A vertical-flow constructed wetland-microalgal membrane photobioreactor integrated system for treating high-pollution-load marine aquaculture wastewater: A lab-scale study.

Individual biological water treatment techniques often prove ineffective in removing accumulated high concentrations of nitrogen and phosphorus in the late stages of biofloc aquaculture. To address this issue, we integrated a previously developed autotrophic denitrification and nitrification integrated constructed wetland (ADNI-CW) with a microalgal membrane photobioreactor (MPBR). Under high nitrogen and phosphorus pollution loads in the influent, the standalone ADNI-CW system achieved removal rates of only 24.17 % ± 2.82 % for total nitrogen (TN) and 25.30 % ± 2.59 % for total phosphorus (TP). The optimal conditions for TN and TP degradation and microalgal biomass production in the Chlorella MPBR, determined using response surface methodology, were an inoculum OD680 of 0.394, light intensity of 161.583 µmol/m2/s, and photoperiod of 16.302 h light:7.698 h dark. Under the optimal operating conditions, the integrated ADNI-CW-MPBR system achieved remarkable TN and TP removal rates of 92.63 % ± 2.8 % and 77.46 % ± 8.41 %, respectively, and a substantial microalgal biomass yield of 54.58 ± 6.8 mg/L/day. This accomplishment signifies the successful achievement of efficient nitrogen and phosphorus removal from high-pollution-load marine aquaculture wastewater along with the acquisition of valuable microalgal biomass. A preliminary investigation of the microbial community composition and algal-bacterial interactions in different operational stages of the MPBR system revealed that unclassified_d__Bacteria, Chlorophyta, and Planctomycetes were predominant phyla. The collaborative relationships between bacteria and Chlorella surpassed competition, ensuring highly efficient nitrogen and phosphorus removal in the MPBR system. This study laid the foundation for the green and sustainable development of the aquaculture industry.

Nitrogen and phosphorus removal performance of sequential batch operation for algal cultivation through suspended-solid phase photobioreactor.

Nitrogen (N) and phosphorus (P) absorbed by algae in the suspended-solid phase photobioreactor (ssPBR) have emerged as an efficient pathway to purify the effluent of wastewater treatment plants (WWTPs). However, the key operational parameters of the ssPBR need to be optimized. In this study, the stability of the system after sequential batch operations and the efficiency under various influent P concentrations were evaluated. The results demonstrated that the ssPBR maintained a high N/P removal efficiency of 96 % and 98 %, respectively, after 5 cycles. When N was kept at 15 mg/L and P ranged from 1.5 to 3.0 mg/L, the system yielded plenty of algae products and guaranteed the effluent quality that met the discharge standards. Notably, the carriers were a key contributor to the high metabolism of algae and high performance. This work provided theoretical ideas and technical guidance for effluent quality improvement in WWTPs.

Continuous cultivation of microalgae yields high nutrient recovery from nitrified urine with limited supplementation.

Microalgae can play a key role in the bioeconomy, particularly in combination with the valorisation of waste streams as cultivation media. Urine is an example of a widely available nutrient-rich waste stream, and alkaline stabilization and subsequent full nitrification in a bioreactor yields a stable nitrate-rich solution. In this study, such nitrified urine served as a culture medium for the edible microalga Limnospira indica. In batch cultivation, nitrified urine without additional supplements yielded a lower biomass concentration, nutrient uptake and protein content compared to modified Zarrouk medium, as standard medium. To enhance the nitrogen uptake efficiency and biomass production, nitrified urine was supplemented with potentially limiting elements. Limited amounts of phosphorus (36 mg L-1), magnesium (7.9 mg L-1), calcium (12.2 mg L-1), iron (2.0 mg L-1) and EDTA (88.5 mg Na2-EDTA.2H2O L-1) rendered the nitrified urine matrix as effective as modified Zarrouk medium in terms of biomass production (OD750 of 1.2), nutrient uptake (130 mg N L-1) and protein yield (47%) in batch culture. Urine precipitates formed by alkalinisation could in principle supply enough phosphorus, calcium and magnesium, requiring only external addition of iron, EDTA and inorganic carbon. Subsequently, the suitability of supplemented nitrified urine as a culture medium was confirmed in continuous Limnospira cultivation in a CSTR photobioreactor. This qualifies nitrified urine as a valuable and sustainable microalgae growth medium, thereby creating novel nutrient loops on Earth and in Space, i.e., in regenerative life support systems for human deep-space missions.

