Vietnam, Bangladesh, and Cambodia, the research countries, are key partners in South Korea's ODA fund. As these nations experience rapid economic development, water pollution has become a major national concern. They are either operating or planning to construct sewerage systems financed by the Economic Development Cooperation Fund (EDCF) of the Export-Import Bank of Korea, the World Bank (WB), the Asian Development Bank (ADB), the Japan International Cooperation Agency (JICA), and other Multilateral Development Bank (MDB) sources. However, South Korean consultants frequently design separate sewage collection systems that do not suit the local conditions. In contrast, local consultants or consultants from other developed countries have accounted for these conditions and chosen combined or partially separate systems. This study analyze the local conditions from the MDB cases to identify appropriate wastewater collection method. It was observed that the cities adopting separate sewage collection systems faced challenges in securing a sufficient quantity of wastewater for the normal operation of the treatment plant, primarily due to the low household connection rates. When deciding between a combined or separate sewer system in developing countries, it is essential to review the local context, and the recipient country’s economic situation.