A review of advances in valorization and post-treatment of anaerobic digestion liquid fraction effluent

Comprehensive review of technologies for separate digestate treatment and agricultural valorisation within circular and green economy

Anaerobic digestion (AD) has the potential to catalyse the shift from a linear to a circular economy. However, effective treatment and management of both solid (DSF) and liquid (DLF) digestate fraction treatment and management require adopting sustainable technologies to recover valuable by-products like energy, biofuels, biochar, and nutrients. This study reviews state-of-the-art advanced technologies for DSF and DLF treatment and valorisation, using life cycle assessment (LCA) and techno-economic analysis (TEA) in integrated digestate management (IDM). Key findings highlight these technologies' potential in mitigating environmental impacts from digestate management, but there's a need to improve process efficiency, especially at larger scales. Future research should prioritize cost-effective and eco-friendly IDM technologies. This review emphasizes how LCA and TEA can guide decision-making and promote sustainable agricultural practices. Ultimately, sustainable IDM technologies can boost resource recovery and advance circular economy principles, enhancing the environmental and economic sustainability of AD processes.

Ammonia oxidizing bacteria (AOB) denitrification and bacterial denitrification as the main culprit of high N2O emission in SBR with low C/N ratio wastewater

A large amount of greenhouse gas nitrous oxide (N2O) will be produced during the biological nitrogen removal process for organic wastewater of low C/N ratio. One of the effective methods to solve this problem is to incorporate inexpensive carbon source. In this study, raw wastewater (RW) from pig farm, that was not anaerobically digested, was utilized as exogenous carbon in both A/O and SBR aerobic reactor to treat liquid digestate with high ammonia nitrogen and low C/N ratio. The results showed that N2O emission in SBR was higher than that of A/O process under the same nitrogen load. The N2O conversion in the biological nitrogen removal process was investigated by the strategy of integrating stable isotope method and metagenomics. The δO18-N2O, δN15-N2O, and SP values of the SBR were closer to the denitrification values of Ammonia-Oxidizing Bacteria (AOB) than those of A/O. The abundance of AOB in the SBR reactor was higher than that in the A/O reactor, while the abundance of denitrifying bacteria was lower. The amoA/B/C gene abundance in the SBR was greater than that in the A/O, and the NOS gene abundance was the opposite. The results indicated that both AOB denitrification and bacterial denitrification led to the increase of N2O emissions of the SBR.

Emerging trends in wastewater treatment: Addressing microorganic pollutants and environmental impacts

This review focuses on the challenges and advances associated with the treatment and management of microorganic pollutants, encompassing pesticides, industrial chemicals, and persistent organic pollutants (POPs) in the environment. The translocation of these contaminants across multiple media, particularly through atmospheric transport, emphasizes their pervasive nature and the subsequent ecological risks. The urgency to develop cost-effective remediation strategies for emerging organic contaminants is paramount. As such, wastewater-based epidemiology and the increasing concern over estrogenicity are explored. By incorporating conventional and innovative wastewater treatment techniques, this article highlights the integration of environmental management strategies, analytical methodologies, and the importance of renewable energy in waste treatment. The primary objective is to provide a comprehensive perspective on the current scenario, imminent threats, and future directions in mitigating the effects of these pollutants on the environment. Furthermore, the review underscores the need for international collaboration in developing standardized guidelines and policies for monitoring and controlling these microorganic pollutants. It advocates for increased investment in research and development of advanced materials and technologies that can efficiently remove or neutralize these contaminants, thereby safeguarding environmental health and promoting sustainable practice.

Meso- and thermophilic posttreatment of press water coming from a thermophilic municipal solid waste digester

An efficient biogas production out of organic (waste) materials is important to contribute to a carbon-neutral future. In this study, thermophilic press water (PW) coming from an organic fraction of the municipal solid waste digester was further digested in a thermo- and mesophilic posttreatment approach using two semicontinuous 14 L digesters. The results showed that the PW can still have considerable high biogas potential-at least during the touristic high season in central Europe. The change in temperature led to an increase in volatile fatty acid concentrations and a decrease in biogas production in the mesophilic approach in the first days. However, the losses in biogas production at the beginning could be compensated thus there were no considerable differences in biogas production between thermo- and mesophilic posttreatment at the end of incubation. This can most probably be contributed to a change in the microbial community, and potentially problematic intermediates like valerate could be better degraded in the mesophilic reactor. Especially the abundance of representatives of the phylum Bacteroidota, like Fermentimonas spp., increased during mesophilic anaerobic digestion.

Saccharina latissima, candy-factory waste, and digestate from full-scale biogas plant as alternative carbohydrate and nutrient sources for lactic acid production

To substitute petroleum-based materials with bio-based alternatives, microbial fermentation combined with inexpensive biomass is suggested. In this study Saccharina latissima hydrolysate, candy-factory waste, and digestate from full-scale biogas plant were explored as substrates for lactic acid production. The lactic acid bacteria Enterococcus faecium, Lactobacillus plantarum, and Pediococcus pentosaceus were tested as starter cultures. Sugars released from seaweed hydrolysate and candy-waste were successfully utilized by the studied bacterial strains. Additionally, seaweed hydrolysate and digestate served as nutrient supplements supporting microbial fermentation. According to the highest achieved relative lactic acid production, a scaled-up co-fermentation of candy-waste and digestate was performed. Lactic acid reached a concentration of 65.65 g/L, with 61.69% relative lactic acid production, and 1.37 g/L/hour productivity. The findings indicate that lactic acid can be successfully produced from low-cost industrial residues.

Anaerobic digestate valorization beyond agricultural application: Current status and prospects

The flourishment of anaerobic digestion emphasizes the importance of digestate valorization, which is essential in determining the benefits of the anaerobic digestion process. Recently the perception of digestate gradually shifted from waste to products to realize the concept of circular economy and maximize the benefits of digestate valorization. Land application of digestate should be the simplest way for digestate valorization, while legislation restriction and environmental issues emphasize the necessity of novel valorization methods. This review then outlined the current methods for solid/liquid digestate valorization, nutrient recovery, microalgae cultivation, and integration with biological and thermochemical processes. The novel valorization routes proposed were summarized, with their challenges and prospects being discussed. Integrating anaerobic digestion with thermochemical methods such as hydrothermal carbonization should be a promising strategy due to the potential market value of hydrochar/biochar-derived products.

A mini-review of biomethane valorization: Managerial and policy implications for a circular resource

The green transition requires renewable energy resources, especially the role of biomass is very crucial as it promotes resource circularity if sustainable substrates are used. This mini-review focuses on green gas derived from biomass called biomethane, which appears to be strategic in the face of soaring energy costs. Hence, combined Strengths, Weaknesses, Opportunities and Threats-Analytic Hierarchy Process analysis is used to compare and evaluate the critical factors. The results provide not only methodological insights through the application of the local-global priority method, but also managerial insights that see biomethane as a winning element for the green transition, fighting climate change and reducing dependence on external energy sources. Subsidies have played a key role in pursuing economic sustainability; however, their use should be reduced over time and measured to the actual contribution related to environmental and social improvement. The results of this work highlight that biomethane development is important to tackle climate change and to be self-sufficient from an energy perspective. This development plan, based on circularity of resources, includes subsidies for small-scale plants, substrates from neighbouring territories, citizen involvement in decision-making processes, valorization of suitable waste from an environmental perspective and stability of political choices.