NC Phase W common mistakes
NC Phase W · Understand the traps and how to avoid them.
When a math item gives mixed units (for example, a flow in gallons per minute and a tank volume in cubic feet, or a dosage in mg/L with a flow in MGD), candidates plug the numbers straight into the formula without converting to consistent units first, and then pick the answer that matches their un-converted arithmetic.
The formula is recalled correctly, but the unit-conversion constants (8.34 lb/gal, 0.785 for a circular area, 7.48 gal/ft³, 1.55 for the cu ft per second to MGD conversion) are applied from memory and often misapplied or skipped under time pressure. The distractor answers are built from exactly these common conversion slips, so an un-converted result is usually present as an option.
What to do instead
Before calculating, write the given quantities with their units in a single column, circle the target unit, and convert every value to that unit system (typically mg/L, MGD, and feet) before touching the calculator. Memorize the small set of conversion factors as a flashcard drill until they are automatic, and after computing, do a units check on the final number (for example, lb/day = mg/L × MGD × 8.34) so a mismatched unit flags the error before selecting an answer.
On disinfection and CT items, candidates treat the required CT value as a single fixed number and ignore that it changes with pH, temperature, and disinfectant type, or they use the peak or design flow when the regulation specifies a different flow basis.
CT tables are memorized as a single value for chlorine, and the candidate does not connect the table to the actual water conditions described in the item. Temperature in particular has a large effect on CT, and items frequently embed a cold-water condition that changes the required value substantially.
What to do instead
For every CT item, first identify the disinfectant, then the pH and temperature given, then look up or compute the required CT for those exact conditions, and finally confirm which flow basis the rule uses (peak hourly, maximum day, or actual). Practice reading a CT table by working items where only temperature changes and noting how the required CT shifts, so the dependence becomes a habit rather than a memorized single number.
Candidates answer coagulation and flocculation items by naming a chemical or a piece of equipment (for example, 'add alum' or 'increase mixing') instead of identifying the specific process control change and the monitoring test that would confirm it, such as jar testing, zeta potential, or streaming current.
Study material often presents coagulation as a list of coagulants and dosages, so the candidate learns the reagents but not the diagnostic logic. Exam items ask what the operator should do next, and the correct answer is usually a test or a targeted adjustment rather than a blanket chemical addition.
What to do instead
Reframe coagulation as a control loop: the observable is settled or filtered turbidity and filter run time; the manipulated variable is coagulant dose and pH; the diagnostic tool is the jar test. For each item, state the symptom, name the jar test as the confirming step, and then state the direction of dose or pH change. Drill items that pair a symptom (rising filtered turbidity, short filter runs, pin floc) with the correct jar-test-based response.
On distribution items involving pressure, candidates confuse psi with feet of head, or they apply the 2.31 ft per psi conversion in the wrong direction, and they also fail to account for elevation change when computing pressure at a point in the system.
The conversion factor 2.31 is well known but is often inverted under time pressure, and elevation is treated as a separate concept rather than being added or subtracted from the pressure calculation. Items that give a pressure at one elevation and ask for pressure at another are missed because the elevation term is dropped.
What to do instead
Always draw a simple sketch showing the two points, their elevations, and the direction of flow, then write pressure head plus elevation head equals a constant along the static system. Convert psi to feet of head by multiplying by 2.31 and convert back by dividing, and say the operation out loud in practice until the direction is automatic. Check the answer for physical sense: pressure should fall as elevation rises in a static system.
Candidates miss coliform and chlorine residual compliance items because they evaluate a single sample against a single limit instead of applying the correct rule structure, such as the presence of any total coliform in a repeat sample, the monthly percentage of positive samples, or the running annual average for chlorine residual.
The rules are multi-part: they depend on sample volume, the number of samples per month, whether a repeat sample was triggered, and whether the limit is per-sample or a running average. Candidates memorize one number (for example, a 4-log removal or a 0.2 mg/L residual) and apply it to every situation without checking which rule governs.
What to do instead
Build a one-page decision tree for each regulated parameter: total coliform (any positive triggers repeat sampling; a positive repeat is a violation), chlorine residual (running annual average of monthly averages against the minimum), and lead and copper (90th percentile against the action level). For each practice item, first name which rule applies and whether the limit is per-sample or averaged, then compute. Rehearse the difference between an acute violation and a monthly or annual exceedance.
