DISINFECTION CONTACT BASIN: COVER IT, OR NOT?

Residual chlorine loss due to ultraviolet sunlight in the chlorine disinfection contact basins were investigated at two full-scale wastewater treatment plants. The investigation compared the chlorine loss in chlorine contact basins that were covered against the basins that were open to direct sunlight. Chlorine decay due to UV-catalyzed reaction was found to be significant. The average chlorine loss across the contact basin due to sunlight photocatalytic decay at peak sunlight hours was 6.6 and 4.7 mg/L for the two plants studied. The total chlorine loss due to sunlight was about 28% and 19% of the total chlorine chemical usage at the two plants. One plant had about 36 percent reduction of total chlorine usage after they covered both chlorine contact basins, which result in approximately 37,000/year savings on chemical purchase. Covering chlorine contact basins also led to more stable chlorine demand throughout the day and throughout the year regardless of the diurnal and seasonal sunlight intensity. Therefore, it offers more stable/accurate chlorine dosage and effluent residual control, and less effort from the plant operators. A mathematical model was developed to calculate the amount of UV-catalyzed chlorine decay. The parameters in the model were determined by calibrating the model to the field observed data. Then the model was employed to evaluate the average UV-catalyzed chlorine decay rate at various average chlorine concentration in the DCB, as well as estimated total chlorine loss due to sunlight at various flow rates.