Abstract
The study presents the results of sulphide control using iron (III) chloride in full-scale anaerobic digesters (ADs) at a large-scale municipal wastewater treatment plant (WWTP). Iron (III) (‘ferric’) chloride was applied at a range of 24–105 mg FeCl3/L with and without alkali solution using different strategies. Introduction points were implemented at the feeding line and the sludge thickener unit. Response of the ADs in terms of biogas H2S reduction over time, solid loading rates (SLR), feed sludge flow rate and pH level were investigated. Reduction of H2S in the biogas reached 4 035 μg/L in directly-dosed AD versus a 1 345 μg/L drop in non-dosed ADs, as a result of internal recirculation among the digesters, where actual values were possibly higher as volatile solid (VS) degradation increased in all ADs during the dosing period. No noticeable effect on biogas production and pH was observed. The degree of H2S production was found to be correlated to the volatile SLR, where primary sludge solids contributed mostly to the organic and proteinaceous content of the thickened sludge and presented a high variation and a strong relationship with H2S production. Correlation analysis based on data for the 17-month period that followed supported the significant role of primary SLR in H2S production.
Highlights
Unionized hydrogen sulphide (H2S) produced in the biogas of anaerobic digesters (ADs) needs to be reduced to a low level before it reaches electric generators, gas storage units and machines in biogas applications, in order to eliminate corrosion
Konya wastewater treatment plants (WWTPs) was brought into operation in September 2010, and from the start biogas H2S reached 8 070 and 6 050 μg/L in the gas and desulfurization outlets, respectively
Designed capacity of the desulfurization unit was exceeded 4 times in the ADs during full operation. This level was unacceptable for the electric generators and the biogas was burned via a flame unit so as to protect the metal equipment and engines
Summary
Unionized hydrogen sulphide (H2S) produced in the biogas of anaerobic digesters (ADs) needs to be reduced to a low level before it reaches electric generators, gas storage units and machines in biogas applications, in order to eliminate corrosion. It can be removed from the biogas via dry or wet oxidation processes (Deublein and Steinhauser, 2008). As ADs are tightly insulated, large-scale reactors, replacement of any metal equipment or maintenance of concrete walls/ceiling are highly problematic and costly, and require long periods of time to make the repairs and bring the reactors back into operation (Erdirencelebi et al, 2015). The sulphate content of the raw inlet wastewater is one source for sulphide produced in the ADs, whereas the protein content can be a more significant factor for strong-character municipal wastewaters or in cases of industrial wastewater intrusion (Stams et al, 2003; Adams et al, 2003)
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