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IWA
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Activated sludge (AS) is a process dealing
with the treatment of sewage and industrial wastewaters and developed around
1912-1914.
o There is a large variety
of design, however, in principle all AS consist of three main components: an
aeration tank, which serves as bio reactor;
o a settling tank
("final clarifier") for separation of AS solids and treated waste
water;
o a return activated
sludge (RAS) equipment to transfer settled AS from the clarifier to the
influent of the aeration tank (Fig. 1).
Atmospheric air or in rare cases pure oxygen
is introduced to a mixture of primary treated or screened sewage (or industrial
wastewater) combined with organisms to develop a biological floc
("Activated Sludge" AS).
The mixture of raw sewage (or industrial
wastewater) and biological mass is commonly known as Mixed Liquor.
Typically, dry solids concentrations of mixed
liquor (MLSS) range from 3 to 6 g/L.
With all activated sludge plants, the
concentration of biodegradable components present in the influent is reduced
due to biological (and sometimes chemical) processes in the aeration tank.
The removal efficiency is controlled by
different boundary conditions, e.g. the hydraulic residence time (HRT) in the
aeration tank, which is defined by aeration tank volume divided by the flow
rate.
Other factors are: Influent load (BOD5, COD,
Nitrogen,...) in relation to the AS solids present in the aeration tank
(Food:Microorganism Ratio, F:M Ratio), oxygen supply, temperature, etc.
At the effluent of the aeration tank, mixed
liquor is discharged into settling tanks and the supernatant (treated waste
water) is run off to be discharged to a natural water or undergo further
treatment before discharge.
The settled AS is returned to the head of the
aeration tank (RAS) to re-seed the new sewage (or industrial wastewater)
entering the tank and to ensure the desired MLSS concentration in the aeration
tank.
Due to biological growth (and solids present
in the raw waste water which are only partly degraded), excess sludge
eventually accumulates beyond the desired MLSS concentration in the aeration
tank.
This amount of solid (called Waste Activated
Sludge WAS) is removed from the treatment process to keep the ratio of biomass
to food supplied (sewage or wastewater) in balance and the F:M ratio in a
defined range.
WAS is stored away from the main treatment
process in storage tanks and is further treated by digestion, either under
anaerobic or aerobic conditions prior to disposal.
Activated Sludge Process
Figure1. The diagram of activated sludge
process
Many sewage treatment plants use axial flow
pumps to transfer nitrified mixed liquor from the aeration zone to the anoxic
zone for de-nitrification.
These pumps are often referred to as Internal
Mixed Liquor Recycle pumps (IMLR pumps).
The raw sewage, the RAS, and the nitrified
mixed liquor are mixed by submersible mixers in the anoxic zones in order to
achieve de-nitrification.
Purpose
In a sewage (or industrial wastewater)
treatment plant, the activated sludge process can be used for one or several of
the following purposes:
o oxidizing
carbonaceous matter: biological matter.
o oxidizing
nitrogeneous matter: mainly ammonium and nitrogen in biological materials.
o removing phosphate.
o driving off entrained
gases carbon dioxide, ammonia, nitrogen, etc.
o generating a
biological floc that is easy to settle.
o generating a liquor
low in dissolved or suspended material
History
The activated sludge process was discovered
in 1913 in the UK by two engineers, Edward Arden and W.T. Lockett, conducting
research for the Manchester Corporation Rivers Department at Davyhulme Sewage
Works.
Experiments on treating sewage in a
draw-and-fill reactor (the precursor to today's sequencing batch reactor)
produced a highly treated effluent.
Believing that the sludge had been activated
(in a similar manner to activated carbon) the process was named activated
sludge.
Not until much later was it realized that
what had actually occurred was a means to concentrate biological organisms,
decoupling the liquid retention time (ideally, low, for a compact treatment
system) from the solids retention time (ideally, fairly high, for an effluent
low in BOD5 and ammonia.)
Activated Sludge Process Variables
The main variables of activated sludge
process are the mixing regime, loading rate, and the flow scheme.
Mixing Regime
Generally, two types of mixing regimes are of
major interest in activated sludge process: plug flow and complete mixing.
In the first one, the regime is characterized
by orderly flow of mixed liquor through the aeration tank with no element of
mixed liquor overtaking or mixing with any other element.
There may be lateral mixing of mixed liquor but there must be no mixing along the path of flow.
In complete mixing, the contents of aeration
tank are well stirred and uniform throughout.
Thus, at steady state, the effluent from the
aeration tank has the same composition as the aeration tank contents.
The type of mixing regime is very important
as it affects
(1) oxygen transfer requirements in the
aeration tank,
(2) susceptibility of biomass to shock loads,
(3) local environmental conditions in the
aeration tank, and
(4) the kinetics governing the treatment
process.
Flow Scheme
o The flow scheme
involves:
o the pattern of sewage
addition
o the pattern of sludge
return to the aeration tank and
o the pattern of
aeration.
Sewage addition may be at a single point at
the inlet end or it may be at several points along the aeration tank.
