Impact of thermobaric pre-treatment on the continuous anaerobic digestion of high-fat cattle slaughterhouse waste

Abstract

Thermobaric pre-treatment of a combination of dissolved air flotation (DAF) sludge and slaughterhouse wastewater was evaluated for performance over 50 days of continuous anaerobic digestion. Continuous digestion was conducted over three phases represented by varying fat, oils and grease (FOG) concentrations and organic loading. In comparison with earlier biochemical methane potential (BMP) investigations using thermobaric treated substrate by Harris, Schmidt and McCabe (Harris et al., 2017) which yielded an 8.32% increase in specific methane production, pre-treated DAF sludge produced negative impacts on digestion under continuous conditions. Average pH was consistently lower by 0.04, and loss of volatile organics during pre-treatment reduced methane yield by 12.1%. H2S concentration was 56% higher on average with 795 ppm compared with 510 ppm in the controls owing to enhanced protein degradation. Alkalinity was low due to insufficient replacement from the substrate. Fresh substrate containing double the fat content (236 mg/L) and reduced organic loading rate (OLR) caused both control and treatment reactors to fail, highlighting the need for consistent substrate characteristics. Magnesium hydroxide addition effectively recovered both pH and biogas production within digesters rapidly, addressing the underlying complication of insufficient alkalinity contribution by the substrate.