SEWAGE
PLANT TECHNOLOGY
The
basic technology generally used for the treatment of sewage is
the activated sludge process. This applies to both small and large
processing plants and the difference lies in the arrangement and
enhancement of the various sections of the process.
The
activated sludge process is a natural process and nature offers
us a unique solution to treat sewage. Nature has provided a special
balance in this process in that the micro organisms present when
the food levels are high, will also consume the largest amount.
This allows the quick breakdown of the BOD levels to more reasonable
levels.
Once
these levels are reached, other micro organisms, which are heavier
and less mobile, will reduce the BOD levels further, until the
final acceptable standards are obtained.
The
fact that the last organisms are large and heavy, allows us in
practical terms to settle these organisms out very efficiently,
producing a clear liquor.
To
balance the process, we can identify four major sections in an
activated sewage plant system:
These
sections are repeated in the different systems available. Only
section 1, the anaerobic stage is not always required, depending
on the solids removal system proposed.
The
plants will produce an effluent in accordance with the Department
of Water Affairs Specification of 5 April 1962, R553, General
Standard. With the addition of a ferric chloride dosing system
and an additional settler, Special Standard can be obtained for
discharge into the local dams.
SMALL
SEWAGE PLANTS - TECHNOLOGY
As
indicated above, the activated sludge technology has been applied
to smaller sewage plants and has proven to be reliable and easy
to maintain. There are a number of alternative designs available
to suit the application on hand.
1. SMALLER SYSTEM : TYPE FMP
The
four sections described above are clearly visible in the construction
of the FMP unit. A typical plant is shown on figure 1:
1.
A collection tank or interceptor tank: this tank is not installed
in all installations, but has certain advantages. The tank ensures
that the feed to the aeration tank is mixed and more even in
character. This assist with better aeration action.
This
tank is also used to store extra sludge generated in the process,
which can then be removed on six monthly or yearly intervals.
2.
The aeration tanks (or tank) are flat-bottomed tanks with a
special aeration ring positioned in the bottom of the tank.
Air is introduced and by means of the special aeration system,
intimate contact is established between the air and the solid
matter.
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3.
The third section of the plant consists of a settler tank where
solids are settled and a clear overflow liquid is produced.
Although more expensive to manufacture, we only use conical
tanks for the settler so that no solids can collect in the bottom
of the tank. If these solids are not returned to the process,
they would become anaerobic and cause foaming in the settler,
generating a dirty effluent.
4.
The final contact tank is a chlorine contact tank, which finally
treats the liquor to produce an effluent in accordance with
the General Standard suitable for discharge into rivers or for
use as irrigation.
For
South African dams and rivers feeding directly into dams, the
Department of water Affairs requires the removal of phosphates
as well as chlorination. This is effected by dosing ferric chloride
and settling the phosphates using another settler similar to
the main process settler.
It
is clear from the above that the basic process used in the FMP
plants is basic and simple. For the evaluation of any system,
it is imperative that the operation of such a plant is also
simple and does not require special equipment or specialised
operators.
The
sludge collected in the settler has to be recycled back into
the main process. The use of pumps would require special operating
skills and will generate high maintenance costs. The FMP system
uses a special airlift system, so that the fan is the only moving
part on the plant. The complete system is thus easy to operate
and requires no special skills to maintain.
To
ensure the complete plant offers the user a long-term viable
alternative two more requirements are to be met:
a.
The plant must withstand the corrosive nature of sewage. This
ensures that the life of the plant is extended as far as possible
without replacement or repairs.
b. The plant must remain economically viable.
This
is accomplished by the use of plastic and/or fiberglass tanks.
The moulded polyethylene tanks are fully corrosion resistant
to all conditions that may occur in the process. They also are
UV resistant and are light and easy to install. Using multiple
tanks, various combinations can be effected to accommodate many
duties.
To
accommodate slightly larger flows, a fibreglass tank system
has been developed. Using a special resin system, corrosion
resistance is guaranteed. The external surface is pigmented
green for better UV resistance, allowing the installation of
the plant above or below ground.
The
FMP system offers a low cost, low maintenance, high efficiency
solution to sewage problems for applications such as : police
stations, hotels, resorts or smaller housing developments, toll
plazas, forester stations and any other smaller development.
The
type FMP is also specifically suited to the many new nature
reserves and game parks that are being developed throughout
Africa. There are a number of aspects that support this
-
As indicated the plant is economical and is suited to both
up-market and more general developments, as it does not affect
the viability of the development.
- The plant is easy to operate and requires no special skills.
Local labour can easily be trained to operate the plant successfully.
- No maintenance skills are required and no spares have to
be held.
- The water being discharged by the plant can be used for
the generation of a small wetland or reed bed, to enhance
bird life in the area. If required, the water can be used
for irrigation of the gardens.
- The solids generated by the plant can be dried in a special
drying system available. These solids can be used as a fertiliser
for the gardens.
If
compared to other systems, it is clear that the FMP process
has many advantages over other systems for the same duty.
For
very small applications such as construction sites or lodges,
the FMPX system contains all of the above, but assembled in
one single tank. It should be noted that these plants can easily
be moved, if required. This allows for example a construction
site to operate to strict ecological standards and relocate
the plant as contracts are completed.
2.
TYPE CSC UNIT FOR LAGER FLOWS
Obviously
the use of separate tanks is limited by the actual size of the
tanks. Larger tanks become more expensive and also become difficult
to transport.
The
cheapest alternative for larger structures is concrete, and
type CSC incorporates a number of advantages:
-
It has all the aspects of a full activated sludge system as
described above.
- It reduces the total construction by arranging the various
elements in a circular fashion. Using this circular arrangement
allows us to maximise the area, while minimising space.
