Rotating Biological Contactors

Compiled by:
Dorothee Spuhler (seecon international gmbh)

Executive Summary
Rotating biological contactors (RBC), also called rotating biological filters, are fixed-bed reactors consisting of stacks of rotating
disks mounted on a horizontal shaft. They are partially submerged and rotated as wastewater flows through. They are used in
conventional wastewater treatment plants as secondary treatment after primary sedimentation of domestic grey- or blackwater, or
any other biodegradable effluent. The microbial community is alternately exposed to the atmosphere and the wastewater, allowing
both aeration and assimilation of dissolved organic pollutants and nutrients for their degradation.

In Out
Blackwater, Brownwater Faecal Sludge, Fertigation Water

A rotating biological contactor. Source: PT. BIOSEPTIC WATERINDO ABADI-CompactWastewater Treatment System.

Rotating biological contactors (RBC) are a conventionalaerobic biological wastewater treatment unit. Conventional biological treatment means activated
sludge systems and fixed film systems such as trickling filters, or RBC (NOLDE 1996). The advantage of all these systems is that they are compact (i.e. in
densely populated urban settings) and that they efficiently reduce organic matter (JENSSEN 2004). However, they are high-tech and generally require skilled
staff for construction as well as for operation. RBC can treat domestic black- or greywater and any other low- or high-
strength biodegradable wastewater (e.g. industrial wastewater from food processors or paper mills). They have been found to be particularly effective for
decentralised applications (on the level of a small to medium community or industry/institution), where electricity and skilled staff are available (METCALF &
EDDY Inc. 2003).

Treatment Process and Basic Design Principles

The disc is made out of light-weighted material such and usually ridged, corrugated, or lattice-like to make as much as surface available for the biofilm to attach. Source: NSFC
(2004)

A series of circular lightweight rotating discs are mounted on a shaft through which wastewater flows. The partially submerged discs rotate through
thewastewater slowly. The disks are most commonly made of high-density plastic sheets (e.g. Polyethylene, polystyrene or polyvinylchloride) and are usually
ridged, corrugated, or lattice-like to increase thespecific surface area (NSFC 2004). The surface of the disks provides an attachment site for bacteria and as
the discs rotate, a film ofbiomass grows on their surfaces (NSFC 2004; WSP2008). This biofilm is alternately exposed to either the air or the wastewater as it
rotates. The oxygen necessary for the growth of these microorganisms is obtained by adsorption from the air as the biofilm on the disk is rotated out of the
liquid (CRITES & TCHOBANOGLOUS 1998; SANIMAS 2005). As the biofilm passes through the liquid phase, nutrients and organic pollutants are taken up. All
oxygen, nutrients and organic pollutants are necessary for the growth of the microorganismand the conversion of the organic matter to CO2. Nitrogen is
removed by nitrification and subsequent denitrification transforming it to gaseous N2, which is released to the air. The process is optimised by adjusting the
speed of rotation and the depth of submergence (METCALF & EDDY Inc. 2003). In some designs, air is added to the bottom of the tank to provide additional
oxygen in case of high-strength influents (CRITES & TCHOBANOGLOUS 1998).
The submerging level varies from 40 to 80 % (CRITES & TCHOBANOGLOUS 1998) and a usual rotating speed is 1 to 2 rpm (U.S.EPA 1980). The common disc
diameter is between 0.6 and 3 m (SANIMAS 2005). The degradation process is similar to the one in a trickling filter with a high rate of recirculation (CRITES &
TCHOBANOGLOUS 1998). The higher contact time in RBCs due to rotation allows up to 8 to 10 times higher levels of treatment than in trickling filters
(WSP 2008). Also because the rotation allows both optimum wetting and oxygen supply, RBCs are generally more reliable than other fixed-film processes.
Additionally, the disc design is made in such a way that large amounts of biofilm can attach, which means that there is a large amount of biological mass
present to degrade the pollutants (WSP 2007). The large amount ofbiomass and the stability of contact also results in an improved stability and a reduced
susceptibility to changes in hydraulic or organic loading compared to conventional activated sludge processes (WSP 2007). As for all fixed-film processes,
primary settling and/or screeningis required for the removal of grit, debris, and excessive oil (U.S. EPA 1980, WSP 2008). Suchprimary treatments are typically
septic tanks, Imhoff tanks or anaerobic reactors. To remove sloughing sludge, a post-settling unit (i.e. a clarifier) is also required.

RBCs are a secondary treatment and as for all fixed-film processes, primary settling as well as sedimentation of sloughedsludge in a tertiary clarifier is required
(example: greywatertreatment in Germany). Source: GTZ (2006)

The performance of RBCsystems depends on the design, the temperature, the concentration of the pollutants, the rotating velocity and the hydraulic
retention time. RBCs can achieve biological oxygen demand (BOD) reductions of 80 to 90 % (SANIMAS 2005;WSP 2007; WSP 2008). The removal
of nitrogen (which is mostly present as ammonia) by nitrification and subsequent denitrification is also high, because
bothaerobic nitrifying bacteriaand anaerobic denitrifyingbacteria can simultaneously live in the attached biofilm (HOCHHEIMER 1998), depending on weather
they are situated on the bottom of the film, close to the disc support (and thus in anaerobic or anoxic conditions) or at the top of the film exposed to the air.

