisms driving formation of microalgal products (such as biofuels, biogas, etc.) and how they are affected by different environmental factors in selected species. Lastly, the quantitative information gathered from the articles were used to estimate the potential benefits gained from microalgae grown on urban wastewater in Campania Region, a region sometimes criticized for poor wastewater management.

Induction of vitamin B12 to purify biogas slurry and upgrade biogas using co-culture of microalgae and fungi

Different gradient concentrations of vitamin B12 (0, 10, 100, 1,000 ng L^-1 ) were used in the symbiosis system (Chlorella vulgaris-Ganoderma lucidum or Chlorella vulgaris-Pleurotus ostreatus) to assess their effect on simultaneous purification of biogas and removal of nutrients in biogas slurry using co-culture of microalgae and fungi. When B12 was added to the symbiosis system, biomass growth, intracellular carbonic anhydrase activity (CA), chlorophyll a content (CHL-a), photosynthetic characteristics of the two cultivation system, and removal efficiency of nutrients in biogas slurry and CO₂ in biogas were significantly higher than those in the control group. The optimal concentration of B12 was determined to be 100 ng L^-1 considering the removal efficiency of nutrients and CO₂ . Maximum mean chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), and CO₂ removal efficiencies were 75.98 ± 6.26%, 78.46 ± 6.21%, 80.21 ± 6.83% and 61.08 ± 5.21% in Chlorella vulgaris-Ganoderma lucidum, respectively. This study showed the potential of microalgae and fungi symbiosis system with B12 addition for nutrient removal and biogas upgrading. PRACTITIONER POINTS: Vitamin B12 had positive effects on algal-fungal pellets growth. The optimal vitamin B12 concentration was 100 ng L^-1 . The highest CO₂ remove rate was 61.08% by G. lucidum/C. vulgaris pellets. Vitamin B12 significantly improved photosynthetic performance of pellets.

Role of extracellular polymeric substances on nutrients storage and transfer in algal-bacteria symbiosis sludge system treating wastewater

This study reported the role and significance of extracellular polymeric substances (EPSs) on nutrients storage and transfer in an algal-bacteria symbiosis sludge (ABSS) system for wastewater treatment, and the novel algae-based sequencing batch suspended biofilm reactor (A-SBSBR, Ra) was selected as model of ABSS system. Results showed that compared to conventional SBSBR, the EPS of Ra performed better storage for NO₂^--N, NO₃^--N, total phosphorus and PO₄^3- -P, with increase ratios of 43.7%, 36.0%, 34.1% and 14.7% in sludge phase and 174.0%, 147.4%, 150.4% and 122.0% in biofilm phase, respectively. The analysis of mechanisms demonstrated that microalgae active transport and uptake for divalent cations could enhance their local concentrations around ABS flocs and partially neutralized negative charge of EPSs, and more anions related to nutrients were absorbed in EPSs. Moreover, O₂ produced by microalgae photosynthesis enhanced bacteria activity and improved the production of EPSs in both sludge and biofilm phases.

Effects of substrate stress and light intensity on enhanced biological phosphorus removal in a photo-activated sludge system

Photo-activated sludge (PAS) systems are an emerging wastewater treatment technology where microalgae provide oxygen to bacteria without the need for external aeration. There is limited knowledge on the optimal conditions for enhanced biological phosphorus removal (EBPR) in systems containing a mixture of polyphosphate accumulating organisms (PAOs) and microalgae. This research aimed to study the effects of substrate composition and light intensity on the performance of a laboratory-scale EBPR-PAS system. Initially, a model-based design was developed to study the effect of organic carbon (COD), inorganic carbon (HCO₃) and ammonium-nitrogen (NH₄-N) in nitrification deprived conditions on phosphorus (P) removal. Based on the mathematical model, two different synthetic wastewater compositions (COD:HCO₃:NH₄-N: 10:20:1 and 10:10:4) were examined at a light intensity of 350 µmol m^-2 sec^-1. Add to this, the performance of the system was also investigated at light intensities: 87.5, 175, and 262.5 µmol m^-2 sec^-1 for short terms. Results showed that wastewater having a high level of HCO₃ and low level of NH₄-N (ratio of 10:20:1) favored only microalgal growth, and had poor P removal due to a shortage of NH₄-N for PAOs growth. However, lowering the HCO₃ level and increasing the NH₄-N level (ratio of 10:10:4) balanced PAOs and microalgae symbiosis, and had a positive influence on P removal. Under this mode of operation, the system was able to operate without external aeration and achieved a net P removal of 10.33 ±1.45 mg L^-1 at an influent COD of 100 mg L^-1. No significant variation was observed in the reactor performance for different light intensities, indicating the EBPR-PAS system can be operated at low light intensities with a positive influence on P removal.

