Some particular considerations regarding water quality characteristics and their implications for challenge testing are as follows:

• High quality water or "particle–free water" with a low concentration of suspended solids (e.g., membrane filtrate) should be used as the matrix for the challenge solution, minimizing the potential for formation of a fouling layer during the challenge test that would enhance removal of the challenge particulate.

• No oxidants, disinfectants, or other pretreatment chemicals should be added to the test solution unless necessitated by process requirements (e.g., acid addition and/or scale inhibitor which may be necessary with NF/RO processes).

• If the challenge particulate is a molecular marker, the water quality of the matrix for the test solution should not interfere with the introduction, dispersion, or measurement of the marker. Thus, the impact of water quality parameters, such as pH and ionic strength, on the chemical characteristics and speciation of molecular markers should be considered in the design of the test solution. This is particularly critical for NF or RO membrane modules, which can concentrate organic and inorganic solutes, potentially interfering with some molecular markers.

• If a microbial challenge particulate is used, it may be necessary to add buffers or other materials to maintain the viability of the organisms. Any additives used must not interfere with any aspect of the test or result in a change in the concentration of the challenge particulate over the duration of the test. In addition, because water quality parameters such as pH and ionic strength can affect microbial aggregation, these solution characteristics should also be considered.

• It is recommended that the challenge test solution be characterized with respect to basic water quality parameters, such as pH, turbidity, temperature, total dissolved solids (TDS), TOC, and any others that are critical to the test or interpretation of the results.