The 3rd International Symposium on Low Carbon Buildings
Introduction and assessment of an integrated method for urban rainwater harvesting in China SUN Yuda Department of Architecture & Built Environment University of Nottingham Nottingham, UK
and periodic variation of water resources; (3) the continuous living standard urbanisation and industrialisation; and (4) the extremely serious pollution in cities.
Abstract—From an ecological stand point and in view of the impending water crisis, rainwater should not be simply drained as waste water. The objective of rainwater harvesting in cities is to maintain the balance of the natural hydrological cycle during the incredibly rapid rate of urbanisation and industrialisation. Thus, rainwater harvesting is of great importance for the sustainable development of cities. In this paper, the necessity and feasibility of using rainwater in urban areas is analysed and a specific method for using rainwater in residential areas is discussed in detail. Harvesting rainwater in residential areas of several large cities in China has great potential. Some method can be applied to harvest rainwater in-site, although the most effective way to realise this is by incorporating essential rainwater management at the planning stages of a city.
Therefore, rainwater has much potential as a relatively simple and attainable water resource that can be widely used to supply fresh water in areas where the existing water supply system cannot meet the demand for water. This article analyses the characteristics of rainwater as a water resource in China to justify its use and to assess its feasibility. The developed methods of rainwater management and their benefits were assessed. This paper presents an example of the integrated design of rainwater utilisation for residential areas. The feasibility, advantages, and limitations of the proposed design are analysed through various methods.
Keywords- rainwater harvesting; urbanisation; water resource; water crisis; water management
INTRODUCTION AND BACKGROUND
CHARACTERISTICS OF RAINWATER AS A WATER RESOURCE
According to the UN Water Development Report, “the water crisis is thus increasingly about how we, …, govern the access to and control over water resources and their benefits.” The total volume of water resources in China is approximately 2720 billion m3, which ranks sixth globally; however, its average water resource per capita is only one-fourth of the world’s average . The water shortage problem affects both rural areas and modern cities in China. Given the rapid urbanisation, the incessant expansion of urban populations, and the rapid improvement of living standards, the problem of water shortage in cities has increased. Therefore, the dramatically increasing demand for water significantly affects the water supply. Among the 712 cities in China, 354 are facing water shortage problems . This problem has become one of the most important factors that may hinder the further economic and social development of China.
The average annual precipitation in China is 648 mm, which is 19% less than the world average of 800 mm; the volume of the average annual precipitation in this country is 6190 billion m3 . The regional distribution of precipitation in China is very uneven, although it shows a general trend of gradually decreasing amounts of precipitation from the southeastern coastline to the north-western hinterland . The annual precipitation in south-eastern coastal areas and some south-western regions is about 2000 mm, whereas that in the north-western China is usually less than 300 mm and less than 25 mm in the Tarim, Turpan, and Chaidamu basins according to the China Statistical Yearbook 2011 . The rates of precipitation in 2010 of several major Chinese cities are shown in Table I. TABLE I.
ANNUAL PRECIPITATION IN SEVERAL CITIES IN 2010 
North-eastern (mm) Beijing Harbin 522.5 591.3 South-eastern (mm) Guangzhou Shanghai 2353.6 1128.9
The city water supply basically comes from two resources: surface water and groundwater. Both water resources are closely related to precipitation, which is the main source of resupplying these reservoirs. The recharge to surface water by precipitation accounts for about 98% via both direct and indirect processes . Therefore, water resource supplies to urban areas is enough though not plentiful. These resources are continuously recharged from rainwater because of the hydrologic cycle. However, water scarcity continues to be a serious problem, which has been attributed to the following reasons: (1) the small volume per capita; (2) the large spatial
North-western (mm) Urumqi Xi’ning 282.4 405.0 South-western (mm) Guiyang Kunming 1010.0 869.1
Aside from the uneven spatial distribution of urban rainwater, its temporal distribution is similarly uneven. However, the demand and supply of water in a city is usually steady. Given the influence of the monsoon climate, precipitation in China is usually concentrated in the summer, especially along the eastern coast where most large cities are
The 3rd International Symposium on Low Carbon Buildings
35 30 25 20 15 10 5 0
methods for using urban rainwater first appeared . Since then, rainwater management has drawn more attention, and different kinds of standards have been proposed. Although it began relatively late, the utilisation of urban rainwater has been continuously developed in China. Aside from a number of medium-sized cities, rainwater utilities have also been established in some of the larger cities, including Beijing, Shanghai, and Dalian. To date, rainwater utilisation technologies can be divided into several classes. These technologies include decentralised/centralised systems for rainwater collection in buildings or residences, decentralised/centralised rainwater infiltration systems, roof garden rain water utility systems, and integrated rainwater utility systems for roof gardens, reclaimed water, filtration, and waterscapes . The example mentioned below belongs to the last group.