Real Wastewater Treatment Using a Moving Bed and Wastewater-Borne Algal–Bacterial Consortia with a Short Hydraulic Retention Time

Algal–bacterial consortium is a promising technology, combined with wastewater treatment plants, because algae produce molecular oxygen for nitrification and organic removal and reduce carbon dioxide emissions. However, algal–bacterial consortia based on suspended growth require a relatively long hydraulic retention time (HRT) of 4 d to 6 d for removal of organic matter and nutrients. For the algal–bacterial consortia in a photobioreactor (PBR) containing a moving bed, the organic matter and nutrient removal and the community structure of algal–bacterial consortia were investigated to determine the performance under a relatively short HRT of 2.5 d. Moving media containing algal–bacterial consortia enhanced the photosynthetic oxygen concentration (0.2 mg dissolved oxygen (DO)·L−1 to 5.9 mg DO·L−1), biochemical oxygen demand removal (88.0% to 97.2%), ammoniacal nitrogen removal (33.8% to 95.3%), total nitrogen removal (61.6% to 87.7%), total phosphate removal (66.4% to 88.7%), algal growth (149.3 mg algae·L−1 to 285.4 mg algae·L−1), and settleability (algae removal efficiency of 20.6% to 71.2%) compared with those of a PBR without moving media (SPBR). Although biomass uptake was the main mechanism for nutrient removal in the SPBR, both biomass uptake and denitrification were the main mechanisms in the PBR with moving media (MBPBR). The bacterial community also changed under the moving media condition. This study shows that moving media might be an essential parameter for PBRs with a short HRT to enhance nutrient removal and settleability.

Optimal influent N-to-P ratio for stable microalgal cultivation in water treatment and nutrient recovery

Species specific nitrogen-to-phosphorus molar ratio (NPR) has been suggested for green microalgae. Algae can store nitrogen and phosphorus, suggesting that the optimum feed concentration dynamically changes as function of the nutrient storage. We assessed the effect of varying influent NPR on microalgal cultivation in terms of microbial community stability, effluent quality and biokinetics. Mixed green microalgae (Chlorella sorokiniana and Scenedesmus sp.) and a monoculture of Chlorella sp. were cultivated in continuous laboratory-scale reactors treating used water. An innovative image analysis tool, developed in this study, was used to track microbial community changes. Diatoms proliferated as influent NPR decreased, and were outcompeted once cultivation conditions were restored to the optimal NPR range. Low NPR operation resulted in decrease in phosphorus removal, biomass concentration and effluent nitrogen concentration. ASM-A kinetic model simulation results agreed well with operational data in the absence of diatoms. The failure to predict operational data in the presence of diatoms suggest differences in microbial activity that can significantly influence nutrient recovery in photobioreactors (PBR). No contamination occurred during Chlorella sp. monoculture cultivation with varying NPRs. Low NPR operation resulted in decrease in biomass concentration, effluent nitrogen concentration and nitrogen quota. The ASM-A model was calibrated for the monoculture and the simulations could predict the experimental data in continuous operation using a single parameter subset, suggesting stable biokinetics under the different NPR conditions. Results show that controlling the influent NPR is effective to maintain the algal community composition in PBR, thereby ensuring effective nutrients uptake.

Integration of Algae to Improve Nitrogenous Waste Management in Recirculating Aquaculture Systems: A Review

This review investigates the performance and the feasibility of the integration of an algal reactor in recirculating aquaculture systems (RAS). The number of studies related to this topic is limited, despite the apparent benefit of algae that can assimilate part of the inorganic waste in RAS. We identified two major challenges related to algal integration in RAS: first, the practical feasibility for improving nitrogen removal performance by algae in RAS; second, the economic feasibility of integrating an algal reactor in RAS. The main factors that determine high algal nitrogen removal rates are light and hydraulic retention time (HRT). Besides these factors, nitrogen-loading rates and RAS configuration could be important to ensure algal performance in nitrogen removal. Since nitrogen removal rate by algae is determined by HRT, this will affect the size (area or volume) of the algal reactor due to the time required for nutrient uptake by algae and large surface area needed to capture enough light. Constraints related to design, space, light capture, and reactor management could incur additional cost for aquaculture production. However, the increased purification of RAS wastewater could reduce the cost of water discharge in places where this is subject to levees. We believe that an improved understanding of how to manage the algal reactor and technological advancement of culturing algae, such as improved algal reactor design and low-cost artificial light, will increase the practical and economic feasibility of algal integration in RAS, thus improving the potential of mass cultivation of algae in RAS.