When asked to size a storage tank, pump, or main, candidates use the average demand or the design flow without applying the correct peaking factor, fire flow allowance, or minimum residual pressure requirement, and they ignore that the governing condition may be fire flow rather than maximum day demand.
Sizing items combine several requirements, and the candidate latches onto the first flow number given rather than identifying which demand scenario governs. Fire flow, maximum day, and peak hour are treated as interchangeable when they serve different design purposes.
What to do instead
For every sizing item, list the candidate governing conditions (average day, maximum day, peak hour, fire flow plus maximum day) and compute the required capacity for each, then select the largest. State explicitly which condition governs and why. Practice items that change only the fire flow requirement so the shift in the governing condition becomes visible, and always check the residual pressure criterion at the end.
On collection system items, candidates treat infiltration and inflow as a single problem and apply the wrong remedy, or they misread slope and velocity items by confusing the pipe slope with the hydraulic gradient or by accepting a velocity outside the self-cleansing range.
Infiltration and inflow are often taught together, so the distinction blurs: infiltration is groundwater entering through defects, while inflow is direct storm or surface water entering through openings such as manhole covers and illegal connections. Slope and velocity items are missed because the candidate computes a slope in the wrong units or does not check the resulting velocity against the design range.
What to do instead
Learn the two definitions with a physical picture: infiltration equals groundwater through cracks and joints, inflow equals surface water through manholes, roof drains, and sumps. For slope items, compute slope as rise over run in consistent units (ft/ft), then compute velocity with the appropriate formula and check it against the self-cleansing minimum (commonly about 2 ft/s) and the scouring maximum. Practice identifying the source of the excess flow from the pattern (wet-weather spike versus steady dry-weather gain) before choosing a remedy.
Candidates answer sewer maintenance and corrosion items by naming the general category of problem (roots, grease, corrosion) without selecting the specific inspection or rehabilitation method that matches the condition described, such as CCTV inspection, chemical root control, or cured-in-place lining.
The methods are learned as a list without the decision criteria that link a condition to a method. Items describe a specific defect (offset joint, cracked vitrified clay pipe, hydrogen sulfide attack at the crown) and the candidate picks a plausible-sounding method that does not fit the defect or the pipe material.
What to do instead
Build a defect-to-method table: for each defect type, record the confirming inspection (CCTV, smoke testing, dye testing) and the appropriate rehabilitation (spot repair, cured-in-place lining, slip lining, chemical grouting). Add the corrosion case explicitly: hydrogen sulfide attacks the crown of the pipe above the flow line, so note where the damage appears and which lining resists it. Practice items that describe a defect and require choosing both the inspection and the repair.
On activated sludge items, candidates answer with a single control action (for example, 'increase the return sludge rate' or 'lower the sludge age') without first diagnosing the cause from the sludge settleability, the mixed liquor suspended solids, and the oxygen profile, and without checking whether the proposed change worsens another parameter.
The control knobs (return activated sludge rate, waste activated sludge rate, dissolved oxygen setpoint, food-to-microorganism ratio) interact, so a change made to fix one symptom shifts another. Candidates who memorize isolated corrective actions rather than the underlying relationships pick an action that treats the symptom but not the cause, or that creates a secondary problem such as a low sludge age with poor nitrification.
What to do instead
For each activated sludge item, first state the diagnosis (for example, filamentous bulking, pin floc, dispersed growth, or low dissolved oxygen), then name the evidence that supports it, then choose the control action and predict its side effect. Learn the direction of each relationship: increasing wasting lowers sludge age and reduces nitrification; decreasing return rate raises the sludge blanket. Practice by writing the predicted effect of each control change on the others before selecting an answer.
On wastewater treatment compliance and reporting items, candidates apply the permit limit to a single sample or to the wrong averaging period, or they confuse the reporting requirement (monthly discharge monitoring report) with the compliance determination, and they overlook the difference between a permit exceedance and a treatment process upset.
Permit limits are expressed as monthly averages, weekly averages, or daily maxima, and the candidate applies whichever number they remember to whatever sample is described. The reporting calendar and the compliance calculation are separate skills, and items that describe a single high sample tempt the candidate to call it a violation without checking the averaging period.
What to do instead
For each permit item, identify the parameter, the limit type (daily maximum, monthly average, weekly average), and the averaging period, then compute against that period. Keep a clear distinction between an exceedance (a calculated violation) and an upset or bypass (an event that may have reporting and defense implications). Practice items that give a month of sample results and ask whether a violation occurred, so the averaging logic becomes routine.