Water Resources--Office of Water Quality
This document is also available in pdf format:
Add sodium thiosulfate (Na2S2O3) to sample bottles before sterilization if the water to be collected contains residual chlorine or other halogens added for disinfection. Residual chlorine can be found in samples collected from sources such as treated potable-water taps, in effluents, and surface-water samples collected from the mixing zones of wastewater-treatment plants. A 10-percent solution of Na2S2O3 is prepared in the following manner. In a volumetric flask, dissolve 100 g Na2S2O3 into 500 mL of deionized or distilled water; stir until dissolved, and fill flask to 1,000 mL (Bordner and Winter, 1978, p. 6; American Public Health Association and others, 1992, p. 9-18). Add 0.1 mL of 10-percent Na2S2O3 solution for every 100 mL of sample. Keep Na2S2O3 refrigerated and in a dark bottle; after 6 months prepare a fresh solution.
Add ethylenediaminetetraacetic acid (EDTA) to sample bottles when water to be collected contains trace elements such as copper, nickel, and zinc at concentrations greater than 10 mg/L (Britton and Greeson, 1989, p. 5-6; Bordner and Winter, 1978, p. 6; American Public Health Association and others, 1992, p. 9-18). A 15-percent solution of EDTA is prepared by dissolving 372 mg in 1,000 mL of distilled or deionized water. Before sterilization, add 0.3 mL of the EDTA solution per 100 mL of sample to sample bottles. EDTA can be combined with the Na2S2O3 solution in the sample bottle before sterilization.
Sterilize the filtration apparatus between sites or for each sample collected at the same site at different times. Autoclaving is the preferred method of sterilization. Use only autoclaves that have temperature, pressure, and liquid- and dry-utensil-cycle controls. Steam sterilizers and vertical autoclaves are not recommended because the temperature cannot be held constant.
Take care to ensure that materials to be autoclaved, such as tubing and containers, are thermally stable. Polymers (such as polycarbonate, polypropylene, polyallomer, and polymethylpentene) and Teflons and Tefzel (such as perfluoroalkyoxy-polymers or PFA, ethylenetetrafluoro-ethylene or ETFE, fluorinated ethylene propylene or FEP, and polytetrafluoroethylene polymers or PTFE) can be autoclaved. Each has different thermal characteristics and tolerances to repeated autoclaving.
Only Millipore Hydrosol field filtration units are designed to be flame sterilized with methanol. Formaldehyde gas, a by-product of methanol combustion, kills all microorganisms in the unit. The following sterilization procedure is acceptable for the Hydrosol unit in field situations where other sterilization techniques are not practicable (Millipore, 1973, p. 48-49).