Biomethane production from waste activated sludge promoted by sludge incineration bottom ash: The distinctive role of metal cations and inert fractions.

Sludge incineration bottom ash (SA), a solid waste generated by incineration of waste activated sludge (WAS), has been demonstrated as an inexpensive additive to increase biomethane production from anaerobic digestion (AD) of WAS. However, how SA improved methanogenic performance of a WAS digester remains elusive. Here, we addressed this question by fractionating the SA into accessible leachate (SA-L) and inert residue (SA-R) and investigating their individual effects. The cumulative biomethane production was increased by 6.7%, 20.2% and 39.6% with addition of SA-L, SA-R and SA, respectively. Mechanistic study showed that release of organic-binding metals (Ca and Fe) from SA dissolution suppressed volatile fatty acids production by increasing the apparent activation energy (AAE) and decreasing the surface binding sites for hydrolytic/acidogenic enzymes during WAS hydrolysis-acidogenesis, while trace elements in SA-L promoted metabolism of methanogens (Methanothermobacter and Methanosarcina). In contrast, the gypsum/silicate-cored SA-R facilitated hydrolysis-acidogenesis with reduced AAE but drastically inhibited methanogenesis due to competition of sulfate-reducing bacteria Thermodesulfovibrio. The comparative analysis of KEGG-based functional genes indicated that the enhanced methane metabolism and reductive CO2 fixation pathways with SA addition could result from the release of trace elements to support key enzyme activities.

Pretreatment-promoted sludge fermentation liquor improves biological nitrogen removal: Molecular insight into the role of dissolved organic matter.

Waste activated sludge (WAS) can be used as carbon sources to support biological nutrient removal (BNR). In this study, thermal-alkaline (THALK), ozonation (OZN), electrolysis (EC) and NaClO-promoted electrolysis (EC-AOP) were investigated to facilitate WAS solubilization and production of volatile fatty acids (VFAs). EEMF-PARAFAC and FT-ICR-MS were employed to characterize the transformation of dissolved organic matter (DOM) in WAS fermentation liquors at molecular level. THALK achieved the highest fluorescence intensity of C1 protein after pretreatment. Proteins and lipids were the dominant DOM in the pretreated WAS, while the DOM shifted towards substances with higher H/C and lower O/C after fermentation. The BNR results showed that THALK (100%) and EC-AOP (96.9%) outperformed other groups (78.9-90.3%) in terms of NO3-N removal, indicating the significant impact of DOM compositions. Overall, these results demonstrated that THALK and EC-AOP effectively enhanced release of VFAs and DOM, which subsequently improved NO3-N removal efficiency.

Sludge-based biochar-assisted thermophilic anaerobic digestion of waste-activated sludge in microbial electrolysis cell for methane production.

The development of microbial electrolysis cells (MECs) for methane production from waste activated sludge (WAS) is arrested due to the limited methane yield and fragile system stability. This study proposed a strategy to accelerate and stabilize MEC via 1.0 g/g DM (dry matter) sludge-based biochar (BC). The results showed that BC clearly accelerated methane production by 24.7% and enhanced VS removal efficiency by 17.9%, compared to control group. Variations of SCOD, proteins, carbohydrates and VFAs indicated biochar promoted hydrolysis and acidogenesis process. Cyclic voltammetry (CV) curves and coulombic efficiency (CE) suggested organic matters degradation and electron generation on anode were enhanced with supplement of biochar. Microbial community analyses revealed that biochar addition could both promote DIET through substituting exoelectrogen (e.g., Thermincola) on anode and enrich hydrogenotrophic methanogens (e.g., Methanothermobacter) on cathode, which is beneficial to development of MEC as to methane recovery from organic matters.

In-situ biogas upgrading by a stepwise addition of ash additives: Methanogen adaption and CO2 sequestration.

Biogas from anaerobic digestion (AD) of waste activated sludge (WAS) limited its utilization due to low value-added. In this study, an innovative addition mode for ash known as stepwise addition was developed to enhance methane production and improve CO2 scavenge from AD of sludge. Experimental results confirmed stepwise addition of ash improved methane content to 79.4%, compared to control group (69.1%). Compared to Pulse addition and Control, the cumulative CH4 production was promoted by 39.2% and 35.4%, respectively. Investigation of the mechanism indicated that stepwise addition of ash could decrease hydrolytic and acidifying enzyme activities but increase activity of coenzyme F420, compared to pulse addition group. Furthermore, stepwise addition of ash not only increased the abundance of Methanomassiliicoccus (34.48%), but also promoted amounts of CO2 capture. This method ameliorate utilization availability of sludge ash for sludge anaerobic digestion through promoting cumulative methane production and increasing CO2 storage capacity.