Understanding Wastewater Treatment
Main sources for water pollution
When developing solutions for wastewater treatment it is necessary to understand the source, type and degree of the pollution first. Generally, experts distinguish three major sources through which natural waters may be polluted: domestic sources in our homes, industrial and agricultural sources and surface flow. All of those wastewaters have different composition and require different treatment technologies.
Wastewater may contain a vast variety of elements depending on regional and individual circumstances. Therefore, it's not possible to exactly specify its composition.
Municipal wastewater for example, with an average contamination by industry and agriculture, consists mainly of the following components:
Solid organic matters like feces, hair, food, paper, wood, etc.
Dissolved organic matters like urine, sugars, pharmaceuticals etc.
Solid inorganic matters like sand, grit, metal etc.
Dissolved inorganic matters like ammonia, different kinds of salts, sulfates etc.
Toxins like pesticides or herbicides
Emulsions like paints, hair colorants etc.
General methods of wastewater treatment
Numerous systems and processes for wastewater treatment are available today. The selection depends on the type of wastewater to be treated and on the required purification result.
Generally, there are three principle types of treatment: mechanical/physical, chemical and biological treatment. In most treatment technologies and plants, combinations of those treatment types are used to reach an appropriate purification result. The basic principles of wastewater treatment are:
Principle MethodsPollution removal |
Physical / mechanical treatment |
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Sedimentation
Flotation
Rake – used to remove solids down to a defined diameter |
Solid organic matters
Solid inorganic matters |
Biological treatment |
Degradation Nitrification
Denitrification
Bacteria is used to consume bio-degradable matters. Bacteria mass is stimulated with oxygen |
Solid organic matters
Dissolved organic matters
Dissolved inorganic matters
Toxins, Emulsions |
Chemical treatment |
Used to precipitate undesirable substances such as phosphate |
Dissolved inorganic matters
Toxins, Emulsions |
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Water quality indicators
BOD, COD, TOC, nitrate, ammonia and phosphates are common ways to express the quality of natural water as well as of wastewater influent and effluent.
BOD (biological oxygen demand) is the main indicator for water quality. BOD5 is a value, which expresses the oxygen demand of bacteria in a water body over a period of 5 days.
COD expresses chemical oxygen demand and indicates how much oxygen is used by chemical purification and / or digestion processes.
TOC expresses total organic carbon.
Flowing water classification
Bodies of flowing water can be classified mainly according to their BOD:
Clear waters: |
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Oxygen content 95-105%; BOD5 approx. 1.0 mg/l. |
Minor load: |
Oxygen content 85-95%; BOD5 1.0-2.0 mg/l; ammonia <0.1 mg/l. |
Moderate pollution: |
Oxygen content >6 mg/l; BOD5 2.0-6.0 mg/l; ammonia <0.3 mg/l. |
Critical pollution: |
Oxygen content <50%; BOD5 5.0-10.0 mg/l; ammonia <1.0 mg/l. |
High pollution: |
Oxygen content <2 mg/l; BOD5 7.0-13.0 mg/l; ammonia >0.5 mg/l up to several mg/l. |
Very high pollution: |
Oxygen Content <1 mg/l; BOD5 10.0-20.0 mg/l; ammonia several mg/l; often toxics. |
Excessive pollution: |
Moderate pollution: |
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Standing water classification
Bodies of standing water are classified mainly according to the figures in the following table:
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oligotrophic |
mesotrophic |
eutrophic |
hypertrophic |
Water clarity [m] |
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5 -10, max. 15 - 20
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1 - 2, max. 5 - 10 |
< 1, max. 2 - 3
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< 1 |
Limit of submerse vegetation [m] |
12 - 30 |
5 - 10 |
< 2
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< 1 |
Phosphate [mg/m 3 ] |
4 - 5 |
10 - 35 |
– 100 |
> 100 |
Nitrate and Ammonia (mgN/l) |
Max. 1 |
Max. 1 |
> 2 |
> 2 |
Annual average of chlorophyll [mg/m 3 ] |
< 3,5 |
< 7,0 |
< 11 |
> 11 |
Oxygen [mg/l] |
> 8 |
6 - 8 |
2 - 4 |
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BOD 2 [mg/l] |
0,5 |
1,1 - 2,2 |
4 - 7 |
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BOD 5 [mg/l] |
Max. 3 |
3 - 5,5 |
> 14 |
> 14 |
COD [mg/l] |
1 – 2 |
8 - 9 |
20 - 65 |
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Number of bacteria [#/ml) |
< 100 |
Approx. 10000 |
< 100000
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> 100000 |
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