Iodine in solution as iodine(0), that is as elemental iodine, has been an accepted water disinfectant for many years. Iodine can be added by using a pre-prepared saturated iodine solution or by addition of chlorine to water previously dosed with iodide salts. Effectively treating potentially contaminated brackish water in field conditions by "hypoiodide" pill addition is well known to the public and extensively tested and proven by the military. US parks and many small private well sources routinely use iodine as the main disinfectant.

The resulting slight taste/odour change, which is different from that of chlorine, is reasonably well tolerated. However, the cost of iodine is higher than that of chlorine or hypochlorite and the difficulties associated with handling iodine, which is a solid, appear to have discouraged widespread use.

Recently, some persistent coliform contamination problems with the (unfiltered) chlorinated municipal water supplies at Gander, and at Botwood, Newfoundland, were reported to have been resolved using combinations of small amounts of iodine with chlorine as disinfectant. In addition, reports from the Canadian Forces Base at Borden, Ontario, suggested that use of this mixed iodine/chlorine disinfectant over many years has eased maintenance of a much oversized and antiquated distribution system.

The principal effects observed at Borden are that a more persistent residual was detectable and hardened deposits on the pipe walls appear to be softened enough so high velocity flushing at 1 to 1.5 m/s appeared capable of washing them out. No significant taste or odour changes occurred as iodine levels were held at around 0.1 mg/L.

The potential value of a chemical mix which could help delay or avoid pigging and lining or replacement of water mains is very large; in Ontario it is certainly in the eight figure range in $/year depending on the level of effectiveness. Furthermore, the combination of chlorine and much smaller amounts of iodine removes the economic and odour/taste objection to iodine use. Also, the equipment used in the above locations for iodine addition appeared simple, robust, easy to control and only of modest cost. However, in all the above locations, all the water supplied was iodinated from a set start date. In this type of situation, direct comparisons of system behaviour with and without iodine are difficult to separate from other changes due to factors such as seasonal water composition changes.

Since the potential for cost savings was so large, the Ontario Ministry of the Environment (MOE) took the initiative to investigate the effectiveness of iodine for distribution system rehabilitation. To do a valid investigation required the owners of a distribution system as a partner for MOE. Tay Township kindly volunteered in 1996 to provide the Port McNicoll distribution system as test area for the iodine application; this offer was made subject to citizen assent and with the permission of the Medical Officer for Simcoe County. (Tay covered much of the direct costs of the trial over 1996 and 1997 and the MOE and Iodine Technologies the remaining costs.)

On investigation, the MOE found that the Victoria Harbour/Port McNicoll water distribution system provided a potentially ideal test area. This is because iodine could be injected in the trunk main separating the communities, both of which are supplied from the same conventional plant in Victoria Harbour. Tracking changes in a community which is receiving iodinated water relative to another community which receives identically the same treated water but without iodine, allows for the elimination of factors related to seasonal and treatment differences.

To carry out a study in a scientifically valid manner required selection of a numerically quantifiable performance related parameter. Since mains which are overgrown and partly blocked by deposits show greater resistance to water flow, hydraulic resistance measurement was selected as the parameter of choice.

The next problem issue was to arrange for suitable health protection measures. Surprisingly, there were no long term epidemiological studies of a large population exposed to iodine(0). As a consequence Ministry toxicologists advised that, though they assessed the health risk of water containing iodine at 0.1 mg/L as slight, alternative non-iodinated supplies should be made available to the population. Tay personnel agreed to do this by fitting a publicly accessible tap upstream of the iodine injection equipment. Citizens were also fully informed on how to reduce their iodine intake to compensate for iodine in the water.

The Simcoe Medical Officer offered assent and followed up to support the activity through participation with Tay and the MOE in planning publication of announcements and by having staff attend public meetings in Port McNicoll to explain the plans and allay unnecessary fears. Public response was judged acceptably positive in part probably because the treatment was a possible alternative to a costly and disruptive water main replacement program.

Measurements of "baseline" hydraulic main resistance were made at six locations in Port McNicoll and at a control location in Victoria Harbour by ministry staff during early summer, 1996. Flows from fire hydrants were measured with a magnetic flow meter and data logger while a sensitive differential pressure gauge connected to hose outlets at pairs of upstream houses was used to indicate flow resistance.

Injection of iodine was planned to occur over 90 days during spring and summer of 1996 using equipment purchased by Tay from Iodine Technologies Inc. (Iodine Technologies is the Ontario company which supplied iodine equipment to Borden, Gander and Botwood.) The hardware installed included a chlorine booster station and an iodine addition station about 400 yards downstream of the iodine injection location on the trunk main.

These installations were fitted by Simcoe Engineering who acted under contract to Tay. Difficulties were experienced with control of the addition of iodine due to very low water velocities and the trial had to be abandoned for a year.

During 1997 iodine injection was re-established using a saturator and a positive displacement pump. The pump was operated on a flow paced basis by using signals from an existing turbine type water meter and compensation was effectively made for the very low water velocities with this set-up. Iodine levels were monitored intensively by MOE using a newly developed and precise plasma/mass spectrometry-based analytical method specially devised for the trial.

In all, the system ran reliably with iodine levels held constant in the 100 to 120 microgram/L range for over 90 days over a period of about four months. Attempts to have a citizen monitoring team use an on-site colourimetric iodine measuring method as described in Standard Methods proved unsuitable for use at these low concentrations.

What was observed? Little or no trace of the hoped for freeing up of pipe deposits was noted during routine summertime flushing from fire hydrants. On retesting the mains hydraulic resistance in fall 1997 at the remaining four accessible McNicoll sites and the Victoria Harbour site, no significant improvement was noted. A very slight slowing of the rate of deterioration of pipe condition in the iodine treated sections relative to that in the non-treated areas may have been detected.

A local citizens' opinion survey run by Tay in November also suggested little or no water quality change as perceived by the general public. Attempts to swab mains in Port McNicoll by experienced A-1 Hydrant staff immediately after the iodine addition period were also unsuccessful indicating that sufficient softening of deposits for easy mechanical cleaning had not occurred.

On the basis of the results, Tay has started a program to replace the most break-prone sections of the Port McNicoll water mains system. The MOE Water and Wastewater Optimization Section which coordinated and carried out the scientific assessment has concluded that further work on a iodine-chlorine combined disinfectant for main rehabilitation is not presently merited. Potential use of the mixed disinfectant in clearing persistent coliform contamination remains to be tested, but there are no plans by MOE to do so.

Iodine Technologies remains interested in treating water with iodine in more difficult conditions and in following up on reasons for effectiveness of the iodine treatment in three locations but not in a fourth.

The cooperation and assistance of the very professional operators from the Ontario Clean Water Agency in Victoria Harbour and the enthusiastic support provided by Tay Township management and staff during this project is gratefully acknowledged as are the persistent efforts of the volunteer "Port" citizen monitoring panel.

In addition, the work of the ministry Laboratory Services Branch staff is gratefully acknowledged. Lab staff devised the required new and precise analytical procedures for trace amounts of iodine and its by-products and also accepted the burden of routine monitoring when on-site methods had been found unacceptable.

By no means the least notable contribution to the successful completion of the project was the commitment and persistence of the Iodine Technologies staff who, among other activities, spent many uncomfortable hours "underground" and, later, followed through professionally and without excuses at the final analysis of this project.