The purpose of this study was to evaluate the hydraulic performance of a full-scale on-site vertical-flow biofilter, consisting of a mixture of peat and carbon-containing ash, and a 500 m3 equalization pond prior to the filter-system. The treatment plant was constructed to clean up leachate from an industrial mono-landfill that contained shredder residues of end-of-life vehicles and white goods. With the limited storage capacity of the equalization pond, peak loading rates exceeded up to five to six times the designed daily hydraulic load limit of the biofilter system. Such relatively short overloading events did not negatively affect the purification efficiency. To provide the designed annual irrigation rate on the biofilter of 97 m3 day— 1 (or 133 mm day—1), with large seasonal variations in precipitation, a relatively large pond would be needed. Calculations showed that a storage volume of about 23 000 m3 would be sufficient for annual leachate volumes up to about 35 000 m3. A combination of sprinkler and drip irrigation with straw insulation of the latter made it possible to run the plant continuously even when the ambient air temperature was below zero for more than a month at a time. The grain size distribution of the biofilter medium was noticeably changed after 4 years of usage due to the loading of suspended solids from the leachate and decomposition of the peat, causing reduced hydraulic conductivity.