Enhanced Methane Yields in High-Solids Anaerobic Digestion Through Inoculation with Pulp and Paper Mill Sludge

Abstract

High-solids anaerobic digestion (HS-AD) is used to recover energy and nutrients from the organic fraction of municipal solid waste. However, the low degradability of lignocellulosic wastes, such as yard wastes, limits the rate and extent of methane production from this process. This study investigated the potential to enhance methane yields from lignocellulosic waste during HS-AD through inoculation with granular anaerobic digester sludge from pulp and paper mill wastewater treatment systems (P&P sludge). P&P sludge contains microbial populations that are acclimated to a lignin-rich waste stream and has been shown to enhance methane production from agricultural residues. In this study, side-by-side bench-scale HS-AD experiments were carried out with yard waste inoculated with P&P sludge and domestic wastewater anaerobic digester (WW-AD) sludge. Average methane yield with the P&P inoculation strategy reached 100.2 ± 2.4 L CH4/kg volatile solids (VS), a 72.7% enhancement compared with WW-AD sludge (58.1 ± 1.2 L CH4/kg VS) over 106 days of batch digestion under mesophilic conditions. Trends in evolution of leachate volatile fatty acid and total ammonia nitrogen concentrations suggested that hydrolysis was accelerated in the P&P-inoculated digesters, causing methanogenesis to become rate limiting. Additional experiments were carried out to determine if the enhancement could be sustained through inoculation of fresh yard waste with digestate from the initial digesters, resulting in a 68.5% enhancement of methane yield. Although observed improvements are comparable to certain pretreatment methods, inoculation with sludge from the pulp and paper industry could be a low cost and less resource-intensive alternative to pretreatment and improve the overall sustainability of HS-AD processes.