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Do you have any recommendations for buying a UV water system for the cottage? I want to draw my drinking water from the lake instead of bringing it with me.

Tom Howell, Duck Lake, Ont.

First of all, lets go back to biology class. The UV light destroys microorganisms by gluing parts of their DNA together, which prevents the double-helix strand from unzipping for replication. No DNA replication means no reproduction.

Your UV system should have a flow rate of eight gallons per minute (this size will suit most cottages), a lamp strength of 40 millijoules/cm2 or more, and a light output of 254 nanometres. Look for systems with an NSF 55A or a CSA B483 certification, says Jerry Capko, manager of the safe water program with the Simcoe Muskoka District Health Unit.

Youll need to pre-filter the water, though. The UV is there for one thing only: to keep you safe from water-borne illness, says Larry Miller of DSMI/Passport Water Purification Products in Thornton, Ont. If the water isnt clear enough, the light cant penetrate the viruses and bacteria.

At minimum, youll need a five-micron sediment filter to reduce turbidity from dirt and algae, says Scott Macdonald, president of Envirogard, a company that manufactures water filters and purifiers in Richmond Hill. You may need other filtersfor example, a carbon filter to remove tannins that darken water (colour blocks the light), or a greensand filter for iron and manganese, which can stain the quartz sleeve covering the units bulb and reduce the systems effectiveness.

A typical UV system costs between $500 and $1,500 (the more expensive ones include alarms that tell you when to replace the bulb). Its worth installing electrical protection (surge protector, voltage regulator, uninterruptible power source) to handle power fluctuationsthats about $60 to $100. Tack on $20 to $50 for a sediment filter, and up to $2,000 for additional pre-treatment. Youre probably saying, Never mind, Ill just drink beer. Dont worry: Many people who use UV need only the sediment filter, say our experts.

Capko has a tip: If lake neighbours use a UV system, find out how they pre-treat the water, and ask to see their sample results. Everybodys straw is in the same trough. If theyre not turning green and growing a second head, its probably working out for them. June 2011

We have always drunk the water directly out of the lake in front of our cabin, which is 320 km northeast of Winnipeg. Last year, I installed a 500-gallon water tank outside the cabin. We filled the tank in the spring, and in August, when I was going to fill it again, I saw that great gobs of algae and other green slime had grown in it. Would the green stuff growing in the tank make the water poisonous to drink, or just unsavoury? The water is filtered through a small screen, does not smell, and does not taste any different. Is there something non-toxic that I could put in the water to keep the stuff from growing? This is not critical if the water is not unhealthy, but it would be nice to have the water look clean.

James Thacker, Windsor, Ont.

The short answer is, dont drink the water. There are species of algae, especially blue-green algae, that release toxic chemicals when they break down, explains Jim Bishop, vice president of Environment Protection Laboratories and a former director of the Ontario Ministry of Environment and Energy (MOEE) Water Resources Branch. Bishop also recommends against drinking the water after it has been treated with a product that would be strong enough to keep the algae from growing.

However, the solution to preventing algae growth may be as simple as keeping light out of the tank. Algae is the most primitive form of vegetation and relies on photosynthesis to live, Bishop explains. Since algae is already growing in the tank, before you use it again youll need to drain the tank and give it a good scrub with an algaecide. You will likely also have to replace the connecting waterlines, which will be contaminated and which, by the way, should be a black PVC or rubber hose (rather than clear tubing) so they too keep out the light.

Even once youve got rid of the algae, its important to keep in mind that to avoid contamination by bacteria, the MOEE recommends that all surface water be disinfected before being used for drinking water. However, it would be an impossible job for you to disinfect your tank and the water in it. Your safest and easiest route is to disinfect the water by boiling, chlorinating, or using a water-treatment device after the water leaves the tank and before you drink it, says Judy Patrick, a drinking water specialist in the ministrys Program Development Branch. She also advises using a filter of 5 microns or less to guard against parasitic contamination by organisms such as Giardia lamblia, which is spread by animals and is most prevalent in northern lakes. August 1995

When I received the results of having my water tested, I read in the accompanying note that water tested by the local health unit is checked bacteriologically only. That suggests, to me, that there may be something else in the water. Should I be worried?

Allan Best, Markham, Ont.

There may be other things in the water but, according to the Ontario Ministry of Health, most cottagers neednt be worried about them. When you send a sample to your local health unit, a provincial lab tests it for two things: total coliform counts and Escherichia coli (E. coli), a type of fecal coliform. If elevated levels (above five counts per 100 ml) of coliforms, or any E. coli, are detected, your water will be deemed unsafe for consumption. Of all the things that could actually be a problem for cottagers, the ones to worry about the most are the two that you get tested for free, says Jim Bishop, a water chemist and president of the environmental consulting firm Beak International in Brampton, Ont. Other bad guys, such as water-borne pathogens like Giardia and Cryptosporidium, are more likely to be present in samples where elevated coliforms are detected.