Long-term trouble-free service on low water content primary sludge

When two primary sewage treatment tanks at Northumbrian Water's Chilton Lane STW are desludged each day, it is essential that the water content of the pumped sludge remains as low as possible in order to minimise the cost of tankerage. Two Watson-Marlow Bredel SP65 hose pumps have handled the duty since July 1998, and to date have required neither spares nor maintenance.

The pumps are fitted with 4 kW cast iron frame motors, run at a fixed speed of 20 rev/min, producing a flow rate of 7.2 m3/h, and operate under control of a simple timer. A sludge density monitor is located in the suction line, designed to override the timer and cut-out the pumps should water content exceed a predetermined figure. Suction is flooded; discharge pressure is 5 bar.

Additional fittings include monitors to detect blockage in suction and discharge lines, suction and discharge pulsation dampers, and a high lubricant level sensor to provide warning in the unlikely event of hose failure. Pump motors are fitted with heaters and thermistors.

The success of the Watson-Marlow Bredel hose pump on sludge and similar abrasive pumping duties lies in the design of the hose - the only component in contact with the sludge - allied to the gentle, roll/squeeze peristaltic action and low running speeds.

The hose is composite construction, manufactured from thick, resilient natural rubber outer, with a soft inner core and an embedded internal reinforcement of braided nylon. This provides an optimum combination of rigidity, elasticity and mechanical strength, allowing operation at pressures up to 16 bar, and ensuring that the hose regains shape immediately occlusion by the pump shoe ceases. The latter is a primary factor in maintaining volumetric accuracy.