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NC Water/Wastewater Operator practice questions

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Question 1Water Treatment

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

  1. Aluminum sulfate (alum) Correct answer.
  2. Sodium fluoride
  3. Calcium hydroxide
  4. Sodium hypochlorite

Explanation

Alum hydrolyzes in water to form positively charged hydrolysis products that neutralize the negative surface charge on colloids, allowing them to agglomerate into floc. The other listed chemicals serve disinfection, pH adjustment, or fluoridation rather than coagulation.

Question 2Water Treatment

Turbidity in finished drinking water is normally measured by which method?

  1. Specific conductance
  2. Platinum-cobalt colorimetry
  3. Nephelometry Correct answer.
  4. Gravimetric analysis

Explanation

A nephelometric turbidimeter measures light scattered by suspended particles at approximately 90 degrees to the incident beam and reports the result in nephelometric turbidity units. Colorimetry, gravimetry, and conductivity measure different properties entirely.

Question 3Water Treatment

A disinfection contact basin maintains a chlorine residual of 2.0 mg/L and provides 30 minutes of contact time. What is the CT value?

  1. 32 mg-min/L
  2. 15 mg-min/L
  3. 60 mg-min/L Correct answer.
  4. 0.067 mg-min/L

Explanation

CT is the product of disinfectant concentration in mg/L and contact time in minutes, so 2.0 mg/L multiplied by 30 minutes equals 60 mg-min/L. The calculated CT is then compared with the required CT for the disinfectant, pH, and water temperature.

Question 4Water Treatment

Which free chlorine species is the more effective disinfectant?

  1. Hypochlorite ion (OCl-)
  2. Hypochlorous acid (HOCl) Correct answer.
  3. Nitrogen trichloride (NCl3)
  4. Monochloramine (NH2Cl)

Explanation

Hypochlorous acid is a small, uncharged molecule that penetrates cell walls readily, making it a far stronger disinfectant than the hypochlorite ion. Because the HOCl/OCl- ratio shifts with pH, lower pH favors faster disinfection.

Question 5Water Treatment

A jar test is performed on a raw water sample primarily to determine which of the following?

  1. The volume of lime softening sludge produced
  2. The disinfection CT requirement
  3. The required filter backwash rate
  4. The optimum coagulant dose and coagulation pH Correct answer.

Explanation

The jar test simulates full-scale coagulation and flocculation on a small scale so the operator can compare several chemical doses and pH values. The dose that produces the best floc formation and clarified supernatant is selected for the plant.

Question 6Water Treatment

Surface overflow rate for a settling basin is normally expressed in which units?

  1. Feet per second
  2. Gallons per minute per foot of weir length
  3. Gallons per day per square foot of surface area Correct answer.
  4. Pounds per day per square foot

Explanation

Surface overflow rate is the flow applied divided by the basin's surface area, giving units of volume per day per unit area. It represents the settling velocity a particle must exceed to be removed in that basin.

Question 7Water Treatment

A filter backwash is generally considered adequate when the media bed has expanded by about which amount?

  1. 100 percent
  2. 2 to 5 percent
  3. 20 to 30 percent Correct answer.
  4. 50 to 75 percent

Explanation

Expanding the bed roughly 20 to 30 percent frees the media so that attached solids can be scoured away without washing media over the troughs. Expansion is observed in a sight glass or through the backwash water clarity.

Question 8Water Treatment

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

  1. Sand has a lower specific gravity, so it floats on anthracite
  2. Anthracite is denser than sand and settles to the bottom
  3. Anthracite is less dense and coarser, allowing solids to penetrate deeper Correct answer.
  4. The two media are the same density and differ only in color

Explanation

Anthracite has a lower specific gravity than sand, so it stays on top after backwashing, and its coarser grains let particles penetrate deeper into the bed. This increases solids storage and lengthens filter runs compared with a single sand bed.

Question 9Water Treatment

When chlorine is fed to water containing ammonia, the free chlorine residual stays near zero as the dose increases until the breakpoint is reached. What best explains this?

  1. Chlorine is insoluble at low doses and settles out of the water
  2. Ammonia raises pH and converts hypochlorous acid to chloride
  3. Fluoride in the water reacts with chlorine before the ammonia does
  4. Chlorine is consumed forming chloramines and other combined residuals Correct answer.

