The effect of mixed-enzyme addition in anaerobic digestion on methane yield of dairy cattle manure

This study investigates the effect of applying a mixture of enzymes (ME) to dairy cattle manure (DCM) as substrate in anaerobic digestion (AD). The aims of this study were to evaluate different methods of ME application to DCM at different temperatures and to investigate the effect of adding ME during the pre-treatment of the solid fractions of dairy cattle manure (SFDCM). The results showed that there was no positive effect of direct ME addition to substrate at either mesophilic (35 C) or thermophilic (50 ◦ C) process temperatures, but there was a significant 4.44% increase in methane yield when DCM, which had been incubated with ME addition at 50 ◦ C for three days, was fed to a digester when compared to a control digester operating at the same retention time. Methane production was detected during the pre-treatment incubation, and the total sum methane yield during pre-treatment and digestion was found to be 8.33% higher than in the control. The addition of ME to the SFDCM in a pre-incubation stage of 20 h at 35 ◦ C gave a significant increase in methane yield by 4.15% in a digester treating a mixed substrate (30% liquid fractions DCM and 70% enzyme-treated SFDCM) when compared with the control digester treating a similar mixed substrate with inactivated enzyme addition. The results indicate that direct physical contact of enzyme molecules and organic material in DCM prior to AD, without the intervention of extracellular enzymes from the indigenous microorganism population, was needed in order to increase methane yields. Keywords: biogas; mixed enzymes; pre-treatment; incubation; manur

Optimisation and inhibition of anaerobic digestion of livestock manure

The objectives of this study were to:(1) optimise methane production of livestock manure during anaerobic digestion, focusing on the addition of mixed enzymes, thermal pre-treatment and codigestion, and (2) develop a better understanding of ammonia and sulphide inhibition during digestion of animal manure . Enzyme addition increased methane yield of manure following incubation prior to digestion. Thermal pre-treatment increased methane yield but was only energetically favourable when a surplus of thermal energy was available. Digestion of acidified manure showed sulphide inhibition but no inhibition was measured when processing solid fractions of acidified manure. Co-digestion of non-acidified manure with the solid fractions of acidified manure was found to be a promising method of increasing methane yield in terms of digester volume. Total volatile acids, isobutyric and isovaleric acid and total ammonia nitrogen were found to be useful indicators of ammonia inhibition