ROTATING BIOLOGICAL CONTACTOR (RBC)
The Rotating Biological Contactor system makes use of one or more rotating contact cylinders for the reduction of the organic matter waste water contains. Each contact cylinder is composed of a shaft, onto which polypropylene discs are fitted; discs are variable in number according to the model. The shaft is rotated very slowly (1 – 5 rpm, depending on the model and the effluent properties) by means of a motor-gearbox set.
The discs are partially immerged (about 40% of their diameter) in a tank through which the water to be treated passes. The contact between the sewage and the discs favours the formation of the biological slime on the latter. Thanks to the continuous rotation of the discs, the biological sludge is alternately immersed in the liquid (from where it acquires the organic matter necessary as nutriment) and brought into contact with the surrounding air (where it is saturated with oxygen, another element fundamental to the biological oxidation process).
Once its life cycle is finished, the biological slime layer detaches itself autonomously from the disc surfaces in the form of flakes, easy to settle.
RBC: advantages and application
Biodiscs: system advantages
- Compactness and adaptability of the installations thanks to the limited dimensions
- Modular installation, which allows subsequent plant expansion
- Rapid and spontaneous start-up: the system reaches the operating regime in a short time (10 ÷ 15 days) without necessity of seeding
- Simplicity of plant management and maintenance: no need for specialist personnel
- Low energy consumption, with consequent reduction of operating costs
- No sensibility to variations in either organic or hydraulic loads: the bacterial slime increases or reduces its thickness according to the loadings
- Absence of noise, vibration, aerosols
Application fields – domestic sewage
- Hotels: they can be annual or seasonal. The unique features of RBC are even more evident in the second case, which includes campsites and tourist resorts. Nevertheless, hotels that are open all year always benefit from using plants that are flexible under any load conditions – both hydraulic and biological – and that require as little work as possible for maintenance and management. As compared to alternative bio-purifying systems, the biodisc type is more quiet and, thanks also to its compact size and the fact that it can be installed underground or enclosed inside a building, it has a much better environmental impact.
- Airports: this is one of the many segments in which the biggest amount of RBC systems have been installed during the nearly thirty years of experience. At times, due to the capacity of RBC to biodegrade small quantities of hydrocarbons, sewage can be treated together with rain water (precipitation) from the outdoor areas. In the latter situation, it is essential to ensure proper degreasing before using RBC system. The system is also widely appreciated for its compact and modular structure (facilitating expansion) and for the possibility of repositioning.
- Hospitals: the absence of aerosol, thus eliminating the risk of bacterial contamination of the surrounding air, is undoubtedly the most interesting feature of RBC for this type of installation, particularly when the plant is located near hospital units. As compared to alternative biological water treatment systems, the biodisc type is more quiet and, thanks also to its compact size and the fact that it can be installed underground or enclosed inside a building, it has a much better environmental impact.
- Motorway service areas: this is one of the applications with the largest number of installations. One of the most important features of the biodisc system is its extraordinary capacity to adapt to fluctuation in the wastewater to be treated, in terms of both flow rate and composition, which can be pretty variable in service areas. Specifically, when handling underloads the system reacts spontaneously without undergoing any rebound reactions. Instead, in plants designed previously with activated sludge technology, the lack of organic material that arises with underloads can trigger shutdowns. A further advantage offered by this type of plant is that it requires low maintenance and little supervision by service station attendants. Finally, the compact size of a biodisc plant and its possibility to modulate it make RBC system ideal for this type of installation.
- Mountain shelters: the customer needs to install the plant indoors and in small places. In addition, the flow of waste water for treatment is highly discontinuous and during the low season, it often happens that there is no wastewater at all for several days. The biodisc system is ideal for this type of installation because it is extremely compact, can be installed indoors and has an extraordinary capacity to adapt to fluctuation in the flow and in the composition of the waste water. In these situations, it is not possible to install either an activated sludge system (space requirements and risk of undersupplying; the lack of organic material caused by underloading can trigger shutdowns) or a trickling filter (space and height requirements). Low power requirements combined with low consumption also make it possible to rely on alternative power sources (e.g. photovoltaic panels).
- Holiday resorts and campsites: In the worst situations, the plant runs for three or four months a year, with a rise (June to August) and fall (August and September) in influx. RBC is undoubtedly the only bio-purifying system sized for maximum capacity that can work under any conditions without the need for corrective intervention. Moreover, it is also fundamental to rely on a process that can start up as quickly as possible with the influx of the first consumers or with the arrival of service personnel prior to opening.