Autotrophic granulation of hydrogen consumer denitrifiers and microalgae for nitrate removal from drinking water resources at different hydraulic retention times

To avoid hydrogen injection and to enhance the settleability of microbial biomass in biological treatment of nitrate-contaminated drinking water resources, a new method based on granulation of a mixture of hydrogen consumer denitrifiers (HCD) and microalgae is introduced. Decreasing hydraulic retention time (HRT) was applied as the selection pressure in an up-flow photobioreactor to increase the speed of granulation and nitrate removal under autotrophic condition during a 50-day operation. Formation of granules occurred at three phases including granule nucleation, growth of granule, and mature granule, with decreasing the values of ζ-potential from -19 mV to -4 mV. Enhancement of microbial attachment within granule formation could reduce the presence of total suspended solids in the effluent. Developed granules of HCD and microalgae could settle down with velocity of 40 ± 0.6 m/h when reaching the average size of 1.2 mm at day 40. Complete NO₃^--N removal from drinking water was achieved from the initial stage of granulation until the end of operation at all HRTs of 3 days-5 h. The clear treated water was obtained at the growth phase when the chemical oxygen demand and phosphate were undetectable. Therefore, the application of HCD-microalgae granule is a promising way for nitrate removal from water.

The effect of hydraulic retention time on microalgae-based activated sludge process for Wupa sewage treatment plant, Nigeria

This study evaluated the efficiency of microalgae activated sludge (MAAS) for wastewater treatment by investigating the influence of hydraulic retention time (HRT) on MAAS using batch regime pilot scale photobioreactors at Wupa Wastewater Treatment Plant. The outcome of the study showed that MAAS has a comparably high wastewater treatment performance in comparison with the current Wupa Wastewater Treatment Plant (WWTPA) activated sludge (AS) method and is capable of treating wastewater to the defined Nigerian effluent standards. It was further revealed that treatment performance for most parameters were optimal from HRT₃ (6-day hydraulic retention time). Precisely, total nitrogen (TN), total phosphorus (TP), and BOD₅ had highest removal efficiency at HRT₃ with average total removal of 81.36%, 91.77% and 87.04% respectively. Correspondingly, the average percentage DO increment peaked at HRT₃ with a value of 269.7%. In addition, there was a general deterioration of SVI with increasing HRT, particularly after HRT₂ (4-day HRT). Notably, SVI₃₀ was significantly good at HRT₁ and HRT₂ with SVI values of 48.6 ml/g and 105.52 ml/g; however, from HRT₃ down to HRT₉, the SVI₃₀ became remarkably increases greater than that of HRT₁ and HRT₂, with values ranging from 685.61 to 1832.46 ml/g, which indicates a badly bulking sludge. The MAAS system is recommended as an attractive alternative to the conventional AS wastewater treatment in Nigeria and by extension West African subregion.

Cultivation of Chlorella vulgaris on unsterilized dairy-derived liquid digestate for simultaneous biofuels feedstock production and pollutant removal

In order to assess viability of microalgae cultivation using unsterilized dairy-derived liquid digestate (DLD) for simultaneous biofuels feedstock production and contaminant removal, four DLD concentrations (25%, 50%, 75% and 100%) were used to grow Chlorella vulgaris in batch photobioreactors (PBRs). The 25% DLD was an ideal alternative medium in that high growth rate (0.69 d^-1), high lipid productivity (112.9 mg L^-1 d^-1) as well as high nutrient removal were attained. The high DLD concentration caused inhibition of microalgal growth, where COD was more inhibitive than ammonium. The presence of bacteria did not influence microalgae production because of limited growth. Microalgal growth reduced the richness and diversity of bacterial community. Furthermore, the species of Bacteroidetes, Candidatus Saccharibacteria, and Chlamydiae rather than Proteobacteria benefited microalgal-bacterial symbiosis. These findings contribute to better application of microalgal-bacterial system for large-scale microalgae cultivation as well as environmental sustainability.

