Modeling to Plan for One Distribution System with Two Disinfectants
Authors:
- Meg Roberts, PE, and Jeff Cruickshank, PE - Hazen and Sawyer
Johnston County North Carolina has a water distribution system with 1,600 miles of pipe serving a large geographic area (796 square miles), some of which is sparsely populated.
This large area and low population density produces high water age and, therefore, the County currently uses chloramine as the residual disinfectant to control disinfection by-products (DBPs). Even with the use of chloramines, DBPs are high, and the County sometimes experiences loss of chlorine residual and nitrification.
Additionally, growing maximum day demand is approaching the capacity of the existing water plant and purchased water supplies. Additional purchased supplies are available, but some use chloramine and others use free chlorine as a disinfectant.
The County plans to split its distribution system in order to separate chloraminated and chlorinated water. However, the chlorinated portion of the distribution system has water age in excess of two weeks, and therefore is more prone to high concentrations of DBPs.
To maintain adequate water supply and meet the Stage 2 Disinfectant / Disinfection By-Products (D/DBP) Rule, the County’s goal is to reduce organic carbon in their water. The design of the first MIEX treatment plant in North Carolina is underway at this time. MIEX will only be used on a portion of the water through the treatment plant and this water will primarily supply the portion of the distribution system where free chlorine is used as the residual disinfectant.
This paper summarizes a transmission main master plan that analyzed the impacts of splitting the distribution system. A hydraulic model was used to compare hydraulic performance and water age before and after the disinfectant split for existing water demands and growth expected by 2020. Johnston County’s DBP sampling was considered as part of this analysis, as well as future regulations expected to stem from the Stage 2 D/DBP Rule.
Several operational and infrastructure changes were recommended based on modeling results. The recommended changes maintain system pressures, lower water age, and/or maximize WTP capacity and pressure zone water supplies.
