Biological treatment systems for small communities and even single-family dwellings have been available in the general water pollution control market for many years. These systems are basically scaled-down versions of the activated sludge process, utilizing suspended growth systems. Historically, the main operational problem associated with these systems has been the management of the sludge solids. In very small plants, flow rate variations can be extremely large and cause unintentional wasting or loss of the biological solids from the suspended growth reactor.
The FAST® (Fixed Activated Sludge Treatment) system eliminates many of the operational problems inherent with conventional suspended growth systems. It utilizes fixed media submerged in the aeration tank upon which the bacteria grow. Hence, the term “Fixed Activated Sludge Treatment.”
The FAST® treatment system consists of a vessel packed with a media that provides a high surface area to volume ratio. The media is fully submerged in the liquid. Air diffusers below the media provide circulation of the waste to be treated through the media, and provide oxygenation to the liquid. The bacteria, unlike conventional activated sludge suspended growth systems, grow on the media while the liquor circulating through the bacteria-laden media is essentially clear and free of suspended solids. As the system operates, bacteria grow and flourish on the media and reach a point where they sluff from the media. The solids that are removed by this sluffing action are not overly gelatinous and slimy but tend to be very large and settle very rapidly.
In larger systems, these sluffed solids can easily be removed by a clarifier. Once removed from the effluent, these solids can be thrown away or returned to the aeration basin. Even without the return of the solids, the effective sludge age for this system is quite long, generally 40 to 100 days, depending upon loading rates, etc. The effective mixed liquid suspended solids levels are in the range of 4000 to 8000 mg/L. The concentration of bacteria is dependent on the concentration of the waste. The system is self-regulating.
The type of media is similar to media used in trickling filter towers. It has discreet channel flow paths, which cause selfcleaning action. The flow through the stacked-type media completely eliminates the need for any media maintenance.