WINNIPEG'S WASTEWATER TREATMENT PROCESS
THE HISTORY OF WINNIPEG'S WASTEWATER TREATMENT SYSTEM
A large scale system for supplying water to the City of Winnipeg
began in 1913 when Shoal Lake, at the Manitoba-Ontario boundary,
was chosen as the source of Winnipeg's water supply. An aqueduct
was built to move water to the city from Indian Bay on Shoal Lake,
and a complex series of pipes, pumps and reservoirs was constructed
to distribute the water throughout Winnipeg. Once used, the
wastewater was released directly into the City's rivers, without being
treated.
During the 1930's, the public became concerned that the untreated wastewater, also called
sewage, polluting the City's rivers was causing health problems. This led to the construction of
Winnipeg's first sewage collection and treatment system, with 12 kilometres of collector sewers,
24 pumping stations and the North End Sewage Treatment Plant.
The North End Sewage Treatment Plant was opened on October 25th, 1937. Winnipeg became
the first city in Canada of over 100,000 people to install sewage treatment. Since then the plant
has been upgraded and expanded to become the North End Water Pollution Control Centre. It is
the largest of three wastewater treatment facilities serving the City of Winnipeg, and provides
primary and secondary activated sludge treatment, and sludge processing.
The North End Water Pollution Control Centre (NEWPCC) treats about 70% of Winnipeg's
wastewater. It services most of the old City of Winnipeg, part of St. Boniface, all of East, West,
North and Old Kildonan, Transcona and part of St. James. The rest of the city is serviced by the
West End Water Pollution Control Centre (WEWPCC) in Charleswood and the South End
Water Pollution Control Centre (SEWPCC) in St. Vital.
This web site focuses on the wastewater treatment processes at NEWPCC. However, the
processes used at the SEWPCC and WEWPCC are very similar, with the exception that these
plants are smaller and have no sludge handling capability. The City of Winnipeg, a pioneer in
wastewater management, has early on determined that it is more cost effective to centralize
sludge treatment processes, so that the SEWPCC and WEWPCC haul their sludge using large
tanker trucks to the NEWPCC for processing. The sludge treatment process is described
further in this site.
SEWAGE COLLECTION SYSTEM
A vast system of underground sewers, force mains and pumping stations are needed to collect the
wastewater from the homes of the city's residents and deliver it to the interceptor sewers, then to
the water pollution control centres.
Individual sewers (service connections located 2.5 meters below ground) from homes and
businesses carry the flow of wastewater into lateral sewers in each neighborhood. The
wastewater from the lateral sewers moves into the trunk sewers located 6 to 9 meters below
ground. Pump Stations, also called Lift Stations, raise the wastewater from the trunk sewers into
the main interceptor sewers. The wastewater flows by gravity through these large interceptor
sewers, which are up to 50 feet deep and 60 inches diameter, into the treatment plants.
About 2400 kilometres of sewers and interceptors, and 72 pumping stations are needed to carry
the wastewater to the water pollution control centres. The pumping stations all have automated
equipment alarms, and in the event of a breakdown, alarms are transmitted to a 24 hour dispatch
center for immediate response, to prevent pollution to the rivers.
Approximately 50% of the city is serviced with Combined Sewers, which discharge diluted
wastewater to the rivers during significant rainstorm events. But thanks to the efficient operation
of the Collection System, the City treats Over 90% of all the wastewater generated in the
City. The Collection System is also responsible for the important function of Flood Pumping
during the annual high river event.
THE IMPORTANCE OF TREATING WASTEWATER
The role of the Water Pollution Control Centres is to help control the pollution of the City's
rivers. It does this by treating wastewater to remove inorganic solids such as sand and gravel, and
by reducing the amount of organic material before it is released to the City's rivers. Treated
wastewater is 90-95 percent free of organic material present in sewers (as measured by the
standard 5-day carbonaceous Biochemical Oxygen Demand (CBOD5) analysis).
The process used to treat sewage, also called wastewater, is very similar to the natural
decomposition that would occur if wastewater was released directly into Winnipeg's rivers.
Bacteria would feed on the organic materials and break them down, using up the oxygen in the
water. This would decrease the oxygen in the river, so that healthy populations of fish and
aquatic life could not live there. As these organic materials decomposed and caused septic
conditions, they would also give off unpleasant odors and create a public health concern.
