Anaerobic digestion of agricultural wastes from liquid to solid state: Performance and environ-economic comparison

Performance and environ-economic impacts were compared for anaerobic digestion (AD) of dairy manure and cucumber residues from liquid to solid state. Environ-economic evaluation of the overall AD process at different total solids (TS) was performed with the estimated treatment capacity of 9600 tons/year and service life of 20 years. Results showed that TS increase from 6% to 22% enhanced both cumulative and volumetric methane (CH₄) production. Further TS increase to 25%, however, reduced CH₄ yield. Environ-economic assessment indicated that TS increase enhanced volumetric waste treatment capacity and thus AD environmental footprints. Environmental credits from digestate and biogas utilization could compensate the adverse environmental impacts of other processes in AD plants. Furthermore, biogas and nutrients in digestate determined AD net-present value. As a result, solid state AD was more profitable with higher CH₄ yield and more nutrients in both biosolids and digested effluent of digestate than its liquid and hemi-solid counterparts.