From Waste to Food: Biochemical Oxygen Demand
Wastewater constitutes of thousands of variety of organics and measuring each one of them is next to impossible. Therefore different analysis methodology has been employed in order to ascertain the major part of organics. The analysis can be illustrated by simply understanding the oxidation of organic matter. The organic matter utilizes oxygen for its degradation and the byproducts include carbon dioxide. The consumed amount of oxygen or the produced amount of carbon dioxide. The former is also known as biological oxygen demand, carbonaceous oxygen demand and total oxygen demand while as the latter is known as total organic carbon. These values (BOD, COD, TOC) can be measured and help us in analyzing the organic matter present in wastewater.
Methods: Different measuring methods have been employed to analyze organic matter but it should be noted that while the methods differed, the yield results were also varying which implies that one single method cannot replace other methods indiscriminately. The choice of analysis has been still a subject of discussion to various researchers in the field of wastewater. Generally quoting, chemical analysis is rapid whilst the biochemical analysis is slow.
Biochemical Oxygen Demand (BOD5)
The five day biochemical oxygen demand is known as BOD5. The analysis was developed in Britain. The main concept behind the developing of this analysis technique was that there is food required to microorganisms in polluted water. This was used to determine the extent of pollution in water. Since the oxygen demand was increasing as the temperature increased, it was decided to standardize the temperature. The standardized temperature chosen at that particular time was 65 degree Fahrenheit (the standard temperature used nowadays is 20 degree Fahrenheit). The analysis was used to measure the oxygen demand for microorganisms (oxidation of organic matter and ammonium). It is noticed that in five days most of the organic matter is oxidized which corresponds to horizontal level of 20 degree.
The measurement of biochemical oxygen demand (classical dilution method) is carried by mixing polluted water with clean aerated water. The mix prepared is poured into the bottle which is corked properly so that there is no entrainment of air above the liquid inside the bottle. The organisms inside the polluted water start to consume oxygen and it must be noted that the oxygen present should be sufficient for 5 days incubation. If the oxygen concentration falls short the yield result isn’t of any use. It is for this reason only that dilution is carried out such that at least one bottle is obtained in which the concentration remains above the threshold level. Based on this, the BOD can be calculated. The units of measurement will be g/m3(mg/l).
The value of measurement represents only a part of organic matter which makes it difficult to use BOD for analysis like mass balance. The complete analysis is obtained by enlarging the time period to 15 to 20 days. It has been studied that BOD at infinity is 40-50 percent higher that BOD at 5 days