An onsite wastewater treatment facility normally has a treatment capacity of 5-200 population equivalents. The small system is receiving increasing, attention in Asia countries to make up for the shortage of public sewer system. While many countries rely on large centralized system, small systems in Taiwan have significantly contributed to the treatment of municipal wastewater (21.6%) to make LIP for the low sewer connection (17.0%). To resolve disputes on the design criteria of primary settlers in small systems recommended by the government, a nationwide survey of 350 permit applications were conducted. This result of the survey revealed that 53% adopted self-proven criteria to reduce the size of the primary settlers using a design flow rate (Q) of 10 m(3)/d or less. The official design criteria were thus analyzed by using two new approaches of design criteria, scale-down factor and sludge blanket height ratio, as proposed in this study. The analysis indicated that sizing of primary settlers must consider the diurnal now fluctuation and storage of settled Sludge in primary settlers for a sufficient period of time, preferably up to 6 months. The official design criteria may be too conservative for Q < 5 m(3)/d, but inadequate for Q > 20 m(3)/d. Based on the result of this study, new measures are suggested to strengthen the onsite program. (C) 2009 Elsevier B.V. All rights reserved.