Abstract
In this study, micro-oxygen was integrated into a rusty scrap iron (RSI)-loaded anaerobic digester. Under an optimal RSI dosage of 20 g/L, increasing O2 levels were added stepwise in seven stages in a semi-continuous experiment. Results showed the average methane yield was 306 mL/g COD (chemical oxygen demand), and the hydrogen sulphide (H2S) concentration was 1933 ppmv with RSI addition. O2 addition induced the microbial oxidation of sulphide by stimulating sulfur-oxidizing bacteria and chemical corrosion of iron, which promoted the generation of FeS and Fe2S3. In the 6th phase of the semi-continuous test, deep desulfurization was achieved without negatively impacting system performance. Average methane yield was 301.1 mL/g COD, and H2S concentration was 75 ppmv. Sulfur mass balance was described, with 84.0%, 11.90% and 0.21% of sulfur present in solid, liquid and gaseous phases, respectively. The Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) analysis revealed that RSI addition could enrich the diversity of hydrogenotrophic methanogens and iron-reducing bacteria to benefit methanogenesis and organic mineralization, and impoverish the methanotroph (Methylocella silvestris) to reduce the consumption of methane. Micro-oxygen supplementation could enhance the diversity of iron-oxidizing bacteria arising from the improvement of Fe(II) release rate and enrich the sulphur-oxidising bacteria to achieved desulfurization. These results demonstrated that RSI addition in combination with micro-oxygenation represents a promising method for simultaneously controlling biogas H2S concentration and improving digestion performance.
Highlights
As the by-product of wastewater treatment plants (WWTPs) the output of waste activated sludge generated in biological wastewater treatment processes has increased continuously in the recent decade
The application of anaerobic digestion of sludge is often limited by low methane yield and sludge reduction rates [5], and the limiting factors are generally associated with the slow hydrolysis of sludge [6]
We have examined the effects of varying concentrations of rusty scrap iron (RSI) additions (i.e., 0, 1, 5, 10, 20, 30 g/L) on anaerobic digestion performance from waste-activated sludge (WAS), and obtained the optimal dosage of 20 g-RSI/L
Summary
As the by-product of wastewater treatment plants (WWTPs) the output of waste activated sludge generated in biological wastewater treatment processes has increased continuously in the recent decade. Anaerobic treatment processes have been widely applied to the treatment of organic solid waste due to their low operational costs and high solids reduction efficiency [1,2,3]. Organic matter is initially hydrolyzed and fermented into volatile fatty acids (VFAs) such as acetic acid, as well as hydrogen, which represent substrates for methanogenic archaea for methane production [4]. The application of anaerobic digestion of sludge is often limited by low methane yield and sludge reduction rates [5], and the limiting factors are generally associated with the slow hydrolysis of sludge [6]. The operating cost of the present pretreatment is high and often unattractive for practical application
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