You might get more answers if you posted to the water treatment forum. You are correct in that you can get the H2S to flash out. The problem is that you will need to get the pH down to 5, so the flash approach is not generally feasible.
You have not presented quite information to make a detailed recommendation. Knowing the flow rate would also be helpful. You also need to obtain a water analysis that specifically includes field measurements of carbon dioxide and H2S as these parameters are usually displaced on the way to the lab.
Some observations regarding H2S removal can be made. Both H2S and CO2, when dissolved in water, are very feebly ionized, and it is possible to displace either one of these substances from its alkaline salt by blowing a stream of the other gas through the solution. In treating sulfur waters, though, the important and unfortunate fact is that the CO2 comes out much more easily than the more soluble H2S. Also, as the CO2 comes out, the pH value (assuming a normally alkaline water) rises, and this upsets the equilibrium between alkaline sulfides and H2S so that the reaction proceeds in the wrong direction for the removal as H2S.
At a pH of 5, 98% of the H2S will air strip. At a pH of 6, 86% of the H2S will air strip. At a pH of 7, 39% of the H2S will air strip. If the pH value of 9, only a little over one-half of one percent would be present as H2S, and the remained would be present as an alkaline or alkaline earth sulfide.
While the chlorination of raw waters containing high concentrations of sulfides is rather expensive, chlorination is of great value in eliminating the small residuals from the effluents of other sulfide removal processes. The amount of chlorine required to oxidize 1 ppm of H2S to the sulfate is over 8 ppm.
