Decentralized Rainwater Management Part of decentralized environmentally sound water and sanitation systems

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1 Decentralized Rainwater Management Part of decentralized environmentally sound water and sanitation systems T Schuetze Technical University Delft, Faculty of Architecture, Berlageweg 1, 2628 CR Delft, Netherlands, t.schuetze@tudelft.nl Abstract Decentralized rainwater management measures can be taken to develop and restore sustainability in rural and urban areas. Examples carried out in densely populated urban areas with water-impermeable soils showed that to decentralize rainwater management is possible. At present this type of decentralized management primarily contributes to a failure-free function of mixed sewer systems and wastewater treatment plants. So far none of the examples observed in existing urban housing in densely populated areas has avoided the connection to urban sewers in favor of environmentally sound integrated systems for decentralized rainwater and domestic sewage water management. The author examined the degree to which the introduction of such systems can contribute to sustainable sewage management and ensure safe drinking water resources in cities worldwide. Different systems in housing estates in Hamburg, Germany, and Seoul, South Korea were examined. The necessary measures can be adopted with relatively low effort when refurbishing buildings. Centralized systems costs can be reduced over the long term by not constructing an urban sewerage system. Keywords: urban development, sewage, decentralized rainwater management, sustainability Introduction Decentralized rainwater management measures can be taken to develop and restore sustainability in rural and urban areas. Nature-oriented technical systems can help harmonize the natural water balance, in terms of evaporation, infiltration and runoff rates. Furthermore, these systems can be applied to retain and use rainwater. Rainwater management can help control floods, preserve drinking water resources and irrigate fields. At the same time, it conserves or regenerates a natural microclimate [1]. And thanks to its decentralized nature, this type of management can be adapted to almost all locations. According to recently performed examples, to decentralize rainwater management is possible and contributes to the failure-free function of mixed sewer systems and wastewater treatment plants. This holds true particularly within urban areas with low and high population density and areas with waterimpermeable soils. So far no densely populated urban housing has proved to be equipped with environmentally sound integrated systems for decentralized rainwater and domestic sewage water management and to have avoided the connection to urban sewers. Chapter Results and Discussion describes the preconditions as well as the degree to which the introduction of decentralized water and sanitation systems can contribute to sustainable sewage management and ensure safe drinking water resources in cities worldwide.

2 Materials and Methods The method of this research is a qualitative and as far as possible quantitative analysis of approved ecological water management and sanitation systems based on recent research findings and the author s own investigations. Different urban housing estates in Germany and South Korea were examined in terms of the applicability and acceptance of such systems. Results and Discussion Existing urban areas often lack space for visible rainwater management systems, e.g. in the form of lakes and streams. Furthermore, the specific spatial planning and the institutional and legal framework often imply major barriers to implementing such urban water systems. Measures to decentralize water management in single and contiguous buildings during refurbishment will soon be feasible. Different housing estates in Hamburg, Germany, and Seoul, South Korea, were examined in terms of the applicability of decentralized water systems when refurbishing the existing premises. The examined housing estates in Hamburg and Seoul have an above-average population density; however, the research findings are transferable to lower density housing. Hamburg s housing estates have a population density of 72,774 people per km²; this is 32 times higher than the city s average population density of 2,280 inhabitants per km². (Hamburg has 1.71 million residents within an area of 750 km²). In Hamburg, the floor area ratio is of 3.6. (Only 10% of all buildings in Hamburg have a floor area ratio higher than 1.2). Seoul s housing estates have a population density of 63,797 people per km²; this is 3.2 times higher than the city s average density of 17,000 inhabitants per km². (Seoul has million inhabitants within an area of 608 km²). Seoul s floor area ratio is of 2.5 before undergoing renovations. Afterwards, when buildings are enlarged, it rises to 3.2. Figure 1: View of the investigated housing estates in Hamburg, Germany Figure 2: View of the investigated housing estate in Seoul, South Korea

