12.2 Bacteriological Testing & Coliform Sampling

Quick answer: Collect approved distribution bacteriological samples with sterile technique, protect the disinfectant residual with the correct bottle chemistry, follow the sample-site plan for routine and repeat locations, and treat total-coliform and E. coli positives as public-health events that trigger defined repeats, investigations, and sometimes boil-water notices—not as paperwork annoyances.

Why coliform monitoring exists

Public water systems cannot culture every pathogen in finished water. Instead, they monitor indicator organisms. Total coliform bacteria are a broad group whose presence suggests that pathways for contamination may exist in the distribution system (or that sample integrity failed). Escherichia coli (E. coli) is a subset more specifically associated with fecal contamination and acute risk. Under the federal Revised Total Coliform Rule (RTCR) framework implemented for Texas PWSs, operators must understand both the microbiology concept and the regulatory response chain.

TCEQ’s public drinking water program expects systems to maintain disinfection, pressure, and sanitary integrity—and to prove distribution water quality with an approved monitoring program. A single positive does not automatically mean customers are sick, but it is never “just lab noise” until investigation and required follow-up are complete.

Presence-absence versus quantitative concepts

Exam questions often contrast method styles:

For operator licensing, master the decision logic: a P/A total-coliform-positive sample triggers repeat sampling and assessment requirements under the rule structure; E. coli detection raises acute concern and can lead to an E. coli maximum contaminant level (EMCL) violation pathway when combined with required repeat results as defined in the rule. Do not invent a plant-specific numeric “pass score” for every method—use the method report and the regulatory definitions your system follows.

Wastewater plants also use bacteriological testing (for example fecal indicator limits in some permits), but drinking-water distribution RTCR logic is the highest-stakes exam topic for PWS operators.

Sample-site plans for Texas PWS distribution monitoring

A Texas PWS maintains a bacteriological sample-site plan (sample siting plan) identifying routine monitoring locations that represent the distribution system, including areas of varied hydraulic conditions as required by rule guidance. Sites are not chosen for operator convenience alone. Typical considerations include:

• Population-served minimum monthly sample counts and scheduling
• Representation of the distribution network (not only the plant clearwell)
• Locations that help detect contamination pathways
• Access, flushing capability, and sanitary sample taps
• Coordination with storage turnover and known problem areas

Routine samples answer: “Is the distribution system free of total coliform indicators this monitoring period at the approved sites?” Repeat samples answer a different question after a positive: they evaluate the original site and nearby locations to characterize whether contamination is confirmed and where it may extend, following the RTCR repeat protocol (original site plus upstream/downstream-type locations as required).

Collection technique that protects validity

1. Use the laboratory’s sterile bacteriological bottle; many contain sodium thiosulfate to neutralize chlorine so organisms are not killed in the bottle after collection.
2. Do not rinse a sterile bottle—rinsing removes sterility and may remove thiosulfate.
3. Select an approved tap; avoid swivel aerators when procedure requires removal; do not sample from contaminated hose bibs or submerged taps.
4. Disinfect the tap as the SOP requires (for example flame or approved chemical wipe where allowed), then flush until the water represents distribution main water—not stagnant premise plumbing—unless investigating a premise issue under a different protocol.
5. Open the bottle only at collection; keep the cap facing down; do not touch the lip; fill to the required volume mark without overfilling instructions that would wash out preservative.
6. Measure and record disinfectant residual as required for the monitoring program; a residual reading is operational evidence alongside the culture result.
7. Keep samples cool and deliver within holding time to a laboratory accredited for the drinking-water microbial analyses your system uses.

Invalid samples

An invalid sample is one the laboratory or rule framework cannot use for compliance because of collection, integrity, or analytical problems (examples: excess holding time, damaged bottle, insufficient volume, laboratory accident, or other documented invalidating conditions). Invalid samples generally must be replaced with another sample from the appropriate site within required timeframes. On the exam, the correct instinct is: do not treat an invalid result as “negative,” and do not ignore the need to recollect.

Also distinguish special samples (repair, new main, complaint, investigative) from compliance routine/repeat samples. Special samples are essential for operations but may not substitute for the compliance set unless the rules and sample designations allow that use.

Positive results, assessment, and boil-water notice overview

When a routine sample is total-coliform positive, the system must collect required repeat samples within the rule’s timeline and conduct a Level 1 or Level 2 assessment when triggered by the RTCR assessment criteria (for example multiple total-coliform positives in a monitoring period, depending on system size and history). Assessments look for sanitary defects: cross-connections, low pressure, storage sanitary issues, treatment breakthrough, sampling errors, and distribution integrity problems.

E. coli positives raise acute risk. TCEQ guidance explains that an EMCL violation can occur when:

• A sample is total-coliform positive and any required repeat is E. coli positive, or
• A sample is E. coli positive and any required repeat is total-coliform or E. coli positive

(Exact combinations follow the federal/state regulatory text; memorize the concept that E. coli plus confirming repeats creates an acute MCL pathway.)

A boil water notice (BWN) tells customers that distribution water may be unsafe because of microbial risk. Triggers include acute microbial situations (such as EMCL pathways), loss of pressure/distribution integrity events, treatment failures that may allow contamination, and other conditions identified in TCEQ BWN instructions. Operators must use current TCEQ templates and delivery rules, notify customers, notify TCEQ (including required submittals such as certificate of delivery), and only rescind after corrective action and required special bacteriological results show the water is again safe under the rescind criteria.

Distribution sampling judgment example

A monthly routine sample from Site B is total-coliform positive and E. coli negative. The correct exam path is not “ignore it if residual was 1.0 mg/L.” Collect the required repeats at the original and required additional sites, review residuals and pressure history, inspect for sanitary defects, and complete any triggered assessment. If a repeat returns E. coli positive, escalate under acute response and public-notice/BWN procedures immediately.

Operator checklist

• Know your approved sites and monthly minimums.
• Protect sterile technique and thiosulfate bottles.
• Record residual, site, time, and collector on every sample.
• Treat positives as investigation triggers, not lab “glitches.”
• Replace invalids promptly; keep special vs compliance designations clear.
• Follow TCEQ public-notice and BWN templates when acute conditions exist.

Official source trail

• TCEQ Revised Total Coliform Rule information
• TCEQ Understanding Boil Water Notices
• TCEQ Boil Water Notice instructions and templates