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What is secondary treatment?

Updated: Aug 14, 2019



Regardless of which disposal option is chosen, land will be needed for the treatment plant, wastewater will be treated to secondary level before being disposed of.


Secondary treatment uses a biological process to digest and remove dissolved organic matter from wastewater. It is an additional stage after pre-treatment and primary treatment.


Pre-treatment removes any large debris from the wastewater stream.


Primary treatment removes organic matter and other solid particles, usually through settlement of the particles in a pond or tank.


Both pre-treatment and primary treatment improve the quality of the wastewater significantly.


Secondary treatment involves a process where natural micro-organisms feed on the organic matter in the wastewater. This process removes the organic matter and significantly improves the quality of the wastewater. There are a number of types of secondary treatment systems – the PMU will recommend a system that:

  • doesn’t require a lot of land

  • is relatively easy to operate and maintain

  • has minimal impact on the community and natural environment, and

  • can be extended to allow for future demand.

If treated wastewater is then disposed of onto land, this acts as an additional treatment as plants take up the nutrients from the soil.


Secondary treatment process produces two products for disposal – treated wastewater, and solid waste called ‘sludge’.


“Both treated wastewater and sludge can be used to enrich the soil because they contain nutrients and organic matter,” said PMU lead environmental scientist Murray Wallis, a soil specialist. “Rather than looking at waste as something to simply ‘dump’ into the environment, there can be great benefit from recycling suitably treated waste to improve soil quality and support agriculture.”


Both treated wastewater and sludge from secondary treatment are suitable and safe to use on the land, when managed properly. For sludge this would involve composting it to a high temperature before applying to the land. For treated wastewater it would involve putting it into the soil without contacting crops.

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