Screenings handling plant
August 2006
United Utilities - Rochdale
With the co-operation of United Utilities, The Longwood Engineering Company have successfully tested their new Combi-Wash screenings handling system at Rochdale Waste Water Treatment Works.
The Combi-Wash is a new screenings handling system designed to wash sewage screenings prior to compaction, but also provide a high through put when storm conditions occur.
The Rochdale site has 3 Longwood Escalators (each 2500mm wide) installed at the inlet to the treatment works and each has a capacity of up to 1000 l/sec from a design population of 197,000. The screenings are discharged into a launder trough, which then delivers to the Combi-Wash.
The Combi-Wash has a theoretical solids handling capacity of 8 m3/hour, however peak loads in excess of this do occur, but the unit has successfully treated all solids that have been delivered to it.
The installation has been visited by engineers from other water companies and a unit has been ordered for a site in the South of England where problems are being experienced with an existing installation.
The Combi-Wash is driven by a single drive unit with a 5.5kw geared motor and incorporates a washing section, a dewatering zone, followed by a compaction section. The washed screenings are then discharged through a tube into a skip for disposal by landfill.
The washing action is achieved by a unique reversing system which agitates and washes screenings prior to entering the dewatering zone. The washed screenings are then discharged into the compaction section. When the screenings are in the compaction section a specially designed clutch system prevents them from being returned to the dewatering zone when the drive unit is operating in the reverse washing mode.
The unit has an automatic control panel which is inverter driven to provide optimum washing of the screenings and to reduce the torque on start up, which in turn reduces wear.
The Combi-Wash incorporates several unique features to provide washed compacted screenings ready for disposal.
Key features
- The unique flight design incorporates a one way drive system that allows greater reverse cycle cleaning / agitation when compared to other solutions. In addition to this, the one way drive system reduces the load on the system whilst operating in reverse; this allows the reverse speed to be much higher.
- At the drive end of the machine organic material is smeared against a static perforated plate. A back wash system located behind this plate ensures the organics are put into solution and washed away via the filtrate pipe.
- The flight design also features a chamfer on the reverse edge, this smears the organics on the reverse stroke and on the forward stroke the organics readily form a solution that is traversed toward the dewatering section.
- The large fully perforated dewatering section provides additional cleaning of the screenings by the use of wash water nozzles that deliver 1 l/s @ 3-4 bar. To ensure that the dewatering section remains effective the flight edges are fitted with a durable and flexible scraper strip.
- The control system ensures a high degree of washing occurs during normal batch mode operation. If storm conditions occur the machine will switch to storm mode operation, in this mode throughput is given priority.
- The machine is suitable for receiving screenings from a launder feed, a conveyor or a direct screen discharge. When used with a conveyor or a direct screen discharge additional wash water may be required to ensure thorough washing of the screenings.