In this study, it was aimed to decrease organic matter and nutrient loads from the waters joining the drinking water reservoirs like creeks and surface flows by a pilot scale serial-operated constructed wetland (CW) system. The serial system consisted of a horizontal-subsurface flow (h-SSF) bed vegetated with Canna, Cyperus, Typhia, and Juncus in the first stage, a free water surface flow (FWS-1) bed vegetated with Salvina, Pistia, and Lemna in the second stage, and a free water surface flow (FWS-2) bed vegetated with Elodea and Egeria in the third stage. Organic matter and nutrient removal performances in different hydraulic and loading rates were examined and contributions of plants on treatment were also investigated. Planted systems operated at the same loading rates have higher performances than unplanted systems. The results showed that the serial system increases removal efficiencies from 67% to 84% for BOD, 56% to 73% for COD, from 41% to 77% for TN and from 40% to 59% for TP. The results of this study indicated that a serial-connected constructed wetland system had a potential for BOD, COD, N, and P removal and the treatment of tertiary treated wastewater.