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The results of laboratory column tests conducted to assess the effect of the mass loading on the clogging of porous media are presented. The tests were conducted using actual leachate from the Keele Valley Landfill under saturated, anaerobic conditions. It is shown that clogging is greatest where there is the greatest mass loading (near the inlet in this case, but likely near the collection pipes in a field situation). An empirical relationship between the hydraulic conductivity and drainable porosity is presented. Even though it is shown that higher flow rates give rise to less efficient bioreactors, the columns with high flow still experience greater rates of clogging than those with low flow. The columns were found to be severely clogged when the drainable porosity had decreased to about 10% of the initial value. The bulk (wet) density of the clog material is found to range between 1.6 and 2 Mg/m3 and, on a dry mass basis, 27% of the clog is calcium and 47% is carbonate. The columns were colonized by a diverse consortium of bacteria including methanogens, sulfate-reducing, and denitrifying bacteria, with methanogens being dominant in the portion of the column where clogging was most severe.Key words: leachate collection, clogging, porous media, mass loading, flow rate, anaerobic, microbial.