Abstract
Hydrolysis is considered to be the rate-limiting step in anaerobic digestion of waste activated sludge (WAS). In this study, an innovative 4 stages cascade anaerobic digestion system was researched to (1) comprehensively clarify whether cascading configuration enhances WAS hydrolysis, and to (2) better understand the governing hydrolysis kinetics in this system. The cascade system consisted of three 2.2 L ultra-short solids retention times (SRT) continuous stirred tank reactors (CSTRs) and one 15.4 L CSTR. The cascade system was compared with a reference conventional CSTR digester (22 L) in terms of process performance, hydrolytic enzyme activities and microbial community dynamics under mesophilic conditions (35 °C). The results showed that the cascade system achieved a high and stable total chemical oxygen demand (tCOD) reduction efficiency of 40–42%, even at 12 days total SRT that corresponded to only 1.2 days SRT each in the first three reactors of the cascade. The reference-CSTR converted only 31% tCOD into biogas and suffered process deterioration at the applied low SRTs. Calculated specific hydrolysis rates in the first reactors of the cascade system were significantly higher compared to the reference-CSTR, especially at the lowest applied SRTs. The activities of several hydrolytic enzymes produced in the different stages revealed that protease, cellulase, amino peptidases, and most of the tested glycosyl-hydrolases had significantly higher activities in the first three small digesters of the cascade system, compared to the reference-CSTR. This increase in hydrolytic enzyme production by far exceeded the increase in specific hydrolysis rate, indicating that hydrolysis was limited by solids-surface availability for enzymatic attack. Correspondingly, high relative abundances of hydrolytic-fermentative bacteria and hydrogenotrophic methanogens as well as the presence of syntrophic bacteria were found in the first three digesters of the cascade system. However, in the fourth reactor, acetoclastic methanogens dominated, similarly as in the reference-CSTR. Overall, the results concluded that using multiple CSTRs that are operated at low SRTs in a cascade mode of operation significantly improved the enzymatic hydrolysis rate and extend in anaerobic WAS digestion. Moreover, the governing hydrolysis kinetics in the cascading reactors were far more complex than the generally assumed simplified first-order kinetics.
Highlights
Waste activated sludge (WAS) is an inevitable by-product generated in biological wastewater treatment plants (WWTPs)
The results showed that the cascade system achieved a high and stable total chemical oxygen demand reduction efficiency of 40–42%, even at 12 days total solids retention times (SRT) that corresponded to only 1.2 days SRT each in the first three reactors of the cascade
The relative abun dance of these metabolic pathways in the different reactors increased when the reactor SRT dropped from 22 to 12 days. They showed a similar trend as the activities of valine and leucine ary lamidase that catalyse the hydrolysis of valine and leucine from peptide chains (Fig. 8d and Fig. 6). These findings suggest that applying a cascade system results in an enhanced microbial metabolism of hydro lytic/acidogenic bacteria that caused the observed acceleration in hy drolytic enzyme activity and subsequent enhanced sludge reduction compared to the reference-continuous stirred tank reactors (CSTRs)
Summary
Waste activated sludge (WAS) is an inevitable by-product generated in biological wastewater treatment plants (WWTPs). Due to quantitative and qualitative extension of wastewater treatment, the annual WAS production has increased in the European Union during the last two decades, from 10 million tons in 2008 to 11.5 million tons in 2015, and is expected to approach 13 million tons by 2020 (Rorat et al, 2019). Anaerobic digestion (AD) is a proven key technology for both stabili zation of WAS and recovery of the biochemical energy stored in the sludge in the form of biogas. Conventional digesters using continuous stirred tank reactors (CSTRs) have to be operated under prolonged sludge retention times (SRTs) exceeding 20 days for an acceptable WAS conversion
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