Inflow and Infiltration

Like many countries, the UK has an ageing sewer system with many parts deteriorating and in desperate need of repair. Older parts of the sewer network may be part of the Victorian combined sewer, whereas newer parts will have separate foul and surface water sewers.

The foul sewer is used to carry wastewater from toilets, dishwashers, washing machines, sinks, or showers in homes and businesses to a sewage works for treatment. The surface water sewer is designed to carry uncontaminated rainwater away and deposit it in rivers and streams.

Only rainwater run off should be connected to the surface water sewer. All other forms of wastewater and process water should be connected to the foul sewer.

Inflow & Infiltration

Inflow and infiltration (I & I) are terms used to describe the ways that surface water enters the foul sewer network and are two of the problems affecting sewer networks across the globe.

Inflow is water that enters the sewer system from above the ground sources. Various sources contribute to the inflow, including downspouts, drains from driveways or even a leaky manhole cover.

Infiltration is groundwater that seeps into the sewer system through cracks and leaks in the sewer pipes from the surrounding soil. Cracks or leaks in sewer pipes may be caused by older pipes that have deteriorated and cracked or collapsed, loose joints, damage caused during installation and maintenance work or root infiltration.

Inflow and Infiltration water is called “clear water” (although it may be dirty) to distinguish it from normal foul sewage water in the sewer system.

Why is inflow and infiltration a problem?

Heavy rainfall or snow thaw events can result in the sewer being forced to transport larger volumes of water than it was designed to handle, potentially leading to the sewer flooding. When that happens, wastewater and potential pathogens are released onto the streets and into waterways, polluting water and increasing the risk to public health.

When groundwater enters the foul sewer, it must be transported and treated like normal waste water. This has outreaching cost implications:

• Treating larger volumes of water in the treatment works, increases operating costs. This could be passed onto consumers and businesses in the form of rate increases.
• Businesses are charged different rates for discharging wastewater, trade effluent and surface water. The rate for surface water is much less than wastewater charges. So businesses that have surface water entering the foul sewer will be paying far more than they need to.

What to do about I&I

Reducing and controlling inflow and infiltration should be part of a long-term monitoring and maintenance program. Once evaluation of the sewer has taken place to determine the extent of inflow and infiltration, flow monitoring instrumentation is placed in the sewer at key points to obtain the desired data.

To measure wastewater flows through the foul sewer system it is important to select the appropriate flow meter, such as the Detectronic MSFM ultrasonic wastewater flow monitor.

The MSFM is a multichannel unit that records flow, level and depth data. Used in conjunction with a rainguage, data is transmitted to Detectronic data analysts, who can then make comparisons between the measured inflow and infiltration and the corresponding rainfall intensity. This analysis will reveal the extent of inflow and infiltration. For an I&I study, we would recommend an 8 week study for optimal results to include several separate rainfall events.

Once a source of inflow and infiltration has been discovered appropriate action to resolve the problem can be taken, including fixing or replacing damaged or leaky sewer pipes and notifying property owners of improper connections.