Explanation

Until the breakpoint, added chlorine reacts preferentially with ammonia to form monochloramine, dichloramine, and then nitrogen gas and nitrate, so no free residual persists. Beyond the breakpoint, the ammonia demand is satisfied and free chlorine begins to appear.

Question 10Water Treatment

A finished water sample has a Langelier Saturation Index of -1.2. What does this indicate?

  1. The water is over-fluoridated and must be blended
  2. The water is at equilibrium with calcium carbonate
  3. The water is supersaturated and will deposit scale
  4. The water is undersaturated and tends to be corrosive Correct answer.

Explanation

A negative Langelier index means the water is undersaturated with respect to calcium carbonate, so it tends to dissolve protective scale and attack pipe metal. A positive index indicates a scaling tendency, and zero indicates equilibrium.

Question 11Water Treatment

In lime softening, calcium hardness is removed mainly by precipitating which compound?

  1. Calcium chloride
  2. Sodium chloride
  3. Calcium sulfate
  4. Calcium carbonate Correct answer.

Explanation

Adding lime raises pH and supplies hydroxide, converting soluble calcium bicarbonate to insoluble calcium carbonate, which settles out in the clarifier. Magnesium hardness is removed at higher pH as magnesium hydroxide.

Question 12Water Treatment

Which chemical is commonly fed to oxidize soluble iron and manganese and to control taste and odor?

  1. Potassium permanganate Correct answer.
  2. Sodium chloride
  3. Calcium hypochlorite
  4. Sodium hydroxide

Explanation

Potassium permanganate is a strong oxidant that converts soluble ferrous iron and manganous manganese to insoluble forms that can be filtered out, and it also destroys many taste- and odor-causing organics. It is typically fed ahead of filtration.

Question 13Water Treatment

Chlorine is fed at 5.0 mg/L and the finished water leaving the clearwell contains 1.2 mg/L of free chlorine residual. What is the chlorine demand of the water?

  1. 1.2 mg/L
  2. 3.8 mg/L Correct answer.
  3. 6.2 mg/L
  4. 4.2 mg/L

Explanation

Chlorine demand is the amount consumed by the water and equals the applied dose minus the residual remaining, so 5.0 minus 1.2 equals 3.8 mg/L. Demand must be satisfied before any free residual can be carried into the distribution system.

Question 14Water Treatment

The optimal fluoride concentration in drinking water for preventing dental caries is approximately which value?

  1. 4.0 mg/L
  2. 0.05 mg/L
  3. 0.7 mg/L Correct answer.
  4. 2.0 mg/L

Explanation

About 0.7 mg/L is the recommended optimal fluoride level for community water systems in the United States, balancing caries prevention against the risk of dental fluorosis. The secondary standard of 2.0 mg/L is a maximum level, not a target.

Question 15Water Treatment

Which treatment approach is most commonly used to remove taste and odor compounds such as geosmin?

  1. Chloramination
  2. Powdered activated carbon addition Correct answer.
  3. Lime softening
  4. Fluoridation

Explanation

Powdered activated carbon adsorbs taste- and odor-causing organic molecules and is then removed with the solids in the clarifier or filters. Softening, fluoridation, and chloramination do not remove these organics and can even intensify odor problems.

Question 16Water Treatment

Which chlorine source contains the highest percentage of available chlorine?

  1. Chlorine gas (Cl2) Correct answer.
  2. Sodium hypochlorite solution
  3. Lithium hypochlorite
  4. Calcium hypochlorite

Explanation

Pure chlorine gas is essentially 100 percent available chlorine by weight, which is why it requires the smallest feed quantity per pound of chlorine delivered. Hypochlorite products contain much lower percentages because of their added carrier compounds and water.

Question 17Water Treatment

Aeration of groundwater is most effective for removing which of the following?

  1. Calcium hardness
  2. Nitrate
  3. Dissolved arsenic
  4. Carbon dioxide and hydrogen sulfide Correct answer.

Explanation

Aeration strips volatile dissolved gases such as carbon dioxide and hydrogen sulfide and adds dissolved oxygen that can oxidize iron and manganese. It does not remove hardness, nitrate, or arsenic to any meaningful degree.

Question 18Water Treatment

Which membrane process is used to remove dissolved ions from water?

