Two covered lagoons on the same dairy, taking the same manure from the same herd, can hand you 1,200 ppm hydrogen sulfide at one flare and 4,000 ppm at the other. The sulfur loading does not have to be different. A pH of 7.6 in one basin and 6.6 in the other accounts for most of that spread by itself.
EFI sees inlet H2S anywhere from 3,000 to over 8,000 ppm across installations that look alike on paper. Some of that spread is feedstock: sulfate in the wash water, gypsum bedding, an acid cleaning cycle that lands in the collection pit. A larger share than most owners expect comes from the chemistry that decides what fraction of the sulfide already in the water is free to leave it.
Sulfide sits in two forms and only one of them is volatile
Dissolved sulfide in a lagoon exists mostly as two species, un-dissociated hydrogen sulfide and the bisulfide ion. The split between them is set by pH and the crossover sits close to pH 7. At pH 7.0 the two are roughly even. At pH 6.0 about 90 percent of the dissolved sulfide is the un-dissociated form. At pH 8.0 about 90 percent is bisulfide.
Only the un-dissociated form leaves the water. Bisulfide carries a charge, stays in solution, and goes out with the effluent. So the gas reading at the flare answers a narrower question than most people read into it. It tells you what share of the basin's sulfide the pH is willing to release, which is a different quantity from the sulfide inventory itself.
The working range matters more than the extremes. A healthy anaerobic lagoon usually runs between pH 6.8 and 7.8. Across that single unit, the free H2S fraction falls from about 61 percent to about 14 percent. That is better than a four to one difference in the driving force behind the gas number with the same pounds of sulfur in the basin.
Temperature moves it in the same direction
Once sulfide is in the free form it still has to cross into the headspace, and how readily it crosses depends on temperature. H2S is roughly twice as volatile in water at 95 degrees F as at 50. A basin reading 1,500 ppm in February and 3,000 ppm in August has not necessarily changed what it is doing biologically. Warmer water also generates sulfide faster, so the two effects compound rather than cancel.
This is the mechanism underneath the daily cycle a continuous analyzer picks up on a covered lagoon. Afternoon warming raises generation and release together. Neither shows up in a grab sample taken at a fixed hour, which is most of why grab samples disagree with each other.
What a liquid sulfide test does and does not settle
A lab sulfide result on a liquid sample comes back as total dissolved sulfide in mg/L. That number is real and on its own it will not predict a gas concentration. Two basins at 40 mg/L total sulfide, one at pH 6.9 and one at pH 7.7, are not carrying comparable gas loads. Ask for pH and temperature with every sulfide result, taken at the same time from the same depth, or the sulfide number cannot carry a design decision.
The reverse case shows up more often in operations. A site calls because outlet H2S has climbed for two weeks with no change in feed, no change in flow, and no alarm on the air injection. Before anyone opens the blower panel, check whether basin pH has moved. A quarter of a unit down, 7.4 to 7.15, raises the free H2S fraction by roughly 45 percent, and that shift is invisible on every other instrument on the site.
Where the sample comes from matters for the same reason. A pH pulled from the discharge structure, after the liquid has had a chance to off-gas and pick up air, is not the pH the sulfide saw. Take it from the basin, below the surface, and take it the same way every time. The absolute value is worth less than the trend, and the trend only exists if the method holds still.
The alkalinity loop
Alkalinity is what holds that pH steady. In a manure lagoon it comes mostly from bicarbonate the digestion itself produces, and its job is to absorb volatile fatty acids generated faster than the methanogens can consume them. Overfeeding, a shock load of high strength waste, or a cold spell that slows methanogenesis all push acid into the basin. Alkalinity takes that hit first and pH follows it down.
A micro-aerated basin has a second draw on the same buffer. Oxidation carried past elemental sulfur to sulfate makes sulfuric acid in the water, and that consumes alkalinity on the way out. On a basin with thin buffer to start with, the consequence is the same pH drift that liberates more H2S from sulfide that was already there. The gas number gets worse for a reason that originated in the treatment.
The practical response is to log alkalinity rather than pH alone. Alkalinity falls first. By the time a pH meter moves, the buffer that was protecting it is largely spent, and recovering it costs more than holding it would have.
Where this changes a decision
- Sizing a treatment train on a single commissioning day reading builds in an error nobody can see later. Take inlet readings across a season with pH and temperature logged beside each one.
- A rising outlet with steady feed is a chemistry question before it is an equipment question. pH and alkalinity are faster and cheaper to check than a blower, and they are more often the answer.
- An offtake sulfur specification written against a winter reading has no margin in it. Basins get worse in warm weather for reasons that have nothing to do with how well the system is running.
- Sample pH, temperature, total sulfide and alkalinity together, same hour, same depth, same method. Any one of the four on its own will not support the conclusion someone wants to draw from it.
EFI has built covered lagoon and digester systems out of Gaston, South Carolina since 1993, with more than 500 systems in service and roughly 82 percent of the US covered lagoon digester market. The O2 injection systems we install routinely bring inlet gas from several thousand ppm down under 100. Whether that is a hard job or an easy one is usually settled in the water, well upstream of the skid.
