Vermifiltration as a green solution to promote digestate reuse in agriculture in small-scale farms.

Digestates from low-tech digesters need to be post-treated to ensure their safe agricultural reuse. This study evaluated, for the first time, vermifiltration as a post-treatment for the digestate from a low-tech digester implemented in a small-scale farm, treating cattle manure and cheese whey under psychrophilic conditions. Vermifiltration performance was monitored in terms of solids, organic matter, nutrients, and pathogens removal efficiency. In addition, the growth of earthworms (Eisenia foetida) and their role in the process was evaluated. Finally, the vermicompost and the effluent of the vermifilter were characterized in order to assess their potential reuse in agriculture. Vermifilters showed high removal efficiency of chemical oxygen demand (55-90%), total solids (60-80%), ammonium nitrogen (83-97%), and phosphate-P (28-49%). Earthworms effectively grew and reproduced on digestate (i.e. earthworms number increased by 183%), enhancing the vermifiltration performance, while reducing clogging and odour-related issues. Both the vermicompost and effluent produced complied with legislation limits established for soil improvers and wastewater for fertigation, respectively. Indeed, there was an absence of pathogens and non-detectable heavy metals concentrations. Vermifiltration may be thus considered a suitable post-treatment option for the digestate from low-tech digesters, allowing for its safe agricultural reuse and boosting the circular bioeconomy in small-scale farms.

Resource recovery and contaminants of emerging concern mitigation by microalgae treating wastewater.

This study aimed to investigate the recovery of agricultural biostimulants and biogas from microalgae treating wastewater, in the framework of a circular bioeconomy. To this end, municipal wastewater was treated in demonstrative raceway ponds, and microalgal biomass (Scenedesmus sp.) was then harvested and downstream processed to recover biostimulants and biogas in a biorefinery approach. The effect of microalgal biostimulants on plants was evaluated by means of bioassays, while the biogas produced was quantified in biochemical methane potential (BMP) tests. Furthermore, the fate of contaminants of emerging concern (CECs) over the process was also assessed. Bioassays confirmed the biostimulant effect of microalgae, which showed gibberellin-, auxin- and cytokinin-like activity in watercress seed germination, mung bean rooting, and wheat leaf chlorophyll retention. In addition, the downstream process applied to raw biomass acted as a pre-treatment to enhance anaerobic digestion performance. After biostimulant extraction, the residual biomass represented 91% of the methane yield from the raw biomass (276 mLCH4·g-1VS). The kinetic profile of the residual biomass was 43% higher than that of the unprocessed biomass. Co-digestion with primary sludge further increased biogas production by 24%. Finally, the concentration of CECs in wastewater was reduced by more than 80%, and only 6 out of 22 CECs analyzed were present in the biostimulant obtained. Most importantly, the concentration of those contaminants was lower than in biosolids that are commonly used in agriculture, ensuring environmental safety.

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.

Assessment of selenium and zinc enriched sludge and duckweed as slow-release micronutrient biofertilizers for Phaseolus vulgaris growth.

Selenium (Se) and zinc (Zn) are essential micronutrients that are often lacking in the diet of humans and animals. Application of mineral Se and Zn fertilizers into soils may lead to a waste of Se and Zn due to the fast leaching and low utilization by plants. Slow-release Se and Zn biofertilizer may therefore be beneficial. This study aims to assess the potential of SeZn-enriched duckweed and sludge produced from wastewater as slow-release Se and Zn biofertilizers. Pot experiments with green beans (Phaseolus vulgaris) and sampling of Rhizon soil pore water were conducted to evaluate the bioavailability of Se and Zn in sandy and loamy soils mixed with SeZn-enriched duckweed and sludge. Both the Se and Zn concentrations in the soil pore water increased upon amending the two biomaterials. The concentration of Se released from SeZn-enriched duckweed rapidly decreased in the first 21 days and slowly declined afterwards, while it remained stable during the entire experiment upon application of SeZn-enriched sludge. The Zn content in the soil pore water gradually increased over time. The application of SeZn-enriched duckweed and sludge significantly increased the Se concentrations in plant tissues, in particular in the form of organic Se-methionine in seeds, without a negative impact on plant growth when an appropriate dose was applied (1 mg Se/kg soil). While, it did not increase Zn concentrations in plant seeds. The results indicate that the SeZn-enriched duckweed and sludge could be only used as organic Se biofertilizers for Se-deficient soils. Particularly, the SeZn-enriched sludge dominated with elemental nano-Se was an effective Se source and slow-release Se biofertilizer. These results could offer a theoretical reference to choose an alternative to chemical Se fertilizers for biofortification, avoiding the problem of Se losses by leaching from mineral Se fertilizers while recovering resources from wastewater. This could contribute to the driver for a future circular economy.

