Jul 19, 2011

World Class Water

Winnipeg water treatment plant adopts eight-step treatment process

Superlatives flow easily when describing the new $300-million water treatment plant (WTP) for the city of Winnipeg, Manitoba, Canada. The world-class facility entered service in December 2009 after a decade of studies, design and construction. With normal upkeep and maintenance, the 12,000-sq-meter plant with an initial capacity of 400 million liters per day has an estimated design life of 75 years. Given that Winnipeg’s population of 650,000 presently needs only 256 million liters per day, the surplus offers the city a potential revenue stream in selling water to other communities. Moreover, the plant will accept expansion up to 600 million liters per day.

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 intake, after terminal storage and again before distribution to the service area. Concerns have arisen, however, about chlorine-resistant pathogens and residual disinfection byproducts in the water utilities industry, and they coincided with encroaching development near the lake. These considerations raised questions about the long-term effectiveness of the multiple-point chlorination process. Therefore, the groundwork for a multi-barrier chain of treatment technologies was examined using a pilot WTP operated in the mid-1990s.

The new plant integrates eight sequential treatment systems: coagulation/flocculation, dissolved-air flotation (DAF) clarification, ozonation, biological filters with granular activated carbon (GAC) media, chlorine disinfection, ultraviolet (UV) disinfection and a final injection of fluoride to protect against tooth decay and orthophosphate to prevent lead from leeching from old piping. Each of the eight processes was proven at the pilot plant and elsewhere within the water utility industry when the new plant entered design development. The result was an inherently safer water supply and a significant reduction of the former taste and odor (T&O) issues that occurred seasonally in the surface water. The utility also now is prepared to fully comply with the likelihood of stricter regulations aimed at disinfection byproducts (DBPs) associated with chlorination.

Tom Pearson served as the utility’s project director and worked in conjunction with CH2M Hill and AECOM Canada Ltd. to produce the complex. The city adopted construction management for project delivery to meet a tight schedule, encourage competitive bidding through smaller contracts and assemble the diversity of expertise needed on the world-class project’s team. Contracts were let for bid starting in 2005 on approximately 20 major construction packages and for dozens of pre-purchased equipment orders requiring long delivery times.

AECOM served as the city’s project manager, overseeing a group of prime contractors and various subcontractors, including Gateway Construction & Eng. Ltd.; Bird Construction and Comstock Canada Ltd. Earth Tech (Canada), a unit of AECOM; and CH2M Hill served as the design engineers. PCL Constructors Canada Inc. received the largest of the eventual 52 contracts. This was for a $56-million package involving 35,000 cu meters of concrete foundations and related structure. Up to 400 tradesmen worked at the site during peak construction.

Renewed City Lifeline

A separate $50-million upgrade of a 137-km aqueduct from Shoal Lake to the Deacon Reservoir was completed just before the construction startup of the new plant. The aqueduct has provided up to 386 million liters of raw water per day since 1919. The water flows by gravity from the lake to four terminal reservoirs. These open earthen storage impoundments hold 8.8 billion liters—a 20-day reserve—which is sufficient for summer peaks and when the aqueduct undergoes maintenance.

The elegant unreinforced concrete arch aqueduct reflects engineering innovations due to steel shortages at the time of design, which coincided with World War I.

The Winnipeg Aqueduct has been recognized as a historic structure by the American Water Works Assn. A future upgrade could extend the present 25-year design life of the aqueduct for another half century.

A View to Future Changes

The additional treatment measures to safeguard the plant’s raw water were proactive rather than reactive to quality management issues. In fact, the water drawn from Shoal Lake presented Winnipeg with a reliable supply of raw water for many decades, but other factors loomed on the horizon.

“The Shoal Lake water quality is actually much better than most surface water sources,” Pearson said. “Because of this, Winnipeg has been able, until now, to defer full treatment and rely only on rough screens and chlorination. We had to take into consideration a recurring problem and some significant future considerations.”

