Pilot-plant experiments were conducted for on-site water treatment along a canal in Bangkok to evaluate the performance of a rock-bed filtration system under different operating conditions. Three different rock sizes with approximate equivalent diameters of 10 cm, 15 cm, and 20 cm were used as filter media in two rectangular reactor units (5.04 m long, 0.5 m wide, and 0.75 m deep). A diffuser network of 1.25 um diameter PVC tubes was installed at the bottom along the length of each reactor to provide aeration. The reactors were fed with raw canal water by a pump through a head tank unit to vary the hydraulic retention time (HRT) from 3 to 13 h. During the 15 months operation, the HRT, filter media size, and aeration locations were changed in five test runs consisting of ten experiments to analyze the filtration mechanism and to determine the optimum operating conditions. The results showed that rode-bed filtration with aeration and a HRT of more than 6 h could successfully improve canal water quality as indicated by reduction in concentrations of suspended solids from 60-120 mg l(-1) to 20-40 mg l(-1) total COD from 50-100 mg l(-1) to 20-50 mg l(-1), and total BOD from 15-30 mg l(-1) to 5-20 mg l(-1). Removal efficiencies for the particulate water quality indices (SS, total and particulate GOD) were much higher than for the soluble ones (soluble COD and BOD). This implied the main removal mechanism to be the sedimentation of the particulate matter. In addition, the aerobic decomposition by the biofilm formed at the surface of the rode-bed assisted in the reduction of soluble organic matter. A combination of three rock sizes arranged in decreasing order along the reactor length showed best results for pollutant removal. The effluent water quality improved with an increase in HRT up to 9 h and showed only minor improvement thereafter. The porosity of rock beds was reduced by approximately 16% over 15 months operation. The solids content in the sediments increased during operation leading to some compaction of the sediment mass. Also, there was high decomposition of the organics in sediments ranging from 60 to 70% as indicated by the decrease in the volatile solids fraction in the sediment over the experimental period. However, the sediments mainly consisted of inorganics and therefore the overall reduction in sediment mass deposits was only 15-25%.