PA Water/Wastewater Operator common mistakes
PA Water/Wastewater Operator · Understand the traps and how to avoid them.
On chlorine problems, reports total chlorine or combined chlorine when the question asks for free chlorine residual, or subtracts demand and calls the result free residual without distinguishing the species.
The three terms sound interchangeable and exam stems often mention dose and demand in one sentence, so candidates grab the first residual number they compute.
What to do instead
Write the definitions before calculating: free = HOCl + OCl-, combined = chloramines, total = free + combined. Set up dose minus demand equals residual, then explicitly label which residual the question requests and check the answer against that label.
Calculates CT using theoretical detention time (V/Q) and peak flow without applying a baffling factor, or uses average flow when the CT table specifies peak hourly flow.
Candidates memorize t = V/Q and stop there, forgetting that short-circuiting reduces effective contact time and that CT tables are flow-specific.
What to do instead
Use effective contact time = theoretical detention time times baffling factor, then CT = disinfectant residual times effective time. Read the table heading for the required flow (usually peak hourly) and state the baffling factor you used.
Miscalculates chemical feed rates by using 8.34 lb/gal instead of 8.34 lb per gallon of water per mg/L, or forgets to convert gpm to MGD before using the pounds formula.
The constant 8.34 is memorized as a number without its units, and gpm-to-MGD conversion is a separate step that gets dropped under time pressure.
What to do instead
Carry units through every line: lb/day = MGD x mg/L x 8.34. Convert gpm to MGD by multiplying by 1,440 and dividing by 1,000,000 before substituting. If the final units are not lb/day, the setup is wrong.
Converts pressure to head in the wrong direction, dividing by 2.31 when converting psi to feet, or multiplying when converting feet to psi.
The constant 2.31 ft per psi is memorized without a reference point, so direction is a coin flip during the exam.
What to do instead
Anchor with 1 ft of water = 0.433 psi, so feet = psi divided by 0.433, which equals psi x 2.31. Sanity-check with a known value: 100 psi is about 231 ft, not about 43 ft.
Misidentifies a backflow scenario as backsiphonage when it is backpressure, or claims a double check valve is acceptable where an air gap is required.
Both are backflow, so candidates treat the terms as synonyms and default to the device they see most often in the field.
What to do instead
Classify by cause: backpressure occurs when downstream pressure exceeds supply pressure (pump, boiler, elevation); backsiphonage occurs when supply pressure drops below atmospheric and creates a vacuum. Remember the air gap is the only absolute protection; reduced-pressure assemblies are testable but not absolute.
Substitutes COD results for BOD5 in loading calculations, or compares a TSS concentration directly against a BOD limit.
All three are reported in mg/L and appear together on lab sheets, so the numbers look interchangeable.
What to do instead
Fix the definitions: BOD5 measures biodegradable oxygen demand over five days; COD measures chemically oxidizable material including non-biodegradable organics; TSS measures suspended solids by weight. Never substitute one for another in a load calculation, and check the permit limit's parameter name first.
Interprets a high SVI (above about 150 mL/g) as good settling, or computes MCRT using only the RAS solids and ignoring solids in the aeration tank.
Higher MLSS and higher SVI both feel like more biomass, and the MCRT formula is recalled as a vague ratio.
What to do instead
Memorize the direction: rising SVI means bulking and poor settling; a low SVI means dense, fast-settling sludge. For MCRT, use total solids in the system (aeration tank plus clarifier) divided by solids leaving per day (waste plus effluent).
Calculates clarifier detention time using plant influent flow only, ignoring the return activated sludge flow that also enters the clarifier.
Influent flow is the headline number on the permit and the flow meter, so it is used by habit even when the process diagram shows RAS re-entering.
What to do instead
Trace the flow diagram before choosing Q. Clarifier feed equals influent plus RAS for a conventional activated sludge system. Write the flow path in one line, then substitute the combined flow.
Computes sewer slope by subtracting invert elevations in the wrong order, or reports percent slope when the question asks for feet per foot.
Invert elevations are often listed downstream first, and candidates treat percent and ft/ft as the same number.
What to do instead
Use slope = (upstream invert minus downstream invert) divided by pipe length, which must come out positive. Keep it as ft/ft unless percent is requested; percent equals ft/ft times 100. Check that a slope of 0.005 is not reported as 0.5 ft/ft.
On confined space items, tests for hydrogen sulfide before oxygen, or describes entering the space to rescue a downed worker without supplied air and retrieval equipment.
H2S is the feared sewer gas so it feels like the first test, and rescue instinct overrides the permit-required procedure under exam wording.
What to do instead
Memorize the test order: oxygen first, then flammables, then toxics (H2S, CO). For rescue, state the non-entry rule: call for help, use retrieval lines, and enter only with SCBA and full confined-space rescue equipment.