Good evening

pls see the attached file

Good evening

pls see the attached file

Good afternoon (Brussels time). First of all you should specify the target concentration of nitrate, ammonia and COD that you want to achieve for water recycle. For example, it is very different if you want to recycle the water for irrigation puropese rather than for industrial purposes, and in the latter case, it depends on WHICH is the industrial process that would recycle the treated wastewater.

Two comments:

Firstly, the core of the problem stated - as I read - it is that you desire to remove organic compounds, NO3-/NO2- and NH4+ while maintaining ferric ions in solution.

You could make an aerobic biological treatment which would remove the organic compounds and oxidise the nitrite and ammonium to nitrate. Unfortunately the bacteria would also oxidize ferric ions to ferro iron which would precipitate.

Theoretically it should be possible to alternate with discontinued aeration and add a small amount of the wastewater. The organic compounds in the untreated wastewater would then react with nitrate and ferro iron to make free nitrogen and reform the ferri ions.

However, the resulting water would have the ammonia from the part of the wastewater that was not aerobically treated and likely some organic compounds would also remain.

This leads to the second comment, as Prof. Canziani states, precise information about the requirement of the reused effluent is essential.

Thank you for the responses, Prof. Canziani and Prof. Andersen.

I need only the 'water containing ferric ions' to recycle in an industrial process for a specific chemical reaction. I probably have to go through nitrification followed by denitrification to remove all the nitrogen, leaving the ferric ions in the water. There is indeed a problem during nitrification due to precipitation of ferric ions.

I was thinking if the precipitation of ferric could be avoided by carrying out nitrification at lower pH? As far as I know, nitrification requires a pH value more than 7 and generates acidity.

Can the simultaneous nitrification-denitrification process be employed in this case?

@Chandrashekhar. There are some examples that nitrification can be achieved at low pH using biofilm systems if the pH is kept constatn. Even if you manage to make the aerobic treatment at a reasonable low pH the iron will be oxidized so you need to reduce it back to ferriiron.

If you consider the alternating aeration, anaerobic system I suggested you could use use methanol to reduce nitrate and ferroiron and achieve nitrogen (ammonia and nitrate) free water.

@Pizarro:

1) If you add H2O2 to this water much of the iron will be oxidized to ferroiron.

2) That is the catalyst and reducing reagent in the process you suggest "catalytic reduction of nitrate and nitrite to convert it into N2" ?

@Dr. Pizarro and Dr. Anderson,

Thank you for your suggestions. They are really very interesting.

But I was thinking if (1) Ferric ions could be used as the electron acceptor for the oxidation of NH4 to Nitrite, making it possible to carry out the process under anoxic conditions (no need to supply oxygen).

(2) Then, the organics along with ferrous ions formed in the above stage can be used as electron donor to reduce all the nitrite/nitrate to N2, also under anaerobic conditions.

So, some part of ferric will first get reduced (1st step) and then again oxidize (2nd step).

Is there any chance to carry out these steps? I am doubtful about the first step, on how nitrite could affect ferric mediated NH4 oxidation,under anoxic conditions. Also, will denitrification take place during the first step ?

@ B. Chandrashekhar: As for ammonium oxidation by ferric ions I found somethingin the literature:

J Biosci Bioeng. 2006 Jan;101(1):70-2. Possibility of anoxic ferric ammonium oxidation. Author: Sawayama S.

Soil Biology & Biochemistry 37 (2005) 2323–2328. Ammonium oxidation coupled to dissimilatory reduction of iron under anaerobic conditions in wetland soils. J. C. Clement, et al.

As for denitrification with ferrous ions, I found that it is possible, but at the cost of green house gas emission:

Water Res. 2011 Nov 15;45(18):5945-52. doi: 10.1016/j.watres.2011.08.056. Epub 2011 Sep 12. Reduced iron induced nitric oxide and nitrous oxide emission.

Kampschreur MJ1, Kleerebezem R, de Vet WW, van Loosdrecht MC.