  1. Granular activated carbon
  2. Ultrafiltration
  3. Microfiltration
  4. Reverse osmosis Correct answer.

Explanation

Reverse osmosis applies pressure greater than osmotic pressure to force water through a semipermeable membrane while rejecting most dissolved ions. Microfiltration and ultrafiltration remove particles and microorganisms, and activated carbon is an adsorbent, not a membrane.

Question 19Water Treatment

Under the Surface Water Treatment Rule, a system using conventional filtration must achieve at least what level of virus removal and inactivation?

  1. 2-log
  2. 5-log
  3. 3-log
  4. 4-log Correct answer.

Explanation

The rule requires conventional filtration systems to achieve at least 4-log removal and inactivation of viruses and 3-log removal and inactivation of Giardia. The required log credits are obtained through a combination of filtration and disinfection CT.

Question 20Water Treatment

A clarifier receives 0.75 MGD and has a total effluent weir length of 100 feet. What is the weir overflow rate?

  1. 75,000 gpd/ft
  2. 7,500 gpd/ft Correct answer.
  3. 1,875 gpd/ft
  4. 13,300 gpd/ft

Explanation

Weir overflow rate is flow divided by weir length, so 750,000 gallons per day divided by 100 feet equals 7,500 gallons per day per foot. Excessive weir loading can cause short-circuiting and carry floc over the weir.

Question 21Water Treatment

A plant treats 2.0 MGD and feeds alum at a dose of 1.5 mg/L. What is the daily chemical usage?

  1. 8.3 lb/day
  2. 25 lb/day Correct answer.
  3. 12.5 lb/day
  4. 50 lb/day

Explanation

The pounds-per-day formula is dose in mg/L multiplied by flow in MGD multiplied by 8.34, so 1.5 times 2.0 times 8.34 equals about 25 pounds per day. The 8.34 factor is the weight in pounds of one gallon of water.

Question 22Water Treatment

A water has an alkalinity of only 20 mg/L as CaCO3 and requires a substantial alum dose. What problem is most likely?

  1. Fluoride feed will become unstable
  2. Chlorine residual will increase unexpectedly
  3. Filter media will expand too much during backwash
  4. pH may drop excessively and coagulation may fail Correct answer.

Explanation

Alum reacts with natural alkalinity, consuming bicarbonate and releasing hydrogen ions; with little alkalinity available, the pH falls sharply and floc formation is impaired. Operators typically add lime or caustic soda to restore alkalinity and pH.

Question 23Water Treatment

What is the first action when a chlorine gas leak is detected in a chemical feed room?

  1. Open the windows and continue working
  2. Pour ammonia solution on the leak
  3. Spray water on the leaking cylinder to absorb the gas
  4. Evacuate upwind and call trained responders Correct answer.

Explanation

Personnel should leave the area immediately, moving upwind, and summon trained responders equipped with self-contained breathing apparatus. Only trained personnel with proper respiratory protection should re-enter to stop the leak.

Question 24Water Treatment

Immediately after backwashing, a filter is often placed in filter-to-waste mode in order to do which of the following?

  1. Prevent a turbidity spike from reaching finished water Correct answer.
  2. Gradually increase the filtration rate
  3. Reduce the volume of backwash water required
  4. Disinfect the filter media with chlorine

Explanation

The first water through a newly backwashed filter carries loosened solids and can show a turbidity spike. Wasting that initial filtrate until turbidity is acceptable protects the finished water and helps meet the combined filter effluent limit.

Question 25Water Treatment

Which treatment change is most commonly used to reduce corrosion in a distribution system?

  1. Increasing the fluoride feed rate
  2. Raising the filter loading rate
  3. Adding a phosphate-based corrosion inhibitor Correct answer.
  4. Shortening the disinfection contact time

Explanation

Phosphate inhibitors form a thin protective film on the interior of pipes that limits metal release and slows corrosion. pH and alkalinity adjustment is often used with them, but the other listed changes have no corrosion-control benefit.

Question 26Water Treatment

What is the primary purpose of rapid mixing in a conventional surface water treatment plant?

