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Biologically Active Filtration

In the management of biological populations, the critical factor is maintaining a population of bacteria of sufficient mass that a ratio of food/microorganisms is maintained, such that the bacteria have enough food to survive, but not so much food that they grow rapidly. This allows for the management of biological cultures to favor desirable ones, and minimizes the production of waste solids requiring dewatering and disposal. One of the differentiating considerations when comparing biological processes is the density of bacterial population that can be maintained, as this translates directly to the size of system required, and in nearly all cases to the cost. In the activated sludge process, air is used to suspend bacteria in the wastewater as relatively fine flocculant particles. In this process, the peak solids concentration that can be maintained in solution before undesirable populations predominate is in many cases 5000 to 10,000 mg/L. This peak solids concentration in solution dictates in most cases the size of the required facility. The solids suspended in the wastewater in the aeration portion of this type of system are allowed to settle out and be recycled from final clarifiers that provide a quiescent environment for settling.

A variety of fixed growth processes exist in which bacteria grow on a fixed surface. The older forms of this process, such as trickling filters or RBCs, have similar volumetric loading criteria to activated sludge, largely because of limitations on the depth of biological growth that can be supported and the surface area available per cubic foot for bacterial growth. To minimize the volume of reactor required, and thus cost, EnCOSS normally provides biological treatment using a process referred to as Biologically Active Filtration. In this process, biomass (bacteria and more advanced life forms) grow inside a vessel which resembles a deep bed filter. The biomass develops on the surface of the filter media, which has a very high surface area to volume ratio, and can thus support large amounts of biomass in a relatively small space. To a lesser extent, biomass also grows as suspended growth in the interstitial spaces between media particles. The media used is a relatively coarse rounded filter media, which is selected due to it's self-cleaning characteristics in backwashing. The advantages of this process are the high volumetric loading rates achievable with the process, and the retention of biomass in the system for long periods of time at a relatively high biomass density, which aids in the development of slow-growing bacteria. The process also provides filtration at the same time that the biological treatment is occurring, so there is not normally a requirement for post-biological treatment clarification or filtration. Exceptional oxygen transfer results from the slow, circuitous path a bubble takes in traveling from the bottom of the vessel to the top, allowing for a reduction in operating cost due to smaller blowers, when compared to conventional systems.

EnCOSS normally provides BAF systems as modular units. These systems consist of a clear well, mud well, and a number of BAF vessels, with associated pumps and blowers. One system designed by EnCOSS personnel has been in operation for over 9 years in a tough industrial environment, and continues to satisfy the customer's needs.

For more information, contact the EnCOSS office nearest you.