A Modeling Framework and Preliminary Results in Assessing Phnom Penh's Sewage Discharges
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 WSSD identified the goal of halving, by the year 2015, the proportion of people worldwide who are unable to reach or to afford safe drinking water or have access to basic sanitation. These goals also were in keeping with the Millennium Development Goals set by the United Nations in 2000. In Cambodia, mortality due to waterborne disease is high, in part reflecting the fact that less than 10% of the rural population and 55% of the urban population has access to adequate sanitation (Irvine et al., 2006).
Globally, constructed wetlands are gaining acceptance as an effective, low cost, and sustainable alternative to sewage treatment (Campbell and Ogden, 1999; U.S. EPA, 2000; UNEP, 2004). The UNEP (2004) noted that the advantages of wetlands treatment includes utilization of solar energy to drive purification processes; longevity of a large system (50-100 years); self-sustaining; effective removal of sediment, pathogens, nutrients, and metals from the wastestream (when properly designed); relatively low cost; low tech maintenance; possiblility of accruing economic benefits from plant harvesting and fishing in some cases. However, Kivaisi (2001) argued that the rate of adoption of wetlands technology for wastewater treatment in developing countries (particularly those in the tropics) has been slower than it should be, in part because donor countries favor technologies that have commercial spin-offs, and in part because technical advisors from developed countries have a tendency to translocate conventional treatment approaches (e.g. secondary or tertiary centralized treatment plants) from their countries rather than adjusting to the realities and cultures of developing nations.
Increasingly, there are studies that have documented the treatment efficiency of a properly designed and maintained wetland. For example, the U.S. EPA (2000) reported reductions in suspended solids concentrations of up to 86% for surface flow wetlands, while BOD5 concentration was reduced by up to 96%. Hiley (1995) reported similar removal rates for BOD and suspended solids. Kootatep et al. (2005) showed reductions in the range of 80-96% for COD, suspended solids, and TKN in test studies of constructed wetlands in Thailand. Nelson et al. (2004) found that wetlands reduced copper and mercury by 80%, lead by 83%, and zinc by 60% from industrial process wastewater and stormwater. Walker and Hurl (2002) found wetlands reduced zinc, lead, and copper, principally through sedimentation, but chromium concentrations remained the same and arsenic levels increased. Results for chromium and arsenic may have been related to their chemical behavior and the role of organic matter in the wetland. Quinonez-Diaz et al. (2001) reported that 90% of all indicator microorganisms (total and fecal coliform, coliphages, Giardia, and Cryptosporidium) were removed from domestic wastewater by their test wetland. Kivaisi (2001) summarized removal efficiency data from a number of developing countries and reported nitrogen was reduced by 30-50%, phosphorus by 20-60%, and indicator bacteria by 60-99%. The forms of the nitrogen and phosphorus were not provided.
The city of Phnom Penh, Cambodia (population 1.3 million) is serviced by a combined sewer system that drains stormwater runoff as well as municipal and industrial discharges. The city does not have a wastewater treatment plant, but most domestic sanitary flow and stormwater runoff is discharged to a system of wetlands located around the city for treatment. Ultimately, these discharges reach the Tonle Sap/Bassac/Mekong River system, hereafter referred to as the Chaktomuk confluence. The Chaktomuk confluence is where the Mekong main stream divides into two branches, the Mekong and the Bassac River, and where the Tonle Sap River drains into the above branches (Figure 24.1). The Mekong River is one of the major rivers of the world, extending approximately 4,800 km from the Tibetan Plateau to the South China Sea and having a drainage area of 795,000 km2. Mean annual flow (1960-2004) for the river at Kratie is 13,200 m3s-1, but during peak rainy season (September), monthly mean flow (1960-2004) is 36,700 m3s-1 (Mekong River Commission (MRC), 2005).
The Tonle Sap/Mekong/Bassac River system is one of the most productive and biodiverse (in terms of species richness) inland fisheries in the world (Dudgeon, 2000; MRC, 2003). It also is an important water source for irrigation and drinking water and there is increasing pressure for hydropower development (MRC, 2001). It is essential that this important river system not become degraded due to urban waste discharge. The Tonle Sap River reverses its flow annually when the Mekong flood starts rising in May-June due to the monsoon rains and snowmelt from Tibet. The Mekong waters flow to the Tonle Sap Lake for about four months, extending its size from the basic 2,500 km2 to 15,000 km2 during high flood years (Fuji et al., 2003; MRC/WUP-FIN, 2006). The lake depth increases from about 1 meter in dry season up to 10 meters during peak flood. The turning of the flow direction also means that the wastewater impacts from Phnom Penh are divided periodically between downstream Mekong and Bassac and ‘upstream’ Tonle Sap River and lake.
The objectives of this chapter are twofold. First, preliminary sample results from Phnom Penh’s sewer system, wetlands treatment system, and receiving waters are presented to provide some understanding about treatment efficiency and impact on the receiving waters. Second, a modeling framework is described, together with some preliminary test runs, to begin examining contaminant fate in the Chatkomuk confluence. The modeling work presented below is part of the work done within the Mekong River Commission Secretariat (MRCS) Project WUP-FIN, which is a complementary, Finnish government funded component to the MRCS Water Utilization Programme (MRC/WUP-FIN, 2005). The objective of the WUP-FIN Project is to develop analytical tools for hydrological, environmental and socio-economic impact assessment in the Lower Mekong Basin (LMB) (e.g. Koponen et al. 2005; Keskinen et al., 2005). An important part of the work is to assess the possible transboundary impacts of basin developments and facilitate the related discussions between the member countries. The Phnom Penh wastewater impacts fall into this category due to their transport further down to the Vietnam part of the Mekong delta. The MRC has been established to provide a constitutional framework for the member country negotiations. The MRC member countries are Cambodia, Laos, Thailand and Vietnam. The other Mekong Basin countries, China and Myanmar are official dialogue partners to the MRC. Capacity building is one of the key components of the WUP-FIN Project which necessitates close cooperation with national institutions, line agencies, and universities. International cooperation in these questions, like the one described in this chapter, are of great help to magnify the effects of single efforts by their synergies and shared motivations.
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