Chlorination process to treat raw water yields to a multi-barrier chain of treatment technologies
A $300-million, 400-million-L-per-day (mLd) water treatment plant in Winnipeg, MB, entered service in 2009. It has an estimated design life of 75 years and can be expanded up to 600 mLd. For decades, the Winnipeg utility used a multiple-point chlorination process to treat raw water drawn from remote Shoal Lake. The water was disinfected at the intake, after terminal storage and again before distribution to the service area.
Concerns eventually arose about the potential presence of chlorine-resistant pathogens—Crytosporidium and Giardia—and residual disinfection byproducts, which coincided with encroaching development near the lake. Taste and odor control problems had also become an issue, despite the fact that Shoal Lake’s water quality is much better than most surface water sources. As a result, the groundwork for a multi-barrier chain of treatment technologies was examined using a pilot water treatment plant.
The new plant integrates eight sequential treatment systems, including coagulation/flocculation, dissolved air flotation (DAF) clarification, ozonation, biological filters with granular activated carbon (GAC) media, chlorine disinfection, ultraviolet disinfection, and a final injection of fluoride to protect against tooth decay and orthophosphate to prevent lead from leeching off old piping.
The utility’s pilot plant tested key treatment options for 16 months to ensure the technologies would meet the utility’s goals. The timetable spanned four different seasonal conditions at Shoal Lake. The testing established a four-stage treatment process: DAF, to remove suspended solids, organics and algae; ozonation as a seasonal barrier against Cryptosporidium (subsequent study revealed that ozone is ineffective in cold water months at the retention times and dose used in Winnipeg) and for taste and odor control; biological active filters with GAC media as an additional barrier against pathogens and to remove additional organic material; and chlorination, although chlorination has not yet been implemented, pending an overall review of disinfection requirements throughout the Winnipeg distribution system. The findings concluded that significant construction savings were attainable with the recommended technologies compared to a conventional plant design.
Raw water entering the plant undergoes flash-mixing and three-stage flocculation ahead of pretreatment by the dissolved-air flotation system.
Treatment starts with flash-mixing sulfuric acid and ferric chloride into the raw water to adjust the pH. This enhances coagulation and flocculation, respectively. Pumps then advance the raw water to eight three-stage flocculation tanks ahead of pretreatment by a Leopold Clari-DAF system. Although the raw water quality usually ranges from only 1 to 1.2 nephelometric turbidity units (ntu), Clari-DAF units can reduce even that level of turbidity down to within a range of less than 0.5 ntu. The energy-efficient, small-footprint Leopold system was selected after alternate equipment was tested for nearly two years.
The Clari-DAF system’s method for removing surface floc further distinguishes it from other clarification methods that rely on particulate matter settling to the bottom of basins for removal. Instead, the Leopold system produces micron-sized bubbles under pressure that carry suspended particulates to the surface, where the floc is skimmed off with mechanical scrapers. The clarified water then advances to the next treatment stage through laterals at the bottom of the basins.
The process flow next reaches two contact tanks, where it undergoes ozonation. Sodium bisulphate is added to the water thereafter to consume leftover ozone before the water is filtered through eight biologically active GAC filters. This step uses beneficial bacteria to remove dissolved organics and any particulates before reaching a chlorine contact chamber. This type of filter was recommended because biomass thrives more on GAC media. The water is then disinfected for bacteria and virus control using onsite-generated sodium hypochlorite before sodium hydroxide is added to raise its pH level to stabilize the drinking water. UV disinfection is then undertaken as a precaution to inactivate any remaining water-borne pathogens. Finally, fluoride and orthophosphate are added to the water before it reaches three regional reservoirs for distribution.
Up to 70% of the organics are trapped by the Clari-DAF pretreatment system before the raw water reaches the plant’s eight biologically active carbon filters, shown here.
Now that the treatment plant is operating, Winnepeg’s drinking water is clearer, smells and tastes better all year, and is of a higher quality than the drinking water guidelines set out by Heath Canada, the federal department responsible for helping Canadians maintain their heath.