Grade 4 Wastewater Treatment Practice Questions

What is the primary purpose of bar screens in preliminary wastewater treatment?

• ATo remove dissolved solids
• BTo remove large debris and rags that could damage downstream equipment
• CTo reduce BOD
• DTo disinfect the influent

A tank holds 50,000 gallons and receives a flow of 100,000 gallons per day. What is the detention time?

• A6 hours
• B12 hours
• C24 hours
• D48 hours

What is the primary purpose of grit removal in preliminary wastewater treatment?

• ATo remove dissolved organics
• BTo remove heavy inorganic material like sand and gravel that could damage pumps or settle in pipes
• CTo disinfect the wastewater
• DTo remove nitrogen

A primary clarifier has a surface area of 1,000 ft² and receives a flow of 1.0 MGD. What is the surface overflow rate?

• A100 gpd/ft²
• B500 gpd/ft²
• C1,000 gpd/ft²
• D10,000 gpd/ft²

Why is dechlorination often required after disinfecting wastewater effluent with chlorine?

• ATo increase the effluent's chlorine residual
• BTo neutralize residual chlorine before discharge, since it is toxic to aquatic life
• CTo reduce BOD
• DTo add nutrients back to the water

A plant receives a flow of 2.0 MGD with a BOD concentration of 220 mg/L. What is the BOD loading in lbs/day?

• A440 lbs/day
• B1,834 lbs/day
• C3,670 lbs/day
• D7,340 lbs/day

What is the main difference between primary and secondary treatment in a wastewater plant?

• APrimary removes settleable solids by gravity; secondary uses biological processes to remove dissolved and colloidal organic matter
• BPrimary disinfects; secondary filters
• CPrimary removes nutrients; secondary removes solids
• DThere is no meaningful difference

In the activated sludge process, what does a high sludge volume index (SVI) typically indicate?

• AExcellent settling sludge
• BPoor settling / bulking sludge
• CComplete nitrification
• DLow dissolved oxygen only

A 1-liter sample of mixed liquor settles to 250 mL after 30 minutes. The MLSS concentration is 2,500 mg/L. What is the SVI?

• A10 mL/g
• B100 mL/g
• C250 mL/g
• D1,000 mL/g

What is the primary function of the secondary clarifier following the aeration basin?

• ATo add oxygen to the mixed liquor
• BTo separate and settle biological solids (sludge) from the treated water, allowing clear effluent to move forward
• CTo disinfect the effluent
• DTo remove grit

An aeration basin receives 1,800 lbs/day of BOD and holds 3,000 lbs of MLVSS. What is the F:M ratio?

• A0.30
• B0.60
• C1.67
• D5,400

Which condition is generally required to promote nitrification (ammonia to nitrate conversion)?

• ALow dissolved oxygen and short SRT
• BAdequate dissolved oxygen and a sufficiently long solids retention time (SRT)
• CHigh organic loading and low pH
• DAbsence of oxygen

What is the purpose of return activated sludge (RAS) in the activated sludge process?

• ATo send treated effluent back to the headworks
• BTo return settled biological solids from the secondary clarifier back to the aeration basin to maintain the microbial population
• CTo dispose of biosolids off-site
• DTo add chlorine to the aeration basin

A trickling filter has a surface area of 2,000 ft² and receives a BOD loading of 100 lbs/day. What is the organic loading rate, in lbs BOD per day per 1,000 ft²?

• A5
• B20
• C50
• D200

What distinguishes Class A biosolids from Class B biosolids?

• AClass A biosolids have met more stringent pathogen-reduction requirements, allowing fewer land-application restrictions
• BClass B biosolids are always safer to use than Class A
• CClass A biosolids must always be incinerated
• DThere is no meaningful difference between them

An aeration basin and clarifier together hold 12,000 lbs of MLSS. The plant wastes 800 lbs/day of solids. What is the mean cell residence time (MCRT)?

• A5 days
• B10 days
• C15 days
• D20 days

A wastewater treatment plant exceeds its permitted effluent limit for BOD. What is the plant generally required to do under its NPDES permit?

• ANothing — minor exceedances are ignored
• BReport the exceedance to the permitting authority and take corrective action per the permit
• CImmediately shut the plant down
• DOnly report if it happens three times in one year

Which method is most commonly used to remove phosphorus from wastewater beyond what biological treatment alone achieves?

• AAdding chlorine
• BChemical precipitation using a metal salt such as alum or ferric chloride
• CIncreasing detention time in the bar screen
• DAdding more grit removal capacity

For NPDES compliance monitoring, why might a permit require a 24-hour composite sample rather than a single grab sample?

• AComposite samples are cheaper to collect
• BA composite sample better represents the average characteristics of a variable wastewater flow over the sampling period
• CGrab samples are not allowed under any permit
• DComposite samples don't need to be refrigerated

A plant wastes activated sludge (WAS) at a flow of 20,000 gpd with a solids concentration of 7,500 mg/L. How many pounds of solids are wasted per day?

• A125 lbs/day
• B625 lbs/day
• C1,251 lbs/day
• D2,502 lbs/day