I set up my first attempt at a biosand filter last night. Alas, it isn't flowing well, so I see minutes, even hours, of playing in the sand, trying to rectify this situation. Shucks...
In the mean time, I went to the internet, to see if there was a quick explanation of why things aren't working. What I came across was information that fine sand (3.5-1.5 mm average size, with a reasonable amount of uniformity) should work.
But I also came across big red additions to one page, warning that you can never know if water from a personal biofilter is actually really safe, particularly if it sources from a roof. The most worrisome issue for the webmaster, it turned out, was the tiny eggs that are found in the leavings of raccoons.
Coons have it great. There is a nematode/worm that lives in their gut (70-90% of coons are so affected). The coons aren't apparently bothered by this worm, but other species who ingest the eggs can suffer weakness and even death. Which means good eating for the fortunate coon that is able to overpower a weakened animal.
Sometime in the mid 1900s, a scientist opined that it would be possible for humans to be affected by this parasite. Since then a dozen or so cases of the illness have been documented, along with five deaths (according to Wikipedia) - allegedly all from toddlers who had played directly with raccoons or raccoon fecal material. It takes a couple of years to kill, but cannot be prevented unless treatment starts within 1-3 days of ingesting the coon poop.
It's a terrible thing for a small child to become weak and slip into a fatal coma. Because this is possible, should all people of all ages across the entire country eschew nature? No home gardens, no playing in the yard, no camping or walking or breathing? That's what it would take to completely and entirely avoid any consequences of injury from nature and the pollutants we humans have introduced to the natural environment around us.
Five deaths were caused by these raccoon-native nematodes over the past forty years. By contrast, road accidents kill about 40,000 people in the US each year. Falls kill 13,000. Drowning kills almost 4,000. Shall we therefore avoid roads, gravity, and water?
By all means, if you find your toddler playing pat-a-cake with a coon or munching on coon scat, do take them to the doctor. Wash their hands after they've been playing in the dirt. Explain to them that sucking on their filthy hands might cause slow brain death, so you'd rather they didn't do it.
But the chances are pretty low of your child getting to be one of the US's 2,000 annual toddler deaths (ages 1-5). And at that they are a thousand times more likely to die from the flu than from coon worms.
In the mean time, I am reminded of Jesus' saying, "Blind guides - you filter out gnats, but swallow camels!" [Matthew 23:24]
And tonight I'll work on why my sand filter is filtering so thoroughly that it's only passing a few ounces per hour, instead of the 10-20 liters per hour that is supposed to be possible.