Natural Wetlands Treatment

Natural Wetlands Treatment of Sewage Discharges from Phnom Penh,Cambodia: Successes and Future Challenges

The 2002 World Summit on Sustainable Development (WSSD) recognized that water and sanitation are vital to protect human health and promote sustainable human settlements. The U.N. proclaimed 2005-2015 as the Water for Life International Decade for Action, with the primary objective of promoting efforts to fulfill international commitments on water related issues, particularly those outlined in the Millennium Development Goals. The U.N. also declared 2008 as the International Year of Sanitation because “the resources needed to tackle the global sanitation crisis have not been forthcoming”. In Southeast Asia, inadequate access to appropriate sanitation has important health and economic impacts.

Constructed wetland systems have been used for wastewater treatment in Denmark since 1983 and are being used increasingly throughout Europe, Asia, Africa, and the United States. Constructed wetlands are regarded as a promising treatment option since they are easy to operate, cost-effective, and a technically feasible approach for treatment of stormwater runoff; municipal, industrial and agricultural wastewater; landfill leachate and faecal sludge. However, recent work done at Asian Institute of Technology (AIT), Thailand, concluded that barriers to adopting innovative,sustainable approaches to wastewater management included lack of managerial capacity and availability of technical skills; ineffective transfer of knowledge from research institutions to decision-makers and practitioners; and a lack of success stories and guidance.

There is relatively little information on the efficiency of natural wetlands in treating wastewater discharges, but many Southeast Asian countries still have an abundance of natural wetlands that could be used for wastewater treatment. A recently-completed study funded by the International Foundation for Science and the Swedish International Development cooperation Agency looked at the efficiency of Phnom Penh, Cambodia’s natural wetlands in treating wastewater discharge, as well as the possible health risks for the peri-urban community living on and using the
wetlands as a food source.

The current population of Phnom Penh is 1.4 million with projection that by 2020 the population of the city will be about 2 million. Phnom Penh is serviced by a combined sewer system consisting of underground pipes that discharge to several main open interceptor sewers. The interceptor sewers subsequently discharge to the natural treatment wetlands that ring the city. This study focused on Boeng Cheung Ek, the the 1,300-2,000 ha natural wetland to the south of the city.

The study showed that the Boeng Cheung Ek treatment efficiency in dry season, dry weather conditions, ranged between 44% for nitrate and 99.97% for E. coli. Reductions for the other sampled parameters were: detergents (86%), total phosphorus (71%), total suspended solids (50%), Cr (89%), and Cu (55%). It is expected that during the rainy season treatment efficiency could be even greater since a freshwater flow enters the wetland at the outlet end, acting as an effective dam that increases residence time. With this treatment performance, the natural wetland is comparable to conventional wastewater treatment systems using advanced techniques in removals of nitrogen, phosphorus, some heavy metals and other toxic compounds including pesticides. Peri-urban crops of Morning glory and Water mimosa are grown extensively in the wetland and appear to be particularly effective in filtering the wastewater within the first 350 m of entering the wetland. The Stormwater Management Model (PCSWMM.NET) was applied to estimate storm flow in the sewer system of central and south Phnom Penh that discharges to Boeng Cheung Ek. Mass loadings of Cu and Cr estimated using the model for an extreme event were less than, but of the same magnitude, as an entire month of dry weather discharge.

Boeng Cheung Ek has an active peri-urban community that lives on, and uses the wetland to harvest a variety of water-based crops (Morning glory, Water mimosa), vegetables, and herbs, as well as snails and fishes. These various foodstuffs were sampled and for Zn, Cu, and Cr from three locations in the wetland. Metals levels in vegetables were low compared to fishes and snails. Sample collection, in association with social surveys of 201 people living on the wetland, were used to conduct a simple risk analysis, using the average daily dose approach. The risk assessment indicated that at this point there is low concern for metals uptake via consumption of Morning glory or fish (with the exception of Cr for children eating fish at one sample site). A higher risk was identified for children consuming snails, with respect to Cr, but not the other metals. Cyprinoid fish species and snails also were sampled at four sites within the wetland and it was found that both could be infected by Opisthorchis viverrini, a liver fluke parasite. This parasite can be passed to humans when they eat undercooked fish.

It can be concluded that Boeng Cheung Ek is effective in treating the waste discharge from central and south Phnom Penh. Some concerns regarding health risk to the peri-urbran community have been expressed, but this study shows that levels of Zn, Cu, and Cr in the vegetables and fish generally do not pose a high health risk (with the exception of Cr for children eating fish at one sample site). These results are consistent with the findings of some recent studies done independently through the Royal Veterinary and Agricultural University, Denmark. Risk management related to certain aspects of food stuff consumption (e.g. education on proper cooking of fish, filtering or boiling water, limiting children’s consumption of snails) could be developed with a community outreach program. Routine testing of the food stuffs should be conducted to confirm results from this study and ensure future health safety.

The city of Phnom Penh is approaching a development cross-road. As it expands, it is reducing its capability of sustainably treating its waste through its existing system of natural wetlands. Already, some of the wetlands to the north of the city are being filled in to accommodate a growing population and small areas of Boeng Cheung Ek also have been filled in since the start of this study. Not only do the wetlands provide wastewater treatment, they also are a natural resource that provides homes, food, and livelihood to a peri-urban community; provide a diverse natural habitat for aquatic plants and animals; and serve an important function in receiving stormwater runoff and reducing urban flooding during the rainy season. We believe that innovative use of multi-functions wetlands seem a viable alternative as an integral element of sustainable sanitation. To optimize multi-functional uses of the wetlands, further intensive study on wetland hydrology, sediment behaviors and its relationships to water quality improvement should be undertaken.

The full details of the study are published as a series of 5 papers in the Asian Journal of Water, Environment and Pollution, 2010, Volume 7, Number 3, pp. 1-48: http://iospress.metapress.com/content/vu0x47882x0p/?p=1a734525e18c4bd3a64de80f6bd8250b&pi=0

Submitted by Dr. Kim Irvine, Department of Geography and Planning; and Director, Center for Southeast Asia Environment and Sustainable Development, Buffalo State, State University of New York; and Dr. Thammarat Koottatep, School of Environment, Resources and Development, Asian Institute of Technology, PO Box 4 Klong Laung, Pathumthani 12120, Thailand.



Measuring flow rates and sampling dry flow in one of the open interceptor sewers discharging to Boeng Cheung Ek (left) and the same sewer during wet weather sampling (right).


Preparing to install sampling equipment in the Morning glory fields, middle of Boeng Cheung Ek (left) and water sampling and flow measurement in the canal that discharges from the treatment wetland to the Mekong River system (right).