The biochemical methane potential test is the most commonly applied method to determine methane production from organic wastes. One of the parameters measured is the volume of biogas produced which can be determined manometrically by keeping the volume constant and measuring increases in pressure. In the present study, the effect of pressure accumulation in the headspace of the reactors has been studied. Triplicate batch trials employing cocoa shell, waste coffee grounds and dairy manure as substrates have been performed under two headspace pressure conditions. The results obtained in the study showed that headspace overpressures higher than 600mbar affected methane production for waste coffee grounds. On the contrary, headspace overpressures within a range of 600-1000mbar did not affect methane production for cocoa shell and dairy manure. With the analyses performed in the present work it has not been possible to determine the reasons for the lower methane yield value obtained for the waste coffee grounds under high headspace pressures.

Cheese whey is considered an important pollutant in dairy wastewaters and an environmental problem to solve. This study aimed to develop a treatment process that combines the advantages of co-digesting cheese whey with manure and the short hydraulic retention time of a high-load anaerobic treatment system by using liquid fractions of dairy manure as a co-substrate. The proposed co-digestion process consists of a one-stage UASB reactor with an external settler and effluent recycling for alkalinity supplementation. Under a constant hydraulic retention time of 2.2 days and increasing proportion of cheese whey in the feed, the system demonstrated stable operation up to a 75% cheese whey fraction in the feed, with an applied organic loading rate of 19.4 kg COD m−3 d−1, obtaining a 94.7% COD removal and a volumetric methane production rate of 6.4 m3 CH4 m−3 d−1. Critical biomass washout was experienced when the cheese whey fraction in the feed was 85%. Operation at a constant 60% cheese whey fraction in the feed mixture enabled stable operation under an organic loading rate of 28.7 kg COD m−3 d−1 and 1.3 days HRT, with 95.1% COD removal and a volumetric methane production rate of 9.5 m3 CH4 m−3 d−1. This new high-load co-digestion method proposed is a promising solution for areas where cheese factories and intensive livestock farming are responsible for environmental pollution caused by unsuitable cheese whey and manure management practices