Channel Experiments on Coastal Water Purification by Porous Bed Using Crushed Stones

Large-scale model channels (30m long × 1m wide × 1.3m deep × 6 channels) were constructed for contact purification experiments for coastal saline water. Each channel was filled with crushed stones which were expected to be coated with natural biofilm. Effects of sedimentation and biological oxidation were observed for different hydraulic retention time (HRT: 1-5 hours) and different stone size (mean d: 10-100 mm). Hydraulic characteristics were also measured. The following was clear after five months' observation in autumn and winter, (a) biofilm was developed for saline (S: 1-2 %) and low COD (COD: 1-5 mg/l) influent, (b) SS was removed by 50-60 % though TOC by 10-20%, (c) effluent quality was clearer with longer HRT and smaller stones, but total removed weight was maximum at the channel of shortest HRT due to the high hydraulic loading(12m³) (d) permeability ranged between 5-20cm/s for bare stones and became smaller as clogging developed, (e) inclination of water surface was a good index for clogging detection.