4.3.1.2 Rotating Biological Contactorr

Categories: 4.3.1 Attached Growth Process
Time of issue: 2022-04-28 18:27:44
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Description:

Information

Rotating Biological Contactor (RBC)

RBC is a fixed-bed reactor consisting of a series of closely spaced, parallel disks mounted on a rotating shaft. The disks are partially submerged in a tank containing wastewater and rotated slowly as wastewater flows through to get in contact with the biofilm attached on the disks.

Treatment Process:

Microorganisms grow on the surfaces of the disks of polystyrene or polyvinyl chloride and form a biofilm. This biofilm is alternately exposed to either the air or the wastewater as the disks rotate, with approximately 40% is immersed in the wastewater. The rotating speed is normally 1 to 2 revolutions per minute. The disk diameter ranges from 60 cm to 3 m. When the disks rotate to the air, microorganisms of the biofilm adsorb oxygen for growth. When the disks rotate into the wastewater, the biofilm gets in contact with the wastewater to enable degradation of organic matters and nutrients in the water. The rotation helps to slough off excess solids. The disk system can be staged in series to obtain nearly any detention time or degree of removal required. The RBC can achieve higher efficiency than the trickling filter.

Design Criteria

General

When designing rotating biological contactors, key factors that shall be taken into consideration include: (i) wastewater characteristics of the influent; (ii) hydraulic loading rate and organic loading rate; (iii) staging of the RBC units.
RBCs should be preceded by professionally designed settling/clarifying facilities to avoid solid built-up on the discs.
RBC units are designed to work in stages. As each stage receives effluent from the previous stage, the organic loading decreases gradually as wastewater passes through the train.
Flow equalization should be provided when the ratio of peak to average daily flow exceeds 2.5.
Permanent covers should be provided to protect the units from sunlight and cold weather and to prevent rains from washing the slime away from the media. Covers should be removable to allow for the replacement of the shaft/media assembly.
Typically, nitrification will occur when soluble BOD in the wastewater is reduced to less than 20 mg/L. As the BOD concentration decreases when passing through the first stages in a RBC system, nitrification will take place in latter stages where BOD concentration is low enough. A dissolved oxygen range of 2.0 to 3.5 mg/L and a pH of 7.0 to 8.5 would be the best for nitrification.
The length of shafts ranges from 1.5 m to 8.2 m, depending on the manufacturer.
Low- or high-density polyethylene is the most used material for RBC disks.
The number of disks is normally selected on the basis of the anticipated organic loading and temperature.

Parallel flow is used to distribute high organic waste flows over several first stage units to reduce shock loads.
Series flow is to reduce the load to minimum and to permit nitrification in later stages.

Operation and Maintenance:

General

A well operated rotating biological contactor shall have:

Healthy biomass that is uniformly brown and distributed in a thin, even layer over the entire media.
No unusual noises from the unit or shaft.
No loping of the RBC disks.

Require a skilled and technical operator to manage the system.
Regular maintenance to all moving parts is important. Lubricate the moving parts, including motors and bearings, and replace seals as required.
All equipment must be tested and calibrated as recommended by the equipment manufacturer. A routine O&M schedule should be developed and followed for rotating biological contactor system.
Ensure continuous supply of power and constant wastewater flow.

Shaft

Always shut down the drive assembly first and allow the shaft to come to a complete stop before attempting any maintenance activities.
Maintenance of the shaft should be performed according to the manufacture’s specifications and intervals.
Check for signs of excessive shaft deflection. If observed, determine if there is excessive growth on the media.
Observe of uniform and continuous rotation of the shaft. If surges are observed, this can lead to high and sudden loading to the shaft, resulting in shaft failure.

RBC disks

Cover the RBC disks to reduce airflow around the bearings, which may increase corrosive effects.
Periodically inspect for excessive deposits in the basin. Use either a small dipper while the RBC disks are shut off or use a small pump to extract a sample from the bottom.
If the basin is of carbon steel construction, monitor for signs of excessive rusting.
Consistent rotational speed must be maintained to achieve uniform media growth.
The rotation speed should be adjusted proportionally to the organic loading.

Biomass growth

The system does not require bacteria inoculation, and the start-up phase is relatively short. It will take about 6 to 12 weeks to reach a good treatment performance.
If the system is designed strictly for BOD removal, brown, shaggy slime growth will be observed on the disks.
If the system is designed for both BOD removal and ammonia nitrogen removal, reddish brown, golden shaggy slime growth will be observed on the disks in latter stages.
White or black biomass is generally not healthy. If white appearing slime, or black appearing slime accompanied by odors and low DO in the effluent is observed, it indicates the organic loading is too high for the system.
If no slime growth on the disks, the temperature or pH of influent shall be checked.
Uneven media growth on the disk media can lead to uneven shaft rotation, which is known as loping. RBC rotation should be checked by timing quarter rotations weekly. An evenly balanced RBC shows quarter rotations with relatively similar times as shown below. The operator shall clean the media when loping is observed.

When excessive growth occurs, the operator can: (i) reverse the rotation direction; (ii) clean the media; (iii) provide supplemental air; (iv) reduce organic loadings; and (v) place additional RBCs in service (if applicable).

Capital Cost and O&M Cost:

Observed costs for RBCs are highly variable depending on climate and location. Generally, RBCs involve high capital costs as not all materials may be locally available and motor and special material for rotation is required. Another cost factor may be manufacture and implementation, which requires skilled experts.

Operation and maintenance costs are relatively high as operation requires a continuous electricity supply and supervision requires semi-skilled labor and professional operator.

Applicability

Suitable in areas where large parcels of land are not available.
Suitable for medium sized communities as the complex design technology is not justified in small installations.