Anaerobic digestion (AD) is an effective biological process to treat organic wastes. A complex anaerobic microbial consortium converts organic matter in the wastes into biogas - a carbon neutral and renewable energy source, and correspondingly alleviates the odor and pathogen problems. It has advantages of simple setup, bioenergy generation, and greenhouse gas emission reduction. However, extensive application of anaerobic digestion technology is confronted by a number of challenges: complicated and expensive H2S removal to upgrade the biogas, nutrients handling in liquid effluent, and solid digestate disposal. Therefore, this study developed an integrated process that simultaneously cleans up biogas, purifies liquid effluent and utilizes solid wastes. The integrated process includes two operation units: a novel biogas-facilitated electrocoagulation (EC) for liquid effluent treatment and biogas purification, and fungal conversion of solid digestate for value-added fuel and chemical production.
Electrocoagulation (EC) is a widely used wastewater treatment, and has been proved highly efficient in reclaiming AD waste effluents with removal efficiencies of >90% and >99% for COD and total phosphorous (TP) respectively. Biogas pumping process has been inserted between multiple stages of EC treatments, which not only neutralized the high pH developed in EC to facilitate following EC processes, but also significantly removed H2S (~488 mg H2S / 1L AD effluent) from raw biogas. Treated liquid effluent with most nutrients removed and turbidity improved, as well as purified biogas with minimal H2S content were obtained. Moreover, the solid wastes from anaerobic digester serves as a potential cellulosic biomass, which could be further degraded into fermentable mono-sugars and other acids to produce value-added biofuels and biopolymers via fungal fermentation. Rhizopus orzyae was the strain applied on the solid digestate for chitosan production. The fermentation results demonstrate that R. oryzae can efficiently utilize pretreated solid digestate and the EC treated water to enhance chitosan production.
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