Wastewater Treatment Plant Operator common mistakes
Wastewater Treatment Plant Operator · Understand the traps and how to avoid them.
Performs loading and efficiency calculations (lb/day, percent removal, solids inventory) by plugging raw lab numbers straight into a formula without converting units or checking the basis of the number first.
Candidates memorize the formulas but rarely practice them with realistic, mismatched inputs (mg/L vs. percent, MGD vs. gpm, dry vs. wet tons), and under time pressure they grab the first number that looks like the right one. A single unit slip produces an answer that is off by an order of magnitude yet still appears on the answer list.
What to do instead
Before solving, write the target unit next to every given value and circle the conversion constants (8.34 lb/gal for water, 1 MGD = 694 gpm, 1 mg/L = 8.34 lb/MG). Do a sanity check at the end: a secondary effluent TSS of 20 mg/L at 1 MGD should be roughly 167 lb/day, and percent removal must fall between 0 and 100. If the result violates either check, find the unit error before selecting an answer.
Responds to a rising secondary clarifier sludge blanket by increasing the return activated sludge (RAS) rate and leaving the waste activated sludge (WAS) rate untouched.
A deep blanket looks like a return-rate problem, and RAS is the control the operator touches most often. The reasoning trap is that RAS only moves solids around inside the system; it changes blanket depth and detention time but does not remove solids from the process, so the underlying cause (usually insufficient wasting and an excessive MCRT/F:M imbalance) persists and the blanket returns.
What to do instead
Check the solids balance before turning a valve: calculate MCRT (aeration + clarifier solids divided by solids leaving in WAS and effluent) and the F:M ratio, and compare to the design window for the specific process (e.g., conventional, extended aeration, oxidation ditch). If MCRT is above target, raise WAS; use RAS to fine-tune blanket depth and clarifier hydraulics, not to fix a mass balance problem.
Treats a permit-required confined space as a non-permit space after a brief odor check or a few minutes of ventilation, and enters without pre-entry testing at multiple depths or continuous monitoring.
Wet wells and digesters usually smell bad without being immediately dangerous, so a 'smells normal today' judgment feels adequate, and ventilation feels like it makes the space safe. But hydrogen sulfide is heavier than air and collects at the bottom; ventilation mixes rather than eliminates, and biological activity in a wet well can regenerate an oxygen-deficient or toxic atmosphere within minutes of the blower stopping.
What to do instead
Treat every wet well, digester, chlorine room, and sewer manhole as permit-required until documented otherwise. Test oxygen, flammables, and H2S from outside the opening at the top, middle, and bottom of the space before entry, ventilate continuously, retest immediately before entry, and keep a calibrated four-gas monitor on the entrant with an attendant outside and retrieval equipment rigged.
Records a DPD chlorine result as 'free chlorine' when the test actually measured total chlorine, or accepts a single residual reading as proof of disinfection without considering demand, contact time, and downstream residuals.
Total chlorine equals free plus combined chlorine, and combined chloramines give a strong DPD response while being far weaker disinfectants. A reading that looks comfortably above the target therefore can hide a failure to actually inactivate pathogens, especially where ammonia is present in the effluent or where the sample sat in the line before being drawn.
What to do instead
Know which DPD procedure the permit and plant use: the standard DPD measures total chlorine; the DPD method with a reagent added first suppresses combined chlorine so the result is free chlorine. Record which one was run, calculate contact time from actual flow and basin volume, and check residual at the point of compliance, not just at the injection point.
Reports a monthly average in a discharge monitoring report for a parameter whose permit limit is a daily maximum, or omits the required percent-removal calculation and just reports concentrations.
Most internal tracking is done as monthly averages, so that is the number already calculated when the report is due. Permits, however, mix limit types (daily maximum, 7-day average, 30-day average, monthly average, and minimum percent removal), and candidates apply the familiar statistic to the wrong limit type or overlook the removal-efficiency requirement entirely.
