Safe drinking water for a major mine site in Western Australia
As the mine site water treatment plant (WTP) and water distribution system was being designed,
OneStone was commissioned to model the chlorination levels throughout the proposed system.
The aim was to see if chlorine dosing at the outlet of the WTP would be sufficient to maintain
appropriate disinfection levels for safe consumption throughout the entire downstream distribution system.
Potable water outlets from 11 water tanks throughout the mine site were considered, with disinfection considered acceptable if the residual chlorination at each outlet was within specific levels.
The initial challenge was to determine the optimal chlorine dosing rate at the WTP to achieve the target residual chlorine levels to maintain water disinfection across all tanks under low, average and high demand conditions. If appropriate residual chlorine levels could not be maintained across the entire distribution system, design, and operational modifications to achieve this were to be explored.
A model of the water distribution system was created, based on the design specifications (tank and pipe sizes, water pumping and valve operation, and tank filling criteria) provided by the water treatment process designer TRILITY Pty Ltd. The projected water demand at each tank outlet under low, average, and high demand conditions was also supplied.
The water age, representing the time taken for water to travel from the point of disinfection at the WTP to each of the distributed water outlets, could then be calculated for each water demand scenario.
Given a chlorination rate at the WTP and the known range of chlorine decay over time, the expected chlorine residual level could be calculated for the aged water at each tank outlet under each water demand condition. In this way the required chlorine dosing rate could be selected to achieve target chlorine residual levels across the distribution system for each demand scenario.
Where acceptable chlorine residual levels could not be achieved, tank operating levels were optimised, and potential design changes were modelled to seek improvements.
From the initial modelling, it was apparent that chlorine residual levels were unacceptably low across most of the tanks under low water demand conditions, despite applying a high residual chlorine level at the WTP. As the tanks were sized to accommodate high water demand, water movement was slow, and chlorine decayed to unacceptably low residual levels under low demand conditions.
A recommendation was made to adjust the operating strategy during low demand periods to reduce the operational volumes in several tanks, shortening the water retention time in these areas and lifting the residual chlorination level. Alternatives were to rechlorinate several tanks during these periods, or to create additional water demand.
Optimisation modelling demonstrated that acceptable chlorine residuals could be achieved across the distribution network in both average and high demand scenarios purely by modifying tank operating levels. Where acceptable residuals could still not be achieved in low water demand scenarios, the level of additional water demand required at each outlet was identified (which was still lower than average demand).
The modelling informed the operational strategy that was implemented once the water distribution system was constructed. Potable water quality criteria are now being met across the mine site year-round.