Waste water treatment

The treatment centre handles waste water from the Ville de Longueuil and three other cities of the agglomeration: Boucherville, Brossard, Saint-Lambert.

Guided Tours

An experienced guide is available to conduct visits around the treatment centre for groups of ten people or more (Ville de Longueuil residents). It should be noted that the centre contains many stairways. Visits last approximately 1½ hours. For information, please contact: Centre d'épuration Rive-Sud.

The South Shore treatment centre

the Roland-Therrien pumping station
One of the pumping stations for the main collector sewer: the Roland-Therrien pumping station.

Water covers three-quarters of our planet's surface in the form of oceans, rivers, streams, lakes and glaciers.

The St. Lawrence is one of the earth's largest rivers. Since the beginning of colonialization, we have built the majority of our towns and factories along its banks, and it is from the river that we take all the water that we need. Until recently, this water, once used, was thrown back into the river untreated, full of domestic and industrial wastes.

Happily, throughout Québec and elsewhere in the world, people have become more conscious of the need to protect the environment. Today, waste water from the region is channelled to the South Shore treatment center to be cleansed before being returned to the river.

In 1984, the municipal administrations of seven of the towns that existed on the South Shore at that time — Boucherville, Brossard, Greenfield Park, LeMoyne, Longueuil, Saint-Hubert and Saint-Lambert — combined their efforts to build a system for treating waste water. Before the construction of the treatment centre could be undertaken, it was first necessary to connect the separate sewer systems that existed at that time and build new interceptor sewers and pumping stations to collect the waste water and transport it to Île Charron.

Financial contributions from the ministère de l'Environnement du Québec (Quebec Ministry for the Environment) together with technical and management expertise from the Société québécoise d'assainissement des eaux (Quebec Waste Water Treatment Corporation) were essential in carrying out this vast project.

Eight years after the initial agreements were signed, Longueuil benefits from a modern centre designed for capacities calculated for the year 2000 and a population of 330,000 to 457,000 people.

Inaugurated in October 1992, the South Shore treatment centre represents a joint project involving both the current and future populations because it addresses a vital preoccupation: the protection of the environment.

A Truly Worthwhile Mission: Purifying Waste Water

What does the term “waste water” mean?

We give the term “waste water” to everything that is carried by the sewers from houses (water from toilets, baths, showers, washing machines, etc.), business premises, industrial sites, office blocks and other buildings. Added to this is some groundwater that infiltrates into the system and surface water such as rain and melted snow.

Water from the river is used for domestic purposes, recreation, and even industrial purposes. Purifying waste water poses an important technical challenge.

The South Shore treatment plant is a collective achievement for which the financial and administrative responsibility rests with Ville de Longueuil. Its mission: to protect the environment and regenerate the St. Lawrence River.


The South Shore treatment centre incorporates four stages:

  1. Treatment of the waste water;
  2. Treatment of the sludge;
  3. Incineration;
  4. Treatment of the air.

These are the steps in providing a safe and efficient treatment for the 330,00 cubic metres of water that is returned to the St. Lawrence River every day. The treatment centre removes 50 tons of pollutants from the waste water each day.



Arrival and Mixing

Waste water arrives at the South Shore treatment centre through three rising mains coming from the borough of Vieux-Longueuil (Roland-Therrien and Lafrance pumping stations) and from the city of Boucherville (Bachand pumping station). These conduits, which pass under the river, have diameters of 1950, 900 and 750 mm respectively.

The outlet of each conduit is designed to ensure that the waste water is well mixed. The flow then passes into three feeder channels designed according to precise hydraulic criteria to equalize the flow without any mechanical input.


This is the stage where large lumps of waste and floating material are removed from the waste water. By passing through the three screens, large solid particles are retained on the metal screens, removed, loaded into semi-trailers and taken to an authorized disposal site.

Grit chambers
Grit chambers

Grit, Oil and Grease Removal

This stage is essential in order to avoid maintenance and repair problems for all the equipment. It comprises the removal of particles, whose size and weight are comparable to gravel or sand, by the injection of air into the water. The oils, greases, floating material and scum are removed from the surface at the exit from the three chambers and taken by semi-trailer to a disposal site.

Measuring the Flow of Waste Water

The waste water then passes into a tank and flows through four "Parshall" flumes that control the distribution of the total flow. This operation ensures optimum performance of the "flocculation - decantation" stages and also, if required, allows for the diversion of the flow when equipment maintenance must be carried out.



Flash Mixer

As the waste water passes through the flash mixer, various chemical products are added in quantities that are proportional to the flow of water to be treated. A mechanical mixer ensures thorough distribution of the chemicals. Waste water stays in the flash mixer for approximately one minute.


The waste water enters the flocculation tanks where it is gently mixed by mechanical paddles that stimulate the formation of a flock (particles that are too light to settle out individually, stick together to form heavier clumps that do settle and can be decanted). The water takes approximately 17 minutes to pass through the flocculation tanks. Skimmers remove scum and floating debris that may accumulate on the surface of the water.


Each flocculation tank also includes two lateral settlement tanks. Water passes from the flocculation section to the settlement section through holes in the lower portions of the tanks.

Next, the water rises towards the collection troughs by passing between closely spaced, flat plates. The flock sticks to the plates and gradually falls to the bottom of the settlement tank to form sludge.

This sludge is continuously removed by bottom skimmers that guide it towards hoppers located at the end of the settlement tanks, from where it is pumped to sludge thickening units.