- Domestic sewage from factories: this application requires flexible plants that won’t be affected by work stoppages on weekends or holidays (Christmas, summer, etc.). There is almost always an equalisation phase for partial lamination of flow, which is normally discharged for 8/9 hours. It is also extremely important to rely on technology that requires very little maintenance and supervision and that can be managed even by non-specialised personnel. The RBC system offers notable advantages in alternative bio-purifying systems (such as activated sludge processes or biofilters) and, depending on the application, these advantages are so evident that planners naturally opt for this solution:extraordinary capacity to adapt to fluctuation in the waste water to be treated, in terms of both flow and composition; low installed power and low power consumption (for the oxidation phase up to 5 – 10 times lower than of alternative systems); less maintenance time and easy management; possibility to modulate if the population to serve increases; low noise level and no aerosol.
- Canteens: treatment with RBC is normally proposed because it is easy to manage and monitor in situations of being understaffed or in areas without qualified personnel. The flow must naturally be equalised since the waste water is discharged only 3 – 4 hours a day. As opposed to other biological treatments, the RBC system is not damaged even if there is no flow for extended periods (weekends, holidays, etc.). Proper pre-treatment for fats and solids is essential.
- Small communities: the optimum field of application for the RBC system in municipal sewage involves communities with a population of 30 to 5.000 equivalent inhabitants, although historically it has been used in installations both above and below this range. In communities with a high influx of tourists, low available electric power, lack of personnel, possible future increases and the plant’s location near the residential area, RBC system offers notable advantages over alternative bio-purifying systems (such as activated sludge processes or biofilters) and, depending on the application, these advantages are so evident that planners naturally opt for this solution: extraordinary capacity to adapt to fluctuation in the waste water to be treated, in terms of both flow and composition. Specifically, when handling underloads the system reacts spontaneously without undergoing any rebound reactions, while in plants designed previously with activated sludge technologies, the lack of organic material that arises with underloads can trigger shutdowns; low installed power and low power consumption (for the oxidation phase, up to 5 – 10 times lower than of alternative systems); lower maintenance costs and easy management; possibility to modulate if the population to serve increases; low noise level and no aerosol.
- Research expeditions: an unusual application of the RBC systems has been chosen – and is still being used – by the Italian expedition (ENEA) in Antarctica. The plant serves approximately sixty people and is installed inside prefab structures (heated). It starts up in October and operates until February. The requirement here was a flexible plant that is easy to start up and to manage and that could also handle a certain level of salinity in the waste water, for the reason that seawater is used for the toilets. The biodisc system fully satisfies these requirements. In addition, the reduced electric power requirements, combined with low consumption, mean that the RBC system can be powered with alternative energy sources (e.g. photovoltaic panels). The application of this kind of plant has similar features as the plants installed in mountain shelters.
- Sports centres: these can range from swimming pools to tennis courts, golf centres, stadiums etc… The flow rate is always extremely discontinuous throughout the day and often involves just a few days a week. Therefore, it is essential to have a system that can operate under different conditions without requiring plant correction. One of the most important features of the biodisc system is its extraordinary capacity to adapt to fluctuation in the waste water to be treated, in terms of both flow and composition. Specifically, when underloading takes place, the system reacts spontaneously without undergoing any rebound reactions. Whereas, for plants with activated sludge technology, the lack of organic material that arises with underloading can trigger shutdowns. Since the plants must be installed under unique environmental conditions, the RBC system is also preferable because it is compact and it can be installed underground or enclosed inside a building as well.
- Rubbish dump waste water: this can be considered a civil application, even though biological treatment in itself is not sufficient for achieving complete purification. Normally, it is integrated with a physical-chemical treatment stage and other pre- or post-treatment. Considering the unpredictability and variability of the water to be treated, the RBC system offers bigger guarantee compared to other biological systems, even in the case of “shock” or poisoning. The system has an extraordinary capacity to adapt to fluctuation in the waste water to be treated in terms of both flow and composition, and the process also starts very rapidly.
Application fields – industrial sewage
- Pharmaceutical industry: this is a sector in which the major number of installations have been made, including some of the largest (16 rollers with a diameter of 2,90 m). The RBC system is not always the only step (biological or other) to be used, since in most cases the water is heavily loaded. Normally, the organic matter to be degraded includes solvents, alcohol, esters, aldehydes and so on. The RBC system is preferable because it takes up less space and adapts better to variations in the features of the influent water, for example during cyclic changes in production. As compared to traditional activated-sludge systems, the RBC system stands out because it is easy to manage and supervise, which means less need for specialised personnel. Obviously, biological treatment cannot be used by producers of antibiotics.
- Oil industry: this sector includes refineries as well as fuel depots. In the literature, there are well-known specific articles about the elimination of oil, hydrocarbons and phenols with biological water treatment using biodiscs, since this is the field in which the first installations were completed and it thus has the largest number of references. The water to be treated is process water (at refineries) or precipitation, tank drainage water or clearance water (in the case of depots). Another reason for using the RBC system instead of other biological technologies involves drainage discontinuity, above all when dealing with water from precipitation. As it has been previously observed, the RBC system does not suffer any damage, even if it is not supplied for extended periods of time. As in the case of most industrial applications, the RBC system has other pre- and post-treatment phases. Nutrient batching is always envisaged.
