Biological treatment systems for small communities and commercial properties have been available in the European and North American 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.

Figure 1: Extended aeration flow scheme. Source: Smith & LovelessFigure 1: Extended aeration flow scheme. Source: Smith & Loveless

A prime example concerns the solids flux, essentially the kilograms of sludge expressed in dry solids removed from the bottom of the clarifier. If the solids flux total, quantified in kilograms per day per square meter of clarifier surface area, is in excess, it will tend to cause solids to back up in the clarifier, overflow the weirs, and go out in the effluent. This causes a drastic reduction in effluent quality. Excessive solids flux creates a high sludge blanket, which decreases the top area of the clarifier necessary for effective settling of the solids. If the sludge is somewhat difficult to settle, it will bulk up quicker with high solids flux and take more room than it should, thus causing settling problems for the plant operator. These problems are often compounded on smaller plants, which generally receive less operator attention and more variable loadings. As a result, a self-regulating, fixed-film activated sludge treatment process is often required and employed in these applications. One such system is the S&L FAST from manufacturer Smith & Loveless, Inc.

Figure 2: Fixed activated sludge flow scheme. Source: Smith & LovelessFigure 2: Fixed activated sludge flow scheme. Source: Smith & Loveless

The S&L FAST process 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 slough from the media. The solids that are removed by this sloughing action are not overly gelatinous and slimy but tend to be very large and settle very rapidly.

Typically, an extended aeration plant has a mixed liquor suspended solids level (MLSS) of 3,000 to 4,000 milligrams per liter (mg/L). At this level, the clarifier is nearly operating at the maximum solids flux. On the other hand with the S&L FAST process, the bacteria grow on the submerged media while the mixed liquor circulates through the bacteria-laden media. The effluent is essentially clear and free of suspended solids, thus reducing the MLSS flow to the clarifier to approximately 100 mg/L to 400 mg/L and greatly reducing the solids flux, which means less stress on the clarifier.

The S&L FAST process does not require sludge to be returned to the aeration zone in comparison to the extended aeration process, which is dependent upon RAS rates of 50% to 150% of Q. This is another reason for the reduction in the MLSS within the S&L FAST zone. This is not saying that extended aeration plants do not operate at a MLSS level of 4,000 mg/L because they have been operating at this level for many years. However, the fixed activated sludge approach offers much more safety and flexibility for the operator, and a more efficient clarification system.

To demonstrate exactly how the two processes differ, see Figure 1 and Figure 2 above. Referring to the extended aeration flow schematic in Figure 1, one can see that an influent flow (Q) enters the aeration zone along with the return sludge flow rate (QRAS). Typically in an extended aeration plant, Q = QRAS; therefore, two Q enters the clarifier with Q exiting the clarifier as effluent and the QRAS returning to the aeration zone. Note that the suspended solids level entering the clarifier is at a level of approximately 4,000 mg/L and is thickened to approximately 8,000 mg/L in the clarifier.

Figure 3: Mass balance equation. Source: Smith & LovelessFigure 3: Mass balance equation. Source: Smith & Loveless

In comparison, the S&L FAST flow schematic shows only Q (no QRAS) level entering the aeration zone and subsequently the clarifier. The suspended solids level entering the clarifier is about 100 mg/L to 400 mg/L, in comparison to 4,000 mg/L in the extended aeration scenario. Also note that the thickened sludge in the clarifier is at the same strength; however, in Figure 2 it exits only as waste activated sludge rather than return activated sludge. The suspended solids level entering the clarifier is from waste solids sloughing off the media. Basically, the only suspended solids in the wastewater are derived from this sloughing action.

Figure 3 demonstrates that the solids flux of the extended aeration clarifier is 20 times greater than the solids flux of the clarifier stemming from the S&L FAST process. Thus, the S&L FAST system places significantly less loading on the clarifier, which allows for simpler, more efficient operation.