COD to BOD factor for fecal sludge - use a factor of 0.8?

  • macador
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COD to BOD factor

Hello all,

I am involved with a small scale FSM plant and have a question about measuring BOD. As a quick and dirty measure, does it make sense to calculate BOD from COD by simply multiplying by 0.8?

We are currently following this method as the reagents for BOD are hard to get a hold of and the test process is more laborious, but do you think it is a useful measure or would it make more sense to only test COD (assuming the BOD test materials do not arrive)?

Thanks,

owen
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  • muench
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Re: COD to BOD factor for fecal sludge - use a factor of 0.8?

I don't have this information available offhand at my fingertips for fecal sludge but we have had discussions around this topic before. Have a look at this thread which I think will be useful to you:
Quality parameters of septage that need to be tested
forum.susana.org/214-vault-content-resea...at-need-to-be-tested

Also more threads of relevance are in this sub-sction:
Pit or vault content research and faecal sludge characteristics
forum.susana.org/214-vault-content-resea...udge-characteristics

The work done and published by EAWAG will probably answer your question.

For conventional municipal wastewater the ratio was usually 2:1 (COD:BOD) but more likely it is smaller for fecal sludge as it's most easily degradable stuff in there, unlike municipal wastewater that also has industrial effluent.

Please let us know how you get on with your question, so that we can all learn together.

Regards,
Elisabeth

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  • kevintayler
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Re: COD to BOD factor for fecal sludge - use a factor of 0.8?

Dear Owen

The quick answer to your question is no. The COD to BOD ratio varies widely, depending on the age of the sludge and the degree to which it has digested in the tank or pit from which it is taken. The COD to BOD ratio for fresh sewage is typically around 2 but in theory should be higher for digested sludge. Actual ratios measured from samples taken in the field vary from around 2 to 10 and above. See Table 3.3 in my book on FS Treatment, which is available online at www.developmentbookshelf.com/doi/book/10.3362/9781780449869 .

EAWAG and others would certainly say that it is better to focus on COD rather than BOD although the equations for some treatment options use BOD rather than COD.

What is the source of your faecal sludge, what is the proposed plant capacity? Answering these questions will help you to determine what treatment processes are appropriate. (See Chapter 4 of the book for an overview.

I hope that this helps

Kevin

Kevin Tayler
Independent water and sanitation consultant
Horsham
UK
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  • macador
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Re: COD to BOD factor for fecal sludge - use a factor of 0.8?

Thanks Elisabeth,

Apologies for the delayed response. The thread you shared has some great links in, so I will keep reading and hopefully get a better understanding soon enough.

Cheers,

owen
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  • macador
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Re: COD to BOD factor for fecal sludge - use a factor of 0.8?

Thanks Kevin,

I feared that may be the case. I had actually read your book, but clearly didn't pay quite enough attention.

The majority of the FS we are collecting is from pit latrines, some emergency some more sustainable, with pretty high filling rates and anal cleansing with water.

Plant capacity is not well understood at the moment as we have made some alterations / additions, but we were processing around 15m3 / day, 5 days / week. The bottleneck seems to be in the drying stage as we are limited with space.

The local guidelines list BOD rather than COD as a parameter so we may have to measure BOD (when we have the materials in country) and COD simultaneously at the final effluent stage to see if there is a reliable enough correlation for using a factor.

Thanks again for your inputs and I will go re-read your book.

Cheers,

owen
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  • muench
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Re: COD to BOD factor for fecal sludge - use a factor of 0.8?

Dear Kevin,

Aha, so I was wrong! I thought faecal sludge would be easy to digest and therefore have a COD:BOD ratio of less than two. I guess I was thinking of "fresh faecal sludge" but maybe sludge from pit latrines and septic tanks is never "fresh". There must be quite a bit of degradation going on in the pit latrine if the COD:BOD ratios are 10 or more! - If we were looking at just faeces, then the COD:BOD ratio is somewhere around 2. *

I add here the table from your book that you mentioned:



Regards,
Elisabeth

* Actually there is a paper by Alison & team which gives data on this: The Characterization of Feces and Urine: A Review of the Literature to Inform Advanced Treatment Technology
www.ncbi.nlm.nih.gov/pmc/articles/PMC4500995/

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This email address is being protected from spambots. You need JavaScript enabled to view it. Twitter: @EvMuench
Sanitation Wikipedia project leader: en.wikipedia.org/wiki/Wikipedia:WikiProject_Sanitation
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