Dataset to assess the shadow effect of an outdoor microalgae culture

This data in brief (DIB) article is related to a Research article [1].

Microalgae biomass absorb the light photons that are supplied to the culture, reducing the light availability in the inner parts of the photobioreactors. This is known as self-shading or shadow effect. This effect has been widely studied in lab conditions, but information about self-shading in outdoor photobioreactors is scarce.

How this shadow effect affects the light availability in an outdoor photobioreactor was evaluated. In addition, advantages and disadvantages of different artificial light sources which can overcome light limitation are described.

Nutrient removal and microalgal biomass production from different anaerobic digestion effluents with Chlorella species

Potential of microalgal cultivation as an alternative approach to the treatment of anaerobic digestion (AD) effluents was examined using two representative Chlorella species, Chlorella vulgaris (CV) and Chlorella protothecoides (CP). Both species effectively removed NH₄⁺-N from the AD effluents from four digesters treating different wastes under different operating conditions. In all experimental cultures on the AD effluents, NH₄⁺-N (initial concentration, 40 mg/L) was completely removed within 10 days without residual NO₃^--N or NO₂^--N in batch mode. Compared to CP, CV showed greater biomass and lipid yields (advantageous for biodiesel production), regardless of the media used. Prolonged nitrogen starvation significantly increased the lipid accumulation in all cultures on the AD effluents, and the effect was more pronounced in the CV than in the CP cultures. On the other hand, compared to CV, CP showed significantly faster settling (advantageous for biomass harvesting) in all media. Our results suggest that the Chlorella cultivation on AD effluents under non-sterile, mixed-culture conditions may provide a viable way to manage and valorize the problematic effluents. Diverse bacteria derived from the AD effluents co-existed and presumably interacted with the Chlorella species in the cultures.

Impact of solids residence time on community structure and nutrient dynamics of mixed phototrophic wastewater treatment systems

Suspended growth, mixed community phototrophic wastewater treatment systems (including high-rate algal ponds and photobioreactors) have the potential to achieve biological nitrogen and phosphorus recovery with effluent nutrient concentrations below the current limit-of-technology. In order to achieve reliable and predictive performance, it is necessary to establish a thorough understanding of how design and operational decisions influence the complex community structure governing nutrient recovery in these systems. Solids residence time (SRT), a critical operational parameter governing growth rate, was leveraged as a selective pressure to shape microbial community structure in laboratory-scale photobioreactors fed secondary effluent from a local wastewater treatment plant. In order to decouple the effects of SRT and hydraulic retention time (HRT), nutrient loading was fixed across all experimental conditions and the effect of changing SRT on microbial community structure, diversity, and stability, as well as its impact on nutrient recovery, was characterized. Reactors were operated at distinct SRTs (5, 10, and 15 days) with diurnal lighting over long-term operation (>6 SRTs), and in-depth examination of the eukaryotic and bacterial community structure was performed using amplicon-based sequencing of the 18S and 16S rRNA genes, respectively. In order to better represent the microalgal community structure, this study leveraged improved 18S rRNA gene primers that have been shown to provide a more accurate representation of the wastewater process-relevant algal community members. Long-term operation resulted in distinct eukaryotic communities across SRTs, independent of the relative abundance of Operational Taxonomic Units (OTUs) in the inoculum. The longest SRT (15 days, SRT 15) resulted in a more stable algal community along with stable bacterial nitrification, while the shortest SRT (5 days, SRT 5) resulted in a less stable, more dynamic community. Although SRT was not strongly associated with overall bacterial diversity, the eukaryotic community of SRT 15 was significantly less diverse and less even than SRT 5, with a few dominant OTUs making up a majority of the eukaryotic community structure in the former. Overall, although longer SRTs promote stable bacterial nitrification, short SRTs promote higher eukaryotic diversity, increased functional stability, and better total N removal via biomass assimilation. These results indicate that SRT may be a key factor in not only controlling microalgal community membership, but community diversity and functional stability as well. Ultimately, the efficacy and reliability of NH₄⁺ removal may be in tension with TN removal in mixed phototrophic systems given that lower SRTs may achieve better total N removal (via biomass assimilation) through increased eukaryotic diversity, biomass productivity, and functional stability.

