Anaerobic treatment of high sulfate wastewater generates sulfide as a terminal end product of sulfate reduction. The sulfide so produced is inhibitory to methane producing bacteria at high concentrations. This will not only lower the methane yield but also lead to treatment performance deterioration and/or failure. There has been lack of an appropriate method to control sulfide toxicity. It is therefore essential to develop a suitable method for sulfide toxicity control by eliminating the sulfide as fast as it is produced....[ Read more ]

Anaerobic treatment of high sulfate wastewater generates sulfide as a terminal end product of sulfate reduction. The sulfide so produced is inhibitory to methane producing bacteria at high concentrations. This will not only lower the methane yield but also lead to treatment performance deterioration and/or failure. There has been lack of an appropriate method to control sulfide toxicity. It is therefore essential to develop a suitable method for sulfide toxicity control by eliminating the sulfide as fast as it is produced.

In this study, an online sulfide toxicity control was achieved through injection of pure oxygen to biogas recirculation stream to oxidize sulfide. Oxidation-reduction potential (ORP) was used as a controlling parameter to regulate the pure oxygen injection. The ORP based sulfide control method was tested in a completely mixed reactor and upflow anaerobic filter for its effectiveness to control the sulfide. The system ORP was precisely maintained within ± 10 mV of the target ORP during oxygenation. The elevation of natural ORP by +25 mV (i.e. Target ORP = Natural ORP + 25 mV) was found appropriate for the online sulfide control for higher sulfate level of 6000 mg/L. The actual needed ORP increase depended on both the influent sulfate level and also wastewater characteristics.

The ORP controlled oxygenation was able to oxidize more than 98.5% of the produced sulfides (gaseous and dissolved) when influent sulfate levels were 1000, 3000, 5000, and 6000 mg/L. During oxygenation, a part of influent chemical oxygen demand (COD) was oxidized by facultative bacteria at all influent sulfate levels. The extent of facultative activity was found strongly dependent on the sulfide levels. When sulfide was low, the facultative activity was high, and vice versa. During oxygenation, the methane yield declined at low influent sulfate levels (when the sulfide levels were below the inhibitory level) due to substrates diversion to facultative activity whereas at high influent sulfate levels (when the produced sulfide was above the inhibitory level), the methane yield improved appreciably due to alleviation of sulfide toxicity in comparison to natural ORP. The removal of total organic carbon (TOC) also improved at all influent sulfate levels during oxygenation with respect to natural ORP.

There was as high as 8.2% increase in effluent sulfate level during oxygenation in comparison to natural ORP, and this was attributed to the substrate diversion to facultative activity instead of sulfide re-oxidation. The major pathway of sulfide oxidation appeared to be abiotic and elemental sulfur was the major end product. This conclusion was supported by a series of batch tests on abiotic sulfide oxidation. The presence of trace metals/nutrients in the aqueous phase was found to have a profound catalytic effect on the sulfide oxidation.