By Dr. Saad Y. Jasim, Windsor Utilities Commission
Surface water supplies are often used as sources of drinking water in Canada. Surface waters are exposed to precipitation events, surface water runoff and contamination with microorganisms resulting from activities in their surrounding areas. In general, surface waters are considered vulnerable to contamination from animal populations that carry protozoan cysts pathogenic to humans.
Cryptosporidium has been recognized as a serious cause of water-borne disease in humans. Cryptosporidiosis outbreaks from surface water supplies have been documented in Canada and the United States. The outbreaks in the last decade caused a major concern to the public. It has been speculated that many other cases of water-borne outbreaks of gastroenteritis may have been caused by Cryptosporidium.
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| The Ozone generator -- 500 kg per day at 12% ozone production. |
Ozone, a strong disinfectant, is superior to chlorine, chlorine dioxide, or chloramines for the inactivation of water-borne pathogens such as Giardia and Cryptosporidium. Disinfection regimes excluding ozone are not capable of the inactivation of Cryptosporidium oocysts at dosages and contact times typically practised, and even filters may allow a few organisms to pass through in treated water.
Many water treatment plants in North America are using ozone to ensure inactivation of Cryptosporidium oocysts, and to produce aesthetically acceptable treated water. However, a chlorine residual of 1 mg/L is maintained as the treated water enters the distribution system.
The Windsor Utilities Commission, in conjunction with the NSERC Chair in Water Treatment, University of Waterloo, and the Civil and Environmental Engineering Department, University of Windsor, have conducted pilot-scale studies since 1992.
A pilot-scale system was used to investigate the improvement of water treatment processes by using ozone. The pilot plant had two identical process trains constructed of organically inert materials (stainless steel, glass, or fluorocarbons). Each side of the pilot plant shared identical physical characteristics, which allowed for direct comparison between the two sides of the plant with a common raw water quality.
Experiments conducted in Windsor, indicated that ozone-induced coagulation and flocculation is possible at the Windsor Water Treatment Centre. Settled water turbidity was always lower on the side of the pilot plant where pre-coagulation ozone was applied. Dual-media (anthracite/sand) filter performance for turbidity was consistently better when pre-coagulation ozone was applied prior to coagulation. The application of pre-coagulation ozonation improved particle removal especially for size ranges of 2-5 µm and 5-10 µm, compared to the non-ozonated control stream.
The Windsor Utilities Commission has been sampling for Giardia and Cryptosporidium since 1997. Cryptosporidium oocysts were detected on a few occasions in raw water samples and other samples contained Giardia cysts, as indicated in Table 1. The presence of these pathogens at these levels in the raw water source caused a major concern to members of the Windsor Utilities Commission.
The Commission decided on July 21, 1999, to implement ozone for drinking water treatment at the A.H. Weeks plant in Windsor, Ontario. The A.H. Weeks plant was built in 1994 at the site of the old plant. Provisions were made for future addition of ozone to the new plant, while studies for the rehabilitation of the old plant included the use of ozone in the treatment process.
The ozone system was designed to achieve 2-log inactivation of Cryptosporidium oocysts. The ozone generators, power supply units, and ozone destruct units were supplied by Ozonia North America. The ozone system was brought on-line in June 2001. There was an immediate improvement of treated water quality. Filtered water turbidity was reduced from 0.06 NTU to 0.03 NTU, on average, despite the fact that alum dosage was reduced.
The average alum dosage for the months of June-December for the years 2000 and 2001 was 52.7 mg/L and 42.5 mg/L, respectively. The dosage for the coagulant aid (Percol LT 22, cationic polymer) was 0.07 mg/L and 0.05 mg/L for the years 2000 and 2001, respectively. The reduction in alum dosage was 19%, and 28% for the Percol LT 22. Particle count for filtered water was noticeably reduced to an average of 10-15 counts/ml. The average turbidity for raw water during that period was 11.15 NTU in the year 2000, while it increased to 20.1 NTU for the year 2001.
The Windsor Utilities Commission used to add powdered activated carbon (PAC) for taste and odour control during the summer. The addition of PAC was eliminated when ozone was used at the A.H. Weeks plant. The elimination of PAC saved the Commission in excess of $400,000 annually. The reduction of alum and Percol LT 22 costs was $59,756, and $2,746, respectively. The saving in operational costs is expected to exceed $200,000 annually.
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| Liquid oxygen storage tank. |
The operation of the ozone system to achieve the design goal of 2-log inactivation of Cryptosporidium oocysts provides a substantial additional log inactivation credit for Giardia and viruses far in excess of maximum values shown in regulatory disinfection tables. The improvement of water quality using a multi barrier system has inspired public confidence, especially after the Walkerton and North Battleford incidents. The implementation of ozone treatment in Windsor was to prevent a potential outbreak of disease from occurring.
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