Roles of immobilized biomass in an anaerobic hybrid reactor

Abstract

An anaerobic hybrid reactor (AHR), comprising an anaerobic filter (AF) upper section and an up-flow anaerobic sludge blanket (UASB) lower section, was used to treat synthetic wastewater under different loading conditions. The AF was then separately investigated for its role in contributing to the performance and process stability of the AHR. The immobilized biomass and population distribution of the different microbial species in the AF were also studied under steady operating and different shock loading conditions. With the incorporation of the AF section in the upper portion of the system, the AHR presented a better capability against the environments of the shock loadings. The AF section of the AHR played a significant role in the removal of COD; particularly when the AHR system was operated under adverse conditions, such as shock loadings. The AF section of the AHR performed as a biofilter to maintain the temporarily inhibited microorganisms on the surfaces of the packing medium and thus prevented a wash out of the inhibited microorganisms under shock loadings. This rendered the AHR more tolerant to shock loadings. During shock loadings at different HRTs, the average COD removed by the AF section was about 36%, ranging from 49 to 21%, whereas the UASB section only accounted for about 10% of the COD removal, ranging from 10.4 to 8.2%. Even under critical shock loading at an HRT 0.5 day, the average COD removed by the AF section was maintained at about 21 %; whereas the COD removed by the UASB section declined to about 9%. In response to the shock loadings, the AHR showed a temporary drop in the efficiency of COD removal, but resumed to steady state operations after the adverse situation ceased. As a general trend, the COD removal rate decreased as the HRTs decreased from 5 to 0.5 days. The efficiency of removal of COD of the AF section was much higher than that of the UASB section during the transient state of shock loadings. The COD removal rate of the AHR was maintained at between 1.5 and 0.4 g COD/L-d at HRTs of 2.5 to 0.5 days before the failure of the AHR at an HRT of 0.25 day. The AF section was then isolated to study its process stability and responses to hydraulic and organic shock loadings. For the hydraulic shock loading experiments, the AF was started-up with synthetic wastewater of 3000 mgCOD/L at 5.0 days of HRT, achieving 98.1% COD removal efficiency. Under 2, 4 and 5 times hydraulic shock loadings, the efficiency of COD removal was temporary reduced, ranging from 92.7 to 89.7%, the pH of the treated effluent and biogas production were also affected. The average pH value dropped from 7.3 to 6.0. The specific methane yield, with an average methane concentration around 69% (v/v), was generated at a rate of between 0.28 to 0.32 L/g COD. The AF recovered from a state of temporary inhibition resulting from the shock loadings, and resumed normal operation within 8 days. Under 10 times hydraulic shock loading, the treatment performance deteriorated drastically. Volatile fatty acids (VFAs) accumulated in the AF liquor, resulting in reactor souring and failure. When the HRT of the AHR was restored to 5 days, the AF recovered within a few days. The ability of the AF to recover from critical hydraulic shock loadings and system failure was attributed to the immobilized-biofilm design, which enabled the temporarily inhibited biomass to be retained in the AF and the temporarily inhibited biomass resumed its activity when favourable conditions were restored. For the organic shock loading experiments, the operation of the AF was steady after 60 days of operation at an HRT of 1.25 days. The COD removal efficiency was 98.2%. The biogas production rate was maintained within the range of 1.9 to 2.0 L/day. Methane concentration was about 70% (v/v). Effluent pH ranged from 6.4 to 6.5. When the organic load of the AF was progressively increased to 115.2 gCOD/L-d, the total COD removal efficiency of the AF was still be maintained above 90%. The AF recovered from various shock loadings equivalent to a sixteen times increase in organic load, which showed that the AF possessed excellent anti-shock capacity. The specific methane yield, with an average methane concentration of 68% (v/v), was at a rate of between 0.22 to 0.12 L/g COD. The methane gas produced from each cubic meter of the trade effluent was equivalent to 2.2 * 10 5 kJ of energy generation. This heat value was in excess of the amount required for the pre-heating of the effluent to 30 oC. Combining the mechanisms of both organic adsorption and biodegradation rendered the AF more stable under various shock loading conditions. The anaerobic pathway of the organic degradation, converting acetate to carbon dioxide and methane by Methanosaeta and Methanosarcina spps., was prone to inhibition under the critical hydraulic shock environment. The results suggest that the hydraulic shocking loading distributed, to some extent, the physical contact between the syntrophs and methanogen, leading to an inhibition of the methanogenic bacteria. This inhibition leads to an imbalance of the microbial ecosystem, including an accumulation of VFAs and a decline in methane concentrations in the biogas. The anaerobic pathways of the conversion of the VFAs to acetate, hydrogen and carbon dioxide by (i) Syntrophomonas spp., the conversion of the acetate to methane and carbon dioxide by (ii) Methanosaeta spp., the conversion of the butyrate to methane and water and the conversion of the carbon dioxide and reduction of the hydrogen mediated by (iii) Methanococcus spp., were prone to inhibition under the critical organic shock loading environment. The degree of inhibition of these three groups of bacteria was found to be different (e.g. iii > ii > i), as evidenced by the concentration of carbon dioxide that increased incrementally as the organic loading increased. As a result, the supply of organic acids, mainly acetate and butyrate, exceeded the assimilative capacity of the methane-forming bacteria, Methanosaeta spp., leading to an accumulation of VFAs. The content of the methane in the biogas decreased and the concentration of carbon dioxide increased. Generally, the failure of the AF under various shock loadings was attributed to process souring, as indicated by the low methane yield and an accumulation of VFAs. The results suggest that the AF contributed a stable environment for the immobilized biomass in the AHR, and helped to improve the performance and process stability of the AHR. Under shock loading conditions, the AF also rendered the AHR more able to recover from unstable operating conditions or even from process failure.