By Charlene Wong, Engeny Water Management Network Planning Engineer and Paul Sherman, Principal Scientist – Water Quality Planning, Queensland Urban Utilities.
This paper describes a new process of using network water quality modelling of secondary disinfection (chloramine) to directly inform investment decision making.
This new approach supports Queensland Urban Utilities’ (QUU) strategy of active water management with restructuring of infrastructure to avoid a traditional network water quality management approach, such as installing a new chemical boosting station.
Water quality modelling was undertaken for QUU’s Chapel Hill water supply zone to understand the cause of its poor secondary disinfection performance. A list of potential active water management and infrastructure investment options to improve water quality outcomes had been developed and modelled.
The water quality modelling was able to identify options that restructured the infrastructure to improve water quality outcomes with capital and operational savings realised, as well as maintaining resilience.
In 2016, water utilities within South East Queensland (SEQ) collaborated on the Regional Secondary Disinfection Optimisation Project (RSDOP), which aimed to improve the secondary disinfection regime on a SEQ regional and sub-regional basis.
As part of the RSDOP, Engeny Water Management assisted in the development of a regional water quality network model to help assess the outcomes of improved secondary disinfection decay stability across SEQ.
Since the completion of that project, Engeny and QUU have incorporated the SEQ network specific chlorine and chloramine decay kinetics, developed during RSDOP, into various ‘all-pipes’ reticulation network models for the QUU water supply reticulation networks.
This paper focuses on the development of a water quality model for the QUU Chapel Hill water supply zone (WSZ) to investigate and improve water quality management within the zone.
The Chapel Hill WSZ is supplied from the Mount Crosby water treatment plant (WTP) via Seqwater’s Green Hill bulk water supply storage tanks (Figure 1). As this zone is relatively close to Mount Crosby, the zone would be expected to have relatively good secondary disinfection outcomes; however, the area has historically seen poor water quality performance.
The objective of this study was therefore to first understand the cause of poor secondary disinfection performance within the Chapel Hill WSZ and second to identify, model and shortlist water quality improvement options.
Prior to the assessment, model validation was undertaken by comparing the modelled results against the recorded total chlorine data within the Seqwater and QUU water supply networks over similar network demand, network operating strategies and water temperature scenario.
The model predictions closely aligned with the corresponding recorded data, providing confidence in using the Chapel Hill WSZ network model to:
- establish the current status of the secondary disinfection performance;
- assess the impact of planned regional secondary disinfection residual stability improvements at the major WTPs as identified during RSDOP;
- develop and assess the effectiveness of several active water management and restructuring the infrastructure options to improve secondary disinfection performance; and
- identify options that are able to achieve the secondary disinfection target.
The existing system performance within the Chapel Hill reticulation network was simulated and is presented in Figure 2. A total chlorine residual of 0.5 mg/L as a measure of chloramine concentration at the customers’ meters was adopted as the target disinfection residual.
As shown on Figure 2, the eastern side of Chapel Hill, the entrance to the zone, is largely supplied directly from the Russell Terrace pump station and therefore maintains relatively high residuals.
In contrast, the western Chapel Hill area, further into the zone, is either supplied from the Russell Terrace pump station when the pump is operating, or is back fed from the Chapel Hill storage tank when the pump is not in operation.
As the Chapel Hill storage tank (10 ML) is oversized compared to the zonal demand (1.2 ML), and is located at the extreme periphery of the zone with a single inlet outlet pipe, very poor disinfection residual has been simulated for the western Chapel Hill area.
Findings from the RSDOP indicated that the combination of improved secondary disinfection stability at the major regional WTPs, including increased organics removal, improved ammonia control and raised pH for chloramine stability, will potentially achieve improved treated water chloramine stability between 40% and 70% (see Figure 3 for decay profiles).
Given the poor network configuration and that the Chapel Hill storage tank is vastly oversized, modelling results predict that the poor water quality performance within the Chapel Hill western area cannot be resolved by improved residual stability.
Several active water management and infrastructure restructuring options to improve secondary disinfection performance have been identified and modelled (see Table 1). The options were selected based on the following principles:
- Active water management – optimising existing infrastructure to deliver safe and reliable water to our customers at the best cost.
- Restructure the infrastructure – delivering safe and reliable water by efficiently locating appropriate infrastructure.
The modelling assessment shortlisted options that are able to achieve the secondary disinfection target of having more than 0.5mg/L total chlorine at the customer’s meter. The shortlist of options has been progressed through to concept design and preliminary costing phase to identify the preferred solution to improve the water quality performance within the Chapel Hill WSZ.
The authors were able to use a water quality model connected to hydraulic model as a planning tool to compare scenarios on how to transform the way existing water systems are managed to deliver safe water to our customers. In this case, this approach demonstrated that restructuring the infrastructure would potentially deliver a systematic reduction in cost to deliver the safe water by more efficiently and resiliently locating appropriate infrastructure.
This process would avoid undertaking risky solutions that would either involve untested active water management approaches or delay building inappropriate and unnecessary infrastructure.