3 replies to this topic

[steven12]

Hi Guys,


My name is Steven, recently I have just finished university and is currently working at a wastewater treatment plant as my first job.

Recently at our plant, I observed an unusually situation where current tunnel scrubber (packing type), which uses a mixture of hypochlorite and Caustic soda solution to scrub odourous gases, mainly H2S, is ejecting white foaming material , as the picture below shows.




We do have a demister for our towers, but I don't quite know whether it is in good condition, my question is basically if anyone knows that Caustic/Hypochlorite mixtures are foamy by nature? One of my suspicison is that we use recycle water as top up water for the scrubber. Our membrane bed reactor digests sewage anaerobically and aerobically and through filtration to produce very good quality recycle water, however I suspect that if recycle water contains high level of Oil and Grease, it might actully saponificate with the caustic scrubbing liquid.

Anyone with experiences with scrubber towers ejecting foams?

Thanks

[siretb]

Yes, I would qualify sodium hypochlorite / caustic as foamy by nature.

To mitigate, as much as possible the effects, yes using a good quality surfactant free water will help.
You have a packed tower. Is the packing irrigation made by nozzles, or troffs and gutters? A liquid distributor, using chanels or pipes is less likely to generate foam than a full cone nozzle.
You may try to change (lower) the pH. as long as no free chlorine appears. I do not know how much freedom you have with that but it is worth trying.
I have never tried that, but also a bit of low cost activated carbon could help by capturing the surfactant agent. The packing and the nozzle will handle that without problem. I have never tried that in this specific case, but we often use activated carbon in the waters of scrubbers.

If your problem is H2S you may also consider to switch to FeCl3 as reagent.

[steven12]

The top up water we use comes directly from recycle water plant so there isn't a lot of choice, one thing I do notice though is that this foaming problem only occur when scrubber has been turned on after fews days due to inactivity (could be due to maintenance etc..)

I have also been told that sometimes the scrubbers are citric cleaned before being turned on, so there could an issue when the sump liquid is still slightly acidic and when turned on, it may mix with NAOH liquid. so there it could be the cause although I am not very sure.

The foaming problem goes away after the scrubber has been in operation for a while.

[siretb]

I cannot be sure, but do not see citric acid as the kind of molecule likely to generate a foaming problem.
An acidic solution reacts with NaClO and yields some chlorine, that may gas out. You could rinse with water the scrubber before startup, because what you describe looks more like that some kind of unwanted component is present at startup.

You state that the top water comes directly ... Yes, but how is this water distributed over the entire surface of the scrubber? Is it that the scrubber is so small that it does not require any liquid distribution at top?