Articles & Case Studies

Flygt Jet-Mixing System lowers lifecycle costs at Creigiau Sewage Treatment Works

Posted: Monday 12th May 2008

Situated to the North West of Cardiff, Creigiau is a small, rural village with a sewage treatment works situated in woodland just outside the village adjacent to a trout fishery.

Due to the layout of the sewage treatment works and because the final effluent from the works enters the same water course used by the fishery, a Flygt Jet-Mixing System was installed to allow easy removal of sludge from the works – resulting in lower lifecycle costs.

The inflow to the works is from the village and the surrounding areas. There are no primary settlement tanks on the site, instead, the inflow is fed to an activated sludge racetrack via screens and a degritting chamber. Artificial lift pumps raise the effluent to the racetrack. Aeration is then undertaken with two rotor type surface aerators and activated sludge is decanted and held in settlement tanks - of which there are three on-site. The outflow has to pass through all three tanks before the final effluent can be discharged to the local water course shared by the fishery.

The final settlement tanks are desludged daily and the sludge is returned to the racetrack. As the total sludge volume approaches a predetermined level, the flow is diverted into a holding tank that is situated at the entrance to the treatment works. Once the sludge has reached a pre-set volume with a dry solids content of 2.5% – 3.0%, the operators request a tanker to take the sludge off site for further treatment at larger works. As the sludge may sit in the holding tanks at Creigiau for a period of time, it will settle out. It is therefore necessary for sludge particles to be re-suspended before the tanker can be loaded.

The Flygt Jet-Mixing System incorporates the market leading Flygt N Pump hydraulics and offers a perfect solution to the problem of re-suspending particles at this final stage of the treatment and removal of sludge.

The N hydraulics are designed to pump municipal sludge of up to 8.0% dry solids, even if polymer is present. The combination of the N hydraulics and the Hydroejector driven Jet-Mixing System ensures full tank re-suspension within five to seven minutes of starting the pump.

The Flygt Jet-Mixing System incorporates the drive pump, discharge nozzle, Hydroejector and interconnecting pipework. The mixing duty is calculated from the thrust required to re-suspend solids in a tank of known geometry. The Flygt MiDS mixer selection software has a thrust calculation facility, which is then converted into a required recirculation flow rate.

The sludge type and solids concentration is entered onto Flygt’s FLYPS pump selection software – especially developed software allowing users to search for the appropriate Flygt pumps for their site’s individual needs. After this information has been input, the sludge module within FLYPS will derate the pump curve according to dry solids concentration and adjust friction loss through pipework and fittings in accordance with the sludge flow behaviour. Once the pump has been selected, the motor power data is adjusted for the sludge concentration.

The Flygt NT3127 vertically mounted drive pump is installed at Creigiau Sewage Treatment Works with an 80mm discharge nozzle and 100mm Hydroejector. This system provides over 300 Newtons of thrust to re-suspend the contents of the tank with optimum use of input power, providing a benefit of lower lifecycle costs. As this particular N pump does not chop the sludge, the wear is minimal and this is reflected within the plant’s running costs. Overall, the efficient use of power to re-suspend the solids at the final stage of removal from the works and the minimal need for maintenance on the Flygt system, has resulted in tangible cost and efficiency savings for Creigiau Sewage Treatment Works.

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