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NM Water/Wastewater Operator practice questions

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Question 1Water Treatment

Which chemical is most commonly used as the primary coagulant in municipal water treatment?

  1. Aluminum sulfate (alum) Correct answer.
  2. Calcium hydroxide
  3. Granular activated carbon
  4. Sodium chloride

Explanation

Alum reacts with water to form positively charged hydrolysis products that neutralize the negative charge on colloidal particles, allowing them to agglomerate. Sodium chloride is not a coagulant, lime is used mainly for pH adjustment or softening, and activated carbon is used for adsorption.

Question 2Water Treatment

In a conventional treatment plant, what is the purpose of flocculation?

  1. Gravity settling that removes settled floc
  2. Chemical oxidation that inactivates pathogens
  3. Rapid mixing that disperses treatment chemicals
  4. Gentle mixing that promotes particle contact Correct answer.

Explanation

Flocculation follows rapid mix and uses gentle, prolonged mixing so that destabilized particles collide and form larger floc. Rapid mixing is coagulation, settling occurs in sedimentation, and oxidation is disinfection.

Question 3Water Treatment

What is the primary purpose of disinfection in drinking water treatment?

  1. Removing suspended solids
  2. Improving taste and odor
  3. Softening hard water
  4. Inactivating pathogens Correct answer.

Explanation

Disinfection destroys or inactivates pathogenic bacteria, viruses, and protozoa; it is the most important barrier against waterborne disease. Solids removal, taste/odor control, and softening are separate treatment objectives.

Question 4Water Treatment

Turbidity in drinking water is reported in which units?

  1. ppm
  2. µS/cm
  3. NTU Correct answer.
  4. mg/L

Explanation

Turbidity is an optical measurement of light scattering by suspended particles and is expressed in nephelometric turbidity units (NTU). mg/L and ppm express mass concentration, while µS/cm measures electrical conductivity.

Question 5Water Treatment

A pH of 7.0 at 25°C indicates that the water is:

  1. Unbuffered
  2. Neutral Correct answer.
  3. Strongly acidic
  4. Strongly basic

Explanation

At 25°C, pH 7.0 is the neutral point, where hydrogen ion and hydroxide ion activities are equal. Values below 7 are acidic and values above 7 are basic; buffering is a separate capacity concept.

Question 6Water Treatment

Hardness in water is primarily caused by dissolved:

  1. Iron and manganese
  2. Sodium and potassium
  3. Calcium and magnesium Correct answer.
  4. Chloride and sulfate

Explanation

Hardness is mainly due to calcium and magnesium ions, which react with soap and form scale. Sodium, chloride, and sulfate contribute to total dissolved solids, while iron and manganese cause staining and taste problems.

Question 7Water Treatment

Why is a granular media filter backwashed?

  1. To remove trapped solids Correct answer.
  2. To increase the filtration rate
  3. To add coagulant to the bed
  4. To disinfect the filter media

Explanation

Backwashing reverses flow through the filter at a rate that expands the bed and flushes out accumulated particles, restoring head loss and run time. It is a physical cleaning step, not disinfection or chemical addition.

Question 8Water Treatment

Which treatment process removes dissolved gases such as carbon dioxide and hydrogen sulfide by contacting water with air?

  1. Sedimentation
  2. Coagulation
  3. Aeration Correct answer.
  4. Filtration

Explanation

Aeration brings water into contact with air so volatile gases can transfer out of solution and oxygen can oxidize iron and manganese. Sedimentation and filtration remove particles, and coagulation destabilizes colloids.

Question 9Water Treatment

What is the main purpose of a jar test in water treatment?

  1. To estimate the chlorine demand of the water
  2. To find the best coagulant dose and pH Correct answer.
  3. To measure the filter backwash rate
  4. To calculate the plant flow rate

Explanation

A jar test simulates coagulation, flocculation, and settling at several chemical doses so the operator can choose the dose and pH that produce the clearest settled water. It does not measure backwash rate, chlorine demand, or plant flow.

