Underestimation of alkaline dosage and precipitate amount during water treatment: Role of inorganic carbon and use of PHREEQ-N-AMDTreat

Abstract

Predicting the dosage of alkaline agents and the amount of sludge for wastewater treatment is crucial for designing and operating treatment facilities. Accurate predictions can contribute to reducing chemical usage and CO2 emissions. However, commonly used calculations based solely on pH and metal concentrations may lead to inaccuracies (e.g., ∼2000%). Thus, theory-based geochemical modeling and its assessment with experiments is necessary. In this study, the key factors influencing the dosage and PHREEQ-N-AMDTreat models were assessed using actual (n = 11) and synthetic (n = 7) mine drainage as well as operational data from pilot- and full-scale facilities, targeting pH of >8.3 to treat Mn. The ratios of the actual and simply calculated dosages had a positive linear relationship with the alkalinity of the raw water. Moreover, PHREEQC modeling revealed that the effect of the dissolved inorganic carbon on the increase of the dosage was more dominant compared to that of calcite precipitation. These findings suggest that the discrepancy in dosage arises because H2CO30 and HCO3− are important components of acidity, particularly when the pH is increased to >8.3. Among the three aeration conditions of PHREEQ-N-AMDTreat, aeration to equilibrium exhibited predictions closest to the measured values, owing to the most accurate prediction of Mn concentrations. Thus, PHREEQ-N-AMDTreat under the condition of aeration to equilibrium could predict the alkaline dosages and sludge amounts for wastewater treatment at pH > 8.3, with average actual/predicted values of 119% and 124%, respectively.