Enhancing phosphorus removal of photogranules by incorporating polyphosphate accumulating organisms.

Photogranules are a novel wastewater treatment technology that can utilize the sun's energy to treat water with lower energy input and have great potential for nutrient recovery applications. They have been proven to efficiently remove nitrogen and carbon but show lower conversion rates for phosphorus compared to established treatment systems, such as aerobic granular sludge. In this study, we successfully introduced polyphosphate accumulating organisms (PAOs) to an established photogranular culture. We operated photobioreactors in sequencing batch mode with six cycles per day and alternating anaerobic (dark) and aerobic (light) phases. We were able to increase phosphorus removal/recovery by 6 times from 5.4 to 30 mg/L/d while maintaining similar nitrogen and carbon removal compared to photogranules without PAOs. To maintain PAOs activity, alternating anaerobic feast and aerobic famine conditions were required. In future applications, where aerobic conditions are dependent on in-situ oxygenation via photosynthesis, the process will rely on sunlight availability. Therefore, we investigated the feasibility of the process under diurnal cycles with a 12-h anaerobic phase during nighttime and six short cycles during the 12 h daytime. The 12-h anaerobic phase had no adverse effect on the PAOs and phototrophs. Due to the extension of one anaerobic phase to 12 h the six aerobic phases were shortened by 47% and consequently decreased the light hours per day. This resulted in a decrease of phototrophs, which reduced nitrogen removal and biomass productivity up to 30%. Finally, we discuss and suggest strategies to apply PAO-enriched photogranules at large-scale.

Effect of phosphorus limitation on Se uptake efficiency in the microalga Nannochloropsis oceanica.

Microalgae are considered an efficient accumulator and promising source of Se for feed additive purposes. This study aimed at investigating, for the first time, the effect of phosphorus limitation on Se accumulation and uptake efficiency in N.oceanica. A range of phosphorus concentrations (0-2470 µM) were tested in either the presence or absence of sodium selenite (0, 5, 30 µM). Se accumulation was increased up to 16-fold and Se uptake efficiency was increased up to 3.6-fold under phosphorus growth-limiting concentrations. N.oceanica was then cultivated in a 1.8L flat-panel photobioreactor in batch operation under two phosphorus growth-limiting concentrations (250 and 750 µM) where the accumulation of Se in the microalgal biomass, as well as its presence in the spent medium were analysed. This study is the first to investigate the effect of phosphorus limitation for increasing Se accumulation in microalgae, and to prevent the release of Se in wastewater.

Growth of Scenedesmus dimorphus in swine wastewater with versus without solid-liquid separation pretreatment.

Scenedesmus dimorphus was cultivated in raw and pretreated swine wastewater (SW) with 6-L photobioreactors (PBRs) to investigate the effect of solid-liquid separation on algal growth. The same aerated PBRs containing no algae were used as control. Moderate COD and nitrogen removal from the SW was achieved with the algal PBRs. However, compared to the control reactors, they offered no consistent treatment boost. Improved algal growth occurred in the pretreated SW, as measured by maximum algal cell count (3202 ± 275 × 106 versus 2286 ± 589 × 106 cells L-1) and cell size. The enhanced algal growth in the pretreated SW resulted in relatively high nitrogen (5.7 %) and organic matter contents in the solids harvested at the end of cultivation experiments, with ∼25.6 % of nitrogen in the SW retained in the solids and ∼9.1 % absorbed by algae. The pretreatment also resulted in elevated phosphorus removal. This study is anticipated to foster the development of microalgae-based SW treatment processes.

