Modelling Escherichia coli removal during domestic wastewater treatment in outdoor pilot scale High Rate Algal Ponds

Abstract

While there is growing evidence that High Rate Algal Ponds (HRAPS) can support disinfection during wastewater treatment, the mechanisms of pathogen removal involved remain unclear. Based on previous findings, the present study aimed at verifying the occurrence and significance of dark decay, alkaline-pH toxicity, and sunlight direct damage in conditions representative of ‘real’ HRAP operation. For this purpose, Escherichia coli cell count was monitored hourly during 24 h periods in outdoor pilot HRAPs fed domestic wastewater. The data generated was used to create 2 distinct data sets that were then used to calibrate and validate a model of E. coli removal in HRAP. By modelling the contribution of the three mechanisms considered, E. coli removal in HRAPs could be predicted with a mean relative absolute error of 5.34% (N = 46). The model was then inputted yearly environmental data obtained from the continuous operations of the pilot scale HRAPs to evaluate the yearlong contributions from each mechanism to overall E. coli removal. Dark decay was thus predicted to support the highest yearlong mean contribution (78.6 – 91.0% of total E. coli decay, at the 95% confidence level) followed by sunlight mediated disinfection (6.01 – 17.8%) and pH toxicity (0.22 – 6.2%). Significant seasonal and daily fluctuations of decay mechanism contributions were evidenced in this study.