Ever wonder how we treat the water that you use after it goes down the sink,
shower or toilet? Let’s find out.
After the wastewater leaves your house it flows through underground pipes to a wastewater
treatment plant. The wastewater treatment plant is where wastewater is cleaned and disinfected.
The wastewater travels through moving screens- these screens catch items that should
never be put down the toilet or drain like dental floss, toys, rags and Q-tips.
Remember you should flush only the three Ps: Pee, Poop and Paper down the toilet.
From here, the wastewater travels to the primary clarifying tanks.
In the primary clarifying tanks, the wastewater is given time to slow down.
Slowing the wastewater down lets heavy materials called sludge sink to the bottom and lighter
materials called scum float to the top. The sludge and scum are removed for further
treatment, and the liquid moves on to the aeration tanks.
These are the wastewater aeration tanks. Here, oxygen is pumped into these tanks
to let good microorganisms live and grow. These microorganisms breakdown nutrients,
organic material, and bacteria in the wastewater. The microorganisms
and the water then move on to the secondary clarifying tanks.
In the secondary clarifying tanks, the microorganisms become very full and heavy.
Heavier particles and solids settle to the bottom of the tanks and form
sludge. The sludge is collected from the bottom of the tanks and sent to the anaerobic digesters.
Some sludge is returned to the aeration tanks to help improve the microorganism population.
The final disinfection stage includes chlorinating the water using sodium
hypochlorite, commonly known as bleach. Then sodium bisulphite dechlorinates the water,
before it is returned to the lake. Wastewater is regularly tested
throughout the treatment process. Before the treated water is returned to the lake,
it is tested one final time to make sure it is safe and meets all regulatory requirements.
The treated water is slowly returned to Lake Ontario through a large pipe
called an outfall pipe. The outfall pipe extends more
than 2 kilometers into the lake and is 70 meters below the water’s surface.
Diffusers in the pipe slowly return the water back into the lake so that it doesn’t
disrupt plants and wildlife. Now that we’ve seen how the liquid
in wastewater is treated, let’s see what happens to the solids that were removed.
Sludge from the secondary clarifiers is pumped to the anaerobic digesters.
The anaerobic digesters are large enclosed tanks without oxygen. What happens in the anaerobic
digesters is similar to what happens in our own digestion systems. Here, microorganisms break
down the organic material in sludge. This helps to eliminate bad odours and reduce pathogens.
Sludge that’s processed in the digester’s is called biosolids.
The biosolids stay in the digester at a temperature of 38 degrees Celsius.
As the microorganisms break down the organic material they create a biogas
that contains carbon dioxide and methane. This biogas is a renewable source of energy that’s
collected and used throughout the wastewater treatment plant for heat and electricity.
The material removed from the digester is called digestate. Digestate is
about 98% liquid and 2% solid. Material from the digesters is sent to
the centrifuges. A centrifuge works like a clothes dryer. It spins the digestate
to remove water and reduce its volume. The drained material is sometimes referred
to as “cake.” Digestate is considered to be cake when its solids concentration is 25% or more.
The cake leaves the centrifuge and enters an incinerator.
The incinerator heats up to 840 degrees Celsius and burns the cake
from the centrifuges, leaving ash behind. Incineration reduces the volume of the leftover
material, leaving behind only inorganic material, such as phosphorus and iron.
The remaining ash is removed and stored in onsite ash lagoons.
At the Region of Peel we are researching ways to beneficially re-use this leftover material,
to support a circular economy.