Selenate and selenite uptake, accumulation and toxicity in Lemna minuta.

The kinetics of Se uptake and toxicity to Lemna were studied over a period of 14 days of exposure to Se(IV) or Se(VI). The growth of Lemna stopped immediately after exposure to 5.0 mg/L of Se(IV) or Se(VI). The content of chlorophyll and phaeopigments of Lemna exposed to 5.0 mg/L of Se(IV) was two to three times less than in the control after 3 d exposure. Lemna took up Se rapidly within the first 3 d. The Se content in Lemna along with the exposure time fitted well the two-compartment and the hyperbolic model, which demonstrates that the mechanism of Se(IV) and Se(VI) uptake in Lemna is not only through passive diffusion, but also through other processes such as ion channel proteins or transporters. The kinetic bioconcentration factors (BCFs) were 231 and 42 for 0.5 mg/L Se(IV) and Se(VI) exposure, respectively. The uptake rate of Lemna reached 263 mg/kg/d and 28 mg/kg/d in the Se(IV) and Se(VI) treatments, respectively. This study showed that Se(IV) has a faster accumulation rate than Se(VI), but a higher toxicity, indicating Lemna could be a good candidate to remove Se(IV) from water, producing Se-enriched biomass which may eventually also be considered for use as Se-enriched feed supplement or fertilizer.

Strategies to Optimize Microalgae Conversion to Biogas: Co-Digestion, Pretreatment and Hydraulic Retention Time.

This study aims at optimizing the anaerobic digestion (AD) of biomass in microalgal-based wastewater treatment systems. It comprises the co-digestion of microalgae with primary sludge, the thermal pretreatment (75 °C for 10 h) of microalgae and the role of the hydraulic retention time (HRT) in anaerobic digesters. Initially, a batch test comparing different microalgae (untreated and pretreated) and primary sludge proportions showed how the co-digestion improved the AD kinetics. The highest methane yield was observed by adding 75% of primary sludge to pretreated microalgae (339 mL CH4/g VS). This condition was then investigated in mesophilic lab-scale reactors. The average methane yield was 0.46 L CH4/g VS, which represented a 2.9-fold increase compared to pretreated microalgae mono-digestion. Conversely, microalgae showed a low methane yield despite the thermal pretreatment (0.16 L CH4/g VS). Indeed, microscopic analysis confirmed the presence of microalgae species with resistant cell walls (i.e., Stigioclonium sp. and diatoms). In order to improve their anaerobic biodegradability, the HRT was increased from 20 to 30 days, which led to a 50% methane yield increase. Overall, microalgae AD was substantially improved by the co-digestion with primary sludge, even without pretreatment, and increasing the HRT enhanced the AD of microalgae with resistant cell walls.

Microalgae conversion to biogas: thermal pretreatment contribution on net energy production.

Microalgal biomass harvested from wastewater treatment high rate algal ponds may be valorised through anaerobic digestion producing biogas. However, microalgae anaerobic biodegradability is limited by their complex cell wall structure. Thus, pretreatment techniques are being investigated to improve microalgae methane yield. In the current study, thermal pretreatment at relatively low temperatures of 75-95 °C was effective at enhancing microalgae anaerobic biodegradability; increasing the methane yield by 70% in respect to nonpretreated biomass. Microscopic images showed how the pretreatment damaged microalgae cells, enhancing subsequent anaerobic digestion. Indeed, digestate images showed how after pretreatment only species with resistant cell walls, such as diatoms, continued to be present. Energy balances based on lab-scale reactors performance at 20 days HRT, shifted from neutral to positive (energy gain around 2.7 GJ/d) after thermal pretreatment. In contrast with electricity consuming pretreatment methods, such as microwave irradiation, thermal pretreatment of microalgae seems to be scalable.

