Organic acids, biochemical oxygen demand and chemical oxygen demand in the soluble fraction of piggery slurry

Abstract Close, largely linear, relationships were found between volatile fatty acids (VFA) determined by gas chromatography, total organic acids (TOA) determined colorimetrically, biochemical oxygen demand (BOD 5 ) and chemical oxygen demand (COD) in the soluble fraction of piggery slurries. Samples were taken from untreated and aerobically‐treated slurries. All correlations were significant at P =0.001, with values of r ranging between 0.88 for the correlation of VFA and COD, to 0.97 for the correlation of TOA and BOD 5 . VFA accounted for up to 69% of BOD 5 and 55% of COD. It is suggested that organic acids could replace soluble BOD 5 in some situations, since the correlations were good. Prediction of COD from organic acids, or BOD 5 from COD, were less accurate, particularly when concentrations were low, since COD contains a biologically inert fraction, which varies with storage. Soluble organic nitrogen was not useful in predicting BOD 5 or COD, correlating only at P =0.02.