The actual oxidation
ditch is an elongated closed loop that resembles a racetrack in
configuration. The mixed liquor flowing in the two channels of the ditch is
separated by a wall that runs down the central axis of the ditch and
terminates at the beginning of the semi-circular ends of the ditch.
Mechanical brush aerators (rotors) span the channels of the ditch, with the
brush partially immersed in the mixed liquor. A steel or concrete bridge
covers the rotors and provides an access walkway that also eliminates the
spray of aerosols into the air. The rotors aerate the wastewater and provide
the mixing which keeps the activated sludge in suspension. Ditches can be
designed to process wastewater flows ranging from 100,000 gallons per day to
millions of gallons per day. Consequently, a wide range of aeration
capability must be available. Kruger manufactures a number of standard rotor
sizes designed to accommodate such a flow range.
Kruger manufacturers rotors that range in length from approximately 2 to 9
meters (m) with motor sizes ranging from 10 Hp to 60 Hp depending on the
size of the rotor. The Princeton Plant uses Maxi Rotors.
The Maxi-Rotor has a diameter of 3'-3 3/8" ± (1 meter) and comes in standard
nominal lengths of 6.0 m, 7.5 m, and 9.0 m. The Maxi-Rotor is mounted on
concrete pedestals that are cantilevered from the inside of the outer wall
and the central baffle wall. Bridges also cover the Maxi Rotor and are
supported by the outer wall and central baffle wall of the ditch. SWD's for
ditches with Maxi-Rotors typically ranges from 8 ft to 12 ft without
submerged mixers. The Princeton Plant uses submerged Mixers. With the use of
submerged mixers to keep the activated sludge in suspension, the SWD can be
extended to 18 ft.
The number of rotors required for the ditch depends upon the characteristics
of the particular wastewater to be treated. Organic loadings, temperature
variations, and effluent requirements combine to establish the oxygen demand
that must be met by the rotors. The Princeton Plant has two (2) Maxi Rotors
in each of the two (2) Oxidation Ditches installed. The use of two (2)
Oxidation Ditches is referred to as Double Ditch Technology. Kruger
maximizes the oxygen transfer efficiency of the aerators by controlling the
rotors via Dissolved Oxygen (DO) and liquid level monitors/control.
Biological Start-Up Procedure
The biological start-up of any wastewater treatment system involves the
growth and proliferation of micro organisms that degrade and remove
pollutants from the liquid phase. These micro organisms, along with
non-biodegradable solids, will comprise the mixed liquor suspended solids
(MLSS). The micro organisms will grow in response to favourable
environmental conditions as well as temperature and waste strength. For
example, nitrifying bacteria, which can only grow only in the presence of
oxygen, will proliferate in the presence of a suitable residual dissolved
oxygen concentration and pH as well as a presence of ammonia. Similarly, a
system treating a stronger waste stream will have a shorter biological
start-up period when compared to a system treating a more dilute stream.
Basis for All Solids Control
The purpose of an activated sludge plant is to convert colloidal and
dissolved solids (substrate) to biological floc. Some of the substrate is
converted to CO2 and H2O, but the remainder is converted to cellular
material or is inert and accumulates within the bacterial floc. It has been
estimated that, under normal operating conditions, about one-third of the
incoming useable substrate is used for oxidation, while the remaining
two-thirds are used for synthesis. Large portions of the incoming wastes are
inert and not easily used. The result is that much of the substrate removed
by the activated sludge floc remains in the floc and accumulates as either
inert or living solids.
Because of this collection and production of solids, eventually the final
settling tanks would fill with solids. If the sludge were not removed from
the final settling tanks, it would flow over the effluent weir. Increasing
the return sludge pumping rate without wasting some of the sludge would not
solve the problem because the sludge would just be moved around in the
system and not disposed. Ultimate control of the system, no matter what
intermediate operating decisions are made, always will be based on solids
wasting. There are three methods commonly used by operators to decide how
much sludge to waste:
1. Control by maintenance of a constant MLVSS.
2. Control by maintenance of a constant F/M ratio.
3. Control by maintenance of a constant sludge age.
CONTROL BY MAINTENANCE OF A CONSTANT
MLVSS
With this method, the operator is maintaining a constant mass of organisms
to use the incoming food supply. For example, if the operator finds that an
MLVSS concentration of 2,000 mg/l works effectively at the plant, that level
will be maintained. If the solids in the aeration tanks increase above 2,000
mg/l, the operator will waste more until the MLVSS level is again 2,000
mg/l. If the MLVSS drops below 2,000 mg/l, the operator will waste less and
allow the solids concentration to increase.
This system of solids control is simple to understand and manage, involves a
minimum amount of lab work, and can produce good results, especially if the
incoming waste strength is stable. This method, however, has a rather severe
limitation in that the important F:M ratio is ignored. For example, suppose
that the BOD5 of the incoming waste increased by 50 percent over a
substantial period of time. This may happen with seasonal loadings, such as
from food processing plants or canneries. The increased solids production
from the higher BOD5 load would be wasted to maintain the constant MLVSS.
The result of this action, however, is that the F:M ratio is 50 percent
higher than the previously maintained ratio. The resulting high F:M or
organic overload easily could lead to process inefficiency or failure.
This limitation may be minimized if the operator has determined from
experience when the MLVSS must be changed to match an anticipated change in
the incoming load.
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