Speeding up and controlling the decomposition of the organic material in sewage inside the
treatment plant helps to maintain a healthy environment for fish and other aquatic life in
Winnipeg's rivers. The odors produced through this decomposition are also contained within the
treatment plant. Without treatment, the organic material in the wastewater would be released to
the river, where it would decompose and reduce oxygen levels in the river to a point where they
may become lethal to biota.
on figure for larger scale) (For PRINTING INSTRUCTIONS, click HERE!)
THE MAIN BUILDING
The Main Building of the NEWPCC contains the administrative offices and the Laboratory
Services Division, in addition to the main pumps. Laboratory Services provide testing and
control services for pollution control as well as water quality monitoring and associated research.
(See section: "Laboratory Services Division")
Sewage enters the treatment plant by flowing through the main interceptor (1, see figure below)
into a wet well (2), 16 meters below ground level in the Main Building. In addition, leachate
from landfills is trucked in and dumped here for treatment. Leachate is a high strength liquid
which collects at the bottom of landfills and must be removed to prevent groundwater pollution.
Also, septage from the septage haulers is dumped here for treatment.
From the wet well, the sewage is drawn into three pump wells (3), each having two pumps. The
number of pumps being used at one time depends on the amount of wastewater flowing into the
plant. Rainfall, run-off from spring thaw and the time of day all affect the amount of flow
entering the plant.
The pumps are programmed to handle the flow coming into the plant and lift it above ground
level into a discharge chamber (4). From this point, all the main flow through the rest of the
treatment plant occurs by gravity. From the discharge chamber it flows into the sewage conduit
(5). The sewage moves through the conduit to the first stage of treatment in the Pre-aeration and
Grit Removal Building.
PRE-AERATION - SCREENING - GRIT REMOVAL
As the sewage flow enters the Pre-aeration and Grit Removal Building, it is divided into four
covered tanks (6, see figure below). Before entering these tanks, the sewage passes through bar
screens (7) with 12 mm openings. These screens remove large objects such as sticks, rags and
garbage. These materials are conveyed to trucks and taken to a landfill.
After passing through the bar screens, the sewage is gently agitated with air in the first part of the
tank. This helps to remove heavier inorganic materials such as sand and gravel, called grit, while
keeping organic matter in suspension for treatment. Once the grit has dropped to the bottom of
each tank, it is removed by a clam-shell bucket (8) and placed into trucks (9-10) for disposal at
a landfill site.
In the second section of the tanks, air is bubbled more vigorously through the wastewater. This is
known as pre-aeration. It helps to remove foul-smelling gases like hydrogen sulfide (rotten egg
smell) which develop in the sewers when the material in the wastewater begins to break down.
These gases are vented to the atmosphere through tall (50 metre) chimney-like stacks.
Waste (Waste Activated Sludge) from a later stage of the treatment process (11) is added to
sewage after (or before) passing through this building, before the sewage goes onto the primary
stage of treatment.
PRIMARY TREATMENT
Primary treatment is the first step in separating the fine solid material from the liquid wastewater.
Primary treatment removes about one half of the solids and one third of the organic pollutants
from the wastewater flow.
Primary treatment takes place in five large settling tanks called primary clarifiers (13, see
figure below). Sewage coming from the Pre-aeration and Grit Removal Building (12) flows into
these five tanks where it stays for at least two hours. During this time about 50 percent of the fine
solid waste material (known as suspended solids) in the wastewater settles out and sinks to the
bottom of the tanks. Once they have settled out, these solids (called Primary Sludge) are
collected from the bottom of the tanks by large mechanical scrapers. These scrapers move the
sludge into hoppers or bins at the bottom of each tank. The sludge is then pumped to another area
(15) for further treatment (see "Sludge Digestion").
Surface scum or grease is skimmed off the top of each tank and taken by a separate system of
pumps and pipes for further treatment with the sludge.
The liquid left in the tanks is called "settled sewage". It flows over the edge of the tanks and onto
the secondary treatment stage (16). Primary treatment is now complete.
