MEASURING THE USABLE CARBONATE ALKALINITY OF OPERATING ANAEROBIC DIGESTERS

An alkalinity titration with an endpoint of pH 5.75 was shown to accurately measure the bicarbonate alkalinity and represent the state of operating anaerobic digesters. Unlike existing titration methods, the proposed test allows for direct determination of usable bicarbonate alkalinity, while excluding interferences due to commonly found volatile acids. Long-term monitoring of operating digesters showed that true bicarbonate alkalinity (TBA) values determined using the proposed endpoint compared well with those determined using the "Standard Methods" technique. Both TBA5.75 and the total alkalinity titrated at pH 5.75 (Alk5.75) were good measures of digestion activity; Alk5.75 values below 1200 mg/L as CaCO3 were indicative of digester trouble. Additional sludge samples also produced TBA5.75 and TBA4.3 values that correlated well (correlation coefficient 0.96). Reproducibility of the analytical techniques required to determine TBA5.75 was good, with generally less than 3% error. However, the error for volatile acids (VA) was as high as 30%. Volatile acids determination is a necessary step in estimating TBA4.3. Considering the difficulty encountered when measuring VA as required in the "Standard Methods" test, the proposed alkalinity measurement to pH 5.75 is superior to the currently accepted technique.