Hi we're working on an oil water separation project using coalescing separators. Flow's at 20 cu.mph & oil concentration is 1000 ppm. The water also contains H2S 68mg/l and this is to be lowered to 2mg/l as well.
I've come across degasifiers for stripping the H2S from water, but I'm not sure if this supports oily water or if the oil has to be removed prior to it. Can someone please suggest if the H2S stripping should follow the oil-water separation or vice versa. What are the other methods applicable for lowering the H2S content in such cases?
Any advise is appreciated.
Many thanks!

Dissolved air flotation will be effective for both the oils and H2S.  But, you need to be careful with accumulation of toxic levels of H2S from the portion of it driven off by the aeration.  Lowering the pH will also help in both parameters.  Again, be careful with the H2S.

Hopefully, Dan Keys will add some more specific suggestions on DAF with high H2S levels.

Application of Advanced Nano Oxidation Process could be an option to weaken both the parameters..by our experience we have been able to address higher values of oil in waste water to ND levels and h2S too..but subject to tertiary treatment post process by ANOT....
Cheers
Madan Iyengar
www.ozosciences.com

Thanks for all the suggestions. But we are trying to find out more about stripping the H2S by Nitrogen injection. Is anyone familiar with this,  inert gas like nitrogen being used to strip H2S from waste water ?

Adjusting the pH of the water to 8.5 or higher will keep the H2S in the water as the HS- anion.  So, it odor or a hazardous environment is created with H2S gassing off in the area of the DAF, a simple pH adjustment may be effective at reducing the levels of H2S in the air.  

Keeping the H2S in the water can allow for treatment either by oxidation (which has some down sides) or with other chemicals such as a cyclic triazine.  A number of companies sell this chemical for removing sulfides from water.

I agree with a DAF or if it is Hydrocarbon processing plant DNF (N for Nitrogen) .
You can simply oxidize H2S in the DAF with application of either Hydrogen Peroxide alone, Iron-Peroxide or Fenton.

1- Hydrogen Peroxide Alone
Depends on the pH , over 8.5 you will generate So4 but need 4 part H2O2 for 1 part Sulfide  and below 8.5 you will need only 1.2 part H2O2 for each part of Sulfide and the product would be elemental sulfur.
The numbers are based on stoichiometric coefficient and since H2O2 is not a selective oxidizer you may need a little more , however in higher temperature you will get better results.
The reaction needs 20 minutes time and normally in DAF you have 30 min to 2 hours depends on the design.
With Peroxide alone you will be able to improve COD reduction and improvement of free and Emulsion oil removal since by adding Peroxide you will have small O2 bubbles (10 to 20 microns) and so they can capture more oil particles with coagulant and flocculant.
In this case you will be able to reduce the amount of flocculant as well.


2- Iron-Peroxide
If you need faster reaction time you can add 20 ppm Iron as Ferric or Ferrous (either chloride or sulfate) to catalyze peroxide and generate hydroxyl radical which is very strong oxidizer. To give you an Idea if the pH is below 8 , try  300 ppm of 35% H2O2 and 20 ppm of ferric chloride at the lab to conduct a treat-ability test, then after 5 min, check H2S residual and H2O2 residual with test strips.

3- Fenton,
Some people think about Iron Peroxide as Fenton reagents, but for Fenton you need to decrease the pH to <4.3 and then at the end neutralize it.
Fenton would be good too as it oxidize Phenol and decrease COD up to 75% but then you need more chemicals and pH control.

I am  currently working in this specific solution with many US Oil refineries and food industries. However there is safety concerns about the application of Peroxide specially due to O2 generation and risk of explosion but with several years in filed experience we know how to apply it safe as turn key full service solution. If you would like to know more visit US Peroxide website at; www.h2o2.com

Mostafa

Sorry,

reaction equation is:

H2S + 2O2 -> H2SO4

BR
biogataw