As　appurtenance　of　existing　building　drainage　systems　the　septic　tank　may　be　the　single　most　important　component　used　in　the　onsite　treatment　and　collection　alternatives.　The　passive　-　energy　free　-　septic　tanks　provide　the　most　cost　efficient　form　of　primary　treatment　available　for　nonindustrial　sewage.　Because　the　septic　tank　is　an　essential　ingredient　to　the　success　of　building　drainage　systems　and　a　failed　septic　tank　can　present　major　inconvenience　and　pose　a　serious　health　threat　as　well　as　are　failing　to　adequately　protect　the　environment,　it＇s　engineered　design,　practical　application　and　maintain　addresses　the　central　problem.　In　this　paper　the　existing　problems　of　the　septic　tank　system　currently　using　in　China　are　summarized　by　means　of　field　survey　and　investigation.　Also　via　the　field　experiments,　the　treatment　efficiency　of　CODcr　has　been　made　an　appraisal.　The　application　of　the　computer　simulation　to　analyze　of　sewage　flow　characteristic　inside　the　septic　tank　is　clearly　described.　Afterwards,　some　new　ideas　and　methods　are　presented　in　order　to　improve　the　performance　of　existing　system.　©　2011　ASCE.