Shared watercourse called for right jet-mixing system

Tony Price of ITT Flygt explains how the sludge-treatment needs of a sewage treatment works, which nestles next to a trout fishery in rural Wales, were met

A sewage treatment works in Wales, which shares a watercourse with a fishery, needed a solution for its sludge-removal predicament.

The works, outside rural village Creigiau, north-west of Cardiff, is adjacent to a trout fishery. Due to the layout of the works, and because the final effluent enters the same watercourse used by the fishery, the right equipment was called for. It was decided to install a Flygt Jet-Mixing System to remove sludgefrom the works.

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.

Final effluent
Aeration is then done with two rotor-type surface aerators and activated sludge is decanted and held in settlement tanks - there are three on-site. The outflow has to pass through all three tanks before the final effluent can be discharged to the local watercourse 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 at the entrance to the 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 away for further treatment at larger works.

As the sludge may sit in the holding tanks at Creigiau for a while, 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 Flygt N Pump hydraulics and can re-suspend particles at the final stage of sludge treatment.

The N hydraulics are designed to pump municipal sludge of up to 8% dry solids, even if polymer is present. The combination of the N hydraulics and the hydroejector-driven Jet-Mixing System means 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. 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 into Flygt's pump-selection software, called FLYPS.

After this information has been put in, the sludge module within FLYPS the will derate the pump curve according to dry solids concentration.

Sludge concentration
It then adjusts 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.

Hence there is efficient use of power to re-suspend the solids at the final stage of removal from the works. Add the minimal need for maintenance on the Flygt system, and there is cost and efficiency savings for Creigiau Sewage Treatment Works.

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