Hi Mr Periyasamy,

Establishment of constant reletionship among measures of COD/BOD depends primarily on the nature of the wastewater and its source. Some COD/BOD ratios of wastewaters are high and biodegradability is low, others are vice versa. Although COD/BOD ratio shows the biodegradability of wastewater I can not introduce a certain index for connecting COD/BOD ratio and biodegradability. For typical untreated domestic wastes, however the BOD5/COD ratio vares from 0.4 to 0.8 and BOD5/TOC ratio varies from 1.0 to 1.6. If you find any index please inform me about that. It should also be noted that these ratios vary considerably with the source of wastewater, for example is it industrial or not?

Generally the wastewater is considered to be easily biodegradable when BOD5/COD is higher than 0.6. Biological treatment is assumed to be appropriate if BOD5/COD is higher than 0.4.

I agree with Adriana and Olcayto in this respect.

In addition to those information, the BOD5/COD ratio can be used to determine the treatment design too.

For and example

(1) If it is greater than 0.6; then those ww may be considered for AEROBIC treatment design.

(2) If the ratio is less than 0.3; then should concentrate more on chemical abatement. Because, those ww will contain more portions of bio-refractory organics.

(3) If the value is in between 0.3-0.6; usually ANAEROBIC treatment design is prescribed.

It should be noted that, in almost all the cases, COD is greater value than BOD5.