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sustainable urban sewerage and drainage
1 SUSTAINABLE URBAN SEWERAGE AND DRAINAGE Viet-Anh Nguyen, Assoc. Prof. Dr. Vice Director, Institute of Environmental Science and Engineering (IESE), Hanoi University of Civil Engineering (HUCE), Vietnam. Add. 55 Giai Phong Rd., Hanoi, Vietnam. Tel. (+84-4) 3628 4509, MP: (+84) 91320.9689. E-mail: vietanhctn@gmail.com ABSTRACT: Urban sewerage and drainage is among hot issues in fast developing urban areas in Vietnam, especially in nowadays where climate change process is creating more and more impacts. Inadequate infrastructure engineering systems in Vietnamese urban areas cannot provide sufficient service for the socio-economic development. At the same time, Vietnamese urban areas have lots of opportunities to apply new approaches which are more appropriate and sustainable in term of service efficiency, financial sustainability, environment friendly, etc. The paper presents new approaches and technical options in urban drainage and sewerage management. Decentralized wastewater management, with low-cost treatment technologies has number of advantages compared with conventional approach. Sustainable urban drainage solutions (SUDS) are also introduced whereas flow diversion and retention techniques can be applied. Rainwater harvesting has shown a very promising adaptive water supply solution. Some good models applied in Vietnam and other countries are presented as examples. Sustainable, integrated urban water management approach is recommended. KEY WORDS: Decentralized wastewater management; rainwater harvesting; urban sewerage and drainage, Vietnam 1. INTRODUCTION Improvement of sanitation systems for urban, peri-urban and rural areas in Vietnam is very challenging and becoming more and more crucial issue in this fast-developing country. According to the Ministry of Construction (MOC), from 760 urban centers, in the large cities, 50-80% of households use septic tanks, treating mostly black wastewater, while 10-20% uses simple pit latrines. In the other cities (class 3-5), 20-50% uses septic tanks and 30-50% uses pit latrines or double vault latrines. The current system does not have the capacity to respond to the demands created by wastewater and run-off from urban areas, industrial centers, and agricultural land. Water bodies such as lakes, streams, and canals increasingly serve as sinks for domestic sewage and industrial wastes. It is estimated that only 10% of urban wastewater is treated (Nguyen V.A., 2009). In number of new urban areas, though domestic wastewater is separated from surface runoff from the buildings, but they are again mixed up during discharge to the city’s combined sewerage and drainage network. This problem is wasting invested money and polluting the environment. Besides, there is poor control on ground level of number of new development areas, roads and surrounding areas, leading to conflicts and negative impacts on each others. Environmental and wastewater fees are too low, not enough to recover the system operation and maintenance expenses. While large centralized sanitation projects are not affordable for most cases in urban and rural areas, the only way to increase sanitation coverage, especially for the poor, is to implement low-cost alternatives with decentralized sanitation management schemes where local community, administrative authorities and private sectors are involved in the decision making 2 as well as in the exploitation process (Nguyen V.A., 2004). This concept is being developed in Vietnam, even though it still requires more efforts and co-operations before it could be widely disseminated in the practice. Climate change is showing more and more evidences and becoming more and more serious challenge for urban planning and engineering infrastructure, especially in coastal cities. According to the World Bank study (2008), up to the year 2050, the most accepted scenario show the sea level in Vietnam will be raised up for 30 cm more. Climate change will also lead to negative consequences such as increased rainfall, changes of unpredictable urban hydrology patterns, etc. what impact on collection and conveyance of urban wastewaters. 2. APPROACHES AND MODELS FOR SEWERAGE AND DRAINAGE IN VIETNAMESE URBAN AREAS Sustainable Urban Drainage Solutions There is a growing tendency to argue that decentralized solutions would be more effective than a sewer-based centralized system for pollution control. Decentralization of wastewater management systems relate to planning and decision-making, design of physical infrastructure and management arrangements for operations and maintenance (Parkinson and Tayler, 2003). The decentralized approach offers important benefits, namely by the possibility of dealing with wastewater locally, and applying pollution control measures at the source. By tackling pollution problems close to their source, the large capital investment of trunk sewers associated with centralized systems can be reduced, thus increasing the affordability of wastewater management systems. Furthermore, decentralized systems allow an incremental approach to provision of facilities. In some cases, the investment may require little more than improvements to existing sanitation infrastructure (Parkinson and Tayler, 2003). Furthermore, in a agriculture-based country like Vietnam, where wastewater reuse and nutrient recycling has a long tradition, decentralized management systems are likely to be compatible with local demands for wastewater reuse in urban and peri-urban