Fixed-film anaerobic digestion of flushed dairy manure after primary treatment: wastewater production and characterization

The characteristics of flushed dairy manure after primary treatment at a Florida dairy were monitored daily over a 1-year period along with cow numbers and water use, with the aim of developing fixed-film anaerobic digestion of flushed dairy manure wastewater (FDMW). Primary treatment included screening through a mechanical separator followed by sedimentation. The FDMW exhibited average total solids (TS), volatile solids (VS) and total chemical oxygen demand (COD) levels of 3580, 2210 and 3530 mg l(-1), respectively, for a milking herd average of 359 cows and a wastewater production of 502 m3 d(-1). Analyses of FDMW parameters and manure excretion estimates indicated that 53% of the TS, 40% of the VS and 58% of the total COD remained in the wastewater after screening and sedimentation. The separation of fibrous solids increased the COD to VS ratio of the FDMW. Wastewater temperature was found to significantly affect both the separation of TS, VS, and total COD, and the partitioning of wastewater parameters between the particulate and soluble phases. Higher FDMW temperatures tended to reduce TS, VS, and total COD removal efficiencies. The ratio of soluble COD to dissolved VS increased with temperature, while the ratio of particulate COD to volatile suspended solids decreased, indicating that significant levels of biological activity occurred in the sedimentation basin at higher temperatures. The VS measurement does not accurately determine the methane potential of components in FDMW compared to COD measurements. In spite of the fact that 60% of the VS is removed by screening and sedimentation, the major portion of the methane potential remains in the wastewater since the solids that are removed are relatively non-degradable fibres and do not contribute to methane production. Model equations were developed to predict total COD levels in flushed dairy manure wastewater as a function of fresh flushwater usage per animal unit for similar dairy operations, providing an important design parameter for implementation of fixed-film anaerobic digestion systems.