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Water and Wastewater

Conventional Wastewater Treatment


Diagram of Wastewater Treatment Process
(PDF 1 page, 399 KB)


Wastewater enters the treatment plant through the headworks, which consists of mechanical screens, screening washer compactors, and grit chambers.

HeadworksMaterial caught by the mechanical screens is removed, cleaned and dewatered before disposal.

The grit chambers remove grit by inducing a vortex pattern in the circular chamber. A drive paddle in the vortex unit maintains circulation under all flow conditions. Grit slurry pumps periodically remove the accumulated grit from the hopper at the bottom of the grit chambers. The grit is dewatered before being removed to disposal bins.


Primary Treatment:

Primary TreatmentSuspended solids (sludge) settle at the bottom of the primary tanks and are swept gently to collection hoppers to be pumped away and processed. Surface skimmers remove floating debris (scum). The sludge and scum are pumped to the Biosolids facility for treatment and disposal. The remaining water (effluent) flows by gravity to aeration tanks for secondary treatment.


Nutrient Removal and Aeration Tanks:

The conventional activated sludge process involves nutrient (phosphorus) removal from the wastewater, aeration using micro-organisms to consume waste, and clarification to settle micro-organisms.

Nutrient (Phosphorus) Removal: As the wastewater enters the aeration tank, ferrous chloride is added to remove phosphorus. The ferrous chloride makes the phosphorus in the wastewater settle to the bottom in the final clarifiers.

Aeration (Feeding the Micro-organisms): Large, dense populations of bacteria, protozoa, and other micro-organisms consume suspended and dissolved organic matter. As they need oxygen to live, air is pumped through diffusers in the base of the tanks. The rising bubbles of air also provide a mixing action, keeping the micro-organisms suspended and in contact with the wastewater.


Secondary Clarifier:

Final SettlingThe wastewater, along with the micro-organisms, flows into secondary clarifiers. Activated sludge is settled and collected at the base of the clarifiers. A portion of the activated sludge (RAS) is pumped back to the influent end of the aeration tanks. The remainder of the activated sludge (WAS) is pumped to the WAS tanks in the biosolids building.


Final Outfall:

Secondary-treated effluent flows over the clarifier weirs. Sodium hypochlorite is added at the beginning of the outfall for disinfection. Sodium bisulphate is added towards the end of the outfall for dechlorination.

The final effluent is discharged to Lake Ontario through a pipe reaching 1,250 metres offshore to a diffuser. This structure ensures the effluent is retained for long enough to be thoroughly disinfected. The 35 separate ports in this 188 metre-long pipe discharge the final effluent over a large area to minimalize environmental impact.


Revised: Thursday May 18 2017


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