Sewage treatment processes - simply explained!


Sewage treatment processDiagrammatic representation of the process of sewage treatment



Sewage treatment at a small activated sludge plantDiagrammatic representation of sewage treatment at a small activated sludge plant





There are various types of treatment systems that can be used to treat sewage more fully than a septic tank.
When untreated sewage is discharged into a watercourse it will:

  • Remove oxygen
  • Kill plants
  • Create a nuisance
  • Cause a risk to health

 And prevent its use for:

  • Water supply
  • Crop Irrigation
  • Fisheries
  • Recreation

The choice of system to be used will depend on many factors, some of which are:

  1. Is the sewage from a few properties / a small or large community?
  2. Is the sewage to be treated likely to have large amounts of other contaminants?
  3. Where is the sewage treatment system to be sited?
  4. What effluent standards are required for discharge of treated liquors?
  5. What facilities are there available to maintain the system?
  6. Are there reasonable amounts of surface water in the sewage?
  7. What is access like to the system?
  8. What power supply is available for the new treatment system?
  9. What are the climatic conditions in the location?
  10. What safety requirements are going to be required at the site?
  11. What are the ground conditions at the proposed site?
  12. What is the topography of the site like?

This above list may be added to by other factors that a designer will have to take into account.

  • Preliminary Treatment
  • Primary Settlement
  • Biological Treatment
  • Final or Humus Settlement
  • Tertiary Treatment
  • Activated Sludge Systems
  • Other Sewage Treatment Processes
  • Reed beds

Preliminary Treatment

In most sewage treatment systems of any size there will be some form of Preliminary Treatment. In smaller treatment systems up to perhaps say 500 / 1000 population equivalent this stage may be omitted.

There are several stages within the preliminary treatment section and these are:


This stage removes all, large floating or suspended solids, rags, plastics, timber, animal carcasses etc. There are various types of screens from manual raked to mechanically raked.

The subsequent screenings have to be dealt with in a suitable manner for removal off site.
As an alternative it is sometimes possible to macerate the screenings to a much smaller size to be removed at later stages, but this may produce problems elsewhere.

Grit Removal

This stage which usually happens at larger works, removes as much as possible of the silt, sand, gravel, cinders, ashes, metal, glass etc and even razor blades in the sewage.

There are various types of grit removal systems from grit tanks, grit channels, and other mechanical systems. The grit once removed has to be dealt with in a suitable manner for removal off site.

Flow Measurement

Flow of sewage to a works is measured to control and monitor treatment and to comply with consents. This can be undertaken by means of an open channel with perhaps a flume or V notch weir, or in a pipe with an external flow meter.

Storm Water Overflows

At some works where there is a high incidence of surface or storm water in the sewage, (usually at works which serve older towns where the surface water readily enters the sewerage system), there is a need to separate the excess storm water flows out from the main sewage to be treated.

This storm water may be bypassed to a separate storm overflow to the watercourse or may be stored in storm tanks, for later treatment once the main storm has passed.  This separation is usually undertaken by means of storm overflow weirs.  If there are to be storm storage tanks then these need to be carefully designed, as some settlement will occur in the tanks.

Primary Settlement

Primary settlement of the sewage is necessary to remove heavy solids, remove grease and scum, balance the load, and performs some biological activity. This is usually carried out by carefully designing tanks which are either of a radial flow, upward flow, or horizontal flow type.

The type of tank used will depend on many circumstances and it has to be carefully designed with all the many factors taken into account. From the primary settlement stage a lot of the coarser suspended solids will have been removed, and the biological content will have been reduced by between 25% and 50%. However from this stage there will be quite a lot of sludge, which must be removed from the tank very regularly and, in a large works, probably continuously. Sludge can be removed by means of, manual, automated systems, or by tank emptying.

Biological Treatment

The sewage after primary settlement still contains quite a large amount of biological pollutants. The aim of the biological treatment stage is to purify the settled sewage by removing dissolved organic material, so that after further settlement in humus tanks, the effluent can be discharged to a watercourse.

Biological filtration is the oldest form of sewage treatment dating back to 1897.  The process relies upon aerobic bacteria and micro-organisms to break down the impurities in the settled sewage. Other forms of life will also colonise the filter bed, for example: protozoa, worms, fly and insect larvae, spiders and even birds, each species feeding off smaller members in the food chain.

To live and to purify settled sewage the bacteria and micro-organisms need:

  • A home
  • Adequate oxygen
  • Adequate food
  • A comfortable environment

 - just like all human beings!

In a good and correctly designed biological filter bed, all these conditions can exist. A biological treatment system must however be correctly dosed with the sewage, and there are many different types of filtration system.

Final or Humus Settlement

Final or Humus Settlement is usually carried out in tanks as they are a means of removing the smaller solids from filter effluent.
Filter effluent contains quantities of biological film that has been washed from the media, dead micro-organisms and worms.
These tanks are very similar in design to primary settlement tanks and can be either of a radial flow, upward flow, or horizontal flow type.
From this stage there will be sludge that must be removed from the tank very regularly, and in a large works probably continuously. Sludge can be removed by means of manual or automated systems, or by tank emptying.
The effluent from Humus Tanks will generally be about 90% to 95% clean, if the works are correctly designed and operated.

Tertiary Treatment

Tertiary treatment is aimed at improving the quality of effluent produced by a conventional biological treatment plant. It is intended that tertiary treatment should polish a good quality effluent. It is not a substitute for inadequate secondary treatment processes.

Tertiary treatment can achieve a reduction in:

  • Suspended solids
  • BOD
  • Ammonia

There are various types of tertiary treatment system, these being:

  • Grass plots
  • Clarifiers
  • Sand filters
  • Micro-strainers
  • Drum filters
  • Reed beds
  • Lagoons
  • Nitrifying filters

The type of system to be used will depend on many factors and the designer will have to make a technical decision on which to use.

Activated Sludge Systems

The aim of the activated sludge process is to purify the settled sewage by removing dissolved organic material, so that after further settlement, the effluent can be discharged to a watercourse.

The process was developed in the early 1900's and relies upon the aerobic bacteria and micro-organisms to break down the impurities in the settled sewage. In some smaller works the sewage may be initially discharged into a treatment tank without settlement first taking place. These organisms, when aerated and agitated produce a culture known as activated sludge.

The activated sludge process needs:

  • Bacteria and micro-organisms
  • Adequate oxygen
  • Adequate food
  • Correct mixing

The important features of an activated sludge works are:

  • Aeration tank
  • Mixed liquor
  • Final settlement tank
  • Returned activated sludge
  • Surplus activated sludge
  • Air System
  • Mixing system
  • Control system

There are various types of systems that offer all of the above in one package and in many cases these can be advantageous for a particular application. The designer must however be very careful in ensuring that the system is appropriate for the installation.

Other Sewage Treatment Processes

There are many other sewage treatment processes available to the designer of a system.
These are

  • Reed Beds
  • Contact Stabilisation
  • Membrane Treatment
  • Rotating Biological Contactors
  • Oxidation Ditches
  • Extended Aeration

Each system has its advantages and disadvantages, and a designer must take into account not only the site conditions but also the many other factors such as operation and maintenance, etc.

Some of these systems are being developed further, so that in the future they could well supersede the existing systems. In addition some of the above systems are now made in packaged type treatment plants, so that the whole plant can be readily installed on site, with hopefully minimum problems.