Valorization of Organic Fraction of Municipal Solid Waste Through Production of Volatile Fatty Acids (VFAs) and Biogas (Preprint)
Authors: Borhany Hojjat 
Published: September 2023

BACKGROUND

Whereas the traditional methods of treating organic wastes through single-step anaerobic digestion (AD), composting, and incineration only yield low market-valued products, the novel approach of multistep of AD preceded by acidogenic fermentation could obtain further products with higher commercial value like volatile fatty acids (VFAs). Here, the necessitation of the lab-scale experiments before a full-scale implementation was addressed to characterize the process in terms of mass and energy recovery along with its amelioration through operational amendments like varied hydraulic retention times (HRTs), addition of carbonaceous material, feedstock/inoculum ratio, and pre-treatment.

OBJECTIVE

The goal was to elaborate and amend the process of obtaining VFAs and biogas from the organic fraction of municipal solid waste (OFMSW) in terms of mass, energy, and kinetics.

METHODS

The methods include fermentation of pre-treated OFMSW in semi-continuous completely stirred conditions followed by batch-scale anaerobic digestion. The process was characterized in terms of VFAs concentration, distribution, volatile solids, and biogas yield. The technical and economic feasibility of an imaginary recovery line with a capacity of 70000 population equivalent (PE) was studied through mass and energy balance analysis. The process kinetics was assayed through calibration of the two most used models of the first order and modified Gompertz. The effectiveness of HRT alteration in obtaining higher VFAs content was affirmed by statistical analysis.

RESULTS

The results (p-value=0.99) indicate that in the fermentation, HRT of 4.5 days could yield higher VFAs content of 30.5 (g-SCOD/L) than HRT of 3 days with 27.66 (g-SCOD/L), without compromising its distribution, which mainly consists of butyric acid (26%-31%), acetic acid (27%), caproic acid (22%-26%), and propionic acid (9.2%-9.9%). Moreover, the AD of residual fermented solids yielded specific methane production of 0.3 (CH_4/m^3*kg−VS). In this part, the feedstock/inoculum ratio of 0.3 (volatile basis) coupled with thermal and fermentative pre-treatment improved the kinetics of the AD process substantially, whereas biochar addition could only improve the maximum obtainable methane content by 8 percent (76%, vol/vol). Furthermore, kinetics study showed that the first order rate model is more precise in describing the AD with biochar (RMSE of 6.7 vs 8.6), while the modified Gompertz is fitted better to data without biochar (9.31 vs 8.33). The mass and energy analysis for the up-scaled line proves that the proposed process could reach profit much sooner, almost 3 years in comparison to a single AD with 11 years of non-profitability. Remarkably, the recovery percentage of volatile solids in terms of mass was found to be around 80%.

CONCLUSIONS

The advantage of the production of VFAs and biogas from OFMSW over the traditional method of single AD was well-detailed, and its economic and technical feasibility was confirmed through mass and energy balance analysis.

CLINICALTRIAL