Dear brother, it is  normal, i mean this ratio for treated sewage is less than raw sewage. For your case 76% of the organic matter in the raw sewage is biodegredable (which contribute to BOD value), and 24% is not biodegredable. During  treatment most of the biodegredable organic matters is consumed by Bacteria , while other organics is not consumed. That is why for the treated water there is little amount of biodegradable matter (only 43%) of the total organics.

In short , there are two types of organics in sewage: biodegradable and non biodegradable. Only the biodegredable is consumed in treatment plant.

biodegradable contribute to BOD

biodegredable and non biodegredable, both together contribute to COD

BOD/COD of normal wastewater around 0.7. But for stabilized landfill leachate may reach to 0.1 or less than 0.1. Biological treatment is inefficient for low BOD/COD, such as for stabilized leachate.

The ratio between BOD to COD is variable with season and geographical location.  generally  it is 0.4 - 0.45 in summer and increases 0.3 - 0.35 in winter.

Depend on the type of your wastewater can vary from 0.6 to 0.8, this rate can be consider biodegradable (biological treatment is recommended). If this ratio is under 0.5, biodegradation is not the best treatment for this wastewater. It is better to use, or start your treatment system with a physical or chemical process. Low rate is commonly found in industrial wastewater. In domestic wastewater is expected to be high rate.

Dear Friends

First of all, I did not understand the wastewater from where-  either domestic, industrial or other.. we cannot generalize the ratio between BOD and COD.  For example wastewater from  domestic, dairy industry, tannery industry

Dear Friends

First of all, I did not understand the wastewater from where-  either domestic, industrial or other.. we cannot generalize the ratio between BOD and COD.  For example wastewater from  domestic, dairy industry, tannery industry

Dear Iswar

thanks for your replay

the wastewater was domestic wastewater

Please be careful of the ratio BOD/COD. It depends the source/type of wastewater. It varies. Better to test every time you collect new sample (place and time). 

BOD to COD ratio is very important parameter it allso called the biodegradabilty of the wastewater. In your case BOD/COD ratio it reduced from 0.73 to 0.46 it is also good biodegradale range. Please check the disposable limit BOD and COD of your county standard it is really matters. In case of matured leachate increasing the biodegrablity ratio is challange and we used various advanced oxidation techniques to improve the biodegradability. But domestic wastewater generally used conventional biological treatment options. If possible provide tratment name and average COD and BOD value so we may get more idea.,  

Dear Shabiimam

the treatment process is conventional activated sludge

the value of BOD and COD in the raw wastewater is 257 and 339 mg/l respectively

and the values in treated wastewater is 21 and 51 mg/l respectively 

For biological treatment system, the BOD/COD ratio of trated wastewater is always less than the one of raw wastewater. 

In activated sludge process the bacteria play a dominant role in degrading the pollutants. The high BOD/COD ration of your raw wastewater indicates that it contain high proportion of easily biodegradable matter. During the treatment process the bacteria oxidize the easily biodegradable organic matter however the refractory organic compounds remains the same. That is the reason why you get a lower BOD/COD ratio after treatment. The reduction in the BOD/COD ration after treatment indicate that your bacteria is doing their job pretty well.

The low BOD/COD ratio of the wastewater is considered to have less impact on the dissolved oxygen level of the receiving water body than the high BOD/COD ratio.

The BOD:COD ratio varies with the wastewater and degree of treatment. With typical domestic wastewater a BOD:COD around 0.5 is common. This does not mean that the COD is 50% degradable. The BOD to degradable COD ratio also varies, with 0.68 a commonly quoted value. If you have a lot of nondegradable COD you will have a low ratio. If you have a lot of particulate degradable COD you will have a low ratio. If you have a lot of glucose you will have a high ratio.

I am looking at one sewage works where BOD is mostly below the limit of detection - to the BOD to COD ratio is somewhere (depending upon the assumption of how to process 'Below LoD') of 0 to 0.025.

The high BOD5/COD indicates the high biodegradability of the  hydrolysable wastewater substrate.The high BOD20/COD and BODu/COD ratios (0,91 - 0,98) indicate the high biodegradability of the slowly hydrolysable wastewater substrate.

The high BOD5/COD indicates the high biodegradability of the  hydrolysable wastewater substrate.The high BOD20/COD and BODu/COD ratios (0,91 - 0,98) indicate the high biodegradability of the slowly hydrolysable wastewater substrate.

Commonly BOD/COD values for municipal wastewater are greater than 0.5. While, for old landfill leachate some times equal to 0.1. BOD/COD has a great effect on treatment process. For wastewater with BOD/COD> 0.5, biological treatment is efficient.

This ratio indicates how much fractions of biodegradable organic matter is there and may vary from place to place.

Could BOD be calculated from COD value without knowing BOD: COD ratio ?

Ajay, as described above you can produce an estimate. The chosen conversion factors will differ based on source and degree of treatment of the wastewater. When you finally measure your BOD you may get a large difference. That does not invalidate the estimation procedure but keeps you aware of its limitations.

In fact, the question is not clear for me, usually researchers have an estimation of BOD:COD ratio and estimate the BOD value based on it, however, it is just an estimation and the exact value could be determined by the BOD test.

If you do not have BOD:COD ratio (as you mentioned), how could you calculate BOD value?

It would be better to measure BOD directly. But COD can be measured quickly and with relatively little equipment, lesser training and no questions over seed sludge and acclimitisation. BOD would only be measured if there was a requirement. The interest in BOD may come later.

Thank you Dr.Jeremy, for you valuable contribution, i would like to know about your opinion if the biological wastewater treatment could be evaluated based on COD only? or BOD in this case is a fundamental parameter?

Most people now evaluate on COD but it becomes more complex. Ideally you want COD as four fractions, soluble and particulate, biodegradable and nondegradable. Measuring the degradable fraction can as difficult as a BOD test. So often simplification based on common ratios are used.

You could evaluate just on COD but usually having an idea of the soluble nondegradable fraction helps understand the limit to treatment, and the particulate nondegradable fraction the performance of the settling tank. The degradable elements are affected by the aeration tank in a similar manner to BOD but with greater differentiation between oxidation, biomass uptake and sedimentation removal.

The thing is we in our ETP plant have the process of chlorination for disinfecting the micro organisms in the final effluent. But however we do not have the provision to check the free chlorine . SInce we have to check BOD in chlorine free effuent , We are having trouble in estimating the amount of sodium sulfite to be added in the effluent ( gross amount) such that the chlorine free effluent is achieved. Please suggest the method to check the free chlorine in the effluent that we are achieving.