Wastewater entering the treatment plant includes items like wood, rock and cloth. Unless they are removed, they could cause problems later in the treatment process. So all incoming water passes through screens that filter our any solid material larger than 2.5 centemeters.
Step 2 - Pumping
The wastewater system relies on the force of gravity to move wastewater from your home to the treatment plant. So wastewater treatment plants are located on low ground. By the time it reaches the treatment plant, wastewater is about eight metres below ground level. Since the plant is built about ground, the wastewater has to be pumped up to the aeration tanks. From here on, gravity takes over to move the wastewater through the treatment process.
Step 3 - Aerating
Wastewater enters a series of long, parellel concrete tanks. Each tank is divided into two sections. In the first section, air is pumped through the water. As organic matter decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygen through the water also keeps the organic material suspended while it forces "grit" - coffeegrounds, sand and other small, dense particles - to settle out. Grit is pumped out of the tanks and taken to landfill sites.
Step 4 - Removing Sludge
Watewater then enters the second section or sedimentation tanks. Here, the sludge (the organic portion of the sewage) settles out of the wastewater. It is pumped out of the tanks. Some of the water is removed in a step called thickening and then the sludge is processed in large tanks called digesters.
Step 5 - Removing Scum
As sludge is settling to the bottom of the sedimentation tanks, lighter materials are floating to the surface. This "scum" includes grease, oils, plastics and soap. Slowmoving rakes skim the scum off the surface of the wastewater. Scum is thickened and pumped to the digesters along with the sludge.
Step 6 - Killing Bacteria
Finally, the wastewater flows into a "chlorine contact" tank. Between May 1 and September 30 (swimming season) the water is chlorinated to kill bacteria which could pose a health risk in the waters off local beaches. The chlorine is mostly eliminated as the bacteria are destroyed. However, sulfur dioxide is added to remove any remaining chlorine. This protects fish and other marine organisms which are susceptible to the smallest amounts of chlorine. The treated water (primary effuent) is then discharged to the Fraser River or the ocean.
*All information on this page was taken from "A Guide to Wastewater Treatment", issued by the Greater Vancouver Regional District.*