3 Climate Conditions The investigation carried out in Hamburg and Seoul can be applied to other European and Asian metropolitan areas, due to the specific and different surrounding conditions in both cities. Seoul s average annual rainfall is 1,283 mm and more than two thirds of it falls during summer months (June to September), varying considerably from year to year (for example, 754 mm in 1939 and 1,782 mm in 1989) [3]. Heavy precipitation events are caused by typhoons, which are common in East-Asia, and elsewhere known as hurricanes. Hamburg s average annual rainfall is 714 mm, evenly distributed throughout the year. One quarter of the year s precipitation falls from July to August. Its average precipitation can also vary from year to year; for example, from 680 mm to 1,070 mm [4]. Decentralized rainwater retention and infiltration The water treatment measures described can be classified into two types: decentralized rainwater management and decentralized wastewater management. To decentralize rainwater management is crucial, since future climate changes are expected to increase and intensify heavy precipitation events. As well, it is the starting point for developing environmentally sound systems in housing estates that can avoid the connection to a sewerage system. The results for the application of different measures in the investigated housing estates are described below [2]. In order to change the examined housing estates into almost sewerage-free areas, the implementation of decentralized rainwater management systems is essential and will enable on-site rainfall discharge without the need of connecting to a sewer system. Therefore the available space for applying decentralized rainwater infiltration measures has been investigated. Figure 3: Hamburg s investigated housing estates. Building s site plan (grey with house numbers). The area s size is approx. 50 x 100 metres. Shallow pit and infiltration ditch systems are placed in the courtyard and adapted for precipitation events that occur once every five years [2] Hamburg s existing housing estates are characterized by a base area to plot area ratio of These values do not vary, because the buildings will not be enlarged during refurbishment. According to Germany s legal requirements for infiltration measures, a specific distance between infiltration ditches or swales and the building has to be maintained in order to prevent the water from damaging the building. Furthermore, the residents use the only available area in the courtyard for onsite rainwater infiltration as garden. Therefore, the available area to apply the necessary measures is only about 50% of the free courtyard area (net infiltration area), equivalent to 27% of the total courtyard area (gross infiltration area). The freeboard to groundwater table is approximately 6m. The water permeability of the soil is 10-5 m/s.

4 The space required for infiltration of precipitation was calculated for precipitation events that occur once every 5, 10 to 100 years. According to research results, retention and infiltration of precipitation events that occur only once every 100 years is also possible without limiting the use of the courtyard, if measures are taken with underground infiltration ditch systems. When applying a combined system of shallow pits and infiltration ditches, the space required would be larger than the net infiltration area. But it would still correspond to the available gross infiltration area, covering only 55% of the total available courtyard area (see also Figure 4). Figure 4: Portion of the infiltration area in the courtyard (gross and net) for swale and ditch systems as well as for ditch systems alone, adapted for precipitation events that occur once every 5, 10 to 100 years. [2] Figure 5: Seoul s investigated housing estates. The area s size is approx. 100 x 130 metres. Underground site plan. The green surface is the area required for an infiltration ditch system necessary for the retention and infiltration of extreme precipitation events (once every 100 years) and for poor water permeability of the soil. [2] The total construction costs of a system designed to cope with precipitation events once every 5 years were divided by the number of residents and compared with the renovation costs of a standard sanitary installation during refurbishment. The rainwater retention and infiltration system costs are 126 Euros per person on average. This amount is equivalent to only 9% of the average sanitary installation costs per person. Seoul s existing housing estates are characterized by a base area to plot area ratio of During refurbishment, buildings will be enlarged and the base area to plot area ratio will afterwards be of Furthermore, an underground car park area will be built, increasing the water-impermeable area to 74%. Hence, only 26% of the total area is available for rainwater infiltration. This portion is always similar to Hamburg s available gross area in the investigated housing estates (27% gross available, see above). Theoretically, the area under the underground car park area could also be used for infiltration purposes. However, according to the investigations herein described, this is not necessary. Research findings may be transferable to lower water-permeability areas; rainwater retention and infiltration systems were calculated on a water-permeability basis of 10-6 m/s. The actual water-permeability is 10-4 m/s and the freeboard to the groundwater table is approximately 10m.

5 Figure 6: area and volume in percent, related to different surface properties and infiltration methods for a seweragefree housing estate. [2] The space required for infiltration of total rainfall was calculated for regularly occurring extreme precipitation events in Seoul (equivalent to precipitation events that occur once every 100 years). According to research results, retention and infiltration is possible without limiting the design and uses of the housing estate after renovations. Therefore, a 0.9m high underground infiltration ditch system created from plastic elements is buried and covered with soil (see also Figure 5). To build larger retention systems, the volume could be easily doubled or tripled by arranging infiltration elements in a pile and by burying them deeper into the ground; for example, for areas with less permeable soils or less available area. By constructing water-permeable footpaths and greened roofs, as well as shallow pits, the underground infiltration system could be reduced in size. Figure 8 illustrates the decrease of area and volume in terms of different surface properties and infiltration methods. By applying partly unsealed surfaces and greened roofs, the system s volume and area could be reduced by almost one third. The total construction costs of a system designed to cope with precipitation events once every 100 years were divided by the number of residents and compared with the renovation costs of a standard sanitary installation during refurbishment. The rainwater retention and infiltration system costs are 453 Euros per person on average for 10-6 m/s water-permeable soil. This amount is equivalent to only 2% of the average building renovation costs per person. In case of 10-4 m/s water-permeable soil, a much smaller system would be required. Such system would cost only 52 Euros per person on average, equivalent to 0.2% of the average costs of building renovation per person. The measures described for rainwater infiltration are also suitable for the complete infiltration of decentralized, treated and purified wastewater from households, because the accumulated volume is relatively small compared to rainfall [2]. Rainwater catchment and utilization Rainwater utilization measures in both cities may not be calculated for designing rainwater retention and flood control measures. But drinking water demand is met at a maximum 26% in Hamburg and 25% in Seoul, because of natural and structural conditions (climate, high population density and relatively-small rainwater catchment area). Due to different precipitation patterns, the tank volume required for storing rainwater in Seoul is almost 8 times greater (0,66m³/inhabitant) than in Hamburg (0,09m³/inhabitant). The portion of collected rainwater is equivalent to 9% of the total water demand of water saving systems in households. Rainwater catchment systems in greened roofs reduce the degree of efficiency even more. The construction work related to rainwater catchment systems and utilization measures does not limit the uses of property in the investigated housing estates [2].

