Thermo-alkaline pretreatment of excess sludge: Effects of temperature on volatile fatty acids accumulation and microbial community

Hydrocyclone combines with alkali-thermal pretreatment to enhance short-chain fatty acids production from anaerobic fermentation of waste activated sludge

The production of short-chain fatty acids (SCFAs) through anaerobic fermentation of waste activated sludge (WAS) is commonly constrained by limited substrate availability, particularly for WAS with low organic content. Combining the hydrocyclone (HC) selection with alkali-thermal (AT) pretreatment is a promising solution to address this limitation. The results indicated that HC selection modified the sludge properties by enhancing the ratio of mixed liquid volatile suspended solids (MLVSS)/mixed liquid suspended solids (MLSS) by 19.0% and decreasing the mean particle size by 17.4%, which were beneficial for the subsequent anaerobic fermentation process. Under the optimal HC + AT condition, the peak value of SCFAs production reached 4951.9 mg COD/L, representing a 23.2% increase compared to the raw sludge with only AT pretreatment. Mechanism investigations revealed such enhancement beyond mechanical separation. It involved an increase in bound extracellular polymeric substances (EPS) through HC selection and the disruption of sludge spatial structure by AT pretreatment. Consequently, this combination pretreatment accelerated the transfer of particulate organics (i.e., bound EPS and intracellular components) to the supernatant, thus increasing the accessibility of WAS substrate to hydrolytic and acidifying bacteria. Furthermore, the microbial structure was altered with the enrichment of key functional microorganisms, probably due to the facilitation of substrate biotransformation and product output. Meanwhile, the activity of hydrolases and SCFAs-forming enzymes increased, while that of methanogenic enzymes decreased. Overall, this strategy successfully enhanced SCFAs production from WAS while reducing the environmental risks of WAS disposal.

Enhanced methane production from anaerobic digestion of waste activated sludge with weak magnetic field: Insights into performances and mechanisms

The impact of weak magnetic field (WMF) on anaerobic digestion (AD) performance of waste activated sludge (WAS) and underlying mechanism were investigated. Results showed that WMF significantly stimulated the methane yield by 12.9∼25.1% with 15 and 30 mT WMF addition, but high WMF (60 mT) attenuated the positive effect. The WMF enriched the anaerobic microbes, especially the acetoclastic and hydrogenotrophic methanogen. Additionally, the WMF dramatically facilitated the metabolic pathways of key enzymes for methanogenesis, which was validated by the significant increase of absolute abundance of anaerobic functional genes (mcrA). The enzyme activities of ATP and F420 were also significantly promoted by 30 mT WMF, but high WMF (60 mT) resulted in increased activity of lactate dehydrogenase. This study reveals that low WMF can promote AD performance of WAS through enhancing microbial activities especially methanogen, but high WMF leads to the loss of cell membrane integrity and attenuates its positive effect.

Temperature-dependent microbial mechanism and accumulation of volatile fatty acids in primary sludge pretreated with peroxymonosulfate

For revealing the influence of temperature on volatile fatty acids (VFAs) generation from primary sludge (PS) during the anaerobic fermentation process facilitated by peroxymonosulfate (PMS), five fermentation groups (15, 25, 35, 45, and 55 °C) were designed. The results indicated that the production of VFAs (5148 mg COD/L) and acetic acid (2019 mg COD/L) reached their peaks at 45 °C. High-throughput sequencing technology disclosed that Firmicutes, Proteobacteria, and Actinobacteria was the dominant phyla, carbohydrate metabolism and membrane transport were the most vigorous at 45 °C. Additionally, higher temperature and PMS exhibit synergistic effects in promoting VFAs accumulation. This study unveiled the mechanism of the effect of the pretreatment of PS with PMS on the VFAs production, which established a theoretical foundation for the production of VFAs.

Roles of quorum-sensing molecules in methane production from anaerobic digestion aided by biochar

Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted.

Control of phosphorus release from sediment by iron/aluminum co-modified zeolite: efficiency, mechanism, and response of microbial communities in sediment

The efficiency of iron/aluminum co-modified zeolite (FeAl-Z) covering and amendment for controlling the internal loading of phosphorus (P) from sediment to the overlying water (OW) and its controlling mechanism were explored. The response of the composition of sedimentary microbial communities in sediment and their function to the FeAl-Z capping and amendment was also examined. FeAl-Z showed good removal performance for phosphate in aqueous solution. The maximum phosphate adsorption quantity for FeAl-Z at pH 7 attained 11.2 mg P/g. The release of sediment endogenous phosphorus to OW can be successfully restrained by the FeAl-Z covering and amendment, and the suppression ability of FeAl-Z covering was stronger than that of FeAl-Z amendment. Under the capping or amendment condition, FeAl-Z can effectively inactivate the labile phosphorus measured by diffusion gradient in thin film (DGT-LP) in the overlying water and surface sediment. The added FeAl-Z transformed redox-sensitive phosphorus (BD-P) to metal oxide-bound phosphorus (NaOH-IP) and residual phosphorus (Res-P) in sediment, which increased the stability of inorganic phosphorus in the sediment. The passivation of soluble reactive phosphorus (SRP) and DGT-LP in the surface sediment by FeAl-Z significantly contributed to the inhibition of sediment endogenous phosphorus release to OW by the FeAl-Z capping, and the passivation of SRP, DGT-LP and mobile phosphorus in the surface sediment played a pivotal role in the control of sediment internal phosphorus release by the FeAl-Z amendment. The FeAl-Z amendment and capping did not increase the liberation risk of Fe from sediment, and the microorganisms in the sediments under the conditions of FeAl-Z amendment and covering still can perform good ecological functions. Results of this research demonstrate that FeAl-Z capping has high application potential in the control of phosphorus transfer from sediment to OW.

Innovative cation exchange-driven carbon migration and recovery patterns in anaerobic fermentation of waste activated sludge

Despite numerous treatments have been developed to enhance anaerobic fermentation of waste activated sludge, the innovative cation exchange (CE) approach has been rarely reported, little attempt was conducted to revealcarbon source fate. The interphase carbon balance was illustrated to clarify endogenous carbon dissolution, biotransformation,and recovery patterns. By CE-mediated divalent cation removal, almost 34.72 % of particulate carbon sources were dissolved in 2-day treatment, corresponding to soluble carbon content of 1165.58 mg C/L. Most of the originally dissolved carbon sources (58.01-66.81 %) were bio-transformed to volatile fatty acids with high bioavailability, while the further transformation to biogas was inhibited, contributing to recoverable carbon source accumulation. Overall, 21.38 % of total solid carbon sources were recovered through 8-day fermentation, the carbon extraction was implemented by solid-liquid separation with carbon loss of 14.21-22.91 %, manifesting the valid carbon recovery of 85.05-87.96 mg C/g VSS. Such CE-driven carbon recovery provided negentropy benefits in sustainable cycle economy.