H2S Over the Mixer: Getting the Headspace Below 5 ppm

The monitor clipped to the ladder read 38 parts per million when the second operator went up to check the batch. He was down the ladder in under a minute and spent the next hour sitting on a pallet in the fresh air, and the site was lucky that the reading was on a personal monitor rather than in an incident report. The batch itself was fine. Gelling a sulphidic leachate stirs up exactly the gas you spend the rest of the year trying to keep in the liquid.

Hydrogen sulphide is the one hazard in leachate work that gets worse the more energetically you do the job.

Why Mixing Releases It

Sulphide sits in equilibrium in the liquid, and the proportion that exists as dissolved gas depends on pH. At pH 7 about half of the total sulphide is in the gaseous form; at pH 8 it is nearer 10 percent; at pH 9 it drops into the low single figures. The gas leaves the liquid at a rate set by the surface area and the turbulence, and a paddle at 60 rpm in an open tank is a very effective stripper.

So the site that doses a sulphidic liquor without thinking about pH and turbulence is running an unplanned stripper tower with the operators standing at the top of it.

The trap is that the gel makes it look safe afterwards. A firm gelled mass releases very little at the surface, so the readings taken after the batch is complete are reassuring and tell you nothing about the ten minutes when it was being mixed.

Controls in Order of Effectiveness

• Raise the pH before mixing. Getting from 7.2 to 8.4 can cut the release by a factor of five, and it is a dosing decision, not a capital one. Watch the effect on the gel: above 9.5 the polymer skins, so there is a window.
• Mix with the tank closed and extract from the headspace. A lid with a 150 millimetre extraction spigot and a fan sized for 15 air changes an hour on the headspace volume will hold the working area near background.
• Mix at the lowest speed that works. If the batch gels in eleven minutes at 45 rpm, do not run it at 70 because it looks faster.
• Keep people off the ladder. Sampling and inspection happen after the batch is gelled and the headspace has been purged, not during.

Personal monitors on everyone in the area, alarm at 5 ppm, evacuate at 10. Fixed detectors in the mixing bay if the site runs batches daily. The numbers are not conservative; at 50 ppm the gas deadens the sense of smell within minutes, which is why odour is not a warning.

The Near Miss Worth Repeating

Two operators on a night shift, one batch, an open-topped tank and a warm night. The first man added 400 kilograms of powder into a vortex and climbed the ladder to check for lumps. The reading on his monitor went from 4 to 38 in about ninety seconds. He got down, but he was unsteady on the last three rungs.

The investigation found three compounding factors. The leachate had been standing in a sealed sump for eleven days and had built up sulphide to around 90 milligrams per litre. The pH was 6.9, lower than usual because a haul of construction waste had come in. And the night crew had opened the tank lid because the vapour was fogging the sight glass.

None of those three was unreasonable on its own. Together they turned a routine batch into an exposure. The site’s fix was a pH trigger written into the batch sheet: below 7.4, no mixing until the pH is corrected.

What the Gel Itself Does

There is a secondary question that comes up on every sulphidic site: does gelling trap the sulphide or release it slowly over the journey? Mostly the former. The gel reduces the surface area available for transfer and the water is bound, so the release rate from a finished batch is low. What comes off during transport is a fraction of what comes off during mixing.

That does not mean a sealed trailer is safe to open without thought. A trailer that has carried sulphidic gel for six hours can have an accumulated headspace concentration that surprises whoever opens the hatch. Vent before opening, and stand upwind.

Records and Monitoring

Log the sulphide number on the batch sheet if you have a field kit, and log the pH on every batch regardless. Note the reading on the personal monitor at the end of mixing. Three fields, thirty seconds, and they are the difference between a control that works and a control that is assumed to work.

Gas monitoring is also the kind of record an inspector will ask for after any odour complaint from a neighbour, not just after an incident. If the complaint log at your site has entries from the boundary on mixing days, the fix is usually pH and extraction, in that order.

The operating guidance for sulphidic leachate sets out the pH window where gel performance and gas control overlap, which is narrower than most sites assume.