Test on Age of Activated Sludge Zhou FanSludge age used as a parameter in design and operation has more advantagesthan others. But the relationship between θ_s and the parameters of plug-flowsystem has been scarcely reported. Results obtained from laboratory tests onsludge age using monosodium glutamate as base medium are presented in thepaper. Plug-flow aeration was simulated in the test. It is shown by the test resultsthat the relationship between θ_3 and related parameters is approximately thesame as that in the test by using complete mixing activated sludge with aerationtime of 4-6 hours generally. In this test condition, the minimum sludge age,θ_3~m, has been found to be 0.55～0.85 days, and the ratio of oxygen absorption ratesof sludge at the begining and at the end of aeration has been found to be 4approximately.