Civil Unit

BIOLOGICAL SEWAGE TREATMENT

1.  PRELUDE

South African waterways have been seriously polluted by Waste water and sewage effluent being one of the biggest contributors;  it is the responsibility of the polluter to ensure that sufficient sewage treatment plants are installed and in working order.

Sewage pollution is an organic substance.  This is when organic matter such as sewage or manure enters the water.  When organic matter grows more and more in a pond, the number of decomposers, (bacteria germs) will increase.  The decomposers grow exceptionally fast and use a lot of oxygen.  A shortage of oxygen kills of plant and aquatic life.

The Water Act demands that where an effluent cannot be treated at the local municipal work, it MUST be treated at the source itself, to such a level that it can safely be discharge directly back into the environment.  The Department of Water Affairs has drawn up specifications regulating the quality of such discharge water.  These are given in The Government Gazette No 20526 of 8 October 1999.

2.  INTRODUCTION

Sewage treatment is the process by which domestic wastewater is cleaned by removing the contaminants introduced into the water by human activity.  Sewage typically includes household waste liquids from toilets, baths, showers, kitchens and sinks etc., that have been disposed of via the sewer.  In many areas sewage can also include liquid waste from industry and commerce.

The bulk of the contaminants consist of organic waste matter and nutrients such as nitrogen, in the form of ammonia, that have been generated as waste products from the human body, as well as phosphate salts from washing powders and soaps.  Removing this organic matter and nutrients from the waste stream and thereafter disinfecting the water to kill pathogens before it is released back into the natural environment, forms the basis of sewage treatment.

Releasing this organic matter into the natural environment untreated in sufficient quantities results in surface water bodies such as dams turning into cess pools of rotting material that break down the ecology of the water body.  Excessive nitrogen and phosphorous nutrients causes eutrophication, which resluts in the overgrowth of the weeds and algae.  Such a rapid growth of algae is unsustainable and eventually most of them die, taking up most of the available oxygen in the water in the process of decomposing, so that the water becomes uninhabitable for useful organisms, that sustain the ecology, of the water body.

Some algal species also produce toxins that contaminate drinking water supplies, in addition to causing deoxygenation of the water.  Pathogens released into a surface or underground water body can eventually make their way back into drinking water supplies.

Although southern Africa has the poorest record for sewage treatment worldwide, with most effluent remaining untreated, South africa's sewage treatment is very advanced and the country has been a world leader in nutrient removal for many years.  Paradoxically though, there are some areas of South Africa thtat are not having their sewage treated, such as many rural areas and informal settlements in the suburban areas of some cities.

In general sewage treatment works can be divided into three main categories:

  • Systems catering for very large populations (say upwards of 2 000 people) corresponding to daily flows of 500kl and more.  These are the centralized municipal type works, typical extensive tanks and settlers, drying beds and other components.  Implication would then be extended reticulation systems, significant civil engineering works and usually quite large areas of rather unsightly dams and tanks.
  • Systems catering for single dwelling or a small number of individuals.  Here the "treatment" is as rudimentary as a "long drop"; up to a septic tank, with or without a French drain.  These systems obviously constitute health hazards as well as the potential fouling of underground water sources.  While these systems were used extensively in the past, the modern environmentally responsible approach is to dissuade their use.

 

The Water Act demands that where an effluent cannot be treated at the local municipal works, it MUST be treated at the source itself, to such a level that it can safely be discharge directly back into the environment.  The Department of Water Affairs has drawn up specifications regulating the quality of such discharge water.  These are given in The Government Gazette No 20526 of 8 October 1999.

3.  LETTER OF GUARENTEE

Effluent outflow is guaranteed to comply with Government Gazette No 20526 of 8 October 1999 as per DWA specifications.

ADVANTAGES

  • The design uses the latest technology creating a higher efficiency, resulting in smaller and less expensive plants.
  • Save on area and capital costs
  • The mode of operation allows for underground placement.
  • The plants are aesthetic and do not spoil the view and can be covered with soil.
  • Maintenance is uncomplicated and is user friendly with unskilled labour.
  • No high level of controls required
  • No off-odours are generated
  • Mode of operation eliminates the build up of top and bottom sewage sludge.
  • Eliminates the use of drying beds
  • Substantially reduced electricity supply.
  • Substantially reduced piping installation.
  • Plants are modular and can be constructed as the phases are developed.
  • The developers cash is not tied up in one big plant from day one thus assisting with cash flow.
  • Adaptability and flexibility of placement.
  • Repairs and maintenance greatly reduced due to the limited working parts.

 

SEWAGE PLANT SKECTH OPERATION

 

                  

 

                   

 

4.  TYPICAL INSTALLATIONS

                     

 

                    

 

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