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.

Phycobiliprotein recovery coupled to the tertiary treatment of wastewater in semi-continuous photobioreactors. Tracking contaminants of emerging concern.

This study evaluated a tertiary wastewater treatment technology using cyanobacteria to recover value-added phycobiliproteins. The presence of contaminants of emerging concern (CECs) in wastewater, cyanobacterial biomass and pigments recovered were also analyzed. For this, a wastewater-borne cyanobacterium (Synechocystis sp. R2020) was used to treat secondary effluent from a municipal wastewater treatment plant, with and without nutrients supplementation. Then, the stability of phycobiliprotein production was assessed by operating the photobioreactor in semi-continuous mode. Results showed similar biomass productivity with and without nutrients supplementation (153.5 and 146.7 mg L-1 d-1, respectively). Upon semi-continuous operation, the phycobiliprotein content was stable and reached up to 74.7 mg gDW-1. The phycocyanin purity ratio ranged from 0.5 to 0.8, corresponding to food grade (>0.7). Out of 22 CECs detected in secondary effluent, only 3 were present in the phycobiliprotein extracts. In order to identify applications, prospective research should focus on CECs removal during pigment purification.

Microalgae harvesting for wastewater treatment and resources recovery: A review.

Microalgae-based wastewater treatment has been conceived to obtain reclaimed water and produce microalgal biomass for bio-based products and biofuels generation. However, microalgal biomass harvesting is challenging and expensive, hence one of the main bottlenecks for full-scale implementation. Finding an integrated approach that covers concepts of engineering, green chemistry and the application of microbial anabolism driven towards the harvesting processes, is mandatory for the widespread establishment of full-scale microalgae wastewater treatment plants. By using nature-based substances and applying concepts of chemical functionalization in already established harvesting methods, the costs of harvesting processes could be reduced while preventing microalgae biomass contamination. Moreover, microalgae produced during wastewater treatment have unique culture characteristics, such as the consortia, which are primarily composed of microalgae and bacteria, that should be accounted for prior to downstream processing. The aim of this review is to examine recent advances in microalgal biomass harvesting and recovery in wastewater treatment systems, considering the impact of consortia variability. The costs of available harvesting technologies, such as coagulation/flocculation, coupled to sedimentation and differential air flotation, are provided. Additionally, promising technologies are discussed, including autoflocculation, bioflocculation, new filtration materials, nanotechnology, microfluidic and magnetic methods.

Production of phycobiliproteins, bioplastics and lipids by the cyanobacteria Synechocystis sp. treating secondary effluent in a biorefinery approach.

Cyanobacteria have been identified as promising organisms to reuse nutrients from waste effluents and produce valuable compounds such as lipids, polyhydroxyalkanoates (PHAs), and pigments. However, almost all studies on cyanobacterial biorefineries have been performed under lab scale and short cultivation periods. The present study evaluates the cultivation of the cyanobacterium Synechocystis sp. in a pilot scale 30 L semi-continuous photobioreactor fed with secondary effluent for a period of 120 days to produce phycobiliproteins, polyhydroxybutyrate (PHB) and lipids. To this end, the harvested biomass from the semi-continuous photobioreactor was transferred into 5 L vertical column batch photobioreactors to perform PHB and lipid accumulation under nutrient starvation. Three hydraulic retention times (HRT) (6, 8 and 10 days) were tested in the semi-continuous photobioreactor to evaluate its influence on biomass growth and microbial community. A maximum biomass concentration of 1.413 g L-1 and maximum productivity of 173 mg L-1 d-1 was reached under HRT of 8 days. Microscopy analysis revealed a shift from Synechocystis sp. to Leptolyngbya sp. and green algae when HRT of 6 days was used. Continuous, stable production of phycobiliproteins in the semi-continuous photobioreactor was obtained, reaching a maximum content of 7.4%dcw in the biomass. In the batch photobioreactors a PHB content of 4.8%dcw was reached under 7 days of nitrogen and phosphorus starvation, while a lipids content of 44.7%dcw was achieved under 30 days of nitrogen starvation. PHB and lipids production was strongly dependent on the amount of nutrients withdrawn from the grow phase. In the case of lipids, their production was stimulated when there was only phosphorus depletion. While Nitrogen and phosphorus limitation was needed to enhance the PHB production. In conclusion, this study demonstrates the feasibility of cultivating cyanobacteria in treated wastewater to produce bio-based valuable compounds within a circular bioeconomy approach.

