Water Conditioning & Purification Magazine


Up or down with a rapid sand filter

Dear Editor:
Please, can you explain to me why using up-flow and down-flow in rapid sand filtration might or might not be recommended? How is it effective at reducing high iron content in raw water?

Binu Abraham
A water treatment student

Editor’s note: A member of WC&P’s Technical Review Committee informs us that down-flow is easier to engineer and lowers the cost. Also, down-flow is easier to backwash off the trapped particulate matter. There are no good reasons he knew of to use up-flow sand filters. High iron in the ferric oxidation state is not very water soluble and is removed by fine sand filtration. There you have it.

Your watershed and your behavior

Dear Editor:
 I work on a research team that’s designing a study assessing, in part, drinking water quality. We are behavioral, not environmental scientists, and I am writing to you with the hope that you will take a moment to answer a few basic questions we have about the assessment of drinking water quality.

1) If we want to determine a subject’s level of exposure to various toxins, will data from the local watershed, or from the various water systems within it be sufficient, or do toxins often contaminate a dwelling’s water supply on a more micro, house-by-house level? If so, which drinking-water toxins commonly vary between plumbing systems?
2) Are you aware of any publicly accessible databases or recent studies that provide detailed information about the levels of toxins within different watersheds and their component water systems?

Noah Susswein
Department of Psychiatry
University of Medicine and Dentistry
of New Jersey

Editor’s note: Two members of the WC&P Technical Review Committee responded to your query. One suggested that being more specific regarding your meaning of the word “toxin” might merit a more specific answer. If you mean health-related contaminants addressed by the Safe Drinking Water Act, they’re controlled by the municipal water provider; and this applies to the water treatment plant as well as the distribution system roughly to where the water leaves the street and enters the house. Toxins that may be produced by microorganism activity in drinking-water distribution systems have been poorly studied, and are generally not regulated. With the exception of lead (which may be in the household plumbing materials of construction), contaminants such as heavy metals and organics are likely to be in the raw water supply, and thus identified and removed at the treatment plant. Of course, households on individual wells are a completely different matter, although these water sources are almost always groundwater and less likely to be contaminated from human activity. This committee member wasn’t aware of any specific database of the sort you mention, but suggested you check U.S. Geological Survey for this information (see http://water.usgs.gov/).

Another review committee member said that lead from older systems and connections between pipes in the plumbing systems may contain lead, and was the first toxin to come to mind for him. An easy way to test this is to scratch the metal making the joint with a screwdriver. If it scratches and appears to be soft, it is likely to contain lead. Testing would be required to confirm that the material contains lead. Even though your plumbing materials may contain lead, you may not have a lead problem in your tap water. Many pipes that might precipitate lead may have a coating such that the flowing water does not have contact with the lead-containing pipe itself. If your water is acidic (lower pH), it increases the risk of lead leaching into your water. The only way you can determine if you have a lead problem is to do laboratory testing. The sampling protocol should have the worst case sampling and the best case sampling. The worst case consists of a sample from the tap after the water has set over night. If lead is leaching into the water this sample will have a high lead number. Also if this sample is low (below 1 ppb), there’s no need to test the best case sample. The best test sample is taken after the tap is allowed to run for 15 minutes. Lead is bad for young developing children, a rich subject for behavioral scientists.

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