Is modelling the cornerstone of a wastewater planning study?

Modelling is widely considered to be the cornerstone for wastewater planning studies. When a catchment is identified as having level of service issues it is common to default to “modelling” as the means to develop a capital investment plan (CIP). A typical programme will commence with flow monitoring, followed by model build/calibration, system performance assessment, options development and analysis and finally delivery of a CIP (or catchment management plan). Most of the costs, resources, and time are dedicated to the modelling component - but can we be confident of the outcomes, and is modelling the most efficient means to the end?

This paper considers seven recent Auckland wastewater studies retrospectively to assess the role flow monitoring, modelling, and alternative analysis tools played respectively. Opportunities to achieve cost and programme efficiencies are considered to achieve the same or greater confidence in the outcomes. Hindsight has 20/20 vision, so hindsight is applied to the studies to assess the confidence in the outputs, and whether there was a “better” way of delivering them. The conditions that contribute materially to these findings are also discussed. The findings demonstrate that :

• Some but not all studies require modelling,
• Sometimes models that are theoretically calibrated to flow monitoring data give answers that are inconsistent with flow monitoring data,
• Flow monitoring data and asset data, not a model, is the cornerstone of understanding system performance,
• Often operations data is useful, but often operations data is also misleading especially because it is incomplete,
• The presentation of models and model findings in reports generally do not allow engineers reading a report to understand what is happening in the network and why it is happening, making it impossible for engineers to review any recommendations.

This paper presents a) some key questions for consideration to assist in the structuring of a wastewater planning study, b) appropriate checks and balances to ensure the findings of the study have a high level of confidence, and c) some basic protocols as to how model and/or network information can be presented to communicate the findings more clearly.