Low strength wastewater treatment using emerging anaerobic treatment processes
Authors:   , Hou Liyuan 
Published: April 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Anaerobic treatment is a promising and energy saving process for low-strength wastewater treatment. Roles of half saturation constant (Ks) and maximum specific growth rate (umax) in anaerobic treatment systems, however, are often overlooked. This study proposed to apply specific affinity (defined as umax/ Ks) as the key performance indicator of anaerobic processes treating low-strength wastewater. Furthermore, this study provided a new insight into the relationship between specific affinity and population of methanogens in an anaerobic membrane bioreactor (AnMBR) treating low-strength wastewater. High abundance of Methanosaeta (85.8% of total archaea) was linked to the high specific affinity (1.6 x 10[superscript -3] L/mg COD/d) in acclimated anaerobic sludge, resulting in low effluent chemical oxygen demand (COD) concentrations. Short hydraulic retention times (HRTs) are preferred for AnMBRs to treat low strength wastewater at a high volumetric organic loading rate with lower capital costs. However, short HRTs become a potential bottleneck in anaerobic treatment processes because of possible interspecies mass transfer limitations and membrane fouling in AnMBRs. Till now, little is known about how short HRTs would affect effluent water quality that is linked to the specific affinity of anaerobic sludge and their microbial community structures in AnMBRs. In current study, the overall performance, specific affinity of anaerobic sludge, and dynamics of community structures of an AnMBR treating synthetic municipal wastewater at decreasing HRTs (i.e., 24 h, 12 h, and 6 h) was investigated. A decrease in HRT resulted in sludge with high specific affinity. Correspondingly, Methanosaeta became the dominant methanogens in the AnMBR. Both the effluent water quality and methane yield were enhanced. Municipal wastewater contains complex organic constituents while multi-step biochemical processes are involved in anaerobic treatment processes. Two identical AnMBR were operated under decreasing HRTs (24 h, 12 h, and 6 h, respectively) treating low strength wastewater containing different substrate (acetate or glucose, respectively). As a result, microbial communities in the two AnMBRs diverged. The effluent quality and methane yield were enhanced in the acetate fed AnMBR while methane yield decreased in the glucose fed AnMBR as HRT decreased. Correspondingly, the abundance of Methanosaetaceae in the acetate fed AnMBR increased, but it decreased in the AnMBR fed with glucose. Interestingly, hydrogenotrophic methanogens have a higher proportion in the glucose fed AnMBR than in the acetate fed AnMBR. Overall, a minimum HRT higher than 6 h may be required to treat wastewater containing complex organic matter to ensure a successful operation. To treat the sulfate-containing low-strength wastewater, we proposed a newly designed anaerobic microbial fuel cell (MFC) system that could be used to produce electricity and remove sulfate simultaneously. A maximum voltage output of 129 mV was observed under the following feed conditions: that the ratio of lactate: sulfate was 60:20 and 0:10 in the anodic chamber and cathodic chamber, respectively. The decrease in the organic substrate/sulfate ratio in anodic chamber had a great effect on the electricity production, which could be resulted from an increasing DvH attaching on the electrode at a higher sulfate concertation contributes more electrons transfer. However, there was no significant electricity production at the ratio of two presumably because sulfate in the anodic chamber obtained all electrons produced by lactate without transferring to cathodic chamber since the stoichiometric ratio of lactate and sulfate is two. To our knowledge, this was the first time to show the electricity generation by using Desulfovibrio vulgaris Hildenborough (DvH) in such a MFC configuration. Electron microscopic analysis indicated that nanoscale filaments could enhance the extracellular electron transfer of DvH. DvH biofilm, which is necessary for extracellular electron transfer, suggesting that DvH has multiple direct electron transfer mechanisms. This could further benefit the application of DvH to enhance the power output and treat the real sulfate-containing low-strength wastewater.