Date of Award

2017

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Civil and Environmental Engineering

Keywords

Anaerobic Sulfate Reduction, Heavy Metal Removal, Inhibition, Precipitation, Single Stage Process, Two Stage Process

Supervisor

Biswas, Nihar

Supervisor

Seth, Rajesh

Rights

info:eu-repo/semantics/openAccess

Abstract

Biological anaerobic sulfate reduction to sulfide by sulfate reducing bacteria (SRB) can be performed in a single-stage reactor in which the biological sulfate reduction to sulfide and metal precipitation occur simultaneously, or in two-stage reactors where the two follow sequentially. The single stage process may be more cost-effective and simpler to operate. However, some factors, such as acidic nature of acid mine drainage (AMD) and the presence of the residual heavy metals in the system may pose an inhibitory and toxic effect on SRB and limit the application of the process. In addition, some studies suggest that beyond a certain level of metal loading, the process of sulfate reduction and the corresponding metal precipitation by the sulfide generated is adversely affected. In the first part of this study, the effect of different concentrations of copper on anaerobic sulfate reduction in semi continuous stirred tank reactors (SCSTRs) at 35±2ºC was investigated. Four parallel SCSTRs received synthetic wastewater containing copper at various concentrations. They were optimized for pH and were operated at a predetermined COD/SO42-. Reactors receiving lower concentration (< 200 mg/L) of copper showed a very little negative effect in their performance. However, at higher concentrations (> 400 mg/L), performance was inhibited, which could be attributed to the presence of metal precipitates in these reactors. Batch kinetic experiments confirmed this inhibition of the sulfate reduction process in the presence of high concentration of metal precipitates. The cultures withdrawn at various process conditions were analyzed for their respective microbial pattern. It showed that certain concentrations of copper precipitates adversely affected the population of sulfate reducers. Reactors receiving 0 mg/L and 200 mg/L of copper showed more similarity in terms of their respective sulfate reducers’ population. The presence of thiosulfate reducers in microbial community may be an evidence for the existence of an alternate pathway in dissimilatory anaerobic sulfate reduction that generates thiosulfate as the intermediate byproducts during the reduction of sulfite to sulfide. In the second part of the study, two upflow anaerobic hybrid reactors (UAHR) were designed to overcome the inhibition of SRB by the metal precipitates. Two identical UAHRs received simulated wastewater with COD/SO4 2- of 1, where the sulfate concentration was 3040 mg/L. One UAHR was used to represent the single-stage process, and the influent contained metal (copper) in the feed. The other UAHR represented the first stage of the two-stage process. The performance of the two processes was compared over different hydraulic, organic, and sulfate loading rates by varying the HRT between 40 and 2.5 days at a temperature of 33±3°C. The results show that both sulfate reduction and copper precipitation in the single stage process were similar to or better than the two stage process over the entire duration of the study. The rate of copper removal in the single stage process was found to reach up to two times of that of the two stage process. This suggests that the proposed UAHR configuration was successful in overcoming the Inhibition of SRB by the metal precipitates. In the single stage reactor for which S/Cu was higher than 1, copper was precipitated in the form of CuS. The same happened for the second stage of two stage process once S/Cu was set to be higher than 1. This was confirmed with the equilibrium calculations using MINTEQ speciation model. However, the results of the MINTEQ showed that at S/Cu of less than 1, additional amount of copper is precipitated in the forms of Cu3(PO4)2 and CuO.

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