A review on the current application of light-emitting diodes for microalgae cultivation and its fiscal analysis.

Microalgae are the promising source of products having a low and high economic value that include feedstock and vitamin supplements. Presently, their cultivation is being carried out by using sunlight in the open raceway ponds. However, this process has disadvantages like fluctuations in irradiance of the sunlight due to climatic changes and bad weather. Artificial lights, exploiting light-emitting diodes are beneficial in increasing the volumetric productivity of the microalgal biomass as it provides continuous illumination in the photobioreactors and assist in the external and internal design. However, the application of light-emitting diodes accrues high input costs. Though the cost of light-emitting diodes was estimated long ago, there is no recent economic analysis of the same. This study aims to enlist the applications of light-emitting diodes in microalgal cultivation with reference to internally illuminated photobioreactors coupled with the evaluation of the cost and energy balance of the artificial lights. The calculation shows that the electrical energy cost incurred during the application of light-emitting diodes for microalgae cultivation is approximately USD 15.19 kg-1 DW. The collective fraction of electrical energy transformed into chemical energy (microalgae biomass) is around 6-8%. The cost of the light-emitting diodes can be decreased by the application of an Arduino-based automated control system to control the power supply to LEDs, photovoltaic powered photobioreactors and additional light. These techniques of input cost reduction have also been explored deeply in the present study. As estimated, they can reduce the cost of light-emitting diodes by 50%.HighlightsDiscussion on the current application of light-emitting diodes for microalgae cultivationA broad discussion on internally illuminated photobioreactors and their modificationsMicroalgae cultivation cost exploiting LEDs' is around USD 15.19 kg-1 DWNet conservation of electrical energy during the cultivation process is 6-8%Photovoltaic powered PBRs and Arduino microcontrollers will decrease cultivation cost.

Enhanced nutrient removal and bioenergy production in microalgal photobioreactor following anaerobic membrane bioreactor for decarbonized wastewater treatment.

This study investigated the nutrient removal, decarbonization potentials, and bioenergy production (i.e., algal biomass and biogas) between a membrane photobioreactor (MPBR) and a sequencing photobioreactor (SPBR) as the post-treatment process of an anaerobic membrane bioreactor (AnMBR) for municipal wastewater treatment. All photobioreactors without aeration showed favourable performance on AnMBR effluent polishing and bioenergy production. In comparison, MPBRs achieved higher removal efficiencies with 98.4 %-99.1 % NH4-N and 74.8 %-88.4 % PO4-P removal compared to the SPBRs with 41.1 %-82.0 % NH4-N and 39.6 %-72.9 % PO4-P removal. MPBRs enhanced more nutrient utilization (24.9-49.3 g(N)/(m3·d) and 3.4-8.1 g(P)/(m3·d)) and CO2 assimilation (22.9-43.4 g(C)/(m3·d)), and concentrated more microalgae with 1.58-1.98 g/L higher than the SPBRs. Moreover, the MPBR effectively upgraded the biogas from AnMBR with superior methane percentage of 89.4 %-93.4 % due to its better CO2 biofixation. The MPBR, with better carbon, nitrogen and phosphorous removal and bioenergy production, following AnMBR is an attractive decarbonized technology for future sustainable wastewater treatment.

The effects of light regime on carbon cycling, nutrient removal, biomass yield, and polyhydroxybutyrate (PHB) production by a constructed photosynthetic consortium.

Microalgae can add value to biological wastewater treatment processes by capturing carbon and nutrients and producing valuable biomass. Harvesting small cells from liquid media is a challenge easily addressed with biofilm cultivation. Three experimental photobioreactors were constructed from inexpensive materials (e.g. plexiglass, silicone) for hybrid liquid/biofilm cultivation of a microalgal-bacterial consortia in aquaculture effluent. Three light regimes (full-spectrum, blue-white, and red) were implemented to test light spectra as a process control. High-intensity full-spectrum light caused photoinhibition and low biomass yield, but produced the most polyhydroxybutyrate (PHB) (0.14 mg g-1); a renewable bioplastic polymer. Medium-intensity blue-white light was less effective for carbon capture, but removed up to 82 % of phosphorus. Low-intensity red light was the only net carbon-negative regime, but increased phosphorus (+4.98 mg/L) in the culture medium. Light spectra and intensity have potential as easily-implemented process controls for targeted wastewater treatment, biomass production, and PHB synthesis using photosynthetic consortia.

