Abstract
The anaerobic thermophilic co-digestion of mixtures of the organic fraction of municipal solid waste (OFMSW) and the liquid fraction from the hydrothermal carbonization (LFHTC) of dewatered secondary sludge was studied. Mixtures with a low OFMSW to LFHTC ratio (50, 75 and 100% LFHTC) exhibited accumulation of volatile fatty acids (VFA) as well as low degradation of organic matter and methane production. However, the mixture containing 25% LFHTC performed quite well in terms of methane production: (179 ± 3) mL CH4 STP g–1 CODadded, which was only slightly lower than the value obtained with 100% OFMSW. The experimental results fitted the modified Gompertz model reasonably well and the maximum methane production rate for the mixture containing 25% LFHTC (11.96 mL CH4 g–1 COD d–1) was 29.3% higher than that obtained with the substrate with 100% OFMSW. Therefore, centralized co-digestion of OFMSW in mixtures with 25% LFHTC seemingly provides an effective method for valorizing the latter substrate.
Highlights
Thermochemical conversion processes are typically used to convert biomass into valuable products or biofuel
We explored the batchwise anaerobic codigestion under thermophilic conditions of mixtures of the liquid fraction from the hydrothermal carbonization of dehydrated sewage sludge and the organic fraction of municipal solid waste in variable ratios with a view to improving methane yields in relation to the processing of either substrate alone
The concentrations of individual volatile fatty acids (VFA) from acetic to heptanoic, iso forms included, were determined on a Varian 430 gas chromatograph equipped with a flame ionization detector (FID) and a capillary column filled with Nukol according to De la Rubia et al (2018a)
Summary
Thermochemical conversion processes are typically used to convert biomass into valuable products or biofuel. Hydrothermal carbonization (HTC) at 180–260◦C under auto-generated pressure is a promising method for converting wet biomass (Tekin et al, 2014; Kambo and Dutta, 2015). HTC converts biomass into a valuable solid product called “hydrochar” in addition to a liquid fraction (LFHTC) and a gas stream. HTC is especially suitable for biomass waste with a high moisture content such as sewage sludge produced in wastewater treatment plants (WWTP). Because this hydrochar possesses a high heating value The resulting liquid fraction (LFHTC) has a total chemical oxygen demand (COD) of nearly 100 g O2 L−1 (Villamil et al, 2018), which justifies its valorization to recover organic matter up to 15% of all initial carbon (Broch et al, 2014)
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