Ecological Engineering for Water in Sustainable Settlements Construction

By Min, Wang Jingsong, Yan; Rusong, Wang

Key words: Human settlement, ecological engineering, water system, sustainable development SUMMARY

The components of a water system in a settlement should benefit the residents, environment and nature. Ecological engineering for water use in settlements is crucial for designing and constructing a sustainable community. This article discusses ecological engineering measures and technology case studies in China for systematic regulation of the multiple functions of water in a sustainable community. These methods include reusing all sources of water (rainwater, wastewater and surface water), separating water that has been used for different purposes, promoting nutrient cycling, and improving the self-purification of water by controlling pollutant loads.

THE WATER SYSTEM AS A PART OF SUSTAINABLE HUMAN SETTLEMENT AND DEVELOPMENT

Sustainable human settlement is an important part of sustainable development, and is governed by specific principles and strategies. These have been discussed at many international meetings (United Nations Conference on Human Settlements 1976, 1996; Agenda 21 1992) and also stressed by China’s central government (The State Council of the People’s Republic of China 1994). The ideal human settlement involves reasonable planning, harmony between man and nature, a sound environment and comfortable living conditions. To meet these requirements, human settlements need to be designed and constructed according to four major ecological principles: coordination between nature and man, following natural laws, maximum production with minimum cost, and satisfying reasonable human needs (Yan and Wang 2003a,b, 2004a, b, c) (Figure 1).

Water is essential to human settlement due to its scarcity and functions in human evolution (Davis 1996). The water system in a settlement includes water resources, the landscape, water safety and habitat, among other things (Yan and Wang 2003b). Residents, the environment and nature all benefit when the various parts of the water system and surrounding environment are in harmony. Water has many functions in the community: drinking, flushing, adjusting hydrology, maintaining biodiversity and reducing the heat island effect. Water system planning and design in a settlement is generally limited to water supply, drainage and water in the landscape (Yan and Wang 2003b). Potable water is usually supplied by local government; rainwater and wastewater are generally discharged directly or after processing at central treatment plants, which transfer pollutants to natural water bodies.

This linear model of resource consumption (Figure 2a) only considers water quality within a community or individual parts of the system. Such a model wastes water resources and transfers the problems of the community to the environment. Using water in landscaping often focuses on the scenery without considering ecological function, which results in high maintenance costs. The semi-circulatory water usage model (Figure 2b) integrates technology and ecological methods according to different water functions, and decreases demand for clean water and discharge of wastewater. Ecological engineering of water is very important for designing sustainable ecosystems to benefit the natural environment and human society, because it will change the mode of water use and create a sustainable water system within the community (Mitsch 1996; 1998; Mitsch and Jorgensen 2003; Yan and Ma 1991).

UNDERLYING CAUSES OF ENVIRONMENTAL PROBLEMS

Usually, water problems, such as excessive consumption, pollution, flooding and waterlogging, result from transient combinations of natural and artificial factors. These environmental problems are caused by stagnancy and exhaustion of resource input- output at spatial and temporal scales that can involve conflicts between local needs versus the whole ecosystem, or short-term yield and economic gain versus long-term sustainability (Yan and Wang 2003a).

The aquatic system is self-balancing, it exchanges materials and energy with the external environment; and water quality is often determined by the kind and concentration of dissolved solutes. Aquatic plants and animals transfer, transform and export dissolved substances, keeping the system in a relatively stable state. When input exceeds output and the self-regulatory capacity of the ecosystem, stagnation develops and solutes accumulate. For example, when runoff or wastewater bring abundant nutrients into the water, beyond the carrying capacity, pollution or eutrophication will occur; while if input is less than output of a substance (as in resource exploitation), this will reduce the concentration of the substance and result in its exhaustion.

Substance transformation and energy turnover depend on the structure and function of the aquatic ecosystem. The structure, which consists of organisms and their relationships in the aquatic ecosystem, is the framework or channel for functions, including substance absorption, transformation and accumulation. Within the structure, materials are converted along a food chain, transferring input to output, which causes changes in the system and its states (Yan and Zhang 1997). Functions develop and sustain structures but, conversely, functions depend upon the structure. In an ecosystem with a fully developed structure, organisms will use and transform all inputs, with no net waste because all by-products and wastes are fully utilized. If function and structure are in disharmony, the transformation will be blocked or weakened, resulting in substance accumulation and eutrophication.

