Future thinking in design of sewage pumping mains

Future thinking in design of sewage pumping mains

By Martin Osborne &
Matt Leedham, WSP

Sewage pumping stations are a vital part of sewerage systems to ensure that low lying areas can be effectively drained. The UK water industry has at least 40 000 sewage pumping stations, many discharging through long pumping mains. It is how the pumps work together with the pumping mains that set the performance of the pumping system.

Pumping mains bursts are a common occurrence with typically 2.5% of pumping mains suffering a burst each year. This often leads to pollution of watercourses and can cause property flooding. In addition it has been estimated that as many as half of all sewage pumping systems have poor hydraulic performance and do not pump as much flow as they should or do not pump it in an energy efficient way. So what is going wrong?

The normal design approach to pumping mains is to design for a minimum velocity of 0.75 m/s to keep the main free of sediment and clean of slime and grease. To achieve this the pumps run intermittently alternating between a high flow rate to give adequate velocity and stopped with no flow.

This has several effects.

The high velocities mean high friction losses and so increased energy use.

The high friction losses mean high pressures in the main and alternating running means pressure shocks that can eventually cause fatigue failure of the main. Up to half of all pumping mains are uPVC and many are approaching the end of their asset life due to fatigue from cyclic loading.

Even at this relatively high velocity the mains (especially small diameter ones) still accumulate fats and slime that reduce their capacity and further increase the pump pressure.

The sewage can become septic while it is standing in the pumping main leading to odour and corrosion problems further downstream.

WSP has recently been involved with trials of a new technique for cleaning pumping mains that can restore their original capacity. This uses ice-pigging, a plug of ice slurry, pushed through the main by pumping water in behind it. The technique was original developed by the University of Bristol for the food and chemical process industries. It was then adapted by Bristol Water for use on water distribution mains. It is a proven technique for these applications used on approximately 700 km of pipe per year across 13 countries.

In pilot projects for Severn Trent Water the technique was successful in restoring the original performance of the pumping systems giving increases in pumping rates of up to 40%; even for pumping mains that had been designed to the conventional 0.75 m/s “self-cleansing” velocity. Perhaps the same behavioural changes that are leading to fatbergs in gravity sewers are also leading to increasing fat deposits in pumping mains.

So if our current design standards are not keeping mains clean, then either we need to increase velocities even more with resulting increase in pressure, energy use and fatigue failure or we accept that pumping mains will need periodic cleaning and design instead for long life and low energy.

With this new approach, pumps would run at lower flow rates but run for longer. This would give lower pressures and also fewer pressure surges as pumps start and stop; so extending the life of the pumping main. The lower velocities have lower friction losses; and so use less energy. If we could halve the velocity we reduce the energy lost to friction to a quarter. Periodically we would need to clean the main of deposits to keep it operating properly; but experience is that we should be doing that anyway; but generally we do not.

In parallel with planned maintenance of the pumping main would be planned maintenance of the air valves installed at high points on the main. Poorly maintained air valves can lead to air pockets that also reduce the pumping capacity and can also increase the transient pressure when pumps start and stop, eventually leading to failure of the main.

So the new approach would move away from a high capex; low maintenance approach where we build a pumping main and assume that it needs no maintenance until it fails and is replaced; to a true best-value totex approach where we design allowing for ongoing maintenance of the pumping main but achieving more efficient operation and a longer life? The current update of the standard Sewers for Adoption perhaps offered a chance to change design practice; but it appears to be a chance missed.

www.wsp.com

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