Biological hydrogen sulfide removal with nitrate

Abstract

Aim: Hydrogen sulfide is a major occupational hazard in agriculture, industry, and sewage processing; its presence leads to corrosion. Thus, hydrogen sulfide removal is obligatory. These investigations aim to evaluate the temperature (25-10 ○C) and N/S ratio influence on simultaneous NO3- and H2S removal and products distribution. The dynamics of microbial communities under temperature stress was examined and a method for indirect H2S measurements was developed.

Materials and methods: The experimental work was performed in an expanded granular sludge bed (EGSB) reactor in two trials. Before the start of each trial, an acclimatization period of around 1 month has been assured to obtain stable conditions when the test started. The reactor was continuously fed with laboratory prepared synthetic wastewater that consisted of nitric acid (HNO3) as the electron acceptor and sodium sulfide nonahydrate (Na2S·9H2O) as the electron donor. The electron acceptor and donor solutions were prepared and supplied in separate tanks. A pH buffer was supplied together with the electron donor while macro-, microelements, and vitamins were supplied dissolved together with the electron acceptor. The first short-term trial was focused on the evaluation whether the process can run at frequent temperature changes (25-10 ○C) and elemental sulfur (S0) can be accumulated in the granular sludge (details are given in Article I). The main trial was performed over 150 days (excluding the acclimatization period). During this trial, the temperature impact (25-10 ○C) in a longer time span and different N/S ratios (0.35- 1.30) were studied (details are given in Articles II-IV). The obtained results were analyzed considering Gibbs free energy and electron balance. The microbial communities in biomass samples were also examined to better understand the observed temperature adaptation.

Results and discussion: Performed experimental studies on temperature and feed composition impact show that the granular sludge bed autotrophic denitrification process can operate in the 25-10 ○C temperature range with high HS- removal rate and extent from 98 % (at 25 ○C) to 89.2 % (at 10 ○C) with a complete NO3- removal. Feed N/S ratio can be tuned to enhance the sludge associated S0 accumulation, so that S0 enriched sludge can be harvested. The temperature influence was not only limited to changes in HS- removal. Changes in temperature also influenced the product characteristics under invariable feeding conditions. Increased SO42- production and decreased of S0 was observed with decreasing temperature. The average S0 yield ranged from 83.7 % at 25 ○C to 67 % at 10 ○C, while the SO42- presence increased from 14.4 % (25 ○C) to 22.1 % (10 ○C). The Gibbs free energy analysis revealed that the changes in HS- removal and products distribution between S0 and SO42- allowed the microbial community to maintain similar reaction energy (for catabolism) at each temperature. This metabolic shift allowed biomass to obtain more energy per HS- consumed. It is hypothesized to be a microbial response to compensate for the temperature changes. The observed metabolic shift could be due to changes in metabolism within the microorganisms or changes in the microbial community. A significant population shift was confirmed by the microbial community analysis at 25 and 10 ○C which showed that under mesophilic conditions (25 ○C) Thauera sp. and Alicycliphilus sp. (both β-Proteobacteria) prevailed and comprised over 57 % of all identified sequences, while ε-Proteobacteria (mostly Sulfurimonas sp., 31.3 %) predominated under psychrophilic conditions (10 ○C). Changes in relative abundance of these Proteobacteria classes are similar to the relative changes in product composition, particularly in case of S0acc. Its production decreased 2.5 times from 25 to 10 ○C, while the presence of β-Proteobacteria (Thauera sp. and Alicycliphilus sp.) decreased by 2.3 times. Thus, it can be suggested that their presence is connected with the HS- oxidation to S0 and its accumulation. Effects of different N/S ratios (0.35, 0.40, 0.60, and 1.30) were studied under psychrophilic conditions (10 ○C). The HS- removal was the highest at the lowest and highest studied N/S ratios, 89.2 % and 89.6 %, respectively. Lower HS- removal was obtained at N/S=0.40 and 0.60 with the lowest 76.9 % at N/S=0.60. Product formation deviated from the theoretical predictions, suggesting that the reactions in continuous flow bioreactors are more complex than assumed in the standard stoichiometric models. Increasing N/S feed ratio increased the SO42- production and decreased of S0. The S0 accumulated at low N/S feed ratio was utilized at higher N/S leading to higher SO42- production. This phenomenon can explain the lower removal of HS- at mid-N/S ratios and the higher total effluent sulfur concentration than fed at N/S=1.30.