The performance of rapid sand filters was evaluated in three tertiary wastewater treatment plants in the State of Kuwait. These plants are located at Ardiya, Rikka and Jahra, and receive municipal wastewater flows of 220,000, 95,000 and 42,000 m(3) d(-1), respectively. The Ardiya plant uses a two-stage activated sludge process for the secondary treatment of wastewater whereas both the Rikka and Jahra plants use the extended aeration process. Daily records were obtained from each plant over a period of 1 year, and the efficiency of the tertiary sand filters was determined based on reductions in SS, VSS, BOD, and COD. Analysis of these records showed that the secondary-treated effluent quality is highly variable. Seasonal variations were observed due to nitrification and denitrification that enhance the production of nitrogen gas and carry over of sludge solids in the effluent during summer, causing more frequent backwashing of the filters. The data were also statistically analyzed using the ANOVA program. The results obtained indicated significant improvements, at 95% and 99% significance levels, in solids (SS, VSS) and organics (BOD, COD) removal by sand filtration. They also showed that, in addition to improving effluent quality, the tertiary filtration played an important role in the stability of effluent quality so as to dampen variations in the quality of secondary-treated effluent. The tertiary effluent consistently satisfied the water quality requirements for irrigation.