THE IMPORTANCE OF MEASURING THE BIOCHEMICAL OXYGEN DEMAND (BOD) IN WATER AND WASTEWATER

As our population grows and industrial activities expand, the demand for water also increases. This heightened demand can put a strain on existing water resources and it often leads to the utilization of water sources that may be contaminated or vulnerable to pollution from industrial processes. Consequently, the combination of rising water demand and industrial activities exacerbates existing pollution levels and poses significant risks to human health and the environment. 

Therefore, monitoring water and wastewater quality is essential to identify pollutants, assess their impact, and implement measures to minimize contamination, ensuring safe water resources. 

One parameter of this monitoring is Biochemical Oxygen Demand (BOD), which helps assess organic pollution levels and protect both human and animal life from harmful contaminants. Understanding BOD is essential for assessing water and wastewater quality, as it reflects the microbial activity and biodegradability of organic pollutants. 

When organic matter decomposes, the microorganisms feed upon this decaying material. They use oxygen for their biochemical processes and produce an equivalent volume of carbon dioxide. If the process develops in a closed system and the carbon dioxide is absorbed by a strong alkali (e.g. KOH), a progressive decrease in internal pressure can be measured. Thus, the BOD value is calculated by measuring this change of pressure: the higher the BOD value, the greater the amount of organic waste, or “food” available for oxygen-consuming bacteria.

The BOD measurement therefore depends on three basic aspects: the amount of oxygen, aerobic microorganisms, and biodegradable organic matter. Biodegradable organic matter, in addition to being a key variable, is also the object of analysis: the greater its amount, the greater the amount of oxygen required by aerobic microorganisms to assimilate and degrade it.

VELP RESPIROMETRIC Sensor measures the internal pressure decrease and directly shows on the software the BOD value, in mg/l (or ppm), with no need for further calculation.

In addition, the consumption of oxygen by microorganisms to degrade biodegradable organic matter depends on the time allowed for the biodegradation process, at a set temperature. A longer time means a greater amount of degraded organic matter and a greater amount of oxygen consumed by the microorganisms.

• The 5-day biochemical oxygen demand (BOD5) represents oxygen consumption after 5 days at 20 °C and it has been adopted as a compromise between a short test period and the detection of a practically complete biological breakdown of organic materials.
• In some countries, the 6-day biochemical oxygen demand (BOD6) and the 7-day biochemical oxygen demand (BOD7) are calculated, representing oxygen consumption after 6 or 7 days at 20 °C.
• The ultimate BOD (BODu), or Last BOD, represents the amount of oxygen required to oxidize the entire organic fraction. The complete degradation (100% BOD) is achieved after 20-30 days at 20 °C: indeed, after 5 days, only 70% of the biologically convertible substances are broken down.

Corresponding to the BOD expected value, it’s necessary to use a different scale; VELP RESPIROMETRIC Sensor features a BOD scale of 4000 mg/l: 
 

BOD SCALE (mg/l)	VOLUME (ml)
90	400
250	250
600	150
999	100
4000	25

Here is an example of the expected BOD5 value on some samples: 

• Very well-treated effluent ~ 3-5 mg/l
• Standard effluent (after treatment) ~ 10-30 mg/l
• Badly treated sewage ~ 40-80 mg/l
• Strong sewage ~ 400-600 mg/l
• Trade effluents (animal and vegetable waste) > 1000 mg/l

To obtain more precise results, it’s recommended to work in the upper half of the scale: if the expected BOD is 200 mg/l, use the scale 250 mg/l, not higher!