Return to the River

The first major step has now been completed: that of treating the waste water. The treated water meets the effluent standards set by the ministère de l'Environnement (Ministry of the Environment) of 334 kg per day of total phosphates and 7,378 kg per day of total phosphate suspended material, measured on an annual basis.

The effluent is released in the centre of the St. Lawrence River after passing through an underground conduit, 2,750 mm in diameter, whose outlet is located 55 metres north of the Louis-Hippolyte-La Fontaine Tunnel.

Sludge Treatment

Sludge Treatment

Sludge Thickening

The sludge is thickened in two large, cylindrical, concrete tanks fitted with mechanical scrapers. The liquid that rises to the surface is collected in a trough running around the outer edge of each tank and is returned to the decantation stage. The sludge, concentrated to approximately 5% solid content, is continuously drawn off at the bottom of the thickening tanks to feed the dewatering chain.

Rotary Presses
Rotary Presses


The sludge, still in a liquid state, is pumped into two homogenization tanks. Here, the sludge, oil, grease, and scum are mixed to form an homogenous matter. These tanks also act as reservoirs to even out the flow of sludge to the rotary presses.


At this stage, chemical products are added to assist in the dewatering process. The sludge is fed into the rotary presses which, by a process of pressure and friction, extract a large part of the water from the sludge to form a relatively solid mass called "sludge cake." This then falls into a hopper located under the press.



Pumps designed to pump concrete, push the dewatered sludge towards a fluidized bed sludge incinerator. This special type of incinerator uses a bed of sand that is held in suspension by a current of air and heated to a temperature of 750 to 800° Celsius. The sludge is normally burned without any other fuel being added.

A small portion of the energy created by the burning process is used to pre-heat the intake of air, while the main portion is recovered as steam to heat the building and operate the air conditioning. Savings are thus possible on the purchase of heating fuel. The final products of this stage are cinders and gas, both of which are recovered in electrostatic precipitators.

The gases are vented to a chimney, while the solid particles are transported by tanker to a local cement factory for recycling.

The cinders produced by the South Shore treatment centre as part of the waste water purification process are now integrated into the manufacture of cement powder. This technological innovation was carried out in December 2000 thanks to an agreement reached with Lafarge Canada Inc. This agreement not only allows for the annual recycling of more than 3,000 tons of cinders, it also produces savings of some $50,000. Research continues to see whether other waste treatment plants can also apply this technology.

From an environmental point of view, the cinder recycling is an example of the 3Rs: reduce, recycle, and reuse.

  • Reduce landfill material and recycle the cinders. Instead of transporting cinders to a landfill site, these are recycled in the cement making process.
  • Reduce the quantity of water and reuse it. When they were transported to a landfill site, the cinders required a moisture content of 25% to 30% to avoid them blowing away during transit and unloading. Thanks to the new method that has been developed, the treatment centre is able to deliver the cinders entirely dry. The water that was previously required to transport the cinders to the landfill can now be returned to the river.
  • Re-use energy. The sludge incineration process that produces the cinders, also allows energy recovery to heat the plant in winter. In summer, this energy is transformed into iced water to provide air conditioning for the administrative section of the plant.

Treatment of air and gases

The system for treating polluted air guarantees both hygienic conditions inside the plant and the absence of annoying factors (odour control) outside, in accordance with objectives set by government authorities. This is achieved by passing the air through stripping towers (not shown) whose primary function is to completely remove odours.

The main treatment method consists of neutralizing, oxidizing, or absorbing the foul smelling gases through contact with reactive agents. The only perceptible odour that may remain, if strong sulphur compounds are present, is a slight smell of bleach.

A Treatment Plant that is Perfectly Integrated into its Environment

Great pains have been taken to preserve the natural beauty of this site that is much appreciated by nature lovers. Because of the unique location of the treatment plant — near to the Boucherville Islands Park and adjacent to the Hôtel des Gouverneurs — the whole plant is enclosed and the air and gases are purified before being released. The treatment centre does not emit any smells whatsoever.

The location covers some 20 hectares — on which are planted over 3,000 trees and bushes — and integrates seamlessly with the natural environment, forming a continuation of the recreational network of the adjoining towns.

Modern Technology

The treatment centre houses a control room fitted with the most modern technology: an automated computer system for process control and data collection, computerized remote control of the plant and of the pumping stations, together with closed circuit television for security surveillance of the loading bays and the outdoor areas around the plant.

The treatment centre uses laminar plate settlement tanks, a technique that allows the settlement tanks to be reduced in size, thus providing savings during the construction of an entirely covered plant.

Designed by the Centre de recherche industrielle du Québec (CRIQ) [Quebec Centre for Industrial Research] and made under licence by a Quebec company, the rotary presses represent, by their simplicity and efficiency, a major innovation in the treatment of waste water.

The pumps used to transport the sludge, which has a 35% solid content, were designed based on pumps used in the construction industry to pump concrete. Their main qualities are that they ensure easy pumping and good odour control.

The treatment centre is self-supporting for its energy requirements; heat from the incinerators is recovered to provide heating and air-conditioning for the building.

Waste water treatment in numbers (CERS)


  • Average flow treated at the CERS: 300,000 cubic metres per day
  • CERS capacity: 447,000 cubic metres per day
  • Maximum daily production: 325,000 cubic metres per day, or the equivalent of 130 Olympic-sized pools
  • Daily average per person: 586 litres per day per person

Load treated

  • Flock removed: 35,000 kg per day
  • Treatment capacity: 38,000 kg of flock per day
  • Flock removal at its peak: 70,000 kg of flock per day