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biomanoj

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How to co-relate COD , Nitrate and Methanol rates
Sep 14^th, 2008, 9:56pm

Hi,
I am trying to draw a relationship between increased amount of Methanol (from 15 ml to 30 ml / reactor) and the increased COD and Nitrate removal rates following the increase in carbon source. Is there a formalae for precisely calculating this ratio/ relationship between COD and Carbon source, particularly Methanol, or a ratio/relationship between COD/nitrate. I do realize that I can always coventionally compare. However, is there a standard format/formulae for drawing such a comparision ?

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DS

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Re: How to co-relate COD , Nitrate and Methanol rates
Reply #1 - Sep 15^th, 2008, 2:07am

Hi vrmanoj,

The relationship between Substrate (as such) and the COD of it is 1.5 g COD/g MeOH.
MeOH (after adaptation!) is readily and completely biodegradable, so now wonder that upon adding MeOH, the (total) degradation rate increases.
MeOH is also a good C-source for denitrification. Often, the availability of C for denitrification is limiting, so it is no wonder that denitrification rate increases upon adding an extra (readily biodegradable) C-source. Theoretically, 2.95 g COD are required to denitrify 1 g of NO3-N. In practice however, higher COD/NO3-N ratios are required (more then 4, also dependent on reactor lay out and operational parameters) since a part of the C added, is stored into the biomass and respired aerobically, so this part is not available for denitrification.

best regards,

DS

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Victor

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Re: How to co-relate COD , Nitrate and Methanol rates
Reply #2 - Sep 15^th, 2008, 11:01am

vrmanoj:

DS is right about the COD/NO3 requirements. The reaction is as follows:

6H⁺+ 6NO3^- + 5H2O + 5CH3OH----->3N² + 5CO²

The theoretical COD demand is 2.85 mg COD/mg of NO³ with an additional COD demand for bacterial growth/cell demands increased to 3.75 mg of COD/mg NO³. This COD demand for denitrification can go up to 10 mg of COD/mg of NO³ and is highly dependant on type of sewage being treated.

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biomanoj

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Re: How to co-relate COD , Nitrate and Methanol rates
Reply #3 - Sep 16^th, 2008, 8:02pm

Thanks DS and Victor.

But, is there a specific formulae for determining the COD/Methanol rate. For example, in my reactor, I get an influent COD of 344 mg/L and No3 of 120 mg/L. If I have understood your comments correctly, 120 mg No3 requires around 440 mg of COD ? The amount of carbon source as Methanol I added initially was 15 mL and I did not get sufficient denitrification rates.. it fell only to 80 mg/L. But, when I increased it to 30 mL, then denitrification rates are very cool indeed and have dropped to as low as 1.5 mg/L. I am having a very difficult time defending my reactor's huge demand for methanol ! My influent tank volume is 40 litres and reactor volume is 15 litres. Please advise. I know something is right, but I am having a hard time defending the same. Sad

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DS

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Re: How to co-relate COD , Nitrate and Methanol rates
Reply #4 - Sep 17^th, 2008, 2:27am

Hi Vrmanoj,

120 mg NO3/L = 27.1 mg NO3-N/L (conversion factor = 14/62)

one mg NO3-N requires theoretically 2.95 mg of (biodegradable) COD for nitrate-N conversion to N2. So you need 27.1*2.95 = 80 mg COD/L, minimum, at the moment you want to start to denitrify. The COD value of the influent is 344 mg/L, so more then enough if that COD is biodegradable.
The nitrate is present in the influent, so in order to remove nitrate, your reactor should have an anoxic zone or period first. Denitrification only takes place in the absence of oxygen (DO = zero!). So no aeration there/then, and be aware of oxygen input due to eventual recirculation from aerated zone.

Dentrification only works with biodegradable COD. Denitrification rates are higher if the COD (or a part of it) is readily biodegradable (like acetate, MeOH,..). If the denitrification rate is too slow with the unspiked (MeOH) influent, this means that the influent COD is not readily or (too) slowly biodegradable. Other causes are: oxygen is not zero or biomass is not yet adapted to anoxic denitrification (this is unlikely...).

The ratio COD/MeOH is 1.5 g COD/g MeOH, so calculate the amount MeOH out of the concentration you use, instead of working with volumes added.
The denitrification rate is something you need to measure yourself in the lab, this is dependent on many factors like: activity of the sludge, influent COD,....

best regards,

DS

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biomanoj

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Re: How to co-relate COD , Nitrate and Methanol rates
Reply #5 - Nov 13^th, 2008, 2:16pm

Oops.. I did not get email notification for your post DS.. so sorry ! But thanks a lot for that detailed clarification. I must therefore now assume theoritically that Methanol has compensated for the non biodegradable COD which was locked to the organisms. Whoah ! this is a brain opener !

I am thinking one way of determining how much organics are in the wastewater would be to see mainly the total and volatile solids among others. I am monitoring that now just to get a consistent idea.

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