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Email Link   Treatment Process and Standards

When the raw sewage or influent arrives at a wastewater treatment plant, it contains a lot of material which has to be removed before effluent can be returned to the river or land. The sorts of material that must be extracted include:

  • Organic matter (eg faeces)

  • Sediment

  • Grease

  • Insecticides

  • Household and industrial chemicals

  • Disease-causing viruses and bacteria

  • Objects (ie nappies, sanitary items, toys, metal and wood)

  • Heavy metals

  • Nutrients such as phosphates and nitrates

The Treatment Process


Primary Treatment - removing solids, grease and sediment

The wastewater is passed through screens to remove inorganic solids. These solids are then buried on site. The velocity of the wastewater is reduced to remove grit and organic solids as they settle. Floating grease and scum are removed by a surface skimmer. During this stage the flow is also measured.

Secondary Treatment - Removing organic matter and nutrients

This is a biological process where naturally occurring micro-organisms break down organic matter and nutrients. This involves the use of biological filters, activated sludge tanks and oxidation ponds, followed by settling tanks or clarifiers to separate micro-organisms from the purified liquid.

Dissolved oxygen is used to break down organic matter in the water. If such water was released into the waterways, then aquatic animals that also need dissolved oxygen to respirate, cannot survive. For this reason the biological oxygen demand (BOD) in wastewater is reduced to an acceptable level before being returned to the environment.

Secondary treatment at most plants include nutrient removal of Nitrogen and Phosphorus. Sometimes this is chemical, sometimes biological, depending upon the size of the plant. Usually, it is more cost-effective for larger plants to use a biological process, with chemical processes as a backup.


Tertiary Treatment - removing pathogens that can pose a risk to human health

The most common methods of tertiary treatment for the effluent are disinfection and filtration. This may be done by:

  • Chlorination followed by dechlorination
  • Holding the effluent for at least ten days in tertiary effluent ponds where sunlight and other micro-organisms reduce the pathogens
  • Filtering using micro filtration or cloth filters
  • Ultra Violet and/or Gamma radiation to disinfect the effluent

Additional treatment may be required if the treated wastewater is reused for purposes such as irrigation of food crops or where close human contact may result.

Water Quality Readings in the Tweed


Normal river quality < 5 mg/L BOD5
Flood river quality > 1000 mg/L BOD5
Sewage into the treatment plants 250 mg/L BOD5 and SS
Sewage after extended aeration 20 mg/L BOD5 and SS
Final effluent discharged < 10 mg/L BOD5 and SS

BOD5 refers to the Biochemical Oxygen Demand of the micro-organisms in a 5 day period.

SS refers to Suspended Solids


Environmental Protection Authority Licence Range

The Environmental Protection Authority (EPA) sets the standards for water quality of the water being discharged.

Generally for BOD5

< 15 mg/L 90% of the time

NFR < 20 mg/L 90% of the time
Oil and Grease<10 mg/L

The licence standards vary between plants depending on the sensitivity of the receiving waters, how well it is flushed and what it is used for. Some examples of the limits are as follows:

pH between 6.5 and 8.5
Faecal Coliforms < 600 cfu/100mL
Total Nitrogen < 5 mg /L
Total Phosphorus< 2 mg/L

Tweed Shire Council publishes their Pollution Incident Response Management Plans and pollution monitoring data. See Environmental Monitoring.


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