Rotating Biological Contactor RBC
Category / type: biofilm, fixed bed, constant flow rate;
Application: civil (or assimilable) and industrial wastewater – secondary treatment.
Population Equivalents (PE) served: on demand, only tailor-made solution.
Normative references and guarantees: EN 12566-3, d.o.p. According to Regulation (UE) n.305/2011.
Material: according to design requirements.
Rotating Biological Contactor (RBC) or biodisc is a fixed bed reactors settled on a horizontal rotating shaft, it’s an aerobic biological treatment for domestic wastewater.
Bacteria grow up on discs partially submerged in wastewater that constantly rotate around a fixed horizontal axis.
They are used as a secondary treatment to treat organic pollution downstream to primary treatment.
The aerobic microbial community that live on discs is alternately exposed to atmosphere and wastewater, allowing for degradation both aeration and assimilation of dissolved organic pollutants and nutrients. It’s a compact and efficient system successfully used in densely populated areas however it’s a very advanced technological solution that requires qualified personnel for startup and maintenance operation.
The system also effective for discharge of domestic sewage from isolated residential complexes (small or medium-sized community or industry), where power supply and the constant presence of qualified personnel are guaranteed.
How It Works
A series of circular rotating discs are fixed on a horizontal shaft; disks are partially submerged in the wastewater and rotate around their horizontal axis with very low angular speed.
On their surface, usually rough in order to allow the grown of aerobic microbial community, the biofilm is alternately exposed to either air or wastewater during rotation.
Bacteria during discs rotation use oxygen, obtained from air, and nutrients and organics matter from wastewater at different times.
Nitrification and denitrification process can take place depending on rotation speed: anoxic, anaerobic and aerobic conditions are possible in different disk area.
Nitrogen is removed by nitrification and denitrification process and then releases in air as N2.
Both phases are critical for microorganisms proliferation and for the purification process.
So the process is optimised by adjusting rotation speed.
The immersion depth is variable from 40% up to 80% and a typical rotation speed is 1 to 2 round per minute. Disc diameter range is very wide, from approximately 60 cm up to 3 m.
The process is like aerobic tricking filter but it gets a higher efficiency.
The large amount of biofilm has a significant resistance to a shock loading but the increase of biomass create a dynamic load to all mechanical components (shaft, bearing and support).
Mechanical pre-treatment or primary sedimentation for sand, grease and oil removal is required as all biofilm processes. Septic tank, Imhoff tank and grease trap can be used for small size plant.
A secondary settling is needed to remove excess of biomass detached from the support.
The main parameters for civil wastewater plants (or equivalent) project is population equivalent (PE); normally it refers to 60 g BOD5 per day and a flow rate from 150 to 250 litre per day.
Population equivalent (PE or sometimes used p.e.) is a conversion value which aims at evaluating non-domestic pollution in reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC concerning Urban Waste Water Treatment) at 60 g/day related to BOD5.
European legislation defines Population total (PT) as a sum of population and population equivalent (PE).
A proper wastewater characterisation is possible for incoming organic loads and nutrients better estimation, considering the possibility of dosing additives for biological reactions.
Sizing is normally based on equivalence population and standard organic load (BOD 5); it is very important to estimate accurately which is the main project parameter for each product and plant. For example, in European countries there are different standard values for inhabitants water consumption.
Operation and maintenance
During operation, the system must be supervised by professional operators.
Disk should be checked for debris accumulation and biomass accumulation; regular maintenance to all moving parts (motors and bearings) is very important.
This kind of plants requires frequent check and maintenance but does not require bacteria inoculation (unlike other biological rectors) and the start-up phase is relatively short. However, it takes from 6 to 12 weeks to reach a good treatment performance.
The biodisc plant, if used as a secondary treatment for domestic or similar wastewater, is conceptually a wastewater treatment system as defined in the European standard EN 12566-3. Then it is subject to EN certification and requires a DoP (Declaration of Performance).
Some local regulations refers to biodiscs as a recommended small domestic wastewater treatment.
It’s more frequently used as industrial and medium sized civil settlements because the complex design technology of the plant is not justified in small installations.
Features and benefits
The biodisc can get a high biological pollutant removal from domestic sewage (grey and black water) and can obtain good results for industrial wastewater (for example, dairies, bakeries, food industry, paper mills, etc.).
A wide variety of application is possible: as secondary treatment or as a post-treatment (downstream of a activated sludge plant).
The occupied area is small, but power and maintenance costs are high.
The system requires a correct sizing analysis and careful operation planning.
- high contact time;
- high effluent quality (BOD and nutrients reduction);
- hydraulics and loads shock resistance;
- large active surface;
- small space required (footprint)
- silent process compared to activated sludge plants blowers;
- no channelling risks;
- lower power supply than conventional plants;
- low excess sludge production.
- high initial and maintenance costs;
- weather protection needed (sun, wind, rain and low temperatures);
- odour problems;
- permanent skilled technical labor needed.