Design Criteria for Decentralized Wastewater Treatment Systems Using Cost-Benefit Analysis: A Case Study of Vientiane Capital, Lao PDR

Abstract

This study established design criteria for decentralized wastewater treatment systems (DEWATS) to advance sustainable sanitation in ASEAN countries, where sewer networks are limited and financial resources are constrained. Using Vientiane Capital, Lao PDR, as a case study, data were collected through interviews and group discussions with local authorities, household surveys and field visits to evaluate the economic feasibility of implementing the DEWATS. The cost-benefit analysis evaluated the Economic Net Present Value (ENPV), Benefit-Cost Ratio (BCR), and Economic Internal Rate of Return (EIRR), with the condition that the EIRR must exceed 5% over a 15-year project period. Based on these findings, three design criteria were proposed: (i) wastewater tariffs can be determined according to households' willingness and ability to pay, (ii) modular, low-maintenance systems can be adapted, and (iii) financial benchmarks ENPV greater than zero, BCR greater than one, and EIRR exceeding 5% should guide investment decisions. These criteria provide a transferable WTP/ATP-CBA analytical framework for similar ASEAN cities.