Towards biomass production and wastewater treatment by enhancing the microalgae-based nutrients recovery from liquid digestate in an innovative photobioreactor integrated with dialysis bag.

Microalgae-based nutrients recovery from liquid anaerobic digestate of swine manure has been a hotspot in recent decades. Nevertheless, in consideration of the high NH4+-N content and poor light penetrability exhibited by the original liquid digestate, uneconomical pretreatment on liquid digestate including centrifugation and dilution are indispensable before microalgae cells inoculation. Herein, aiming at eliminating the energy-intensive and freshwater-consuming pretreatment on liquid digestate and enhancing microalgae growth, the dialysis bag which permits nutrients transferring across its wall surface whereas retains almost all matters characterized by impeding light transmission within the raw liquid digestate was integrated into a column photobioreactor (DB-PBR). Consequently, light availability of microalgae cells in DB-PBR was elevated remarkably and thus contributed to a 357.58% improvement on microalgae biomass concentration in DB-PBR than the conventional PBR under 80 µmol m-2 s-1. Likewise, superior nutrients removal efficiencies from liquid digestate were obtained in DB-PBR (NH4+-N: 74.84%, TP: 63.75%) over the conventional PBR (NH4+-N: 30.27%, TP: 16.86%). Furthermore, higher microalgae biomass concentration (1.87 g L-1) and nutrients removal efficiencies (NH4+-N: 95.12%, TP: 76.87%) were achieved in the DB-PBR by increasing the light intensity to 140 µmol m-2 s-1. More importantly, the DB-PBR may provide a simple and greener solution to purify other kinds of wastewater.

Buffering culture solution significantly improves astaxanthin production efficiency of mixotrophic Haematococcus pluvialis.

Sodium acetate (NaAc) supplementation, often used to increase the growth of H. pluvialis under low light, but promotes cell death under high light; its underlying reasons and solutions are rarely reported. Here, NaAc supplementation was found to rapidly increase pondus hydrogenii (pH) of culture solution, elevate reactive oxygen species (ROS), and cause cell death immediately under higher light. Adjusting pH of NaAc supplemented culture solution with 10 mM Tris-HCl once before high light significantly reduced cell mortality and increased astaxanthin yield. When verified in a 5-litre photobioreactor, this novel method produced over 4.0% of dry weight (DW) astaxanthin within only 8-10 days. In summary, this study explained reasons underlying NaAc supplementation-induced cell death and provided an rapid, easy and effective method to produce high amount of astaxanthin in H. pluvialis.

Coelastrella terrestris for Adonixanthin Production: Physiological Characterization and Evaluation of Secondary Carotenoid Productivity.

A novel strain of Coelastrella terrestris (Chlorophyta) was collected from red mucilage in a glacier foreland in Iceland. Its morphology showed characteristic single, ellipsoidal cells with apical wart-like wall thickenings. Physiological characterization revealed the presence of the rare keto-carotenoid adonixanthin, as well as high levels of unsaturated fatty acids of up to 85%. Initial screening experiments with different carbon sources for accelerated mixotrophic biomass growth were done. Consequently, a scale up to 1.25 L stirred photobioreactor cultivations yielded a maximum of 1.96 mg·L-1 adonixanthin in free and esterified forms. It could be shown that supplementing acetate to the medium increased the volumetric productivity after entering the nitrogen limitation phase compared to autotrophic control cultures. This study describes a promising way of biotechnological adonixanthin production using Coelastrella terrestris.

Outdoor phycoremediation and biomass harvesting optimization of microalgae Botryococcus sp. cultivated in food processing wastewater using an enclosed photobioreactor.

