Stage 3 - Tertiary treatment

Tertiary treatment is the final cleaning process that improves wastewater quality before it is reused, recycled or discharged to the environment.

The treatment removes remaining inorganic compounds, and substances, such as the nitrogen and phosphorus.

Bacteria, viruses and parasites, which are harmful to public health, are also removed at this stage.

Let's take a look.

Wastewater flows from the biological reactor and IDAL to a pumping station and meet in the flash mixer.

Alum is used to help remove additional phosphorus particles and group the remaining solids together for easy removal in the filters.

Wastewater from the flash mixer is gravity fed through the sand filters.

Because of the addition of alum, tiny particles cluster together in masses called floc.

Floc is trapped by the sand, while the clear water is gravity fed to the chlorine contact tank.

The filters are backwashed every 24 hours to remove the floc that has accumulated. The backwash water is returned to the primary treatment stage to go through full treatment.

The chlorine contact tank disinfects the tertiary treated wastewater.

Chlorine removes microorganisms in treated wastewater including bacteria, viruses and parasites such as Giardia and Cryptosporidium.

The chlorinated water slowly winds its way along the discharge tank, giving it time to react.

Any remaining chlorine is removed by adding sodium bisulphite just before it's discharged.

Chlorine is removed from the water since it can be harmful to water quality and aquatic life when in high concentrations.

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