agriculture. Another advantage of decentralized systems in developing countries is that the systems are mostly developed and run with more community-based approach, where users are involved from the early stages of infrastructure system planning. Experience from number of infrastructure development projects in this region have shown risk of failures of centralized waste management systems due to there are no ‘’willingness-to-connect’’ and ‘’willingness- to-pay’’ from the local users. Local resource contribution in decentralized wastewater management systems make the system financial requirements affordable and feasible, the user participation and the decision-making process more committed. Besides availability of appropriate technologies, those conditions are necessary for sustainability of infrastructure system. The Decentralized Sanitation (DESA) study team at the Institute of Environmental Science and Engineering (IESE), Hanoi University of Civil Engineering has been developing number of decentralized sanitation solutions applicable for Vietnamese conditions. The team has developed the package wastewater treatment plant BASTAFAT, a combination of anaerobic treatment step in a baffled septic tank with anaerobic filter, and aerobic treatment unit with attached growth treatment process. The tanks are made from a durable material of fibro- reinforced plastics (FRP). The system BASTAFAT can achieve effluent standard QCVN 14/2008-BTNMT, Class A, appropriate for reuse or discharge to the environment (Figure 1). 3 Figure 1. On-site, prefabricated package wastewater treatment system BASTAFAT Sustainable urban drainage solutions – SUDS Urbanization process has been creating negative impacts on the natural drainage, such as change of natural flow patterns, loss of natural retention capacity of the green spaces and soil, which have been replaced by impermeable surfaces of roofs, concrete and asphalt. The latest covers significantly increase surface runoff flows (Figure 2.a) which again result in increase of soil erosion and mud sedimentation. The above mentioned factors impact on environment, flooding and aquatic life. Figure 2. Principle of sustainable urban drainage (a) Change of flow concentration due to changed urban surface; (b) Return to original natural flow patterns thanks to surface flow retention solutions; (c) Reduction of flow to be drained off thanks to infiltration and flow retention solutions. Conventional drainage system is often designed in order to convey surface water away as soon as possible. Costs for investment, operation and maintenance of such systems are often very high, while their capacity and upgrading possibility are often limited. The consequences are high probability of flood, erosion and pollution in the downstream areas, as well as loss of chance for the on-site recharge of the valuable groundwater sources. In recent years, new engineering alternative solutions of been developed in order to overcome the mentioned drawbacks. The solutions are based on the following principles: retaining of natural patterns of intensity, volume and quality of the flows, maximum control of the flows at source, minimization of direct drain off areas, enabling of on-site flow retention and infiltration associated with pollution control. Those are principles of SUDS. The SUDS flow retention approach is to avoid flow concentration in a short period of time (Figure 2), as limited cross section diameter of the collector, though expensive, cannot be sufficient in case of heavy rainfall, what may cause overflows and flooding. Combination of flow retention solutions often work best, using regulation ponds on the catchment and conveyance area, using of surface of the city itself for infiltration through green spaces. Those BASTAFAT 4 solutions also bring to added values of recreational landscaping and micro-climate conditioning (Figure 2.b, c). In case of limited on-site flow control, SUDS can be applied by diversion of flows from sub- basins, conveyance of flows by open, shallow canals, retention of flow in some ponds, and infiltration through soil, swales, constructed wetlands, etc. Recommendations for sewerage and drainage systems in Vietnamese urban areas A scheme of urban sewerage and drainage should be based on the local specifics. The following general principles of urban sewerage and drainage planning are recommended: - For the existing urban areas: in coming decade, the existing combined sewerage and drainage is still to be utilized, whereas ring or by-pass sewers are to be built for collection of wastewater and first flush storm water and to convey them to the wastewater treatment plants, while preventing direct discharge of those flows into the rivers and lakes. In order to avoid overloads of wastewater treatment plants combined sewer overflows chambers are built on those collectors in order to discharge high flows of storm water and diluted wastewater over weirs to the receiving water bodies. The selection of centralized or decentralized wastewater management systems, or combination of them, with low-cost of high-tech technologies, is based on local natural conditions, population density, land availability, wastewater reuse needs, etc. - For the new development areas, separate sewerage system is to be built whereas wastewater should be treated at adequate level before its discharge to the environment. - In coming decades, where the separate sewerage and wastewater treatment is still not affordable for most of cities in Vietnam, the role of septic tank for treatment of