The anaerobic digestion (AD) process can generally be classified into two categories based on the total solids (TS) content of material in the digester: solid-state anaerobic digestion (SS-AD) which has TS content of 20% and higher, and liquid AD which has TS content of 15% and lower. SS-AD has the advantages of using a smaller reactor volume and less moving parts as agitation is generally not required. Energy demand for heating the material in digester is therefore reduced with a smaller volume of materials to be heated.
Lignocellulosic biomass has been considered as one of the suitable feedstock for SS-AD as it is easily available and issues encountered in liquid AD such as stratification can be avoided. The challenge of utilizing lignocellulosic biomass in AD is the recalcitrant properties of such material. Pretreatment is therefore necessary to improve the biodegradability of lignocellulosic biomass.
In this study, SS-AD of four types of feedstocks was tested under batch operation. The SS-AD was conducted under 37°C for 30 days. The feedstocks selected were corn stover, wheat straw, fallen tree leaves, and yard waste. As inoculation is essential in SS-AD, effect of substrate to inoculum (S/I) ratio (volatile solids of substrate to volatile solids of inoculum) on methane yield was investigated. For all feedstocks tested, the highest methane yield was obtained at S/I ratio of 2. The highest methane yield of 81.2 L/kg volatile solids (VS) was obtained from SS-AD of corn stover followed by wheat straw (66.9 L/kg VS), leaves (55.4 L/kg VS) and yard waste (40.8 L/kg VS). An inverse linear relationship was obtained between the methane yield and lignin content of lignocellulosic biomass. The methane production of the feedstock in SS-AD fits the simple first-order kinetic model.
SS-AD of leaves with simulatanoues alkaline (NaOH) treatment was further studied to improve the methane production. The highest methane yield of 81.8 L/kg volatile solids (VS) was obtained at NaOH loading of 3.5% and S/I ratio of 4.1. However, it was not significantly different from that of control (without NaOH addition). At S/I ratio of 6.2, NaOH loading of 3.5% enhanced the methane yield by 24-fold over the control. The AD process at S/I ratio of 8.2 were failed. In addition, increasing the total solid (TS) content from 20% to 26% reduced biogas yield by 35% in reactors at S/I ratio of 6.2 and NaOH loading of 3.5%. Cellulose and hemicellulose degradation and methane yields during the 30-day AD process are highly related.
The results obtained from this study showed the feasibility of utilizing lignocellulosic biomass as feedstock in SS-AD for biogas production. In addition, alkaline pretreatment with NaOH was also shown to enhance methane production from leaves in SS-AD.