5 replies to this topic

[tellme116]

Good day guys!


I would just like to ask if there is really an effect on COD due to the amount of sodium sulfate and sodium sulfite present in wastewater? If there is, what is the relation between the two? I am currently having a hard time trying to figure this out. If the COD is too high what wastewater treatment process could be used in order for it to reach the standard.

The components found in the wastewater are Na2So3, Na2SO4, NaOH, H2O.



Thank you very much!

[kkala]

Not having experience on waste water treatment, I will try an answer, not so far received. Properties of NaSO3 can be found at http://en.wikipedia.org/wiki/Sodium_sulfite.
Na2SO3 can be oxidized by atmospheric oxygen, Na2SO3 + 1/2 O2 --> Na2SO4. If the waste water contains 126 mg/l of Na2SO3, the corresponding COD (Chemical Oxygen demand) due to Na2SO3 would be 1/2 mgmol O2/l, that is 16 mg O2/l.
COD is usually referred to organic material, http://en.wikipedia.org/wiki/Chemical_oxygen_demand, but oxidation of S in Na2SO3 can be realized even by air, so COD due to Na2SO3 has to be considered.
On the contrary S in Na2SO4 cannot be further oxidized, so Na2SO4 has no effect on COD of the waste water.
A possible way to reduce COD to permissible limit could be to pass the waste water through aerators, so that atmospheric oxygen will convert Na2SO3 to Na2SO4.
Note: Biological oxidation of aqueous Na2SO3 to Na2SO4 (similar to thiosulphates) has been searched in Web with unclear results.

Edited by kkala, 23 June 2012 - 09:43 AM.

[tellme116]

Thank you very much kkala!

So most probably the amount of Na2SO3 triggers excess COD content of the wastewater.

Is there other way other than an aerator that can be used?

If i added Lime in the wastewater and did flocculation will it help reduce the amount of COD??

[kkala]

-Solubility products (see http://www.csudh.edu/oliver/chemdata/data-ksp.htm) indicate that Na2SO3 will precipitate using Ca(OH)2, and so will Na2SO4. Quantitative estimate can give figures of precipitated CaSO3 and CaSO4 (in usual temperatures it will form gypsum - CaSO4*2H2O), taking actual Na2SO3, Na2SO4 concentrations into account.
-In theory any oxidizing agent can be used to convert Na2SO3 to Na2SO4 (KMnO4, Cl2, NaOCl, etc), but excess of that agent could create a problem worse than that of Na2SO3. See http://www.csudh.edu/oliver/chemdata/data-ksp.htm, where SO2 is used to reduce remaining chlorine. Actual Na2SO3 concentration might affect choice. Someone with experience in waste water treatment could advise best practical techniques to remove SO3⁼.

Edited by kkala, 12 July 2012 - 08:53 AM.

[tellme116]

Ohh!! what could be the optimum amount of oxygen to be added so that it would only convert the Na2SO3 to Na2SO4?? Can I use the stoichiometric ratio of 126/16= 7.9 ppm from http://www.biosorption.net/WP/ch4.pdf to be able to determine the amount of O2 needed? is there a way that I can calculate the time so that I will be able to know if the amount of oxygen supplied to the wastewater is enough. And if I aerated the Na2So3 with NaOH, H2O and Na2SO4 would there be a problem or will Na2SO3 be the only one to react with the O2?

Thank you very much for the help.

[kkala]

Stoichiometric demand of O2 would be 16 mg/126 mg Na2SO3=0.127 (post No 2), but I assume that aerators would use a big excess of atmospheric air. However potential ways could be aeration, precipitation (post No 4), probably biological oxidation (post No 2), even something else. It is the task of a specialist (e.g. a hired professional) to chose the best applicable treatment for the case, specifically considering flow rate, concentration, temperature and specifying process parameters (e.g. excess of air or chemicals, depending of treatment). And giving general advice on the matter of Na2SO3.
In case of choosing aeration, air excess would not create a problem. Na2SO4, NaOH, H2O cannot be oxidized.

Edited by kkala, 17 July 2012 - 01:32 AM.