Implementing WSAA's Wastewater I/I Guidelines Here in NZ - Why?, What?, How?

With the La Nina weather cycles increasing rainfall along most of eastern Australia and New Zealand in the past 3 years, the reduction of rainfall dependent inflow and infiltration (I/I) as a means of reducing wastewater overflows is now firmly back on the agenda of wastewater system managers on both sides of the Tasman. The Water Services Association of Australia (WSAA) is an industry membership organisation that represents the 25 largest water companies/authorities in Australia and New Zealand. To respond to the evolving market need of its member organisations, WSAA recently engaged GHD, in association with local consulting partner Urban Water Solutions (UWS) to develop a Best Practice Guideline Document on I/I Management.

To the authors’ knowledge and understanding, this project effectively represents the definitive worldwide research work into I/I management practices. The resulting Good Practice Guideline Document can now be used by wastewater system managers to more confidently implement I/I reduction programs as a means of better managing wastewater system overflows.

Carried out in two stages of work, the project outputs define both theory and practice of good I/I management. This first stage work considered the processes and outputs of a range of real project case studies to develop the Good Practice Guideline document.

The second stage of the project has more recently been completed and brings forward the outputs of the first stage works into a more detailed and definitive outputs that include not only the background and theory on I/I management but also predictive guidelines on likely I/I reduction outcomes and a range of easily implementable calculation tools. Having consistent approaches and means of quantifying I/I has been proven in the case studies to be a key to the success of I/I reduction programs.

What the WSAA work found was that from the significant number of projects whose outcomes have now been analysed, there is now a robust knowledge base upon which to make far more reliable and informed predictive estimates of I/I reduction levels achieved from different levels of system rehabilitation. The work in New Zealand has evolved to a point where a predictive model now exists of the likely level of I/I reduction that can be achieved from existing levels for different extents of system rehabilitation. This is considered to represent a significant advancement worldwide in the planning of wastewater system improvement and management programmes.

Of greatest value to the wastewater systems industry is that the work gives wastewater planners and managers a far more robust basis upon which to undertake I/I reduction programmes through system rehabilitation with the expectation of certain outcomes for a given financial investment. This paper will therefore be of interest to the wide range of delegates challenged by the issue of infiltration and inflow in the effective management of their wastewater systems.