Selenium recovery from wastewater by the green microalgae Chlorella vulgaris and Scenedesmus sp.

Selenium (Se) is an important element for many living organisms and its supplementation may be needed in food, feed, and soil to make up for its deficiency. At the same time, high selenium concentrations can harm the environment, thus its management in sewage and the study of its removal from waste streams are important. Microalgae-based systems may be used for wastewater treatment and nutrients recovery, while producing biomass for bioproducts or bioenergy. In this study, Chlorella vulgaris and Scenedesmus sp. grown in urban wastewater with different selenium concentrations (50-1000 µg Se/L) were evaluated for their resistance and selenium removal/recovery efficiency. Chlorella vulgaris and Scenedesmus sp. were able to remove up to 43 and 52 % of Se from wastewater, respectively. Chlorella vulgaris accumulated up to 323 mgSe/kg DW (in urban wastewater with 1000 µg Se/L). The Se-rich biomass produced may be applied to the supplementation of animal feed or used for biofortification of crops.

Life cycle assessment of microalgae systems for wastewater treatment and bioproducts recovery: Natural pigments, biofertilizer and biogas.

The aim of this study was to assess the potential environmental impacts associated with microalgae systems for wastewater treatment and bioproducts recovery. In this sense, a Life Cycle Assessment was carried out evaluating two systems treating i) urban wastewater and ii) industrial wastewater (from a food industry), with the recovery of bioproducts (i.e. natural pigments and biofertilizer) and bioenergy (i.e. biogas). Additionally, both alternatives were compared to iii) a conventional system using a standard growth medium for microalgae cultivation in order to show the potential benefits of using wastewater compared to typical cultivation approaches. The results indicated that the system treating industrial wastewater with unialgal culture had lower environmental impacts than the system treating urban wastewater with mixed cultures. Bioproducts recovery from microalgae wastewater treatment systems can reduce the environmental impacts up to 5 times compared to a conventional system using a standard growth medium. This was mainly due to the lower chemicals consumption for microalgae cultivation. Food-industry effluent showed to be the most promising scenario for bioproducts recovery from microalgae treating wastewater, because of its better quality compared to urban wastewater which also allows the cultivation of a single microalgae species. In conclusion, microalgae wastewater treatment systems are a promising solution not only for wastewater treatment but also to boost the circular bioeconomy in the water sector through microalgae-based product recovery.

The use of solar pre-treatment as a strategy to improve the anaerobic biodegradability of microalgal biomass in co-digestion with sewage.

Sustainable sewage treatment plants (STPs) have been intensively investigated in search for low-cost, environmental-friendly options. Anaerobic-aerobic treatment solutions, as upflow anaerobic sludge blanket (UASB) reactors followed by high rate algal ponds (HRAP) have already proved to be efficient for pollutants and micropollutants removal, as well as for energy recovery from the co-digestion of raw sewage and microalgal biomass. Since microalgae cells have complex structures that make them resistant to anaerobic digestion, pre-treatment techniques may be applied to improve microalgal biomass solubilisation and methane yield. Among the thermal pre-treatments, the use of solar energy for biomass solubilisation has yet to be investigated. Therefore, this study aimed at evaluating the performance of a solar thermal microalgal biomass pre-treatment prior to the anaerobic co-digestion with raw sewage, comparing a UASB reactor feed only raw sewage and other UASB reactor feed with raw sewage and pre-treated microalgal biomass. The results showed that, the solar pre-treatment step reached an organic matter solubilisation of 32% (COD). Furthermore, the methane yield was increased by 45% (from 81 to 117 NL CH4 kg-1 COD), after the anaerobic co-digestion with pre-treated microalgae as compared to the mono-digestion of raw sewage, indicating significant difference between the evaluated UASB reactors. The energy assessment showed a positive energy balance, as the total energy produced was twice the energy consumed in the system.

