Why not Chlorination?

My brother Steve came to visit us in Ranquitte, along with my mother Lyn and a group of friends from our church back home (and other friends). He was gung ho to put in a water treatment facility for the school kids and the community.

The system he wanted to put in (by Living Waters for the World) was not the system that Christian Flights International wanted to put in (the McGuire water purification system), however, so he agreed to come and put in their system.

It became apparent to me, however, that CFI was focused on treating the water for those living on the campus, and those groups visiting the campus -- not the community, and not even for the students. I have to admit that this peeved me, and I set out on a quest to find a way to treat water for our students.

There were several suggestions that looked promising:

water pasteurization was one: simply heat water to 65 degress celsius for a few minutes (see the activity below). Traditionally this is done with wood or charcoal, however, and that's a no-no for Haiti.
Solar pasteurization was another. Ordinary soda bottles on tin painted black can be used to render water potable (SODIS -- using plastic bottles for water pasteurization).
I tried solar pasteurization via a parabolic collector. Didn't work too well (didn't get to the desired temperature), but I did get a nice warm shower out of it! This might be adapted to work, but in the end I decided that it was going to be too much trouble (what Haitian lady is going to want to be sitting around focusing her parabola all day?).

In the end, however, I elected to go with Biosand filters. Sometimes the solution is not the beautiful mathematical one, based on lovely parabolas, but rather the simple one based on brute force....

Now here's an activity that shows how heat treatment results in a "dilution" of nasty creatures. It's an example of exponential decay (which is the flip side of exponential growth). If you plot the curves that result, you'll know what I mean!

Activity

Thanks to: suggested by the web page at solarcooking.wikia.com/wiki/Water_pasteurization

Keywords: exponential decay, discrete dynamical system

Overview:

The chart below indicates the temperatures at which the most common waterborne pathogens are rapidly killed, thus resulting in at least 90 percent of the microbes becoming inactivated in one minute at the given temperature. (The 90 percent reduction is an indicator frequently used to express the heat sensitivity of various microbes.)

Microbe Killed Rapidly At

Worms, Protozoa cysts (Giardia, Cryptosporidium, Entamoeba) 55^oC (131^oF)

Bacteria (V. cholerae, E. coli, Shigella, Salmonella typhi), Rotavirus 60^oC (140^oF)

Hepatitis A virus 65^oC (149^oF)

(Significant inactivation of these microbes actually starts at about 5^oC (9^oF) below these temperatures, although it may take a couple of minutes at the lower temperature to obtain 90 percent inactivation.)

To do/Questions:

If 90% of microbes are killed in the first minute at the given temperature, how many microbes are killed in two minutes? three minutes?
How long will it take to kill 90% of Worms at 55^oC? How long will it take to kill 99.9% of them?
Write an equation that shows the percentage of microbes killed x_i at each minute, starting from x_o=0.
Draw a graph, showing percentages of Hepatitis A virus killed in the first 5 minutes if heated at 149^oF.
Harder: what percentage of microbes are killed in a half minute at these temperatures? (Hint: it's not 45%!)

Something to think about/discuss: suppose that "Significant inactivation of these microbes actually starts at about 5^oC (9^oF) below these temperatures", and that in fact it takes three minutes. Should one heat the water less and keep it hot for a longer period of time, or push to the higher temperatures and spend less time? What considerations will drive your decision?

Links:

SODIS -- using plastic bottles for water pasteurization
the McGuire water purification system
Living Waters for the World
Exponential decay of bacteria or viral populations by heating

Website maintained by Andy Long. Comments appreciated.

Microbe	Killed Rapidly At
Worms, Protozoa cysts (Giardia, Cryptosporidium, Entamoeba)	55^oC (131^oF)
Bacteria (V. cholerae, E. coli, Shigella, Salmonella typhi), Rotavirus	60^oC (140^oF)
Hepatitis A virus	65^oC (149^oF)
(Significant inactivation of these microbes actually starts at about 5^oC (9^oF) below these temperatures, although it may take a couple of minutes at the lower temperature to obtain 90 percent inactivation.)