Performance of a haloalkaliphilic bioreactor under different ${\text{NO}}_3^{-}/{\text{SO}}_4^{2-}$ ratios

Highlights

• Haloalkaliphilic microorganisms were used to reduce sulfate and nitrate.

• Sulfide concentration reached up to 703 mg/l.

• There was no sulfide inhibition to haloalkaliphilic microorganisms.

• Bacterial community of haloalkaliphilic bioreactor was studied.

Abstract

Effects of ${\text{NO}}_3^{-}/{\text{SO}}_4^{2-}$ ratio on denitrification and sulfate removal efficiency were investigated in model experiments applying haloalkaliphilic bioreactor. The reduction of both substrates performed well at different ${\text{NO}}_3^{-}/{\text{SO}}_4^{2-}$ ratios ranging from 17.6 to l.5. The removal rates of nitrate and sulfate were 6 and 1.39 kg m⁻³ d⁻¹, respectively, at ${\text{NO}}_3^{-}/{\text{SO}}_4^{2-}$ ratio 3.0, while sulfide concentration reached up to 703 g m⁻³. The major sulfate-reducing and denitrifying bacteria were Desulfonatronovibrio sp. and Halomonas campisalis, respectively. Decrease in ${\text{NO}}_3^{-}/{\text{SO}}_4^{2-}$ ratio led to obvious changes in bacterial community. Although the sulfate reducers became dominant, the population of denitrifying ones also increased as it was demonstrated by analysis of PCR-amplified 16S rDNA fragments, which suggested that SRB and DB coexisted well in bioreactor.

Introduction

The main components of flue gas are sulfur dioxide (SO₂) and nitrogen oxides (NO_x). Conventional methods used to treat flue gas can be very efficient. However, they generate secondary pollution as well as require a lot of energy inputs and a large room to work (Philip and Deshusses, 2003). Therefore, environmentally friendly and more economic alternative is needed. The flue gas can be treated by ozone (Wang et al., 2007b) followed by the washing with alkaline solution (Chu et al., 2001). SO₂ and NO_x can be converted to sulfate and nitrate, respectively. Since there were sulfate and nitrate in the alkaline solution, the wastewater needs to be dealt with before they were discharged to environment. Since sulfate and nitrate exist in the absorption solution of flue gas, it is important to simultaneously remove sulfate and nitrate. Nitrate can be reduced to N₂ by denitrifying bacteria (DB) or Thiobacillus denitrificans (Chen et al., 2008). Sulfate can also be reduced to sulfide by sulfate-reducing bacteria (SRB), and sulfide subsequently being oxidized to element sulfur (Sorokin and Kuenen, 2005, Zhou et al., 2011). However, in the reaction system at about pH 6.0, sulfide can penetrate into cells easily generating a direct toxicity to SRB and other microorganisms (Reis et al., 1992) and it need strip sulfide with N₂ (Vallero et al., 2005) generating additional costs. Distinctly, in haloalkaliphilic system, the presence of hydrogen sulfide in solution is in the form of HS⁻ (Yongsiri et al., 2005) that can not penetrate into cells easily (Mora-Naranjo et al., 2003). In an engineering point of view, if the flue gas is treated by haloalkaliphilic microorganisms at pH about 9.5, it will need less cycle index because of higher gas absorption efficiency than common system and the expenses can be decreased. Thus the biological treatment technology of flue gas can be used to replace conventional treatment (Philip and Deshusses, 2003). No reports on simultaneous removal of sulfate and nitrate by haloalkaliphilic microorganisms are available in the literature, thus the use haloalkaliphilic microorganism to treat wastewater containing sulfate and nitrate is a novel method.

Sulfate reduction inhibition by nitrate reduction has been recognized as potential drawbacks in common system and must be solved firstly in order to reduce sulfate and nitrate simultaneously by haloalkaliphilic microorganisms. This inhibition is caused by the fact that nitrate reduction is preferential over sulfate reduction and there is a competition for electron donor (Ontiveros-Valencia et al., 2012, Zhang et al., 2008). However, it was reported that the competition for electron donor can be reduced when the electron donor concentration available to SRB was high (Greene et al., 2003, Ontiveros-Valencia et al., 2012).

The composition of the microbial community could influence the performance of anaerobic reactors, and to optimize the treatment capacity of bioreactor, it is necessary to control the microbial acting microbial consortium. However, the diversity and structure of microbial community in haloalkaliphilic bioreactor has not been reported yet. Denaturing gradient gel electrophoresis (DGGE), – a powerful tool to evaluate the diversity of a microbial community (Muyzer et al., 1993, Yamashita et al., 2011), – was used to follow changes in relationships of bacterial species taking part in reduction processes.

In present study, sulfate was added into the denitrification bioreactor to determine the ability of simultaneous removal of sulfate and nitrate by haloalkaliphilic microorganisms with sufficient electron donor. The aim was to explore the effects of ${\text{NO}}_3^{-}/{\text{SO}}_4^{2-}$ ratio on the diversity and structure of bacterial community, denitrification efficiency and sulfate removal efficiency in haloalkaliphilic denitrification bioreactor.