An extended aeration sewage treatment plant on an island camp in Georgian Bay performed fair to well for BOD, TSS and TP removal, but was unable to nitrify ammonium to compliance levels for surface discharge. The raw sewage is from residential and kitchen sources, but is often high strength (cBOD >500 mg/L and TKN >100 mg/L), and the natural alkalinity is low.A proposal was made to retrofit a Waterloo Biofilter absorbent trickling filter to polish the treated sewage, similar to polishing lagoon effluent. The influent mass rate to the BIOFILTERS for design purposes was 4.0 kg/d NH4-N and 0.6 kg/d cBOD (influent design peak of 160 mg/L NH4-N and 15 mg/L cBOD at 25,000 L/d). The design criteria for the effluent out of the BIOFILTERS were 0.075 kg/d NH4-N for compliance (3.0 mg/L), and 0.025 kg/d NH4-N (1.0 mg/L) as a target.Performance guarantees were requested but field experience polishing this particular wastewater type (low BOD and high NH4-N) at the required high hydraulic loading rates was not available. To predict how this technology would perform, empirical relationships of influent and effluent data of two different wastewaters were examined, with the intent to interpolate to the expected conditions at the camp and to predict performance.Existing polishing data from low-strength secondary clarifier effluent at high hydraulic rates (<1700 L/d per m3 filter medium) indicated sustainable and complete nitrification. The same could be done with 15 m3 filter medium at the island camp, easily placed in the limited space available. From this data, an effluent discharge of 0.0013 kg/d NH4-N was predicted at peak flow, less than the target.From operational data at a second, high-strength, surface-discharge BIOFILTER, an effluent discharge of 0.013 kg/d or 0.5 mg/L NH4-N was predicted, also within target limits. With this performance prediction, the BIOFILTERS were installed in 2002 and the results for BOD, TSS, and NH4-N were all within compliance limits and all but one month within target limits. During the 2003 operational season, all above noted parameters were within target limits. Analysis of empirical relations from operational data from two dissimilar wastewaters such as low BOD/low NH4-N and high BOD/high TKN can successfully predict the performance of a retrofitted Polisher on a third wastewater with low BOD/high NH4-N, suitable for surface discharge of the polished effluent