250 Precipitation (mm)
located . The relationship between the average precipitation and temperature for every month in Shanghai and Beijing from 2006 to 2012 are shown in Figures 1 and 2, respectively. By contrast, this uneven distribution has its own benefits because the demand for water is usually higher in the summer than at any other time of a year.
200 150 100 50 0 0 1 2 3 4 5 6 7 8 9 101112 Month
The first reason for using rainwater has been reiterated above, which is to alleviate the water shortage in cities. Simultaneously, the other benefits of rainwater should not be ignored. These benefits or aims are inalienable parts of the completed rainwater management system. One aim is to rehabilitate the natural behaviour of rainwater in urban areas, including rainfall, runoff, evaporation, and infiltration. Consequently, the groundwater is gradually recharged to alleviate its scarcity and land subsidence. Furthermore, a completed rainwater management system can mitigate the pressure on existing sewage networks and subsequently reduce flooding in cities. All these benefits are clearly related to the environment, economy, and ecology, which should be considered in parallel with flourishing urbanisation.
30 25 20 15 10 5 0 -5 0 1 2 3 4 5 6 7 8 9 101112 Month
140 120 100 80 60 40 20 0 Precipitation Temperature Precipitation (mm)
Figure 1. Five-year average monthly precipitation and temperature in Shanghai (from 2006 to 2010)
An analysis of the different existing methods for rainwater collection showed that each method has its own limitations and advantages. However, not all methods are applicable in urban residential areas in China because of their specific characteristics. These characteristics include its centralisation, high population density, and the dominance of high-rise apartments. Thus, a promising method of collecting and using rainwater is proposed below, which takes into consideration these characteristics. The proposed method combines green roof technologies and a variety of waterscape facilities for the purification, storage, and utilisation of rainwater by installing rainwater cistern on the roof of a building. The rainwater harvesting process is shown in Figure 3.
Figure 2. Five-year average monthly precipitation and temperature in Beijing (from 2006 to 2010)
Therefore, rainwater can be treated as a water resource because of the following reasons:
Rainwater has the fastest regeneration speed among all water resources.
Rainwater is not concentrated in a particular area. This expanded distribution of rainwater allows for its accessibility everywhere.
The treatment cost for rainwater is low. Such treatments can often be used directly, apart from a number of special cases, especially in several rural areas where the air quality is high.
The supply of rainwater is always more in the summer, when the demand is also higher.
Using rainwater incurs less cost because less or no infrastructure needs to be constructed. III.
SAMPLE INTEGRATED DESIGN OF RAINWATER
CURRENT RAINWATER UTILITY AND BENEFITS
In the 1980s, several countries have regarded rainwater as a part of its water resources. During this time, modern
Figure 3. Rainwater harvesting process
The 3rd International Symposium on Low Carbon Buildings
A. Feasibility To analyse the feasibility of collecting rainwater, we briefly calculated parameters for a selected residential department in Shanghai as an example. The building is assumed to be five-storeys-high, with two families residing in each storey. An average of 3 people live together in each family. Thus the building is assumed to have a total of 30 people. The overall effective catchment area on the roof is 300 m2; the average annual precipitation in Shanghai is 1189.0 mm, as derived from the five-year statistics from 2006 to 2010 . The theoretical total rainwater collected throughout a year is calculated according to Equation (1) by assuming an effective ratio of 80%. ⁄
2) Integrated design with the building After flowing through the vegetation modules and being partially purified by them, water with large gravitational potential energy is accumulated in the cistern. Water in the cistern on the roof is more easily extracted as compared with the below-ground systems, which often require a pump. Moreover, detecting the leaks and faults underground is difficult, whereas tree roots or overhead traffic may damage the below-ground water storage . A well-designed rooftop cistern is cheaper than burrowing a reservoir underground. In addition to the location and size of the cistern, the pipe systems should be carefully considered along with the architectural design. The pipework should achieve the following requirements: (1) Any leakage and any kind of insufficient sealing should be avoided; (2) The flow rate into each household and out of the building is well controlled; (3) Water from the municipal supply is imported when the water level is low; (4) The pipes correctly respond when collected water is more than sufficient; and (5) Pipe surfaces are kept clean and have their own ability to reduce any kind of pollutants.
