NSF 350 - Evaluation of Log Reduction Targets

Some states and local jurisdictions have already mandated evaluation of these systems using a risk-based framework, including the state of Colorado, and the cities of San Francisco, CA and Austin, TX. Further, California, Washington, Alaska, Hawaii, and Minnesota are progressing toward adoption of similar regulations or policies with Texas, Ohio, Oregon and Alaska eyeing similar steps in that direction.

Risk-based frameworks for the evaluation of onsite non-potable water systems is based on a log reduction target and crediting system. Log reduction is a mathematical expression of the relative number of organisms eliminated by given treatment system. A 1 log₁₀ reduction indicates inactivation of 90% of target microbes, a 2 log₁₀ reduction indicates 99% removal, a 3 log₁₀ reduction indicates 99.9% reduction, and so on.

Risk-based frameworks used to evaluate onsite non-potable water reuse systems specify minimum log-reduction targets (LRTs) for pathogen removal based on the source water type and proposed end-use applications. Log reduction credits (LRCs) are assigned for each specific process used in the treatment train based on the validated log-reduction capability of the individual process and the ability to continuously monitor surrogate parameters for ongoing system performance. To demonstrate adequate treatment and public health protection, the sum of the LRCs must meet or exceed the specified LRTs required.

NSF/ANSI 350 has been updated to include optional criteria that incorporate a risk-based framework using log-reduction targets (LRTs) and log-reduction credits (LRCs). The updates are aligned with the recommendations from the National Blue-Ribbon Commission on Onsite Non-Potable Water Systems and are a component of EPA’s Water Reuse Action Plan (Action 2.18). NSF can evaluate onsite non-potable water systems according to this framework and verify that systems meet the minimum log reduction targets for pathogen reduction as specified for its applicable end use. These optional criteria apply to commercial and multi-family systems only and systems must additionally comply with the standard requirements in NSF/ANSI 350, including product testing.

Systems that are evaluated and found to be compliant with these optional criteria will be authorized to bear an endorsement on their product marking and public product listing.