Tuke & Bell Ltd

Tuke & Bell Ltd

Rotating Biological Contactors


Established in 1910, Tuke and Bell Ltd. have been one of the U.K's major manufacturers of sewage purification equipment, the range includes grit plants, scrapers, aerators, filter distributors and picket fence thickeners as well as rotating biological contactors. We have a wealth of experience in the design, manufacture and installation of our complete range. Our Engineers use the latest computer design systems to individually suit our customers needs. Rotating Biological Contactors can be supplied as part of an integral package plant to treat sewage from various communities. Integral units are provided in sizes of up to 500 population equivalent. A smaller version is also available for small private installations.

Modular systems can also be adapted to cater with populations of any number.

In the U.K. multiple units have been used for populations in excess of 5000.

Each plant is designed to meet the specific requirements of the site and the effluent quality required.

The units are totally enclosed by GRP covers, which are environmentally acceptable and durable. These covers and the tanks are manufactured by our subsidiary company, Exallot, who are specialist GRP manufacturers.

Operating Principles

The principles of the rotating biological contactor originated in the early 1900's but its application to sewage treatment did not occur until the 1960's when the present system was developed. The process employed relies on the well-established principle of biological oxidation using naturally occurring organisms to ensure that even the most stringent effluent standards can be achieved.

Primary Settlement Zone

Incoming flows of crude sewage enter the RBC primary settlement zone, which is designed to have a buffering capacity of balancing flows up to 6DWF.

Settlement solids are retained in the tank's lower region whilst the partially clarified liquor passes forward to the biozone where it makes contact with the slowly rotating disks.


Rotating Biological Contactors are available in sizes from 1100mm diameter up to 3800mm in diameter. The media packs that form the rotors are manufactured from vacuum formed black polyethylene sheets supported on the central shaft with a galvanised steel framework. The central shaft is manufactured from mild steel tube, protected internally against corrosion and fitted with end stub shafts, which are supported on split bearings.

Rotation is provided by a shaft mounted gearbox and motor fitted at one end.


The rotor assembly is suspended within the biozone with 40% of the diameter submerged in the liquor at any one time. The disks slowly rotate and the continuous alternate exposure to air and sewage results in a growth of organisms known as biomass which adheres to the disks. These organisms occur naturally in the sewage and carry out the purification process by feeding off the impurities present in the sewage. As they have a short life cycle, these organisms are continually shearing off the rotating disks and pass from the biozone to the final zone.

The biozone is fitted with a series of baffles between each bank of media, this is to prevent short circuiting and ensure maximum performance.

Final Settlement Zone

The recently completed installation at Culbokie, for North of Scotland Water

The biomass passes from the biozone into the final settlement zone where it settles to form a humus sludge. This is then regularly pumped out using either an air lift system or submersible pumps and returned to the primary zone.

The clarified liquid decants from the top of the tank as effluent that can be discharged to a reed bed for further clarification or direct to a watercourse.

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