Thus, such a system can collect 285.4 m3 rainwater every ⁄ year, which is equal to 782.0 l every day ( ). Moreover, we assumed that each person uses the toilet on an average of four times a day, which uses 5 l during each instance. Therefore, a total of 600 l water is ⁄ ⁄ used every day ( ⁄ ). Compared with the amount of rainwater collected, this amount of water is sufficient for the people living in the apartment to flush their toilets for as long as rainwater can be harvested effectively. In these months whose precipitations are not plentiful with the amount of about 70mm per month, the rainwater collected a month can be: ⁄ ⁄ . It is a little insufficient comparing with .
3) Integrated design with waterscapes Waterscape design in a residential district is necessary. However, consuming a large amount of well-treated or even potable water that is intended for domestic used is unnecessary. A portion of the rainwater collected by the apartment can be used as the major of supplementary source for waterscape use. When the water for the landscape needs to be changed and discharged, subsurface irrigation or infiltration techniques may help to irrigate the vegetation, recover the recharging ability of groundwater, and gradually compensate for land subsidence . The schematic of the integrated design is shown in Figure 4.
Thus, when considering generally uneven and discontinuous of inter-year distribution of urban rainwater. The magnitude of rainwater cistern should be designed based on 20 times to 30 times of the amount required for one day. Therefore, if the rainwater is only used as lavatory water, the size of the cistern should be 12-18 m3. Its maximum full weight should be more than 12 tones. Such a size is suitable for a total roof area of 300 m2. This assumption is more realistic if 1 cistern is replaced by 2 or 4 smaller cisterns. B. Advantages 1) Integrated design with green roof Rooftops, as the fifth façade of a building, have long been neglected by people. Although establishing green roof is still a relatively young practice in China, this kind of roof has developed rapidly since 2000 . Rainwater harvesting on the top of the building acts as a more innovative technology than can be designed to function in combination with a green roof. An environment-tolerant green roof vegetation layer can be designed to be the first filter of rainwater before the water flows into the cisterns from specific channels or pipes. Apart from the intrinsic benefits of a green roof, including its being aesthetical and environment-friendly, this kind of integrated design can function as a thermal insulation layer to replace the conventional ones. Simultaneously, the vegetation can be arranged as different modules for easy maintenance.
Figure 4. Schematic of the integrated design
C. Limitations and further suggestions The aforementioned example is relatively simple and basic. However, a number of limitations exist, which necessitate its further study. First, the rainwater cisterns are constantly exposed to the environment, which makes it more susceptible
The 3rd International Symposium on Low Carbon Buildings
to damage. In addition, the water quality should be prudently guaranteed because of its exposure to the elements. Second, several adjustments are difficult if the pipework design is not sufficient from the initial stages of the architectural design. Consequently, modifying an existing building is not easy. Third, all pipework, equipment, cisterns, green roofs, and others that are used in the integrated design increase the initial cost as compared with traditional construction. Furthermore, effective cooperation between the architects, landscape designer, construction engineers, and building service engineers will significantly ensure a successful system. Finally, although the harvested rainwater can easily meet the demands of residential lavatory use during the hottest months, a number of problems may exist in winter, when the amount of precipitation might not be sufficient.
must be identified, which include the refurbishing of residential areas, as discussed above.
