ABC Class II Water Treatment common mistakes
ABC Class II Water Treatment · Understand the traps and how to avoid them.
Works process-control math without tracking units - plugs gpm into a formula expecting MGD, multiplies mg/L by flow in gpm and calls it lb/day, or mixes metric and US units mid-calculation, then selects the answer closest to the wrong number.
Formulas are memorized as symbol strings (C x Q x 8.34) without the unit bookkeeping that makes the 8.34 factor meaningful. Under a 60-90 second per-item clock, candidates skip the unit check and rationalize a plausible-looking distractor.
What to do instead
Write the unit on every number and cancel them explicitly before dividing: (mg/L)(MGD)(8.34 lb/gal) = lb/day. Convert flow to MGD first and say it out loud on scratch paper. Keep a one-card reference of the 8.34 factor, 1 MGD = 694 gpm, 1 ft3 = 7.48 gal, and 1 mg/L = 8.34 lb/MG, and do a one-line order-of-magnitude sanity check on every answer.
Computes CT by simply multiplying residual times contact time, ignoring the CT table's temperature and pH corrections, using peak flow instead of the flow the table requires, or applying free-chlorine CT credit to a chloraminated system.
The concept ('residual times time') is easy and feels complete once the multiplication is done; the correction factors live in tables that are tedious to read under time pressure, and free vs. combined chlorine look interchangeable on the page.
What to do instead
Before calculating, write three decisions on scratch paper: disinfectant type, the pH and temperature that index the table, and which flow (peak hourly for most CT determinations) is required. Then take the table value for the 10th-percentile residual, not the average. Practice 15 mixed free/combined problems until temperature and pH correction becomes automatic, and never accept a CT answer that has no correction step shown.
Sets a coagulant feed rate using the jar-test dose in mg/L directly as lb/day, or as gallons per day of neat product, without correcting for commercial product strength (e.g., 48% alum) and specific gravity.
Bench dose and plant feed rate both get spoken of as 'the dose,' and product data sheets bury percent strength and specific gravity. Candidates who learned one formula on neat chemical carry it over to as-shipped product.
What to do instead
Use the two-step chain every time: lb/day = dose(mg/L) x flow(MGD) x 8.34, then gal/day = lb/day / (8.34 x specific gravity x decimal strength). Circle the words 'as shipped' or 'as Al2O3' in the stem before starting, and rehearse with a 48% alum and a 70% caustic example so the strength divisor is never omitted.
Diagnoses every filter problem as breakthrough: calls high effluent turbidity with normal headloss 'breakthrough' without checking for a rising headloss with good turbidity (surface blinding), short runs with air release at the start of backwash (air binding), or localized turbidity spikes right after backwash (media upset).
Turbidity is the number everyone watches, so it becomes the only symptom used. The distinguishing data - headloss trend, run length, backwash turbidity curve, filter inspection - require pulling a second number that the question hands over but the candidate ignores.
What to do instead
Build a three-column mental table before answering any filter question: effluent turbidity trend, headloss trend, run length. Breakthrough = turbidity up, headloss normal. Surface blinding = headloss up fast, turbidity still low. Air binding = short runs, negative pressure, air in the effluent line. Then choose the action each pattern implies (end run vs. backwash vs. fix the backwash/air source). Also confirm whether the run should end on turbidity (0.3 NTU, or 1 NTU if the state allows for conventional filtration) or on terminal headloss, whichever comes first.
Treats MCL, MCLG, MRDL, and treatment technique as interchangeable - applying the 4.0 mg/L chlorine MRDL as if it were an MCL or a goal, or answering a turbidity question with an MCL number when turbidity is a treatment technique with its own reporting triggers (e.g., single-filter effluent above 1.0 NTU, or 0.3 NTU 95th percentile for conventional filtration).
All four are 'limits' in casual speech and appear in the same regulation tables. Candidates learn the numbers without learning which mechanism enforces each one, and the exam deliberately offers the sibling number as a distractor.
What to do instead
Memorize a four-box classification: health goal (MCLG), enforceable limit at the tap (MCL), disinfectant limit (MRDL), and required process performance (treatment technique, with its own monitoring and reporting triggers). When a question asks for a number, first name the category out loud, then retrieve the value. Write the turbidity treatment-technique triggers and their reporting deadlines on a card until they are automatic.
