Hydraulic model testing: The right benching angle for a waste water treatment plant.

Solids deposits can impair pump operation significantly.

Waste water transport in treatment plants needs to run smoothly. However, transport can be hindered by the solids that are frequently contained in sewage, grey water or surface water. The problem: When the waste water leaves the inlet line, the flow velocity drops. Depending on the velocity distribution in the structure, this can result in sedimentation, i.e. particles settling out due to gravity.  The waste water pumps in the treatment plant are then unable to draw in and transport the deposited solid particles together with the water. If the sedimentation continues to build, it can jeopardise waste water transport by changing the flow through the structure or clogging pumps.

As a preventive measure, many operators of plants in which fluids with a high solids content have to be transported have their systems designed with sloped floors, so-called "benching". The benching ensures smooth processes as it facilitates the transport of solid particles through the piping. 

What benching angle is required to prevent deposits?

Standard 134 of the German Association for Water, Wastewater and Waste (DWA) recommends benching angles of around 60 degrees. This will result in relatively high building costs as the sump will need to be much deeper for the same sump fill volume. Would a flatter slope work? And if so: How much flatter? KSB investigated these questions in a hydraulic model test.

For the model test, KSB specialists simulated the behaviour of different solids with different floor slope angles and different surface qualities in the intake area of a pumping station. To do this, they continually changed the benching inclination angle with a servomotor. The solids they used were differently sized stones, broken glass and metal particles. In addition, the sliding behaviour of these solids on different surfaces was examined: on a polyethylene (PE) plate, on a ceramic plate and on a concrete surface.

The results of the benching angle model test provided valuable insights for plant engineers, not only for avoiding future clogging events but also for significantly saving space and costs when designing the structure. The model test revealed: The required benching angle largely depends on the coating of the benching surface.

Conclusion of the benching angle model test

Especially effective were benching surfaces made of polyethylene (PE): On smooth plastic surfaces, coarse solids start sliding off the benching at an angle of less than 40 degrees. By contrast, on slopes with concrete or ceramic surfaces, the tested materials started sliding much later: Angles exceeding 45 degrees were required in the model test to prevent deposits.

This means, if benching surfaces are coated accordingly, a much smaller angle can be used and, therefore, the depth of the structure can be significantly reduced.

Here is another tip from the KSB experts for plant engineers: If the pump sump floor in a waste water treatment plant is designed with a very flat slope, specific flow guiding measures may be useful to achieve a flushing effect. The flow velocity can be increased, for example, by localised changes in cross-section. 

Model testing by KSB: Thinking small, building big! 

In general, KSB conducts model tests for an object or procedure whenever the measurement of original conditions is not technically or economically feasible. Experimental investigations enable KSB to uncover new potential for increasing efficiency in pump systems.

The benching angle hydraulic model test is just one of the many model tests with which KSB has gained valuable knowledge that we are happy to pass on to our customers. Expand your specialist knowledge also with the results of our other hydraulic model tests on topics such as air pockets, solids transport or air intake.

Good to know: When planning, performing and evaluating all model tests and transferring the results to the original-size machine and/or the original operating conditions, the affinity laws are observed and applied. Apart from maintaining geometric similarity, this involves considering changes in length due to elastic and thermal deformations, converting test results on the basis of affinity laws, observing fluid properties, and much more.

In other words, everything that allows a model test to optimise the daily operation of a pump system in real life.

Do you have any questions? Please contact us. We look forward to hearing from you.