The aquatic ecosystem in a community is human-dominated by technological and social behaviour, sustained by natural life- support systems, and vitalized by ecological processes. It is a Social-Economic-Natural Complex Ecosystem (SENCE, Figure 1) (Ma and Wang 1984). Modern capital-dominated society, since the industrial revolution, has operated on the assumption that the carrying capacity of natural resources and the environment is infinite, and that the effects of any economic activity can be measured in monetary terms and remedied from the profits (Wang and Yan 1998). The resulting shortsighted preoccupation with short-term yield and economic gain weakens or damages the ecosystem’s metabolic network, or changes its processes, which results in resource waste and excessive discharge of pollutants.

ECOLOGICAL ENGINEERING FOR WATER IN SUSTAINABLE COMMUNITY DEVELOPMENT

Ecological engineering for water in a sustainable human community seeks an environmentally sound, economically productive and systematically responsible form of construction. It integrates planning, design and technologies for sustainable development (Wang et al. 1994; Yan and Wang 2001). Some basic concepts and principles of ecological engineering include design consistent with ecological principles, site-specific contexts, independent functional requirements, efficiency in energy and information, and acknowledgement of the values and purposes that motivate design (Odum 1992; Mitsch and Jogensen 2003; Bergen et al. 2001). In China, based on systems ecology, scholars have recognized the principles of holism, harmony, self-resilience, regeneration and recycling (Ma and Yan 1989; Yan and Ma 1991; Yan and Zhang 1997) in developing residential communities such as Hai De (HDC) in Yangzhou, Hui Jing (HJC) in Guangzhou, and Hua Fa (HFC) in Zhuhai.

As aquatic systems are a sink for terrestrial runoff (Birkett and Rapport 1998), a plan with a systematic perspective focuses on the integration of source, flow and sink (Yeang 2000), and on the harmonious relationship of structure and function is required. Combined with the necessary techniques and facilities, the plan should not only arrange the supply and drainage system and the recycling of water from different uses, but also design ponds in the community as ‘nutrient sinks’ (Tilley and Brown 1998). The goals of such plans are to save water resources, beautify the landscape, and maximize economic gains by means of reduction, reuse and recycling (Yan and Wang 2003b). Water conservation is one of the most effective methods to solve tins problem.

Separation of water systems by end use

Water of differing quality can meet different goals in residential areas, which can save tapwater and reduce stress on the water supply and sewage treatment infrastructure. For example, Hai De (HDC) in Yangzhou has 750 families, with a population of 3062, covering 21 hm^sup 2^. The average daily water consumption was 1089 m^sup 3^ (Table 1). The community adopted a strategy of separating water for different uses. Two separate water supply systems were built, a municipal tap water system, and a supply of lower quality water for irrigation, toilet flushing, and landscaping use. The latter came from collected rainwater and treated sewage. The landscaping water was used to collect and store rainwater and treated wastewater. An on-site, low-power treatment system was established to treat wastewater, which was used for irrigation. In this way, the community saved tapwater and reduced discharge by about 140,000 m^sup 3^ annually, about 35% of its original total supply of tapwater. Water-efficient appliances

Washing clothes, flushing toilets and badiing account for almost 80% of a family’s total water consumption. Therefore, water- efficient appliances are effective in saving water.

1. Efficient toilets. Lavatories account for 25-30% of domestic water usage. In HDC, 3/6-liter toilets have been used, reducing water use from the traditional 9 liters to 3 liters for urine and 6 liters for excrement.

2. Efficient taps could save 40% of tapwater.

3. Efficient showerheads could save 50% of bathing water.

4. Recycling facilities for swimming pools, through filtration and sterilization, could save 75% to 80% of water use.

Adaptive rainwater management and on-site wastewater treatment

Black water contains most resources in domestic wastewater, and grey water contains less organic matter and only small amounts of plant nutrients (Brandes 1978; Jenssen 1996). Rainwater, black water and grey water in the community are discharged by different systems and collected for treatment and recycling, respectively. The open loop of water usage is partially closed to promote sound recycling and reduce discharge.

The speed and volume of surface runoff are key factors leading to downstream flooding. Too much surface sealing disturbs the hydrology of the watershed (Schueler and Galli 1992). Sustainable community development should consider using rainwater. In HDC, rainwater was discharged into vegetation and ponds for infiltration and collection. The vegetation comprised more than 40% of the area and was 0.10 m lower to allow it to receive runoff from sealed surfaces. Trenches 1-2-m wide and 0.1-0.3-m deep (Wang and Li 2002) were constructed in open spaces, filled with filtering materials, and planted with grass. Permeable paving was used in over 10000 m^sup 2^ of roads and parking lots. All these methods helped to decrease surface runoff and flooding.

