Hydrogen sulfide (H2S) removal from biogas using anoxic bioprocesses are economic and efficient compared to physico-chemical H2S removal or other biogas upgrading technologies. Most of these biotechnologies have used nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) as the dominant microorganism for H2S removal. Anoxic sulfide removal technologies have been widely applied for both liquid and gaseous pollutants, particularly for biogas clean-up, because it is more practically applicable than the conventional aerobic systems in terms of ease of use and operational costs (Almenglo et al. 2016; Fernández et al. 2014; Soreanu et al., 2008). In this study, the performance of an attached growth bioreactor, i.e. a fluidized bed reactor (FBR) and a combined attached and suspended growth bioreactor, i.e. a moving bed bioreactor (MBBR), were tested under different operating conditions and the bioreactors were compared for their ability to perform sulfur oxidation coupled to autotrophic denitrification.In anoxic sulfide-oxidizing reactors, a crucial factor is the nitrogen/sulfur (N/S) ratio, which affects the metabolism of nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) and the ratio of the end products of sulfide oxidation such as elemental sulfur and sulfate. Thus, the objective of this study was to evaluate the effect of the N/S ratio on the thiosulfate removal efficiency in two different anoxic biofilm bioreactors, i.e. a MBBR and a FBR, as shown in Figure 1. Both the lab-scale MBBR and FBR were operated for 250 days, at room temperature (~20 ºC) and at a feed pH of 7.0 ± 0.2. The dissolved oxygen (DO) concentrations in the MBBR and FBR were 0.51 ± 0.09 and 0.26 ± 0.06 mg L-1, respectively. The FBR used in this study was previously used for thiosulfate-driven denitrification (Di Capua et al. 2017). The MBBR was inoculated by using the biomass obtained from that FBR containing Thiobacillus denitrificans as the dominant microorganism. The performance of the MBBR and FBR were evaluated under three different N/S ratios (0.5, 0.3 and 0.1). Thiosulfate was used as a substrate for sulfide-oxidizing bacteria at a constant concentration of 200 mg S-S2O32- L-1, whereas the concentration of the electron acceptor, nitrate, was decreased stepwise from 40 to 10 mg N-NO3- L-1. The performances of the MBBR and FBR can be compared in Table 1. The removal efficiency of thiosulfate was > 98% and nitrate was completely consumed during the operational time in both bioreactors at N/S ratio of 0.5. Under the nitrate-limiting conditions tested, i.e. N/S ratio of 0.3 and 0.1, the thiosulfate removal efficiencies in the MBBR (83.4 and 37.8%) were higher than those observed in the FBR (77.8 and 26.1%), resulting in a higher sulfate production.The higher DO concentrations observed in the MBBR compared to the FBR likely played a role in enhancing thiosulfate oxidation due to T. denitrificans, a dominant microorganism in the inoculum, being a facultative anaerobe which enables to use oxygen as alternative e- acceptor to oxidize the thiosulfate. Additionally, it was probably because of the different bioreactor configuration and mixing conditions.
The MBBR and FBR can be operated at
room temperature (~20 ºC) for achieving high removal efficiencies of
thiosulfate (> 98%), under autotrophic denitrification conditions, at a HRT
of 5 h, feed pH of 7 and a N/S ratio of 0.5. However, the MBBR resulted in
higher thiosulfate oxidation rates than the FBR after the nitrate-limiting
conditions were applied. The reactor performance at a N/S ratio of 0.1 and the evaluation
of the microbial community composition at different N/S ratios require further investigation.