- The aeration tank is situated on the outside and feeds a
settler positioned in the centre of the plant.
- The walls of the construction are manufactured from precast
concrete panels using special patented construction methods.
This minimises the amount of site work, and reduces the total
cost of the plant.
The
plant is shown in figure 2 and the various sections can be identified
as follows:
The
total plant is thus compact and all the advantages described
for the smaller plastic system again apply.
The plants can be installed above or below ground, depending
on the topography of the area.
This
type of plant can be adapted for populations up to 8000 people,
making it suitable for small villages or portions of larger
developments.
3.TYPE CHC
WITH FLAT BOTTOM DESIGN
The
building of the conical section in outlying areas can add to
the total cost of a plant quite considerably. It was thus the
aim to design a plant, which could be installed on a flat concrete
base.
After
many designs and much testing, the type CH plant was developed
which is shown on figure 3.
Details
are:
- The
circular format has been retained, so that all the advantages
of this configuration remain in force.
- The
settler has now been placed on the outside of the aeration
tank. This is quite logical, as the settler action is based
on the total area available for settling and the overall upward
velocity achieved in the settler. The outside of the circle
represents the largest area per metre of width. We can thus
obtain very large settling areas, without increasing the size
of the plant significantly.
- The
patented sludge return system ensures that the sludge is returned
automatically from the settler section into the aeration tank.
The
sludge return system is the secret of this design. It ensures
that there is a constant balancing between the flow into the
settler and the return flow of the solids back towards the aeration.
The advantages of the system are obvious, and it offers:
- Easier
civil work in that only a flat reinforced concrete floor is
required.
- Walls
can be built in concrete panels, using the same system as
described for the CSC system.
- It
incorporates all the aspects of a successful activated sludge
design.
The
only problem with some of the applications, is that the concrete
panels are difficult to transport and difficult to handle due
to their weight. For this reason the CHF system was developed,
which uses fibreglass panels for the manufacture of the circular
container.
The
fibreglass panels can easily be transported on a truck or containerised
so that plants in outlying areas can be reached economically.
This system also allows the system to be exported readily, as
shipping costs are reduced drastically. Panels can be containerised
without any problem.
NEW
DEVELOPMENTS
1.
SMALLER PLANTS
In
the case of smaller developments, the choice of plant is usually
easy as it is determined by the layout and the number of people
to be catered for.
The
above systems are easy to specify, simple to install and have
very short contract periods. This allows a plant to be built
within minimum time at minimum cost.
2.
HOUSING AND TOURISM
In
the case of housing or tourist developments, it is often better
to install a number of smaller plants and allow the sewage plant
facilities to grow with the development. This minimises the
capital requirements and allows the developer to phase in facilities
as required. This has also a technical advantage in that the
sewage plant will never be underutilised, which often causes
problems with the quality of the effluent produced.
It
should be noted that in case of tourist developments, the use
of an interceptor tank or anaerobic area is advantageous.
As
indicated earlier, this tank evens the feed to the aeration
tank. During low off-peak periods, the interceptor tank will
allow the plant to keep in balance when loading is low, not
withstanding the fact that they have to be designed for the
peak loads during weekends or holiday seasons.
3.
LARGER HOUSING DEVELOPMENTS
It
has been general practice up to now, to install large central
plants for larger housing developments. The development of the
plants presents the developer with an alternative,
which in some cases may have definite advantages.
Using
the more traditional method of single central sewage plants,
it can be understood that the distances between the plant and
the individual houses becomes greater. Depending on the topography
of the land, various possibilities exist:
- A
gravity system is employed, but due to the distances involved
trenches tend to become very deep. To install these expensive
machinery is required, and the cost of total reticulation
system becomes very high.
- If
the land cannot allow a gravity system, local collection points
will collect the sewage by gravity from where it is pumped
to the main treatment plant.
The
cost of these types of distribution systems is normally more
than the cost of the sewage plant itself.
If
however a number of satellite sewage plants are used, we find
that:
- Distances
are much smaller and pumping is not required
- Piping
does not have to run as deep. Local labour can be used to
install the piping instead of large civil contractors as required
above. Besides the fact that this will assist local communities,
costs are reduced.
- The
building of smaller plants as and when the community grows
will assist with the capital requirements for such a project.
This is in stark contrast to the high capital investment required
from the outset with a central plant.
- As
capital costs are lower, interest charges for the overall
project will be reduced significantly.
- As
the smaller sewage stations do not produce a smell, they can
be installed strategically within the new housing schemes
and do not have to be separated, as is the case with central
plants. This further reduces costs. A plant currently operating
in the Kempton Park municipality confirms this point.
- As
the delivery of these smaller units is quicker, planning becomes
easier and can be continuously adapted to the actual progress
of the housing scheme. A typical delivery to completion is
around 3 months.
- The
fact that plants are built as required, results in better
loading of the plant, making them more efficient during the
initial stages of the project. Large unused plant capacity
can often lead to operational problems.
OPERATION
As
can be seen from the above, smaller units do reduce the total
capital cost of a development. However it is also important
to realise that the operational costs are also reduced.
All
designs offered, as can be noted from the above, are simple
in principle. Operation can be handled by local labour without
the need for assistance and/or supervision by specialists.
No
expensive instrumentation or computer-aided equipment is required
to operate the plants. Moving parts are minimised and little
maintenance is required.
It is also common practice that the solids produced in the plant
are dried on site. A clean system of drying beds has been developed
for this purpose. These solids can be used as a fertiliser,
which can be sold to the local community for crop production.
Water
is produced in accordance with the General Standard, the reuse
of the effluent water for irrigation should be considered.
