Microbac Bioreactors are designed specifically for biological treatment and detoxification of industrial and municipal wastewater streams. In addition, the units can be used for bioremediation of oil spills, oily sludges or land polluted with organic residues and for the decontamination of polluted groundwaters.
The Bioreactor unit consists of a coated steel or cast-in-place concrete tank, which contains a rigid, submerged, structured plastic matrix and a membrane air distribution system. It also has provisions for effluent discharge to maintain a fixed operating liquid level in the tank.
Diffused Aeration-Mixing System
This unique system provides an excellent growth environment for bacteria. The Bioreactor tank is full of rigid matrix, with the exception of the water freeboard above the matrix and the small water-filled space below the matrix. In the latter space, several air distribution membranes cross the width of the tank to supply air from blowers via header pipes.
The matrix is designed in such a manner as to provide a series of separate vertical columns spaced along the long axis of the Bioreactor unit. These segments are the alternating areas shown in Figure 1. The alternate vertical columns serve as up-flow (those located over an air distribution membrane) and down-flow (those having no air distribution membrane) in these stationary biological film reactors.
In the vertical columns of the matrix located over the air distribution membranes, the air and water flow upwards, driven by the airlift effect of the bubbles. In these up-flow columns, oxygen transfer from the air bubbles to the wastewater occurs very efficiently. The latter is due to the shearing and reshearing of air bubbles rising through the plastic matrix towards the surface.
Liquid reaching the surface through the up-flow columns flows horizontally to the next vertical column and then flows downwards through the matrix. While there is no direct oxygenation of the liquid flowing downward through the matrix, the dissolved oxygen levels in the liquid which was previously aerated in the up-flow vertical columns are sufficient to supply the oxygen requirements of the bacteria in the down-flow column.
Bacteria grow rapidly in the Bioreactor units under properly engineered conditions, utilising the organic substrates present in the wastewater as food. As the bacteria grow, viscous polysaccharides are secreted by the cells, which enable the organisms to adhere to the surface of the matrix and form a fixed-film.
In the standard Bioreactor system, the rigid plastic matrix provides 150m2 of surface per m3 of volume on which the fixed-film can grow. The high surface area to volume ratio of the Bioreactor allows for the accumulation of substantial concentrations of bacteriain a film in a relatively small treatment unit. Furthermore, the accumulated viable bacteria are fixed in the system and do not need to be recycled, as is the case in conventional biological treatment plants, such as activated sludge systems.
In the Bioreactor system, the water is constantly flowing through the rigid plastic matrix on which the biological film is attached. As the water flows past the biological film, the bacteria in the film absorb the organic chemicals as well as dissolved oxygen, nitrogen, phosphorous and other trace nutrients required for their growth. The diffusion process which takes the organics and nutrients to the bacteria is depicted in figure 2.
As the bacteria grow on the matrix and as more chemicals are added to the Bioreactor unit in the wastewaters, the stationery biological film will continue to build in thickness (Figure 3). As the film becomes thicker, the depth of penetration by diffusion of the absorbed materials, such as dissolved oxygen or other nutrients, is insufficient to reach the base of the bacterial film. At this point, the inner layer of the film becomes starved of oxygen and nutrients and the organisms lose their ability to adhere to the matrix surface. The shear forces of the water and air bubbles flowing through the matrix will ultimately become great enough to tear this portion of the biological film loose from the matrix. This process is called sloughing. The solids which slough from the media will flow out of the system with the treated effluent. The exposed portion of the matrix surface will repeat the process of growth and sloughing, ensuring that a new and vigorous biomass is maintained to promote optimal biodegradation activity.
In actual operation of the Bioreactor unit, the biological film will be in a state of dynamic, continuous growth and sloughing, so that at any given time, portions of the matrix are always at some points between forming a new film and sloughing.
Effluent Quality and the Bioconversion Process
The effluent discharge from the Bioreactor system will contain the sloughed biological solids but will be relatively free of soluble organic chemicals. The quantity of biological solids in the effluent will depend substantially on the quantity of suspended solids entering the Bioreactor system and the concentration of BOD5 and COD entering the unit.
To determine the efficiency of the biological conversion of chemicals to bacterial mass, it is essential to monitor concentration of chemicals in the influent wastewater and in filtered (0.45µ porosity paper) samples of the treated effluent. If the discharge requirements for BOD5, COD or TOC are low, then it will be necessary to provide a means of separating the sloughed biomass from the treated effluent. Usually a simple clarifier attached to the Bioreactor system will provide the needed solids separation and subsequent reduction in the effluent BOD5, COD or TOC.
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