6 Decentralized wastewater management The author examined the application of three different systems of water supply and decentralized sewerage treatment in both housing estates. System 1 comprises vacuum toilets and an anaerobic digestion facility for the fermentation of black water (6 litres waste water from the toilets per person and day) and kitchen waste (1 litre per person and day), biogas production and its utilization as fuel in a combined heat and power generator. System 2 corresponds in general with System 1. The difference is that the black water is not fermented together with organic kitchen waste but is treated together with grey water from kitchen in a Membrane Bio Reactor (MBR). System 3 comprises the installation of urine separation toilets with yellow water (urine) collection. The remaining brown water (sewage from toilets without urine) is treated together with the grey water from kitchen in a Membrane Bio Rector. The three systems are still connected to a central drinking water source and they comprise the retention, evaporation and infiltration of rainwater and purified wastewater as well as the recycling of grey water generated from bathrooms for service water supplies. Measures to recycle grey water generated from bathrooms (using Sequencing Batch Reactors or Membrane Bio Reactors) and to substitute the use of drinking water can successfully meet service water demands (for toilet flush, laundry and cleaning) in the investigated housing estates in Hamburg and Seoul by 100% (that is almost 37% and 26%, respectively, of total water demand). Grey water from kitchen processes is first filtered (remaining solids are precomposted in a retting container) and afterwards treated either separately (in system 1) or together with black water (in system 2) or together with brown water (in system 3). Hence, the systems differ mainly in the applied principles and technologies for the treatment of black water (formed by brown and yellow water). Collected rainwater is not used for drinking water because the water supply is much less than reclaimed grey water (only 25 26% in Seoul and Hamburg) (see above) [2]. Conclusion Measures to decentralize rainwater management and the possibility to avoid the connection to sewerage systems can significantly help develop sustainability of urban water systems, for example by flood control, water storage, aquifer recharge and evaporation. Together with measures to decentralize wastewater management and to encourage the adoption of green buildings policies, positive synergetic effects can be achieved. The necessary steps towards almost sewerage-free housing estates are feasible with relativelylow effort when refurbishing buildings and using alternative sanitary installations. According to the research findings, in Germany and South Korea, a high user acceptance of the described decentralized water systems may be expected. The implementation of these systems will save drinking water resources. It would also allow nutrients to be reused, treating pollutants appropriately. Therefore, the investigated systems comply with the principles of closed loop recycling management. Centralized systems costs can be reduced over the long term by not constructing an urban sewerage system. The basic requirements to achieve a wide application of decentralized water systems can be fulfill due to the very different climate conditions and the transferability of single measures herein described. Hence, alternative water systems and sewerage-free housing estates may be implemented in cities worldwide under different natural and structural conditions.

7 References [1] Schuetze, T. (2006). Decentralized Rainwater management Solution for sustainable (re)development and independency. Pages Proceedings IWA World Water Conference. Rainwater Harvesting Task Force, Rainwater Harvesting and Management September, Beijing, China. [2] Schuetze, T. (2005). Dezentrale Wassersysteme im Wohnungsbau internationaler Grossstaedte am Beipiel der Staedte Hamburg in Deutschland und Seoul in Sued-Korea (Decentralized system of water supply and wastewater in house building. An example: Hamburg, Germany & Seoul, South Korea). PhD Dissertation. University Hannover, Department Landscape and Architecture. Hannover, Germany [3] Korea Water Resources Corporation (2002). Basic Features of Water Resources in Korea. Information Brochure. Daejon, South Korea [4] Mueller, M. (1983). Handbuch ausgewaehlter Klimastationen der Erde (Handbook of selected climate stations of the world). Sourcebook. Trier, Germany.