Copa Rotating Biological Contactor
A Rotating Biological Contactor (RBC) is a process used to remove soluble, biodegradable, organic materials from sewage thereby achieving a high quality effluent.
There are three types of plant:
(i) Package plant (Integral)
(ii) Semi package Systems
(iii) Modular plant
Package Plants
A package RBC plant consists of three separate unit processes: primary settlement, biocontactor and final clarification, combined into one tank, and are generally provided for small and medium population ranges. Where the population is too large for a single package RBC, multiples of the unit can be used.
Typical Small RBC
Several package (integral) RBCs are often placed in series or parallel to serve larger populations
All Copa RBC systems meet the minimum process requirement of BS6297 (British Standard - Small Sewage Treatment Works) or Severn Trent Water Limited Standard, with every zone or tank being individually tailored to suit the calculated design parameters.
The single tank composite packaged plant is one of the most economic units in the small and medium population ranges. Civil requirements are simple and installation times short, thereby reducing site costs and work to an absolute minimum. In addition power, lubrication and desludging frequencies are low when compared with alternative treatment solutions.
Grit removal
Screening and detritus removal can be collected in the primary tank thereby avoiding the need for separate on site facilities.
Semi-Packaged Plants
As packaged plants increase in size, the format of the single tank starts to impose design and handling constraints. This can be easily overcome by the provision of a separate final settlement stage in the form of a purpose designed GRP conical tank. The septic tank and Biozone are still combined into one tank, which is totally enclosed by low profile covers, identical to those used on composite single tank plants.
Modular RBCs
Alternatively a modular tank system can be adopted. In this case the primary and final are separate from the biozone. These are usually used for larger populations.
Individual tanks for each of the three unit processes often provide the greatest flexibility in process design and site layout or for the utilisation of any existing facilities.
The Primary section can either be a sealed septic tank to retain the desludging advantages of the smaller system, or a conical upward flow settlement tank. Biological treatment takes place in the self contained Bio-Contactor, with the final settlement in an upward flow clarifier. Copa conical tanks have an external effluent launder provided with conventional weirs or multiple dip pipe outlets.
Consents
RBCs are capable of producing a high quality effluent achieving a 95% ile limit of 5 ammonia all year round.
Cranfield University
Severn Trent Water Ltd set the standard for RBCs in the 1990s by employed Cranfield University, School of Mechanical Engineering to advise on the key parameters to achieve a long life plant, defect and maintenance (other than lubrication) free for a minimum of 20 years.
The Copa Rotating Biological Contactor (RBC) frames and shafts were designed by Cranfield University School of Mechanical Engineering, especially for Severn Trent Water Ltd. They are fully designed and constructed in accordance with Severn Trent Water Ltd's Regional Mechanical Specification for RBC's and Cranfield recommendations and are validated at 1.0 rpm.
The rotating metal assembly is designed to last a minimum of 20 years at a speed of rotation of 1 rpm and a specified biomass thickness of between 5mm and 3mm spread evenly across the total surface area of rotating discs. This is in accordance with Cranfield's recommendations and Severn Trent Water Ltd Specification.
A 20 year plant is substantially more cost effective than replacement say every 10 years based on 'whole life cost' scenario's.
However, it is also possible to achieve a much shorter life if specified or required. Where this may be the case we recommend that careful evaluation is given to the whole life costs where consideration to the cost of temporary treatment facilities are required during the interim period of replacing or refurbishing the RBC plant.
Typical Costs schedule of rotor failures to consider
- Rotor replacement
- Labour
- Temporary treatment (SAF Hire/Tankering etc.)
- Operator Supervision
- Crane Hire
- Mechanical and Electrical involvement
- Recommissioning
Life
Both Biomass thickness and speed of rotation are fundamental to the design, and consequently the life of an RBC plant.
Biomass Thickness
The biomass thickness may vary from site to site but is the uniformly distributed load which acts on the frame. The fatigue stress and hence the life is proportional to the thickness squared. Consequently specifying thickness is important. Speed of Rotation
Research has shown that rotating at less than 1.0 rpm can assist the development of Beggiatoa which is a nuisance bacteria. This not only affects process performance but also reduces the life of the plant.
Applications
- Treatment of domestic sewage effluent
- Treatment of industrial effluent
Operation Principle
A typical RBC system comprises of three separate unit processes, which can either be combined in a single package tank or into individual modular tanks. The unit processes are:
(i) Primary Zone:
Integral RBC
Crude sewage discharges into a two stage, horizontal flow primary tank settling out gross solids including screenings and grit. The large volume of this zone ensures that the settled solids are retained for a long period, reducing the number of tanker visits. Some anaerobic digestion may take place.
Modular Unit
In modular units separate primary and final settling tank are employed.
In such cases sludge should be withdrawn daily to a holding tank.
(ii) Bio Zone: The biological treatment process takes place in the biozone and is common to both designs. Sewage enters the biozone from the primary zone. A multitude of discs attached to a shaft form a rotating rotor assembly which is partially submerged in the trough. This creates a perfect environment to develop and sustain an active biomass which releases s the solids in suspension creating a clear liquid and solids the rotor rotates at approximately 1.0 rpm thereby ensuring the biomass is exposed alternatively to the settled sewage and atmospheric oxygen. Excess Biomass is continually sloughed from the discs.
The aim of the rotor design is to contain a high surface area for biological growth per unit volume whilst ensuring that the film on adjacent surfaces cannot meet and so block the unit. The media therefore comprises polyethylene discs in either corrugated or corrugated and flat design.
The sizing of the disc media depends on the population equivalent and the final effluent standard required from the overall treatment process. Typical rotor sizes range from 1.0m diameter to 4.0m diameter and lengths up to approximately 6m. Package plants can range from 2.8m wide to 4.0m wide tanks. At the current time 3.6m and 4.0m dia rotor assemblies can not be accommodated into a package unit.
(iii) Final Tanks:
The solid matter produced in the biozone is settled in the final tank. The clear effluent is discharge directly to the stream or to separate tertiary treatment process dependant on consent. In this zone settlement of the mixed liquor and excess Biomass admixture takes place.
The humus sludge is co- settled in the primary zone.
Remediation
Copa Ltd has developed a procedure for investigating and repairing defective RBCs. This procedure is unique to Copa, and developed in association with Cranfield University . It involves the assessment of the remaining life of key aspects of the plant and where ever feasible defective components can often be replaced in the most cost effective manner thereby avoiding unnecessary expenditure by total rotor replacement.
It is more cost effective to install new 20 year life rotors than to replace the existing RBC process by some other.
We have worked with clients developing their preferred process design where the site is constrained.
