AWWA ACE95268

The work presented in this paper was done to investigate alum and acid enhanced coagulation at full-scale as a method for stabilizing molecular ozone concentrations in a subsequent ozonation step, through lower pH levels and reduce ozone demand. Reduced ozone demand was achieved through the improved NPOC removal prior to ozone addition. Enhanced coagulation was found to have a positive impact on ozone stability, NPOC removal, turbidity removal and aluminum residuals in the treated water may be reduced during enhanced coagulation. Two classes of common ozonation byproducts, aldehydes and carboxylic acids, were found to form in the ozone contactor. The compounds with a smaller number of carbon atoms were formed preferentially. A Bayesian statistical method was used for part of the full-scale experimental design. This method uses prior knowledge to maximize the amount of information gained in a minimum of experimental runs. In this case, the prior knowledge was gained from lab-scale and preliminary full-scale experiments. A range of ozone doses was investigated to allow the development of a relationship between the level of enhanced coagulation, ozone dosage, and calculated level of Cryptosporidium inactivation. Simple models based on carbonate system equilibrium are used successfully to predict the level of acid addition required to achieve a specified pH level after coagulation, and the amount of caustic (NaOH) needed to increase the pH after treatment. An empirical model was developed to describe the removal of NPOC through pretreatment. Cost and inactivation contour curves were developed for transferred ozone dosage at specified temperatures and alkalinity. Alkalinity was stable through each of the seasons, so the curves established were for specified temperatures during the seasons of interest. The contours show cost or inactivation vs alum dosage and pH after coagulation at a given temperature and ozone dosage. These curves permit site-specific costing for a specified level of Cryptosporidium inactivation. A general philosophy for ozone contactor design is highlighted by the work.