Generation and Emissions of Volatile Fatty Acids and Alcohols during Anaerobic Storage of Dairy Manure

Abstract

Production of volatile fatty acids (VFAs), alcohols, and several other compounds in batch stored dairy manure was experimentally studied. The test conditions included three initial total solids (TS) contents of dairy manure (2%, 4% and 9%) and three storage temperatures (15, 25 and 35oC). Under each set of test conditions, the concentrations of methanol, ethanol and VFAs in the manure and the amount and the composition of biogas produced were measured over one month period. Results showed that the higher TS content of the manure resulted in more VFAs and less biogas production. Higher temperatures had positive effect on methanogenic activity especially in the manure of 2% TS. At 35oC, the total VFAs reached 16,500 and 23,000 mg[COD]/L in the manure of 4% and 9% TS, respectively. Acetic acid represented over 42% of the total VFAs as COD. The highest methanol and ethanol concentrations were found in the manure of 9% TS stored at 35C at 30 mg/L and 230 mg/L, respectively. A mathematical model was developed to calculate the generation and emission rates and cumulative emissions of acetic acid from dairy manure storage under different temperature and wind velocity conditions. Modeling results for a 1,000-cow dairy farm located in Davis, California showed that the cumulative acetic acid emissions from a lagoon receiving the manure of 2% TS were about 4.8 and 11.2 g/cow/month under winter and summer conditions, respectively. More research is underway to calibrate and validate the developed emission model over a longer storage period.