The Impact of Preservation Techniques on Methane-Arrested Anaerobic Digestion of Nutrient-Rich Feedstocks

The carboxylate platform is a promising biomass-to-energy pathway that uses methane-arrested anaerobic digestion (MAAD) to convert biomass to carboxylic acids, which can be chemically converted to industrial chemicals and liquid fuels. Lignocellulose is an energy-rich carbon source, but lacks nutrients necessary for microbial growth. Chicken manure (rural waste) and sewage sludge (urban waste) are rich in nitrogen and useful macronutrients; therefore, co-digesting these wastes with lignocellulose improves MAAD performance. However, waste nutrients must be digested immediately, or preserved. This study investigated the effects of various preservation techniques - frozen (fresh), air-dried, and baked - on chicken manure and sewage sludge. Batch experiments were performed with office paper (carbon source) and chicken manure or sewage sludge (nutrient source) with different methods of preservation. Fresh substrates produced higher acid yields and biomass conversion (the amount of biomass consumed during digestion) than dried substrates. Baked chicken manure showed reduced conversion and total acid production, which suggests that oven-drying reduces digestibility. From the batch data, the Continuum Particle Distribution Model (CPDM) predicted results of a four-stage countercurrent digestion. The data are displayed on maps showing the impact of liquid residence time (LRT) and volatile solids loading rate (VSLR) on conversion and product concentration. Co-digesting office paper and wet chicken manure at a non-acid volatile solid (NAVS) concentration of 300 g/L_liq, the model predicted a high total acid concentration of 52.8 g/L and conversion of 0.89 g NAVS_digested/NAVS_fed at a volatile solid loading rate of 4 g/(L_liq·day) and liquid retention time of 35 days.