Why Treat Wastewater?
Health
If water is not properly cleaned it can carry disease. Since we live, work and play so close to water, harmful bacteria have to be removed to make sure the water returning to waterways is clean. Clean water is safe water.
Habitat for Wildlife
The Fraser River has the largest salmon run in the world. Each year, millions of fish migrate upstream to spawn. After hatching, the young salmon that survive the journey downstream spend time in the Fraser River estuary. The Burrard Inlet and the Georgia Strait support a variety of shellfish and fish species, all of which are sensitive to pollution. Our rivers and ocean waters teem with life that depends on shorline, beaches and marshes. They are critical habitats for hundreds of species of fish and other aquatic and marine life. Migratory water birds also use the areas for resting and feeding.
What is Wastewater?
Wastewater is used water. Commonly know as "sewage", it includes substances such as human waste, animal waste, food scraps, oils, soaps and chemicals. In homes this includes water from sinks, showers and bathtubs, toilets, washing machines and dishwashers. Businesses and industries also use water for a wide variety of purposes. Most wastewater flows through an extensive network of underground pipes into wastewater treatment plants.
How is Wastewater Treated?
The solid materials suspended in wastewater use oxygen as they break down naturally in the environment. However, the availability of dissolved oxygen in water is key to the survival of aquatic and marine life. (The term used to describe the oxygen used up as suspended solids decay is "Biochemical Oxygen Demand" or BOD). The major aim of wastewater treatment is to remove as much of the suspended solids as possible before the remaining water, called effluent, is discharged into waterways. This reduces the BOD. Treatment plants can remove various levels of suspended solids and BOD to purify wastewater. The level of treatment chosen depends on how much we need to aid the waterway's natural purifying ability.
Primary Treatment
Primary treatment removes about 60% of suspended solids from wastewater. Treatment also involves forcing air into the wastewater to reduce the BOD by about 35%. Click here for more information...
Secondary Treatment
Secondary treatment removes up to 90% of suspended solids and also reduces the BOD by about 90%. Click here for more information...
Tertiary Treatment
Tertiary treatment removes ammonia and/or nutrients from wastewater. While these substances occur naturally in rivers and oceans, too much in effluent can disrupt the natural balance. The excess ammonia and nutrients can harm or kill aquatic life. Tertiary treatment can remove either or both of these substances from wastewater effluent so they remain at safe levels in the rivers and oceans. Greater Vancouver produces millions of litres of wastewater every day, without these treatments it would overwhelm the natural purifying mechanisms of our local waters.
What Happens to the Solids Removed from the Wastewater?
The other aspect of wastewater treatment is dealing with the organic solids, or sludge, removed from the water. At the treatment plants, sludge is processed into biosolids, a nutrient-rich soil conditioner. Thickened sludge is pumped into large, heated and enclosed tanks called digesters. In these tanks, bacteria digest the sludge in much the same way that the human stomach digests food. Sludge is continually fed into the digesters to keep this biological process active. Digestion kills virtually all the harmful bacteria in sludge. It also reduces the sludge volume by half and substantially reduces odours. Methane gas produced as a byproduct of digestion is used to produce power for the treatment plants.
*All information on this page was taken from "A Guide to Wastewater Treatment", issued by the Greater Vancouver Regional District.*
