Rheological properties and dewaterability of anaerobic co-digestion with sewage sludge and food waste: effect of thermal hydrolysis pretreatment and mixing ratios

Overcoming deep-dewatering challenges in food waste digestate with polyethylene oxide as an innovative conditioning agent

The effective treatment of food waste digestate is critical for reducing environmental pollution and mitigating carbon emissions, with deep dewatering playing a pivotal role. Conventional dewatering agents such as polyaluminum chloride (PAC) and polyacrylamide (PAM), commonly employed in municipal sludge treatment, exhibit limited efficacy when applied to food waste digestate due to the latter's high salinity and advanced fermentation stages. This study introduces polyethylene oxide (PEO) as a novel conditioning agent and investigates its dewatering performance in comparison to PAC and PAM, elucidating the underlying mechanism. PEO conditioning markedly improves deep-dewatering, reducing digestate moisture content from 93.11 % to 56.71 % and lowering specific resistance to filtration (SRF) by 90.3 %. In contrast, PAM, PAC, and their combination achieve moisture reductions to 81.18 %, 84.49 %, and 87.07 %, respectively, with significantly lower SRF improvements. PEO promotes the release of bound water by weakening solid-liquid binding energy, facilitating the transition of bound water to free water and enhancing overall water mobility. Moreover, compressibility coefficient analyses and X-ray computed tomography (X-CT) reveal that PEO treatment significantly increases filter cake porosity, with an effective porosity rate of 56.65 %, resulting in superior drainage performance. The enhanced dewatering efficiency of PEO stems from its ability to improve water permeability within the filter cake during compression, distinguishing its mechanism from traditional flocculation (PAM) and coagulation (PAC) approaches. This work highlights the potential of PEO as a highly effective solution for food waste digestate treatment in solid waste management, with its salt-resistant properties further extending its applicability to high-salinity waste streams.

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.