Thermophilic biogas production from microalgae-bacteria aggregates: biogas yield, community variation and energy balance

Trends and social aspects in the management and conversion of agricultural residues into valuable resources: A comprehensive approach to counter environmental degradation, food security, and climate change

The circular economy is essential as it encourages the reuse and recycling of resources while reducing waste, which ultimately helps to reduce environmental pollution and boosts economic efficiency. The current review highlights the management of agricultural and livestock residues and their conversion into valuable resources to combat environmental degradation and improve social well-being. The current trends in converting agricultural residues into useful resources emphasize the social benefits of waste management and conversion. It also emphasizes how waste conversion can reduce environmental degradation and enhance food security. Using agricultural residues can increase soil health and agricultural output while reducing pollution, greenhouse gas emissions, and resource depletion. Promoting sustainable waste-to-resource conversion processes requires a combination of strategies that address technical, economic, social, and environmental aspects. These multiple strategies are highlighted along with prospects and considerations.

A critical review on phycoremediation of pollutants from wastewater-a novel algae-based secondary treatment with the opportunities of production of value-added products

Though the biological treatment employing bacterial strains has wide application in effluent treatment plant, it has got several limitations. Researches hence while looking for alternative biological organisms that can be used for secondary treatment came up with the idea of using microalgae. Since then, a large number of microalgal/cyanobacterial strains have been identified that can efficiently remove pollutants from wastewater. Some researchers also found out that the algal biomass not only acts as a carbon sink by taking up carbon dioxide from the atmosphere and giving oxygen but also is a renewable source of several value-added products that can be extracted from it for the commercial use. In this work, the cleaning effect of different species of microalgae/cyanobacteria on wastewater from varied sources along with the value-added products obtained from the algal biomass as observed by researchers during the past few years are reviewed. While a number of review works in the field of phycoremediation technology was reported in literature, a comprehensive study on phycoremediation of wastewater from different industries and household individually is limited. In the present review work, the efficiency of diverse microalgal/cyanobacterial strains in treatment of wide range of industrial effluents along with municipal wastewater having multi-pollutants has been critically reviewed.

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.

An integration of algae-mediated wastewater treatment and resource recovery through anaerobic digestion

Eutrophication is one of the major emerging challenges in aquatic environment. Industrial facilities, including food, textile, leather, and paper, generate a significant amount of wastewater during their manufacturing process. Discharge of nutrient-rich industrial effluent into aquatic systems causes eutrophication, eventually disturbs the aquatic system. On the other hand, algae provide a sustainable approach to treat wastewater, while the resultant biomass may be used to produce biofuel and other valuable products such as biofertilizers. This review aims to provide new insight into the application of algal bloom biomass for biogas and biofertilizer production. The literature review suggests that algae can treat all types of wastewater (high strength, low strength, and industrial). However, algal growth and remediation potential mainly depend on growth media composition and operation conditions such as light intensity, wavelength, light/dark cycle, temperature, pH, and mixing. Further, the open pond raceways are cost-effective compared to closed photobioreactors, thus commercially applied for biomass generation. Additionally, converting wastewater-grown algal biomass into methane-rich biogas through anaerobic digestion seems appealing. Environmental factors such as substrate, inoculum-to-substrate ratio, pH, temperature, organic loading rate, hydraulic retention time, and carbon/nitrogen ratio significantly impact the anaerobic digestion process and biogas production. Overall, further pilot-scale studies are required to warrant the real-world applicability of the closed-loop phycoremediation coupled biofuel production technology.

