Assessing Performance Below the Ground – Is there a systematic approach to inflow and infiltration management in sewer networks or is beauty in the eye of the beholder

Sarah-Kate Dakin, Ed Couriel - NSW Public Works MHL , 30 October, 2014

Asset Managers face the long standing problem of groundwater and stormwater entering the gravity pipes of the wastewater reticulation system, known as inflow and infiltration. Often there is an initial qualitative assessment of sewer performance based on the number of overflows or the number of public complaints or treatment plants being under capacity during wet events.

To effectively assess sewer network performance it is essential to be able to quantify flows during dry weather and a range of wet weather events. Hydrography in the sewerage system has many challenges but many obstacles can be overcome by understanding the behaviour of sewerage flow at each individual site and knowing the limitations of gauging equipment. This paper reviews five local Councils across NSW that have undertaken sewage flow and rainfall monitoring and inflow and infiltration analysis and how the data collected was used to assess targets and inform remedial measures. Based on quantitative results; it supports a shift away from simplified design standards such as 5 x average dry weather flow towards an overflow containment risk standard that takes into account rainfall intensity of the location, soil type and the likelihood of defects, with infield verification.