OH Water/Wastewater Operator common mistakes
OH Water/Wastewater Operator · Understand the traps and how to avoid them.
Applying the wrong regulatory threshold (for example, answering with a secondary maximum contaminant level, or a treatment-technique limit, when the question asks about a primary MCL), or quoting an outdated value for a contaminant such as nitrate, arsenic, or TTHM.
Ohio's drinking water rules layer primary MCLs, secondary MCLs, and treatment technique requirements on top of each other, and several limits have been revised over time. Candidates often memorize one number per contaminant and then reuse it in the wrong context. The exam deliberately offers the other limits as distractors.
What to do instead
Build a one-page table with columns for primary MCL, secondary MCL, treatment technique, and monitoring frequency, and fill it only from the current Ohio Administrative Code chapter on primary and secondary drinking water standards. For each contaminant, write one sentence explaining why the limit exists (health versus aesthetics). When you miss one, note which category you confused it with.
Calculating CT (disinfection contact time) or chlorine residual credit without converting units, or using the wrong detention time for the basin geometry, so the answer is off by a factor of 10 or by the baffling factor.
CT problems combine flow in gallons per minute or million gallons per day, basin volume in gallons or cubic feet, and residual in mg/L. Candidates rush the unit conversion and forget that the baffling factor reduces theoretical detention time. The answer choices usually include the unconverted result.
What to do instead
Work every CT problem in a fixed sequence: compute volume, convert to gallons, compute theoretical detention time at the design flow, multiply by the baffling factor, then multiply by the residual to get CT. Write the units on every line and cancel them explicitly. Practice with three basin shapes (rectangular, circular, baffled) until the sequence is automatic.
Treating turbidity as a single grab-sample number rather than applying the combined filter effluent and individual filter effluent limits, including the requirement that a filter exceeding the threshold triggers a specific follow-up action.
Candidates learn one turbidity limit and apply it everywhere. Ohio's filter performance rules distinguish combined filter effluent from individual filter effluent, set different numeric limits, and require actions such as filter-to-waste or reporting when an individual filter exceeds its limit. The exam tests whether you know which limit applies to which sample.
What to do instead
Write out the filter turbidity rule as a decision tree: which sample, which limit, what action, what reporting. Memorize the individual filter effluent trigger and the required response, and the difference between a treatment technique violation and a monitoring violation. Rehearse with a scenario where one filter spikes while the combined effluent stays compliant.
Confusing the purpose and chemistry of coagulation, flocculation, and sedimentation, such as answering that a polymer is added to raise pH or that lime is added to increase alkalinity when the question is about coagulant aid or pH adjustment for corrosion control.
The four processes are taught together and share chemicals. Candidates memorize chemical names without tying each to its function and its effect on pH, alkalinity, and corrosion. Exam items swap the roles to test whether the candidate understands the treatment train rather than the vocabulary.
What to do instead
For each chemical used (alum, ferric chloride, polymer, lime, caustic soda, carbon dioxide, phosphate), write what it does, where it is dosed, and how it shifts pH and alkalinity. Then trace a jar-test result through the plant: which dose change would fix the problem and which side effect it would create. This links chemistry to operations rather than to a list.
Miscalculating storage, fire flow, or pressure results by ignoring the difference between static and residual pressure, or by failing to account for head loss and elevation change in a distribution system problem.
Distribution questions often present a pressure gauge reading and ask what it means for available flow. Candidates use static pressure as if it were residual pressure, or forget that pressure in psi converts to feet of head by multiplying by about 2.31, and that elevation changes subtract from available pressure.
What to do instead
Practice converting between psi and feet of head in both directions until it is reflex, and sketch the system: source, elevation change, pipe, customer. Label static pressure, residual pressure, and head loss separately on the sketch before computing. For fire flow questions, state the residual pressure criterion first, then solve.
Answering cross-connection and backflow questions with the wrong protection device, such as specifying a double check valve assembly where an air gap or reduced pressure zone assembly is required, or missing that a thermal expansion control issue follows installation of a backflow preventer.
Candidates learn device names but not the hazard-level hierarchy. The exam tests whether the candidate can classify a hazard as high or low and match the device. Thermal expansion from a closed system is a secondary effect that is easy to overlook.
What to do instead
Rank the devices from highest to lowest protection (air gap, reduced pressure zone, double check, pressure vacuum breaker, atmospheric vacuum breaker) and write one example of a high-hazard and a low-hazard connection for each. Add a note that a closed system created by a backflow preventer requires thermal expansion control. Practice classifying ten real connections by hazard level.
Confusing coliform monitoring requirements, such as the total coliform rule repeat-sample triggers, the difference between a total coliform positive and an E. coli positive, and when a public notice is required.
The coliform rules have layered triggers: a routine positive requires repeat samples, a repeat positive escalates, and an E. coli positive triggers a specific assessment and public notice timeline. Candidates collapse these into one step and pick the wrong follow-up action or notice deadline.
What to do instead
Draw the coliform decision tree from routine sample through repeat samples, assessment, and public notice, with the numeric trigger and the required timeline at each branch. Distinguish total coliform from E. coli and fecal indicators explicitly. Then answer ten scenario questions using only the tree, so the logic, not memory, drives the answer.
Applying the wrong solids or loading calculation to an activated sludge problem, such as computing food-to-microorganism ratio with the wrong mixed liquor volatile suspended solids basis, or confusing mean cell residence time with sludge age in days.
Wastewater process control uses several ratios that look alike: F/M, MCRT, sludge volume index, and solids loading. Candidates plug numbers into the formula they remember without checking whether the denominator is mixed liquor suspended solids or volatile suspended solids, or whether the flow is influent or return sludge.
What to do instead
Write each formula with a labeled diagram of the aeration tank showing exactly which stream and which solids measurement goes where. Compute one full example for each ratio using consistent units (pounds per day for mass, days for time). Then solve a problem where the SVI changes and predict the effect on return sludge rate and wasting.
Mixing up nitrification and denitrification conditions, or answering that anaerobic and anoxic zones serve the same purpose, when the question asks about nitrogen removal or filamentous bulking control.
Nitrogen removal and phosphorus removal both use staged zones with different dissolved oxygen conditions. Candidates memorize the zone names but not the oxygen status or the organisms involved, so they assign the wrong condition to the wrong tank and miss the alkalinity and oxygen demand effects.
What to do instead
Build a table with columns for zone, dissolved oxygen range, organisms, what is converted, and effect on alkalinity and oxygen demand. Note that nitrification consumes alkalinity and oxygen while denitrification recovers alkalinity and reduces oxygen demand. Trace one influent nitrogen load through each zone to see where the mass goes.
Answering collection system questions with the wrong pipe slope, minimum velocity, or infiltration versus inflow classification, such as treating a roof drain connection as infiltration or ignoring the self-cleansing velocity when checking a gravity sewer.
Collection system items test the boundary between design criteria and observed conditions. Candidates know the terms but not the numeric criteria, and infiltration and inflow are easy to swap because both add clear water to the sewer. Slope and velocity questions punish unit errors in feet per foot versus percent.
What to do instead
Memorize the self-cleansing velocity range and the minimum slope criteria, and practice converting between slope as a ratio and slope as a percent. For infiltration versus inflow, write a definition plus three examples of each and classify a list of sources. Then solve two slope problems where the pipe diameter and flow are given and the velocity must be checked.