11.3 Microbiology, Pathogens & Waterborne Disease

The Four Groups of Waterborne Agents

DEP's General Overview module states the exam-level framing directly: the three categories of microorganism agents of waterborne disease are bacteria, protozoa and viruses. Helminths and algae are added below because operators encounter them, but a question asking for the categories of waterborne disease agents is asking for those three. DEP also teaches two facts that surprise candidates: drinking water disease outbreaks most often involve groundwater systems, not surface water plants, and the common sources of waterborne pathogens are waterfowl droppings, septic fields and farm runoff.

Named Organisms and the Diseases They Cause

Exam items pair the organism with its illness. The list below follows DEP's disinfection and general-overview modules.

What Makes a Usable Indicator Organism

DEP lists the properties an indicator must have before it can substitute for direct pathogen testing. An exam question that asks why total coliform is used instead of testing for Salmonella is testing this list. An ideal indicator is:

• Present in large numbers in fecal matter
• Present whenever the pathogen is present
• Present at a high ratio of indicators to pathogens
• Stable and non-pathogenic itself
• Absent in uncontaminated water
• At least as resistant to treatment as the pathogen, and similarly affected by natural conditions
• Detectable by a simple, inexpensive, fast and accurate laboratory test suitable for all water types

How Each Organism Is Regulated

Only one microbiological parameter carries a maximum contaminant level. Everything else is controlled by a treatment technique, which is why operating performance rather than a finished water number is the compliance evidence.

The Chlorine Resistance Ladder

Ranked from easiest to hardest to inactivate with free chlorine:

1. Vegetative bacteria (easiest)
2. Enteric viruses
3. Giardia cysts
4. Cryptosporidium oocysts (essentially chlorine resistant at practical doses)
5. Bacterial spores

This ranking is the entire justification for the multiple barrier approach. Because free chlorine cannot be relied on for Cryptosporidium, the barrier for that organism is physical removal by filtration, supplemented where required by ultraviolet light, which inactivates protozoa at very low doses, or ozone. It is also why a turbidity excursion is treated as a public health event rather than an aesthetic one.

Indicator Organisms and What a Positive Actually Means

Operators do not test for every pathogen. They test for indicators:

• Total coliform is a broad group of bacteria common in soil, vegetation and the intestines of warm-blooded animals. A total coliform positive in the distribution system indicates a possible pathway for contamination, such as a main break, a cross connection, loss of pressure, biofilm or a sampling error. Under the Revised Total Coliform Rule it triggers repeat sampling and, if repeats confirm, a Level 1 assessment performed by the system or a Level 2 assessment performed by a party acceptable to DEP.
• Escherichia coli is specific to fecal contamination. A confirmed E. coli MCL violation is an acute violation requiring Tier 1 notification within 24 hours.
• Heterotrophic plate count measures general bacterial population; a rising HPC with a falling chlorine residual signals biofilm growth and excessive water age.

Sampling Rules and the Assessment Triggers

A coliform result is only defensible if the sample was collected and handled correctly, and those handling rules are testable:

• The standard sample volume is 100 mL regardless of the analytical method used.
• Systems determine presence or absence only; a density or count is not required.
• Time from collection to the start of test-medium incubation may not exceed 30 hours.
• When the water carries a chlorine residual, sodium thiosulfate is added to the bottle before sterilization so residual chlorine cannot keep killing organisms in transit and produce a false negative.

Under Pennsylvania's Chapter 109 adoption of the Revised Total Coliform Rule, the results drive a find-and-fix assessment rather than an automatic violation:

A Level 1 assessment is performed by the system itself; a Level 2 assessment must be conducted by a party approved by DEP. Either way, any sanitary defect found must be corrected, and the correction is the treatment-technique compliance action.

Distribution System Biology

Two problems dominate:

• Biofilm. Bacteria colonize pipe walls inside a protective polysaccharide matrix that shields them from disinfectant. Biofilm consumes residual, causes coliform positives without any external contamination, and is controlled by maintaining residual, flushing dead ends and reducing water age.
• Nitrification. In chloraminated systems, excess free ammonia feeds ammonia-oxidizing bacteria that convert ammonia to nitrite. The signature is a falling total chlorine residual with rising nitrite and rising HPC, usually first in a low-turnover storage tank. Controls are tighter chlorine-to-ammonia ratio management, tank turnover, flushing, and a temporary free chlorine burn.
• Legionella grows in warm, stagnant water and inside biofilm, particularly in premise plumbing and in poorly turned-over storage. Residual maintenance and storage turnover are the operator's tools.

Wastewater Microbiology

An activated sludge basin is a managed ecosystem, and the microscope is a process control instrument:

Two functional groups matter most. Heterotrophs consume carbonaceous BOD quickly and tolerate a wide range of conditions. Autotrophic nitrifiers (Nitrosomonas converting ammonia to nitrite, Nitrobacter converting nitrite to nitrate) grow slowly, need a long solids retention time, consume about 7.1 mg/L of alkalinity as calcium carbonate per mg/L of ammonia nitrogen oxidized, and stall in cold water or at low pH. Losing nitrification in January is a microbiology problem before it is a permit problem.