Introduction The Lao People’s Democratic Republic (Lao PDR) is committed to providing safe and affordable drinking water for all under Agenda 2030. However, there are challenges to achieving this goal, including the high prevalence of open defecation, especially in rural areas; disparities in sanitation services coverage between rural and urban areas; and water safety and quality issues. Untreated wastewater threatens surface water, which is the main source of supply in urban areas. Investment in water, sanitation, and hygiene remains inadequate. Water treatment and sanitation, in particular, require more attention and resources, as well as support and funding from the government and development partners. The major constraints in addressing wastewater treatment issues in the Lao PDR include the following: (i) inadequate legal framework; (ii) lack of specific standard for wastewater discharge; and (iii) lack of financial and technical resources necessary for the proper installation, operation, and maintenance of wastewater treatment facilities. Decentralized Wastewater Treatment Solutions (DEWATS) offer a way to overcome these constraints. It can complement or provide a strong alternative to centralized wastewater treatment and management systems. It is considered eco-efficient, flexible, and cost-effective, and it can be customizable based on the type of wastewater produced in urban areas. DEWATS is being successfully implemented in several projects in the Lao PDR and can also be applied or adapted according to the needs of urban and peri-urban communities in other developing countries. What is DEWATS? Decentralized wastewater treatment was first recognized and promoted as a suitable low-cost solution for low- and middle-income areas in the 1990s. In Southeast Asia, the Bremen Overseas Research and Development Association promoted the approach under the name DEWATS. DEWATS complements conventional treatment systems for more sustainable and effective environmental sanitation services and can treat both household and industrial wastewater. It is based on the principles of decentralization, simplicity, and reuse of treatment products. Simplicity is achieved through onsite treatment without chemicals or electrochemical equipment or energy input, and with low-maintenance requirements. Individual onsite systems can serve one household while shared facilities can serve up to about 2,500 households (1,000 cubic meters per day) or public or commercial facilities. DEWATS can be adapted to suit diverse local conditions; can be built with local materials by the local workforce; provides reliable and efficient treatment of domestic and process wastewater; and can be designed and built within a short period. However, it can release some odor as wastewater pond occupies open land; requires relevant knowledge and care during construction; and is usually not suited for the treatment of industrial wastewater with high heavy metal or chemical content. Operating and maintaining DEWATS systems can be inexpensive and unsophisticated compared to conventional centralized wastewater treatment plants. Maintenance activities can be done by service providers or by supervised and trained maintenance personnel onsite. For community-based DEWATS, local residents can operate and maintain the plants. What makes a decentralized system the best option? Lao PDR needs low-cost, low-technology, and energy efficient nature-based wastewater treatment solutions that can be effectively managed even with limited human, technical, and financial resources. DEWATS may be regarded as the best option to improve wastewater treatment and sanitation in selected urban and peri-urban communities in the country. It is useful for areas where centralized wastewater collection and treatment approaches and systems are not available. Almost all households in major urban areas in the Lao PDR use onsite facilities and only a very small portion are served by sewerage systems. Households typically have a pour-flush toilet connected to a septic tank or soak pit that can be emptied or replaced when full. As a result, water in the drainage system is contaminated with fecal matter and coliforms from latrines and septic tank effluent. Several pilot programs supported by nongovernment organizations aimed to improve wastewater management and sanitation in urban towns. These involved improving drainage systems and developing stabilization ponds, but most were unsuccessful due to inadequate technical and financial resources to operate and maintain the facilities. In comparison, a decentralized system (i) complements conventional treatment systems for more sustainable and effective environmental sanitation services; (ii) can efficiently treat domestic and process wastewater; (iii) requires low operation and maintenance costs; and (iv) has limited requirements for operation and maintenance. DEWATS demonstration projects have been developed in several towns and provinces through an ongoing cooperation between Bremen Overseas Research and Development Association and the Lao PDR’s Department of