Techno-economic analysis of single-stage and temperature-phase anaerobic co-digestion of sewage sludge, wine vinasse, and poultry manure

Anaerobic co-digestion (AcoD) is a mature and consolidated waste management technology that can transform agro-industrial by-products into biogas and digestate. This study conducted a techno-economic assessment of bioenergy and agricultural fertilizer production from AcoD of sewage sludge, wine vinasse, and poultry manure. In this case study, three configurations were investigated: i) Scenario 1, AcoD in thermophilic temperature; ii) Scenario 2, AcoD in mesophilic temperature; and iii) Scenario 3, AcoD in a temperature phase (TPAD) system, where the digestate produced in the first reactor (thermophilic) feeds the second reactor (mesophilic). The process was designed to manage 24,022 m³ wine vinasse y^-1, 24,022 m³ sewage sludge y^-1, and 480 m³ poultry manure y^-1. The major cost was the fixed capital investment for the single-stage (320,981 USD) and TPAD processes (379,698 USD). The TPAD process produced the highest electricity (1058.99 MWh y^-1) and heat (4765.47 GJ y^-1) with the lowest cost of manufacturing for electricity (84.99 USD MWh^-1), heat (0.019 USD MJ^-1), and fertilizer (30.91 USD t^-1). Regarding the profitability indicators, the highest net present value (509,011 USD) and the lowest payback time (4.24 y) were achieved for Scenario 3. In conclusion, TPAD is a profitable and sustainable waste-to-energy management technology that can be applied in a circular economy framework to recover bioenergy and fertilizer, contributing to decreasing the carbon footprint of the agri-food sector.

The effect of heat pre-treatment on the anaerobic digestion of high-solid pig manure under high organic loading level

Since more and more large-scale farms appear in China and changes in fecal sewage source disposal, the production of high-concentration solid manure waste is also increasing, and its conversion and utilization are gaining attention. This study investigated the effect of heat pre-treatment (HPT) on the thermophilic anaerobic digestion (AD) of high-solid manure (HSM). Pig manure (PM) feed with a total solids of 13% was used for the HPT and subsequent anaerobic digestion (AD) test. The HPT was carried out at 60°C, 80°C, and 100°C, respectively, for 15 min after the heating reached the set temperature. The results show that HPT led to PM feed COD solubilization, observing a maximum increase of 24.57% after pretreated at 100°C, and the treated PM feed under this condition received the maximum methane production potential of 264.64 mL·g−1 VS in batch AD test, which was 28.76% higher than that of the untreated group. Another semi-continuous AD test explored the maximum volume biogas production rate (VBPR). It involves two organic loading rates (OLR) of 13.4 and 17.8 g VSadded·L−1·d−1. The continuous test exhibited that all the HPT groups could produce biogas normally when the OLR increased to the high level, while the digester fed with untreated PM showed failure. The maximum VBPR of 4.71 L L−1·d−1 was observed from PM feed after pre-treated at 100°C and running at the high OLR. This reveals that thermal treatment can weaken the impact of a larger volume of feed on the AD system. Energy balance analysis demonstrates that it is necessary to use a heat exchanger to reuse energy in the HPT process to reduce the amount of energy input. In this case, the energy input to energy output (Ei/Eo) ranged from 0.34 to 0.55, which was much less than one, suggesting that biogas increment due to heat treatment can reasonably cover the energy consumption of the pre-treatment itself. Thus combining HPT and high-load anaerobic digestion of PM was suitable.

Low-Temperature Pretreatment of Biomass for Enhancing Biogas Production: A Review

Low-temperature pretreatment (LTPT, Temp. < 100 °C or 140 °C) has the advantages of low input, simplicity, and energy saving, which makes engineering easy to use for improving biogas production. However, compared with high-temperature pretreatment (>150 °C) that can destroy recalcitrant polymerized matter in biomass, the action mechanism of heat treatment of biomass is unclear. Improving LTPT on biogas yield is often influenced by feedstock type, treatment temperature, exposure time, and fermentation conditions. Such as, even when belonging to the same algal biomass, the response to LTPT varies between species. Therefore, forming a unified method for LTPT to be applied in practice is difficult. This review focuses on the LTPT used in different biomass materials to improve anaerobic digestion performance, including food waste, sludge, animal manure, algae, straw, etc. It also discusses the challenge and cost issues faced during LTPT application according to the energy balance and proposes some proposals for economically promoting the implementation of LTPT.

