It might surprise you to know that we all drink and bathe in recycled water. The water we flush down the toilet is turned into potable water, and it is the job of ST plants to make this water fit for human consumption or release into rivers and oceans.

Wastewater from sinks, baths, washing machines, toilets and other appliances has to go somewhere. After traversing miles of sewer network, it ends up in sewage treatment plants whose job is to treat and discharge it.

Primary stage

The primary stage involves separating solids from liquids. The sewage is pumped into sedimentation tanks, where gravity forces solids to the bottom of the tank. The water is then released, leaving behind a sludge/slurry.

The sludge is a by-product of primary treatment and can sometimes be repurposed as a fertiliser, but it requires treatment such as de-watering to stabilise it. Incineration is the most likely destination for heavily contaminated sludge. 

Secondary treatment

Secondary treatment is the biological treatment stage that breaks down organic contaminants in wastewater.

The two most frequently used processes are activated sludge (aerated ponds) and filter beds (sewage tricked over aggregate), where ‘good’ bacteria in the sludge/aggregate break down the pathogens in the wastewater.

After secondary treatment, wastewater can sometimes be released, providing there’s a low risk to human and animal life and the environment.

Tertiary treatment

Wastewater is considered clean after secondary treatment, but tertiary treatment returns it to an even higher quality for release in protected waters.

The type of tertiary treatment depends on the wastewater. For example, suppose we should release wastewater into bathing or shellfish waters. In that case, it requires disinfection, and nutrients in the water, like phosphorous, must also be removed.

Types of tertiary treatment include:

• Microfiltration (where water passes through tiny holes at high pressure).
• Ion exchange (where ions in the water are exchanged for other ions).
• Activated carbon adsorption (which removes organics).
• Disinfection (where UV light or chemicals kill organic pathogens leftover).