Abstract
Biogas production from sewage sludge volatile solids (VS) by anaerobic digestion slows down towards the end of the process, among inhibitory factors being pH increase upon ammonia accumulation, poorly digestible biomaterials, and high fixed solid (FS) content. The possibility of concentrating the digested sludge VS (41.7–56.6% on a dry weight basis) by surface and bottom layer separation with biogas post-production was studied. Furthermore, the potential to recycle concentrated VS and digested sludge back to the process after adjusting pH 7.0 to optimal for biogas-producing microbes and after acid, alkali, thermal, and sonolytic treatments was examined. In general, pH 7.0 control alone improved biogas production from the recycled digested sludge the most. An equally good improvement in biogas production was achieved by recycling the digested sludge, which had been heated until ammonia had evaporated and the pH dropped to 7.0 (1–2 h, 75 °C), and at the same time, VS was degraded. The biogas production from the sonicated and recycled sludge was almost as good as from the pH-adjusted, or heat-treated recycled sludge. After the acid and base treatments of the digested sludge, the recycled sludge yielded often the lowest biogas volume, as the added chemicals increased the FS concentration, which proved to be a more important inhibitory factor than poorly degradable VS. The high FS content significantly reduced the benefits of the treatments. By separating the surface and bottom layers with biogas post-production, the surface layer of VS was concentrated to 51.6–61.8%, while different compositions of the layers affected the biogas production.
Highlights
The risk of hazardous compounds limits the utilization of sewage treatment plant sludge, biogas production being one of the most common applications (Chen et al 2008; Chen et al 2014; Tyagi and Lo 2011)
To improve digested sludge volatile solids (VS) consumption, the VS of Low solid sludge (LSS), Dewatered sludge (DS), and High solid sludge (HSS) were separated to surface and bottom layers using flotation by post-produced biogas, followed by adjusting the pH of the sludges to the optimum pH 7.0 for biogas-producing microbes; VS digestion with acid, alkali, heat, or sonolytic treatment; and biogas production measurements from recycled sludges
The biogas yield was best after the sole pH 7.0 adjustment, and after ammonia evaporation and VS digestion by heat treatment at 75 °C until the pH dropped to 7.0 (1–2 h), sonolytic treatment being almost as effective
Summary
The risk of hazardous compounds limits the utilization of sewage treatment plant sludge, biogas production being one of the most common applications (Chen et al 2008; Chen et al 2014; Tyagi and Lo 2011). Environ Sci Pollut Res (2020) 27:28155–28168 wastewater sludge volatile solids (VS) into biogas; other possible substrates include manure, energy crops, and municipal solid waste (Appels et al 2008; Gaida et al 2017; Weiland 2010). Anaerobic biogas production is a four-stage microbial process consisting of hydrolysis, acidogenesis, acetogenesis/ dehydrogenation, and methanation, of which hydrolysis is considered to be the rate-limiting step (Nguyen et al 2015; Weiland 2010; Feki et al 2015; Grübel and Suschka 2015). The final biogas product consists mainly of methane (60–70% in maximum) and carbon dioxide (Appels et al 2008)
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