Hydrogen enrichment as a bioaugmentation tool to alleviate ammonia inhibition on anaerobic digestion of phenol-containing wastewater
Introduction
There are various industrial wastewaters containing extremely high concentration of phenolic compounds and ammonia, such as coal gasification wastewater, coking wastewater, and petrochemical wastewater (Rosenkranz et al., 2013, Wang et al., 2012). As the major pollutants of these wastewaters, the concentration of phenolic compounds and ammonia can even exceed over 4000 and 3000 mg/L, respectively (Gai et al., 2008). The development of technique to achieve the harmlessness and energy recovery of phenolic compounds is still quite urgent. Anaerobic digestion is regarded as a technical feasible and economically attractive strategy of phenol-containing wastewater treatment (Na et al., 2016, Rosenkranz et al., 2013), but phenol degraders are easily inhibited by the high concentration of ammonia (Wang et al., 2017b). Therefore, developing an efficient way is necessary to enhance anaerobic digestion of phenol under high ammonia concentration condition.
Interestingly, hydrogen enrichment is used as a tool for in-situ biogas upgrading, as hydrogen introduced into the anaerobic digester can be converted together with carbon dioxide into methane by hydrogenotrophic methanogens (Luo et al., 2012). Meanwhile, hydrogen enrichment is considered a great way to enhance the hydrogenotrophic methanogenic activity of sludge, and the increase of relative abundance of hydrogenotrophic methanogens occurs during this process (Luo and Angelidaki, 2012). Hydrogenotrophic methanogens are more tolerant to ammonia and phenol toxicity compared with acetoclastic methanogens (Wang et al., 2016, Yenigun and Demirel, 2013). Importantly, the syntrophic conversion of phenol to methane was proposed only favorable via the hydrogenotrophic pathway at high inhibitory levels (Poirier et al., 2016a). The well-established syntrophic interactions between syntrophs and hydrogen consuming microorganisms (i.e. methanogens or homoacetogens) can accelerate finial conversion of phenol (Karlsson et al., 2000). Nevertheless, the rate-limiting step of anaerobic phenol degradation is reduction to benzoate (Veeresh et al., 2005). High hydrogen partial pressure (HPP) can suppress degradation of benzoate thermodynamically (Zhuang et al., 2015). Therefore, it is not clear whether hydrogen enrichment can be used as a bioaugmentation tool to enhance anaerobic digestion of phenol under high ammonia concentration.
The aims of this study were to investigate the effect of hydrogen enrichment on anaerobic digestion of phenol under high ammonia concentration condition, and to discuss the influencing factors and potential mechanisms of hydrogen enrichment on phenol degradation and methane production. Furthermore, more insights were given into the shift and mechanism of microbial community occurring in response to hydrogen enrichment.
Section snippets
Inoculum and wastewater composition
Anaerobic granular sludge obtained from an UASB reactor treating phenolic wastewater was used as the inoculum (Wang et al., 2017c). The total suspended sludge (TSS) of seed sludge was 50.91 g/L and the ratio of volatile suspended sludge (VSS) and TSS was 0.80. Phenol was used as the sole carbon resource in the medium, and the concentration of phenol was set at 1000 mg/L in all batch tests. Ammonium chloride was used as ammonia source. The macronutrients and micronutrients in the medium were
Impact of hydrogen enrichment on anaerobic digestion of phenol under high ammonia concentrations
The impact of hydrogen enrichment on phenol removal efficiency under different ammonia concentrations is shown in Fig. 1. When the concentration of ammonia was increased from 2 to 4 g NH4+-N/L, the corresponding phenol removal efficiency of the control group was significantly decreased from 41.38% to 8.49% (P = 3.0 × 10−5 < 0.05). Furthermore, when the sludge was exposed to 6 and 8 g NH4+-N/L, the phenol degradation in the control group tended to be completely inhibited, and the corresponding
Conclusion
Hydrogen enrichment was successfully developed to alleviate ammonia inhibition on anaerobic digestion of phenol. The endurance to ammonia toxicity of sludge was greatly enhanced by introducing hydrogen under high ammonia levels (from 2 to 8 g NH4+-N/L). Phenol utilization rate, the acetoclastic and hydrogenotrophic methanogenic activities of sludge were gradually improved with the increase of initial HPP at high ammonia concentration. Moreover, three hydrogen-consuming pathways including
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51878232) and CAS Key Laboratory of Urban Pollutant Conversion, University of Science and Technology of China (KF201702).
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