black wastewater, or combination of black and grey wastewater from households, offices and commercial points, is still highlighted. There should be adequate design, construction and management of the septic tanks. Figure 3 introduces pre-fabricated reinforced concrete septic tank designed and made by IESE in cooperation with the Vinaconex Xuan Mai Concrete Company. The tank is now under commercializing and mass production. Figure 3. Pre-fabricated reinforced concrete septic tank - For the mountainous urban areas, with sufficient slope along the roads, the separate sewerage system is recommended. The low-cost simplified sewerage, with small bore sewers, lay along pavements, plus cross quarter (back-yard) connection scheme and maximum utilization of existing channels for the surface runoff collection and conveyance is recommended. - For the urban areas in flat, plateau region, where the sewer slop is limited, it is recommended to use increased sewer diameter, maximum utilize urban water bodies for the flow retention and conveyance as well as for reduction of sewer depth. 5 - For the urban areas along the coast, where flat topography is associated with limited number of urban water bodies due to sandy loam prevalence in the soil structure, the tidal gate system is recommended. Such manual or automatic functioning devices can be used for daily drain off and flush of the sewerage network thanks to natural tidal run. - SUDS principle should be applied as soon as possible by integration into urban planning. In urban infrastructure and land use management, ground level should be strictly controlled. Integration and harmonization of urban sewerage and drainage among urban hydrology as a component of the whole basin whereas water resources, irrigation, flood prevention and hydropower activities in up- and down-streams are being considered. In the urban area, basic formula for determination of storm water flow to be drained is as follows: Q = ψ.q.F (l/s), where ψ is flow coefficient, depending on the surface type; q is rain intensity (l/s/ha) and F is drainage surface area (ha). Controllable factor, which may lead to reduction of flow concentration to the collector, is a flow coefficient ψ. The ψ values of the roof, asphalt, concrete surface is 0,9 - 0,95; of the earthen road: 0,4 - 0,5; of the park, green space: 0,1 - 0,3. By reduction of ψ from 0.4 to 0.8, the flow will be 2 times less. Storm water regulation ponds and open canals also play important role in SUDS application. Assuming the urban drainage basin area is 100 ha; with average flow coefficient ψ is 0.6. In case of the 1- year return period, 1-day rainfall of 120 mm, we have: Total volume of storm water to be drained off is W 1 = 72,000 m 3 . If there are 7.0 ha of lakes in the basin, with storm water regulating height before and after the rain is 0.5 m, the total regulating volume is W 2 = 35,000 m 3 . Then, the total volume of storm water to be drained off is only W 3 = 37,000 m 3 . Therefore, if the water surface in the city is 7% of the total surface area of the basin, the total water volume to be directly drained off remains half. Besides, there are other added values of the lakes such as micro-improvement of the climate conditions, landscaping, ecology and biodiversity, etc. Large public spaces like squares, parking, pavements, and even roads, should be built from permeable materials whereas surface runoff can be penetrated through the surface to the underground infiltration trenches with the gravel bed. Along the sides of the highways the filter strips should be designed for flow retention and removal of pollutants. From economics point of view, damages caused by the flooding during the rains can be much more than investment and operation and maintenance costs for such SUDS facilities. The damage caused by the historical storm in Hanoi in October 2008 has been estimated as VND 8,000 billions, which is equal to total investment cost of the phase 1 and phase 2 of the Hanoi sewerage and drainage improvement project (1996 – 2015). Figure 4. Filter strip for flow retention and pollutants control in public places in the UK Rainwater harvesting One of interesting counter measures of flooding remediation is a public participation in rainwater harvesting. If each household in Hanoi has a 6 m3 rainwater tank, the water volume is sufficient for toilet flushing around the year. Rainwater tanks in households and public 6 buildings will significantly delay the flow intensity of the surface runoff. This rainwater can be used also for gardening, car and motorbike washing, fire protection, groundwater recharge, etc. Underground rainwater tanks can be built in each public place. There are number of successful eco-city models where SUDS, rainwater harvesting are applied in integration and harmonization with urban planning and other engineering infrastructure components. 