What happens after you flush? Find out with this overview of primary and secondary wastewater treatment processes in our region. Join WWTP operator Craig Mayer for an explanation of the steps taken before our wastewater is returned to the environment.
Wastewater can be treated in two ways. One method is called primary treatment and the other is called secondary treatment. Both processes are used on the wastewater that flows to the Annacis Island Wastewater Treatment Plant.
Craig Mayer, a Metro Vancouver Wastewater Treatment Operator at the Annacis plant, explains the steps that transform sewage back into safe water before it is returned to the environment.
“The first stage of treatment takes place at the Influent Pumping Station, where solid debris is removed from the wastewater. We start by removing all the things that people put down the toilet.”
Sticks, cloth, plastics, and a wide array of personal hygiene products, ranging from sanitary napkins to dental floss, are separated from the wastewater by the bar screens at the Influent Pumping Station.
In the second step, the wastewater is sent to aeration tanks — to settle out the sand and grit, which settles to the bottom of the tanks. Now the water is sent to the sedimentation tanks. In this third step, all the scum, fat, oil, and grease floats to the surface. It’s collected at the front of the tank. The heavy organic matter falls to the bottom of the tank.
At some wastewater plants, this would be the end of the process and water is discharged to the river or ocean. These are called primary wastewater plants. Other plants, including Annacis, continue treating the wastewater even more. These are called secondary wastewater treatment plants.
“We have four large domes,” says Mayer. “Here the wastewater goes to the trickling filters. Inside the trickling filters we have about fifteen feet of honeycomb-shaped media. As the water filters through this media a slime starts to grow where microscopic organisms start to consume the soluble organic matter that we couldn’t settle out in the sedimentation tanks. After the wastewater leaves the trickling filters, it goes to the solids contact tanks.”
This is the fourth step in the process, where bacteria consume the remaining soluble materials. Then the bacteria clumps together, gets heavy and sinks.
“Here at the secondary clarifiers, the wastewater slows down,” explains Mayer. “It’s going to reside in the tanks for about four and a half hours. In that time it’s going to slowly settle out all the heavy material, so the water on the top is very clean. That’s the water we are going to end up sending out to the river.”
Here’s the five steps again.
- Large materials and grit are scraped and settled out
- Oils are skimmed and sludge is settled out
- Bacteria consume tiny organic materials
- Bacteria clumps settle out
- Clarified water flows to Fraser River.