Reading Passage 2
Water Treatment 2: Reed Bed
TRUE/FALSE/NOT GIVEN 3题
Table Completion 3题
Nowadays subsurface flow wetlands are a common alternative in Europe for the treatment of wastewater in rural areas. Mainly in the last 10 to 12 years there has been a significant growth in the number and size of the systems in use. Compared to common treatment facilities, wetlands are lower in cost investment, lesser to maintain, and are ideal for densely populated rural or suburban areas rather than urban areas.
The Common Reed has the ability to transfer oxygen from its leaves, down through its stem and rhizomes, and out via its root system. As a result of this action, a very high population of micro-organisms occurs in the root system, with zones of aerobic, anoxic, and anaerobic conditions. Therefore with the waste water moving very slowly and carefully through the mass of Reed roots, this liquid can be successfully treated.
A straightforward definition of a reed bed is if you have dirty water in your pool or water, which is heavily polluted, Reed Beds will be planted to make the water clean again. This is good for ecology and living organisms and fish in the water. Reed Beds have a wide range of qualities and are acceptable for cleaning everything from secondary to tertiary treatment of mild domestic effluent, to rural waste and even heavy industrial contaminants. The reason why they’re so effective is often because within the bed’s root sector, natural biological, physical and chemical processes interact with one another to degrade or remove a good range of pollutants. Reed beds can be built in a number of variants, but mainly they are of the horizontal flow or vertical (down) flow configuration where water flows through the beds horizontally or vertically.
HORIZONTAL FLOW REED BED SYSTEMS
Horizontal-flow wetlands may be of two types: free-water surface-flow (FWF) or sub-surface water-flow (SSF). In the former the effluent flows freely above the sand/gravel bed in which the reeds etc. are planted; in the latter effluent passes through the sand/gravel bed. In FWF-type wetlands, effluent is treated by plant stems, leaves and rhizomes. Such FWF wetlands are densely planted and typically have water-depths of less than 0.4m. However, dense planting can limit oxygen diffusion into the water. These systems work particularly well for low strength effluents or effluents that have undergone some form of pretreatment and play an invaluable role in tertiary treatment and the polishing of effluents. The horizontal reed flow system uses a long reed bed, where the liquid slowly flows horizontally through. The length of the reed bed is about 100 meters. The downside of the horizontal reed beds is that they use up lots of land space and they do take quite a long time to produce clean water.
VERTICAL FLOW REED BED SYSTEMS
A vertical flow reed bed is a sealed, gravel filled trench with reeds growing in it (see the picture below). The common reed oxygenates the water, which helps to create the right environment for colonies of bacteria to break down unwanted organic matter and pollutants. The reeds also make the bed attractive to wildlife. How a vertical flow reed bed works?
In vertical low (down flow) reed beds, the wastewater is applied on top of the reed bed, flows down through a rhizome zone with sludge as substrate, then the root zone with sand as substrate and followed by a layer of gravel for drainage, and is collected in an under drainage system of large stones. The effluent flow onto the surface of the bed and percolates slowly through the different layers into an outlet pipe, which leads to a horizontal flow bed and is cleaned by millions of bacteria, algae, fungi, and microorganisms that digest the waste, including sewage. There is no standing water so there should be no unpleasant smells.
Vertical flow reed bed systems are much more effective than horizontal flow reed-beds not only in reducing biochemical oxygen demanded (BOD) and suspended solids (SS) levels but also in reducing ammonia levels and eliminating smells. Usually considerably smaller than horizontal flow beds, but they are capable of handling much stronger effluents which contain heavily polluted matters and have a longer lifetime value. A vertical Reed bed system works more efficiently than a horizontal reed bed system, but it requires more management, and its reed beds are often operated for a few days then rested, so several beds and a distribution system are needed.
There are several advantages of Reed Bed Systems over traditional forms of water treatment: first, they have low construction and running costs; second, they are easy management; third, they have an excellent reduction of biochemical oxygen demand and suspended solids; last, they have a potential for efficient removal of a wide range of pollutants.
Reed beds are natural habitats found in flood plains, waterlogged depressions and estuaries. The natural bed systems are a biologically proved, an environmentally friendly and visually unobtrusive way of treating wastewater, and have the extra virtue of frequently been better than mechanical wastewater treatment systems. In the medium to long term reed bed systems are, in most cases, more cost effective in installment than any other wastewater treatment. They are robust and require little maintenance. They are naturally environmentally sound protecting groundwater, dams, creeks, rivers and estuaries.
14-16 TRUE/FALSE/NOT GIVEN
14. Conventional methods are best for urban areas. F
15. Algae was helpful in purifying water. T
16. Reed beds are best for newly grown plants. NG
17-19 Table Completion
20-24 Summary Completion（有选项）
20. H more effective
21. A has several beds
22. F does not work at the same time
23. B requires less cost
24. D less attention
25-26 Multiple Choice（多选）
25. B it requires little installation cost
26. D it is aesthetically pleasing