Treatment of low C/N ratio wastewater and biomass production using co-culture of Chlorella vulgaris and activated sludge in a batch photobioreactor

The aim of this work was to study the performance of pollutants removal and biomass production by co-culture of Chlorella vulgaris and activated sludge in a batch photobioreactor (PBR), compared with their single system to treat a low C/N ratio (COD/N = 4.3) wastewater. The co-culture system surpassed activated sludge system in terms of nutrients removal and outperformed microalgae alone system in regard to COD removal. Biomass productivity of the co-culture system was 343.3 mg L^-1 d^-1, and the harvested biomass could be developed as biofuels, animal feeds or soil conditioners due to the improved calorific value and cellular composition compared with activated sludge. The low C/N ratio wastewater enabled bacteria to maintain a relatively low level, hence in favor of microalgae enrichment and nutrient recovery.

Wastewater treatment and microbial community dynamics in a sequencing batch reactor operating under photosynthetic aeration

A sequencing batch bioreactor (SBR) treating municipal wastewater was photosynthetically aerated using microalgae cultivated in a photobioreactor (PBR). Symbiotic interactions and CO₂/O₂ exchange were established between activated sludge in the SBR and microalgae in the PBR through hydrophobic hollow fiber membranes. Photosynthetic aeration enhanced COD removal in the SBR from 52.2% (without external aeration) to 90.3%, whereas N-NH₄⁺ and P-PO₄^3- removal increased by 63.5% and 90.4%, respectively. The SBR performance under photosynthetic aeration was comparable to that under mechanical aeration. However, no nitrification was observed in the SBR, indicating oxygen limitation and poor growth condition for nitrifiers. In the PBR, there was a rapid increase in biomass concentration and it stabilized at 3.0 g/L after 22 days of operation. High nitrogen demand in the PBR indicated the steady flow of inorganic carbon from the SBR through the membranes. Prolonged oxygen limitation and massive sludge attachment on the membranes resulted in low suspended sludge concentration in the SBR. Microbial community analysis indicated gradual enrichment of facultative and strictly anaerobic microorganisms in the SBR. These results highlight the potential of microalgae in lowering the cost of wastewater aeration and underline the challenges in sustaining symbiotic gas exchange during long-term.

Light intensity affects the mixotrophic carbon exploitation in Chlorella protothecoides: consequences on microalgae-bacteria based wastewater treatment

Microalgal-bacteria consortia application on wastewater treatment has been widely studied, but a deeper comprehension of consortium interactions is still lacking. In particular, mixotrophic exploitation of organic compounds by microalgae affects gas (CO₂ and O₂) exchange between microalgae and bacteria, but it is not clear how environmental conditions may regulate algal metabolism. Using a respirometric-based protocol, we evaluated the combined effect of organic carbon and light intensity on oxygen production and consumption by C. protothecoides, and found that the chemical oxygen demand (COD) was not consumed when incident light increased. Batch experiments under different incident lights, with C. protothecoides alone and in consortium with activated sludge bacteria, confirmed the results obtained by respirometry. Continuous system experiments testing the combined effects of light intensity and residence time confirmed that, under limiting light, mixotrophy is preferred by C. protothecoides, and the nutrient (COD, N, P) removal capability of the consortium is enhanced.

Effects of hydraulic retention time on the performance of algal-bacterial-based aquaponics (AA): focusing on nitrogen and oxygen distribution

The effects of hydraulic retention time (HRT) on the performance of algal-bacterial-based aquaponics (AA) were investigated in this study. Both the highest fish growth and algal biomass increase were observed in the AA system at 2-day HRT, resulting in the highest nitrogen utilization efficiency (NUE) (39.28%) in this microcosm. On the contrary, ammonia oxidation bacteria (AOB) abundance at 4-day HRT was approximately ten times higher than that at 2-day HRT, since longer HRT would benefit bacterial growth. The ¹⁵N labeling study showed that microalgae assimilation was the main pathway of NH₄⁺ removal in the AA system, and oxygen produced by microalgae could in situ support complete nitrification, thus leading to much lower NH₄⁺ concentrations at 2-day HRT. Accordingly, better water quality was achieved at 2-day HRT. Considering all the factors, HRT of 2-day was considered to be optimal for the AA system.

