Optimum loading ratio for co-digested wastewater sludge and brewery spent yeast

Abstract

Anaerobic digestion (AD) is one of the most widely used technologies in the world for converting solid organic waste into energy and other value-added products. In the treatment of municipal wastewater, the disposal of sewage sludge is a problem of growing importance, as it contributes significantly to the operating costs of a wastewater treatment plant. It has also been recognised as a pollutant, albeit one with energy potential. In the brewing industry, a significant proportion of the raw materials used also end up as secondary raw materials or waste. Due to increased energy prices, brewer’s yeast has attracted great interest as an energy substrate, despite its difficult degradability under anaerobic conditions. AD offers the possibility to produce biogas and digestate from sewage sludge, resulting in a reduction in sludge volume and disposal costs. However, mono-digestion of sewage sludge often yields only a small amount of biomethane. To improve the quality and quantity of biogas, co-digestion is often used, leading to an increase in the economics of AD. However, the feedstock and co-digestion ratio need to be understood and carefully selected because they can affect the optimal methane production. Therefore, this study investigates the proportion of co-substrate that can optimise biogas yield. The co-fermented samples with different loading ratios of 1:1, 1:2 and 2:1 were characterised in terms of C/N ratio, total solids (TS), volatile solids (VS), moisture content and pH. They were measured by the standard method. In addition, the methanogenic potential was determined based on the biomethane potential (BMP) using the AMPTS 11 test. From the study, it was found that the optimum co-fermentation ratio was 1:2 of WWS: BSY with the highest biomethane production of 338.2 NmL CH4/g VS. Consequently, it was found that co-digestion of WWS with BSY can improve biomethane production and that brewer’s waste yeast could mitigate some of the limitations associated with mono-digestion of sewage sludge.