Mechanistic study of on-line bicarbonate (hydrogencarbonate) monitoring of anaerobic sewage sludge digesters

Abstract

A study is reported of a simple method for bicarbonate (hydrogencarbonate) alkalinity determination in an anaerobic sewage sludge digest based on the measurement of pressure increase in a constant-volume vessel on acidification of a sludge aliquot. The method provides the basis for a simpler and more robust instrument than the current constant-pressure designs, an important consideration for unattended operation in commerical installations. The factor that has led to the previous dismissal of constant-volume instruments has been the perceived problem of carbon dioxide solubility in water and the proportionality of the solubility with carbon dioxide partial pressure. Pre-saturation of the sample aliquot with carbon dioxide was thus inappropriate for such instruments. In fact the instability of aqueous sodium bicarbonate is shown to give rise to errors in bicarbonate alkalinity determination which are significantly larger than those resulting from failure to pre-saturate with carbon dioxide, provided that carbon dioxide partial pressures do not exceed typically 50 mbar over a 0.08 mol dm–3 aqueous sodium bicarbonate solution, typical of a digester sludge. Furthermore, carbon dioxide pre-saturation is shown to exacerbate significantly the effect of sludge sulfide content on the accuracy of bicarbonate alkalinity determinations. A theoretical basis is presented for this phenomenon and the conclusions are supported by the results of a factorial analysis experiment. A standard additions calibration of carbon dioxide pressure increase resulting from the neutralization of 10 cm–3 of a typical digested sludge, with additions of 6.756 g dm–3 aqueous sodium bicarbonate solution, was linear with a correlation coefficient of greater than 0.999 between sodium bicarbonate concentration and pressure increase. This calibration has been used in the monitoring of a digester by daily triplicate bicarbonate alkalinity analyses which have had an average relative standard deviation of approximately 2.7% over a 2 month period. Comment is made regarding a modification to this prototype instrument that will allow the simultaneous monitoring of sludge sulfide content by means of an electrochemical sensor, the sulfide content measurement to be applied as a correction to the bicarbonate alkalinity determination.