INTRODUCTION

South Africa requires the building of new housing developments throughout the country. Water born sewage is a requirement by the new householders and the use of septic tanks has proven to be too costly in the long term.

With the limitations in the availability of capital, new alternatives have to be found for the provision of sewage treatment.

At the same time many smaller developments outside the central municipal areas will also require full sewage treatment to protect our rivers from pollution and in this way protect the many people living from the South African rivers.

FAMSYSTEMS offers a wide range of sewage plants, which can be adapted to suit most applications offering high tech system at reasonable costs. The company offers over 30 years experience in the design of small to medium sewage treatment plants.

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:

      1. Collection and anaerobic storage
      2. Aeration of the sludge
      3. Settling of the sludge removing all solids
      4. Chlorination and phosphor removal to bring the final effluent up to the   required standard.

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.

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 LARGER 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:

      A. Interceptor tank area 
      B. Aeration section
      C. Settler
      D. Chlorine contact

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.


 

 


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