Electrochemical sulfide removal from wastewater: microbial interactions and process development

Sulfide is commonly present in domestic and industrial wastewater. As it is toxic, corrosive and odorous, it often needs to be removed prior to discharge to sewer or in the sewer system itself, and certainly before discharging into the environment. The scope of this thesis was to develop and demonstrate a novel, low energy electrochemical technique for the removal and recovery of sulfide from wastewater. In addition, this study aimed to evaluate the influence of inorganic sulfur species on organics oxidation in bioelectrochemical systems. The results demonstrate that sulfide oxidation to elemental sulfur can generate net electrical power in an electrochemical system. However, while the process effectively removed the sulfide from the wastewater, the elemental sulfur was deposited on the electrodes and deactivated them over time. Sulfide removal rate decreased from its initial value 80±2% to 62±4% after 8 days of operation when a lab scale reactor operated continuously in fuel cell mode (external resistance 10 Ω) with a loading rate of 0.43 ± 0.04 kg-S m-3 d-1 of total anodic compartment (TAC). The removal rate was constant for the following 50 days of operation and significantly decreased to about 10% after 90 days. On average, the power production was 5±1 W m-3 TAC with the coulombic efficiency of 88±5% but the maximum power production capacity of the reactor was 78 W m-3 TAC using potassium ferricyanide cathode. However, the deposited sulfur could be effectively removed and recovered as a concentrated sulfide/polysulfide solution by reversing the polarity of the electrode with low electrical energy input. The results also demonstrate that microbial consortia that developed due to the organic electron donors in the wastewater, negatively affected the performance of the sulfide removal process. The microorganisms were using the electrodeposited sulfur as a preferred electron acceptor over soluble sulfate and the electrode. This process was converting sulfur back to sulfide irrespective of the electrochemical conditions. In batch systems, the sulfide produced in this way could be re-oxidized at the anode and therefore the obtained coulombic efficiency was 97±2% for acetate oxidation. However, in continuous systems, depending on the operational conditions and wastewater characteristics, the sulfide could leave the system in the effluent. By applying cell polarity reversals at a sufficiently high frequency, it was possible to avoid biofilm formation and hence the re-generation of sulfide from the deposited sulfur. To confirm the effectiveness of the electrochemical sulfide removal in real wastewater, the process was demonstrated on the effluent of an anaerobic digester of a paper mill. Sulfide was removed from 44±7 to 8±2 mg-S L-1 at a removal rate of 0.845±0.133 kg-S m-3 TAC d-1 and a recovery rate of 75±4% with the voltage input of 0.52 to 1.3 V. Periodic switching in every 24 hours intervals between anode and cathode was an effective technique to maintain a good sulfide removal performance and avoid unwanted biofilm formation at the anode. Sulfide present in the wastewater could therefore be effectively removed from the liquid phase and harvested as elemental sulfur deposit on the electrode