High Rate Algal Ponds Research Articles

Knowledge Mapping of High-Rate Algal Ponds Research

High-rate algal ponds (HRAPs) are a highly promoted wastewater treatment system that uses sunlight as an energy source to provide the oxygen needed in the system through photosynthesis and has a high nutrient and organic matter removal capacity. In addition, the microalgae in the system can use wastewater as a growth substrate to produce valuable bioproducts, biomaterials, and bioenergy, so it is receiving more and more attention. This review uses bibliometric analysis to explore current research hotspots and future research trends in this emerging technology. By analyzing research papers related to HRAPs published in the Web of Science (WOS) from 1987 to 2021 based on the co-occurrence and clustering of keywords, it shows that the research hotspots of HRAPs are mainly focused on wastewater treatment, nutrient removal, microalgal biomass, biofuel, and biogas upgrading. In the future, in-depth research will continue to be added on the contribution of HRAPs to environmental sustainability, including E. coli removal, biogas upgrading and oxygen removal, treatment of aquaculture wastewater, purple phototrophic bacteria, aqueous biorefineries, and biorefineries. The results assist scholars in systematically understanding the current research status, research frontiers, and future trends of HRAPs from a macro perspective.

Removal of pharmaceutical compounds from real urban wastewater by a continuous bio-electrochemical process at pilot scale

A coupled bio-electrochemical treatment was designed by combining a microalgae-based system with an electro-oxidation (EO) process for removing pharmaceutical compounds in real urban wastewater. First, a preliminary study was carried out at bench scale to identify the best configuration for the coupled treatment, which involved determining whether EO should be used as a pre or post-treatment of the microalgae-based system. The results showed that using EO as a post-treatment provided the best removal results for most of pharmaceuticals, with an average of 58.8% compared to 47.7% obtained with EO as a pre-treatment. Based on these results, a bio-electrochemical treatment was designed using a high rate algal pond (HRAP) followed by an EO process at pilot scale. EO process was evaluated using a Boron Doped Diamond electrode anode and cathode (BDD/BDD cell) with a current density of between 20 and 30 mA cm−2 and an electrolysis time of one hour. This coupled treatment showed removal efficiencies above 80% for hypolipidemic, analgesics and anti-inflammatory drugs. However, antiepileptic drugs showed the lowest removal rate between 36.8% and 69.1%. It is noteworthy that carbamazepine and 10,11-Dihydro-10,11-dihydroxycarbamazepine (CBZ-Diol) showed almost five times the removal rate in the coupled treatment compared to HRAP alone. Overall, the coupled bio-electrochemical treatment proved to be efficient in removing the studied pharmaceutical compounds. The HRAP pre-treatment reduced the presence of organic and inorganic load by 77.4%, allowing the EO process to remove the more recalcitrant target contaminants.

Exploring resistomes and microbiomes in pilot-scale microalgae-bacteria wastewater treatment systems for use in low-resource settings

Antibiotic resistance genes (ARGs) released into the environment are an emerging human and environmental health concern, including ARGs spread in wastewater treatment effluents. In low-to-middle income countries (LMICs), an alternate wastewater treatment option instead of conventional systems are low-energy, high-rate algal ponds (HRAP) that use microalgae-bacteria aggregates (MABA) for waste degradation. Here we studied the robustness of ARG removal in MABA-based pilot-scale outdoor systems for 140 days of continuous operation. The HRAP system successfully removed 73 to 88 % chemical oxygen demand and up to 97.4 % ammonia, with aggregate size increasing over operating time. Fourteen ARG classes were identified in the HRAP influent, MABA, and effluent using metagenomics, with the HRAP process reducing total ARG abundances by up to 5-fold from influent to effluent. Parallel qPCR analyses showed the HRAP system significantly reduced exemplar ARGs (p < 0.05), with 1.2 to 4.9, 2.7 to 6.3, 0 to 1.5, and 1.2 to 4.8 log-removals for sul1, tetQ, blaKPC, and intl1 genes, respectively. Sequencing of influent, effluent and MABAs samples showed associated microbial communities differed significantly, with influent communities by Enterobacteriales (clinically relevant ARGs carrying bacteria), which were less evident in MABA and effluent. In this sense, such bacteria might be excluded from MABA due to their good settling properties and the presence of antimicrobial peptides. Microalgae-bacteria treatment systems steadily reduced ARGs from wastewater during operation time, using sunlight as the energetic driver, making them ideal for use in LMIC wastewater treatment applications.

