How Water Purification Really Works
Water Purification Methods
Classification of Purification Methods
A variety of water purification methods are employed, depending on the conditions involved.
The water quality of a water area can be maintained if decomposition and oxidation of the inflowing substances and existing substances is repeated and if the speed of circulation is moderate. However, artificial water purification methods must be added for support if excessive pollutants cause the loss of this water quality maintenance capability.
Three general categories of purification methods exist:
1) Physical methods
Physical methods consider the river’s self-maintenance characteristics. The flow speed is controlled and artificial settlement, sweeping, solid-liquid separation, and aeration are performed. Solid materials are separated and removed by use of screens.
2) Chemical methods
To remove dissolved substances and matter that cannot be removed through physical methods, chemical methods are employed in which chemical substances are added to the water to coagulate, settle, and destroy harmful organisms.
3) Biological methods
Biological methods artificially condense microorganisms living in flowing water, and promote the decomposition and oxidation of organic matter. Such methods also absorb and solidify nutrient salts by means of certain aquatic organisms.
River Purification
To purify rivers, both physical methods, including a method of promoting natural settlement by means of controlling the speed of flow through weir/dams; and biological methods are employed. However, biological methods are mainly used.
The principal method employed is gravel catalytic oxidation. A number of types are available, from use of real gravel to use of exclusively developed plastic filter material.
Basically, the velocity of the flow through the filter material is utilized to efficiently separate the solids and liquids. At the same time, the dissolved oxygen in the water decomposes the BOD components.
To further promote this action, an aeration-employed gravel catalytic oxidation method is available. With this method, air is supplied into the spaces between the pieces of gravel to vigorously increase the number of microorganisms, which then promotes biodegradation.
In many cases construction takes place using the riverbed. In small rivers in cities, there are also methods that treat water in fixed quantities with plant-like facilities that apply gravel catalytic oxidation.
In city drainage systems and other rivers with high BOD loads, fluidized-bed biological diaphragm filters (leg packs) are being applied.
Lake Purification
In small lakes and marshes (with capacities of several tens of thousands of tons) direct purification through circulation is a realistic method. However, with multipurpose control dams and reservoirs having capacities of several tens of millions of tons decomposing or separating pollutants directly is not practical.
To remove pollutants, the only effective method is the thorough implementation of actions to eliminate the source of the pollution load upstream.
Within the lakes themselves, rather than removing the nutrient salts and pollutants, emphasis should be put on controlling plant plankton growth to prevent secondary problems.
Thus, plant plankton ecology, especially the growth mechanism, should be understood in order to use such methods as water fountain and circulation to disturb the growth conditions as much as possible to control plankton growth.
Basically, an effective way to maintain the water quality in lakes and marshes is to maintain overall DO and its flow, in bottom areas too. Employing floating aerators (“swannies”) and submersible aerators can produce this effect.
Sea Area Purification
Of all water areas, sea areas are the furthest downstream; thus, all pollution loads of rivers and lakes flow downstream and accumulate in the sea areas. If purification upstream is insufficient, pollution of the sea is unavoidable.
At present there is no definitive method of removing inflowing pollution load sources from the sea and producing clean water. If the water area is of a size that can be handled by sand filter or biological filter systems, purification is possible. In many cases, however, water areas are linked together. To constantly handle inflowing loads, extremely large-scale filtering systems are needed.
Consequently, practical methods of purifying sea areas include the following:
- Introducing relatively clear seawater and diluting
- Controlling the growth of red tide and other plankton
- Producing artificial gravel and beach to restore natural purification capabilities
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