OR Water/Wastewater Operator common mistakes
OR Water/Wastewater Operator · Understand the traps and how to avoid them.
Working process-control problems by plugging numbers into a memorized formula without carrying units, so answers come out off by a factor of 8.34, 1,000, or 60 (e.g., using gpm where the formula needs cfs, or reporting a dosage in lb/day when the question asks for mg/L).
Formulas are memorized as symbol strings (C x Q x 8.34) rather than as relationships with built-in unit requirements, and under time pressure candidates skip the dimensional check that would have caught the mismatch.
What to do instead
Write every quantity with its units and cancel them line by line before dividing; keep a one-page conversion card (1 mg/L = 8.34 lb/MG, 1 ft3 = 7.48 gal, 1 cfs = 448.8 gpm, 1% = 10,000 mg/L) and do a final sanity check that the answer's magnitude is plausible for the plant size given.
Using hydraulic retention time where the question is actually asking for solids retention time (MCRT) in an activated sludge system, or computing SRT from aerator volume alone while ignoring the clarifier, return sludge, and solids lost in the effluent.
Both are 'time' calculations with similar-looking divisions, and HRT is the number candidates practice most, so the formula gets borrowed even when the question describes wasting and sludge inventory.
What to do instead
Anchor the definitions: HRT = tank volume / flow; SRT = total solids mass in the whole system (aeration + clarifier + return) divided by solids leaving per day (wasted + effluent). Before calculating, circle the words 'wasting', 'sludge age', or 'MCRT' in the stem as the trigger for the mass-balance formula.
Equating chlorine dose with chlorine residual, or applying a CT value to a dose rather than to the measured residual at the actual contact time and pH.
In day-to-day plant talk 'we feed 2 mg/L' and 'we have 2 mg/L' get used interchangeably, and the demand component disappears from the mental model.
What to do instead
Always write the balance as dose = demand + residual, subtract demand first, and for CT problems confirm which disinfectant species and pH the table applies to; check that contact time is baffling-adjusted (T10), not theoretical detention time.
Ignoring the alkalinity and pH consequences of coagulant addition, or reading a jar test only for the clearest supernatant while missing that the optimum dose drove pH below the range where the coagulant works.
Jar tests are often evaluated purely on turbidity removal, and alum/ferric reactions are taught as 'coagulation' without the stoichiometry, so the alkalinity consumed per mg/L of coagulant is not in the candidate's calculation set.
What to do instead
Pair every coagulant dose question with an alkalinity check (alum consumes roughly 0.45–0.5 mg/L alkalinity as CaCO3 per mg/L of alum), and when a jar test shows poor floc at increasing dose, consider pH depression and supplemental alkalinity before assuming underdosing.
For a routine total-coliform-positive sample, treating the response as 'increase chlorine and move on' instead of identifying the required repeat samples within the required timeframe and the distinction between a total coliform positive and an E. coli positive.
The rule's tiered structure (routine, repeat, triggered, assessment) is memorized as a list rather than as a sequence of deadlines and consequences, and the MCL is confused with the individual sample result.
What to do instead
Build a one-page decision tree: routine positive triggers the specified number of repeats at the original and adjacent sites within the regulatory window; E. coli positives escalate further. Test yourself by writing the tree from memory, then map each branch to its deadline and MCL type.
Entering a wet well, vault, or digester-adjacent structure after only a combustible-gas check, or testing in the wrong order and accepting a marginal oxygen reading as 'close enough to enter'.
Oxygen and toxicity meters are sometimes treated as one reading, and candidate familiarity with routine entries breeds the assumption that a permit is paperwork rather than a gating condition.
What to do instead
Memorize the test order - oxygen first, then flammables, then toxics - because high oxygen skews the combustible reading; then verify ventilation, a standby attendant, retrieval equipment, and the permit before entry, and treat out-of-range readings as a stop, not a caution.
Sizing or evaluating gravity sewers using percent slope interchangeably with feet-per-foot, or assuming design velocity is met without checking partially full flow conditions.
Percent slope feels intuitive and the conversion is treated as a formality, and the Manning-based velocity relationship at partial depth is often learned as a formula without the concept that velocity at half-full is not half the full-flow velocity.
What to do instead
Convert slope to ft/ft before any Manning calculation (2% = 0.02), then check the resulting velocity against the self-cleansing range rather than the slope itself; practice a few partial-full problems so the depth-of-flow versus velocity relationship becomes a picture, not a formula.
Treating hardness and alkalinity as the same parameter, or taking a reported value 'as CaCO3' and using it directly in a lime-softening or corrosion-control calculation without confirming which parameter and which expression the number represents.
Both are reported in mg/L as CaCO3, both are described as 'minerals in the water', and the two are frequently correlated in real source waters, so the distinction gets blurred.
What to do instead
Define them separately and out loud: hardness is the sum of divalent cations (mainly calcium and magnesium); alkalinity is the acid-neutralizing capacity (bicarbonate, carbonate, hydroxide). When a stem gives a number as CaCO3, state what it is and what it is not before selecting a treatment target.
Mixing units across a solids-handling calculation - pounds of solids per day into a percent-solids formula that calls for mass, or sludge volume in gallons used where a dry-solids mass is required - and reporting a volatile solids reduction without the correct feed and product basis.
Sludge problems chain several conversions (mg/L to lb/MG, volume to mass, wet basis to dry basis), and candidates often skip the intermediate step where the unit actually changes.
What to do instead
Lay the problem out as a chain with an explicit unit on every arrow: concentration, then volume or flow, then mass of dry solids, then percent. Convert percent solids to a decimal before dividing, and label volatile solids as a fraction of total solids at each stage so feed and product terms cannot be mismatched.
Collecting samples with the wrong container, missing the preservative or dechlorinating agent, filling a bacteriological bottle to the brim or without the required headspace, or letting a sample exceed its holding time because the cooler was packed but not chilled.
Sampling is treated as a field chore separate from the exam's 'real' content, so the parameter-specific requirements (chlorinated samples needing a reducing agent, dissolved metals needing filtration, residual chlorine having essentially no holding time) are learned vaguely.
What to do instead
Make a parameter-to-requirement table (container, preservation, holding time, headspace) and drill it, then apply the rule that any sample for a volatile or dissolved parameter must not be aerated during collection; when a question describes a sampling error, identify the specific requirement violated rather than answering 'improper technique'.