Generation of Typical Meteorological Sequences to Simulate Growth and Production of Biological Systems

Numerical simulation applied to agriculture or wastewater treatment (WWT) is a complementary tool to understand, a priori, the impact of meteorological parameters on productivity under limiting environmental conditions or even to guide investments towards other more relevant circular economic objectives. This work proposes a new methodology to calculate Typical Meteorological Sequences (TMS) that could be used as input data to simulate the growth and productivity of photosynthetic organisms in different biological systems, such as a High-Rate Algae Pond (HRAP) for WWT or in agriculture for crops. The TMS was established by applying Finkelstein-Schafer statistics and represents the most likely meteorological sequence in the long term for each meteorological season. In our case study, 18 locations in the Madrid (Spain) region are estimated depending on climate conditions represented by solar irradiance and temperature. The parameters selected for generating TMS were photosynthetically active radiation, solar day length, maximum, minimum, mean, and temperature range. The selection of potential sequences according to the growth period of the organism is performed by resampling the available meteorological data, which, in this case study, increases the number of candidate sequences by 700%.

Nature-based solutions for wastewater treatment and bioenergy recovery: A comparative Life Cycle Assessment

The aim of this study was to assess the environmental impacts of up-flow anaerobic sludge blanket (UASB) reactors coupled with high rate algal ponds (HRAPs) for wastewater treatment and bioenergy recovery using the Life Cycle Assessment (LCA) methodology. This solution was compared with the UASB reactor coupled with other consolidated technologies in rural areas of Brazil, such as trickling filters, polishing ponds and constructed wetlands. To this end, full-scale systems were designed based on experimental data obtained from pilot/demonstrative scale systems. The functional unit was 1 m3 of water. System boundaries comprised input and output flows of material and energy resources for system construction and operation. The LCA was performed with the software SimaPro®, using the ReCiPe midpoint method. The results showed that the HRAPs scenario was the most environmentally friendly alternative in 4 out of 8 impact categories (i.e. Global warming, Stratospheric Ozone Depletion, Terrestrial Ecotoxicity and Fossil resource scarcity). This was associated with the increase in biogas production by the co-digestion of microalgae and raw wastewater, leading to higher electricity and heat recovery. From an economic point of view, despite the HRAPs showed a higher capital cost, the operation and maintenance costs were completely offset by the revenue obtained from the electricity generated. Overall, the UASB reactor coupled with HRAPS showed to be a feasible nature-based solution to be used in small communities in Brazil, especially when microalgae biomass is valorised and used to increase biogas productivity.

Modelling Escherichia coli removal during domestic wastewater treatment in outdoor pilot scale High Rate Algal Ponds

While there is growing evidence that High Rate Algal Ponds (HRAPS) can support disinfection during wastewater treatment, the mechanisms of pathogen removal involved remain unclear. Based on previous findings, the present study aimed at verifying the occurrence and significance of dark decay, alkaline-pH toxicity, and sunlight direct damage in conditions representative of ‘real’ HRAP operation. For this purpose, Escherichia coli cell count was monitored hourly during 24 h periods in outdoor pilot HRAPs fed domestic wastewater. The data generated was used to create 2 distinct data sets that were then used to calibrate and validate a model of E. coli removal in HRAP. By modelling the contribution of the three mechanisms considered, E. coli removal in HRAPs could be predicted with a mean relative absolute error of 5.34% (N = 46). The model was then inputted yearly environmental data obtained from the continuous operations of the pilot scale HRAPs to evaluate the yearlong contributions from each mechanism to overall E. coli removal. Dark decay was thus predicted to support the highest yearlong mean contribution (78.6 – 91.0% of total E. coli decay, at the 95% confidence level) followed by sunlight mediated disinfection (6.01 – 17.8%) and pH toxicity (0.22 – 6.2%). Significant seasonal and daily fluctuations of decay mechanism contributions were evidenced in this study.

