Using I/I reduction and modelling to shape a wastewater renewal programme - Palmy's experience

Palmerston North City Council is well aware of significant inflow and infiltration issues throughout the city of Palmerston North, and its impact on the spare capacity of the trunk wastewater network, the capacity at the Totara Road Wastewater Treatment Plant, and associated operational costs.

A major capacity upgrade to the Totara Road Wastewater Treatment Plant is scheduled for 2029, which will meet the discharge consent that will be renewed by 2030. As part of managing the future capacity upgrade of the wastewater treatment plant, the Council is committed to reducing Inflow and Infiltration by 30% through rehabilitation of the wastewater network.

This paper investigates how fourteen years of data from seven different flow monitoring programmes was analysed to determine three key Inflow and Infiltration parameters for each wastewater catchment. These parameters were then used to rank and prioritise a city-wide rehabilitation programme to reduce Inflow and Infiltration. This has reshaped Council’s wastewater renewals works programme.

The programme began by carrying out pilot rehabilitation studies in the two worst catchments to better gauge the likely success of the works in reducing Inflow and Infiltration city-wide.

To achieve their 2030 target, it is estimated that the Council is likely to be required to double their renewals budget to target the Inflow and Infiltration alone. The impact of Inflow and Infiltration on treatment and power costs was quantified and used in a cost/benefit analysis of the works programme to determine how much of the works could be offset through savings in power and chemical costs.