What to do instead
Before filling in any DMR, read the limit table column by column and label each parameter with its limit type, then compute only that statistic. For secondary limits, calculate percent removal as (influent - effluent)/influent x 100 and verify it meets the minimum (commonly 85 percent for BOD and TSS) as a separate check. Sign and date only after a second pass comparing each reported value to the reported limit.
Diagnoses a rising secondary treatment load as a biological upset without first checking primary effluent TSS, primary percent removal, and sludge blanket depth.
Symptoms show up in the aeration basin and the secondary clarifier, so attention goes there. But a short-circuited or overloaded primary clarifier passes solids and BOD forward, and the biological system is blamed for a problem that started upstream. Seasonal temperature changes, sludge blanket buildup, and sludge collector or scum problems in the primaries are easy to skip when the visible problem is downstream.
What to do instead
When secondary loading rises, pull the primary data first: influent and primary effluent TSS/BOD, calculated percent removal, sludge blanket depth, and sludge pumping records. If primary percent removal has dropped from its normal range, correct blanket control, pumping schedule, and hydraulic loading before touching return rates or aeration.
Estimates lift station or wet well flow from pump run time using the nameplate or design pump curve, without field-verifying actual pump capacity or subtracting the volume that bypasses or drains back during the off cycle.
Run-time hour meters are convenient and always available, so pump hours get multiplied by a rated capacity that may be decades old, worn, or based on a different static head. Actual delivered flow falls as impellers wear and as head changes, and check-valve leakage or backflow inflates apparent throughput.
What to do instead
Establish actual capacity by drawing the wet well down between known levels with the inflow isolated (or by a calibrated flow meter), and record that measured capacity for the station. Recheck it after any impeller trim, pump change, or persistent discrepancy, and apply infiltration/inflow analysis to wet-weather flow spikes rather than assuming the extra water is inflow alone.
Raises digester feed rate to increase gas production without checking volatile solids loading, mixing, or the VFA-to-alkalinity ratio.
Gas production is the most visible digester indicator, and more feed intuitively means more gas. But an overloaded or poorly mixed digester accumulates volatile acids faster than methanogens can consume them, alkalinity is consumed as a buffer, pH falls, and the digester sours. Because pH lags the volatile acid buildup, the failure looks sudden when it was developing for days.
What to do instead
Trend volatile solids loading, volatile acid concentration, alkalinity, and the VFA-to-alkalinity ratio together, and treat a rising ratio (roughly above 0.3) as an early warning even while pH is still normal. Change feed in steps, verify mixing and heating before increasing load, and hold feed steady until the ratio and gas composition stabilize.
Acts on a single dissolved oxygen, pH, or level reading from a field instrument without verifying the instrument against a grab sample or checking its calibration and maintenance history.
Instruments are treated as ground truth because they are digital and always on. A fouled DO membrane, a drifted pH electrode, a coated probe, or a probe left in a calibration buffer produces a plausible but wrong number, and the resulting process change (cutting blower output, adding chemicals) makes the real problem worse.
What to do instead
Adopt a verify-before-act habit: compare the in-line reading to a bench or grab-sample measurement whenever a number drives a process change, and check the calibration date, last cleaning, and any active alarms or fault codes. Log both readings and the verification so drift patterns become visible at the next calibration.
Judges membrane or filter performance by effluent turbidity alone and skips the integrity, pressure-decay, or headloss and turbidity-trigger checks the equipment requires.
Low turbidity looks like proof of a sound barrier, and turbidity is the number on the display. But a compromised membrane or a filter with a breakthrough path can pass organisms while turbidity stays low, because the particles carrying pathogens are a small fraction of the light-scattering load and an intact-looking effluent can hide a mechanical breach.
What to do instead
Follow the equipment-specific verification program: perform direct integrity testing (pressure decay or diffusive air test) on membranes at the required interval, and for filters use headloss, effluent turbidity, and run time together to trigger backwash rather than the clock alone. Record the integrity results with the run data so a trend can be seen before a compliance exceedance occurs.