Carbon supplementation in domestic sewage via mixing with paint booth effluent: Influence on the performance of bioremediation and algal biomass production from high-rate algal ponds

The paint booth effluent (PBE) was used as a carbon supplement to evaluate the treatment of domestic sewage (DS) via microalgae biotechnology. The present study aimed to assess the performance of different mixtures of PBE and DS in the treatment and production of algal biomass in high-rate algal ponds (HRAPs), seeking to determine the optimal mixture between the effluents. The blends were: D1 (0% PBE, 100 % DS), D2 (18.75 % PBE, 81.25 % DS), D3 (37.5 % PBE, 62.5 % DS), D4 (56.25 % PBE, 43.75 % DS) and D5 (75% PBE, 25 % DS). As a result, in the D2 blend, with better carbon, nitrogen, and phosphorus ratio, there was a higher primary and total biomass productivity (0.08 g chl-α/m2.day and 2.0 g VSS/m2.day). The regression adjustment corroborated the results of the D2 mixture, showing PBE dosages between 18 and 25 % as better for algal growth. The insertion of PBE in the medium positively affected the bioremediation performance, increasing the removals with the addition of PBE. The nutrient and organic matter removal efficiencies ranged from values close to 50 %, of total organic carbon in the D1 mixture, and reaching 100 % in the removal of ammoniacal nitrogen. The D2 mixture stands out with the highest nitrogen recovery via the assimilation of organic nitrogen, reaching 19 % more in relation to the initial value. Thus, PBE contributed to the supply of organic carbon to the biomass, favoring the consortium between algae and bacteria.

Copper multifaceted interferences during swine wastewater treatment in high-rate algal ponds: alterations on nutrient removal, biomass composition and resource recovery

Microalgae cultivation in swine wastewater (SW) allows the removal of nutrients and biomass production. However, SW is known for its Cu contamination, and its effects on algae cultivation systems such as high-rate algal ponds (HRAPs) are poorly understood. This gap in the literature limits the proposition of adequate concentrations of Cu to optimise SW treatment and resource recovery in HRAPs. For this assessment, 12 HRAPs installed outdoors were operated with 800 L of SW with different Cu concentrations (0.1–4.0 mg/L). Cu's interferences on the growth and composition of biomass and nutrient removal from SW were investigated through mass balance and experimental modelling. The results showed that the concentration of 1.0 mg Cu/L stimulated microalgae growth, and above 3.0 mg Cu/L caused inhibition accompanied by an accumulation of H2O2. Furthermore, Cu affected the contents of lipids and carotenoids observed in the biomass; the highest concentration was observed in the control (16%) and 0.5 mg Cu/L (1.6 mg/g), respectively. An innovative result was verified for nutrient removal, in which increased Cu concentration reduced the N–NH4+ removal rate. In contrast, the soluble P removal rate was enhanced by 2.0 mg Cu/L. Removal of soluble Cu in treated SW reached 91%. However, the action of microalgae in this process was not associated with assimilation but with a pH increase resulting from photosynthesis. A preliminary evaluation of economic viability showed that the commercialisation of biomass considering the concentration of carotenoids obtained in HRAPs with 0.5 mg Cu/L could be economically attractive. In conclusion, Cu affected the different parameters evaluated in this study in a complex way. This can help managers consort nutrient removal, biomass production, and resource recovery, providing information for possible industrial exploitation of the generated bioproducts.

Preparation of simple biodegradable, nontoxic, and antimicrobial PHB/PU/CuO bionanocomposites for safely use as bioplastic material packaging

AbstractPlastic waste in Egypt is causing a problem. An innovative solution to reduce this pollution is inevitable. Microalgae were considered a new resource for developing biobased bioplastics. This study aims to prepare a simple biodegradable film able to use in packaging and from polyurethane (PU) nanocomposite with Poly 3-hydroxybutyrate (PHB) in the presence of various concentrations of CuO-NPs. The Poly 3-hydroxybutyrate compounds were extracted from the dominant microalgal species in High rate algal pond Microcystis sp. Algal extracts have antimicrobial activities against Escherichia coli, Salmonella Typhimurium, Pseudomonas aeruginosa, Staphylococcus aureus, Listeria monocytogenes, Enterococcus faecalis, and Candida albicans. Films prepared from PHB, and PU showed no cytotoxic impact on human tumor cell lines in terms of cell viability. The mechanical properties of the films were studied, and it was found that the final contact angle values were improved from 77° to 87° as CuO-NPs loading raised from 2 to 4%, respectively. XRD displays no difference in the intensity of CuO-NPs peaks by increasing CuO-NPs loading in the PHB/PU matrix. FTIR spectra of all blends were recorded in the range of 400 to 4000 cm−1. Tensile properties were improved with the addition of 40 wt.% PHB.