Question 10Water Treatment

In disinfection, the CT value used to demonstrate pathogen inactivation is calculated as:

  1. Chlorine dose divided by contact time
  2. Turbidity multiplied by chlorine residual
  3. Contact time divided by flow rate
  4. Residual concentration times contact time Correct answer.

Explanation

CT is the product of disinfectant residual concentration (C) and effective contact time (T), and it is compared with required CT values for the target pathogen. A higher residual or longer contact time gives a larger CT.

Question 11Water Treatment

Chloramines, used as a secondary disinfectant, are formed when chlorine reacts with:

  1. Calcium carbonate
  2. Ammonia Correct answer.
  3. Sulfate
  4. Dissolved oxygen

Explanation

Adding chlorine to water containing ammonia forms monochloramine and other chloramine species. Chloramines are weaker primary disinfectants than free chlorine but persist longer and form fewer regulated disinfection byproducts.

Question 12Water Treatment

A filter has reached its terminal head loss. What does this mean?

  1. The filter is clogged and needs cleaning Correct answer.
  2. The filtration flow rate is set too low
  3. Filter media has been lost from the bed
  4. The filter has been properly disinfected

Explanation

Terminal head loss occurs when accumulated solids make it too hard to push water through the filter, so the filter must be backwashed. Media loss, disinfection status, and low flow are unrelated conditions.

Question 13Water Treatment

Why is anthracite placed above sand in a dual-media filter?

  1. It disinfects the water as it flows
  2. It is denser and settles below sand
  3. It is lighter and traps coarse solids Correct answer.
  4. It raises the pH of the filtrate

Explanation

Anthracite has a lower density than sand, so in a stratified dual-media bed it sits on top and captures larger floc particles in its coarser pores. This arrangement uses the bed depth efficiently and extends filter runs.

Question 14Water Treatment

In a sedimentation basin, the surface overflow rate is calculated as:

  1. Flow multiplied by basin depth
  2. Flow divided by surface area Correct answer.
  3. Basin volume divided by flow
  4. Sludge volume divided by flow

Explanation

Surface overflow rate, also called surface loading rate, is flow divided by the basin's horizontal surface area and has units of velocity. It represents the settling velocity of the smallest particle that can be completely removed.

Question 15Water Treatment

Why is sufficient alkalinity important when alum is used as a coagulant?

  1. It removes iron and manganese
  2. It increases the turbidity of the water
  3. It disinfects the water during coagulation
  4. It buffers pH for effective coagulation Correct answer.

Explanation

Alum consumes alkalinity as it hydrolyzes, which can lower pH outside the range where coagulation works well. Adequate alkalinity buffers the water and keeps pH stable; if it is too low, lime or another base may be added.

Question 16Water Treatment

How does ion exchange softening remove calcium and magnesium from water?

  1. It removes them with a sand filter
  2. It oxidizes them into settleable particles
  3. It drives them off as gases in an aerator
  4. It exchanges them for sodium on a resin Correct answer.

Explanation

In a softener, hardness ions in the water displace sodium or hydrogen ions held on the resin; the resin is later regenerated with salt or acid. Oxidation, sand filtration, and aeration do not remove dissolved hardness by ion exchange.

Question 17Water Treatment

What is the purpose of adding fluoride to drinking water?

  1. To remove nitrate from the water
  2. To control corrosion in pipes
  3. To improve the taste of the water
  4. To reduce tooth decay Correct answer.

Explanation

Fluoridation adjusts the natural fluoride concentration to an optimal level that helps prevent dental caries. Corrosion control, nitrate removal, and taste improvement are separate treatment goals.

Question 18Water Treatment

Which practice is commonly used for corrosion control in a distribution system?

  1. Raising pH and feeding phosphate Correct answer.
  2. Raising free chlorine above 10 mg/L
  3. Increasing finished-water turbidity
  4. Adding alum at the clearwell

Explanation

Corrosion control usually involves adjusting pH and alkalinity and adding an inhibitor such as phosphate or silicate to form a protective film on pipe surfaces. High chlorine, alum addition, and elevated turbidity do not protect pipes from corrosion.

