Abstract: 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 m3 d1, obtaining a 94.7% COD removal and a volumetric methane production rate of 6.4 m3 CH4 m3 d1. 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 m3 d1 and 1.3 days HRT, with 95.1% COD removal and a volumetric methane production rate of 9.5 m3 CH4 m3 d1. 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.

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