  1. To disinfect the water before it reaches the filters
  2. To aerate the water and strip dissolved gases
  3. To disperse coagulant rapidly through the raw water Correct answer.
  4. To build large, dense floc particles that settle quickly in the clarifier

Explanation

Rapid mix provides intense, brief turbulence so the coagulant is spread uniformly and can react with particles before flocculation begins. Floc formation and growth are the job of the later, slower flocculation stage.

Question 27Water Treatment

A jar test is performed at a water treatment plant primarily to determine which of the following?

  1. The chlorine demand of the finished water
  2. The backwash expansion needed for the filters
  3. The optimum coagulant dose and coagulation pH Correct answer.
  4. The filter loading rate that should be used

Explanation

A jar test simulates mixing, flocculation, and settling on small samples at several chemical doses and pH values so the best operating combination can be selected. It is a bench-scale tool for chemical treatment optimization, not for hydraulic or disinfection decisions.

Question 28Water Treatment

What is the function of the graded gravel layer in a rapid sand filter?

  1. It removes dissolved minerals from the water
  2. It supports the media bed and spreads backwash flow evenly Correct answer.
  3. It traps the majority of the turbidity removed during a filter run
  4. It provides a surface for biological treatment of the water

Explanation

The gravel bed keeps the finer sand or anthracite from washing down into the underdrain and helps distribute backwash water across the bottom of the filter. It is a support layer, not a significant treatment layer.

Question 29Water Treatment

Turbidity of treated water measured with a bench meter is reported in which units?

  1. Microsiemens per centimeter
  2. Nephelometric turbidity units Correct answer.
  3. Standard pH units
  4. Milligrams per liter

Explanation

Nephelometric turbidity units express the light scattered at 90 degrees by suspended particles in a calibrated turbidimeter. Milligrams per liter is a mass concentration, pH is a logarithmic acidity scale, and microsiemens per centimeter measures conductance.

Question 30Water Treatment

Which reagent is used in the common colorimetric field test for free chlorine residual?

  1. DPD Correct answer.
  2. Sodium thiosulfate
  3. EDTA
  4. Phenolphthalein

Explanation

DPD reacts with free chlorine to form a pink color whose intensity is read against a color comparator or photometer. It is the standard indicator reagent for chlorine residual testing in the field.

Question 31Water Treatment

What is the main limitation of ultraviolet light as a primary disinfectant in a public water system?

  1. It does not provide a disinfectant residual Correct answer.
  2. It requires much longer contact time than free chlorine
  3. It cannot inactivate bacteria
  4. It leaves a strong taste and odor in the water

Explanation

UV light inactivates organisms at the lamp, but nothing remains in the water to protect against contamination farther downstream in the distribution system. Many utilities therefore use UV together with a residual-producing disinfectant such as chlorine or chloramine.

Question 32Water Treatment

Why do bacteriological sample bottles contain sodium thiosulfate?

  1. To supply nutrients that help bacteria grow
  2. To preserve the sample at a low pH
  3. To keep the sample from freezing during transport
  4. To neutralize any chlorine residual in the sample Correct answer.

Explanation

Sodium thiosulfate destroys the disinfectant residual at the moment of collection so that any organisms present survive until the sample is analyzed. Without it, chlorine in the sample could continue killing bacteria in the bottle and produce a false negative.

Question 33Water Treatment

A sodium-form cation exchange softener is returned to service condition by passing which solution through the bed?

  1. Sulfuric acid
  2. Sodium hydroxide
  3. Brine (sodium chloride solution) Correct answer.
  4. Lime slurry

Explanation

Brine is used to regenerate sodium-form cation exchange resin, replacing the calcium and magnesium captured during service with sodium. The hardness ions are flushed to waste during the rinse that follows.

Question 34Water Treatment

A settling basin is 100 feet long, 40 feet wide, and holds water 12 feet deep. At a flow of 0.5 MGD, what is the detention time?

  1. About 1.4 hours
  2. About 8.6 hours
  3. About 4.8 hours
  4. About 17.2 hours Correct answer.

Explanation

The basin volume is 100 x 40 x 12 = 48,000 cubic feet, which equals about 359,000 gallons or 0.359 million gallons. Dividing that volume by 0.5 MGD gives about 0.72 days, or roughly 17.2 hours.

Question 35Water Treatment

Ozonation of water that contains bromide can produce which disinfection byproduct of concern?