Advances in algal-prokaryotic wastewater treatment: A review of nitrogen transformations, reactor configurations and molecular tools

The synergistic activity of algae and prokaryotic microorganisms can be used to improve the efficiency of biological wastewater treatment, particularly with regards to nitrogen removal. For example, algae can provide oxygen through photosynthesis needed for aerobic degradation of organic carbon and nitrification and harvested algal-prokaryotic biomass can be used to produce high value chemicals or biogas. Algal-prokaryotic consortia have been used to treat wastewater in different types of reactors, including waste stabilization ponds, high rate algal ponds and closed photobioreactors. This review addresses the current literature and identifies research gaps related to the following topics: 1) the complex interactions between algae and prokaryotes in wastewater treatment; 2) advances in bioreactor technologies that can achieve high nitrogen removal efficiencies in small reactor volumes, such as algal-prokaryotic biofilm reactors and enhanced algal-prokaryotic treatment systems (EAPS); 3) molecular tools that have expanded our understanding of the activities of algal and prokaryotic communities in wastewater treatment processes.

Assessment of membrane photobioreactor (MPBR) performance parameters and operating conditions

Membrane photobioreactor (MPBR) technology is an emerging algae-based wastewater treatment system. Given the limitations due to the general use of conventional analytical approaches in previous research, this study aims to provide a more comprehensive assessment of MPBR performance through advanced characterisation techniques. New performance parameters are also proposed, encompassing five important aspects of MPBR system efficiency (i.e. biomass concentration, composition, production, nutrient uptake and harvesting potential). Under initial standard operating conditions, performance parameters, such as cell count/MLSS ratio, cell viability, proportion of bacteria and biomass yield coefficient, were found to offer new insights into the operation of MPBR. These parameters were then used, for the first time, to systematically investigate MPBRs operated under different hydraulic retention times (HRTs) and solids retention times (SRTs). Applying shorter HRT and SRT was observed to increase cell viability and productivity (up to 0.25 × 10⁷ cells/mL·d), as anticipated due to the higher nutrient loading. It was noted that the faster growing algal cells featured lower requirement for nutrients. On the other hand, extending HRT and SRT resulted in a more heterogeneous culture (lower cell count/MLSS ratio and higher proportion of bacteria), achieving a higher degree of autoflocculation and greater NO₃-N and PO₄-P removals of up to 79% and 78% respectively. The results demonstrate the trade-off between applying different HRTs and SRTs and the importance of fully characterising system performance to critically assess the advantages and limitations of chosen operating conditions.

Enhanced nitrogen and phosphorus removal from domestic wastewater via algae-assisted sequencing batch biofilm reactor

This study proposed a potential strategy for enhancement of nutrients removal from domestic wastewater by adding algae to sequencing batch biofilm reactor (SBBR) to form a novel algal-bacterial symbiosis (ABS) system. Results indicated that the algae-assisted SBBR increased the total nitrogen and phosphorus removal efficiencies from 38.5% to 65.8%, and from 31.9% to 89.3%, respectively. The carriers fixed at the top of the reactor were favorable for both formation of ABS system and algae enrichment. The chlorophyll-a increased to 3.59 mg/g at stable stage, which was 4.07 times higher than that in suspension. Moreover, the bio-carrier replacement and sludge discharge were independent, indicating that the sludge and algae retention time could be separated. The mechanisms analysis suggested that the enhanced nitrogen and phosphorus mainly attributed to the enrichment of both algae biomass and total biomass in biofilm. This study highlights the significance of developing ABS system for wastewater treatment.

Improving nitrogen utilization efficiency of aquaponics by introducing algal-bacterial consortia

Aquaponics is a promising technology combining aquaculture with hydroponics. In this study, algal-bacterial consortia were introduced into aquaponics, i.e., algal-bacterial based aquaponics (AA), to improve the nitrogen utilization efficiency (NUE) of aquaponics. The results showed that the NUE of AA was 13.79% higher than that of media-based aquaponics (MA). In addition, higher NO₃^- removal by microalgae assimilation led to better water quality in AA, which made up for the deficiencies of poor aquaponic management of nitrate. As a result of lower NO₃^- concentrations and dramatically higher dissolved oxygen (DO) concentrations caused by microalgae photosynthesis in the photobioreactor, the N₂O emission of AA was 89.89% lower than that of MA, although nosZ gene abundance in MA's hydroponic bed was approximately 30 times over that in AA. Considering the factors mentioned above, AA would improve the sustainability of aquaponics and have a good application foreground.

