Biochemical oxygen demand

The Biochemical Oxygen Demand ( BOD, and biological oxygen demand, Eng. Biochemical oxygen demand, BOD) indicates the amount of oxygen that is required for biotic degradation present in the water of organic substances under certain conditions and within a certain time. In particular, the biological oxygen demand serves as a pollutant parameter for the assessment of pollution by sewage.

Water Pollution Monitoring

In wastewater monitoring only BOD is often from the breakdown of carbon compounds determined (carbon BSB), with the addition of allyl thiourea ( ATH), nitrification is inhibited to prevent an oxygen consumption for the breakdown of nitrogen compounds. In the water monitoring the entire BOD is measured as the sum of the reduction of carbon and nitrogen compounds often. Both measurement methods lead to different results. In surface water can reach 3 times the value of the inhibited BOD in the summer of uninhibited BSB.

Usually, the BOD5 is used. This value is the amount of oxygen in mg / l, the bacteria present in the water and all other microorganisms consume at a temperature of 20 ° C within five days, from which one concludes that the amount of this decomposed organic matter. In addition, occasionally even the BSB2 and the BOD ∞ is determined, indicating the oxygen demand within two days or until the expiry of breathing, so hypothetically to the removal of all biotic degradable organic substances. As a rule of thumb, BOD5 is about 70 % of the BOD ∞.

Chemical Oxygen Demand

To assess the water quality in addition to the biochemical oxygen demand is also the chemical oxygen demand (COD ) is important. Here, everyone, even the non- biotic degradable substances by the strong oxidizing agent potassium dichromate oxidized. COD is therefore always greater than the BOD ( Exceptions: strongly emphasized nitrogen wastewater from food manufacturing as the slaughterhouses or sausage factories ).

Wastewater constituents

The ratio of the two values ​​gives an indication of the type of wastewater constituents:

  • Is BOD5 = (50 ... 100) % COD, the ingredients are good biotic degradable.
  • Is BOD5 <50 % COD the ingredients can only be degraded badly biotic and therefore long remain in the environment or they have a toxic effect on micro-organisms and therefore may be poorly degraded.
  • BOD5 = ( 12 ... 25) % COD: this ratio has waste water usually after a biological treatment.

The BOD5 is to capture only the organic carbon compounds. Which can be disturbed by the biotic oxidation of ammonium ions and ammonia, that is, by nitrification. A possibly occurring oxygen consumption by nitrification, therefore, must be suppressed by the addition of nitrification inhibitors ( allyl thiourea ).

Determination methods

  • Manometrically: The water to be examined is introduced into a bottle ( Sapromat ), wherein a major part of the bottle is filled with air. The bottle is sealed and determined the pressure in the vessel with a pressure gauge. The CO2 produced is chemically bound. The consumption of oxygen causes a decrease in pressure, resulting in the biological oxygen demand can be calculated. The apparatus is maintained at a constant temperature. Advantage of the method is the possibility of the current record of the development of the oxygen demand, and in that the water to be analyzed in its original concentration enters the test ( inhibition and influence toxin ). A disadvantage is the significant equipment expense.
  • Dilution method: The water to be examined is diluted with water containing oxygen, resulting in the expected oxygen demand is less than the dissolved oxygen in the diluted sample. With the diluted sample to a flask ( Wheaton Karlsruhe bottle or bottle) is completely filled, sealed and stored dark at constant temperature. The oxygen content is measured at the start of the determination, and after five days. The difference between the oxygen contents of the biological oxygen demand is calculated. Advantage of the method is the ease of use and low cost apparatus. Disadvantage is that there is no continuous detection of the development of the oxygen demand and possible inhibitory and toxins are also diluted.
  • Sapromatmethode: The oxygen consumption and carbon dioxide by absorption (eg potassium hydroxide ) caused negative pressure triggers a pulse is produced by the electrolytic oxygen is added to the measurement vessel. From the number of pulses can then be directly derived the amount of oxygen consumed. The advantage over the first two methods is that the measurement in the original wastewater occurs without limiting the measurement time at constant oxygen concentration and therefore the standard method dar.

Population equivalent

The population equivalent value specifies the load of domestic wastewater with biotic oxidizable substances, expressed as BOD5 per capita per day. He is about 60 g BOD5 per capita per day.

The unified framework for the measurement of the biological oxygen demand are defined in DIN EN 1899-1 and 1899-2.

Together with the chemical oxygen demand are the BSB clues to the quality of the pollutants contained in the wastewater and is therefore an important parameter in the design, dimensioning and operational control of wastewater treatment plants.

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