FAST Overview0 pages

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FAST® Process
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S M I T H & L OV E L E S S I N C .

Fixed Activated Sludge Treatment (FAST®) Process Technology Overview

B

iological 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.”

1.

Conceptually, what occurs in the system is a bit different
in theory than a conventional suspended growth system.
The bacteria, unlike conventional activated sludge
suspended growth systems, grow on the media and the
liquor circulating through the bacteria-laden media.

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.
The FAST® system offers numerous advantages:

The system handles peak shock and toxic loads, along
with low loadings as a biological system.

This growth pattern results in subsurface anaerobic microbes
and surface aerobic microbes, both attached to the media.
In case of a peak shock or toxic load, only the surface
aerobic microbes would die and sluff off. The subsurface
anaerobic microbes once exposed to air will quickly
convert to aerobic microbes and degrade the incoming
organics.
If the loading conditions to the FAST® system varies, the
microbial mass will adjust to these conditions but, unlike
a conventional active sludge system, the excess mass is
not lost in the effluent, but rather remains in the system
attached to the media.
2.

The system can handle a very high biomass population.
In a conventional activated sludge system, the Sludge
Volume Index (SVI) is a key factor in the system design.
Indirectly it limits the reactor basin MLSS concentration
and, in turn, the MLVSS that can be achieved because it
controls the settling tank underflow concentration. Thus,
for a given (SVI) and return sludge rate, the maximum MLSS
and MLVSS is fixed with narrow limits.
Because of the conceptual difference of the FAST® process,
the system does not depend on the return sludge rate and,
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