EQUIPMENT AND SUPPLIES--Sterilization of Microbiological Equipment 

In Reply Refer To:                        August 30, 1985 
WGS-Mail Stop 412


Subject: EQUIPMENT AND SUPPLIES--Sterilization of 
                                 Microbiological Equipment

Recent Water-Quality program reviews have indicated a 
recurring problem in some Districts--the use of non-approved 
methods for sterilization of bacteriological filter 
assemblies; specifically, ultraviolet irradiation.  The 
U.S. Geological Survey's approved method is described in 
section 7.3, page 37 of TWRI, Methods for collection and 
analysis of Aquatic Biological and Microbiological Samples.  
It requires autoclaving for 15 minutes at l2l degrees C at 
15 psi.  Incomplete combustion of methanol, to produce 
formaldehyde gas, is permitted for field sterilization, 
although autoclaving in the laboratory is preferred.

The justification most often given for using UV irradiation 
instead of autoclaving is that the Environmental Protection 
Agency (EPA) lists ultraviolet radiation, among other 
techniques, as a method of sterilization acceptable to them 
in Microbiological Methods for Monitoring the Environment; 
Water and Wastes, 1978.  This manual however, is designed for
use by trained microbiologists operating in a laboratory 
setting.  And these alternative methods of sterilization are 
usually used as adjuncts to primary sterilization to maintain 
sterility in hoods and glove boxes.  That UV irradiation does 
not guarantee sterility is shown in the EPA manual on pages
198 and 199.  Quality control of UV Units requires frequent 
monitoring with a shortwave UV meter and at least quarterly 
measurement of effectivness on a coliform culture.  The 
manual states, "UV-irradiated plates should show 99% 
reduction in the count of the control plate.  If count 
reduction is less than 80%, replace lamp."  Further, users of 
UV units should be aware of the safety hazards attendant to 
their use.

The special media we use for culturing fecal coliform and 
fecal streptococci promote the growth of the microorganisms 
we are seeking and inhibit or kill all others.  This is why 
we can get away with performing filtrations in the field 
under grossly unsanitary conditions.  This tolerance of the 
media however, must not be abused.  We must assure that our
glassware and everything else that we can control, that comes 
in contact with the sample, is sterile--not almost sterile.

If cost of multiple stainless steel filtration units is a 
problem, Districts can use the polycarbonate plastic units 
made by Nalgene and other companies.  They can be autoclaved 
and sealed in kraft paper bags, used once at the field site, 
and returned to the laboratory for autoclaving.  They are 
cheap enough that dozens can be taken on an extended field 
trip if necessary.

The sampling and analytical methods described in our TWRI's 
are the methods currently approved for use in obtaining data 
that will be placed in WATSTORE.  Our data users, both within 
and outside the Survey, have a right to expect that the 
methods we say we use in obtaining data are the methods
we actually use.

                        David A. Rickert
                        Acting Chief, Quality of Water Branch

Distribution: A, B, FO, PO

This memorandum does not supersede any other memorandum