IL Water/WW Operator common mistakes
IL Water/WW Operator · Understand the traps and how to avoid them.
Computes pounds per day as mg/L × gpm × 8.34 without converting gpm to MGD, producing an answer roughly 694 times too large.
The candidate memorized the lb/day formula but not the unit conversion; the exam stem supplies flow in gpm to test unit tracking.
What to do instead
Write the cancellation: mg/L × MGD × 8.34 lb/gal. Convert gpm to MGD by multiplying by 0.00144 (or dividing by 694.4) before multiplying, and check that the answer is reasonable for the plant size.
Uses 'total chlorine' and 'free chlorine' interchangeably when answering disinfection residual or CT questions.
The DPD test reports both, and combined chlorine equals total minus free; regulations specify which residual counts for compliance.
What to do instead
Define free chlorine as HOCl plus OCl−, combined chlorine as chloramines, and total chlorine as free plus combined. Before answering, identify whether the rule or question requires free or total residual.
Inverts detention time by dividing flow by tank volume instead of volume by flow.
The candidate mixes up the relationship, and the units cancel only one way.
What to do instead
Set up D = V/Q with V in gallons and Q in gpm, then convert minutes to hours. Sanity-check that a larger tank gives a longer detention time for the same flow.
Treats the lead action level of 0.015 mg/L as an MCL and compares each individual sample directly to it.
The Lead and Copper Rule uses a 90th-percentile calculation, not a single-sample maximum contaminant level.
What to do instead
Rank the required first-draw samples, calculate the 90th percentile, and compare that value to 0.015 mg/L for lead and 1.3 mg/L for copper. Do not call the action level an MCL.
Forgets the 24-hour repeat-sample deadline under the Total Coliform Rule or uses the wrong number of repeat samples.
The repeat count depends on the number of routine samples collected, and the deadline is short enough that candidates conflate it with other monitoring schedules.
What to do instead
Memorize that repeat samples must be collected within 24 hours of notification and that the number of repeats equals the number of routine samples, with a minimum of three for systems collecting fewer than five routine samples per month.
Assumes any well is 'groundwater' and therefore not subject to surface-water treatment requirements, even when it is under the direct influence of surface water.
The candidate overlooks the groundwater-under-direct-influence classification and treats all wells the same.
What to do instead
Check for a GWUDI determination. If a well is under direct influence of surface water, apply the required filtration and disinfection requirements unless the state approves an avoidance determination.
Holds a chlorine residual sample for later analysis or records a residual from a sample that sat in the truck before testing.
Chlorine is volatile and reacts with organic and inorganic material, so residual changes quickly after collection.
What to do instead
Analyze chlorine residual immediately on site by DPD or amperometric titration. Do not hold or transport the sample for later residual testing.
Answers filter backwash questions using a fixed bed-expansion percentage or a fixed backwash rate without adjusting for water temperature.
The backwash rate needed to achieve a given expansion depends on water viscosity, which changes with temperature.
What to do instead
Use the filter supplier's or state-approved backwash criteria with temperature correction. Colder water requires a lower backwash rate to achieve the same bed expansion.
States that the 20-psi minimum pressure applies only at the plant discharge or only at the customer tap, rather than at all points in the distribution system.
The candidate confuses plant operating pressure with the distribution-system minimum pressure requirement.
What to do instead
Memorize that the minimum pressure requirement applies at all points in the distribution system, not just at the treatment plant or a single tap.
Tests a confined space for oxygen only, or tests for hydrogen sulfide before oxygen, and enters without continuous atmospheric monitoring.
Confined spaces present multiple atmospheric hazards, and the order of testing matters because combustible-gas instruments depend on adequate oxygen.
What to do instead
Test oxygen first, then flammables, then toxics; ventilate and retest. Use continuous monitoring and follow a permit-required confined-space entry procedure.