Benefits and risks of agricultural reuse of digestates from plastic tubular digesters in Colombia.

The aim of this study is to characterize the digestates from three plastic tubular digesters implemented in Colombia fed with: i) cattle manure; ii) cattle manure mixed with cheese whey; iii) pig manure. All the digesters worked under psychrophilic conditions. Physico-chemical characteristics, heavy metals, pathogens, and agronomic quality were investigated. All the digestates were characterized by physico-chemical characteristics and nutrients concentration suitable for their reuse as biofertilizer. However, these digestates may only partially replace a mineral fertilizer due to the high nutrients dilution. Heavy metals were under the detection limit of the analytical method (Pb, Hg, Ni, Mo, Cd, Chromium VI) or present at low concentration (Cu, Zn, As, Se) in all the digestates. Biodegradable organic matter and pathogens (coliform, helminths and Salmonella spp.) analysis proved that all the digestates should be post-treated before soil application in order to prevent environmental and health risks, and also to reduce residual phytotoxicity effects. The digestate from pig manure had a higher nutrient percentage (0.2, 0.6 and 0.05 % w/w of total N, P2O5 and K2O, respectively), but also higher residual phytotoxicity than the other digestates. Co-digestion seemed not to significantly improve the digestate fertilizing potential. Finally, further studies should address how to improve fertilizing potential of digestates from plastic tubular digesters, avoiding environmental and health risks.

Health and environmental impacts of drinking water choices in Barcelona, Spain: A modelling study.

Quantitative evidence of health and environmental tradeoffs between individuals' drinking water choices is needed to inform decision-making. We evaluated health and environmental impacts of drinking water choices using health impact and life cycle assessment (HIA, LCA) methodologies applied to data from Barcelona, Spain. We estimated the health and environmental impacts of four drinking water scenarios for the Barcelona population: 1) currently observed drinking water sources; a complete shift to 2) tap water; 3) bottled water; or 4) filtered tap water. We estimated the local bladder cancer incidence attributable to trihalomethane (THM) exposure, based on survey data on drinking water sources, THM levels, published exposure-response functions, and disability-adjusted life years (DALYs) from the Global Burden of Disease 2017. We estimated the environmental impacts (species lost/year, and resources use) from waste generation and disposal, use of electricity, chemicals, and plastic to produce tap or bottled drinking water using LCA. The scenario where the entire population consumed tap water yielded the lowest environmental impact on ecosystems and resources, while the scenario where the entire population drank bottled water yielded the highest impacts (1400 and 3500 times higher for species lost and resource use, respectively). Meeting drinking water needs using bottled or filtered tap water led to the lowest bladder cancer DALYs (respectively, 140 and 9 times lower than using tap water) in the Barcelona population. Our study provides the first attempt to integrate HIA and LCA to compare health and environmental impacts of individual water consumption choices. Our results suggest that the sustainability gain from consuming water from public supply relative to bottled water may exceed the reduced risk of bladder cancer due to THM exposure from consuming bottled water in Barcelona. Our analysis highlights several critical data gaps and methodological challenges in quantifying integrated health and environmental impacts of drinking water choices.

Valorization of selenium-enriched sludge and duckweed generated from wastewater as micronutrient biofertilizer.

