What is Wastewater?
Wastewater is a combination of used water that comes out of homes, schools, businesses, restaurants, car washes and industries. Wastewater contains a number of different substances, most importantly Total Suspended Solids (TSS). Suspended solids in the wastewater, if not removed and discharged into the environment, can decay or break down naturally by consuming oxygen. This demand for oxygen commonly known as Biological Oxygen Demand (BOD) is a measure of how clean the wastewater is.
Why Treat Wastewater?
There are many reasons to treat the wastewater before discharging back to the environment:
- Marine life and aquatic habitat protection – untreated wastewater can consume oxygen that is available for plants and animals, and cause them to suffer and may even kill them.
- Recreation and quality of life – untreated wastewater can affect swimming, sport fishing, boating, picnicking etc. by imparting undesirable colour and unpleasant odour to the receiving water.
- Health concerns – untreated wastewater can cause disease causing microorganisms to enter into receiving water that we may use for drinking and recreation.
How is Wastewater Treated?
Wastewater is treated in many different ways. All kinds of wastewater treatment involve removing suspended solids and reducing BOD in the wastewater. Please read through the following steps to understand how wastewater is treated by the City of Abbotsford.
Primary Treatment involves mainly mechanical processes such as headworks and primary sedimentation that removes up to 40 percent of BOD and 50 percent of TSS from the wastewater.
Wastewater that comes into the plant is pumped to the headworks area of the plant by two screw pumps. The headworks consists of screening and grit removal systems where large objects such as rags, sticks, plastics, stones and heavier dirt are removed from the wastewater and sent to the landfill. The screened wastewater, know as Primary Influent, flows by gravity to Primary Sedimentation tanks.
Primary Sedimentation is performed in seven large open-air tanks. The wastewater is held in these tanks in a quiescent state for 2 to 7 hours. Lighter particles float and are skimmed off and heavier solids sink to the bottom. The solids (Primary Solids) removed from the sedimentation tanks are sent to Digesters for further treatment. The overflow, known as Primary Effluent, from these tanks goes to the secondary process for further treatment.
Secondary Treatment is a biological process that removes up to 90 percent of BOD and TSS.
Trickling Filters and Solids Contact Tanks
Trickling Filters are large tanks that are packed with natural media such as rocks or synthetic media such as plastic plates. The packed media serve as a support structure on which naturally occurring microbes can grow and eat the dissolved organic material present in the wastewater. Wastewater is usually introduced at the top of the filter and treatment occurs as the wastewater trickles down to the bottom of the filter, hence the name Trickling Filter.
JAMES Plant has two rectangular tricking filters packed with synthetic plastic media. The Primary Effluent is pumped to the top of the filter and distributed over the plastic media plates where dissolved organics are eaten by the growing biomass and particles are filtered out or trapped.
Each Trickling Filter at the plant has a Solids Contact Tank. The function of Solids Contact Tank is to facilitate aggregation of finely divided microbes in the trickling filter effluent into heavier and easily settleable microbes. The trickling filter effluent and settled solids from secondary clarifiers are combined in this tank and aerated for microbes’ aggregation. The liquid flowing out of the Solids Contact Tanks flows into the Secondary Clarifiers.
JAMES plant has three circular Secondary Clarifiers, which are large open-air tanks where the fully grown microbes, having done their work of eating the organic material, settle to the bottom of the tanks as sludge (Secondary Solids). A portion of this sludge is returned to the Solids Contact Tanks as mentioned above and the remainder is sent to the Digesters. The clear liquid at the top of the clarifiers, also known as Secondary Effluent, overflows to the next stage of treatment called Disinfection.
Disinfection is a process by which microorganisms present in the wastewater are killed using chlorine, Ultraviolet light or Ozone.
The secondary effluent is pretty clean, but it still contains large number of invisible harmful microorganisms that need to be destroyed before discharge into the Fraser River. To destroy the microbes, JAMES Plant currently practices disinfection by injecting chlorine gas into the wastewater.
Treated Wastewater Discharge
After disinfection, the residual chlorine in the wastewater is neutralized by injecting sulphur dioxide gas. The treated wastewater is then discharged by gravity or pumping into the Fraser River via a 1200 mm diameter outfall pipe.
There are three digesters at the JAMES plant. They are large enclosed tanks where the skimmings and settled sludge from primary sedimentation tanks and secondary clarifiers are digested or cooked in the absence of air or oxygen. This digestion process greatly reduces the organic content and odour of the sludge. The solids that enter in the Digesters are pre-pasteurized at 65oC to reduce the pathogenic bacteria to negligible concentrations. The end products of the digestion process are methane gas and well digested sludge, also called “Class A” Biosolids.
The methane gas is used as a fuel to produce heat and electricity at the plant. Biosolids are dewatered by filter press and the dewatered biosolids are temporarily stored on site before being routinely disposed off at reclamation sites in BC Interior and elsewhere. Some of the biosolids are also mixed with woodchips and sand to provide a marketed top soil product called Val-E-Gro.