|
Bacteria (viable)
Bacteria can grow in any room temperature water system given some
organic carbon (above 0.5 ppb) and can proliferate above a level of
5 ppb. Bacteria occur as single-celled organisms or as aggregates of
many cells bound together by a gelatinous, polysaccharide matrix.
Bacteria may be as small as 0.2um by 0.5um or as large as 2um by 10
um dependent on environmental conditions. In ultra-pure water
systems, the smaller sizes are usually seen under high flow
conditions and the larger sizes are usually seen under stagnant
water conditions.
Low flow rates are defined as below 5 linear feet per second. Flows
below this number are automatically destined to have a significant
number of bacteria and the systems with these low flows will need
cleaning much more often than systems with flow-rates above 5 linear
feet per second. The ultimate low flow situation is the dead leg
which pipe with no continuous flow that is greater than six time
its’ diameter. A ¾ inch pipe diameter will be a dead leg if it is 4
inches or longer.
When you get to pipe diameters below ½ inch, even 5 linear feet per
second cannot keep the pipe clean. These thin bore pipes or tubing
can be a serious source of contamination if they are not flowing a
significant portion of each day. Spin rinser driers in the
electronic industry and automated diagnostic equipment in the
medical device/diagnostic industries are particularly susceptible to
this type of microbial contamination. The contamination of these
pieces of equipment can be due to low equipment usage or inadequate
flow-rates leading to the equipment. Routine cleaning of the
equipment may not have the desired results of reducing microbial
contamination. Instead practical experience i.e. trial and error is
the only way to determine the optimal flushing regimen for this type
of tubing in equipment.
|
