Section 930. Quantification of microbial inactivation

A. Microbial inactivation ("kill") efficacy is equated to "Log₁₀ Kill," which is defined as the difference between the logarithms of number of viable test microorganisms before and after treatment. This definition is equated as:

Log₁₀ Kill = Log₁₀ I(cfu/g) %68 Log₁₀ R(cfu/g) where:

Log₁₀Kill is equivalent to the term Log₁₀ reduction.

"I" is the number of viable test microorganisms introduced into the treatment unit.

"R" is the number of viable test microorganisms recovered after treatment.

"cfu/g" are colony forming units per gram of waste solids.

B. For those treatment processes that can maintain the integrity of the biological indicator carrier (i.e., ampules, plastic strips) of the desired microbiological test strain, biological indicators of the required strain and concentration can be used to demonstrate treatment efficacy. Quantification is evaluated by growth or no growth of the cultured biological indicator.

C. For those treatment mechanisms that cannot ensure or provide integrity of the biological indicator (i.e., chemical inactivation/grinding), quantitative measurement of treatment efficacy requires a two step approach: Step 1, "Control"; Step 2, "Test." The purpose of Step 1 is to account for the reduction of test microorganisms due to loss by dilution or physical entrapment.

1. Step 1.

a. Use microbial cultures of a predetermined concentration necessary to ensure a sufficient microbial recovery at the end of this step.

b. Add suspension to a standardized medical waste load that is to be processed under normal operating conditions without the addition of the microbial inactivation agent (i.e., heat, chemicals).

c. Collect and wash waste samples after processing to recover the biological indicator organisms in the sample.

d. Plate recovered microorganism suspensions to quantify microbial recovery. (The number of viable microorganisms recovered serves as a baseline quantity for comparison to the number of recovered microorganisms from wastes processed with the microbial inactivation agent).

e. The required number of recovered viable indicator microorganisms from Step 1 must be equal to or greater than the number of microorganisms required to demonstrate the prescribed Log reduction as specified in 9VAC20-120-910 (i.e., a 6 Log₁₀ reduction for vegetative microorganisms or a 4 Log₁₀ reduction for bacterial spores). This can be defined by the following equation:

Log₁₀RC = Log₁₀IC %68 Log₁₀NR

where: Log₁₀RC is greater than or equal to 6 for vegetative microorganisms and is greater than or equal to 4 for bacterial spores and where:

Log₁₀RC is the number of viable "Control" microorganisms (in colony forming units per gram of waste solids) recovered in the nontreated processed waste residue.

Log₁₀IC is the number of viable "Control' microorganisms (in colony forming units per gram of waste solids) introduced into the treatment unit.

Log₁₀NR is the number of "Control" microorganisms (in colony forming units per gram of waste solids) that were not recovered after processing. Log₁₀NR represents an accountability fad-or for microbial loss.

2. Step 2.

a. Use microbial cultures of the same concentration as in Step 1.

b. Add suspension to the standardized medical waste load that is to be processed under normal operating conditions with the addition of the microbial inactivation agent.

c. Collect and wash waste samples after processing to recover the biological indicator organisms in the sample.

d. Plate recovered microorganism suspensions to quantify microbial recovery.

e. From data collected from Step 1 and Step 2, the level of microbial inactivation (i.e., "Log₁₀ Kill") is calculated by employing the following equation:

Log₁₀Kill = Log₁₀IT %68 Log₁₀NR %68 Log₁₀RT, where:

Log₁₀Kill is equivalent to the term Log₁₀ reduction.

Log₁₀IT is the number of viable "Test" microorganisms-(in colony forming units per gram of waste solids) introduced into the treatment unit. Log₁₀IT = Log₁₀IC.

Log₁₀NR is the number of "Control microorganisms (in colony forming units per gram of waste solids) that were not recovered after processing.

Log₁₀RT is the number of viable "Test" microorganisms (in colony forming units per gram of waste solids) recovered in treated processed waste residue.

Historical Notes

Derived from VR672-40-01:1 § 11.10, eff. June 29, 1994.

Statutory Authority

Chapter 14 (§ 10.1-1400 et seq.) of Title 10.1 of the Code of Virginia.