CO2 Addition and Semicontinuous Feed Regime in Shaded HRAP—Pathogen Removal Performance

The influence of CO2 addition and feeding regime (continuous versus semicontinuous) on the removal of Pseudomonas aeruginosa, Clostridium perfringens, Staphylococcus, Enterococcus faecalis, and Escherichia coli (E. coli) from three shaded high-rate algal ponds (HRAPs) treating raw sewage (RS) was studied. The three HRAPs were operated at an analogous hydraulic retention time (HRT) for 5 days and with shading of 50%. The CO2 addition and feeding regime had no statistically significant influence on the removal of Pseudomonas aeruginosa, Clostridium perfringens, Staphylococcus sp., and Enterococcus faecalis, with 2.39–3.01, 2.07–2.31, 3.02–3.38, and 3.14–3.45 log units, respectively. However, the removal of E. coli decreased significantly with the feeding regime of 0.1 h d−1 and 2.23–3.29 log units. The productivity and the total suspended solids (TSS) removal efficiency were significantly improved with the semicontinuous feeding regime and CO2 addition. The highest productivity was obtained in the semicontinuous feeding regime, 5.93 g m2 d−1, while the TSS removal efficiency was similar between the semicontinuous feeding regime and CO2 addition (31–36%). The control of light intensity led to greater variability in the algal community, and was present in the three reactors, in different proportions, in the form of the microalgae Scenedesmus acutus, Scenedesmus obliquus, and Chlorella sp.

Microalgae-based wastewater treatment for micropollutant removal in swine effluent: High-rate algal ponds performance under different zinc concentrations

Despite the social and economic importance of swine farming, this activity has great polluting potential due to the large wastewater generation composed of high micropollutant concentrations, such as trace metals (Zn and Cu). Microalgae can integrate the solutions to overcome these challenges by obtaining value-added biomass, closing the loop for an environmentally and economically sustainable wastewater treatment technology. Therefore, this study aimed to identify the interference of different Zn concentrations (0.5 to 70 mg/L) in SW on the performance of high-rate algal ponds considering real treatment and biodegradation conditions via microalgae. The innovative results showed that N-NH4+ removal was compromised with high Zn concentrations due to nitrification inhibition. Oppositely, the removal of soluble P was enhanced under conditions that favored algal growth (40 and 55 mg Zn/L). Moreover, Zn conferred an increase in soluble chemical oxygen demand that changed from 190 mg/L at the beginning of the treatment to 624 mg/L at the end due to extracellular polymeric substances production. Sequential chemical extraction demonstrated that biomass growth had an important contribution to Zn removal; however, it did not exceed the contribution of carbonates (28.7–44.2 %). The biomass characterization showed that Zn did not significantly affect the levels of proteins (41.2–53.5 %) and carbohydrates (34.0–43.5 %), while the lipids content was higher (18.2 %) in 55 mg Zn/L. The effects of Zn on nutrient removal and the biomass's biochemical composition must be pondered to combine the recovery of metals and nutrients and the production of value-added biomass.

Modification of hydraulic retention time in open raceway ponds to prevent rotifers and cladocerans establishment

Cost effective strategies to control zooplankton density in open pond algal cultures are not currently available, and those successfully tested at pilot-scale are generally difficult to implement at full-scale.Here we propose a method to calculate the hydraulic retention time (HRT) for open raceway pond algal cultures, below which undesirable zooplankton species would be prevented from establishing. We hypothesised that a HRT shorter than the generation time (GT) of zooplankton species should prevent their establishment by flushing the individuals out of the pond before they can reproduce. We reviewed published GT data of zooplankton species known to be detrimental to open raceway ponds performance and plotted GT against temperature over the range typical of algae pond operation (10 to 30 °C). We fitted trend lines to describe how the shortest, average, and longest GT of each zooplankton genera or species varies with temperature. We then validated the method against 29 months of operational data from two identical 8 m3 wastewater high rate algal ponds. Finally, we tested the method in a laboratory experiment performed under favourable growth conditions.Validation of the method against actual operation data showed that rotifers did not establish when HRT was shorter than their GT. However, cladoceran establishment was not predicted by the method likely due to their stratification in the water column depending on the intensity of the solar radiation and resulting avoidance of the pond outflow. The laboratory experiment confirmed that rotifers cultured under favourable growth conditions established at the shortest GT predicted by the method and HRTs shorter than the GT eradicated rotifer populations.We propose that operating ponds with HRTs that are up to ∼20 % shorter than the actual pond HRTs will control densities of zooplankton species evenly distributed in the water column to low levels.