Question 19Water Treatment

Under the federal turbidity rule for conventional filtration, what performance must a system meet?

  1. Turbidity below 1 NTU in 95% of monthly samples
  2. 95% of monthly samples ≤0.3 NTU and none >1 NTU Correct answer.
  3. Turbidity below 5 NTU in every monthly sample
  4. Turbidity below 0.1 NTU at all times

Explanation

For conventional and direct filtration, the turbidity limit is 0.3 NTU in at least 95% of monthly measurements, with a maximum of 1 NTU. The 5 NTU and 0.1 NTU figures do not match the rule.

Question 20Water Treatment

What is the maximum contaminant level for total trihalomethanes (TTHM) in finished drinking water?

  1. 80 µg/L Correct answer.
  2. 10 µg/L
  3. 80 mg/L
  4. 60 µg/L

Explanation

The Stage 2 DBP rule sets the TTHM MCL at 80 µg/L and the HAA5 MCL at 60 µg/L, both measured as locational running annual averages. Confusing mg/L with µg/L is a common unit error.

Question 21Water Treatment

Water has a chlorine demand of 1.5 mg/L. What chlorine dose is needed to leave a 0.8 mg/L free chlorine residual?

  1. 1.5 mg/L
  2. 2.3 mg/L Correct answer.
  3. 0.8 mg/L
  4. 0.7 mg/L

Explanation

Dose equals demand plus desired residual, so 1.5 + 0.8 = 2.3 mg/L. The demand consumes chlorine before any free residual remains.

Question 22Water Treatment

A filter measures 4 ft by 8 ft and receives 0.20 mgd. What is the loading rate in gpm/ft²?

  1. 2.2 gpm/ft²
  2. 4.3 gpm/ft² Correct answer.
  3. 8.7 gpm/ft²
  4. 12.5 gpm/ft²

Explanation

Convert flow to gpm: 0.20 mgd × 1,000,000 ÷ 1,440 = 139 gpm. Filter area is 4 × 8 = 32 ft², so 139 ÷ 32 = 4.3 gpm/ft².

Question 23Water Treatment

What should be the first action when a chlorine gas leak is detected at a treatment plant?

  1. Enter the room and close the valve
  2. Evacuate the area and call for help Correct answer.
  3. Add ammonia to neutralize the gas
  4. Spray water on the leaking cylinder

Explanation

Chlorine gas is highly toxic, so personnel should leave the area, move upwind, and summon trained responders with self-contained breathing apparatus. Only trained personnel should attempt to stop the leak.

Question 24Water Treatment

Which condition most often shortens a granular filter run?

  1. Proper coagulation and settling ahead of the filter
  2. Adequate backwashing between runs
  3. Low influent turbidity to the filter
  4. Excess floc carryover onto the filter Correct answer.

Explanation

When coagulation or sedimentation is poor, floc carries onto the filter and plugs the media quickly, causing early head loss or turbidity breakthrough. Good pretreatment and backwashing lengthen runs.

Question 25Water Treatment

A clearwell holds 250,000 gal and receives a peak flow of 2.0 mgd. A tracer study gives a baffling factor of 0.5. If the chlorine residual is 1.0 mg/L, what is the effective CT in mg/L·min?

  1. 180
  2. 45
  3. 30
  4. 90 Correct answer.

Explanation

Detention time is 250,000 gal ÷ 2,000,000 gal/day = 0.125 day = 180 min. Effective contact time is 180 × 0.5 = 90 min, so CT = 1.0 mg/L × 90 min = 90 mg/L·min.

Question 26Water Treatment

What is the main purpose of rapid mix in a conventional surface water treatment plant?

  1. To disperse coagulant quickly through the water Correct answer.
  2. To build large, settleable floc particles
  3. To provide disinfection contact time before filtration
  4. To settle out the heavier suspended solids

Explanation

Rapid mix supplies brief, intense turbulence that spreads the coagulant throughout the water so its hydrolysis products can contact and destabilize particles before flocculation begins. Gentle, prolonged mixing for floc growth happens later in a separate basin.

