In the context of sewerage systems:

• inflow refers to rainwater entering the sewerage system via defective or inappropriate plumbing of stormwater to the sewerage network and can be identified as the fast response component of a system’s total response to wet weather events.
• infiltration refers to groundwater entering underground sewerage system infrastructure via cracks and faulty joints [and can be identified as the delayed response component of a system’s total response to wet weather events]; and

Inflow and infiltration has many undesirable impacts upon wastewater systems, including:

• System overflows during wet weather events
• Increased pump station operating costs
• Increased treatment plant operating costs
• Excessive flows at treatment plants causing washout of biomass or bypass and consequent breach of compliance standards
• In coastal areas, high salinity levels that affect treatment plant processes and compromise the potential for effluent re-use

A well-designed and properly targeted infiltration and inflow reduction program can serve many purposes, including:

• Reducing the number and frequency of system overflows, and the resultant environmental pollution and public health and safety risks;
• Effective increase in system capacity resulting in deferral of system augmentation works;
• Reduction in operating costs at pump stations and treatment plants;
• Satisfaction of imposed standards and/or conditions at key system assets

Developing an in-depth understanding of the extent to which wet weather infiltration and inflows contribute to under-performance of wastewater systems is therefore a vital step for system managers when considering renewal programs.  A well-built dynamic hydraulic model can accurately predict the levels and impacts of infiltration and inflows occurring within a sewerage system enabling a targeted and cost-effective program of field investigation work to be developed.  As such, the model is a useful tool to use prior to committing to expensive field monitoring programs for the purpose of developing an infiltration and inflow reduction program.

Urban Water Solutions was the co-author of the industry standard – the WSAA Management of Wastewater System Infiltration and Inflow Good Practice Guideline and is Australia’s leading expert in the use of dynamic hydraulic modelling to predict infiltration and inflows in wastewater systems.