Microalgae cultivation is well known as a sustainable method for eco-friendly wastewater phycoremediation and valuable biomass production. This study investigates the feasibility and kinetic removal of organic compounds and nutrients from food processing wastewater (FPW) using Botryococcus sp. in an enclosed photobioreactor. Simultaneously, response surface methodology (RSM) via face-centered central composite design (FCCCD) was applied to optimize the effects of alum and chitosan dosage and pH sensitivity on flocculation efficiency. The maximum growth rate of Botryococcus sp. cultivated in FPW was 1.83 mg day-1with the highest removal of chemical oxygen demand (COD), total organic carbon (TOC), and total phosphorus (TP) after 12 days of phycoremediation of 96.1%, 87.2%, and 35.4%, respectively. A second-order polynomial function fits well with the experimental results. Both coagulant dosage and pH significantly (p < 0.05) affect the flocculation efficiency of Botryococcus sp. biomass cultivated in FPW. The highest flocculation efficiency (92.4%) was obtained at a dosage of 166 mg L-1and pH 12 for alum coagulant, while 94.9% flocculation efficiency was achieved with optimum chitosan dosage and pH of 30 mg L-1and 5.54, respectively. In general, Botryococcus sp. shows a great removal efficiency of FPW contamination, whereas RSM provides excellent analysis for biomass harvesting optimization using a flocculation technique.

Eutrophication is caused by an overabundance of organic compounds and nutrients such as chemical oxygen demand, total nitrogen, phosphorus and total organic carbon in food processing wastewater (FPW) that is discharged into natural water systems. Although, there has been few research on the phycoremediation using Botryococcus sp. in wastewater treatment. Hence, the current study was carried out to investigate Botryoccocus sp. biomass harvesting efficiency using alum and chitosan as flocculants after the nutrients and organic compounds removal were analyzed during phycoremediation. The use of response surface methodology also offers excellent statistical analysis for flocculation optimization.

Potassium supplement enhanced cadmium removal in a Microcystis aeruginosa photobioreactor: Evidence from actual and simulated wastewater.

In this study, a Microcystis aeruginosa-based photobioreactor (M. aeruginosa-based PBR) was developed for the removal of cadmium (Cd2+) from diluted actual mine wastewater (DW) and Cd2+-contained simulated wastewater (SW), with a uniform Cd2+ concentration of 0.5 mg/L. For the DW and SW, both K+ -abundant (DWA & SWA) and K+-insufficient (DWB & SWB) treatments were conducted. It was found that continuous supplementation of K+ benefited Cd2+ removal. The Cd2+ removal efficiency in SWA reached 70% during the 41 days of operation, which was 20% higher than that in the SWB. The K+ addition triggered great higher Cd2+ removal efficiency (90%) in the DWA in comparison to the SWA. The Cd2+ assimilation by M. aeruginosa and Cd2+ retention on M. aeruginosa surface were the primary processes involved in the PBR system. The K+ starvation triggered a 45% and 43% loss of M. aeruginosa biomass in the DWA and the DWB, respectively. Hence, the Cd2+ removal efficiency in DWB increased significantly, and this was attributed to the increased abundance of non-living cells and enhanced bioretention of Cd2+. The results revealed that continuous K+ supplementation enhanced the Cd2+ removal efficiency in the M. aeruginosa-based PBR jointly by prompting algal cell growth, Cd2+ assimilation and biosorption, as well as Cd2+ retention on the algal cells.

Photoautotrophic and Mixotrophic Cultivation of Polyhydroxyalkanoate-Accumulating Microalgae Consortia Selected under Nitrogen and Phosphate Limitation.

