A comparison of methods for wastewater system performance assessment

Most organisations responsible for separate sanitary and combined sewer systems have target Levels of Service (LOS). Hydraulic models are frequently used to predict LOS performance and identify areas where targets are not met.

This paper compares three methods for system performance analysis, each of which has advantages and disadvantages:

• Design storms are simple to apply, however may not consider spatial variation of rainfall or back-to-back storm events.
• Long time series can be time consuming, but provide a more robust statistical understanding of how the system will perform for different conditions.
• A typical year is relatively quick to run and often uses actual rainfall; however determination of the typical year can be complex.

Modellers need to understand the system in order to understand the consequences and limitations of their choice of methodology. It is important to carry out sensitivity analysis, for example through the use of more than one method, and to understand how the models will ultimately be used to support development of potential solutions.

Current New Zealand standards leave the decision of methodology largely up to the organisation or modeller. This can lead to a lack of consistency and an inability to make meaningful comparisons of LOS.