Question 27Water Treatment

What does the uniformity coefficient of a filter medium describe?

  1. The rate of backwash flow that just fluidizes the bed
  2. The particle size at which 10 percent of the media by weight passes a sieve
  3. The depth of media needed to achieve a target effluent turbidity
  4. The ratio of the 60th-percentile media size to the 10th-percentile size Correct answer.

Explanation

The uniformity coefficient is d60 divided by d10, so it describes how wide the range of grain sizes is; a value close to 1.5 indicates a closely graded bed. The d10 value itself is the effective size, a separate media specification.

Question 28Water Treatment

Which process removes dissolved taste- and odor-causing organic compounds from water?

  1. Ion exchange demineralization
  2. Chemical precipitation
  3. Adsorption onto activated carbon Correct answer.
  4. Rapid sand filtration

Explanation

Activated carbon has an enormous internal pore surface that adsorbs dissolved organic molecules, including many taste, odor, and disinfection byproduct precursor compounds. Filtration and precipitation act on particles, not dissolved organics.

Question 29Water Treatment

Which instrument is used to measure turbidity in finished drinking water?

  1. A nephelometric turbidimeter Correct answer.
  2. A colorimeter
  3. A conductivity meter
  4. A spectrophotometer set at 254 nm

Explanation

Nephelometry measures light scattered at roughly 90 degrees by suspended particles, and the result is reported in nephelometric turbidity units. It is the approved method for compliance turbidity measurement.

Question 30Water Treatment

After the breakpoint is passed during chlorination, what happens as more chlorine is applied?

  1. Combined chlorine residual continues to increase steadily
  2. Free chlorine residual rises almost in proportion to the dose Correct answer.
  3. Chlorine demand increases sharply with each added dose
  4. The water becomes undrinkable and must be wasted from the plant

Explanation

Below the breakpoint, applied chlorine is consumed by reducing substances and ammonia; once those are destroyed, additional chlorine survives as free chlorine residual. The residual then increases nearly one-for-one with further dose.

Question 31Water Treatment

Under the federal Surface Water Treatment Rule, a conventional filtration system must achieve what overall removal and inactivation of Giardia lamblia?

  1. 99 percent (2-log)
  2. 99.99 percent (4-log)
  3. 99.9 percent (3-log) Correct answer.
  4. 90 percent (1-log)

Explanation

Conventional filtration systems must demonstrate at least 99.9 percent (3-log) removal and inactivation of Giardia lamblia, combined across filtration and disinfection. The virus requirement for these systems is higher, at 99.99 percent (4-log).

Question 32Water Treatment

How are iron and manganese normally removed from a groundwater supply?

  1. Oxidation followed by filtration Correct answer.
  2. Simple settling in a conventional clarifier
  3. Reverse osmosis applied to the whole plant flow
  4. Chlorination alone, with no filtration step

Explanation

Iron and manganese are dissolved in oxygen-poor groundwater, so they must first be oxidized to insoluble oxide particles and then captured on a filter. Settling alone cannot remove the dissolved forms.

Question 33Water Treatment

Why are tube or plate settlers installed in a sedimentation basin?

  1. They reduce the sludge production of the basin
  2. They increase the effective settling area Correct answer.
  3. They provide disinfection between basins
  4. They add coagulant chemicals to the influent

Explanation

Closely spaced inclined tubes or plates create many shallow settling zones, so the basin behaves as though it had a much larger settling area. That allows a higher overflow rate with the same particle removal.

Question 34Water Treatment

A flocculation basin holds 90,000 gallons and receives a flow of 3.0 mgd. What is the detention time in minutes?

  1. 22 minutes
  2. 43 minutes Correct answer.
  3. 65 minutes
  4. 30 minutes

Explanation

Detention time equals volume divided by flow: 90,000 gal divided by 3,000,000 gal/day is 0.03 day. Multiplying 0.03 day by 1,440 minutes per day gives about 43 minutes.

