Wu-Low Chi produces 9.7 m3/sec of average annual flow and 13800 kg/d of BOD5 load, is the most important pollution source of Kao-Ping Chi. Water quality of the Creek is so bad that average annual BOD5 concentration is 16.5 mg/L, and in the range of 17mg/L to 80mg/L in the dry season。In order to decreasing the pollution loads of Kao-Ping Chi, Environmental Protection Agency signed this project to purify the Water using an enforced nature purification process, saying contact aerated lagoon. The creek water was pumped into settling basin first, and then treated by a aerated lagoon which has 1600m3 of volume and packed with a lot of ropes as media. Four runs of experiment according to 31.6 hours ， 62.4 hours , 15.8hours and 12.2hours of hydraulic detention time (HRT) or their respective 3 g/m2/d , 0.9g/m2/d , 8.2g/m2/d and 5.8g/m2/d of media BOD5 loads had been finished , and results are shown on the following： Hydraulic detention time should be not greater than 16hrs and the total removal efficiency of SS, COD and BOD are 63% , 57% and 70%,respectively ；Hydraulic-surface area load model is employed and got BOD Model ＝＞ COD Model ＝＞ Where LR is removed mass per unit volume of lagoon , g/m3/d；LS is organic load per init surface area of media , g/m3/d ；and QH is HRT of lagoon ,day .