Septic tank with sedimentation and anaerobic processes in the same tank is commonly used as a domestic wastewater treatment technology in individual households and communal systems. Although simple to construct, such a system has some problems, such as effluent not meeting the quality requirement and blocking by trash and suspended solid before the second treatment chamber. This research aims to develop a new septic tank design that is simpler to construct and improve performance. The new design uses HDPE material, which is easier to build and standardized compared to the conventional concrete structure. The performance of the new design was compared to the conventional septic tank. The start-up process was monitored for flowrate, COD, TSS, NH3N, PO4-P parameters and evaluated against effluent standards. The study was conducted at public toilet facilities at Wisdom Park UGM and Sunday Morning Market. Results from the study show that the new design effectively improves effluent quality and overcomes the trash problem.