NE Water/Wastewater Operator common mistakes
NE Water/Wastewater Operator · Understand the traps and how to avoid them.
Subtracts chlorine demand from the residual instead of from the dose, so a problem giving a 3.0 mg/L dose and a 1.2 mg/L demand gets answered as 4.2 mg/L residual (or 1.2 mg/L).
'Demand' and 'residual' both describe chlorine that is used versus chlorine that is left, and the mass-balance direction is easy to flip when working quickly under time pressure.
What to do instead
Before touching the numbers, write the balance on scratch paper: Dose = Demand + Residual. Then rearrange only after the words are on paper, and label which quantity the question is actually asking for.
Computes a CT value by multiplying a contact time that was never converted into the same time unit as the required CT table (e.g., uses hours against a mg/L·min table), and skips the baffling factor when the basin geometry is given.
Basin volume typically arrives in gallons and flow in MGD, so detention time is a two-step conversion; candidates collapse it into one step and lose the unit or forget that only a fraction of the basin volume counts as effective contact.
What to do instead
Do detention time as its own labeled line: gallons ÷ (MGD × 1,000,000 ÷ 1440) = minutes. Apply the baffling factor on the next line, then multiply by the residual concentration. Check the final unit is mg/L·min.
Converts pressure head in the wrong direction - multiplying psi by 0.433 to get feet, or dividing feet by 2.31 to get psi - when asked for pressure at the bottom of a standpipe or the elevation of a pressure zone.
The two constants (0.433 psi/ft and 2.31 ft/psi) are reciprocal, and both appear on formula sheets, so the candidate grabs whichever one is remembered rather than testing the direction.
What to do instead
Anchor the direction physically: water is lighter than a column of mercury, so a given column of water produces a smaller psi number than its foot value. Feet → psi multiplies by 0.433; psi → feet multiplies by 2.31. A 100-ft column must come out near 43 psi, not 231 psi.
Treats a single routine total coliform-positive sample as an immediate MCL violation and selects the public-notice/penalty answer, instead of the repeat-sampling answer.
Candidates memorize 'coliform positive = violation' as a single association and do not separate routine positives, repeat positives, and the presence of E. coli, which carry different consequences.
What to do instead
Sort the scenario first: routine positive triggers repeat samples within 24 hours; the MCL is driven by repeat-sample results (or failure to collect repeats). For systems collecting fewer than 40 samples per month, a routine plus a repeat positive is an MCL violation. Match the answer to the stage named in the question.
Computes removal efficiency as (influent − effluent) ÷ effluent, or adds a BOD percent removal to a TSS percent removal and reports the total as plant performance.
The subtraction step is the memorable part and the denominator gets filled in by habit; multi-parameter questions invite averaging or summing percentages that are not additive.
What to do instead
Write the formula as a fraction before substituting: (In − Out) ÷ In × 100. Sanity-check that each answer is under 100% and that efficiency rises when effluent quality improves. Compute BOD and TSS efficiencies as two separate answers.
Uses the pounds formula lbs/day = mg/L × MGD × 8.34 but plugs in a flow given in gpm or gpd without converting, producing an answer off by a factor of 694 or 1,000,000.
The formula sheet shows one constant with MGD, while the problem statement deliberately supplies gpm (pump rates) or gpd (small systems), and the conversion is easy to skip when the arithmetic looks familiar.
What to do instead
Make 'convert flow to MGD' a mandatory first line of every loading calculation (gpm × 0.00144 = MGD; gpd ÷ 1,000,000 = MGD). Never write 8.34 until the flow line above it ends in MGD.
Calculates solids retention time using only the aeration-tank inventory and the waste sludge, leaving out solids carried out in the secondary effluent (and sometimes substituting MLSS where MLVSS is required for F/M).
Wasting is the visible, controllable solids loss, so effluent suspended solids are mentally filed as a treatment result rather than as part of the solids leaving the system; MLSS and MLVSS are both routinely reported and look interchangeable.
What to do instead
List all solids-loss terms before dividing: SRT = aeration-tank solids ÷ (waste sludge solids + effluent solids per day). Check whether the question says volatile (MLVSS/VSS) or total (MLSS/TSS) and stay consistent on both sides of the ratio.
Interprets a 2% sewer grade as 2 ft of fall per foot of pipe (or reports a 0.004 ft/ft slope as 0.4%), and assumes doubling the slope doubles the velocity.
Percent, decimal, and ft/ft are three notations for the same quantity and the percent sign is dropped casually; velocity feels intuitively proportional to slope even though it varies with the square root of slope in Manning's equation.
What to do instead
Convert grades out loud: 2% = 0.02 ft/ft = 2 ft per 100 ft. For velocity, use Manning's n and remember velocity scales roughly with the square root of slope, so doubling slope raises velocity about 1.4 times, not 2 times.
Labels a roof drain connected to a sanitary manhole as infiltration, or labels groundwater seeping through a cracked joint as inflow, when asked to classify a flow source.
Both are 'extra water that shouldn't be there,' and the everyday meanings of the words overlap even though the regulatory definitions are mutually exclusive.
What to do instead
Classify by pathway, not by water type: infiltration enters through defects in pipe, joints, or manholes (groundwater, soil moisture); inflow enters through direct openings or connections (roof leaders, yard drains, sump pumps, unsealed covers). Ask 'is there a pipe or opening?' - if yes, it is inflow.
Answers a filtration turbidity question with an MCL value, or mixes up the 0.3 NTU / 95% performance criterion with the 1 NTU never-exceed ceiling for conventional or direct filtration.
Turbidity limits are the most-quoted numbers in drinking water, and candidates recall the digits without recalling that for filtered systems they are treatment technique requirements with two separate conditions, not a single maximum contaminant level.
What to do instead
Memorize the pair as a sentence: '95% of monthly measurements at or below 0.3 NTU, and no single measurement above 1 NTU,' and tag it as a treatment technique. Scan answer choices for 'MCL' as a red flag when turbidity is being discussed.