In estimating the BOD₅ of Food Industrial Waste water and Treated Waste water etc., there are considerable errors in calculating numerically from COD the measured values of the turbidimeter and the transparency meter.
Therefore, visible ray absorption photometry was adopted as the method which can express the turbidity in the quantitative form, in order to avoid growth in noticable by using a transparency meter or turbidimeter, and in order to estimate both simultaneously the pollution and turbidity in water, the COD must be converted to a COD of visible ray having an absorbancy of 1.0 by absorb ancy measurement.
In other words, the converted COD method was presented as a new method.
As this method can not only be used to construct many regression lines for the relation between BOD₅ and COD, but also between absorbancy and BOD₅, absorbancy and SS with a high level accuracy, it can be especially considered as an accurate method of estimation of BOD₅.
By applying the converted COD method to the influent and efluent of Food Industrial Waste water by using a Rotating Biological Contactor, we were able to confirm that this method was very useful.
For example, in a correlative graph with the relation between COD and BOD₅ of alkaline influents to treatment plant equipment of Food Industrial Wast water, three definite regression lines were clearly drawn.
And in the correlative graph with the relation between absorbancy (x) and SS (y), a COD concentration gradient of the converted COD was the reverse condition of distribution depending on the kinds of Waste water for the gradient between the x-axis and the y-axis on a correlative graph.
In the correlative graph with the relation between absorbancy and BOD₅ of efluents by the three stage Rotating Biological Contactor, the regression lines of efluents which contained the sludge suspended in water was quite different from the regression lines of the supernatant efluents which settled down after about thirty minutes.

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