They are commercially available on the market, polyethylene biomedia with a specific density between 0.94 and 0.96 g.cm-³, with a specific surface area ranging from 450 to 1200 m2.m-³, allowing, when in terms of equivalence with the concentration of suspended solids present in the tank. At polyethylene biomedia differ mostly by providing a protected area for the creation of the biofilm, and/or by the material used in its manufacture.
Regarding the available area, the larger the area, the greater the amount of sludge to be formed and the smaller the amount of polyethylene biomedia to be applied per reactor volume. The protected area shall be calculated by the internal surface area of the polyethylene biomedia, multiplied by the number of pieces per cubic meter. The outer surface is exposed to collision with other polyethylene biomedia making biofilm formation difficult, so it should not be considered as a useful area in the MBBR modality.
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The number of pieces must be calculated by perfect arrangement, quantifying the number of pieces positioned next to each other and one on top of the other in a 1 m³ container (1m x 1m x 1m).
The random arrangement is obtained by calculating the percentage that would fill the same container, but in a disordered way. The percentage should be obtained by sampling and counting in smaller containers, for example 50 to 250 liters. This percentage ranges from 78% to 85% depending on the size of the polyethylene biomedia. The smaller the dimensions, the higher the percentage relative to the perfect arrangement.
In turn, the selection of polyethylene biomedia depending on the protected area to be used, it depends on the organic load of the effluent, the volumes of existing tanks or those to be built, the desired effluent quality and the hydraulic detention time of the reactor. It is important to note that filling the tanks with polyethylene biomedia MBBR, vary between a minimum of 30% and a maximum of 70% and this may limit the area of polyethylene biomedia to be used.
"An polyethylene biomedia with a lot of area, for example 1000 m2/m³, or greater, for filling 30 % of the tank volume, considered minimum, requires an aeration tank with a very small volume, it is necessary to consider the hydraulic detention time of the effluent (TDH ) which should not be less than a certain value, as the contact time between the effluent to be treated and the microorganisms would be insufficient for the metabolic processes involved”, explains Catino.
He also points out that special attention should be given to the geometry of the part, since, polyethylene biomedia which have larger areas, in large part, have very restricted and small openings, do not allow the flow of the substrate and consequently the formation of a healthy biofilm. Several media on the market, which offer large areas, do not have the interior areas available, as the superficial biofilm blocks the flow of liquid to the lower layers.
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As for the manufacturing material, there are on the market polyethylene biomedia Made of PP (Polypropylene) and HDPE (High Density Polyethylene), both compatible with MBBR technology. These are the most used, and since they are made from virgin raw material 100 %, they have a longer life of 20 years.
It is worth noting that some MBBR manufacturers opt for recycled raw material, black in color and of very low quality, greatly compromising the quality of the process, interfering with the image of MBBR technology. “We believe that the effluent treatment market, which is increasingly competitive, has observed this issue very appropriately” – emphasizes Catino.
Campos also explains that in general, the polyethylene biomedia present in the MBBR system basically contribute to maintaining a high concentration of active biomass, resulting from the control of the biofilm thickness as a result of shocks and shear caused by the energy of the mixture; for the ability to retain highly specialized biomass for each type of condition imposed on the reactor, regardless of the sludge age parameter; and to allow the ability to adjust to high diffusion rates resulting from turbulent conditions imposed on the reactor.
It is right inside the polyethylene biomedia biofilm develops, an active part in the treatment process. These are small ecosystems generally made up of three layers. According to Campos, in the first layer of colonization, the macromolecules present in the sewage are adsorbed, promoting clarification, since they are transported to the interior of the polyethylene biomedia; in the second layer, the aerobic processes of stabilization of organic matter are observed with maximum growth rate of the microorganisms involved.
He points out that the production of exopolymers that promote stability and adhesion to the biofilm is also observed. In the third layer, due to the unavailability of DO, anaerobic/anoxic processes occur, in addition to possible endogenous respiration due to the low amount of substrate.
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