biological sewage treatment

Sewage Plant


In most of the southern countries drinking water is a precious asset, this is a reason why it has to be saved. Waste water should be treated until it is safe to be infiltrate and will not contaminate drinking water resources. At the same time it can be collected for further use.

Different processes are needed to treat waste water in a biological sewage plant. In comparison to sewage plants in western countries generally no chemicals are used and the plant should run without auxiliary energy.


In such a system different types of water can be processed. Mainly two types are treated / separated:

  • Grey water: Hand wash basin, shower, kitchen and laundry
  • Waste water: Flushing toilets, urinals (if urine is not separated!)


The main steps of processes in a BSP are:


Primary or Pre-treatment => mechanical

The collected waste water is discharged to the collection unit (usually with 2 or 3 chambers) where fluids and solids are collected. Through the process of sedimentation, solids are collected at the bottom of the chamber. In the last chamber liquids remain and overflow to the next step of treatment in the feeding chamber.


Secondary treatment => biological

The feeding chamber flushes periodically waste water from the last chamber to the biological treatment area. The mechanical element flushes to the sand filter by gravity.

A sand filter infiltrates flushed water. The input of oxygen during flushing is essential.

  • Top layer – 70-80cm of river sand (rough)
  • 2nd layer – 20-30cm of gravel (4/8)
  • 1st layer – 20-30cm of gravel (8/16)
  • Bottom – Sand layer of 5cm, drainage pipe has to be placed on top of the sand.

Retention time of one day should be guaranteed for the discharged water in the biological treatment area. The oxygen is starting the biological processes.

Vegetation is planted on top of the sand filters. Plants (reed, elephant grass, sugar can) are using the free nutrients and potassium. The quality of the treated water can be increased, if more sand filters are connected in series.


Final treatment => re-use or infiltration

Treated water is now discharged from the drainage to the final treatment. The treated water can be discharged and infiltrated in a trench which is harmless to the ground water resources and the environment. Another option is the collection eg. for irrigation or for flushing toilets (NO DRINKING WATER!)

Key facts
  • Solution for small scale (hospitals, boarding schools etc.) and large scale (e.g. communities, municipalities or townships)
  • No chemicals used for water treatment
  • Maintenance and operation essential
  • Awareness raising has to be provided (new system to many users)


  • No contamination of drinking water
  • Construction done with local materials
  • Further use of treated water (irrigation, flushing the toilet)
  • Hygienically a very good solution, water is in a closed system until no contamination of the water has to be feared


  • High construction costs
  • Maintenance has to be done periodically
  • Auxiliary energy has to be used for flushing, if no gradient is available in the surroundings