Hydrogen sulfide removal by nitrate reducing and sulfide oxidizing bacteria in anoxic bioreactors

Abstract

The strain MAL 1HM19, a nitrate reducing and sulfur oxidizing bacteria (NR-SOB) was successfully isolated from Mae Um Long Luang hot spring from Mae Hong Son province (Thailand) using hydrogen sulfide (H2S) and nitrate (NO3-) as an electron donor and acceptor, respectively. Among the isolates of NR-SOB from different sources, the strain MAL 1HM19 was the most promising novel strain of NR-SOB because of its ability to yield ~100% H2S removal, under anoxic conditions, within 5.5 h at an initial H2S concentration of 650 ppmv. The identification of strain MAL 1HM19 based on the 16S rDNA nucleotide sequence suggests that this strain is closely related to Paracoccus versutus and is the member of Alphaproteobateria with 99.93% sequence similarity. The P. versutus strain MAL 1HM19 was also able to grow at various NaCl concentrations (0.03-7%), in a pH range of 7.0-9.0 and at temperatures of 20-50°C. The ability of H2S removal by the P. versutus strain MAL 1HM19 under the influence of different initial NO3--N concentrations (60, 120 and 240 mg NO3--N/L), at 35°C, was investigated for 96 h. The results showed that 100% of H2S oxidation was attained within 10 h, irrespective of the different initial NO3--N concentrations. The final end product [sulfate (SO42-) or elemental sulfur (S°)] depended on the concentrations of NO3--N. In the long-term experiments, i.e. in a biotrickling filter (BTF), biological H2S removal was investigated by inoculating the pure cultures of P. versutus strain MAL 1HM19 in anoxic BTF for 188 d. The BTF was packed with polyurethane foams (PUFs) cubes and the reactor was operated under anoxic conditions in both fed-batch and continuous modes. The H2S inlet concentrations were varied between 100 and 500 ppmv during steady-state experiments, while during H2S shock load tests, the concentrations were increased to 1000, 2000, 3000 and 4000 ppmv, respectively. The removal efficiency (RE) of H2S varied between 17 and 100% depending on operational mode of the BTF and the addition of the C source. The maximum elimination capacity (ECmax) was achieved at 121.83 ± 0.1 g S/m3 h (RE - 96.5 %) during H2S shock load experiments at 4000 ppmv. The results from this study demonstrated that both free and immobilized cells of P. versutus strain MAL 1HM19 can be efficiently used in industrial situations to remove H2S and NO3- under a wide range of operating conditions.