  1. Haloacetic acids
  2. Chlorite
  3. Bromate Correct answer.
  4. Total trihalomethanes

Explanation

Ozone oxidizes naturally occurring bromide to bromate, a regulated byproduct, and the amount formed increases with ozone dose, contact time, and pH. Trihalomethanes, haloacetic acids, and chlorite are associated with chlorine-based disinfectants rather than ozone.

Question 36Water Treatment

Compared with free chlorine, what is an advantage of using chlorine dioxide as a disinfectant?

  1. It is safer to generate and store at the plant
  2. It produces a more persistent residual in the distribution system
  3. It forms fewer trihalomethanes Correct answer.
  4. It costs less per pound of applied disinfectant

Explanation

Chlorine dioxide reacts mainly by oxidation rather than by substituting into organic molecules, so it produces far less trihalomethane than free chlorine. Its drawback is that it must be generated on site and does not leave as durable a residual.

Question 37Water Treatment

Installing tube settlers in an existing settling basin increases its capacity mainly because the tubes do what?

  1. Add coagulant chemicals to the flowing water
  2. Increase the hydraulic detention time of the basin
  3. Reduce the distance a particle must settle before it is captured Correct answer.
  4. Increase the horizontal velocity of flow through the basin

Explanation

Closely spaced inclined tubes give particles a short vertical settling path, so a given particle is removed in much less time than in the full basin depth. The surface area available for settling is effectively increased without enlarging the basin footprint.

Question 38Water Treatment

Dissolved air flotation is generally preferred over settling for which type of water?

  1. Water with very high hardness and alkalinity
  2. Water containing algae and light, low-density floc Correct answer.
  3. Water containing high concentrations of sand and grit
  4. Water with high concentrations of dissolved iron

Explanation

Dissolved air flotation attaches microscopic air bubbles to particles and lifts them to the surface, which works well for algae and floc that settle slowly or tend to float. Heavy, dense solids are handled more economically by plain sedimentation.

Question 39Water Treatment

In a slow sand filter, the biologically active layer that develops on the surface of the sand is known as the what?

  1. Schmutzdecke Correct answer.
  2. Anthracite cap
  3. Underdrain
  4. Support gravel

Explanation

The schmutzdecke is a thin mat of silt, algae, and microorganisms that forms on top of slow sand media and does much of the treatment through biological action and fine straining. When it becomes too restrictive, the filter is cleaned by scraping off the top layer of sand.

Question 40Water Treatment

A river supply carries very high turbidity after storms. What is the purpose of a presedimentation basin ahead of the coagulation process?

  1. To soften the water by precipitating calcium carbonate
  2. To disinfect the raw water before coagulation
  3. To remove a large share of heavy solids before chemical treatment Correct answer.
  4. To lower the dissolved oxygen content before filtration

Explanation

Presedimentation is a plain settling step that drops out sand, silt, and other heavy material so the coagulation and filtration stages are not overwhelmed. This reduces chemical demand and solids loading on the rest of the plant.

Question 41Water Treatment

After excess-lime softening, carbon dioxide is added to the water in order to do which of the following?

  1. Stabilize the water and bring the pH back down Correct answer.
  2. Oxidize dissolved iron and manganese
  3. Raise the pH so that more magnesium precipitates
  4. Disinfect the water and provide a residual

Explanation

Recarbonation with carbon dioxide converts the excess lime and caustic alkalinity to carbonate and bicarbonate, lowering the pH and making the water less scale-forming in the distribution system. Without it, the finished water would tend to deposit calcium carbonate in pipes and on filter media.

Question 42Water Treatment

A clearwell provides 40 minutes of theoretical contact time with a baffling factor of 0.5, and the free chlorine residual at the outlet is 1.5 mg/L. What is the CT value in mg-min/L?

  1. 60
  2. 22.5
  3. 80
  4. 30 Correct answer.

Explanation

CT is the disinfectant residual multiplied by the effective contact time, which is the theoretical time times the baffling factor. Here the effective time is 40 x 0.5 = 20 minutes, and 20 x 1.5 = 30 mg-min/L.

Question 43Water Treatment

What is the most reliable way to determine the actual output of a chemical metering pump?

