CO Water Operator common mistakes
CO Water Operator · Understand the traps and how to avoid them.
Calculating a chlorine or chemical feed problem by plugging numbers straight into 'dose = residual' - the candidate solves for the target residual (say 0.5 mg/L) and reports that as the pounds-per-day to feed.
Dose and residual appear in the same word problem, and in daily plant talk operators say 'we're running 0.5' meaning residual. Under time pressure the familiar number gets attached to the wrong side of the equation.
What to do instead
Before computing, write the sentence 'Dose = Demand + Residual' at the top of the scratch work and label the two givens. If the problem gives only a target residual and a demand, add them first; if it gives a dose and a residual, subtract to find demand. Practise a half-dozen problems each way until the sentence is automatic.
Dropping or misplacing the 8.34 lb/gal factor and the million-gallon conversion when moving between mg/L, gallons, and pounds per day, so the answer comes out off by factors of 100 or 1,000,000.
The feed formula is memorised as a string of symbols rather than as a chain of units, so when the flow is given in gpm instead of MGD the candidate multiplies or divides by whatever 'feels' right.
What to do instead
Carry units through every line and cancel them before touching the calculator: (MGD x mg/L x 8.34) has units of lb/day, and gpm x 1440 gives gpd. Convert flow to MGD in a separate, labelled step. Check the magnitude of the answer against a sanity estimate (a few pounds per day for a small plant, not thousands).
Evaluating disinfection contact time (CT) credit using average or design-average plant conditions - average flow, warmest water temperature, mid-range pH - and declaring the CT requirement met.
The candidate treats CT like a single check rather than a worst-case compliance calculation, and the arithmetic is easier with the nice round average numbers printed on the operating log.
What to do instead
Compute CT at the conditions that make credit smallest: highest flow through the contact basin, lowest water temperature, highest pH in the range, and the lowest measured residual at the outlet of that basin. Confirm the resulting CT against the required value for the target log inactivation, and remember each treatment step earns its own credit only if the required residual is maintained through it.
Converting pressure to head or head to pressure with the 2.31 factor upside down, e.g. dividing by 2.31 when going from psi to feet.
2.31 is remembered as 'the number for pressure' without the direction being attached to it, and both operations look equally plausible on paper.
What to do instead
Anchor it with a physical fact you can re-derive in seconds: a column of water about 2.31 ft tall weighs 1 psi at the bottom, so feet of head is always the larger number and psi is the smaller. If the answer gives more psi than feet for the same head, the factor was inverted. Sanity-check with 100 ft of head equals roughly 43 psi.
Treating the routine coliform sample count as a fixed number (commonly four per month) instead of determining it from the population served, and then also missing that any positive routine sample triggers repeat samples at specified locations.
The number four is quoted so often in study material and small-system conversation that it hardens into a rule, and the repeat-sampling requirement is a separate table the candidate has not rehearsed.
What to do instead
Memorise the population-band table so the count is looked up, not recalled, and note the band boundaries on your scratch sheet at the start of the exam. Pair every routine-sample rule with its consequence: one positive routine sample means repeat samples at the original tap plus upstream and downstream locations, with the count tied to the number of routine samples taken.
Computing food-to-microorganism ratio by putting MLSS (or a mixed set of units) in the denominator where MLVSS belongs, or by using pounds of BOD per day against a solids number still expressed in mg/L.
MLSS is the number printed on the daily lab sheet and is used in almost every other calculation, so it gets substituted automatically; the unit mismatch is invisible because both quantities look like 'solids' and 'food'.
What to do instead
Write the F/M ratio with its units spelled out before substituting: lb BOD applied per day over lb of volatile solids in the aeration tank. Convert the aeration tank volume to gallons, then to pounds using 8.34, then multiply by MLVSS in mg/L - never leave mg/L sitting against lb/day. Verify the result lands in the normal operating range for the process type.
Confusing mean cell residence time with hydraulic retention time - concluding that because wastewater sits in the aeration basin for, say, eight hours, the sludge age is also eight hours.
Both are 'time in the system' and both are computed from the same aeration tank volume, so the two ideas collapse into one during recall.
What to do instead
State the difference out loud: HRT is how long the water stays; MCRT is how long the bugs stay, and it is set by how much solids you waste, not by tank volume alone. Practice problems by computing both from the same data set so the differing denominators - influent flow versus pounds of solids wasted plus effluent solids - become distinct.
Diagnosing every high-SVI, poor-settling sludge as filamentous bulking and answering with chlorine or selector questions, even when the scenario describes a young sludge, a low-DO condition, or a hydraulic overload.
Filamentous bulking is the most heavily studied bulking mechanism and the one most practised, so it becomes the default answer whenever a clarifier acts up.
What to do instead
Run a fixed decision sequence on every settleability scenario: check MCRT first (very low means young sludge, wasting too much), check dissolved oxygen and the presence of a septic or low-F/M condition, then look for the mixed-liquor and clarifier clues given. Only after those are excluded should filamentous organisms be named as the cause.
Expressing sewer grade in the wrong form - reporting a slope of 0.005 ft/ft as 0.5 percent when it is 0.5 percent only if written as 0.005, or inversely computing 1 percent as 0.1 ft/ft - and using plan (horizontal) distance from a map instead of the actual pipe length.
Percent grade, ft/ft, and inches of fall per foot all describe the same slope and the decimal shifts are easy to lose; map distances are the numbers most readily available and the offset from pipe length is small enough to go unnoticed.
What to do instead
Fix the identity in memory: 1 percent grade equals 0.01 ft/ft, which is about 1/8 inch per foot. Convert to one form only, write the conversion out, then solve. For pipe length, add the vertical drop to the horizontal run when the grade is steep, or use the surveyed length when it is given; note that on flat grades the difference is negligible but the method must still be stated correctly.
Classifying a storm drain, roof leader, or sump pump connection as infiltration when reporting inflow and infiltration, and classifying groundwater seepage through cracked pipe joints as inflow.
The two terms are always spoken together as 'I&I', so the distinction gets blurred; both appear as clear water entering the system.
What to do instead
Use the source test: infiltration is groundwater that enters through defects without a direct surface opening; inflow is surface water or direct connections that respond immediately to rain. Sort each item in a scenario by asking 'did this come from the ground or from a pipe/drain on the surface?' before answering.