SC Water/Wastewater Operator common mistakes
SC Water/Wastewater Operator · Understand the traps and how to avoid them.
Plugging numbers into a memorized formula without converting units first - for example computing detention time by dividing a volume in gallons by a flow in MGD, or computing pounds per day without the 8.34 factor - and then choosing an answer choice that matches the raw arithmetic.
Candidates memorize formula shapes (V/Q, mg/L × MG × 8.34) but not the units each formula expects, and under time pressure they skip the conversion step because the numbers 'look right'.
What to do instead
Before substituting, write the target unit on the right side of the equals sign and convert every given value to the units the formula demands (gallons, MGD, mg/L, feet, psi). Carry units through the arithmetic so they cancel; if the leftover unit is not the one asked for, the setup is wrong. Then sanity-check magnitude - a 500,000-gallon clearwell at 2 MGD cannot have a 6-hour detention time.
Treating every drinking water limit as an MCL - applying a maximum contaminant level to turbidity or to a disinfection byproduct rule requirement that is actually a treatment technique or a running annual average, and misjudging when public notice is required.
MCL is the most familiar regulatory term, so candidates default to it, and they conflate 'exceeded a limit' with 'must notify the public within 24 hours' without separating violations from monitoring/reporting lapses.
What to do instead
Build a three-column recall sheet: MCL (contaminant concentration limit), treatment technique (required process or performance, e.g., turbidity and filtered water limits), and MRDL (disinfectant residual limit). For each, note the averaging period and the tier of public notice. Remember that Tier 1 notice is generally 24 hours, Tier 2 is 30 days, and Tier 3 is annual, and that a monitoring violation still requires notice even with no bad sample.
Assuming a groundwater source is automatically 'protected' and needs no treatment, or classifying a spring or shallow well as pure groundwater without asking whether it is groundwater under the direct influence of surface water.
Textbook language about soil filtration as a natural barrier leads candidates to skip treatment and disinfection questions tied to groundwater systems, and GWUDI is a term candidates meet late in study.
What to do instead
Anchor on the rule that all public water systems must provide treatment that includes disinfection (and filtration for surface water or GWUDI), regardless of source. When a scenario mentions a shallow well, spring, karst terrain, or a source with turbidity that tracks rainfall, flag it as GWUDI until a site-specific determination says otherwise, and expect treatment requirements similar to surface water.
Placing unit processes in the wrong order or mixing up process rate units - putting disinfection ahead of coagulation and settling, or computing a filter loading rate in gpm when the standard unit is gpm per square foot, or confusing backwash rate with filtration rate.
Each unit process is studied in isolation, so the train order is never rehearsed end to end; rate problems are also where unit labels (per square foot, per square meter) get dropped.
What to do instead
Recite the conventional train: source, screening, coagulation, flocculation, sedimentation, filtration, disinfection, storage, distribution. Write surface loading rates and filter rates with the area term attached (gpm/ft², gpm/ft² backwash at the design percent expansion) and check that the backwash rate is several times the filtration rate - if your answer is not, the units or the area is wrong.
Using psi and feet of head interchangeably, and answering pressure questions as if higher pressure means higher flow - for example claiming a customer at the end of the main gets less water because 'pressure drops' without separating pressure from head loss and flow.
Gauges read psi while hydraulic calculations use feet, and the 2.31 ft per psi of water relationship is easy to half-remember or invert.
What to do instead
Commit to the conversion for water: 1 psi = 2.31 ft of head, so head (ft) = psi × 2.31. Sketch the system and label where head loss occurs (friction in mains, elevation, fittings). State separately that pressure is potential energy per unit weight and flow depends on pressure difference and resistance, so a pressure drop along a main reflects friction loss, not a shortage of water.
Using infiltration and inflow as synonyms, and sizing or evaluating gravity sewers without checking slope, velocity, and the self-cleaning range.
Both terms describe clear water entering the collection system, so the distinction feels academic; slope and velocity are often computed with the wrong vertical distance or without the minimum-velocity criterion.