Both aerobic and anaerobicmicroorganisms can live in the biofilm and contribute to the removal of pollutant form the water.
Source:http://en.wikipedia.org/wiki/Rotating_biological_contactor[Accessed: 18.03.2010]
Some other microorganisms which can transform ammonia(NH3) in one single step to gaseous N2 under anaerobicconditions have also been discovered in
biofilms growing onRBC. These bacteria were called annamox and resulted in the development of innovative aerobic ammonia removal andwastewater
treatment processes. Little is known about the removal of phosphorus in RBCs, but it can be presumed that large parts of the phosphorus present is either
accumulated in the biofilm or in the settled and collected sludge.
RBCs can be arranged in a variety of ways depending on specific effluent characteristics and the secondary clarifierdesign (e.g. specifically for BOD removal
or nitrification, NSFC 2004).
Excess biomass sloughs off the discs by the shearing forces exerted as the discs rotate, combined with the force of gravity (WSP 2008). The rotation movement
helps to keep sloughed solids in suspension so they can be carried to aclarifier (gravity settler) for secondary settling. The collected sludge in
the clarifier requires further treatment (WSP 2008) for stabilisation, such as anaerobic digestion, composting,constructed wetlands, ponds or drying. Very
often in small installations, accumulated sludge is also directed back to the septic tank for storage and partial digestion (U.S.EPA 1980).
Effluents from RBC do not contain high levels of nutrients and are therefore not particularly interesting for agriculture, although they constitute a source of
water. However, due to reduced removal of microorganisms (1 to 2 log units, U.S. EPA 2002), RBC effluents require a further treatment, such as sand
filtration, constructed wetlands or another form of disinfection (e.g. chemical disinfection or UV disinfection).
RBCs are usually designed on the basis of hydraulic and organic loadings derived from pilot plants and other full-scale installation (WSP 2007). Hydraulic
retention times (HRTs) generally lye within some hours up to two days. Even though RBCs are resistant to shock loading, long-term high organic loading may
cause anaerobic conditions, resulting in odour and poor treatment performance (U.S.EPA 1980).

Example of an underground RBCfor the decentralised treatment of domestic blackwater. Source:http://web.deu.edu.tr/atiksu/ana52/ani4043.html[Accessed: 18.03.2010]

Recirculation is not normally practised in package fixed-film systems since it adds to the degree of complexity and isenergy and maintenance intensive.
However, recirculation may be desirable in certain applications where minimum wetting rates are required for optimal performance (U.S.EPA 1980).
Units may be installed at or below ground depending upon site topography and other adjacent treatment processes. Access to all moving parts and controls is
required, and proper venting of the units is paramount, especially if naturalventilation is being used to supply oxygen (U.S.EPA 1980).
RBCs are often covered with a fibreglass housing to protect the disks from sunlight, wind, rain and low temperatures as performance of RCS drops considerably
at air temperaturesbelow 12°C (U.S.EPA 2002; NSFC 2004).

Costs Considerations
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
(SANIMAS 2005).

Operation and maintenance costs are also relatively high, because operation requires a continuous electricity supply and supervision requires semi-skilled
labour (U.S.EPA 1980) and professional operator (SANIMAS 2005).

Operation and Maintenance

Figure 5: Large-scale RBCs are often covered to protect them form cold temperatures, rain, wind and sun. Sometimes, artificial aeration is required to keep the
process aerobic when the systems are covered. Source:http://web.deu.edu.tr/atiksu/ana52/ani4043.html[Accessed: 19.03.2010]

During operation, the system must be supervised by professional operators (SANIMAS 2005). Maintenance includes lubrication of moving parts, motors and
bearings; replacing seals, motors, servicing bearings; and cleaning the attached-growth media (spray-washing of discs and purging of settledsludge) (METCALF
& EDDY Inc. 2003; WSP 2007). The discs may be also checked for debris accumulation, ponding and excessive or not sufficient biomass accumulation (U.S.EPA
2002).
Although fixed film units such as RBC and trickling filters are operation- and maintenance-intensive, they do not requireseeding with bacterial cultures (as
do anaerobic processes such as anaerobic baffled reactors, septic tanks, upflow anaerobic sludge blanket reactors or anaerobic digesters) and the start-up
phase is therefore considerably shorter. However, it takes 6 to 12 weeks for the biofilm to establish for a good treatment performance (U.S.EPA 2002).

Health Aspects
For correct operation, RBCs are covered and thus protected from contact with humans or animals. However, excess sludge as well as the effluent require post-
treatment to remove pathogenic microorganisms.
In any case (i.e. for operation and maintenance) direct contact with the biomass growing on the discs, the effluent or the sludge should be avoided.
For discharge or reuse, a treatment unit allowing further pathogen removal should be considered as mentioned above.

Applicability
RBCs can achieve a high removal of biodegradable organic pollutants form domestic black- orgreywater as well as from high-strength industrial
wastewater (e.g. from dairies, bakeries, food processors, pulp, paper mills, WSP 2008). A great variety of applications are known, either as post-treatment
for activated sludge processes in conventional domestic wastewater treatment plants, or for decentralized application at the level of small to medium-sized
communities, industries or institutions (WSP 2007; WSP 2008). They are adapted for urban areas mostly: land requirements are low, but continuous and
consequent energy supply as well as semi-skilled labour are indispensable. Some of the material may be locally available, however, the system can only be
planned and implemented by experts (SANIMAS 2005)
Advantages
 High contact time and high effluent quality (both BOD and nutrients)
 High process stability, resistant to shock hydraulic or organic loading
 Short contact periods are required because of the large active surface
 Low space requirement
 Well drainable excess sludge collected in clarifier
 Process is relatively silent compared to dosing pumps for aeration
 No risk of channelling
  Low sludge production

Disadvantages
 Continuous electricity supply required (but uses less energy than trickling filters or activated sludge processes for comparable degradation rates)
 Contact media not available at local market
 High investment as well as operation and maintenance costs
 Must be protected against sunlight, wind and rain (especially against freezing in cold climates)
 Odour problems may occur
 Requires permanent skilled technical labour for operation and maintenance