Day: September 19, 2014

Friday, 19 September 2014

09:38 – With Barbara leaving at 0400 Sunday morning, I’m trying to keep her from exhausting herself before she leaves. These tours are often physically demanding, with lots of walking. Barbara said she’d stop at the supermarket on the way home from work, then make dinner, then go out and cut the grass. Tomorrow, she’s planning to clean house in addition to all the preparations for the trip. I’m trying to convince her to take it easy.

The second bottle of Polar Pure showed up in yesterday’s mail. The confusion could have been avoided if the vendor had simply stated that the two items would ship separately. Instead, it said they’d ship together in one package and they provided only one tracking number.

Incidentally, I was wrong about using strong Lugol’s solution for water disinfection. As it turns out, the triiodide ion is much less effective than free iodine. I should have remembered that, because I read Iodine Disinfection in the Use of Individual Water Purification Devices several years ago. That PDF is well worth reading if you have any interest in the topic.

The Polar Pure bottles are each supposed to contain 8 grams of crystal iodine. Polar Pure considers this amount adequate to disinfect 2,000 quarts/liters, which means an iodine concentration of 4 mg/L. I’m more comfortable at 8 mg/L or even 16 mg/L, so I think I’ll modify these Polar Pure bottles by adding more crystal iodine to each bottle and updating the instructions to achieve a final concentration in the 16 mg/L range. Alternatively, you can use an unmodified Polar Pure bottle simply by doubling or quadrupling the recommended amounts, which of course cuts the capacity down to 1,000 or 500 liters. Even at 500 liters, that’s still a 250 person-day supply at 2 L/day, which isn’t bad for that small bottle.

Even at the higher concentrations, Cryptosporidium remains a problem. Three interrelated factors affect disinfection effectiveness: iodine concentration, temperature, and contact time. Achieving even a 2-log reduction in Cryptosporidium requires by one source a CT of 1,015 mg-min/L, presumably at 20C. In other words, to kill 99% of the Cryptosporidium oocysts at 16 mg/L, the contact time required would be 1015/16 = ~ 64 minutes. At Polar Pure’s recommended 4 mg/L, a 2-log reduction takes more than four hours. Much better just to boil the water if at all possible.

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