Housing and Urban Planning under the Ministry of Public Works and Transport. These projects have treated 155,745 cubic meters of wastewater per year (BORDA Lao PDR 2018). It has been successfully implemented in two areas in the country. Navieng Village, Xumnuea District of Houphan Province lacked a sewer system to treat greywater (i.e., wastewater from kitchen and bathrooms). To address this issue, DEWATS was constructed under a sanitation project supported by Bremen Overseas Research and Development Association in partnership with the ASEAN SDGs Frontrunner Cities Programme funded by the Japan-ASEAN Integration Fund. Completed in 2015, it has a treatment capacity of 14 cubic meters per day, which can serve 200 people. Navieng also established a community-based organization and developed an action plan to effectively operate and maintain the decentralized water treatment system. Meanwhile, Theun-Hinboun Operator Village, Nahin Village in Khounkham District, Bolikhamxay Province had difficulties operating and maintaining its previous wastewater treatment system, which it used from 1995 to 2009. It failed to treat wastewater according to the national environmental standard for effluent and produced a bad odor. Thus, the village switched to DEWATS in 2011. It treats wastewater from 82 operator households with 300 people. ADB-Financed Projects in the Lao PDR DEWATS systems are part of several Asian Development Bank (ADB)-financed urban development projects in the Lao PDR. These include the Greater Mekong Subregion East-West Economic Corridor Towns Development Project, the Second Greater Mekong Subregion Corridor Towns Development Project, and the Fourth Greater Mekong Subregion Corridor Towns Development Project. It will also be implemented under the Second Strengthening Higher Education Project for the treatment of wastewater discharged from all Savannakhet university campuses and facilities. DEWATS can treat 31.7 cubic meters per day to 4,619 cubic meters per day of wastewater discharged from each facility, or equivalent to the daily consumption of 396 persons per day to 57,737 persons per day. Although the ADB-financed projects are still under preparation and/or in the early stages of implementation, the sustainability of DEWATS facilities, as well as the improvement of the environment, is promising. The installation cost of wastewater treatment facilities has been less than that of conventional treatment facilities. Operation and maintenance costs of the plants are also expected to be lower and more acceptable. Since DEWATS can be applied to all types of wastewater generated by residential areas, schools, hospitals, small and medium-sized enterprises, and other commercial establishments, it can be considered and supported under ADB-financed projects that involve wastewater treatment.  An onsite sanitation system is a system in which excreta and wastewater are collected and stored or treated on the plot where they are generated. This usually consists of pit latrines and simple septic tanks.  A stabilization pond is a pond where the wastewater undergoes a biological process by which the organic matter in the sludge produced from the primary settling and biological treatment of wastewater is stabilized, usually by conversion to gases and cell tissue. Also see McGraw-Hill Education. 2004. Wastewater Engineering, Treatment and Reuse: Fourth Edition. Resources Asian Development Bank (ADB). Lao People’s Democratic Republic: Fourth Greater Mekong Subregion Corridor Towns Project. ADB. Lao People’s Democratic Republic: Greater Mekong Subregion East-West Corridor Towns Development Project. ADB. Lao People’s Democratic Republic: Second Strengthening Higher Education Project. ADB. 2021. Wastewater Treatment Solutions for Urban and Peri-Urban Communities in the Lao People’s Democratic Republic. Manila. Bremen. 2017. DEWATS Implementation by Borda. Water Sanitation Program (WSP). 2010. A Rapid Assessment of Household Sanitation Service, Vientiane, Lao PDR- Final Report for WSP. Ask the Experts Vongphet Soukhavongsa Senior Safeguards Officer, Southeast Asia Department, Asian Development Bank Vongphet Soukhavongsa works on various environmental issues of ADB projects in the Lao People’s Democratic Republic. Prior to ADB, he worked as an environmental inspector for several large hydropower projects. He also served as a consultant, primarily on water quality and environmental impact assessment, at an environmental consulting firm and at the Ministry of Natural Resources and Environment. Asian Development Bank (ADB) The Asian Development Bank is committed to achieving a prosperous, inclusive, resilient, and sustainable Asia and the Pacific, while sustaining its efforts to eradicate extreme poverty. Established in 1966, it is owned by 68 members—49 from the region. Its main instruments for helping its developing member countries are policy dialogue, loans, equity investments, guarantees, grants, and technical assistance. Follow Asian Development Bank (ADB) on Leave your question or comment in the section below: View the discussion thread.