Constructed wetland

I have studied at Guizhou University in China for four years; this period is a very important experience in my life. What impressed me is the comfortable climate in the summer day, air-conditioners or fans are not necessary for the dormitory. Because there is a nature reserve which is called Hua’xi wetland locating just about one kilometer away from the campus. The existing of this wetland regulates regional climate, so in hot days the temperature is lower than the predicted.

 

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Hua’xi wetland at autumn

 

A very rare storm occurred in the upstream area makes me realize that the comfortable climate in summer day is not the only benefit the wetland brings to my university. One day morning everyone who lives in the dormitory noticed that the water becomes turbid. Then university office published a notice told us due to the extremely rare storm in the upstream area, the wetland cannot treat dramatically increased water flow, and the simple equipment of water plant of the university also cannot purify the water thoroughly. This is the first time I know that our water for life (not include drinking water) comes from the wetland directly after simple purification and sterilization. Our university extracts water from the wetland to meet our daily demand such as teeth brushing and toilet flushing. Apparently, this wetland meets the standard of water sources area. This wetland saves lots of money for our university to get clean water, since we don’t need to buy clean water from the water company.

This wetland not only can treat the stormwater from upstream area and provide clean water, but also can effectively remove soluble pollutants. Our daily waste water after the solid material being separated out can be discharged into downstream part of the wetland area.

Moreover, more than a thousand different vegetation spices and about five hundred animal species including birds, fish, amphibians etc. live in this wetland area. It provides a pleasant view of a scene and fresh air for the residents and visitors.

 

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Hua’xi wetland 

What Hua’xi wetland brings to us is not only clean water and reduced expense on waste water treatment, but also a lovely sensual experience.

I believe that the Hua’xi wetland is a good example; it proves a wetland in the urban area can bring both economic, environmental and social benefits. But, in many cities, there is no such naturally formed wetland; thus, constructing wetland is an excellent choice.

The constructed wetland is not a new concept, its application can be traced back to 1903 in Yorkshire, people constructed the first artificial wetland for sewage treatment at Earby. Before this wetland being stopped operation, it is successfully running 89 years (Hiley, 1995). In the 1970s the root zone treatment technology is developed in Germany, and it is proved to be a reliable method to treat waste water (Nanda, 2017).

In the past decade, constructed wetlands in the UK, Germany, France, Australia, Brazil, and the Netherlands have developed rapidly. It has not only become an important waste water treatment measure for small and medium-sized cities, but also has become an important technology for rainwater treatment (Carleton, Grizzard, Godrej, & Post, 2001) and industrial waste water treatment (Vrhovšek, Kukanja, & Bulc, 1996).

The constructed wetland has many economic advantages in treating waste water. Constructing an artificial wetland is cheaper than building a water treatment plant. A study in China shows that we need to invest 100~150 US dollars to acquire 1-ton waste water treatment capacity, but to the same capacity for constructed wetland, the cost is only 25 US dollars (Song, Bi, & Cao, 2003). And for the constructed wetland, labor cost is very limited, if there is no dramatic change in waste water quality and water flow volume, the constructed wetland can automatic running without power plant. But to the traditional waste water plant, both human operation and power consumption are necessary. A good example is in Washington, Indiana. To solve the storm-related sewer overflow, the community needs a more powerful or efficiency waste water treatment measurement. One choice is upgrading the traditional waste water plant which will cost them about $54 million. But, after the firm Bernardin Lochmueller & Associates designed a constructed wetland for the community, the budget for waste water treatment capacity upgrading is nearly halved. Also, the operating cost is saved which is about $1.6 million for each year. In fact, the 27 acres constructed wetland not only just saves money for the local community, but also brings environmental benefit; it is the first time in years that minnows, frog, and other wildlife go back to this area (Casey, 2013).

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The Lujiao Creek Constructed wetland at New Taipei City.  The site was an abandoned waste dump. After the construction, the water purification project of the creek was carried out using aquatic plants and natural purification methods, and the wetland ecological environment was also created.                   

However, we have to concede that the constructed wetland has some drawbacks. The constructed wetland requires much more land to develop. Compared with the traditional waste water plant, if a constructed wetland wants to have the same treatment capacity, it will need larger land area. Especially in the urban area, the land for construction becomes more valuable with the increase in population.

