· by Michael Blair, Nashville's Water Filtration
A well that smells like rotten eggs is telling you two things, and most people only listen to the first. The first is that hydrogen sulfide is in the water. The second, which matters more for the fix, is where it is getting in and how much of it there is. A whole-house filter bought on the smell alone can be too small for the level in the water, or aimed at the wrong end of the plumbing entirely. Here is how we sort it out on a well water treatment job, starting at the kitchen faucet and ending at a number.
Does the smell come from the hot tap, the cold tap, or both?
Run the cold tap and smell it, then run the hot tap and smell that, and the answer splits the problem in two. Hot only points at the water heater. Hot and cold both points at the well, the aquifer or the plumbing ahead of the heater. Do it at a tap that has sat for a few hours, such as first thing in the morning, and again after the water has run for a couple of minutes, because a smell that fades with running often lives in the lines or the heater rather than in fresh water from the well.
Penn State Extension, in its guide to hydrogen sulfide in water wells, notes that most people can smell the gas at levels well below 0.5 mg/L. So a nose test is sensitive enough to tell you where the smell is. It cannot tell you how much is there, and that number is what picks the equipment.
What is the water heater doing to hot-side water?
Most tank water heaters carry a magnesium rod that sacrifices itself so the tank does not rust, and Penn State explains that the magnesium can chemically reduce sulfates in the water to hydrogen sulfide. Swapping it for an aluminum rod, the same guide says, should end the odor and still protect the tank. Pulling the rod out with nothing in its place is the shortcut to avoid: Penn State warns it can shorten the heater’s life and is likely to void the warranty.
There is a second hot-side smell that people confuse with sulfur. Texas A&M AgriLife’s leaflet Hydrogen Sulfide in Drinking Water (L-5312) describes a sour, damp-rag odor that often starts after the thermostat is turned down, because odor-causing bacteria grow in the warm tank. It says running the heater at about 160 degrees for 8 hours will usually kill them, but only after confirming the pressure relief valve works and warning everyone in the house about scalding water. A lower everyday setting can stay, with a hot flush repeated whenever the odor comes back.
A water softener is worth a look in the same visit. Penn State says that in rare cases a softener gives sulfur-reducing bacteria a comfortable place to grow, so a smell that began soon after one went in deserves a test on both sides of it.
How is hydrogen sulfide measured before a filter is chosen?
It is measured on site or in a sample bottle that holds it in place, because the gas escapes from water quickly. The Texas A&M leaflet says the cause has to be settled before testing and treatment, that samples should go in bottles with stabilizing chemicals from the lab doing the analysis, and that labs generally want the sample refrigerated and delivered within 48 hours. Results come back in mg/L, which is the same as parts per million. A jar filled at the sink and carried around for a week reads low and leads to an undersized system.
The same test should cover what tends to travel with sulfur. Texas A&M lists iron, manganese and pH as things that change which treatment works, and says water below pH 6.7 may need neutralizing before an iron filter will do its job. That is the reason our water testing on a well does not stop at the smell. The sulfur number decides the size of a well water system; the iron, manganese and pH decide the type.
Sulfate is a separate reading from hydrogen sulfide. The Connecticut Department of Public Health notes there is no federal drinking water standard for hydrogen sulfide, while the EPA’s recommended secondary limit for sulfate is 250 mg/L. Sulfate in the water is what the magnesium rod has to work with, so a high sulfate reading backs up a hot-side diagnosis.
Which treatment fits which level of hydrogen sulfide?
The level in mg/L sorts the options, and the extension guides draw the lines in slightly different places. Where they disagree, we plan to the more cautious figure.
At trace levels, carbon does the job. Texas A&M puts activated carbon at roughly 0.05 to 0.3 mg/L and says moderate to high levels mean frequent replacement. Penn State gives carbon up to about 1.0 mg/L and recommends a large whole-house tank, 1.0 to 1.5 cubic feet, for that work. It adds that catalytic carbon reaches higher levels but needs at least 4.0 mg/L of dissolved oxygen in the source water. A carbon tank of that kind is the same equipment as our whole-home filtration, sized for the sulfur rather than for chlorine.
In the low single digits, an oxidizing filter takes over. Penn State says manganese greensand and similar filters remove up to 2 to 3 mg/L, need potassium permanganate to regenerate and regular backwashing, and that Birm media is not recommended for hydrogen sulfide. Texas A&M stretches greensand to 10 mg/L and says the recharge runs every 1 to 4 weeks depending on the water. Connecticut lists aeration as effective below 2.0 mg/L, and Texas A&M calls the pump-and-storage-tank aeration systems complex and not recommended for a household.