Transforming wastewater treatment plants in sustainable units coupled with local economies: Microalgae as resource recovery agents

Besides reducing the sewage organic and nutrient load, microalgae-based wastewater treatments promote the recovery of important natural resources, such as N and P, which can be used for biofuels and animal feed production. In this context, the present study proposes a treatment system based on a circular economy and resource recovery of waste streams: an up-flow anaerobic reactor (UASB, widely used in developing countries such as Brazil) followed by high-rate algae ponds (HRAPs). Geographic Information Systems tools were used to select suitable areas for allocating the proposed system in Minas Gerais (Brazil). Two regions showed excellent suitability for allocating the UASB + HRAP system, with more than 70% coverage each. These areas cover 59 municipalities, with a production capacity of 43,565 m³/day of raw sewage, a volume that, after treatment, can be used for crop irrigation and fish farming. In addition, municipalities can produce 6621.9 kg/day of microalgae biomass. This biomass can generate 13.96 MWh/day of electrical energy, 1316.7 kg/day of nutrients for animal feed, 3.97 kg of β-carotene, 2576.3 kg of biodiesel, 132.4 kg of Omega-3, 198.7 kg of glycerol or 1125.72 kg of ethanol. Productivities were calculated from three by-product valorization routes of the suggested system: energy, agriculture, and high-value products. The proposed system improves current wastewater treatment standards in the selected regions, with the potential to generate valuable resources compatible with attractive economic activities from these areas.

Can microalgae grown in wastewater reduce the use of inorganic fertilizers?

Alternatives to conventional inorganic fertilizers are needed to cope with the growing global population and contamination due to the production and use of those inorganic compounds. The recovery of nutrients from wastewater and organic wastes is a promising option to provide fertilization in a circular economy approach. In this context, microalgae-based systems are an alternative to conventional wastewater treatment systems, reducing the treatment costs and improving the sustainability of the process, while producing nutrient-rich microalgal biomass. The aim of the present study is to evaluate the use of microalgal biomass produced during domestic wastewater treatment in high rate algal ponds as a biofertilizer in basil crops (Ocimum basilicum L.). Wastewater was successfully treated, with removal efficiencies in the secondary treatment of 69, 91 and 81% in terms of chemical oxygen demand (COD), total inorganic nitrogen (TIN) and phosphates (PO43–P), respectively. The microalgal biomass, composed mainly by Scenedesmus, presented the following composition: 12% of dry weight and nutrients concentration of 7.6% nitrogen (N), 1.6% phosphorus (P) and 0.9% potassium (K). The study compared the performance of 3 different fertilizers: 1) microalgae fertilizer (MF), 2) inorganic fertilizer (IF) as positive control and 3) the combination of both microalgae and inorganic fertilizer (MF + IF). Comparable plant growth (i.e., number of leaves, shoot fresh and dry weight and leaf fresh weight) was observed among treatments, except for leaf dry weight, which was significantly higher in the IF + MF and MF treatments (28 and 27%, respectively) in comparison with the control. However, the microalgae treatment provided the lowest chlorophyll, N and K leaf content. In conclusion, this study suggests that combining microalgae grown in wastewater with an inorganic fertilizer is a promising nutrients source for basil crops, enhancing the circular bioeconomy.

