Sri Lanka employs aquatic plants to clean up its heritage lake
THE highly polluted Kandy lake of Sri Lanka is in news again, and this time for good reasons. The government has joined hands with University of Peradeniya in Kandy city and is creating artificial floating wetlands to clean up the historic water body.
Built in 1800 AD by the last king of Kandy, the lake used to be a major source of irrigation, and drinking water and recreation centre for the hilly city.
However, all activities other than cruising have been prohibited in the lake since the 1960s after studies showed that the lake water has high levels of faecal contamination and carcinogenic heavy metals. It is also loaded with phosphorus and nitrogen compounds that lead to dense bloom of toxic cyano bacteria causing eutrophication, or depletion of oxygen in lake water, and killing of aquatic organisms. Several fish kills were reported in 2009. Pollution in the lake has already contaminated the nearby Mid Canal and wells around it.
L L A Peiris, deputy general manager of the National Water Supply and Drainage Board (NWSDB), blames such severe pollution on the absence of a sewerage plan for the hilly city. Barring a few patches of forest, the lake is surrounded by densely populated housing colonies and hotels, which release wastewater directly into the lake or dump the solid waste in open, which eventually drains into the lake.
Wastewater should be treated before it is released into the lake. But it is difficult to set up sewage treatment plants in the highly urbanised catchment area around the Kandy, says S K Weragoda, chief engineer of NWSDB. The floating treatment wetland is an appropriate technology for the Kandy, he adds.
The floating treatment wetland mimics an artificial wetland ecosystem. It is created by using a perforated floating mat for supporting the vegetation, a frame constructed by PVC pipes, coconut coir pith for growing the vegetation and anchors to keep the wetland in tact.
University of Peradeniya researchers chose two native aquatic plants—Typha angustifolia and Canna iridiflora for the purpose. They first allowed the plants to grow in potable water for a week so that they develop roots, and then placed the wetland units in the lake next to the outlet of a storm water drain. In case of a floating wetland, the aquatic plants grow on the water surface rather than being rooted in sediments. The plant roots hanging beneath the floating mats provide an extensive surface area for trapping suspended particulate matter.
Since the plants are not rooted in the soil, they are forced to acquire nutrition directly from the water, which enhances the removal of contaminants, notes the study paper published online in Society of Wetland Scientists on August 29, 2012. Weragoda says the floating wetlands removed the biological oxygen demand of the lake water, nitrogen and phsphorus by 90 per cent within 50 days of setting up the wetland units. “One-third of the lake should have such floating wetlands for cleaning up the entire lake,” says Weragoda, co-author of the study.
The irrigation department now manages the wetlands by harvesting the vegetation once a year to allow regrowth. Weragoda says the colourful flowers of the aquatic plants have added to the beauty of the lake. The lake, which used to emanate foul smell, now attracts tourists who come to visit the nearby famous Buddha temple.