  
Based on our analysis, cities face the unprecedented challenge of water shortage. Rainwater as a novel water resource for urban areas has its own features. These special characteristics contribute to the great potential of rainwater to help solve the water crisis. The most efficient and effective way to realise this goal is to include rainwater management as part of the planning department of a city. However, the existing sewage and drainage system implies that managing rainwater will not be easy. Therefore, alternative methods
Trade Mission to China, Foreign Trade Administration, Ministry of Industry, Trade & Labor, “Water resource in China- Facts and figures,”, based on information of the Ministry of Water Resource, P. R. China. G. Zhu and Z. Duan. (2003), “Research of city rainwater resource comprehensive utilization” in Advancement of Sience and Technology and Countermeasure, vol V, pp 53-55. Chapter of Resources and Environment (2011), China Statistical Yearbook. Z. Zheng, C. He. (2003), “Cheng shi shui wu guan li (Urban water management)”, China Hydrologic Conservancy & Electric Press. J. Ma (2004), “Zhongguo shui weiji ( China’s water crisis)”, EastBridge, Norwalk, CT. B.Zeng, H. Tan, and L. Wu. (2007), “A new approach to urban rainwater management”, in Journal of China University of Mining & Technology, vol. 17, no. 1, pp. 82-84. J.Mcintire. (2006), “China's Learning to Love Green Roofs”. Available from: http://www.treehugger.com/corporate-responsibility/chinaslearning-to-love-green-roofs.html [Accessed: September 1, 2012]. K. Heather. (2007), “Design for water : Rainwater harvesting, stormwater catchment, and alternate water reuse”, New Society Publishers, Gabriola Island.
THE 3rd INTERNATIONAL SYMPOSIUM ON LOW CARBON BUILDINGS IN CHINA 第三届中国低碳建筑国际研讨会
Introduction and assessment of an integrated method for urban rainwater harvesting in China 一种中国城市雨水利用方法的提出和评价 SUN Yuda Department of Architecture & Built Environment University of Nottingham
Abstract—Because of either an ecological point of view or the reality of water crisis, rainwater should not be simply drained as waste water. The objective of rainwater harvesting in cities is to maintain the balance of natural hydrological cycle during incredible fast urbanization and industrialisation. Due to the characteristics of precipitation in some part of China, it seems reasonable to use rainwater in some urban residential areas.
Why do we suffer from water scarcity? (1) the volume per capita is small, (2) a great variation of water resource in time and space, (3) the continuously growth of population, development of urbanization and industrialization (4) the extremely serious pollution.
F.1: 1000m3 per person per year
F.2: Serious water scarcity in China
F.3: groundwater pollution in China
An example of integrated design of rainwater utilization Through the study and analysis of different methods of rainwater collection, it is easy to conclude that all the methods have their limitations and advantages. While the specific characteristics of urban residential areas in China decide some methods are feasible but some are not. When considering these characteristics, one promising method of collecting and using rainwater is preferred and will be advised below. This method is to take the combination of green roof and variety of waterscape facilities in purifying, storage and utilizing rainwater by installing rainwater cistern on the roof. The rainwater harvesting process is shown in Figure 6.
F.6: Rainwater harvesting process
Why rainwater can be treated as a water resource in some urban areas?
5 6 7 Month
9 10 11 12 Precipitation Temperature
F.4: 5-year average monthly precipitation and temperature in Shanghai (from 2006 to 2010)
• Rainwater has the fastest regeneration speed among all the water resource. • Rainwater is not concentrated in a particular area, the property of its expanded distribution decides its accessible usage in every corner. • The treatment cost of rainwater is low. It can be always used directly apart from some special cases especially in some rural areas where the air quality is high. • The supply of rainwater is always more in summer when the demand is also higher. • It always costs less to use rainwater because less or no infrastructure needs to be constructed.
9 10 11 12 0
F.5: 5-year average monthly precipitation and temperature in Beijing (from 2006 to 2010)
F.6: The schematic of the integrated design
Integrated design with green roof： The environmental tolerated green roof vegetation layer can be well designed to be the first filter of. Apart from the own benefits of green roof like aesthetical and environmental friendly, this kind of integrated design is another form of thermal insulation layer Integrated design with the building After flowing through and being purified by the vegetation modules, water with large gravitational potential energy is cumulated in the cistern. Water collected on the roof is more easily extracted compared with the below-grade system. Integrated design with waterscapes A portion of the rainwater collected by the apartment can be used as the source for waterscape use in a residential district. When the water for landscape use need to be changed and discharged, subsurface irrigation or infiltration techniques can be used. Figure 1, 2, and 3 are obtained from following website: http://lesjoyeusesboucanieres.net/2010/09/13/water-shortages-reach-crisis-levels-in-china-by-the-cnn-wire-staff/ http://www.businessinsider.com/china-water-risk-2011-11?op=1