A pilot-scale study of the integrated phycoremediation-photolytic ozonation based municipal solid waste leachate treatment process

Municipal solid waste (MSW) leachate is a highly polluted liquid that accumulates in the landfill and contains a high concentration of toxic pollutants which can pollute the surrounding surface water and groundwater as well, if not treated properly. In this study, an integrated approach of phycoremediation with photolytic ozonation was employed for the leachate collected from the MSW dumpsite which has high Chemical Oxygen Demand (COD) and ammonium (NH4+) levels. Photolytic ozonation treatment was employed as a pre-treatment step under operating parameters of pH: 9.0; Ozone dosage: 5 g/h; UV-C: λ = 254 nm; and contact time: 60 min, in which the COD and NH4+ in the leachate was reduced up to 81% and 95%, respectively. The selected algae Chlorella vulgaris (C.vulgaris) was employed in a lab-scale study to optimize the inoculum conditions in the photolytic ozonated leachate (POL). The specific growth rate of C.vulgaris was observed as 0.14/d in the POL at the optimized condition (inoculum size of 25% (T25)) during the study period of 11 days. High-rate algal pond (HRAP) was employed for the pilot-scale study in controlled environmental conditions as in the T25 experimental run for the assessment of POL treatment and biomass production. C.vulgaris reduced the concentration of pollutants COD, NH4+, and heavy metals (Cu, Fe) in the POL up to 93%, 94%, and 71%, respectively, with the dry biomass productivity of 0.727 g/L/d which is 3 times higher than the biomass productivity of C.vulgaris in freshwater conditions. The biochemical composition (carbohydrates, proteins, and lipids) of the harvested biomass has higher lipid production with lipid productivity of 120 mg/L/d which can be used as a feedstock for the production of value-a dded products.

Effect of a LED-enhancement on microalgal and bacterial communities treating digestate in a deep high rate algal pond

High rate algal ponds (HRAPs) can efficiently treat digestates but large area footprints limit their wider application. Deep HRAPs combining sunlight with light-emitting diodes (LEDs) can reduce area requirements if enough light is supplied for microalgae photoaeration. In this work, microalgae and bacterial communities were characterized in-depth to assess the feasibility of a LED-enhanced deep HRAP (h= 1.2 m, 1.25 m3) to efficiently treat digestate at concentrations of 5%, 25% and 50% (v/v). The LEDs effects were evaluated by comparisons with a conventional HRAP (h= 0.3 m), with the same working volume but 75% more area, operated in parallel. The LED-enhanced HRAP achieved similar or superior COD (74–76%) and TKN (66–67%) removals during operation at 5%-digestate, when supporting high cell densities (>5.0 log cell·L−1) of similar communities of photosynthetic chlorophytes, such as Chlorella sp. and Dictyosphaerium sp. Despite achieving lower removals at 25% and mainly at 50%-digestate, due to limited light availability for microalgae photoaeration, LEDs supported a more mixotrophic community along with higher chlorophyll and carotenoids content in the cells. For both HRAPs, chlorophyll-a:carotenoids ratios above 1 were concomitant to high photoaeration, achieved in the LED-enhanced HRAP when at least 0.015 mol·g TSS−1·d−1 were supplied in the aphotic zone. Similar bacterial communities dominated both HRAPs, with Burkholderiaceae and Verrucromicrobia indicating high photoaeration. Nitrosomonadaceae, Nitrincolaceae and Denitratisoma were likely the main responsible for nitrogen removal. Combining red LEDs and sunlight in deep HRAPs has a high potential for digestate treatment when extensive areas are prohibitive.

Photosynthetic upgrading of biogas from anaerobic digestion of mixed sludge in an outdoors algal-bacterial photobioreactor at pilot scale

Anaerobic digestion can biotransform the biodegradable fraction of sewage sludge into biogas, while the symbiotic action of algal-bacterial consortia can remove both the CO2 and H2S from biogas and nutrients from digestate. A 100 L anaerobic digester operated at 20 days of retention time coupled with a 180 L high-rate algal pond (HRAP) engineered to upgrade the biogas and treat the liquid fraction of the pilot digester was optimized along four operational stages: (I) operation with a greenhouse during winter; (II) operation without greenhouse; (III) process supplementation with NaHCO3; (IV) process supplementation with Na2CO3. The biogas produced was composed of 63.7 ± 2.9% CH4, 33.7 ± 1.9% CO2, 0.5 ± 0.3% O2 and 1.6 ± 1.1% N2. An average methane productivity of 324.7 ± 75.8 mL CH4 g VSin−1 and total COD removals of 48 ± 20% were recorded in the digester. The CH4 concentration in the biomethane gradually decreased to 87.6 ± 2.0% and 85.1 ± 1.3% at the end of stage I and II, respectively, attributed to the loss of inorganic carbon in the HRAP. The supplementation of NaHCO3 and Na2CO3 mediated an increase in the CH4 content to 90.4 ± 1.5 and 91.2 ± 0.7% in stages III and IV, respectively. Steady state CO2 removals of 90% and 88% in stages I and II, and 95.7 and 97.6% in stages III and IV, respectively, were recorded. A constant biomass productivity of 22 g m−2 d−1, set by daily harvesting 26.5 g dry algal-bacterial biomass from the bottom of the settler, was maintained concomitantly with a complete removal of the N and P supplied via centrate.

