The conventional activated sludge (CAS) process is widely applied for wastewater treatment but it produces large amounts of waste activated sludge (WAS) that must be treated and disposed of. The oxic-settling anaerobic (OSA) process was developed for in-situ sludge reduction within the CAS process by inserting an anaerobic side-stream reactor (SSR) in the sludge return line. However, low reaction rates and the resulting large SSR volume impede its application. This study proposed the development of a sulfidogenic oxic-settling anaerobic (SOSA) process via sulfidogenesis bioaugmentation in an SSR to accelerate the sludge destruction rate and improve the sludge reduction performance.

First, the feasibility of the SOSA process was investigated by operating a laboratory-scale SOSA process...[ Read more ]

The conventional activated sludge (CAS) process is widely applied for wastewater treatment but it produces large amounts of waste activated sludge (WAS) that must be treated and disposed of. The oxic-settling anaerobic (OSA) process was developed for in-situ sludge reduction within the CAS process by inserting an anaerobic side-stream reactor (SSR) in the sludge return line. However, low reaction rates and the resulting large SSR volume impede its application. This study proposed the development of a sulfidogenic oxic-settling anaerobic (SOSA) process via sulfidogenesis bioaugmentation in an SSR to accelerate the sludge destruction rate and improve the sludge reduction performance.

First, the feasibility of the SOSA process was investigated by operating a laboratory-scale SOSA process for 205 d in parallel with an anoxic/oxic (AO) CAS process and a conventional OSA process as control systems. The effluent quality of the SOSA process was not negatively affected, as it had soluble chemical oxygen demand (SCOD) and total nitrogen (TN) removal efficiencies of 98% and 99%, respectively. The SOSA process with an observed sludge yield of 0.204 g SS/g COD_removed was found to reduce sludge production by 57% and 30% compared with that of the AO and OSA processes, respectively. Sulfidogenesis bioaugmentation not only improved the sludge settleability and dewaterability but also encouraged sludge decomposition with greater destruction of extracellular polymeric substances (EPS). The SOSA process promoted the growth of slow-growing bacteria including sulfur cycle-related and hydrolytic/fermentative bacteria.

Second, the underpinning mechanism of sludge reduction in the SOSA process was identified via operating three laboratory-scale systems at the same sludge retention time (SRT) of 46 d, namely one anoxic/oxic extended aeration (EAO) process, and two EAO-based in-situ sludge reduction processes, i.e., the conventional OSA (OSA) process and the SOSA process. The SOSA process reduced sludge production by 29% and 20% compared with that of the EAO and OSA processes, respectively, and had a 14% and 8% higher oxygen demand than the EAO and OSA processes, respectively, thereby indicating that sludge reduction in the SOSA was caused not only by EAO-based aerobic digestion in the mainstream but also by bioaugmentation of sulfidogenesis. The roles of sulfidogenesis were further studied, and the results indicated that the SOSA biomass had a faster endogenous decay rate (0.097 d^-1) than the OSA biomass (0.045 d^-1), and sulfidogenesis accelerated anaerobic solubilization, hydrolysis, acidogenesis, and acetogenesis by 2.3 - 3.1 times, 6 - 22 %, 22 - 60% and 6 - 22%, respectively.

The combination of electrochemical pretreatment (EPT) with the SOSA process (named ESOSA) was proposed to develop a high-rate in-situ sludge reduction process. The optimal sulfate concentration for EPT at constant conditions of 15 V and 5 min was first determined via batch tests. The ESOSA process, which consisted of an AO system and an EPT-assisted sulfidogenic SSR (ESSR), was then operated to investigate the beneficial effects of EPT and sulfidogenesis on the wastewater treatment performance, sludge reduction efficiency, and sludge properties. Batch experiments showed that the optimal sulfate concentration of 125 mg S/L enhanced the performance of EPT and anaerobic digestion by 27.5% and 26.6% respectively, compared with that of the control. The long-term operation lasted for 202 d, and the results showed that the ESSR operated in a short SRT of 2.5 d had a good performance with a carbonaceous SCOD concentration of 888.1 ± 149.8 mg/L and a sulfide concentration of 37.1 ± 7.4 mg S/L. The ESOSA process reduced sludge production by 61% with high SCOD and TN removal efficiencies of 96% and 70%, respectively. The sludge in the ESOSA mainstream reactor with a smaller mean particle size of 88.9 ± 5.1 μm and lower EPS concentration of 97.5 ± 6.8 mg COD/g VSS indicated that coupling of EPT and sulfidogenesis in the ESSR could assist sludge floc decomposition and degradation.

This research systematically investigated the feasibility, mechanism, and potential strategies for improvement of the SOSA process. The results can guide for achieving high-rate and cost-effective in-situ sludge reduction technology.