COD/BOD Ratio Calculator

Chemical oxygen demand and biochemical oxygen demand are both reported as milligrams of oxygen consumed per litre, which is what makes their quotient dimensionless and comparable across very different effluents. COD is the oxygen equivalent of everything a boiling acidic dichromate digestion will oxidise in about two hours, so it captures refractory material such as lignin, synthetic dyes, chelating agents and many solvents. BOD5 counts only the fraction a mixed microbial seed metabolises in five days at 20 degrees C. The ratio therefore acts as an inverse biodegradability index. Raw municipal sewage typically falls between 1.5 and 2.5, dairy and brewery effluent near 1.5 to 2.0, tannery and textile streams often 4 to 10, and an aged methanogenic landfill leachate can exceed 10 even after its absolute COD has dropped. The ratio is only meaningful when both tests run on the same unsettled aliquot. Reduced inorganics such as sulfide, nitrite and ferrous iron are oxidised by dichromate and inflate COD without representing organic carbon, while chloride must be masked with mercuric sulfate or it produces a large positive bias. On the BOD side, nitrifying bacteria can exert oxygen demand in the later part of the incubation unless allylthiourea is added, and a toxic or unacclimated sample suppresses BOD and pushes the ratio upward for reasons that have nothing to do with molecular structure. Single BOD5 determinations commonly carry 10 to 20 percent relative error, so treat ratio differences smaller than about 0.3 as noise, and confirm borderline cases with respirometry or a Zahn-Wellens inherent biodegradability test.

The five-day incubation at the heart of this ratio comes from the British Royal Commission on Sewage Disposal, which sat from 1898 to 1915. The commission chose five days because no major river in Great Britain was thought to take longer than that to reach the sea, so five days of oxygen demand approximated the worst load a receiving water would actually experience. Its Eighth Report of 1912 produced the widely copied 20:30 standard, twenty parts BOD and thirty parts suspended solids per million, which anchored effluent regulation in many countries for decades. The incubation temperature was later standardised at 20 degrees C in the American practice codified by Standard Methods for the Examination of Water and Wastewater, first published by the American Public Health Association in 1905. The dichromate COD test was developed as a fast surrogate because operators could not wait five days for a control decision. Dichromate replaced earlier permanganate oxygen-absorbed methods, which oxidised organics only partially and gave poor recovery. A sealed-tube, small-volume version measured colorimetrically was introduced by Jirka and Carter in 1975 and now dominates routine laboratories. Once both numbers were routinely available on the same sample, sanitary engineers began using their ratio as a screening rule for whether an industrial discharge could be sent to a biological plant at all, and the practice was folded into industrial pretreatment programmes that followed the United States Clean Water Act amendments of 1972.