3. SOME URBAN WATER MANAGEMENT EXAMPLES Example Thailand: Treatment and reuse of wastewater on Tourist Island On the Tourist Island of Phi Phi, which was heavily affected by the Tsunami disaster in 2005, a system was implemented to treat septic tank effluent and greywater from hotels, restaurants and residents. The system was designed with a capacity of 400 m3/d. Wastewater is fed into series of vertical-flow and horizontal subsurface flow constructed wetlands before it is fed to free water constructed wetlands and a stabilization pond. The treated effluent is reused for irrigation of nearby gardens. The system is a nice example that illustrates how a decentralized wastewater treatment system can be installed close to residential areas without affecting the living conditions of the neighborhood. Figure 5. Bird-eye-view of the “Flower and Butterfly” wastewater treatment system implemented on Phi Phi Island, Thailand (Photo: T. Koottatep) Example Vietnam: Eco-Park new town, Van Giang district, Hung Yen province Figure 6. Eco-Park new town, Van Giang district, Hung Yen province Eco-Park new town is among largest eco-towns under construction in. Eco-Park keeps about 30% of total 500 hectares for the green space and water. Separate sewerage system is designed where wastewater is collected and treated. Storm water is collected and conveyed via high volume canal network across the town. The canals are designed with the natural shape and continuous flow for landscaping and enhancement of the self-purification capacity. Recharging water flows through vegetated pond. Number of permeable surfaces, green spaces and infiltration swales are well allocated in the town. 7 Figure 7. Permeable surface in villas and public places Example Korea: New rainwater management in Seoul The city of Seoul announced a new regulation to enforce the installation of a rainwater harvesting system in December 2004. The main purpose is to mitigate urban flooding. The secondary purpose is to conserve water. This is expected to ensure the safety of the city and to improve the well-being of citizens as a result. Citizens are asked to cooperate by filling and emptying rainwater tanks according to directions from the disaster prevention agency. A special feature of the new system is the provision of a network for monitoring the water levels in all water tanks at the central disaster prevention agency in the City Office, which are gathered from each Gu-office, which is a regional organization in the City. Depending on the expected rainfall, the central disaster prevention agency may issue an order to building owners to empty their rainwater tanks, either fully or partially. An incentive program is planned for those who follow the order and some penalties for those who do not. After a storm event, the stored water can be used for firefighting and/or miscellaneous purposes such as toilet flushing and gardening. A specific rainwater system was designed for a recently constructed building (Figure 5) at the Star City Project in Kwangjin-Gu, Seoul. A 3000 m 3 rainwater tank was installed in the basement and divided into three sections of 1000 m 3 each. The first section collects rainwater from the unpaved ground surfaces. It should be kept empty most of the time except when there is heavy rain. The second 1000 m 3 section collects rainwater from the roof, which should be used for toilet flushing and landscaping purposes. The third 1000 m 3 section should be filled with fresh water and used for supply during emergencies such as firefighting or accidents (Han, 2006). Example China: Yunnan International Agri-food Technopark Qujing (7 Colors Park) Figure 8. Master plan of the 7 Colors Park 8 Figure 9. Integrated water management model of the 7 Colors Park This is among the first large-scale practical implementation of an integrated water management and SUDS concept. Portable water is produced by membrane technology. Separate sewerage conveys black wastewater to the tertiary treatment and reused for toilet flushing and other domestic purposes. Grey wastewater is reused for irrigation after simple treatment. Part of surface water runoff is discharged to the Bai Shi River, whereas the remaining part is treated in free water surface constructed wetland before it is used to feed the eco-lake and water supply station. Organic wastes from households and farming activities are collected for composting, which is sent back to the farmers as well as to the green spaces in the Park. The construction of this 300 ha complex has been initiated in December 2009. Its commissioning is expected in the year of 2011. 4. CONCLUSIONS There are different technical solutions of drainage, sewerage, wastewater treatment, which should be applied based on the certain local circumstances. Solutions discussed and recommended in the paper are: decentralized wastewater management, with low-cost sewerage and treatment technologies, SUDS with keeping of natural surface water flow patterns by flow retention, integrated management of surface water runoff, wastewater, solid waste and water supply. Biogas recovery from sludge, organic waste and wastewater treatment should be considered as a promising, sustainable energy alternative. Waste should not be considered as a waste, but a resource. These solutions bring multiple benefits such as water and soil pollution control, disease prevention, flood and erosion prevention and mitigation, diversifying and enhancing of aquatic biodiversity, groundwater recharge, rive flow stabilization, economizing water supply source thanks to rainwater harvesting, urban ecology and landscaping improvement, increase of commercial value of the land and improvement of living standards. In order to implement sustainable wastewater management in the urban areas, the investors should be aware of its importance and long-term benefits. Besides, it is necessary to update and improve the current water and wastewater design codes in order to adapt to the patterns 9 associated with the challenging climate change. Integrated, basin based management approach should be applied from early stages of urban planning and management process. REFERENCES 1. Nguyen Viet Anh et al. 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