A Model for the Methane Yields of Dairy Cattle Manure

Background: The dairy sector is responsible for 4% of global anthropogenic greenhouse gas (GHG) emissions. These emissions must be reduced. Anaerobic digestion [AD] converts volatile solids [VS] in cattle manure to methane, carbon dioxide and reactor effluent. AD has been proposed as a technology to reduce the emissions of methane from manure storage at the farms. There is a need to predict the methane yield for anaerobic digestion [AD] of dairy cattle manure in order to optimise the size of the AD reactors. Method: The model is in analogy of the decay of soil organic matter with a modified time dependent decay constant. The model has been tested with experiments on dairy cattle manure in batch and continuously stirred reactors. The model enables not only the prediction of the methane yield but also the calculation of the effect of AD on the effective organic mass [EOM] of the soil. The EOM is important for its fertility. A costs model has been developed for the reactor size. Results: The test results have been divided in four groups (Two for batch reactors, lab-scale reactors and farm scale reactors. The destruction of VS is in the reactors is ten times faster, than that in the soil. The model has a limitation as for each VS content in equation a decay constant can be fitted with the same root mean square [RMS]. A reduction of around 25 % in EOM has been calculated for a research farm using AD of the manure. Conclusion: The model presented can predict the methane yield of cattle manure with an accuracy of 10 - 15 %. It needs a minimum of input (manure production and retention time).The model enables the calculation of the effect of AD on the effective organic mass [EOM] in the soil. The methane production costs are nearly the same for a range of reactor sizes ( between 500 m3 and 1 500 m3) for a manure input of 7 500 m3/a.