The Effect of Effluent Recirculation in a Semi-Continuous Two-Stage Anaerobic Digestion System

Abstract

Abstract:The effect of recirculation in increasing organic loading rate (OLR) and decreasing hydraulic retention time (HRT) in a semi-continuous two-stage anaerobic digestion system using stirred tank reactor (CSTR) and an upflow anaerobic sludge bed (UASB) was evaluated. Two-parallel processes were in operation for 100 days, one with recirculation (closed system) and the other without recirculation (open system). For this purpose, two structurally different carbohydrate-based substrates were used; starch and cotton. The digestion of starch and cotton in the closed system resulted in production of 91% and 80% of the theoretical methane yield during the first 60 days. In contrast, in the open system the methane yield was decreased to 82% and 56% of the theoretical value, for starch and cotton, respectively. The OLR could successfully be increased to 4 gVS/L/day for cotton and 10 gVS/L/day for starch. It is concluded that the recirculation supports the microorganisms for effective hydrolysis of polyhydrocarbons in CSTR and to preserve the nutrients in the system at higher OLRs, thereby improving the overall performance and stability of the process.

Treatment of coal gasification wastewater by a two-phase anaerobic digestion

“…reactors [18]. However, the effluent recirculation led to a rapid decline in treatment efficiency, especially in terms of TP removal when 50% of the effluent was recirculated to the methanogenic reactor.…”

A review of anaerobic digestion systems for biodegradable waste: Configurations, operating parameters, and current trends

“…In the present, there are two different viewpoints about separating digestion-steps into two reactors. The first perspective supposes that the first tank allows the hydrolysis/acidogenesis to occur, and the second tank optimizes the acetogenesis/methanogenesis [49,76]. Meanwhile, the second perspective proposes that the acetogenesis should be in the first reactor and the second reactor only optimizes methanogenesis [18,54].…”

Anaerobic degradation of dairy wastewater in intermittent UASB reactors: influence of effluent recirculation

“…[22,23] When the performance of UASB reactors is assessed in terms of methanization efficiency, it is apparent that presently, these systems face significant challenges in what concerns their applicability to the treatment of complex lipid-rich wastewater of which dairy wastewater is an example. [24] Nadais et al [22] verified that to keep the methanization efficiency above 70%, continuous UASB reactors inoculated with flocculent biomass and used for treating dairy wastewater should not be operated at loading rates above 2.5 g COD L −1 d −1 . Jeganathan et al [25] reported that the continuous operation of an UASB reactor used for treating food processing wastewater resulted in 75% substrate biodegradation at a load of 2.5 kg COD m −3 d −1 and that the system performance degraded sharply for higher loads.…”

“…[37,38] The recirculation of the treated effluent in a semicontinuous two-stage system combining continuous stirred tank reactor (CSTR) and UASB resulted in a higher yield of methane and suggested that the recirculation could support the hydrolysis step as well as avoiding nutrient loss at higher OLR, and thus improving the performance and the stability of the process. [24] The upflow velocity (V up ) is an important operational parameter in UASB reactors for maintaining the mixing of the substrate and biomass. [14] Several studies have demonstrated the importance of the liquid upflow velocity upon the kinetics of high-rate anaerobic systems.…”