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The use of effluent filters is increasing. But, a greater understanding of the septic tank-effluent filter primary treatment system and a means of quantifying effluent filter performance under field conditions is needed. In this paper, observations about septic tank effluent particle sizes and site specificity are discussed. Also, two approaches are described which were used to assess filter performance under septic conditions. One approach involved the use of smaller sample sizes and total suspended solids (TSS) assessment, and the other used a solids sampler that retains large solids from an entire flow through a sampling period. Small samples and TSS assessment showed inconsistent filter effects. This was likely due to many particles being too small to filter during many sampling events, high variability in flows, small sample sizes not representing the overall flow, and sloughing of effluent solids from the filters themselves confounding the TSS assessment. Using the solids sampler, however, showed a significant difference in filtered and unfiltered effects in a replicated system. Filtered and unfiltered effluent means (n=12) were 2.52 and 15.5-g, respectively, and were significant at the 0.01 level. Effluent filters are beneficial at consistently retaining large solids and thus enhancing primary treatment. Residential septic tank flows can be as high as 41,000-l per month. Residential flows can be highly variable over time in terms of volume and waste composition. Using the latter means of sampling, large volumes of effluent are assessed and the fewer larger particles that the filter was designed to prevent from leaving the can be captured and quantified. Effluent filters with surface access for maintenance are recommended to prevent large solids from affecting down stream components.