Photo fermentative biohydrogen production potential using microalgae-activated sludge co-digestion in a sequential flow batch reactor (SFBR)

Biohydrogen (bioH₂) is a sustainable energy source that can produce carbon-free energy upon combustion. BioH₂ can be generated from microalgae by photolytic and anaerobic digestion (AD) pathways. The AD pathway faces many challenges when scaling up using different bioreactors, particularly the continuous stirred tank reactor (CSTR) and sequential flow batch reactor (SFBR). Therefore, the performance characteristics of SFBR were analysed in this study using Chlorella vulgaris and domestic wastewater activated sludge (WWAS) co-culture. An organic loading rate (OLR) of 4.7 g COD L^-1 day^-1 was fed to the SFBR with a hydraulic retention time (HRT) of five days in the presence of light under anaerobic conditions. The pH of the medium was maintained at 6 using a pH controller for the incubation period of 15 days. The maximum bioH₂ concentrations of 421.1 μmol L^-1 and 56.6 μmol L^-1 were observed in the exponential and steady-state phases, respectively. The effluent had an unusually high amount of acetate of 16.6 g L^-1, which remained high with an average of 11.9 g L^-1 during the steady state phase. The amount of bioH₂ produced was found to be inadequate but consistent when operating the SFBR with a constant OLR. Because of the limitations in CSTR handling, operating a SFBR by optimizing OLR and HRT might be more feasible in operation for bioH₂ yield in upscaling. A logistic function model was also found to be the best fit for the experimental data for the prediction of bioH₂ generation using co-culture in the SFBR.

Carboxylic acids production via anaerobic fermentation: Microbial communities' responses to stepwise and direct hydraulic retention time decrease

Carboxylic acids, traditionally produced from fossil fuels, might be generated from renewable biomass resources via anaerobic fermentation. Considering that the microbial activity of this bioprocess is ruled by the imposed hydraulic retention time (HRT), this investigation explored the relationship between process stability and microbial community. Stepwise and direct HRT reduction strategies were assessed in terms of waste bioconversion into volatile fatty acids (VFAs). Microbial community dynamics revealed a microbial specialization along the HRT decrease. The direct implementation of low HRT resulted in drastic microbial fluctuations, leading to process failure at HRT below 6 days. Stepwise strategy for HRT reduction favored microbial adaptation, supporting maximum bioconversions efficiencies (32 % VFA_COD/tCOD_in) at low HRT values (HRT 4 days). Microbial similarity analysis revealed Clostridiales, Lactobacillales and Bacteroidales orders as keystone microorganisms involved in VFAs production, being responsible for protein degradation and propionic acid accumulation.

Respirometry tests in wastewater treatment: Why and how? A critical review

Respirometry tests are a widely employed method in wastewater treatment field to characterize wastewater streams, assess toxic/inhibitory effects to the biomass, calibrate mathematical models. Respirometry can allow to fractionize the chemical oxygen demand (COD) in biodegradable and inert fractions, but also provide information related to biomass kinetics and stoichiometry through standardized laboratory techniques. Considering the increasing number of emerging contaminants detected in wastewater effluents, such as pharmaceuticals, personal care products and pesticides, respirometry can be a useful tool to promptly assess any toxic or inhibitory effect in wastewater treatment plants (WWTPs) operations. Beside conventional activated sludge (CAS), in recent years respirometric methods have been applied to innovative fields, such as moving-bed bio-reactors (MBBRs), fungi and microalgae, exploiting natural remediation methods. In particular, respirometry application to microalgae, through the so-called photo-respirometry, has been investigated in the latest years in the treatment of high-nutrient loaded streams, allowing resource recovery in biomass form. In this work, respirometric methods are first introduced from a theoretical basis and then critically discussed by considering the experimental apparatus, the available characterization protocols and the fields of application; the most recent literature findings on respirometry are coupled with authors' experience in the field. A comparison between physicochemical methods and respirometry is made, considering common protocols for WWTP modelling and calibration. The future research needed on the topic is finally outlined, including the coupling of respirometry with microbial community analysis, potentially leading to an enhanced process understanding, an extended respirometry utilization to get specific kinetic and stoichiometric parameters for modelling purposes, and a wider respirometry application as diagnosis tool in WWTP operations.