Primary Treatment Facts
Number of tanks *
5
Total volume
24,300 m3
Total ultimate flow
830 ML/d
* 3 circular tanks; 2 tanks are 36 m diameter, 1 tank is 44 m diameter
2 rectangular tanks are 66.5 m x 23 m; all tanks are 3.6 m deep
The settled sewage is still not clean enough to release
into a natural body of water such as the Red River.
Secondary treatment is needed to reduce the amount of
organic matter and pollution before it can be released
to the river.
SECONDARY TREATMENT
Secondary treatment is the second step used to remove remaining organic matter from the
wastewater before it flows from the treatment plant to the river. This process is generically
called the Activated Sludge Process.
Oxygen Reactor Tanks
In the first part of secondary treatment, the settled sewage (16, see figure below) flows into six
oxygen reactor tanks (17) arranged into three trains. Here the incoming wastewater is vigorously
mixed with high-purity oxygen (21) and sludge (24) (called return activated sludge, RAS)
containing large amounts of bacteria. In nature, these bacteria need oxygen (aerobic bacteria) to
survive and feed on organic material. This natural process is speeded up in the oxygen reactors
where the bacteria in the sludge use the high-purity (90-95% pure) oxygen to feed on the
organic material in the settled sewage.
High-purity oxygen for the oxygen reactor tanks is produced in the cryogenic air separation plant
(19) on site. This plant is operated by a private company under a long-term contract.
(Click on figure for larger scale) (For PRINTING INSTRUCTIONS, click HERE!)
Final Settling Tanks (Secondary Clarifiers)
From the oxygen reactor tanks the mixture of bacteria and water (called "mixed liquor") flows
(18) into the final settling tanks (23), also called final clarifiers. Here the bacteria laden sludge
settles to the bottom of the tanks. After settling is complete, the water in the tanks is 90 - 95%
free of polluting materials. This final effluent can now be safely released into the river (25).
The settled bacteria laden sludge is now called activated sludge, because it has been "activated"
with bacteria which clean the wastewater. This sludge is removed from the bottom of the tanks
by underwater scrapers and pumps. Most of the activated sludge is returned (24) to the oxygen
reactor tanks to supply the bacteria needed for the secondary treatment. This portion is called
Return Activate Sludge (RAS). The excess, called Waste Activated Sludge (WAS), is sent to be
mixed with the effluent from the pre-aeration stage, where it flows to the primary clarifiers.
There, the Waste Activated Sludge will settle out with the Primary Sludge, and in doing so, it
will thicken from less than 0.5% to over 3% solids. This process, thickening the WAS with
Primary Sludge, is called "co-thickening". The settled sludge is then pumped to the digesters.
Oxygen Reactor Facts
Number of oxygen tanks = 6
Secondary Clarifier Facts
3
(30130 m total)
Number of trains* = 3
Number of clarifier tanks** = 26
Total hydraulic capacity = 600 ML/d
Amount of oxygen used = 33 tonnes/day, 90 t/d
capacity
Mixed liquor suspended solids = 2000-3000 mg/l
*each tanks is 70 m x 17.5 m x 4.5 m deep. Total volume = 31,200 m3
**(10 circular square clarifiers are 20 m diameter; 16 rectangular clarifiers are 70.5 m x 9.1
m. Total volume = 41, 275 m3
SLUDGE DIGESTION
Sludge from the bottom of the primary clarifiers (15, see figure below) is sent to the sludge
digesters (26). Sludge from the South and West End Plants is trucked in and added here as well,
because those plants don't have digesters. In the digesters, bacteria that do not need oxygen
(anaerobic bacteria, the same ones as in your large intestines) begin to feed on the sludge in the
oxygen-free environment inside the digesters. Heat exchangers (27) are used to regulate the
temperature inside the digesters, keeping it around 38 Celsius (same as your body temperature).
The technical name for this process is "Mesophylic Anaerobic Digestion".
The content of the digesters is mixed continuously. The bacteria feed on the sludge for 10-20
days and decompose (stabilize) it. This reduces the odor and organic matter in the sludge. The
digested sludge (called "biosolids"), which is still mostly liquid, is stored in holding tanks (29)
before it is sent to the dewatering system (3) where some of the liquid is removed.
