Converting Organic Municipal Solid Waste into Volatile Fatty Acids and Biogas: Experimental Pilot and Batch Studies with Statistical Analysis

Abstract

BackgroundItaly can augment its profit from biorefinery products by altering the operation of digesters or different designs to obtain more precious bio-products like volatile fatty acids (VFAs) than biogas from organic municipal solid waste. In this context, recognizing the process stability and outputs through operational interventions and its technical and economic feasibility is a critical issue. Hence, the present study is an anaerobic digester in Treviso in northern Italy. ObjectivesThis research compares the novel line, consisting of pretreatment, acidogenic fermentation, and anaerobic digestion, with single-step anaerobic digestion regarding financial profit and surplus energy. Therefore, a mass flow model was created and refined based on the outputs from the experimental and numerical studies. These studies examine the influence of hydraulic retention time (HRT), pretreatment, biochar addition, and fine-tuned feedstock/inoculum (FS/IN) ratio on bio-products and operational parameters. MethodsVFA concentration, VFA weight ratio distribution (VWRD), and biogas yield were quantified by gas chromatography. Then, a t test was conducted to analyze the significance of dissimilar HRTs in changing the VFA content. Further, a feasible biochar dosage was identified for an assumed FS/IN ratio with an adequate long HRT using the First-Order (FO) rate model. Accordingly, the parameters for a mass flow model were adopted for 70,000 population equivalents to determine the payback period and surplus energy for two scenarios. We also explored the effectiveness of amendments in improving the process kinetics. ResultsBoth HRTs were identical concerning the VFA/SCOD (0.88 kg/kg) and VWRD: mainly acetic acid (40%), butyric acid (24%), and caproic acid (17%). However, a significantly higher mean VFA content was confirmed for HRT 4.5d than the quantity for HRT 3d (30.77 vs 27.66, g-SCOD/L) using a t test (t =- 2.68, df=8, P=.03, CI=95%). In this research, 83% of the fermented volatile solids were converted into biogas to obtain a specific methane production of 0.133 (/kg-VS). While biochar addition improved only the maximum methane content by 20% (86% v/v), the feedstock/inoculum ratio of 0.3 (volatile solid basis) with thermal plus fermentative pretreatment improved the hydrolysis rate substantially (0.57 vs 0.07, 1/d). Furthermore, the biochar dosage of 0.12 (g-biochar/g-VS) with HRT 20d was identified as a feasible solution. Principally, our novel line payback period would be almost 2 years with surplus energy of +2251 (MJ/d) compared to 45 years and +21567 (MJ/d) for single-step anaerobic digestion. ConclusionsThis research elaborated on the advantage of the refined novel line over the single-step anaerobic digestion and confirmed its financial and technical feasibility. Further, changing HRT and other amendments significantly raised the VFA concentration and the process kinetics and stability.