Integrating electrocoagulation (EC) and biological routes to convert organic residues into value-added chemicals

Organic residues such as food wastes and animal manure are often treated by anaerobic digestion (AD), which is an effective biological pathway to sequester carbon, control odor, and produce energy (methane). However, the conversion of organic carbon into methane is not thorough, and AD has limitations to remove other nutrients (P and N) in the organic residues. The AD effluent is still an environmental liability that needs to be further processed. Therefore, this study developed an integrated system combining electrocoagulation (EC) and fungal fermentation on AD effluent to produce value-added chemicals of phosphorus fertilizer, chitosan, and organic acids (lactic acid, citric acid, and gluconic acid). A two-stage EC process was developed to treat the liquid AD effluent. The experimental result demonstrated that 90% COD and > 99% total phosphorous (TP) were removed from the liquid effluent, a solid phosphorus fertilizer was produced, and the water was reclaimed. The reclaimed water and solid AD digestate (rich in cellulose and hemicellose) were mixed as the feedstock and treated by a novel co-hydrolysis process followed by fungal fermentation to produce chitosan, organic acids. Aspergillus niger and Rhizopus oryzae were the fungal strains used by this study for various organic acid production. The experimental results indicated that EC solution has a positive effect on fungal fermentation in terms of accumulating chitosan-rich fungal biomass and co-producing organic acids.