The sludge return may be directly from the
settling tank to the aeration tank or through a sludge reaeration tank.
Aeration may be at a uniform rate or it may
be varied from the head of the aeration tank to its end.
Types of plants
There are a variety of types of activated
sludge plants. These include:
Package plants
There are a wide range of other types of
plants, often serving small communities or industrial plants that may use
hybrid treatment processes often involving the use of aerobic sludge to treat
the incoming sewage.
In such plants the primary settlement stage of
treatment may be omitted. In these plants, a biotic floc is created which
provides the required substrate.
Package plants are commonly variants of
extended aeration, to promote the 'fit & forget' approach required for
small communities without dedicated operational staff.
There are various standards to assist with
their design.
Oxidation ditch
In some areas, where more land is available,
sewage is treated in large round or oval ditches with one or more horizontal
aerators typically called brush or disc aerators which drive the mixed liquor
around the ditch and provide aeration.
These are oxidation ditches, often referred
to by manufacturer's trade names such as Pasveer, Orbal, or Carrousel.
They have the advantage that they are
relatively easy to maintain and are resilient to shock loads that often occur
in smaller communities (i.e at breakfast time and in the evening).
Oxidation ditches are installed commonly as
'fit & forget' technology, with typical design parameters of a hydraulic
retention time of 24-48 hours, and a sludge age of 12-20 days.
This compares with nitrifying activated
sludge plants having a retention time of 8 hours, and a sludge age of 8-12
days.
Deep Shaft
Where land is in short supply sewage may be
treated by injection of oxygen into a pressured return sludge stream which is
injected into the base of a deep columnar tank buried in the ground.
Such shafts may be up to 100 m deep and are
filled with sewage liquor.
As the sewage rises the oxygen forced into
solution by the pressure at the base of the shaft breaks out as molecular
oxygen providing a highly efficient source of oxygen for the activated sludge
biota.
The rising oxygen and injected return sludge provide
the physical mechanism for mixing of the sewage and sludge.
Mixed sludge and sewage is decanted at the
surface and separated into supernatant and sludge components.
The efficiency of deep shaft treatment can be
high.
Surface aerators are commonly quoted as
having an aeration efficiency of 0.5-1.5 kg O2/kWh, diffused aeration as
1.5-2.5 kg O2/KWh.
Deep Shaft claims 5-8 kg O2/kWh.
However, the costs of construction are high.
Deep Shaft has seen greatest uptake in Japan, because of the land area issues.
Deep Shaft was developed by ICI, as a
spin-off from their Pruteen process.
In the UK it is found at three sites:
Tilbury, Anglian water, treating a wastewater with a high industrial
contribution; Southport, United Utilities, because of land space issues; and
Billingham, ICI, again treating industrial effluent, and built (after the
Tilbury shafts) by ICI to help the agent sell more.
Deep Shaft is a patented, licensed, process.
The licensee has changed several times and, currently (2007), it is Aker
Kvaerner Engineering Services.
Surface-aerated basins
A Typical Surface-Aerated Basing (using
motor-driven floating aerators)
Most biological oxidation processes for
treating industrial wastewaters have in common the use of oxygen (or air) and
microbial action.
Surface-aerated basins achieve 80 to 90%
removal of BOD with retention times of 1 to 10 days. The basins may range in
depth from 1.5 to 5.0 m and utilize motor-driven aerators floating on the
surface of the wastewater.
In an aerated basin system, the aerators
provide two functions: they transfer air into the basins required by the
biological oxidation reactions, and they provide the mixing required for
dispersing the air and for contacting the reactants (that is, oxygen,
wastewater and microbes).
Typically, the floating surface aerators are
rated to deliver the amount of air equivalent to 1.8 to 2.7 kg O2/kWh.
However, they do not provide as good mixing
as is normally achieved in activated sludge systems and therefore aerated
basins do not achieve the same performance level as activated sludge units.
Biological oxidation processes are sensitive
to temperature and, between 0 °C and 40 °C, the rate of biological reactions
increase with temperature.
Most surface aerated vessels operate at
between 4 °C and 32 °C.
Aeration methods
Diffused Aeration
Sewage liquor is run into deep tanks with
diffuser blocks attached to the floor. These are like the diffuser blocks used
in tropical fish tanks but on a much larger scale.
Air is pumped through the blocks and the
curtain of bubbles formed both oxygenates the liquor and also provide the necessary
stirring action.
Where capacity is limited or the sewage is
unusually strong or difficult to treat, oxygen may be used instead of air.
Typically, the air is generated by some type of blower or compressor.
Surface aerators
Vertically mounted tubes of up to 1 m
diameter extending from just above the base of a deep concrete tank to just
below the surface of the sewage liquor.
A typical shaft might be 10 m high. At the
surface end the tube is formed into a cone with helical vanes attached to the
inner surface.
When the tube is rotated, the vanes spin
liquor up and out of the cones drawing new sewage liquor from the base of the
tank.
In many works each cone is located in a
separate cell that can be isolated from the remaining cells if required for
maintenance.
Some works may have two cones to a cell and some large works may have 4 cones per cell.
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