I think you can start a research program on this topic.

Thank you, Prof. Canziani. I will definitely look forward for it.

@Pizarro

1) Oxidation of acetate at pH 4-4½ is not difficult. You will get other microorganisms than at pH 7. Possible fungus.

I assume in my previous answer that the acidity is caused by the organic acids so a biological process that continuously mineralise the organic acid in a stirred tank would run at a higher pH.

2) Is that economically feasible? Any reason to not just add methanol and let the biology take care of the catalysis. While nitrification is pH dependent, denitrification is quite robust.

@Chandrashekhar

I believe the reaction of ferric ammonia oxidation can occur and it properly occurs in groundwater: 6Fe3+ + NH4+ + 2H2O -> 6Fe2+ + NO2- + 8H+

The practical concern is how - in the suggested wastewater - can you make it run without competition from the reduction of ferric ions by acetatic acid? E.g: 8Fe3+ + C2H4O2 + 2H2O -> 8Fe2+ + 2CO2 + 8H+

To place things in perspective it is probably useful to make an electron balance for your wastewater. Here goes:

Equimolar amounts of NO2- and NH4+ are ideal for anammox: NH4+ + NO2- → N2 + 2H2O

Thus you would like to convert 1 mM NH4+ into 1 mM NO2-. (so you end up with 3mM of each).This requires 6mM electron equivalents.

20 mM Fe3+ represents 20 mM reducing equivalents. Thus the amount of Fe3+ is more than enough to oxidize 1 mM of NH4+; indeed the challenge would be to engineer the process in such a way that not all NH4+ is oxidized to NO2-, or even further, to NO3-.

So far so good.

However the wastewater also contains organic COD.

1600 mgCOD/L. is equivalent to (1600/16)x2 = 200 mM reducing equivalents.

Clearly this oxygen demand is much higher than the 20mM offered by 20mM Fe3+.

Ethanol and acetate are energetically much more favorable sources of reducing equivalents than NH4+. In a typical biological process micro-organisms that use ethanol and acetate as electron donor and iron as electron acceptor will outcompete organisms that use ammonia as electron donor and iron as electron acceptor when iron is limiting (as implicitly stated above by Henrik Andersen).

Thus the challenge is to engineer a multi-step biological process in which you remove the organic COD first e.g. by supplying a limiting amount of oxygen, followed by a step in which you use Fe3+ to oxidize a limited amount of ammonium to nitrite, and finally a step in which equivalent amounts of ammonium and nitrite are converted in innocuous nitrogen gas.

I am surprised that no one has suggested nitrification/denitrification.  Nitrification needs oygen and uses autotrophs to accomplish.  High COD in the form of acetate and methanol are prerequisites for denitrification under anoxic conditions.  If you put the high COD effluent into the aerobic (nitrification) zone first you will get heterotrophic growth and waste the acetate and ethanol.  If you put the effluent into the anoxic zone first this is backwards to the requirement for nitrification first and then denitrification.  The solution is for an internal recycle of about 5 to 6Q from the aerobic to anoxic zones.  Many STPs do this and is quite common.  I suspect this question is too good to be true (that is usually acetate and ethanol have to be added to achieve good denitrification).

You could try plants but it depends of the volume you have off course. It works really good.

@Angus Chu. Nitrification/denitrification was discussed in the comments back in April. Move to the first page of the answers to see it.

I've never seen wastewater containing any nitrite, just nitrate.  It is very difficult to nitrify to nitrite without full nitrification to nitrate, at least in sewage treatment plants.   Anammox has been attempted in a number of ways to produce equimolar concentrations of nitrite and ammonia in many sewage treatment plants around the world using selector bioreactors and such but is still not common or, I suspect, very hard to consistently achieve.

In sewage treatment plants nitrite mostly occur during startups or as a consequence of toxic pulses.

If one requires nitrite for anammox it is suggested to be possible throught selecting for the ammonia oxidiseres against the nitrite oxidisers by varying the aeration according to modeling of the organismes growth.

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