Effect of alkaline pre-treatment on the anaerobic biodegradability of coffee husk

Abstract

The present paper deals with the mono-digestion of coffee husk during the operation of batch reactors and semi-continuous stirred tank reactor (S-CSTR) using both pre-treated residual at 3% Ca(OH)2 and without pre-treatment. The semi-continuous operation was carried out in 4 stages at different organic loading rate (OLR, [gVS L-1 d-1]) and hydraulic retention time (HRT, [d]) (OLR:HRT) ratios (0.1:90, 0.2:90, 0.2:45 and 0.2:30). As a result of the pre-treatment, up to 223% of the organic matter solubilisation was obtained. However, pre-treatment did not provide better results in biodegradability, observing a negative trend in methane yield when operating in S-CSTR, due to the accumulation of compounds from the fractionation of the lignin as is the case of polyphenols. The operation was further developed using the residual without pre-treatment. The highest methane yield was obtained at 45:0.2 when using the residual without pre-treatment being with 277 mLCH4 gVS -1, for a methane productivity of 1.1 L L-1 d-1. When implementing the 30:0.2 ratio, a biomass washout was observed in the system, so it is recommended to operate at low OLR and high HRT when treating solid coffee wastes.