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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







Rake – used to remove solids down to a defined diameter

Solid organic matters

Solid inorganic matters

Biological treatment




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




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:





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:




Standing water classification

Bodies of standing water are classified mainly according to the figures in the following table:


oligotrophic mesotrophic eutrophic hypertrophic

Water clarity [m]





5 -10, max. 15 - 20

1 - 2, max. 5 - 10

< 1, max. 2 - 3

< 1

Limit of submerse vegetation [m]

12 - 30

5 - 10

< 2

< 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  
BOD 2 [mg/l] 0,5 1,1 - 2,2 4 - 7  
BOD 5 [mg/l] Max. 3 3 - 5,5 > 14 > 14
COD [mg/l] 1 – 2 8 - 9 20 - 65  

Number of bacteria [#/ml)

< 100

Approx. 10000

< 100000

> 100000





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