Effect of thermal pretreatment on chemical composition and biogas production from kitchen waste

Biogas has been well known with its wide range of benefits in terms of greenhouse reduction,energy security, creating jobs and generating revenue. However, biological conversion of several biogas feedstocks to biogas under anaerobic digestion has proven inefficient due to low conversion efficiency. Pretreatment of certain type of biomass has been known to improve the biodegradability and methane yield of the pretreated biomass. In this study, the effect of thermal pretreatments viz., pressure-cooking with steam (PC1), pressure-cooking with water (PC2), steam pretreatment in steamer and hydrothermal pretreatments on chemical composition and methane potential of kitchen waste was investigated. Grab samples of kitchen waste were collected from Yliotö restaurant located on the Ylistönrinne campus of the University of Jyväskylä. Methane production from untreated and pretreated kitchen waste was determined in batch experiments at 35 °C and 55 °C. Chemical analyses showed that untreated kitchen waste, which was mainly composed of vegetables trimmings (56%), fruit skins and spoilt fruit (43%) and bread (1%), had total solids (TS) content of 12.6 % with volatile solids (VS)/TS ratio of 0.94. PC1, PC2 and steam pretreatment resulted in an increase in TS and VS by 1.6-20.2 % and 0.8-21.1 %, respectively. On the other hand, TS and VS content of hydrothermally pretreated kitchen waste remained more or less unchanged. All pretreatments resulted in an increase in soluble chemical oxygen demand (SCOD), ammonium nitrogen (NH4-N) and total kjeldahl nitrogen (TKN) concentration. However, complete loss of NH4-N was noticed in PC2. Loss of volatile fatty acid (VFA) (41.5-85.7%) was also noticed from all pretreated samples due to high pretreatment temperature used in the study. From untreated kitchen waste, methane yields of 426 ml/gVSoriginal at 35 °C and 452 ml/gVSoriginal at 55 °C were obtained. Similarly, methane yields obtained from pretreated kitchen varied from 410-439 ml/gVSoriginal at 35 °C and 406-462 ml/gVSoriginal at 55 °C. The results indicate that the studied thermal pretreatment methods had affected the chemical characteristics of kitchen waste to a large extent but no or little effect on methane yields. An increase in methane yields of 1.9-3.1 % was noticed in pretreatments viz., PC1, PC2 and steam pretreatment. The low or no increase in methane yield was probably due to loss of VFA during thermal pretreatment and the possibility of inhibitory products formation.