A method includes: disposing a filter into a body of wastewater containing active sludge, the filter including a supporting member having an outlet, and a filter media enclosing the supporting member and supported by the supporting member so as to have a predetermined firm shape, the filter media including a non-woven substrate and a fibrous filtering layer attached to the non-woven substrate and having a pore diameter ranging from 0.001 to 15 .mu.m and a layer thickness ranging from 10 to 200 .mu.m, the fibrous filtering layer being made from fibers having a fiber diameter ranging from 20 to 1000 nm; and driving the wastewater to pass through the filter using a pump connected to the outlet of the supporting member of the filter.