Above that, chlorine. Penn State says chlorination can treat any level and is most often used above 6.0 mg/L; Texas A&M puts it up to 75 mg/L with an automatic feed pump. Both call for at least 20 minutes of contact time in a tank or pipe coil, and Texas A&M gives a starting dose of about 2.2 times the hydrogen sulfide concentration. A carbon filter after the contact tank takes out the leftover chlorine for drinking and cooking.
Sulfur is rarely the only thing a well brings with it, and our broader post on treating well water in Middle Tennessee covers the bacteria, iron and hardness that often come along. If your water smells and you want a number before anyone sells you a filter, ask us for a free estimate and we will test it at the tap.
- Hot tap only: the heater’s rod or bacteria in the tank, before any filter
- Traces, under about 1 mg/L: activated carbon, sized for the whole house
- Up to 2 to 3 mg/L (to 10 by one source): greensand or another oxidizing filter
- Above about 6 mg/L: continuous chlorination, a 20-minute contact tank, then carbon
Sources
- Penn State Extension, Hydrogen Sulfide (Rotten Egg Odor) in Water Wells — Odor threshold below 0.5 mg/L; magnesium rod and aluminum replacement; softener and bacteria; carbon to about 1.0 mg/L, catalytic carbon and dissolved oxygen; oxidizing filters to 2 to 3 mg/L, Birm not recommended; chlorination above 6.0 mg/L with 20 minutes of contact; shock chlorination odor returning.
- Texas A&M AgriLife Extension, Hydrogen Sulfide in Drinking Water (L-5312) — Sour hot-water odor and the 160-degree flush; sampling and the 48-hour refrigerated delivery; treatment by concentration (carbon, aeration, greensand to 10 mg/L, chlorination to 75 mg/L); pH 6.7; chlorine dose of 2.2 times the H2S level; shock chlorination repeated twice.
- Connecticut Department of Public Health, Hydrogen Sulfide and Sulfate in Private Drinking Water Wells — No federal standard for hydrogen sulfide; corrosion and staining at 1.0 mg/L; EPA secondary limit for sulfate of 250 mg/L; aeration below 2.0 mg/L.
FAQ
Rotten Egg Smell in Well Water: Hot Side or Both? — questions
How do I tell whether the smell comes from the water heater or the well?
Smell the cold tap and the hot tap separately, after the water has sat for a few hours and again after it has run for a while. A smell on the hot side only points at the water heater. A smell on both sides means the hydrogen sulfide is arriving from the well or the plumbing ahead of the heater.
Will a carbon filter remove the rotten egg smell?
Only at low levels. Penn State Extension gives activated carbon up to about 1.0 mg/L in a large whole-house tank, and Texas A&M AgriLife puts it nearer trace amounts. Above that, carbon loads up fast and needs frequent replacement, so a stronger reading calls for an oxidizing filter or chlorination instead.
How much hydrogen sulfide can a greensand filter handle?
Penn State Extension says oxidizing filters such as manganese greensand remove up to 2 to 3 mg/L, while Texas A&M AgriLife gives greensand up to 10 mg/L. Either way, the media has to be regenerated with potassium permanganate, every 1 to 4 weeks by the Texas A&M figure, and the water’s pH may need correcting first.
Why does the sulfur smell return after shock chlorinating a well?
Shock chlorination is a single heavy dose, and the bacteria that make hydrogen sulfide often survive it or come back. Penn State Extension says the odor returns quickly in most cases, and Texas A&M AgriLife says that if it persists after two attempts, continuous chlorination with a feed pump may be needed.
Is water that smells like sulfur safe to drink?
At the levels found in most wells it is a nuisance more than a hazard. Penn State Extension says people smell hydrogen sulfide well before it reaches a level of health concern, and that the bacteria behind it do not cause health problems. It still corrodes pipes, tarnishes copper and leaves black or yellow stains.
Do I need a test before buying a sulfur filter?
Yes, because the level in mg/L decides which kind of system works and how big it needs to be. Texas A&M AgriLife advises testing on site or in a stabilized lab bottle, refrigerated and delivered within 48 hours, and checking iron, manganese and pH in the same sample.