Quantification of the effect of CO₂ transfer on titrimetric techniques used for the study of biological wastewater treatment processes

Titrimetric methods are considered to be useful for the study of biological wastewater treatment processes, particularly those processes that have negligible influence on the dissolved inorganic carbon pool. However, the application of titrimetric methods for studying biological processes that produce/consume carbon dioxide is not straightforward as microbial activity affects the total amount of dissolved inorganic carbon with a proportioned change (determined by pH) in the concentration of every species of inorganic carbon. In this work, the impact of adjustments to the inorganic carbon pool on titrimetric data was assessed by considering a pH-stat titration of heterotrophic carbon oxidation. It was confirmed that at typical operating conditions (pH 7.5 and K_La_CO2 ≈ 22.5 h^-1) carbon oxidation causes a marked increase in the rate of carbon dioxide transfer and consequently has impact on titrimetric data. Model simulation was used to quantify the impact for a wide range of operating conditions. It was found that only when a titration is operated at pH > 8 with a K_La _CO2 < 10 h^-1 can the interference that results from action of the bicarbonate system be neglected (< 5% error induced). Outside these operating conditions it is suggested that the interference be accounted for by either measurement or modelling of carbon dioxide transfer.