The germicidal wavelength of UV (253.7 nm) does penetrate H2O, however it
is readily adsorbed by contaminants in the water compounded by the shielding
effect of particulate matter. UV treatment has received "bad press" over the
last 30 years basically because of the "cowboy" manufacturers who think that
it is as simple as bunging a lamp in a drainpipe. and quoting astronomically
high kill rates obtained from bench studies The biofilm effect observed by
David Jenkins compounds this. There are one or two good manufacturers out
there though. Who sensibly coat the lamps in transparent Teflon type
material to prevent clouding. I would be surprised if a standard UV system
would inactivate Cryptosporidium, the dose required is approximately
400mW.s/cm2 (typical UV systems put out around 35mW.s/cm2. A properly
designed UV system, however can completely inactivate even this tough
parasite (The Water recovery's WR 5000 system can put out in excess of
10,000mW.s/cm2) but beware of the cowboys!
Personally I would prefer my drinking water to be sterilised by UV and UV
alone, the only system that does not produce nasty bi-products (a trace
residual chlorine might be sensible)
Andrew Campbell