Multiple-point disinfection with chlorine and the addition of fluoride for prevention of tooth decay had been adequate due to the raw water’s quality. The major problem in the past had been seasonal T&O and clarity issues common to other surface water during algae blooms at both Shoal Lake and Deacon Reservoir. The problem existed for about two weeks every year. However, there was heightened concern about chlorine-resistant Crytosporidium and Giardia following the infamous outbreak in 1993 in Milwaukee. Confirmed incidents of both pathogens were reported in recent years at other utility intakes along Shoal Lake, according to Pearson. From the conceptual stage, therefore, the utility was receptive to proven technologies that would deliver not only an effective disinfection barrier against the viruses but also mitigate the recurring T&O. Another design consideration was the likelihood of stricter Canadian Drinking Water Guidelines to prevent the ongoing buildup of chlorine DBPs. In addition to disinfection at the Shoal Lake intake, the chlorine was used to combat potential problems caused by zebra mussels in the aqueduct. Any amended use of chlorine would require alternative disinfection.

In fact, the risk of those pathogens brought the early installation of a UV disinfection system at the Deacon Reservoir pump station in 2007, before the new plant’s completion. The UV system consists of six reactors with nine 20-kW lamps that can handle 88 million liters per day. By using the existing pump station building, the utility achieved $20 million worth of construction savings by eliminating the need to build a standalone UV facility as part of the new plant’s complex.

Pilot Plant Field Tests Process Chain

The utility’s pilot plant tested key treatment options for 16 months to ensure that 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:

1. DAF (to remove suspended solids, organics and algae);

2. Ozonation as a seasonal barrier against Crytosporidium (a subsequent study revealed that ozone is ineffective in cold water months at the retention times and dose used in Winnipeg) and for T&O control;

3. Biological active filters with GAC media as an additional barrier against pathogens and to remove additional organic material; and

4. Chlorination, although it 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.

From Two- to Eight-Step Treatment

The adopted design produced a highly automated plant that monitors and controls a wide variety of instruments, mechanical equipment and electrical equipment, including 40 processors, 140 pumps, 2,300 valves and 1,400 instruments. Two operators can manage the process train, which has two parallel treatment trains, to improve reliability. There is also sufficient standby power generation installed on site to power one complete train that would supply half of the plant capacity during a protracted power outage. Treatment starts with flash-mixing sulfuric acid and ferric chloride into the raw water to adjust the pH, enhancing coagulation and flocculation. Pumps then advance the raw water to eight three-stage flocculation tanks ahead of pretreatment by the Leopold Clari-DAF system. The Leopold brand units are the largest ever supplied by ITT Water & Wastewater. The system removes 70% of the organics at the Winnipeg plant, which also improves filtration and extends the intervals between filter backwashes.

Although the raw water quality usually ranges from only 1 to 1.2 NTU, the Clari-DAF units can reduce even that level of turbidity to less than 0.5 NTU. The technology has corrected it at plants elsewhere to as little as 0.24 NTU. The Leopold system was selected after other equipment was tested for almost two years. Its decisive merits were its energy efficiency and the manufacturer’s extensive experience in DAF and filter installations.

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. DAF units also require a smaller footprint than traditional settling basins, which will help accommodate the future expansion of the plant infrastructure.

The process flow next reaches two contact tanks in which it undergoes ozonation. Sodium bisulphate is added to the water thereafter to consume leftover ozone, and the water is then 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 waterborne pathogens (e.g., Giardia and Crytposporidium). Finally, fluoride and orthophosphate are added to the water before it reaches three regional reservoirs and distribution to the utility’s feeder mains, pump stations, water mains and customer plumbing systems.

The zero-discharge plant benefits from the cold Canadian climate, which facilitates disposal of residual waste solids. They are pumped through settling tanks and two gravity thickeners before freezing in storage ponds over the winter cycle. The freezing fractures the cell walls of the residuals and aids further water drainage. The separated solids dry during the following summers before landfill disposal. The resultant material is dry enough to be handled using conventional front-end loaders.

This high-performance plant presents a model for other utilities that confront similar issues in the changing water treatment field.

About the author

Steve London is president of Steve London Associates. London can be reached at [email protected]
or 215.361.3630.