The time costs is one of the defaults of the constructed wetland. A long lifespan constructed wetland requires a complex system. As we know, the ability of resistance to outside interference depends on the biodiversity of the system; for wetland system, if it has more spices and more complex structure and composition, it will have a more stable system and more anti-interference ability. Then, the constructed wetland can acquire a longer lifespan. But, it asks long time to build a well-developed biodiversity system. The long time to modern people who pursue the high efficiency is not acceptable.

The constructed wetland in some region is not always efficiency, its efficiency changes with seasons. In winter, due to the low temperature, the bioactivity will decline to decrease the wetland waste water treatment efficiency.

To some people, the constructed wetland for treating waste water is not an excellent choice considering its time costs and efficiency issue.

However, money is not the only thing the wetland brings; let’s imagine a scenario:

In a drizzling morning. You are walking along the bank of wetland and breathing the fresh, moist air. And colorful flowers, shining green leaves are on both sides of your path. The stream passes through the cliffs and crosses the bridge, creating turbulence, waterfalls, and flat beaches. You can clearly see the plants on the river bottom; and at distance, you also can see the looming wild birds swimming on the river.

For this scenario, how much do you think it worth?

Reference

Carleton, J. N., Grizzard, T. J., Godrej, A. N., & Post, H. E. (2001). Factors affecting the performance of stormwater treatment wetlands. Water Research, 35(6), 1552-1562.

Casey, T. (2013). Meet A Nifty Little Green Sewage Treatment Plant.   Retrieved from https://cleantechnica.com/2013/12/07/constructed-wetland-offers-low-cost-wastewater-treatment/

Hiley, P. (1995). The reality of sewage treatment using wetlands. WATER SCIENCE AND TECHNOLOGY, 32(3), 329-338.

Nanda, S. (2017). What is Root Zone Waste Water Treatment?   Retrieved from https://www.thehamlet.in/single-post/2017/02/21/What-is-Root-Zone-Waste-Water-Treatment

Song, Z., Bi, X., & Cao, J. (2003). Application of constructed wetlands in sewage treatment in small cities in China. Chinese Journal of Ecology, 22(3), 74-78.

Vrhovšek, D., Kukanja, V., & Bulc, T. (1996). Constructed wetland (CW) for industrial waste water treatment. Water Reseaich, 30(10), 2287-2292.

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Urban ecological infrastructure: multi-functional design

The global population is expected to grow to 8.9 billion in 2050 as predicted; more than 60 percent of people will live in the city in the year of 2025 worldwide, and in 2050 this number will increase to 70 percent (Cohen, 2003). As the rapid development of urbanization and population growth, the urban area will inevitably expand to gain more construction land from nature. It can be predicted that in the future the problems such as urban heat island effect, ashes effect, water population, and eutrophication will be more serious than today’s situation. Thus, it is important to develop and modify urban ecological infrastructure.

Traditional city is usually assembled by ‘grey infrastructure’ (figure 1). Grey infrastructures refer to structures and buildings such as hospital facilities, water supply systems, storm water systems and commercial buildings; and so far, modern structures are made of concrete which presents the color of grey. These grey infrastructures provide the basis of the normal function of a city.

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Figure 1 grey infrastructures: concrete jungle

However, the grey infrastructure has an apparent flaw which is single functional designed. To us, our infrastructures have many functions; but, they have on function to the ecosystem. For example, the design of river channel is often for flood protection; indeed, the river channel should also have other functions such as maintaining biodiversity to the ecosystem. The problems such as urban flood with high frequency and traffic congestion reveal that grey infrastructures not only cannot solve the complicated issues a city faces but also would bring ecological issues.

In the mid-1980s, the concept green infrastructure is put forward to solve urban issues by constructing with nature (Wikipedia). Green infrastructure is a green network system consisting of parks, forests, wetlands, conservative areas, green belts and other ecological corridors within the city. It plays a significant role in urban flood control, rainwater regulation and local climate regulation. A study shows that green infrastructure, i.e. the green space network, is important in dealing with climate change and improving surface runoff of rainwater (Gill, Handley, Ennos, & Pauleit, 2007). Also, Margaret Bryant finds ecological greenways and parks can play a critical role in biodiversity in urban areas (Bryant, 2006).

Green infrastructure is not single functional design; it can bring multiple benefits. For instants, green roof can effectively reduce the room temperature by 3°C-6°C, not only reducing the urban heat island effect, diminishing electricity consumption, but also has many ecological benefits such as water storage and emission reduction, purification of roof runoff pollution, and beautification of the environment (Getter & Rowe, 2006).