Wastewater treatment and utilization

Usually, sewage is treated in a central treatment plant, resulting in high cost and a burden to the city (Gong and Sun 1996; Otterpohl et al. 1997). Combining centralized sewage treatment with decentralization is the major direction for wastewater treatment. In a sustainable community, sewage should be treated and used on site, and treatment should be decentralized as much as possible by adding some linkage to wastewater circulation. This internalizes environmental protection in production and consumption, and includes wastewater treatment in water utilization.

In HDC, a biogas digester was used to treat black water (Figure 3). Underground digesters, 15-50 m^sup 3^ in size, were built to take in black water 5-10-times daily. The total digester volume exceeds 1000 m^sup 3^. By adopting water-efficient toilets, total black water production was reduced from 210 m^sup 3^ to 120 m^sup 3^ per day, and black water was retained in digesters for 5 to 9 days. The effluent reaches the standard of wastewater treatment plants (GB8878-1996): COD^sub Cr^, BOD^sub 5^ and SS decreased from more than 400mg/l, 200mg/l and 200 mg/1 to less than 100 mg/l, 50 mg/l and 50 mg/1, respectively. Mortality of ascarid eggs was 100% and the bacillus count was less than 10/1. The digester controls fly and mosquito propagation and effectively prevents infectious diseases. The effluent is safe, good fertilizer and a natural pesticide (especially to aphids). Most of the effluent is used on vegetation.

Grey water and treated effluent of black water were treated with an SPFS to improve water quality. In HFC, an SPFS of 102,500 m^sup 2^ was constructed on top of underground garages. The treatment capacity was about 4,180 m^sup 3^/d, with a hydraulic load of 4 cm/ d. Wastewater first flows into the balance tank, then into a sedimentation and filtration pond. The water is distributed evenly into the SPFS. After treatment, the COD^sub Cr^, BOD^sub 5^, SS, NH^sub 3^-N and fecal coliforms in the effluent were 70-120 mg/l, 30- 50 mg/l, 40-70 mg/l, 20-40 mg/l and 104, decreasing 80-95%, 90-98%, 60-80%, 90-98% and 60-90%, respectively. The reclaimed water volume is 1.15 million m^sup 3^ per year, about 80-90% of the total influent. Of this reclaimed water, 415,600 m^sup 3^ are used for vegetation irrigation, sprinklers and other uses.

Protecting and utilizing landscape water

Landscape water is a major component of a sustainable community. Its location should be based on the specific landform and features of the site. In HJC, the lower places and ditches were used for landscape water. Four major water bodies of 1870 m^sup 2^, 3920 m^sup 2^, 5925 m^sup 2^ and 19,000 m^sup 2^ are distributed in the community to modify the microenvironment. In HDC, the adjacent river was dammed with a lock to maintain a relatively stable water level. The river is used as a receptor for rainwater and treated wastewater. In HDC and HFC, the surface water reached 20,000 and 9,063 m^sup 2^ with available volumes of 40,000-50,000 and 10,000 m^sup 3^, respectively. Surface water bodies help to control flooding in the lower part of the basin.

The littoral zone is a very important part of a river (Baker et al. 2000). Littoral vegetation reinforces the soil with fibrous roots, increases soil shear strength by reducing pore pressures through transpiration, and anchors slopes through deep root penetration (Li and Eddleman 2002). Vegetation can also decrease runoff flow velocities, dissipate flow energy and absorb pollutants in the runoff. Sealed banks destroy vegetation and its dependent functions. Ecologically, a bank should be protected by vegetation instead of hard impermeable surfaces. Many methods of building ecological banks have been suggested, such as live staking, live fascines, brush layers, vegetated geo-grids and geo-gabions., Staking and geo-gabions were used in HDC. Self-purification capacity improvement methods can prevent stagnation and regulate and increase the removal of water pollutants:

1. Increasing dissolved oxygen. Fountains, artificial waterfalls and cascades were used in HDC, HJC and HFC to increase dissolved oxygen.

2. Planting submerged vegetation. This can reduce suspended mud, absorb nutrients and compete with phytoplankton to improve water transparency and increase dissolved oxygen (Qiu et al. 2001; Pu et al. 1998). In HDC, where 3/5 of the water body was seeded with Vallisneria asiatica and Potamotegon, transparency improved from 0.4- 0.5 m to 1.5-2.0 m.

