Two very different results
A lab report saying “coliform present” and one saying “E. coli present” are not the same finding, and conflating them causes both panic and complacency.
| Result | What it means |
|---|---|
| Total coliform positive | “a contamination pathway exists between a source of bacteria…and the water supply” |
| E. coli positive | “much more serious… it indicates that human or animal waste is entering the water supply” |
Penn State Extension’s water team — Bryan Swistock, Stephanie Clemens and William Sharpe — are careful about the first one: “Most types of coliform bacteria are harmless to humans” and “drinking water that is contaminated with coliform bacteria does not always cause illness.”
That is not a reason to ignore it. Coliform is an indicator: it tells you something can get in. What gets in next time may not be harmless.
The standard is absolute regardless — “the drinking water standard requires that no coliform bacteria be present in drinking water.”
Shock chlorination is a test as much as a treatment
The usual first step is shock chlorination: “introducing a high concentration of chlorine to the water for a short time.”
What matters is the retest afterwards, because it distinguishes two different problems:
- Follow-up tests negative → likely a one-time contamination event, now cleared.
- Bacteria return → “you will need to consider a continuous disinfection treatment system.”
That fork is the whole decision. Skipping the retest and buying a UV lamp immediately means paying for continuous treatment you may not need — or worse, installing it while the actual pathway stays open.
When UV works, and when it does not
UV is effective and low-maintenance in the right conditions. Two published limits define them:
- Concentration. “UV light units are not recommended for water supplies where total coliform bacteria exceed 1,000 colonies per 100 mL.”
- Clarity. “The untreated water entering the unit must be completely clear and free from any suspended sediment or turbidity to allow all of the bacteria to be irradiated by the light.”
The second is why UV sits last in the treatment sequence, after iron removal and sediment filtration. A UV lamp installed ahead of particulate removal has bacteria hiding in the shadow of suspended solids. It will still draw power and still show a green light.
Mistakes that cost the most money
Buying UV on a single positive. One result can come from a sampling error or a one-off intrusion. Retesting is free compared with a lamp you run forever.
Installing UV before particulate removal. Bacteria sit in the shadow of suspended solids. The lamp still draws power and still shows a green light.
Treating the symptom instead of the well. A failed cap, casing or seal keeps letting contamination in. UV disinfects what arrives without closing the path.
Running UV above its concentration limit. UV is not recommended where total coliform exceeds 1,000 colonies per 100 mL.
Forgetting the lamp is a consumable. Output falls before the lamp fails, so a unit can be running and no longer disinfecting.
Three situations, and what changes
A single positive, nothing inspected. Check the cap, casing and seals, then retest. This is where the decision belongs, not at a product page.
Repeat positives on clear water. UV is appropriate. Size it on peak flow and accept the replacement schedule.
Repeat positives on cloudy water. Particulate removal comes first. UV installed ahead of it is treating water it cannot penetrate.
The objections worth answering
“Wouldn’t chlorination be simpler?” It is an alternative, but it adds contact-time requirements and taste. UV adds no chemical and no byproducts, which is why it suits a household that has fixed its well.
“The light is on, so it must be working.” Lamp output degrades before failure, and turbidity blocks it regardless. An indicator confirms power, not disinfection.
“Can I skip the retest and just install it?” You can, and you may be paying to run continuous treatment for a problem a seal replacement would have ended.
Who this is for
Private-well households with a coliform result, deciding between fixing the well and installing continuous disinfection.
Choose UV if
- A repeat sample confirms coliform after the cap, casing and seals are checked.
- Total coliform stays below 1,000 colonies per 100 mL.
- The water reaching the lamp is clear, with iron and sediment removed upstream.
- You accept a lamp replacement schedule and a power draw that never stops.
Choose to fix the well first if
- This is a single positive result and nothing has been retested.
- The well cap, casing or seals have not been inspected.
- The water is cloudy or carries visible sediment. A lamp behind turbidity leaves bacteria sitting in the shadow of suspended solids.
Test annually, because nobody else will
Penn State Extension recommends all private supplies be tested for total coliform every year.
Private wells fall outside the enforceable standards that cover public water systems. No utility runs this test, no report arrives, and nothing tells you when a casing seal fails.
Where bacteria treatment sits relative to everything else is worked through in the sizing method, and the rest of the cluster starts at well water treatment.
→ Sizing the sediment filter that has to come first → Turbidity, and why clarity is a precondition for UV → Legionella, which grows downstream of any treatment → Coliform on a public supply, where the rule does the work