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Figure 2 Green roof

Thus, it is important to integrate grey infrastructure and green infrastructure to provide collaborative ecological infrastructure. By optimizing grey infrastructure through green infrastructure and the concept of multi-function design, a better maintenance of integrity and vitality of urban ecosystem service can be acquired. Moreover, the infrastructure system which is the combination of grey and green infrastructure can interconnect urban artificial system and natural system; and a more integrated, stable and sustainable urban infrastructure system can be formed.

Urban infrastructure can be divided into four parts: wetland, green land, structure and contaminate and waste exits.

Wetland includes rivers, lakes, reservoirs, ponds and other wetlands. In some urban area, the river is designed to provide flood protection, satisfy water consumption or just for aesthetic needs. But, a river in nature has more functions: regulating runoff, degrading pollution, purifying water quality, and maintaining biodiversity. Also, in some parks, the ponds or reservoirs exist without a connection. If these ponds and reservoirs can be connected, the storage capacity of water will increase through water flow. Besides, good connectivity can bring fresh water and material to facilitate aquatic organism survival and reproduction.

Green land which is also known as the lung of the city includes urban forests, grass land, green corridors, green gardens, and parks. Although in many cities, the quantity of green land is large, but the quality is lower than expected. Grassland is a typical example; most cities build lots of grass lands for the citizens, but the grass lands only provide green space for leisure time and nothing else. Some grass lands only have extremely limited water storage capacity, under heavy rain situation, these grass lands can hardly help to absorb excessive rainwater. In fact, grass land can be a good place for water storage capacity, water purification and local climate regulation, not just a place for people’s leisure time. It also can be transformed from landscape green land to functional green land by connecting to green land.

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Figure 3 flood in the urban area

The structure includes buildings, pavement, road, riverbed, and embankment, etc. To human beings, a building may be multi-functional just like a shopping mall providing shopping service to entertainment service, even education service. However, to the ecosystem, the building just a useless concrete box. The impermeable surface of building blocks gas and water exchange between air and land. Also, the hardened road surfaces, ditches, and embankment are made of impermeable and dark color materials. These surfaces significantly absorb more solar radiation which causes the break of material and energy balance in the urban area. The intuitive manifestation of this consequence is urban heat island effect (figure 4 & 5). Mitigation of urban heat island effect can be accomplished by designing the green roof and changing the material of surface to reduce the absorption of solar radiation. Moreover, the impermeable surfaces limit the discharge of rainwater; especially in stormy days, lots of rainwater is accumulated within the urban area and cannot be discharged (figure 3). So, the multi-functional road is necessary, it not only satisfies the traffic demand but also can facilitate water and gas exchange between air and earth which is under the road.

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Figure 4 Urban heat island effect

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figure 5 How the heat island phenomenon occurs

For contaminant and waste, traditional sewage system transports all contaminant and waste to the suburbs. People build garbage storage site to pile up the pollutants. But, the accumulation of garbage beyond the carrying capacity of suburbs. The waste disposal by landfill merely wipes the scene of garbage out from people’s horizon. What we need is a system which contains the function of storage, purification, treatment, and recycling.

A city is a complex and comprehensive ecological system. Under the pressure of increasing population, limited resource and climate change, single function infrastructure cannot help us to deal with these issues. In the ecosystem, no one exists with single function; for instant, a river is not just a channel with flowing water, it can provide fresh water to lives, help us to transport cargo and conduct energy exchange. Our tradition infrastructures only have one function which is to satisfy our need. They can do nothing except providing room for our activity. But, they are useless to the rest of world. In fact, we need multi-functional infrastructure; it can not only support our activity but also can provide ecological services to the ecosystem.

Reference

Bryant, M. M. (2006). Urban landscape conservation and the role of ecological greenway at local and metropolitan scales. Landscape and Urban Planning, 76, 23-44.

Cohen, J. E. (2003). Human population: The next half century. Science, 302(5648), 1172-1175.

Getter, K. L., & Rowe, D. B. (2006). The role of green roofs in sustainable development. HortScience, 41, 1276-1286.

Gill, S. E., Handley, J. F., Ennos, A. R., & Pauleit, S. (2007). Adapting cities for climate change: the role of the green infrastructure. Built Environment, 33, 115-133.

Wikipedia. Green infrastructure.   Retrieved from https://en.wikipedia.org/wiki/Green_infrastructure