Sewage Treatment

Sewage Treatment
Where does the wastewater go?
When water goes down your sink or toilet, it travels through small pipes to much larger trunk sewers. The sewer systems are built to the slope of the ground around the Elbow and Bow Rivers. This slope allows most of the waste to go down by gravity, rather than expensive pumps, to one of four plants. To handle low areas, which do not have enough slope, pumping stations have been made.

How is wastewater treated?
Wastewater treatment in Calgary involves the following processes:
Preliminary treatment:
The water flow is slowed down to get the sand, gravel and other materials to settle out into grit tanks. The waste is then strained by bar screens, which remove large solid objects like sticks. All material collected by grit tanks and bar screens is washed and then taken to a landfill.
Primary treatment:
The screened water flows into settling tanks, allowing more solids to settle to the bottom of the tanks. Also at this stage, “scum” (oils and greases) are scraped off the top of the water. This is pumped to large tanks, for disposal.
Secondary treatment:
The half treated water then goes by gravity to covered aeration tanks where it is mixed with “activated sludge” which contains aerobic bacteria. The bacteria eat the organic things remaining in the water. In order to provide a good environment for the bacteria to multiply, air is pumped and spread into the water by blowers. The water, air and the activated sludge are mixed in the aeration tanks for four to six hours.
Phosphorus removal and disinfection:
The mixture then flows into sedimentation tanks where the activated sludge is settled out. An iron salt solution is added to the water to remove phosphorus to stop overgrowth of plants. Finally chlorine is added to the wastewater to kill micro-organisms before the treated water flows back into the river.

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What happens to the sludge?
Some of the activated sludge is reused as “seed” to help the bacteria in the aeration tanks. The rest of the sludge is pumped into anaerobic digestion tanks. Then anaerobic bacteria get rid of the sludge, producing methane. This gas is used as an extra source of fuel for the plant. The digested sludge containing 95% water then goes through a “dewatering” process to lower its water content before it is incinerated or used on land. The sludge is sent to centrifuges to remove its water, producing a “sludge cake.
What happens to the sludge cake?
Unrecycled sludge cake is burned in large hearth incinerators. The exhaust passes through heat recovery boilers, which allow their heat to be used in other heating uses in the plant. Finally the water is mixed with dry incinerator ash to make a mixture, which is put in lagoons where the ash is allowed to settle. The water overflows from the lagoons to the pipes carrying the rest of the water out into the river. The settled ash is eventually taken away and disposed of at a landfill.
Why is sludge cake incinerated?
Unrecycled sludge cake is incinerated to reduce its volume before disposal, and to recover its heat.
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