Abstract: The effect of multi-stage constructed wetland-pond system in treating contaminated water with different hydraulic retention time (HRT) is researched. The pollutants in effluent water are tested for comparison. Results show that in early spring with low temperature, the removal rates of COD, TN and NH₄⁺-N increase significantly with the increasing of HRT. Under the premise of ensuring the treatment effect, four-day HRT is reasonable. Twelve months of test with four-day HRT were conducted, and the test results show that as the seasons change, the purification effect of multi-stage constructed wetland-pond system fluctuates within a reasonable range: the COD, TN and NH₄⁺-N removal rates in summer are better than those in winter. From the test results we conclude that the present system could achieve satisfactory result in removing contaminants, with the best quality of COD-removed effluent water reaching grade IV, and NH₄⁺-N-removed effluent water close to grade IV of surface water.

Key words: multi-stage constructed wetland, pond, hydraulic retention time, water contaminant, nitrogen removal rate