Comparative study of the effect of loading increments on the mesophilic codigestion of waste activated sludge and food waste: Reactor performance, stability analysis, and microbial community

Co-digestion and co-treatment of sewage and organic waste in mainstream anaerobic reactors: operational insights and future perspectives

The global shift towards sustainable waste management has led to an intensified exploration of co-digestion and co-treatment of sewage and organic waste using anaerobic reactors. This review advocates for an integrated approach where organic waste is treated along with the sewage stream, as a promising solution to collect, treat, and dispose of organic waste, thereby reducing the environmental and economic burden on municipalities. Various efforts, ranging from laboratory to full-scale studies, have been undertaken to assess the feasibility and impacts of co-digestion or co-management of sewage and organic waste, using technologies such as up-flow anaerobic sludge blankets or anaerobic membrane bioreactors. However, there has been no consensus on a standardized definition of co-digestion, nor a comprehensive understanding of its impacts. In this paper, we present a comprehensive review of the state-of-the-art in liquid anaerobic co-digestion systems, which typically operate at 1.1% total solids. The research aims to investigate how the integration of organic waste into mainstream anaerobic-based sewage treatment plants has the potential to enhance the sustainability of both sewage and organic waste management. In addition, utilizing the surplus capacity of existing anaerobic reactors leads to significant increases in methane production ranging from 190 to 388% (v/v). However, it should be noted that certain challenges may arise, such as the necessity for the development of tailored strategies and regulatory frameworks to enhance co-digestion practices and address the inherent challenges.

Synergistic effect of hydrothermal sludge and food waste in the anaerobic co-digestion process: microbial shift and dewaterability

While thermal hydrolysis technology is commonly employed for sewage sludge treatment in extensive wastewater treatment facilities, persistent challenges remain, including issues such as ammonia-induced digestive inhibition and reduced productivity stemming from nutrient deficiency within the hydrothermal sludge. In this study, the effects of hydrothermal sludge-to-food waste mixing ratios and fermentation temperatures on anaerobic co-digestion were systematically investigated through a semi-continuous experiment lasting approximately 100 days. The results indicated that anaerobic co-digestion of hydrothermal sludge and food waste proceeded synergistically at any mixing ratio, and the synergistic effect is mainly attributed to the improvement of carbohydrate removal and digestive system stability. However, thermophilic digestion did not improve the anaerobic performance and methane yield. On the contrary, mesophilic digestion performed better in terms of organic matter removal, especially in the utilization of soluble carbohydrates, soluble proteins, and VFAs. Microbial community analysis revealed that the transition from mesophilic to thermophilic anaerobic co-digestion prompts changes in the methane-producing pathways. Specifically, the transition entails a gradual shift from pathways involving acetoclastic and hydrogenotrophic methanogenesis to a singular hydrogenotrophic methanogenesis pathway. This shift is driven by thermodynamic tendencies, as reflected in Gibbs free energy, as well as environmental factors like ammonia nitrogen and volatile fatty acids. Lastly, it is worth noting that the introduction of food waste did lead to a reduction in cake solids following dewatering. Nevertheless, it was observed that thermophilic digestion had a positive impact on dewatering performance.

Promoting the Circular Economy on an Island: Anaerobic Co-Digestion of Local Organic Substrates as a Possible Renewable Energy Source

The local waste co-digestion is an interesting option to tackle in reduced and isolated areas like the islands. The islands have limited territory and scarce fuel production. Moreover, organic waste can create serious environmental problems in soil, water and air. Anaerobic co-digestion (AcoD) is a technology fulfilling the concept of waste-to-energy (WtE) based on local resources. The valorisation of organic waste through AcoD on an island would prevent environmental impacts, while being a source of renewable energy. In this study, cow manure (outdoor and indoor), pig slurry, bird manure, kitchen waste, sewage sludge and oily lacteous waste produced on Island Terceira (Portugal) were tested in mesophilic -35 °C- Biochemical Methane Potential (BMP) co-digestion assays. The goals were to analyse the recalcitrant and high potential produced waste and to estimate the energetic supply source on the island. The cow manure and pig slurry were used as inocula and specific methanogenic activities (SMAs) were carried out. The results showed that both substrates have a significant methanogenic activity-SMA 0.11 g-COD/(g-VSS.d) and 0.085 g-COD/(g-VSS.d), respectively. All the studied combinations were feasible in AcoD, showing TS removals in the range of 19-37%; COD removals in the range 67-78% and specific methane yields from 0.14 to 0.22 L/gCOD removed, but some differences were found. The modified Gompertz model fitted the AcoD assays (R² 0.982-0.998). The maximum biogas production rate, Rmax. was highest in the AcoD of Cow+Pig+Oily and in the Cow+Pig+Sludge with 0.017 and 0.014 L/g-VS_added.day, respectively, and the lowest in Cow+Pig+Bird with 0.010 L/g-VS_added. In our AcoD studies, the bird manure limited the performance of the process, since it was recalcitrant to anaerobic degradation. On the other hand, the oily lacteous waste showed a great potential in the anaerobic digestion. The estimated biogas production, from the best-studied condition, could cover the 11.4% of the energy supply of the inhabitants. These preliminary results would prevent the environmental impact of organic waste on the island and promote the use of local waste in a circular economy scenario.