The CHWQP is a regional biofilter located in Seattle, Washington that was designed to treat 153 Mgal of stormwater annually. One of the three monitored biofilter blocks was configured with a polishing layer to reduce phosphorus export from the compost-based filter media. The biofilter with polishing layer reduced influent TP concentrations by an average of 75.8 percent. TP reductions were -58.4 and 27.6 for the two biofilters without polishing layers. This is the first full-scale field test of a polishing layer to reduce TP export from a compost-based biofilter.