Microalgae consortia were photoautotrophically cultivated in sequencing batch photobioreactors (SBPRs) with an alteration of the normal growth and starvation (nutrient limitation) phases to select consortia capable of polyhydroxyalkanoate (PHA) accumulation. At the steady state of SBPR operation, the obtained microalgae consortia, selected under nitrogen and phosphate limitation, accumulated up to 11.38% and 10.24% of PHA in their biomass, which was identified as poly(3-hydroxybutyrate) (P3HB). Photoautotrophic and mixotrophic batch cultivation of the selected microalgae consortia was conducted to investigate the potential of biomass and PHA production. Sugar source supplementation enhanced the biomass and PHA production, with the highest PHA contents of 10.94 and 6.2%, and cumulative PHA productions of 100 and 130 mg/L, with this being achieved with sugarcane juice under nitrogen and phosphate limitation, respectively. The analysis of other macromolecules during batch cultivation indicated a high content of carbohydrates and lipids under nitrogen limitation, while higher protein contents were detected under phosphate limitation. These results recommended the selected microalgae consortia as potential tools for PHA and bioresource production. The mixed-culture non-sterile cultivation system developed in this study provides valuable information for large-scale microalgal PHA production process development following the biorefinery concept.

Kinetic modeling of autotrophic microalgae mainline processes for sewage treatment in phosphorus-replete and -deplete culture conditions.

A kinetic model of autotrophic microalgal growth in sewage was developed to determine the biokinetic processes involved, including carbon-, nitrogen- and phosphorus-limited microalgal growth, dependence on light intensity, temperature and pH, light attenuation and gas exchange to the atmosphere. A new feature was the differentiation between two metabolic pathways of phosphorus consumption according to the availability of extracellular phosphorus. Two scenarios were differentiated: phosphorus-replete and -deplete culture conditions. In the former, the microalgae absorbed phosphorus to grow and store polyphosphate. In the latter the microalgae used the stored polyphosphate as a phosphorus source for growth. Calibration and validation were performed with experimental data from a pilot-scale membrane photobioreactor (MPBR) fed with the permeate obtained from an anaerobic membrane bioreactor (AnMBR) pilot plant fed with real urban wastewater. 12 of the model parameters were calibrated. Despite the dynamics involved in the operating and environmental conditions, the model was able to reproduce the overall process performance with a single set of model parameters values. Four periods of different environmental and operational conditions were accurately simulated. Regarding the former, light and temperature ranged 10-406 µmol·m-2·s-1 and 19.7-32.1 °C, respectively. Concerning the later, the photobioreactors widths were 0.25 and 0.10 m, and the biomass and hydraulic retention times ranged 3-4.5 and 1.5-2.5 days, respectively. The validation of the model resulted in an overall correlation coefficient (R2) of 0.9954. The simulation results showed the potential of the model to predict the dynamics of the different components: the relative proportions of microalgae, nitrogen and phosphorus removal, polyphosphate storage and consumption, and soluble organic matter concentration, as well as the influence of environmental parameters on the microalgae's biokinetic processes. The proposed model could provide an effective tool for the industry to predict microalgae production and comply with the discharge limits in areas declared sensitive to eutrophication.

Comparative life cycle assessment of a mesh ultra-thin layer photobioreactor and a tubular glass photobioreactor for the production of bioactive algae extracts.

This study aimed at the comparison of two different photobioreactors with focus on technology and sustainability. The mesh ultra-thin layer photobioreactor (MUTL-PBR) exhibited around 3-fold biomass based space-time-yield and an around 10-fold specific antioxidant capacity than the traditional reference photobioreactor. Life cycle assessment (LCA) was done under autotrophic conditions in both pilot scale reactors with focus on biomass production and on antioxidant capacity of the biomass, respectively. Biomass production within the reference reactor showed a lower environmental impact in most categories. A significantly higher energy demand for mixing and cooling of the cell suspension within the MUTL-PBR is the major reason for its environmental burden. This relates to high impacts in the categories "non-renewable energy" and "global warming potential" per kg biomass. Comparing algal antioxidant capacity, environmental impact of the MUTL cultivation was 5-10 times lower. This clearly illustrates the potential of MUTL-PBR for sustainable production of bioactive substances.

Treatment of Rose Oil Processing Effluent with Chlorella sp. Using Photobioreactor and Raceway.

