DC Water Operator common mistakes
DC Water Operator · Understand the traps and how to avoid them.
Calculating chlorine dose or residual using the wrong demand/contact-time relationship, e.g., treating a 1.0 mg/L residual target as the dose without adding chlorine demand, or applying CT credit for inadequate contact time.
Candidates memorize the target residual number and skip the step of subtracting demand, or they confuse free chlorine with total chlorine when a combined residual is present, so the arithmetic looks right but the chemistry is wrong.
What to do instead
Write the dose equation explicitly every time: Dose = Demand + Residual (in mg/L), then convert to lb/day with lb/day = mg/L × 8.34 × MGD. Before claiming disinfection credit, verify the required CT (residual × contact time) against the actual basin volume and flow, and state whether the residual is free or total.
Reading the wrong point on a jar test or coagulant dosing curve, or recommending a coagulant change without adjusting pH and alkalinity first.
Jar test data is presented as a table or curve and candidates grab the lowest turbidity value without checking whether that dose is within the plant's feed capability or whether pH dropped into a range that hurts floc formation.
What to do instead
Evaluate jar test results in order: identify the dose range that produces settled turbidity below the goal, confirm finished pH and alkalinity remain in target range, then select the lowest effective dose. If alum or ferric is used, check that alkalinity consumed (roughly 0.45–0.5 mg/L as CaCO3 per mg/L of alum) does not drive pH out of range.
Misapplying the filter run and backwash criteria, such as backwashing on a fixed time schedule instead of on headloss or effluent turbidity triggers, or using the wrong backwash rate units.
Candidates default to the schedule they saw at one plant and don't convert between gpm/sq ft and m/h, or they miss that a filter should be returned to service only after a filter-to-waste or ripening period.
What to do instead
State trigger-based criteria: backwash when headloss reaches the setpoint, effluent turbidity exceeds the limit, or a maximum run time is reached, whichever comes first. Convert backwash rates with 1 gpm/sq ft ≈ 2.44 m/h and confirm the rate is high enough for bed expansion (typically 20–30 percent) without washing media to waste.
Confusing hydraulic retention time with actual detention time in a distribution storage tank or clearwell, and ignoring short-circuiting when computing contact time.
The volume divided by flow gives a theoretical number, and candidates report it as the real contact time even when inlet/outlet arrangement, baffling, or fluctuating level makes actual detention much shorter.
What to do instead
Use T10 (the time for 10 percent of a tracer to pass) or the baffling factor from the basin configuration rather than V/Q when disinfection credit matters. Note the tank level used, since falling level reduces volume and contact time, and state the baffling condition (unbaffled, poor, average, superior) that justifies the factor.
Sizing or evaluating a distribution main without checking velocity and headloss at design flow, or mixing up the Hazen-Williams C-factor direction.
Candidates plug numbers into the formula but don't sanity-check the result, and they forget that a lower C means rougher pipe and higher headloss, not lower.
What to do instead
After any headloss calculation, check velocity against typical design range (about 2–5 ft/s for transmission mains, up to 10 ft/s briefly) and confirm the C-factor chosen matches pipe material and age. Remember: lower C = more friction loss. Show the headloss per 1,000 ft so the reviewer can verify the magnitude.
Failing to distinguish a cross-connection hazard from a backflow incident and choosing the wrong protection device (air gap vs. RPZ vs. double check vs. PVB).
The terms overlap in everyday speech, and candidates pick the device with the highest protection level without considering the actual hazard, whether it is a health hazard or a non-health hazard, and whether backpressure or backsiphonage is possible.
What to do instead
Classify the hazard first: health hazard vs. non-health, and backpressure vs. backsiphonage. Then select: air gap for the highest health hazard, RPZ for health hazards where an air gap is impractical, double check for non-health hazards only, PVB for backsiphonage-only health hazards. State the classification reasoning, not just the device name.
Reporting BOD or TSS results without confirming the dilution, seed correction, or the difference between filtered and unfiltered samples.
Candidates treat BOD as a single number and forget that a sample outside the 2–300 mg/L working range needs dilution, that seeded dilution water requires a correction, and that TSS is measured on an unfiltered sample while dissolved solids are not.
What to do instead
For BOD, verify depletion in the 2 mg/L minimum and 1–2 mg/L residual dissolved oxygen range, apply the seeded correction (D1 − D2 minus seed correction, times dilution factor), and flag any result outside the valid range. For TSS, state that the sample is unfiltered, dried at 103–105 °C, and reported in mg/L from the weight difference.
Solving activated sludge problems by changing SRT or wasting rate without checking F/M ratio, MLSS, and the sludge volume index together.
Candidates reach for one lever, usually wasting, and don't see that bulking sludge, low MLSS, and high F/M can point in opposite directions; they also confuse MCRT with hydraulic retention time.
What to do instead
Work the set as a system: compute F/M (lb BOD applied per lb MLSS per day), SRT (solids inventory divided by solids leaving per day), and SVI (settled volume in 30 minutes × 1000 / MLSS). If SVI is high with low F/M, suspect filamentous bulking or nutrient deficiency; if low MLSS with high F/M, reduce wasting. State which variable you are changing and predict the effect on the other two.
Misreading the disinfection requirement for wastewater effluent, such as applying drinking-water CT concepts or ignoring the effect of ammonia on chlorine effectiveness.
Candidates carry over water-treatment habits, and they don't account for the fact that wastewater chlorine demand is high and ammonia forms chloramines that are much weaker disinfectants.
What to do instead
For wastewater, base dosing on the required fecal coliform or E. coli limit and the measured chlorine demand after nitrification status is known. Check ammonia: if present, chlorine forms chloramines and free chlorine residual will be low, so report total residual and note the limitation. Confirm the required contact time at peak flow, not average flow.
Designing or evaluating a gravity sewer using the wrong slope direction or ignoring the minimum self-cleansing velocity and the difference between peak and average flow.
Candidates invert the rise/run relationship or use average daily flow when the pipe must carry peak hourly flow without surcharging, and they forget that flatter slopes require checking for deposition.
What to do instead
Compute slope as fall divided by horizontal run, in the same units, and confirm the pipe carries design peak flow at partial depth (typically about 0.5–0.75 full) without exceeding the crown. Check minimum velocity at average flow (about 2 ft/s) for self-cleansing and maximum velocity (about 10 ft/s) for scour. State both the peak and average flow checks.