•Alkaline fermentation significantly reduced target ARGs in sludge.•Shifted community structures of potential ARGs hosts were observed at pH 10.•The potential of horizontal gene transfer was blocked by fermentation at pH 10.Alkaline fermentation has been reported to be an effective method to recover valuable products from waste sludge. However, to date, the potential effect of alkaline pH on the fate of antibiotic resistance genes (ARGs) during anaerobic fermentation of sludge has never been documented. In this study, the target ARGs in sludge was observed to be removed effectively and stably when sludge was anaerobically fermented at pH 10. Compared with the control (without pH adjustment), the abundances of target ARGs at pH 10 were reduced by 0.87 (sulI), 1.36 (sulII), 0.42 (tet(O)), 1.11 (tet(Q)), 0.79 (tet(C)) and 1.04 (tet(X)) log units. Further investigations revealed that alkaline fermentation shifted the community structures of potential ARGs hosts. Moreover, alkaline fermentation remarkably decreased the quantities and the ARGs-possessing ability of genetic vectors (plasmid DNA, extracellular DNA and phage DNA), which might limit the transfer of ARGs via conjugation, transformation and transduction. These results suggest that the shifted compositions of gene hosts and restricted gene transfer potential might be the critical reasons for the attenuation of ARGs at pH 10.Download high-res image (404KB)Download full-size image

This paper reported an efficient method to significantly reduce nitrous oxide (N2O) and nitric oxide (NO) generation in anaerobic−aerobic (low dissolved oxygen) processes. It was found that by the use of waste-activated sludge alkaline fermentation liquid as the synthetic wastewater−carbon source, compared with the commonly used carbon source in the literature (e.g., acetic acid), the generation of N2O and NO was reduced by 68.7% and 50.0%, respectively, but the removal efficiencies of total phosphorus (TP) and total nitrogen (TN) were improved. Both N2O and NO were produced in the low dissolved oxygen (DO) stage, and the use of sludge fermentation liquid greatly reduced their generation from the denitrification. The presences of Cu2+ and propionic acid in fermentation liquid were observed to play an important role in the reduction of N2O and NO generation. The analysis of the activities of denitrifying enzymes suggested that sludge fermentation liquid caused the significant decrease of both nitrite reductase activity to NO reductase activity ratio and NO reductase activity to N2O reductase activity ratio, which resulted in the lower generation of NO and N2O. Fluorescence in situ hybridization analysis indicated that the number of glycogen accumulating bacteria, which was reported to be relevant to nitrous oxide generation, in sludge fermentation liquid reactor was much lower than that in acetic acid reactor. The quantitative detection of the nosZ gene, encoding nitrous oxide reductase, showed that the use of fermentation liquid increased the number of bacteria capable of reducing N2O to N2. The feasibility of using sludge fermentation liquid to reduce NO and N2O generation in an anaerobic−low DO process was finally confirmed for a municipal wastewater.

Recently, waste activated sludge (WAS) fermentation for short-chain fatty acids (SCFAs) production has drawn much attention because the waste biosolids produced in wastewater treatment plants (WWTP) can be reused, and the produced SCFAs can be applied to promote biological nutrient removal (BNR). Usually, after WAS fermentation, the fermentation liquid is separated and then the recovery of ammonium and phosphorus, which are released during WAS fermentation, is conducted to prevent the increase of nitrogen and phosphorus loadings to WWTP. As an alternative to the traditional process, this paper investigated the recovery of ammonium and phosphorus in the formation of struvite before sludge−liquid separation, and its positive effect on the following sludge−liquid filtration separation. First, the conditions for ammonium and phosphorus recovery from the WAS fermentation mixture were optimized by response surface methodology (RSM). Then, the effect of ammonium and phosphorus recovery on sludge filtration dewatering was investigated. With ammonium and phosphorus recovery, it was observed that the specific resistance to filtration (SRF), the capillary suction time (CST), and the sludge volume after filtration reduced by 96.9, 99.6, and 88.7%, respectively, compared with no ammonium and phosphorus recovered sludge. Third, the mechanisms for ammonium and phosphorus recovery significantly enhancing sludge dewatering capacity were investigated. The formation of struvite, the neutralization of ζ potential, the increase of magnesium ion, which was added during ammonium and phosphorus recovery, and the decrease of sludge polymeric substance caused the improvement of sludge dewatering. Finally, the fermentation liquid was used as the additional carbon source of BNR, and the nutrient removal efficiency was obviously enhanced.

•SRT or temperature increase benefited the hydrolysis of fermentation substrates.•SCFAs, especially propionic acid, accumulated most at SRT 8 d and 37 °C.•The activities of key enzymes were in accordance with SCFAs production.•The ratio of Bacteria to Archaea was improved at SRT 8 d and 37 °C.During anaerobic fermentation of waste activated sludge (WAS), the production of short-chain fatty acids (SCFAs), especially propionic acid which is considered as the most preferred carbon source for enhanced biological phosphorus removal, can be improved by controlling the suitable mass ratio of carbon to nitrogen (C/N) and pH in batch mode. In this study the influences of solids retention time (SRT) and temperature on WAS hydrolysis and acidification in the continuous-flow systems in which the C/N ratio of WAS was modified by carbohydrate addition were investigated. Experimental results showed that the increase of SRT and temperature in a pertinent range benefited the hydrolysis of fermentation substrates and the accumulation of SCFAs, and SRT 8 d and temperature 37 °C were the most preferred conditions for the production of SCFAs, especially propionic acid. As there were more consumption of protein and carbohydrate and less production of methane at SRT 8 d and temperature 37 °C, more SCFAs were accumulated. Also, both the activities of key hydrolases and acid-forming enzymes and the ratio of acidogenic bacteria to methanogens showed good agreements with SCFAs production.