- Dairies and creameries: the water to be treated is particularly biodegradable, although it often involves heavy loads. Depending on the size of the production facilities or the types of drainage restrictions to be observed, the RBC system is used either as the sole treatment or it is combined with another biological systems. A typical example is rough treatment with biorulli® and finishing with an activated sludge process. What stands out in the first case is the easy management and operation of the plant, an essential feature for small farms, while the distinctive feature in the second case is the ability of RBC to work even with large load rate. Compared to traditional systems such as activated sludge processes, the RBC system is distinguished by its compactness, modularity (permitting future expansion), low electric power consumption, and easy control and management, minimising the need for specialised personnel.
- Dyeworks: used prevalently at dyeworks, the RBC system is also used for fabric print works, wool mills, felt mills, etc. The key parameter to be degraded almost always involves surface-active agents, particularly non-ionic ones. Although it is known that during biological oxidation with the RBC system there is a rather short retention time, excellent percentages have nevertheless been observed in the removal of these pollutants. The compact size of the system, the easy operation and its low electric power consumption are other reasons for which it is often preferred over alternative bio-purification systems. Another advantage offered by this system is that it significantly lowers phenomena connected to the formation of foam, which is made possible by its low operating speed and the absence of any air insufflation.
- Slaughterhouses: these are plants in which the waste water has a heavy load rate of pollutants, even when blood is recuperated, thus leading to the need to have different treatment phases. As in the dairy industry, the RBC system is often used as a rough, initial treatment for the first biological step followed by another, involving oxidation-nitrification. The slaughter industry often runs on an irregular basis (few days a week), operating for just a few hours a day. As a result, it requires a system that is always active even without influents for several days: RBC is a system like that. In addition to these aspects, slaughterhouses often need a plant that will take up as little room as possible and that requires a very easy operating procedure: the biodisc technique fully satisfies both of these needs.
- Biscuit & confectionery industries: there are various types of this field of industry. Our references are among the oven-baked products (biscuits, cakes, etc.) factories. The RBC system, often combined with other technologies, guarantees good operation even when the different seasonal productions change, thanks to its flexibility. Its modular features, which facilitate expansion, combined with simple operation, have made various end-user firms and project designers to utilise this technology.
- Commercial laundries: comparable at times to the plants used by various textile industries, the equipment is designed to degrade surfactants and organic matter derived from the different types of items being washed (from restaurants, hospitals, etc.). RBC system is proposed because it is easy to manage and monitor, without any need for specialised personnel. Since no additives are required (as in physical-chemical treatment), operating costs are significantly lower. The little amount of space required to install the RBC system and the option of positioning it on a wide variety of sites has promoted the substantial development of the system, above all in small- and medium-sized commercial laundries.
- Wineries: the sewage in this industry is not easy to purify, particularly if one considers the fact that for a few months a year (harvesting and decanting), the organic load can be more than ten times higher than the annual standard level. The ideal solution in these cases is a RBC biological system that operates all year, with integration (biological or non-biological) for peak months. The result is a very low electric power consumption for most of the year, with very simple plant management.
- Paper mills: the choice of the type of plant is affected by the size of the paper mill and the extent to which the production cycle is closed (% of recycling upstream), which determines the influent concentration to the treatment plant. RBC system is used mainly in cases with low or medium concentrations of COD (more open cycles) or as a final step, downstream from other processes, in the case of high levels of concentration (more closed cycles). In both cases, RBC is the preferred system because of its compact size and the fact that it does not require specialised operators. In mills that handle paper for recycling, an additional advantage of the system is that it can easily be adapted for the different types of waste.
- Chemical industry: the water to be treated is more or less similar to that from the related pharmaceutical industries. The companies involved produce substances derived from organic synthesis and as a result, the pollutants are mostly biodegradable. RBC system is used extensively to eliminate hydrocarbons, alcohol, esters etc., and system “performance” is more evident here in comparison with other biological processes. In this field, RBC system also offers the advantage of being easy to adapt to the various types of waste water, which change in relation to current production. Other advantages of RBC that must also be considered are its compact size and easy management.
- Livestock farms: the process varies depending on the type of animals being raised. There are no major problems with cattle, sheep and chicken, but pig raising requires combined plants that are extremely overburdened in order to remain within legal limits. Easy management and monitoring have made RBC technology the leading reference point. At times, the system is used only as a coarse pre-treatment since no sewage restrictions are involved.
- Photo and film processing laboratories: in this industrial field, various installations were completed in the early 1980s, but the demand decreased following the recession that hit this sector. RBC system is used to purify the rinse water (following elimination and recovery of silver) containing even inputs of small concentration. Performance is also highly satisfactory for the nitrification of ammonia. Since many companies in this field are quite small, the compact size of the RBC system is a decisive factor, as well as the easy management of the biological part.