Selenium (Se) is an essential trace element for humans and animals with a narrow window between deficiency and toxicity levels. Application of conventional chemical Se fertilizers to increase the Se content of crops in Se deficient areas could result in environmental contamination due to the fast leaching of inorganic Se. Slow-release Se-enriched biofertilizers produced from wastewater treatment may therefore be beneficial. In this study, the potential of Se-enriched biomaterials (sludge and duckweed) as slow-release Se biofertilizers was evaluated through pot experiments with and without planted green beans (Phaseolus vulgaris). The Se concentration in the bean tissues was 1.1-3.1 times higher when soils were amended with Se-enriched sludge as compared to Se-enriched duckweed. The results proved that the Se released from Se-enriched biomaterials was efficiently transformed to health-beneficial selenoamino acids (e.g., Se-methionine, 76-89%) after being taken up by beans. The Se-enriched sludge, containing mainly elemental Se, is considered as the preferred slow-release Se biofertilizer and an effective Se source to produce Se-enriched crops for Se-deficient populations, as shown by the higher Se bioavailability and lower organic carbon content. This study could offer a theoretical reference to choose an environmental-friendly and sustainable alternative to conventional mineral Se fertilizers for biofortification, avoiding the problem of Se losses by leaching from chemical Se fertilizers while recovering resources from wastewater. This could contribute to the driver for a future circular economy.

Production of selenium-enriched microalgae as potential feed supplement in high-rate algae ponds treating domestic wastewater.

This study assessed the selenium (Se) removal efficiency of two pilot-scale high-rate algae ponds (HRAPs) treating domestic wastewater and investigated the production of Se-enriched microalgae as potential feed supplement. The HRAP-Se had an average Se, NH4+-N, total phosphorus and COD removal efficiency of, respectively, 43%, 93%, 77%, and 70%. Inorganic Se taken up by the microalgae was mainly (91%) transformed to selenoamino acids, and 49-63% of Se in the Se-enriched microalgae was bioaccessible for animals. The crude protein content (48%) of the microalgae was higher than that of soybeans, whereas the essential amino acid content was comparable. Selenium may induce the production of the polyunsaturated fatty acids omega-3 and omega-6 in microalgae. Overall, the production of Se-enriched microalgae in HRAPs may offer a promising alternative for upgrading low-value resources into high-value feed supplements, supporting the drive to a circular economy.

Pretreatment and co-digestion of microalgae, sludge and fat oil and grease (FOG) from microalgae-based wastewater treatment plants.

The aim of this study was to assess the co-digestion of residual biomass flows generated in microalgae-based wastewater treatment plants: microalgae, primary sludge and fat, oil and grease (FOG), with and without microalgae thermal pretreatment. The results evidenced the high methane yield of FOG (563 mL CH4/g VS) as compared to microalgae (140 mL CH4/gVS) and sludge (299 mL CH4/g VS). The methane yield of microalgae and sludge co-digestion (50-50% VS) was increased by 25 and 42% by adding 10 and 20% VS of FOG, respectively. Moreover, co-digestion trials improved the anaerobic digestion first-order kinetics by up to 67%. Regarding the thermal pretreatment, it increased the methane yield of microalgae by 60%, and 15% upon co-digestion with sludge and FOG. Therefore, co-digestion of microalgae, primary sludge and FOG appears as a promising strategy to enhance the biogas production, hence bioenergy recovery from wastewater, even without pretreatment.

Upflow anaerobic sludge blanket in microalgae-based sewage treatment: Co-digestion for improving biogas production.

Upflow anaerobic sludge blanket (UASB) reactors are widely used to treat domestic sewage and frequently require post-treatment. Little is known about the use of high rate algal ponds (HRAP) for post-treating UASB reactors' effluent. This study aimed to evaluate a UASB reactor followed by a HRAP in terms of sewage treatment efficiency and biogas production, during one year at demonstration-scale. The UASB reactor co-treated raw sewage and the harvested microalgal biomass from the HRAP, which was recirculated to the reactor. An identical UASB reactor, treating only raw sewage, was used as control. The results showed an overall removal of 65% COD and 61% N-NH4 in the system. Furthermore, methane yield was increased by 25% after anaerobic co-digestion with microalgae, from 156 to 211 NL CH4 kg-1 VS. An energy assessment was performed and showed a positive energy balance, with a net ratio of 2.11 to the annual average.