Anaerobic digestion of food waste coupled with biogas upgrading in an outdoors algal-bacterial photobioreactor at pilot scale

This work aimed at integrating the anaerobic digestion of food waste (FW) with photosynthetic biogas upgrading at pilot scale in order to obtain a high quality biomethane and a nutrient-laden algal biomass as the main byproducts from FW treatment. The performance of a 100 L anaerobic digester treating food waste integrated via raw biogas and digestate injection with a 1.2 m2 outdoors high-rate algal pond (HRAP) was evaluated. Biogas production in the digester averaged 790 ± 89 mL g VSin-1 (68 ± 8 L d-1) (35 °C, 1 bar) at a loading rate of 0.86 g VS L-1 d-1 and a steady state chemical oxygen demand removal efficiency of 83 ± 7%. The biogas produced (60% CH4 / 39% CO2) was upgraded in a 2.5 L absorption column interconnected with the HRAP via culture broth recirculation at a liquid to biogas ratio of 2, resulting in a maximum CO2 removal efficiency of 90% and a maximum CH4 content of 93.9%. The HRAP, supplied with the centrifuged liquid digestate supplemented with synthetic wastewater (5.0 ± 1.1 L d-1, Total nitrogen (TN) = 793 ± 110 mg N L-1, P-PO43- = 39 ± 19 mg P L-1), supported TN and total phosphorus maximum removal efficiencies of 100% in both cases. Pseudoanabaena sp. and Chlorella vulgaris were identified as the dominant species.

Single-step mixotrophic algal wastewater treatment: Microbial assessment of effluent

Viability of a single-step mixotrophic algal wastewater treatment (A-WWT) system in delivering discharge-quality effluent has been previously demonstrated. This report presents inactivation performance of the A-WWT system and compares it with that of high-rate algal ponds, activated sludge, and membrane bioreactors. Log reduction per unit energy input/volume of wastewater by the A-WWT system is shown to range 6.4–14.6 m3/kWh, while that in the above alternatives ranged 1.3–3.5 m3/kWh. Pathogenicity and virulence of the algal effluent and the potential for formation of disinfection byproducts (DBPs) under chlorination are discussed. Concentrations of seven of the common nitrogenous DBPs in the algal effluent were <50 ng/L.

Study from microcosms and mesocosms reveals Escherichia coli removal in high rate algae ponds during domestic wastewater treatment is primarily caused by dark decay.

While high rate algal ponds (HRAPs) can provide efficient pathogen removal from wastewater, the mechanisms involved remain unclear. To address this knowledge gap, the mechanisms potentially causing Escherichia coli (E. coli) removal during microalgae-based wastewater treatment were successively assessed using laboratory microcosms designed to isolate known mechanisms, and bench scale assays performed in real HRAP broth. During laboratory assays, E. coli decay was only significantly increased by alkaline pH (above temperature-dependent thresholds) due to pH induced toxicity, and direct sunlight exposure via UV-B damage and/or endogenous photo-oxidation. Bench assays confirmed alkaline pH toxicity caused significant decay but sunlight-mediated decay was not significant, likely due to light attenuation in the HRAP broth. Bench assays also evidenced the existence of uncharacterized 'dark' decay mechanism(s) not observed in laboratory microcosms. To numerically evaluate the contribution of each mechanism and the uncertainty associated, E. coli decay was modelled assuming dark decay, alkaline pH induced toxicity, and direct sunlight-mediated decay were independent mechanisms. The simulations confirmed E. coli decay was mainly caused by dark decay during bench assays (48.2-89.5% estimated contribution to overall decay at the 95% confidence level), followed by alkaline-pH induced toxicity (8.3-46.5%), and sunlight-mediated decay (0.0-21.9%).