lift station

The principle of a wastewater collection system is very simply. Water flows downhill. To make it go uphill, it has to be pumped.

Take a typical house in the suburbs. Wastewater flows down into the sewer main, which collects wastewater from many houses, and feeds (discharges) into a lift station. The lift station then "lifts" the wastewater up to the next lift station which has probably also received wastewater itself from nearby houses.

This network of lift stations pumps into a Sewerage Treatment Plant (STP), where it is treated, and finally discharged into waterways or the ocean. Usually, the closer you get to the STP the larger the lift stations are, as they have to pump larger volumes than the outlying areas.

So what's in a lift station?

The essential elements are:

-a well (a hole in the ground usually) with an inlet
-2 pumps (sometimes more) connected into a discharge pipe
-a level sensing device (sometimes more than one)
-a control panel to turn the pumps on and off

At its simplest, when the level reaches a preset point - the activation point, or ON point, one pump is started. This empties the well under normal circumstances. When the well level reaches a lower preset point - the deactivation point, or OFF point, the pump is stopped.

In the next cycle, the other pump is started, evening out the wear between the two pumps. This is called alternation. If the running pump is faulted, or for any reason the level carries on rising, the other pump will be turned on.

Very simple, isn't it?

However, wastewater is pretty nasty stuff and anything that can go wrong, does go wrong, many times. Operations staff who look after lift stations will tell you about ragging of pumps, sticks in pumps, impellors of pumps falling off, cables burning out, level sensing devices giving false reading, pumps overheating..

The consequences are serious, as sewerage overflows create an environmental hazard - for example, children playing in the street come into contact with sewerage and get sick. It's a reality, so government regulatory bodies take sewerage spills extremely seriously, and often impose hefty fines on organizations who have spills.

All of this leads to an important point..

Control and Monitoring of lift stations is a complex process which requires a lot of thought, a lot of planning and a real attention to detail.

The original technology for lift stations was ball floats. These are extremely simple devices that hang down in the well, and when the rising liquid lifts them up, they tilt and a contact closes. The control device senses this contact closure.

Even though they are still widely used in the US, they are very unreliable as level sensing devices, due to the harsh nature of sewerage. They frequently tangle, get caught in one position and give false readings.

Main technologies across the US, UK and Australia:

-Ball Floats: widely used in the US
-Conductive Probe: widely used in the US and Australia
-Hystrostatic Pressure Transducers: widely used in the US and Australia
-Ultrasonic: dominates the UK water/wastewater industry, not as high a penetration of lift stations as pressure in Australia and the US