Pumping primary sludge

At a sewage treatment plant in Holland primary sludge is handled with a solid content of 4.5% to 9.5%. The circumstances for pumping this product are severe, making the handling and treatment difficult.

Previously, an eccentric screw pump was applied to pump the sludge, but experienced several problems, not least because of the severe suction conditions. Sludge is thick and therefore difficult to handle.

The problem

It was impossible to control the flow, as the increase in solid content has the effect of raising the viscosity, which in turn caused a drop in the capacity of the pump. Higher pump speed would resolve the problem, but as solid content then decreased, as a result the lower viscosity increased the flow. This pump type is a positive displacement pump, yet in these circumstances the performance did not maintain a constant relationship between flow and pump speed.

Another problem occurred regularly when a partial blockage in the suction line would cause the pump to run dry for short periods, resulting in a burned stator. In addition, the abrasive solids in the product caused high wear in the pump, stator and rotor life were significantly below expectation.

For this application the eccentric screw pump not only resulted in high maintenance cost, but also failed to offer good performance.

Process data

The required capacity for the pump is 5 m3/hr and the geodetic height is approx. 18 meters. The pump cannot be positioned near to the fluid inlet, leaving a longer suction line.

The solid content varies dependent upon the incoming effluent. Solid content affects the viscosity – 4,5% solids the laminar viscosity of the fluid is 0.78-2.71 Pas (average high level of 747-2593 cSt) but with the 9,5% solids the viscosity is 6.42-22.3 Pas (average high level of 5894-20350 cSt).

These circumstances are far from ideal for any pump type. But Verder offers a solution: the peristaltic hose pump; Verderflex.

The solution

Following a site visit, Verder’s specialist engineers identified the key requirements; positive displacement pump function, good suction characteristics and the capability of dry running. Last year a Verderflex peristaltic hose pump VF65 was installed. Due to its good performance a second Verderflex hose pump VF65 has been installed.

In this specific application, the ability of the Verderflex industrial peristaltic tube pump to handle sludge of varying thickness and viscosity is a result of the high quality hose working at the heart of the pump. The high suction capability generated by the hose is matched by its ability to handle the abrasive solids without adverse wear and tear. The result is great performance with minimal maintenance.

After installing the Verderflex VF65 hose pumps, maintenance cost has dropped significantly and pump performance is completely controllable. The peristaltic pump has been used for over 4000 hours and it is still running well.