Comparing investments decisions using different system performance assessment methods for wastewater networks

Target levels of service for the performance of reticulated wastewater networks under wet weather flows vary widely throughout the country, as does the approach for assessing whether these target levels of service are likely to be met. Generally target levels of service will require flows resulting from a wet weather event of certain magnitude to be passed without overflow, or restrict the frequency of overflows to a specified frequency.

Calibrated hydraulic models provide a powerful tool for assessing the likely performance of a wastewater network under a range of wet weather events and future growth scenarios. But more importantly, these models enable of a number of network upgrade options to be investigated to provide the desired level of service, and the cost of providing this level of service to be quantified.

A number of options are available to a wastewater modeller for assessing the system performance of a wastewater network, including the use of synthetic design storms, historical storms, a typical year rainfall time-series or a long rainfall time-series rainfall.

This paper compares and analyses the system performance results from a number of wastewater models using different methods. It also compares the costs of network upgrades required to provide a given level of service using different system performance assessment methods, and explores the different investment decisions which might be made on the back of using one of these methods over another.

The results presented are based on actual wastewater modelling projects undertaken throughout New Zealand and Australia, and involve wastewater networks encompassing a wide range of current system performance standards and target levels of service.