Background: Slow degradation kinetics of long-chain fatty acids (LCFA) and their accumulation in anaerobic digesters disrupt methanogenic activity and biogas production at high loads of waste lipids. In this study, we evaluated the effect of effluent solids recirculation on microbial LCFA (oleate) degradation capacity in continuous stirred-tank sludge digesters, with the overall aim of providing operating conditions for efficient co-digestion of waste lipids. Furthermore, the impacts of LCFA feeding frequency and sulfide on process performance and microbial community dynamics were investigated, as parameters that were previously shown to be influential on LCFA conversion to biogas. Results: Effluent solids recirculation to municipal sludge digesters enabled biogas production of up to 78% of the theoretical potential from 1.0 g oleate l⁻¹ day⁻¹. In digesters without effluent recirculation, comparable conversion efficiency could only be reached at oleate loading rates up to 0.5 g l⁻¹ day⁻¹. Pulse feeding of oleate (supplementation of 2.0 g oleate l⁻¹ every second day instead of 1.0 g oleate l⁻¹ every day) did not have a substantial impact on the degree of oleate conversion to biogas in the digesters that operated with effluent recirculation, while it marginally enhanced oleate conversion to biogas in the digesters without effluent recirculation. Next-generation sequencing of 16S rRNA gene amplicons of bacteria and archaea revealed that pulse feeding resulted in prevalence of fatty acid-degrading Smithella when effluent recirculation was applied, whereas Candidatus Cloacimonas prevailed after pulse feeding of oleate in the digesters without effluent recirculation. Combined oleate pulse feeding and elevated sulfide level contributed to increased relative abundance of LCFA-degrading Syntrophomonas and enhanced conversion efficiency of oleate, but only in the digesters without effluent recirculation. Conclusions: Effluent solids recirculation improves microbial LCFA degradation capacity, providing possibilities for co-digestion of larger amounts of waste lipids with municipal sludge.

The textile industry is one of the largest environmental polluters in the world. Although waste management via anaerobic digestion (AD) is a sustainable strategy to transform waste into clean energy and water recovery, the efficiency of the AD process is reduced by the presence of recalcitrant materials, chemicals, and toxic contents. This study aims to investigate the performance of several chemical, physical, and biological pretreatments applied to improve the biodegradability of textile waste. We performed a meta-analysis with 117 data extracted from 13 published articles that evaluated the efficiency of pretreatments applied to textile waste prior to AD to increase biogas production measured as methane (CH₄) yield. Even though the majority of the studies have focused on the effect of chemical and physical pretreatments, our results showed that the application of biological pretreatments are more efficient and eco-friendlier. Biological pretreatments can increase CH₄ yield by up to 360% with lower environmental risk and lower operating costs, while producing clean energy and a cleaner waste stream. Biological pretreatments also avoid the addition of chemicals and favor the reuse of textile wastewater, decreasing the current demand for clean water and increasing resource circularity in the textile industry.

The large global generation and improper management of waste lead to the pollution of the environment and efforts toward reducing the impacts of anthropogenic activities on aquatic environments should be prioritized. The United Nations declared 2018-2028 as the international decade for action on “Water for Sustainable Development” and integrated management of water resources. Several international initiatives, such as the UN 2030 Agenda, the Sendai Framework for Disaster Risk Reduction and the Paris Agreement, have highlighted and strongly recommended the development of new technologies to reverse the current environmental scenario of global water bodies. The use of anaerobic digestion for treating organic wastes can minimize and avoid several adverse effects on aquatic environments while promoting nutrient cycling and the production of biogas, a renewable energy source that can replace fossil fuels and therefore decrease the emission of greenhouse gases. We performed a systematic review to evaluate the contribution of anaerobic digestion in preventing and reducing human impacts on aquatic ecosystems. China (15.1%), Spain (7.3%) and Italy (7.3%) are countries with a pronounced research focus on this topic, indicating their awareness on the importance of managing and preserving their water resources. The integration of co-digestion and pretreatment methods into anaerobic digestion improved the production of byproducts (especially energy and biofertilizer). Thus, this review highlights the success of AD technology as a waste treatment strategy, while reducing the damage inflicted to aquatic systems and its consequences to human health and aquatic biodiversity.

Anaerobic digestion (AD) of livestock manure stands out as a sustainable approach for energy generation and nutrient recovery while mitigating greenhouse gas emissions and reducing potential contaminants. However, the recalcitrant lignocellulosic content in cattle manure poses a challenge to methane (CH ₄ ) production, requiring pretreatments to enhance biodegradability. While previous studies have investigated manure pretreatment using various alkali agents and temperature conditions, the effects of Ca(OH) ₂ as an alkaline source combined with moderate-temperatures over extended durations still need to be explored. This study evaluated the effects of alkaline, thermal, and combined thermal-alkaline pretreatments on CH ₄ production from cattle manure and assessed the cost–benefit associated with the implementation of these strategies. Pretreatments were conducted at moderate temperatures (55 °C and 70 °C) to minimize energy consumption. The highest CH ₄ yields were achieved with the combined thermal-alkaline and alkaline (9% Ca(OH) ₂ based on dry matter) pretreatments, increasing methane production by up to 15.2 ± 3.3% and 10.6 ± 2.5%, respectively, compared to raw manure. In contrast, thermal pretreatment alone did not significantly enhance CH ₄ production. The economic assessment indicated that the net revenue for the thermal-alkaline pretreatment was negative, whereas alkaline pretreatment showed financial feasibility, surpassing conventional AD without pretreatment. These findings highlight alkaline pretreatment as a promising strategy for enhancing CH ₄ production while maintaining economic feasibility in biogas systems.