PEENOXTM media is cylindrical. It is divided into chambers and its internal structure is shaped like a honeycomb. Only internal surfaces are considered for the design of a treatment system.
The plastic material used to make the media is chemically inert and resistant to UV radiation. Its relative density is of the order of 0.95 which allows it to float. The geometry used makes it possible to offer an excellent ratio of "specific surface area" to "total surface area" while ensuring high structural strength. The media is not subject to abrasion and is not itself abrasive.
Studies have shown that it is not enough to provide the maximum surface area theoretically available for biomass growth, but that the surface area must be maximized while minimizing the risk of clogging the media (high velocity flow of water in the openings of the media).
Growth on unprotected (external) surfaces is not accounted for, in particular because biomass is continuously removed by shocks and surface friction with other PEENOXTM support media.
The biofilm average thickness is 1 mm, which allows the water to circulate freely through the media chambers. The dimensions of the chambers make it possible to avoid clogging problems. It thus becomes possible to obtain very large concentrations of "fixed" biomass within a relatively small volume.
The concentration of free biomass in the liquid must be added to this value, which varies in each application. It should be noted that protected areas allow the establishment of slow-growing species, which is a significant advantage when it is necessary to nitrify the effluent to be treated.
The media volume must be sufficient to obtain a fluidized bed. In a standard project, the filling volume is between 30 and 60% of the useful basin volume. Depending on the application, the load to be treated and the level of purification sought, this volume may vary between 25 and 70%.
The aeration system has three functions:
- Feed the system with oxygen to allow total or partial mineralization of the organic matter.
- Ensure efficient mixing of the media to create the fluidized bed effect.
- Homogenize the water to be treated in order to ensure a good distribution of the load.
Depending on the application and/or existing equipment, the air distribution system may provide coarse bubbles or fine bubbles. In the case of coarse bubbles, they are sheared into small bubbles as they pass through the fluidized bed formed by the media. This crossing of the media slows the rising rate of the bubbles.
A large part of the bubbles is intercepted by the media and brought back to the bottom. These two phenomena make it possible to increase the contact time with water and thus improve the oxygen transfer rate
Retaining grids must be installed at the outlet of the basin to retain PEENOXTM media in each stage of the bioreactor. Given the size of the media, the openings of the grids are relatively large, about 10 mm.
The grid area allows a flow rate generally comprised between 150 to 300 m/h (50 to 100 gpm/ft²) which limits the pressure drop. Debris accumulation on the grid is limited by constant collisions with the media that is continuously moving in the basin.
If the inlet pipe is submerged or at water level, it must also be protected by a grid of the same opening size.
Generally, air is used as an oxygen source. The size of the blower depends on the following elements:
- The pollution load to be treated (BOD5, nitrogen, suspended solids)
- The airflow required to mix the media
- The volume and geometry of the basin
- The volume of media used in the basin.
In summary, the key elements of the SMBRTM fluidized support bioreactor are:
- BOD/COD and nitrogen reduction
- Complete nitrification
- Low loads (200 mg/L BOD or less)
- High loads (10 000 mg/L BOD and more)
SMBRTM, commonly called MBBR, combines the benefits of activated sludge with those of fixed biomass systems.
The main advantages over conventional activated sludge are:
The concentration of biomass is much higher
A wider variety of microorganism species are present
Its simplicity of operation
In addition, the high biomass concentration makes it possible to obtain excellent purification performance with minimal footprint. In the case of an existing activated sludge system, it is possible to increase the treatment capacity by 50 to 100% per basin, without affecting effluent quality.
The aeration system is installed at the bottom of the basin and ensures constant agitation of the media during the rise of the air bubbles. As the density of the media is very close to that of the water, it is easily carried by the different currents present in the basin and uniformly distributed.
When starting an installation, the microorganisms present in the effluent to be treated will attach themselves on the media. The development of biomass is fast, allowing all specific spaces to be colonized in a few days.
Organic pollution in wastewater is essentially degraded by the biomass fixed on the media (biofilm). Also, this pollution is partially degraded by free suspended biomass.
Part of the upper layer of the biofilm is continuously released into the basin by normal processes of attrition and growth of the lower layers.
The free biomass will be carried with the flow to a clarifier after the bioreactor. Sludge production from the SMBR® represents a fraction of the biomass present in the bioreactor, reducing the mass load to the decanter.
SMBR® or MBBR can be arranged in several ways, depending on the water to be treated and the degree of treatment required:
- SMBRTM bioreactors with or without sludge return
- Media installed in existing activated sludge (IFAS)
- SMBRTM bioreactors installed in series (treatment stages)
- Bioreactors installed at the head of facultative aerated lagoons.