What to do instead
Define infiltration as groundwater entering through defective pipe, joints, and manholes, and inflow as direct surface or storm connections such as roof leaders, sump pumps, and yard drains. For sewer hydraulics, always use the invert drop over the run for slope, convert to ft/ft, and check velocity against the design range (typically about 2 ft/s minimum for self-cleansing and 10 ft/s or less to limit scour).
Confusing solids retention time with hydraulic retention time, and confusing return activated sludge with waste activated sludge - describing wasting as 'returning sludge to the aeration tank' or controlling sludge age by adjusting the return rate.
Both times are computed from tank volume and a flow, and RAS/WAS both move sludge, so the purpose of each stream blurs under exam pressure.
What to do instead
Separate by what each controls: HRT is tank volume divided by influent flow and governs contact time; SRT (MCRT) is the mass of solids in the system divided by the mass leaving per day and governs which organisms survive. Return sludge preserves the biomass inventory, while wasting removes it - SRT is changed by wasting rate, not by RAS rate. Note the direction: raise SRT by wasting less, lower SRT by wasting more.
Computing CT with the wrong residual - using the residual measured at a distant tap or at the end of the system instead of the residual at the disinfection point - or adding chlorine demand to the required residual after already starting from the dose, so the dose, demand, and residual triangle inverts.
CT tables and demand problems both involve chlorine numbers, and candidates memorize 'multiply residual by contact time' without fixing which residual and which contact time the rule specifies.
What to do instead
Write the identity first: dose = demand + residual, so residual = dose − demand. For CT, use the residual and contact time at the actual disinfection point (baffling-adjusted contact time in the basin, not system-wide travel time), then compare to the required CT for the measured pH, temperature, and log inactivation credit. Check that a higher temperature lowers required CT and higher pH raises it for free chlorine.
Mixing up NPSH available with NPSH required, or answering pump curve questions as if two identical pumps in series double flow while pumps in parallel double head.
Both quantities are written NPSH and both curves shift under combinations, and candidates rarely work a curve problem by hand, so the axis labels blur.
What to do instead
Fix the definitions: NPSHr is a property of the pump from the manufacturer; NPSHa is a property of the system and must exceed NPSHr to avoid cavitation. On curves, series adds head at the same flow and parallel adds flow at the same head. Practically, compute NPSHa as atmospheric plus static minus friction and vapor pressure, and remember a pump operating far right of its curve, with high suction lift or a throttled suction, is the cavitation candidate.
Describing confined space entry as a single step - open the hatch, test the air, go in - without a permit, an attendant, continuous ventilation, or retesting, and testing the atmosphere after entry or from the top of the space without pulling a sample from the bottom and mid-depth.
Sample questions often focus on one instrument or one gas, and the sequence gets compressed; candidates also assume a reading at the opening represents the whole space.
What to do instead
Recite the order as a chain and never skip a link: identify and classify the space, obtain and post the permit, isolate and lock out energy and piping, ventilate, test the atmosphere from outside with calibrated instruments sampling at multiple depths, verify oxygen (about 19.5 to 23.5 percent) and flammables and toxics, then enter with an attendant stationed outside, a retrieval system, and continuous monitoring and communication. If the monitor alarms, exit and re-evaluate.
Answering 'what does the operator do next' questions with the technically desirable long-term action instead of the immediate required action - for example recommending a study or a capital project when the scenario calls for an immediate boil water advisory, isolation of a main, or notification of the primacy agency.
Candidates reason like engineers rather than operators, and the phrase 'next' is skimmed as 'what ultimately fixes this'.
What to do instead
Underline the question word before answering: 'next', 'first', 'immediately' all point to the protective action within the operator's authority. Practice sorting scenario responses into three buckets - immediate protective action, notification and documentation, then long-term correction - and choose the earliest bucket that the question asks for. If two answers are both protective, the one that protects public health first wins.