  1. Test the concentration of the chemical in the storage tank
  2. Assume the manufacturer's rated capacity for that model
  3. Measure the volume delivered into a graduated cylinder over a timed period Correct answer.
  4. Read the stroke setting printed on the pump dial

Explanation

A drawdown or timed-volume test measures what the pump actually delivers under current suction, discharge, and chemical conditions, which is what the dose calculation requires. Dial settings and rated capacities are only indications and drift as valves and diaphragms wear.

Question 44Water Treatment

Which chemical is commonly used to fluoridate drinking water?

  1. Sodium thiosulfate
  2. Aluminum sulfate
  3. Fluorosilicic acid Correct answer.
  4. Potassium permanganate

Explanation

Fluorosilicic acid is one of the standard fluoridation chemicals, along with sodium fluorosilicate and sodium fluoride. Aluminum sulfate is a coagulant, sodium thiosulfate is a dechlorinating agent, and potassium permanganate is an oxidant.

Question 45Water Treatment

A filter measures 20 feet by 25 feet and treats a flow of 1.0 MGD. What is the filtration rate in gallons per minute per square foot?

  1. 2.00 gpm/ft²
  2. 3.47 gpm/ft²
  3. 1.39 gpm/ft² Correct answer.
  4. 0.72 gpm/ft²

Explanation

The filter area is 20 x 25 = 500 square feet, and 1.0 MGD equals 1,000,000 gallons divided by 1,440 minutes, or about 694 gallons per minute. Dividing 694 by 500 gives about 1.39 gpm/ft².

Question 46Water Treatment

After coagulant is added, the measured zeta potential of the particles moves from about -25 mV toward zero. What does this indicate?

  1. The particles are gaining a stronger negative charge and will repel each other more strongly
  2. Charge neutralization is occurring and the particles can now agglomerate Correct answer.
  3. The dose is far above optimum and is restabilizing the particles
  4. The alkalinity of the water has been completely consumed by the coagulant

Explanation

Zeta potential measures the electrostatic charge at the particle surface, and particles with a strong like charge repel each other and stay dispersed. As the coagulant neutralizes that charge and the potential approaches zero, the repulsive barrier drops and particles can collide and stick together.

Question 47Water Treatment

Which treatment combination is most effective for controlling Cryptosporidium oocysts in a surface water supply?

  1. UV disinfection or ozone, together with physical removal by filtration Correct answer.
  2. Chloramination with a long contact time
  3. Aeration followed by plain sedimentation
  4. Free chlorine alone at normally required CT values

Explanation

Cryptosporidium oocysts are highly resistant to chlorination at practical doses but are susceptible to UV light and ozone, and filtration physically removes them from the water. Utilities therefore rely on a combination of robust particle removal and a disinfectant that actually inactivates the organism.

Question 48Water Treatment

A filter run ends after only 8 hours because of rapidly rising head loss, while effluent turbidity stays low. Which condition best explains this?

  1. The filtration rate fell well below the design loading rate
  2. The filtered water turbidity was rising steadily throughout the run
  3. Fine floc or polymer has blinded the surface of the media Correct answer.
  4. The effluent turbidity standard was exceeded early in the run

Explanation

Short runs with steep head loss and good effluent quality point to surface plugging, in which fine material or excess polymer seals the top of the bed instead of penetrating into it. The cure is usually better floc formation or reduced polymer dose, not more frequent backwashing.

Question 49Water Treatment

Under enhanced coagulation requirements, the percentage of total organic carbon that must be removed is set primarily by which raw water characteristic?

  1. Alkalinity Correct answer.
  2. Dissolved oxygen
  3. Turbidity
  4. Temperature

Explanation

The required TOC removal is determined from the source water TOC concentration together with its alkalinity, because high alkalinity water is harder to coagulate effectively at low pH. Utilities then must demonstrate the required removal or show that they are using enhanced coagulation practices.

Question 50Water Treatment

For a system using conventional filtration, combined filter effluent turbidity must be at or below what value in at least 95 percent of monthly measurements?

  1. 1.0 NTU
  2. 5.0 NTU
  3. 0.3 NTU Correct answer.
  4. 0.5 NTU

Explanation

The rule sets 0.3 NTU as the combined filter effluent limit that must be met in at least 95 percent of the measurements taken each month. Any single measurement may not exceed 1 NTU, and individual filters have their own reporting and assessment triggers.