Cascade anaerobic digestion system treating waste activated sludge under short sludge retention time: process performance and role of recirculation

Abstract

Hydrolysis, where the complex insoluble organic matter is hydrolyzed by hydrolytic enzymes, is regarded as rate-limiting step for anaerobic digestion of waste activated sludge (WAS). The cascade system, which consisted of small-reactors in series, was developed in this study to enhance the hydrolysis rate. Degradation efficiency of sludge was strongly related to the sludge retention time (SRT) and recirculation ratio (RR). Therefore, this study investigated the effect of SRT and RR on process performance and hydrolysis enzymatic activity of WAS treatment in the cascade system and conventional single continuous stirred tank reactor (CSTR) system. The cascade system’s chemical oxygen demand (COD), volatile solid (VS), protein and carbohydrates reduction, and methane production could be maintained despite the decrease of SRT, which was associated with an increase in organic loading rate (OLR) (15 d = 3.33 gCOD/L/d, 12 d = 4.16 gCOD/L/d). The performance of reference CSTR system deteriorated at shorter SRT. Hydrolysis, acidogenesis and methanogenesis analysis indicated that shorter SRT accelerated the hydrolysis rate of cascade system from 1.29±0.23 gCOD/L/d (SRT=15 d) to 1.59±0.19 gCOD/L/d (SRT=12 d), leading to the increase of overall degradation of WAS. The enhancement of hydrolysis rate was supported by the increased protease and cellulase activity detected at shorter SRT. Recirculating the effluent from reactor3 (R3) to reactor 1 (R1) was also essential to maintain the performance. The results not only showed deteriorating performance of cascade system regarding lower COD, VS, protein, carbohydrates reduction and methane production, but also presented lower hydrolysis enzymatic activity at 2% RR compared to 10% RR. 10% recirculation of solid phase, which contained biomass and sludge-attached enzymes, was beneficial to recover the performance of cascade system but the performance was not as good as that with 10% recirculation of both solid and liquid phase of R3 effluent. The cations analysis revealed that the required quantities of Cobalt (Co), Nickel (Ni) and Zinc (Zn) was highest in R1 while the concentration of soluble Co, Ni and Zn was lowest in R1. The concentration of Co, Zn and Ni was increased along AD of cascade system, providing the opportunities of offering increment of trace elements to the growth of microbial community and enzymes formation in R1. In conclusion, cascade system had better performance at shorter SRT compared to single CSTR. The recirculation played an important role to maintain the superior performance of cascade system under high OLR.