Use of WatPro Model for Infrastructure Planning
A First Nation band in Northwestern Ontario draws its drinking water from a lake of moderately high TOC and colour. The water supply is susceptible to taste and odour complaints during summer months, when algae levels are at their peak. In early 1997, an outbreak of Cryptosporidiosis occurred in the community. At present, treatment of the raw water consists only of chlorination prior to discharge to the distribution system. Sediment has been observed periodically in tap water. In 1998, the band retained an engineering consultant to review the existing water supply and distribution system, and to recommend appropriate technology for upgrading the system. Concerns to be addressed included high colour, turbidity, odour and potential hazard of Cryptosporidium.
A review of water filtration and disinfection technology constituted a significant part of the study. Filtration processes were assessed to characterize microbial reductions, as reflected by the parasite Giardia, while the evaluation of disinfection processes involved both microbial inactivation and formation of chemical disinfection by-products, such as trihalomethanes (THMs).
In the consultantís assessment, the WatPro water treatment plant model was used to simulate the various new water treatment trains under consideration, including microfiltration (low pressure membranes) only, coagulation plus slow sand filtration, and coagulation plus microfiltration. The model was used to predict the levels of Giardia reduction and total THM formation at the point of entry to, and at selected end-user points in, the distribution system. Scenarios were developed to predict total THM formation based on water supply rates corresponding to average and peak flows one year after commissioning, and at the end of the design life of 20 years
Data entry to the WatPro model included:
Because of the high total organic content of the source water, microfiltration alone resulted in predicted total THM concentrations that were well over the target level of 0.10 mg/L of total THMs in the treated water leaving the plant. Substantial increases in the THM levels were predicted in the distribution system, especially under warm weather conditions, as illustrated by the figure below.
Coagulation using alum had a significant beneficial effect on the formation of trihalomethanes. The model simulations suggested that under most flow and temperature scenarios investigated, the total THM target level of 0.10 mg/L could be achieved. Although the terms of reference for the project did not include use of WatPro to optimize coagulant and disinfectant dosages to minimize THM formation, such evaluations could easily be accomplished with the model.
While chlorine may be less effective for inactivation of Cryptosporidium cysts than for other pathogens, use of a microfilter was judged to provide a highly efficient barrier to the Crypto cysts. Application of chlorine thus did not need to be applied for control of Cryptosporidium. Disinfection with chlorine after microfiltration was also judged to be effective in preventing growth of other microorganisms in the distribution system.
On the basis of the technical review and model simulations with WatPro, the recommended process train for upgrading the bandís water treatment system was coagulation, microfiltration and chlorine disinfection.
With use of the WatPro simulator, a large number of potential scenarios were quickly and easily addressed. The model was very useful in identifying the process conditions under which formation of total THMs might be a concern, especially when forecasting future trends.
Predicted Formation of TTHMs in First Nation Band Distribution System, based on Average Flow after 20 Years