Fast food restaurants typically produce a low volume grease-containing wastewater, generated by their daily kitchen activities, for which there is currently no acceptable treatment technology. This paper describes the performance of a novel bioreactor, the weir tank reactor, for the treatment of fast food restaurant wastewater at an organic loading rate (OLR) of 5 kg wastewater m(-3) d(-1). Two different mixed microbial cultures (one designated MC1, the other an activated sludge), were used together with fast food grease residues from two different sources (Woking, Surrey, and Birmingham, West Midlands). The reactor achieved a high (84-96%) removal of FOG (fats, oils and greases) irrespective of the microbial inoculum, the source of the FOG or the alkalinity (low or high) of the water. This high performance was attributed to a combination of the effective mechanical mixing regime and the periodic removal of a portion of microbial solids from the weir tank reactor liquor, in the form of an effluent, during the weir tank reactor studies.