Pumping primary sludge under difficult circumstances

Leiden-Noord, The Netherlands - At the AWZI, sewage plant treatment in Leiden-Noord primary sludge is handled with a solid content of 4.5 up 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 the duty, but the operator experienced several problems, not least because of the severe suction conditions. Flow proved impossible to control, 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 decreased, the resultant 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 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 being significantly less than expectation.

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

The required capacity for the pump is 5 m3/hr and the geodetic height was about 18 meters. The pump could not be positioned near to the fluid inlet, leaving a rather long 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 Pa.s (average high level of 747-2593 cSt) but with the 9,5% solids the viscosity is 6.42-22.3 Pa.s (average high level of 5894-20350 cSt).

Following a site visit, Verder engineers identified the key requirements; positive displacement pump function, good suction characteristics and the capability of dry running. A Verderflex VF65 hose pump was installed last year, followed by a second VF65 more recently.

In this specific application, the ability of the Verderflex 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 performance with minimal maintenance.

According to the plant manager of sewage plant treatment AWZI, Leiden-Noord: Following the installation, maintenance cost has dropped significantly and pump performance is completely controllable. The original hose in the first pump is still running. We have used this pump for over 4000 hours and it is still running well.”