Question 35Water Treatment

A plant treats 2.5 mgd and applies alum at a dose of 25 mg/L. How many pounds per day of alum are used?

  1. 521 lb/day Correct answer.
  2. 208 lb/day
  3. 260 lb/day
  4. 1,042 lb/day

Explanation

The standard feed formula is pounds per day equals mg/L times 8.34 times flow in mgd. Here 25 times 8.34 times 2.5 equals about 521 lb/day.

Question 36Water Treatment

A filter measuring 10 ft by 20 ft is backwashed at 15 gpm/ft². What is the backwash flow rate?

  1. 3,000 gpm Correct answer.
  2. 1,500 gpm
  3. 4,500 gpm
  4. 600 gpm

Explanation

The filter area is 10 ft times 20 ft, or 200 ft². Multiplying 200 ft² by 15 gpm/ft² gives 3,000 gpm.

Question 37Water Treatment

A finished water sample has 0.2 mg/L free chlorine and 1.1 mg/L total chlorine. What does this indicate?

  1. Total chlorine is below the required minimum
  2. The chlorine dose has clearly passed the breakpoint
  3. Most of the residual is combined chlorine Correct answer.
  4. The sample was collected ahead of the clearwell

Explanation

Combined chlorine equals total chlorine minus free chlorine, which here is 0.9 mg/L. That means chloramines dominate the residual and only a small free chlorine fraction remains, so disinfection strength is weak.

Question 38Water Treatment

How many pounds of calcium hypochlorite that is 65 percent available chlorine are needed to supply 50 lb of chlorine?

  1. 65 lb
  2. 115 lb
  3. 77 lb Correct answer.
  4. 33 lb

Explanation

Divide the required pounds of available chlorine by the decimal purity of the product. Here 50 lb divided by 0.65 equals about 77 lb of product.

Question 39Water Treatment

A dye tracer test shows that water reaches the outlet of a settling basin far sooner than the calculated detention time. What does this indicate?

  1. The weir loading is higher than design
  2. The sludge blanket has become too deep
  3. The flow rate is far below the design value
  4. The basin is short-circuiting Correct answer.

Explanation

When tracer appears at the outlet much earlier than the theoretical detention time, part of the flow is traveling directly from inlet to outlet. That short-circuiting reduces the effective settling volume and degrades performance.

Question 40Water Treatment

Which chemical is used to neutralize chlorine residual in a sample bottle before bacteriological testing?

  1. Sodium thiosulfate Correct answer.
  2. Alum
  3. Potassium permanganate
  4. Sodium fluoride

Explanation

Sodium thiosulfate reduces free and combined chlorine so disinfection does not continue inside the bottle after collection. This preserves the bacteriological condition of the water at the moment of sampling.

Question 41Water Treatment

A filter shows a sharp turbidity spike right after backwash, then returns to normal. What is the most appropriate control measure?

  1. Increase the coagulant dose permanently
  2. Reduce the backwash rate below fluidization
  3. Backwash more frequently during the day
  4. Filter to waste until turbidity drops Correct answer.

Explanation

A brief turbidity rise after backwash is normal because fines and residual solids are flushed out first. Returning the filter to service through a waste line until effluent turbidity is acceptable keeps that material out of the finished water.

Question 42Water Treatment

Which process removes dissolved salts such as sodium and nitrate from drinking water?

  1. Reverse osmosis Correct answer.
  2. Coagulation and flocculation
  3. Aeration and degasification
  4. Granular media filtration

Explanation

Reverse osmosis forces water through a semipermeable membrane that rejects dissolved ions, so it can remove sodium, nitrate, and other dissolved salts. Media filtration and coagulation only remove particulate material.

