Biochemical oxygen demand

Manometric determination of the BOD with the OxiTop device

The Biochemical Oxygen Demand ( BOD , also biological oxygen demand ; English demand oxygen biochemical , BOD ) is the amount of oxygen, which in the biotic degradation water existing organic materials under certain conditions and within a certain time is needed. In particular, the biological oxygen demand serves as a pollutant parameter for assessing the pollution of waste water .

Sewage and water monitoring

In wastewater monitoring, often only the BOD is determined from the breakdown of carbon compounds (carbon BOD), with the addition of allylthiourea (ATH), nitrification is inhibited in order to prevent oxygen consumption due to the breakdown of nitrogen compounds. In general water monitoring , the total BOD is often measured as the sum of the breakdown of carbon and nitrogen compounds; this is known as unrestrained BOD . Both measurement methods lead to very different results: In surface waters, the uninhibited BOD can reach 3 times the inhibited BOD in summer.

Usually the BOD _{5 is} used. This value is the amount of oxygen in mg / l that bacteria and all other microorganisms present in the water consume within five days at a temperature of 20 ° C, from which one can deduce the amount of organic substances broken down. In addition there will occasionally BSB ₂ and the BOD _∞ determined that indicate the oxygen demand within two days or until the expiry of breathing, hypothetically ie until removal of all organic substances biotic degradable. As a rule of thumb: BOD ₅ is around 70% of BOD _∞ .

Waste water constituents

The relationship to the chemical oxygen demand provides information about the type of waste water constituents:

if BOD is ₅ = (50… 100)% COD, the ingredients are readily biodegradable.
If BOD ₅ <50% COD, the ingredients are only poorly biodegradable and therefore remain in the environment for a long time or they have a toxic effect on microorganisms and are therefore difficult to break down.
BOD ₅ = (12… 25)% COD: wastewater usually has this ratio after biological treatment.

The BOD ₅ should only record the organic carbon compounds. The result can be falsified by the biotic oxidation of ammonium ions or ammonia, i.e. by nitrification. Any oxygen consumption due to nitrification must therefore be suppressed by adding nitrification inhibitors (allyl thiourea).

Furthermore, the informative value during the analysis about the organic degradability of the wastewater constituents can be falsified by the wrong choice of microorganisms. An adaptation of the microorganisms is only possible to a limited extent within 5 days, as a result of which the metabolism is reduced and the analysis thus leads to an incorrect result about the degradability.

Determination methods

Manometric:
The water to be examined is placed in a bottle, while a large part of the bottle remains filled with air. The bottle is sealed airtight and the pressure in the vessel is determined with a manometer . The resulting CO ₂ is chemically bound. The oxygen consumption causes a pressure decrease, from which the biological oxygen demand can be calculated. The apparatus is kept at constant temperature. The advantage of the method is the possibility of continuously recording the development of the oxygen demand and that the water to be examined is included in the experiment in its original concentration (inhibitory and toxin influence). The disadvantage is the considerable expenditure on equipment and the inaccuracy of ± 5 to ± 10%.

Dilution method:
The water to be examined is diluted with oxygen-containing water to such an extent that the expected oxygen demand is less than the oxygen dissolved in the diluted sample. A bottle (Karlsruhe bottle or Wheaton bottle) is completely filled with the diluted sample, sealed tightly and stored in the dark at a constant temperature. The oxygen content is measured at the beginning of the determination and after five days. The biological oxygen demand is calculated from the difference in the oxygen content. The advantage of the process is the simple handling and the low expenditure on equipment. The disadvantage is that inhibitors and toxins are also diluted.

Sapromat method :
The negative pressure caused by the consumption of oxygen and the absorption of carbon dioxide (e.g. from potassium hydroxide) triggers an impulse which generates electrolytic oxygen that is added to the measuring vessel. The amount of oxygen consumed can then be derived directly from the number of pulses. The advantage over the first two methods is that the measurement is carried out in the original wastewater with the same oxygen concentration without limitation of the measurement time.

Population equivalent

The population equivalent indicates the pollution of domestic wastewater with biotically oxidizable substances, expressed as BOD ₅ , per inhabitant and day. It is around 60 g BOD ₅ per inhabitant and per day.

The uniform framework conditions for measuring the biological oxygen demand are specified in DIN EN 1899-1 and 1899-2.

Together with the chemical oxygen demand, the BOD provides information on the quality of the pollutants contained in the wastewater and was therefore also an important parameter in the dimensioning, dimensioning and operational control of wastewater treatment plants . With the appearance of the new DWA worksheet 131 in June 2016, the BOD ₅ was no longer used as a dimensioning parameter for single-stage activated sludge plants , as the BOD ₅ does not allow a complete accounting of the sludge accumulation and the oxygen demand and is no longer measured across the board in practice.

literature

Klaus Riedel, Gotthard Kunze, Andreas König: Biosensors for environmental control . Oldenbourg Industrieverlag, 2003, ISBN 978-3-486-26509-5 ; limited preview in Google Book search
Pfestorf, Kadner: Chemistry, a textbook for universities of applied sciences . Verlag Harri Deutsch, 2000, ISBN 3-8171-1632-2

Individual evidence

↑ Rosenwinkel, KH, Kroiss, H., Dichtl, N., Seyfried, CF, Weiland, P .: Anaerobic technology: wastewater, sludge and residue treatment, biogas production . 3. Edition. Springer, Berlin / Heidelberg 2015, ISBN 978-3-642-24895-5 , pp. 64 .

[1] Rosenwinkel, KH, Kroiss, H., Dichtl, N., Seyfried, CF, Weiland, P .: Anaerobic technology: wastewater, sludge and residue treatment, biogas production . 3. Edition. Springer, Berlin / Heidelberg 2015, ISBN 978-3-642-24895-5 , pp. 64 .