Question 1

A water treatment plant must dose a reservoir with chlorine to achieve 2.0 mg/L free chlorine residual. If the reservoir holds 500,000 gallons and the chlorine demand is 1.5 mg/L, how many pounds of chlorine (as 100% pure Cl2) are required?

Accepted Answer

Total chlorine dose = demand + residual = 1.5 + 2.0 = 3.5 mg/L. Formula: lbs = mg/L x MGD x 8.34 lbs/gal. Volume = 0.5 MG. lbs = 3.5 x 0.5 x 8.34 = 14.6 lbs of pure Cl2. This formula (mg/L x MG x 8.34) is fundamental for all chemical dosing calculations on the operator exam.

Question 2

Under the Lead and Copper Rule (LCR), the action level for lead in drinking water is:

Accepted Answer

The Lead and Copper Rule action level (AL) for lead is 15 µg/L (ppb) at the 90th percentile of tap water samples from high-risk homes. If the AL is exceeded, the system must optimize corrosion control treatment, conduct public education, replace lead service lines, and take other corrective actions. The MCLG for lead is zero — no safe level is known.

Question 3

What is the detention time of a sedimentation basin that holds 2 million gallons (MG) if the plant flow rate is 4 million gallons per day (MGD)?

Accepted Answer

Detention Time (HRT) = Volume divided by Flow Rate = 2 MG divided by 4 MGD = 0.5 days = 12 hours. Typical sedimentation basin detention times are 2 to 8 hours for conventional treatment. Longer detention allows more complete settling of floc particles. This formula also applies to reservoirs, contact tanks, and other vessels.

Question 4

The Safe Drinking Water Act (SDWA) requires water systems to use which of the following to regulate drinking water contaminants?

Accepted Answer

The SDWA requires EPA to set National Primary Drinking Water Regulations (NPDWRs) for contaminants that may pose health risks. For each regulated contaminant, EPA sets either an MCL (maximum contaminant level, the highest level allowed in drinking water) or a Treatment Technique (TT) requirement when measuring the contaminant is not feasible. States may set standards stricter than EPA's.

Question 5

A water operator calculates the percent removal of turbidity across a filter. The influent turbidity is 8.0 NTU and the effluent is 0.2 NTU. What is the percent removal?

Accepted Answer

Percent Removal = [(Influent minus Effluent) divided by Influent] x 100 = [(8.0 minus 0.2) divided by 8.0] x 100 = [7.8 divided by 8.0] x 100 = 97.5%. Percent removal calculations appear frequently on operator exams for turbidity, BOD, and other parameters. The Surface Water Treatment Rule requires combined filter effluent 0.3 NTU or below, making this filter compliant.

Question 6

A water plant feeds a 12.5% sodium hypochlorite solution at a rate of 80 gallons per day to treat a flow of 2.0 MGD. What is the chlorine dose in mg/L?

Accepted Answer

Step 1 — Convert solution volume to lbs of Cl2 per day: 80 gal/day x 8.34 lb/gal x 0.125 (12.5% strength) = 83.4 lb Cl2/day. Step 2 — Convert to dose in mg/L using: mg/L = (lb/day) divided by (MGD x 8.34) = 83.4 divided by (2.0 x 8.34) = 83.4 divided by 16.68 = 5.0 mg/L. Chemical dosing math using the 8.34 lb/gal conversion factor is a core competency tested on all water operator licensing exams.

Question 7

Under the Lead and Copper Rule (LCR), what action must a water system take if the 90th percentile lead level exceeds the action level of 15 µg/L?

Accepted Answer

Exceeding the Lead and Copper Rule action level triggers a series of required responses: (1) optimize corrosion control treatment (e.g., adjust pH, add orthophosphate or silica inhibitors); (2) expand tap water monitoring; (3) conduct public education and notification; and (4) replace lead service lines on a required schedule. The Lead and Copper Rule Revisions (LCRR, 2021) and Lead and Copper Rule Improvements (LCRI) strengthen these requirements further.

Question 8

A water plant is treating 5.0 MGD. The operator needs to add alum at 40 mg/L. Alum is available as a liquid solution weighing 10.8 lb/gallon and containing 47% alum. Approximately how many gallons per day of liquid alum solution should be fed?

Accepted Answer

Step 1 — Pounds of alum needed per day: lbs/day = dose (mg/L) x flow (MGD) x 8.34 = 40 x 5.0 x 8.34 = 1,668 lb/day. Step 2 — Gallons of solution per day: gpd = lbs/day divided by (solution weight lb/gal x fraction strength) = 1,668 divided by (10.8 x 0.47) = 1,668 divided by 5.076 = approximately 329 gpd. Chemical feed rate calculations combining the 8.34 lb/gal factor with solution strength are commonly tested on water treatment operator exams at Grade 2 and above.

Question 9

What does the EPA's Surface Water Treatment Rule (SWTR) require as the minimum total log inactivation/removal credit for viruses?

Accepted Answer

The SWTR requires 4-log (99.99%) inactivation/removal of viruses through combined treatment. Conventional filtration with chemical coagulation receives 2-log filtration credit for viruses, leaving 2-log to be achieved by disinfection. Giardia requires 3-log total, and Cryptosporidium requirements are specified by the Long Term 2 (LT2) rule based on source water monitoring bin classification.

Question 10

A water system must report a maximum contaminant level (MCL) violation to the state primacy agency within how many hours of learning of the violation, if the violation poses an acute risk to public health?

Accepted Answer

For acute MCL violations or treatment technique violations that pose an immediate risk to public health (e.g., E. coli MCL violation, total coliform trigger with confirmed E. coli), the water system must notify the state primacy agency as soon as possible but no later than 24 hours after learning of the violation. Public notification (Tier 1) must also be issued within 24 hours. For non-acute violations, Tier 2 notification must be provided within 30 days.

Regulations, Math & Chemical Dosing Practice Questions

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