The integration of treatment of wastewater from agro-based industries with microalgae cultivation can reduce costs associated with cultivation while treating wastewater to meet the discharge limits for chemical quality of irrigation in agriculture and to obtain biofertilizers. Rose Oil Processing Effluent (ROPE) can be utilized as a growth medium for Chlorella sp. and thus can be used for biofertilizer production. The present study is aimed at determining the feasibility of the cultivation of Chlorella sp. in ROPE using a tubular photobioreactor with a capacity of 50 L and a raceway to treat ROPE while consuming less energy. The optimum mixing ratio ([ROPE/(ROPE + Bold Basal Medium (BBM)] × 100) was determined as 50% using 2-L Erlenmeyer flasks based on the COD removal efficiency. Better removal efficiencies with regard to COD, BOD5, NH4+-N, and NO3--N were obtained from the raceway compared to the tubular photobioreactor. The effluents from both systems met the chemical quality of irrigation water. The results of the biomasses harvested from both systems in macro and microelements revealed that they have a potential as a biofertilizer in agriculture. The energetic analysis of the ROPE treatment using the tubular photobioreactor and raceway showed that the raceway system had a better net energy ratio while consuming less energy and producing more energy during cultivation. Overall, the raceway appeared to be a better option to treat ROPE with production of biofertilizer and irrigation water quality while consuming less energy.

Nitrogen, phosphorus and high CO2 modulate photosynthesis, biomass and lipid production in the green alga Chlorella vulgaris.

Climate change could impact nutrient bioavailability in aquatic environment. To understand the interaction of nutrient bioavailability and elevated CO2, Chlorella vulgaris cells were grown in ambient air or 5% CO2 in different concentrations of nitrogen and phosphorus in a photobioreactor. The chlorophyll content, photosynthesis and respiration rates increased in 5% CO2 to support higher biomass production. The nutrient limitation in the growth media resulted in reduced photosynthetic rates of the algal cells and their PSI, PSII, and whole chain electron transport rates and biomass production. Conversely, their lipid content increased partly due to upregulation of expression of several lipid biosynthesis genes. The order of downregulation of photosynthesis and upregulation in lipid production due to nutrient limitation was in the order of N > P. The N-50 and 5% CO2 culture had only 10% reduction in biomass and 32% increase in lipids having 85% saturated fat required for efficient biofuel production. This growth condition is ideal for generation of biodiesel required to reduce the consumption of fossil fuel and combat global warming.

Start-up of a trickling photobioreactor for the treatment of domestic wastewater.

A stand-alone trickling photobioreactor (TPBR) was seeded with activated sludge and microalgae to treat domestic wastewater. The TPBR was started-up at 12-h hydraulic retention time at room temperature with 12:12 h light:dark cycle. The light was provided by blue LED strips. The reactor has a total volume of 30 L and is divided into six segments. Each segment is 30 cm long and has a diameter of 15 cm. Each segment was packed with polyurethane foam sponge cubes (2.5 × 2.5 × 2.5 cm3 ) with 40% occupancy. The chemical oxygen demand (COD), total organic carbon (TOC), total nitrogen (TN), and phosphorus (P) of domestic wastewater varied in the range of 164-256 mg/L, 84.4-133.8 mg/L, 34.2-55.6 mg/L, and 24.7-39.3 mg/L, respectively, during this period. The COD, TOC, TN, and P concentrations in the effluent after 45 days of operation were 30.24 ± 3.36 mg/L, 7.69 ± 0.09 mg/L, 16.67 ± 0.39 mg/L, and 17.48 ± 0.5 mg/L, respectively. The chlorophyll-to-biofilm biomass ratio increased during the experimental period. The above results indicate that the algal-bacterial symbiotic relationship is beneficial for carbon and nutrient removal from domestic wastewater. PRACTITIONER POINTS: Trickling photobioreactor works on natural ventilation and has low power requirements and a small footprint. The porous sponge media helped in immobilizing and subsequent harvesting of biomass. The reactor conditions favored the growth of diatoms (brown algae) over green algae.