3. Using emergent plants and floating cultivation beds. Water hyacinth (Eichhornia crassipes) was introduced into the water body to increase transparency (Zhou and Yang 1984) and feed fish. In Yangzhou, maximum production of water hyacinth reached 900t/hm^sup 2^, and it can absorb 1580 kg N, 358 kg P and 198 kg S from the water (Ma and Yan 1989). Harvested hyacinth is used to raise grass carp (Ctenopharyngodon idellu), Wuchang fish (Megabbrame amblycephala) and tilapia (Sarotherodom mossambica and S. nilotica). Floating square or triangular hydroponic beds (2 x 2 m) were also used for plant cultivation, functioning similarly to water hyacinth. In HDC, the floating beds were planted with iris (Iris ensata), canna (Canna indica) and aquatic morning glory (Ipomoea aquatica).

4. Raising animals that consume phytoplankton and organic debris. Transparency is an important feature of landscape water and phytoplankton reduces transparency. Animals that consume phytoplankton are one link in the food web that reduces phytoplankton and organic debris, and promotes transfer and transformation of pollutants (Yan and Zhang 1994; Yan and Wang 2003). Silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis) and silver crucian carp (Carassius auratus gibelio) feed on phytoplankton and organic debris, consuming 25-30 kg of plankton per kilogram weight gain (Yan and Zhang 1994). In HDC, the production of these fish was 1.8-2.2 tons, consuming 45-66 tons of phytoplankton. The annual fish output from the river is 1.5- 1.8 tons, equal to 54-66 kg of N and 7.2-9.8 kg of P. The clams, leach (Cristaria plicata) and lea (Hyriopsis cumingii), can filter plankton of 0.05-1.0 mm. Each kilogram of clams can filter 12-13 m^sup 3^ of water per annum. In HDC, 400 kg of clams were raised (2 tons of production would clarify 240,000-260,000 m^sup 3^ of water, five- to six-times the volume of the water body) consuming 15-20 tons of algae and debris. 400 kg of snails (Cipangopoludinas chinensis) were put into the water. These ate algae and secreted chemicals that coagulate suspended organic matter.

CONCLUSIONS AND RECOMMENDATIONS

Ecological engineering of water use is important in constructing a sustainable community. Its general principles are holism, harmony, self-resilience and recycling. Links between technologies are integrated and the structure and functions are coupled. In traditional engineering, the elements of the system are single- purpose, open loops of material flow, and rigid products and technological processes that reduce the efficiency of resource use and increase consumption. For example, unnecessarily using tapwater increases the water requirement and sewage discharge and reduces water use efficiency of the community as a system. Without considering multi-step and multi-functional utilization, such as recycling sewage for reuse on site, the living space of the community is also wasted. Multifunctional ponds and canals should be emphasized for raising plants and fish that can purify, beautify and improve the microenvironment.

The societal and ecological benefits of ecological engineering of water use are obvious. It is engineering not only for the inhabitants and locale, but also for the benefit of future generations and the natural environment. Environmentally sound design and engineering improve environmental quality. At the same time, however, costs increase, raising market prices. In general, people prefer a house with a good environment and are willing to spend more for it, but when most of the ecological benefits are hidden behind the physical structure, without economic benefit, people have no incentive to pursue it. Experiences in China show that ecological engineering collapses in a community where the technology does not match institutional and human behaviour (Wang and Yan 1998). HDC and HFC confronted these issues. For example, wastewater is treated on site with little or no drainage to the city treatment plant; however, the municipal administrative department has not reduced the drainage fee and still charges either according to the water supply or a fixed fee. The low price of the water supply will also affect application of ecological engineering in the community. Though China is short of water (Wang et al. 2000), tapwater is still cheap. Saving water by engineering is almost the same or even less than the operating cost of water collection and on- site treatment systems. Economic gains must be considered; dierefore, mutually concordant policies and regulations for reducing wastewater and rainwater discharge should be established to promote ecologically sound development of the water environment. Although there are successful experiences and case studies of ecological engineering of water use in communities, the techniques, measures and useful species in a specific location are not universally applicable. For example, the most useful aquatic species differ between nordiern and southern China because of variations in climate and human preferences. Therefore, there is no standard solution or construction model for different places (Van der Ryn and Cowan 1996). The choice and application of successful techniques and measures must consider local conditions, even though the general principles are universally applicable.

ACKNOWLEDGEMENTS

This research was funded by the National Natural Science Foundation of China, under contract number 39930040. We appreciate the support of the developers of HDC, HFC and HJC. We also thank the reviewers and Charles Crane, William Mitsch and Ruthmarie Mitsch for their inspiring and constructive comments and for improving the English of the paper.

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Wang Min1,3, Yan Jingsong2 and Wang Rusong1

1 Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing

2 Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing

3 Eco Land Company, Beijing

Correspondence: Wang Min, P.O.Box 123, Beijing Forestry University, Beijing 100083, China. Email: wangm48@163.com

Copyright Sapiens Publishing Dec 2007

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