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Sewage Treatment Process

Diagrap depicting a Water Pollution Control Plant - Sewage Treatment Process

Origins of Sewage

Wastewater is created within residences, institutions, hospitals, commercial and industrial establishments.  It is comprised of sanitary waste (brown water) and other utility uses (grey water). Wastewater may also include some surface water due to infiltration and some residential drainage.  Wastewater is primarily water with only 0.1% solids. The Durham Region does not have combined sewers, which consist of storm water runoff entering the wastewater collection system, rather the storm sewer collection system is separated from the wastewater collection system and discharges to natural waterways such as rivers and storm retention ponds.

1- Preliminary Treatment


Wastewater is initially strained to remove paper products and large objects that could harm pumps and other process equipment.  Straining is typically accomplished using automated mechanically raked bar screen.  The screenings are compacted and transported for disposal to landfill.

Grit removal:

Following the screening, the sewage then flows to a grit removal component.  Grit removal is the process of removing sand, grit, and stones while keeping the majority of the organic matter suspended.  This can be accomplished by carefully controlling the wastewater flow in a channel or a tank, or by introducing air to the wastewater within a tank or channel and/or also by dewatering equipment (e.g. centrifugal separator).  The grit is then transported to landfill along with the screenings.

2- Primary Treatment

In this stage, the wastewater flows through large tanks commonly called ‘primary clarifiers’.   These tanks are designed to remove larger particles by letting them settle to the bottom (sludge) while lighter and floatable materials such as grease and oils (scum) rise to the surface where it will be skimmed off.   This process removes up to 60% of the suspended solids.  Both the sludge and the scum are removed from the tanks and pumped to further sludge treatment processes (see Sludge Treatment).

3- Secondary Treatment


The secondary treatment stage of the process is where the components of the wastewater are substantially degraded.  This is accomplished using microorganisms, oxygen and the remaining organic materials found in the wastewater.   The microorganisms, bacteria and protozoans, consume some of the organic contaminants and bind much of the remaining organics into floc.  A coagulant is added to assist in bonding the smaller flocs into larger settleable formations.  This also aids in removing phosphorus.  Excessive release of phosphorus could lead to an algal bloom which can be harmful to the receiving waters.

Secondary Clarifiers:

The final part of the secondary treatment stage is to settle out the biological floc to produce a clear and clean effluent.   The floc (activated sludge) that settles to the bottom of the sedimentation tanks, often called ‘secondary clarifiers’, are rich in microorganisms and is returned to the bioreactor to repeat the biological degrading process.   To prevent an overgrowth of microorganisms which will upset the process, some of the activated sludge is sent to the primary clarifiers to co-settle with the sludge and be further treated prior to disposal.

Tertiary Treatment


When the treated wastewater is discharged into small or sensitive receiving water course, it will often undergo a third level of treatment called tertiary treatment. This is also called ‘effluent polishing’.   The Tertiary treatment used by the Region is sand filtration.  This removes most of the remaining suspended solids.

4 - Disinfection

This stage is intended to substantially reduce the number of microorganisms in the effluent to be discharged back to the receiving waters.   Two methods are used in the Region:


The effluent is dosed with chlorine and sent into a tank to allow the chlorine to come into contact with the microorganisms, killing them or inactivating them.   At some plants chlorination is followed by dechlorination of the effluent prior to its discharge to the receiving waters.

2- Ultraviolet light (UV):

For this process to be efficient, the effluent must first undergo Tertiary Treatment to ensure the effluent is as clear as possible. Then the water is bombarded by radiation from a series of UV lights which damages the genetic material within the bacteria, rendering them unable to reproduce.  It leaves no residual entering the receiving bodies. This process is very clean, however also very expensive.

5- Sludge Treatment

Sludge, scum and activated sludge collected from the clarifiers are either pumped to digester tanks, where it will either be further treated on site for eventual land application (biosolids), or stored in holding tanks until transportation to Duffin Creek Water Pollution Control Plant in Pickering for incineration or landfill disposal.

Both processes thicken the sludge creating a lighter liquid supernatant.  This supernatant is pumped to the head of the plant to be processes with the wastewater.

Sludge Digestion/Treatment:

In this process, the sludge is digested by bacteria in the absence of oxygen (anaerobic digestion) to reduce the amount of organic matter,disease-causing microorganisms and odours.   This is a fermentation process to stabilize the remaining complex organic matter into simpler organic compounds rendering it suitable for land application on farms within the region’s farming community.  The digested sludge is often referred to as biosolids.

Another beneficial major by-product is biogas, or methane, which can be used in generators or boilers within the plants reducing energy consumption.


Once the sludge is stabilized, it is called biosolids and can be applied on agricultural land as fertilizer.  The biosolids and agricultural land to be applied on must meet rigorous criteria set by the Ministry of the Environment (M.O.E.) and the Ontario Ministry of Agriculture, Food, and Rural Affairs (O.M.A.F.R.A.).  

Biosolids contain nutrients such as phosphorus and nitrogen which are essential in productive agricultural soils.   It is also high in organic matter which improves moisture retention, soil workability and enhances plant uptake of nutrients and minerals. Farmers benefit from the biosolids by reducing the need for commercial fertilizers which is resource and energy intensive to manufacture.  Municipalities also benefit from this program by recycling a treatment process by-product in a cost-effective manner.