Located northeast of Montreal, Canada, the city of Terrebonne needed to upgrade its water treatment plant to increase capacity and meet environmental regulations. The plant is the last drinking water intake of the many cities along the Rivière des Milles-Iles (River of the Thousand Islands). Given the effluent waters from the cities upstream, the plant needed a treatment process that ensured optimal water quality.
First established in 1861, the Terrebonne treatment plant has evolved tremendously in recent years and has required new infrastructure to serve the constantly growing population. Today the RAIM treatment plant (La Régie d’aqueduc intermunicipale des moulins) distributes high quality potable water to 125,000 users in the cities of Terrebonne and Mascouche.
Efforts to modernize and expand the plant began in 2008, and involved the addition of a new state-of-the-art innovative multi-barrier process to treat the water. Due to constantly changing waters coming from the Rivière des Milles-Iles, a multi barrier approach was required to ensure that the drinking water met the high standards in place. The combination of ozone and UV disinfection solutions from Xylem was an integral part of the plant’s new treatment process.
Running smoothly since its completion in 2011, the plant now has a capacity of 120,000 m³/d, allowing RAIM to serve more than 250,000 consumers in the fifty years to come.
A successful multi-barrier process
While only minor civil work was required, the upgrade was a sound investment for RAIM in collaboration with the partnering municipalities. The ozone solution helps lower operating expenses and energy consumption (see below), and the UV disinfection solution surpasses environmental requirements.
The log reduction for cryptosporidium, giardia, and viruses are regulated to be 2, 5, and 6 logs, respectively, but at RAIM, the use of this multi-barrier approach has led to the plant achieving log reduction levels far beyond the indicated regulations. Given these results, the plant has since established its own objectives and benchmarks.
Gilles Comtois, Director of RAIM, reported that, “The success of this multi-barrier process is due to the fine-tuning of the operators, but the effectiveness of the UV disinfection stage is also a major factor in surpassing the required objectives by such a wide margin.”
How the ozone and UV solution works
Water passing through the plant using the new solution takes four to six hours from intake to the final treatment step. Here is how it works:
1. The process includes a grit removal phase, followed by raw water pumping of 160,000 m³ per day and three sand ballasted clarification systems settling out suspended matter from the water.
2. Two ozone generators oxidize organic compounds, and eleven biofiltration basins digest the sugars and eliminate micro-particles.
3. A UV disinfection step eliminates pathogenic microorganisms and feeds a 19,000m³ reserve stored in three underground reservoirs.
4. Three pumping stations feed the distribution network covering 35 kilometers serving the area’s needs.
The solution: ozone oxidation
The use of ozone following clarification serves a number of purposes, primarily to break down organics into more simple and biodegradable products, which are then readily removed by subsequent treatment steps such as biofiltration. An additional benefit is that ozone breaks down the heavy polymer used in the flocculation step of the sand ballasted clarifiers. This significantly reduces the quantity of chemical products reaching the biological filters, which helps to prevent fouling and extends carbon lifetime.
To ensure continuous flow and supply to the residents, there are two WEDECO SMO700 generators at the facility, one in constant operation and one on standby. The ozone generators are fed by liquid oxygen (LOX), housed outside the plant, and servicing is performed two times per year by a Xylem service provider. Using ozone at Terrebonne results in efficiently oxidizing organics and reducing chemical usage, which provides for longer carbon lifetimes and reduced wash frequency. This also lowers operating expenses and reduces the overall energy consumption at the drinking water treatment plant.
The solution: UV disinfection
Ultraviolet (UV) disinfection is the first step of disinfection performed following biofiltration. As the filtered water circulates through the UV reactors, the UV light penetrates the cell walls of bacteria and viruses to permanently alter the DNA of the microorganisms, rending them “inactivated” and unable to reproduce. The consulting engineers on the project selected ultraviolet disinfection for its effective results and low operating expenses.
Nine WEDECO UV reactors are installed at RAIM: five BX3200 with 32 lamps and four BX1800 with 18 lamps, all with automatic wipers preventing fouling of the lamps. The BX series is a u-shaped closed-vessel technology for water and wastewater applications with capacity up to 14.7MGD (55,000 m³/d), and a UVT range of 80% to 98%.
Liquid chlorine residual is then added as a second step of the disinfection process. Finally the pH levels are corrected to meet the required standards to the treated water.