Question 43Water Treatment

Cavitation in a raw water pump is most often caused by:

  1. A badly worn impeller wear ring alone
  2. A discharge valve that is fully open
  3. Insufficient net positive suction head Correct answer.
  4. Pumping against a lower static head than design

Explanation

Cavitation occurs when pressure at the pump suction drops below the vapor pressure of the water, forming vapor bubbles that collapse and damage the impeller. Adequate net positive suction head available above that required prevents it.

Question 44Water Treatment

A utility converts from free chlorine to chloramines as its secondary disinfectant. What is the primary operational concern?

  1. Nitrification in the distribution system Correct answer.
  2. Excessive free chlorine residual in the clearwell
  3. Loss of all measurable chlorine residual
  4. Increased trihalomethane formation at the plant

Explanation

Chloramines are formed with ammonia, which nitrifying bacteria can use as a food source, so nitrification becomes the key monitoring and control issue. Utilities typically watch ammonia, nitrite, and nitrate and may periodically switch back to free chlorine.

Question 45Water Treatment

After excess-lime softening, water may have a pH near 10.5. What is commonly added to stabilize it?

  1. Sodium hydroxide
  2. Carbon dioxide Correct answer.
  3. Chlorine gas
  4. Sodium chloride

Explanation

Carbon dioxide reacts with the excess hydroxide from lime softening to form bicarbonate, bringing pH down into a range that is neither scale-forming nor corrosive. This step is called recarbonation.

Question 46Water Treatment

A filter reaches 0.5 NTU turbidity breakthrough at 18 hours while head loss is still well below terminal. What is the most likely cause?

  1. The media bed is too deep for the flow rate
  2. The underdrain is plugged with media
  3. The backwash rate is too low to clean the bed properly
  4. Poor coagulation is letting floc pass the bed Correct answer.

Explanation

Turbidity breakthrough with plenty of head loss remaining means particles are not being captured in the bed rather than filling it. That usually points to floc that is too small or weak to attach to the media, so coagulation and flocculation need adjustment.

Question 47Water Treatment

How many gallons per day of 12.5 percent sodium hypochlorite solution (specific gravity 1.20) are needed to dose 1.5 mgd at 4.0 mg/L?

  1. 20 gal/day
  2. 80 gal/day
  3. 50 gal/day
  4. 40 gal/day Correct answer.

Explanation

Available chlorine per gallon is 0.125 times 1.20 times 8.34, or about 1.25 lb/gal. The required dose is 4.0 times 8.34 times 1.5, or 50 lb/day, and 50 divided by 1.25 gives 40 gal/day.

Question 48Water Treatment

Continuous regeneration of a manganese greensand filter used for iron and manganese removal requires:

  1. Continuous feed of sodium hypochlorite upstream of the filter
  2. A daily air scour of the media bed
  3. Periodic regeneration with sodium chloride brine
  4. Continuous feed of potassium permanganate as an oxidant Correct answer.

Explanation

Greensand is coated with manganese dioxide that oxidizes dissolved iron and manganese, and permanganate feed continuously renews that oxidizing capacity. Without it the media becomes exhausted and metals pass through.

Question 49Water Treatment

Which set of conditions most favors trihalomethane formation?

  1. High TOC, low pH, cold water, short contact time
  2. Low TOC, high pH, cold water, long contact time
  3. High TOC, high pH, warm water, long contact time Correct answer.
  4. Low TOC, low pH, cold water, short contact time

Explanation

Trihalomethanes form when free chlorine reacts with organic precursors, so more precursor material, higher pH, warmer water, and longer contact all increase formation. Treatment strategies aim to reduce precursors and contact time before the distribution system.

Question 50Water Treatment

A settling basin receives 2.0 mgd and has 300 ft of effluent weir length. What is the weir loading in gallons per day per foot?

  1. 10,000 gpd/ft
  2. 6,670 gpd/ft Correct answer.
  3. 4,440 gpd/ft
  4. 3,330 gpd/ft

Explanation

Weir loading equals flow divided by weir length. Converting 2.0 mgd to 2,000,000 gpd and dividing by 300 ft gives about 6,670 gpd/ft.