Domestic　sewage　treatment　is　a　hot　research　topic　in　recent　years.　The　testing　methods　for　the　quality　of　treated　water　are　usually　based　on　chemical　techniques,　which　are　more　accurate,　but　are　time-consuming,　complex　operation,　and　may　lead　to　secondary　pollution.　In　this　paper,　we　propose　an　electrical　method　to　detect　domestic　sewage　on　different　stages　of　water　treatment.　A　portable　inspection　system　based　on　coplanar　capacitive　sensor　is　designed.　The　system　includes　data　acquisition　module,　data　conversion　module,　system　control　module　and　data　display　module.　In　order　to　verify　the　effectiveness　of　the　designed　system,　a　comparison　experiments　are　completed　by　using　impedance　analyser.　Four　water　samples　including　original　domestic　sewage,　the　first　stage　of　treated　water,　reclaimed　water　and　running　water　are　tested,　respectively.　The　experimental　results　illustrate　that　the　system　can　effectively　distinguish　different　stages　of　the　treated　domestic　sewage.