Modelling sunlight disinfection in a high rate pond

Studies of disinfection in conventional waste stabilisation ponds (WSPs) suggest that sunlight is an important factor, sometimes interacting with elevated dissolved oxygen and pH. Shallow depth and mixing in ecologically engineered high rate ponds (HRPs) enable greater exposure of wastewater to sunlight than in conventional WSPs, and we hypothesised that the reported efficient disinfection in HRPs may reflect this. We conducted two experiments on disinfection in a pilot-scale HRP treating dairy farm wastewater. The HRP was operated in batch mode so that removal of Escherichia coli could be followed by sampling over time (2 days). In both experiments, E. coli removal was rapid during daylight hours and slow overnight. The data were well-fitted by a simple model with a dark die-off term evaluated using night data, and a sunlight exposure term evaluated using day-time data. Dissolved oxygen and pH appeared to have little influence on inactivation rate over the measured range of pH (8.0-9.2) and DO (0-22 m(-3)). Overall, about 75% of the total E. coli inactivation in the HRP was attributable to sunlight action. (C) 2004 Elsevier B.V. All rights reserved.