Enhancing Methane Production Through Co-Digestion of Sewage Sludge, Citrus Waste and Brewery Spend Grain With Natural Zeolite: Mechanisms and Microbiome Analysis

Abstract

The presented research is focused on evaluating the influence of adding clinoptilolite (Z) on the multi-component co-digestion (AcoD) of sewage sludge (SS), citrus waste represented by orange peel (OP) and brewery spent grain (BSG). The experiments were conducted under mesophilic conditions at 37°C in batch mode; 7 series with differing feedstock configurations were performed. The positive effect of Z was shown in each configuration, most strongly with SS:OP:BSG:Z. The production of methane was enhanced by 81%, in comparison to the co-digestion of SS:OP:BSG without Z, reaching 362.4 mL CH₄/g volatile solids (VS). The kinetics was also improved as the methane production rate increased to a similar extent, amounting to 17.3 mL CH₄/g VS d. Moreover, it was observed that the lag phase was shortened and the contents of both limonene and phenols were reduced. The composition of the microbial communities was significantly altered by the addition of Z in AcoD of SS:OP and SS:OP:BSG. Therein, a unique microbiome structure with the highest percentage of unidentified sequences was found. The beneficial effect of Z was multifaceted and related to the reduction of the stress caused by the presence of limonene and phenols as well as the syntrophic relation between the microorganisms.