Handles compliance samples casually - grabs a coliform bottle without checking thiosulfate, fills a chlorine residual bottle to the brim and lets it sit in sunlight, exceeds the holding time for a nitrate or bacteriological sample, or submits a grab sample where the monitoring plan requires a flow-weighted composite.
Sampling is treated as a chore done while other tasks run. Preservation, dechlorination, headspace, and holding time are invisible in the moment and only matter weeks later in the result, so they are never reinforced by feedback.
What to do instead
Before every sample, run a fixed five-question check: correct container, preservative present, dechlorinating agent present if required, headspace/zero-headspace requirement, holding time and temperature. Read the monitoring plan for grab vs. composite before filling anything. Practice the checks on a written scenario bank until the five questions are reflex, and always ask what the result is used for - compliance, process control, or both.
Answers a chlorine or ammonia release question with 'spray it down with water,' approaches the leak from downwind, or sends an entry team in without SCBA and a buddy, or evacuates toward the low ground where the gas has pooled.
Water-on-spill is right for many chemical releases and gets generalized to halogens; the responder instinct to move toward the problem and the habit of respiratory protection 'if it gets bad' override the gas-specific rules.
What to do instead
Anchor two non-negotiables for chlorine: never apply water to a chlorine leak (it forms corrosive acid and spreads the cloud) and never enter without SCBA and a second person. Approach and evacuate from upwind and uphill, establish the exclusion zone first, and call for the chlorine kit/A/B/C kit rather than improvising. Rehearse the same frame for ammonia (water fog is acceptable for knock-down, but still upwind, PPE, and no entry without air).
Attributes every pump problem to cavitation: calls a worn impeller, a partially closed suction valve, a plugged strainer, or air entrainment 'cavitation' without distinguishing the sound (gravel vs. air rush), the gauge readings (suction vacuum vs. normal), or whether the symptom appeared after a valve change.
Cavitation is the one pump failure mode taught with a vivid noise, so it becomes the default label. Gauge data and the pump curve are available in the stem but require more work to interpret than matching a remembered symptom.
What to do instead
Use a symptom-to-cause grid: gravelly noise plus falling discharge pressure and a suction gauge pulled toward vacuum = cavitation/NPSH problem; air rush and erratic flow at start = air entrainment or suction leak; steady flow loss with normal gauges = wear ring/impeller wear; vibration only at one flow = operating off the curve. Always locate the duty point on the curve from the given flow and head before choosing an answer, and check suction-side restrictions first.
Runs a jar test by dumping coagulant into all six jars at once, skipping the rapid-mix stage or making it too gentle, letting jars settle for inconsistent times, or reading the result only by supernatant clarity without checking settled floc size, pH drift, and whether the chosen dose produces a floc that will survive filtration.
The jar test looks like a simple side-by-side comparison, so the controlled variables - rapid mix intensity and duration, slow mix, settling time held constant, sampling depth - get treated as optional. Candidates also forget that the winning dose must be tied to finished-water goals, not just to the clearest jar.
What to do instead
Follow a fixed protocol: same rapid-mix speed and time, same flocculation speed and time, same settling period, sample at the same depth, and measure pH and turbidity in each jar rather than eyeballing. Rank jars on settled turbidity plus floc character, then confirm the choice against the target filtered turbidity and alkalinity consumed. When a question shows a jar-test table, look for the jar that is best at the lowest dose rather than the clearest at any dose.
Reads source water conditions as background instead of as a signal - misses that a storm runoff event means higher turbidity, TOC, and pathogen load with lower alkalinity and pH stability, or that an algae bloom raises pH and causes filter clogging, and then selects a 'no change to treatment' answer.
Source water is studied early as descriptive material (watershed types, contaminants) and never gets linked to the operational adjustments it forces, so questions framed as 'what should the operator do' get answered from the operations half of the question only.
What to do instead
For every source water question, convert the condition into three operational consequences: coagulant dose, pH/alkalinity adjustment, and disinfection demand or filter loading. Practice the specific pairs - rain event means raise coagulant and watch alkalinity; algae bloom means expect high pH, taste and odor, and shorter filter runs; turnover or low temperature means poorer settling and changed disinfectant demand. Also tie each source to its watershed protection duty (sanitary survey findings, source monitoring, restriction of activities in the recharge area).