The pervasive plant design in Nepal is a constructed wetland — a shallow bed of gravel, stone, and specialized reeds that filter contaminants. Some of the treated wastewater is reused for toilet flushing, and the dried sludge applied as fertilizer on land. More recently, biogas reactors affixed to treatment plants have provided additional energy recovery. Plants can serve communities of up to 2,000 people.
Developing countries facing rapid urbanization often cannot afford the capital expenditure, energy, and technical expertise required to pump wastewater through sophisticated sewerage networks and treat it in large, central plants. As a result, wastewater treatment is often abandoned — only 5 percent of sewage is treated in Nepal and the situation is equally dismal throughout South Asia. “This has both short-term and long-term impact on public health and on the environment, in particular the irreversible damage to the essential and irreplaceable resources of good-quality groundwater and river water,” said Jelle Van Gijn, Urban Water Supply and Sanitation Advisor at the Asian Development Bank. Nitrogen and phosphorus enrichment of aquatic ecosystems from sewage discharge in the Region is expected to increase more than four-fold by 2050, which can lead to algal proliferation and eventually fish mortality, according to a 2009 study in the journal, Global Biochemical Cycles.
The alternative — decentralized systems — reduce transportation costs and allow for localized management and modular construction of wastewater infrastructure to suit a city’s budget.
Decentralized systems are recommended in a five-year restoration plan launched in 2009 for a major river, the Bagmati, running through the capital Kathmandu; and the national Sanitation and Hygiene Master Plan includes a strategy to build decentralized treatment in urban areas. But some question whether Kathmandu will be able to construct such facilities on a large scale. Institutions would need to be revamped to adapt to decentralized management scenarios. And this may only happen with more sophisticated technologies, such as traditional sewage treatment plants, since natural treatment requires sizable land area, which is in short supply in dense urban cores. “There is just no space for low-tech solutions, land costs are simply too high,” said Van Gijn, who believes that decentralization using smaller, traditional treatment plants is the future for Asian cities.
Spurred by organizations like the Environment and Public Health Organisation (ENPHO), Nepal has furnished to other countries its expertise in decentralized technologies that treat wastewater naturally, with minimal energy and maintenance. A workshop was held in Kathmandu last year for engineers from nine countries, including Pakistan, Mongolia and Bhutan, and some Nepali experts have consulted abroad.
Developments in Nepal reflect a regional shift away from investment in large, centralized treatment infrastructure. “It is clear that any city with a GDP below $5,000 per capita would never be able to afford centralized systems,” said Pedro Kraemer, Program Coordinator for South Asia at the Bremen Overseas Research and Development Association (BORDA). BORDA has supported the construction of more than a thousand natural, decentralized treatment facilities across Asia, and is making inroads in Africa and Latin America.