Sink or Swim

It is said that all good things come to those who wait. However, if your job is to supply the largest of British Nuclear Fuels' UK sites with 900m3/d of high quality demineralised water from varying sources then conventional methods of sedimentation and filtration just won't keep up. After all, sedimentation time is money.

Sellafield, in Cumbria, holds the world's first industrial scale nuclear power station, Calder Hall. Also on the site are the Magnox reactor fuel reprocessing plant, Fellside CHP power station and the thermal oxide reprocessing plant (THORP), plus waste treatment plants and storage facilities. Not suprisingly, these facilities demand the purest water attainable at high volumes even when quality of the source is poor.

The existing treatment plant was hampered by reliability problems and associated costs, so it became vital to find a new process which could handle the changeable water source.

Water for Sellafield is taken from a number of sources - wastewater, old mine workings, and the rivers Calder and Ehen. The amount extracted varies from day to day, causing treatment difficulties when quality is below par. Heavy silts can be present during storm conditions, and filter-blocking algae such as Melosira sp., Anabaena sp. and Microcystis, during the summer blooms.

Paterson Candy completed construction of a water treatment plant for BNFL earlier this year, which incorporates the COCO DAFF (counter current, dissolved air flotation and filtration) process to counteract the problems.

Conceived with Thames Water Authorities, the COCO DAFF can be turned on when problems are expected, and off when water quality improves. Paterson Candy claim the small flocculator stage allows an existing plant to be upgraded and can even retrofit the unit to existing filters. Thus the unit is essentially modular and may be incorporated into more typical systems, using coagulation and flocculation followed by sedimentation and filtration. The DAFF process uses flotation as an alternative to sedimentation.

Raw water is delivered to a series of mixing chambers, where chemical dosing occurs, inducing coagulation and flocculation. This semi-flocculated water enters the unit where particles, rather than sinking, float upwards.

The theory of sedimentation is simple enough - floc particles are heavier than water and will settle. However, if a gas bubble attaches to a particle of suspended matter it creates a composite particle which is lighter than water and will float to the surface.

The terminal velocity of a particle attached to a bubble, like settling rates, is controlled by Stoke's Law. The benefits of flotation are due to the difference between particle and water density higher (in magnitude) in flotation than in sedimentation. Thus particles float to the surface faster than floc can sink.

Coagulated water enters the COCO DAFF unit through distributors towards the surface of the tank. Supersaturated water, recycled from further down the treatment chain, is distributed through nozzles above the filter media, releasing an air bubble blanket in a fine milk-like emulsion which fills the area of the tank. The more bubbles, the higher the probability that one will collide with a particle.

Coagulated water entering the unit at the top must pass down through the depth of the rising bubble blanket before reaching the filter bed. This extends the time for particle capture and even aids flocculation within the tank. Paterson Candy point out that flocculation within the bubble blanket halves flocculation time compared to conventional flotation devices. Typical flocculation requires 20-30 minutes while COCO DAFF does the job in 15 minutes.

The floating sludge layer, supported by the deep bubble blanket, is removed at predetermined intervals. Any fallout which occurs, returns upwards through the flotation process.

Once past the bubble blanket, remaining particulate matter is removed through the filtration bed. During trials, backwash of the filter media was needed only every 24 hours compared to the usual four-hour frequency.

Paterson Candy claim the unit offers a reduction in civil engineering costs, because its design incorporates filtration and eliminates a separate flocculation chamber, resulting in a smaller 'footprint' than other plants.