"Sludge gas" (also called "Biogas") consisting of about 65% methane and 30% carbon dioxide is
produced by the anaerobic bacteria during digestion. This gas is highly combustible, so it is
stored (33) in the gas storage sphere, and used as fuel in boilers to heat (35) the treatment plant,
as well as the sludge in the digesters. Excess methane is released using the waste gas burners
(34). In this manner, all the methane, a serious greenhouse gas, is converted to carbon dioxide
and water vapor. On very cold winter days, when there is not enough biogas produced to heat
the plant, natural gas is also used (36).
BIOSOLIDS DEWATERING SYSTEM
Digested sludge, called "Biosolids", are pumped (30, see figure below) from the holding tanks to
centrifuges (38) in the dewatering building. Before it enters the centrifuges, special organic
chemicals known as polymers are added (44) to aid in the separation of liquids and solids. The
centrifuges spin the sludge at very high speeds to separate the liquid from the solids.
To learn more about the
centrifuges used by the
City, please click here
(Courtesy Alfa Laval)
Once they are separated, the liquid (called centrate), which has a high concentration of
ammonia, is returned to the main interceptor to enter the plant for treatment. The dewatered
biosolids (called cake) is pumped through the biosolids cake line (40) to the biosolids cake
storage bins (42). The biosolids cake is temporarily stored in these covered bins until it is loaded
onto trucks (43) inside the dewatering building. It is then taken to agricultural land where it is
applied as fertilizer.
Biosolids are a relatively stable product, and rich in nutrients such as nitrogen and phosphorus.
This makes it a valuable fertilizer for agricultural lands. The City of Winnipeg operates a
successful program called "WinGRO" where dewatered biosolids are recycled by applying it to
agricultural land to fertilize and condition the soil.
THE WinGRO PROGRAM
What is WinGRO?
WinGRO is the program operated by the City of Winnipeg under which dewatered biosolids
from the North End Water Pollution Control Centre (NEWPCC) are hauled to and spread on
agricultural land. The WinGRO program is operated in compliance with terms and conditions
prescribed in a License issued under the Manitoba Environment Act to the City of Winnipeg.
Extensive research has been completed to ensure that the WinGRO program is safe and
beneficial to the environment. The City, in collaboration with academic and private entities, is
continuously researching technologies to improve the program.
Why do farmers want WinGRO?
WinGRO biosolids are an excellent source of organic nitrogen
and micronutrients such as copper. They are also a very good
soil conditioner enhancing the water holding capabilities of the
soil and making it less susceptible to wind erosion.
FINAL EFFLUENT DISINFECTION
Although Wastewater Treatment Plants remove pollution from wastewater, their final effluent
contain bacteria which are pathogenic to humans if ingested. At the NEWPCC, conceptual
design of final effluent disinfection is underway and is expected to be implemented in 2006. At
the SEWPCC, final effluent disinfection is accomplished using Ultra-Violet Radiation (UV
Disinfection), commissioned in 2000. At the WEWPCC, disinfection is accomplished by
diverting the final plant effluent through large open basins, where the pathogens are killed
naturally by solar radiation.
Wastewater management is an expensive enterprise for any large city. For 2000, the total cost
for providing wastewater services in Winnipeg was approximately $74 million. This cost is not
only for the supplies and the operators, mechanics, technicians, electricians, and programmers
needed to directly operate/maintain the facilities, but also for all the support staff such as
engineers, technologists, laboratory, customer services and corporate support. In addition, the
City borrows money to pay for capital expenses, such as treatment plant expansions (over $200
million in the past 20 years), which must be paid back. For a breakdown of these costs, click
here.
"Wastewater Services" is a utility, much like the more familiar gas and electric utilities. The cost
for wastewater management is paid for by the utility's customers. Each customer has a water
meter, which is used to determine how much water is used. The meter is read quarterly, and the
City sends an invoice to the customer. The invoice indicates how much water was used for the
past quarter and the cost for the water. The same invoice also indicates the cost for wastewater
services, based on the same water meter reading. Contrary to popular belief, Wastewater
Services are not paid by municipal taxes.
In general, the sewer rate is determined by dividing the total annual cost of providing
wastewater services, by the total annual amount of water used by the community. The current
(2004) rate is $3.11 per 100 cubic feet.
http://www.theprovince.com/news/Washington+governor+raises+stink+over+Victoria+sewage+t
reatment/9929081/story.html