Comparative evaluation of piggery wastewater treatment in algal-bacterial photobioreactors under indoor and outdoor conditions

This work evaluated the performance of four open algal-bacterial photobioreactors operated at ≈26days of hydraulic retention time during the treatment of 10 (×10) and 20 (×20) times diluted piggery wastewater (PWW) under indoor (I) and outdoor (O) conditions for four months. The removal efficiencies (REs) of organic matter, nutrients and zinc from PWW, along with the dynamics of biomass concentration and structure of algal-bacterial population were assessed. The highest TOC-RE, TP-RE and Zn-RE (94±1%, 100% and 83±2%, respectively) were achieved indoors in ×10 PWW, while the highest TN-RE (72±8%) was recorded outdoors in ×10 PWW. Chlorella vulgaris was the dominant species regardless of the ambient conditions and PWW dilution. Finally, DGGE-sequencing of the bacterial community revealed the occurrence of four phyla, Proteobacteria being the dominant phylum with 15 out of the 23 most intense bands.

Municipal wastewater treatment via co-immobilized microalgal-bacterial symbiosis: Microorganism growth and nutrients removal

A symbiotic microalgal-bacterial system may be an optional technology for wastewater treatment. In this study, co-immobilized of a bacterium isolated from a municipal wastewater treatment plant (Pseudomonas putida) and a microalgae Chlorella vulgaris was used in the study of cell growth and nutrient removal during wastewater treatment under batch and continuous culture conditions. Under batch culture conditions, co-immobilization treatment significantly increased the cell density of C. vulgaris and P. putida compared with other treatments. The co-immobilized treatment also showed higher removal of ammonium, phosphate and COD than any single treatment, indicating that the nutrient uptake capability of C. vulgaris and P. Putida was mutually enhanced mutually. When tested in continuous mode, the treatment with a hydraulic retention time of 24h at the organic load rate of 1159.2mgCODL^-1d^-1 was most appropriate for wastewater treatment.

Simultaneously upgrading biogas and purifying biogas slurry using cocultivation of Chlorella vulgaris and three different fungi under various mixed light wavelength and photoperiods

In order to purify biogas slurry and biogas simultaneously, three different fungi, Pleurotus geesteranus (P. geesteranus), Ganoderma lucidum (G. lucidum), and Pleurotus ostreatus (P. ostreatus) were pelletized with Chlorella vulgaris (C. vulgaris). The results showed that the optimal light wavelength ratio for red:blue was 5:5 for these three different fungi-assisted C. vulgaris, resulting in higher specific growth rate as well as nutrient and CO₂ removal efficiency compared with other ratios. G. lucidum/C. vulgaris was screened as the best fungi-mialgae for biogas slurry purification and biogas upgrading with light/dark ratio of 14h:10h, which was also confirmed by the economic efficiency analysis of the energy consumptions. These results will provide a theoretical foundation for large-scale biogas slurry purifying and biogas upgrading using microalgae.

Enhanced microalgal biomass and lipid production from a consortium of indigenous microalgae and bacteria present in municipal wastewater under gradually mixotrophic culture conditions

The goal of this study was to investigate the influences of gradually mixotrophic culture conditions on microalgal biomass and lipid production by a consortium of indigenous microalgae and bacteria present in raw municipal wastewater. Lab-scale photobioreactors containing the consortium were operated in repeated batch mode. Initial cultivation (phase I) was performed using only the municipal wastewater, then 10% and 25% of the reactor volumes were replaced with the effluent from a sewage sludge fermentation system producing volatile fatty acids (SSFV) at the beginnings of phase II and phase III, respectively. The highest biomass productivity (117.1±2.7mg/L/d) was attained during phase II, but the lipid productivity (17.2±0.2mg/L/d) was attained during phase III. The increase in the effluent from the SSFV influenced microalgal diversity with a preference for Chlorella sp., but bacterial diversity increased significantly during phase III.

Enhanced sludge anaerobic fermentation using microwave pretreatment combined with biosurfactant alkyl polyglycoside

This study investigated the effect of microwave combined with biosurfactant alkyl polyglucose (APG) on sludge anaerobic fermentation in a lab-scale sequencing batch reactor.