Why we treat wastewater
The goal of wastewater treatment is to protect and maintain healthy rivers and oceans. If pollutants in wastewater are not removed, they flow directly into our waterways. This can threaten public health, fisheries, wildlife habitat, recreation opportunities and ultimately, our quality of life. Two of the important components of wastewater addressed through treatment are:
Total Suspended Solids (TSS)
Total suspended solids refers to particles of matter – both large and small – that can be present in wastewater. These particles carry bacteria that can make wastewater harder to disinfect and can use up oxygen in the water as they break down. Solids can also sink, blanketing the bottom of rivers and oceans and destroying habitat for aquatic and marine life.
Biochemical Oxygen Demand (BOD)
When organic materials— both solid and dissolved—break down in wastewater, they use up oxygen. Biochemical oxygen demand measures the amount of oxygen consumed by these materials in water as they biodegrade. If the biological oxygen demand is high and too much oxygen is being consumed, it can cause problems for fish and other aquatic life that also need to use the oxygen in the water.
The amount of total suspended solids and biochemical oxygen demand removed from wastewater is used to gauge the effectiveness of wastewater treatment plants. Wastewater is
regularly monitored to comply with provincial standards.
How wastewater is treated
Primary treatment uses various mechanical processes to remove materials that settle or float. It removes 50 to 60 per cent of the total suspended solids (TSS) and 30 to 50 per cent of the biochemical oxygen demand (BOD). The Iona Island and Lion’s Gate wastewater treatment plants provide primary treatment.
Secondary treatment is an additional treatment step that takes place after primary treatment. Secondary treatment is a biological process that uses aerobic bacteria to consume suspended solids and dissolved organic materials in wastewater. Secondary treatment plants remove about 95 per cent of the organic materials in wastewater. The Lulu Island, Annacis Island and Northwest Langley wastewater treatment plants provide secondary treatment.
Where possible, Metro Vancouver tries to recover resources as part of the treatment process, such as producing heat, electricity and biosolids.