Practice Problems: Wastewater Systems
Question 1
A wastewater engineer is designing a primary clarifier for a municipal treatment plant. The plant receives an average flow of 5.0 MGD with a peak flow of 12.5 MGD. The design specifies a surface overflow rate of 800 gpd/ft² at average flow. What minimum surface area is required for the primary clarifier?
(a) 4,250 ft²
(b) 6,250 ft²
(c) 10,500 ft²
(d) 15,625 ft²
Question 2
A civil engineer is evaluating the hydraulic retention time (HRT) for an activated sludge aeration basin. The basin volume is 850,000 gallons and receives an average influent flow of 3.5 MGD. What is the hydraulic retention time of the aeration basin?
(a) 4.1 hours
(b) 5.8 hours
(c) 7.3 hours
(d) 9.2 hours
Question 3
A wastewater treatment facility operator measures the following parameters for an activated sludge system: MLSS = 2,800 mg/L, aeration basin volume = 1.2 MG, and influent flow = 4.0 MGD with BOD₅ = 180 mg/L. What is the food-to-microorganism (F/M) ratio?
(a) 0.21 day⁻¹
(b) 0.38 day⁻¹
(c) 0.54 day⁻¹
(d) 0.67 day⁻¹
Question 4
A design engineer is sizing a rectangular primary sedimentation tank for a wastewater treatment plant with an average flow of 8.0 MGD. The design criteria specify a detention time of 2.0 hours and a depth of 12 feet. What is the required tank volume?
(a) 89,000 ft³
(b) 133,400 ft³
(c) 178,000 ft³
(d) 222,500 ft³
Question 5
A municipal wastewater treatment plant has an activated sludge system with the following characteristics: aeration basin volume = 2.5 MG, MLSS = 3,200 mg/L, waste activated sludge (WAS) flow = 0.15 MGD, and WAS concentration = 8,500 mg/L. What is the solids retention time (SRT)?
(a) 5.2 days
(b) 6.7 days
(c) 8.4 days
(d) 10.2 days
Question 6
A wastewater engineer is designing a trickling filter for secondary treatment. The filter receives a flow of 2.5 MGD with an influent BOD₅ of 150 mg/L. The filter volume is 45,000 ft³. What is the organic loading rate on the trickling filter?
(a) 52 lb BOD₅/1,000 ft³-day
(b) 69 lb BOD₅/1,000 ft³-day
(c) 87 lb BOD₅/1,000 ft³-day
(d) 104 lb BOD₅/1,000 ft³-day
Question 7
A civil engineer is evaluating a gravity thickener for waste activated sludge. The thickener receives 0.25 MGD of sludge at 0.8% solids concentration and produces thickened sludge at 3.5% solids. Assuming 95% solids capture, what is the thickened sludge flow rate?
(a) 0.048 MGD
(b) 0.054 MGD
(c) 0.062 MGD
(d) 0.071 MGD
Question 8
A wastewater treatment plant operator needs to calculate the oxygen requirements for an activated sludge system. The plant treats 6.0 MGD with influent BOD₅ = 200 mg/L and effluent BOD₅ = 15 mg/L. The waste activated sludge production is 1,200 lb/day. What is the theoretical oxygen demand (not accounting for nitrification)?
(a) 7,830 lb O₂/day
(b) 8,450 lb O₂/day
(c) 9,240 lb O₂/day
(d) 10,120 lb O₂/day
Question 9
A design engineer is sizing a secondary clarifier for an activated sludge plant with a peak flow of 15.0 MGD and MLSS of 3,500 mg/L. The design specifies a maximum solids loading rate of 35 lb/ft²-day at peak flow. What minimum surface area is required?
(a) 10,500 ft²
(b) 12,800 ft²
(c) 15,000 ft²
(d) 18,750 ft²
Question 10
A wastewater engineer is designing a chlorine contact basin for disinfection. The design flow is 4.5 MGD and requires a contact time of 30 minutes at peak flow with a length-to-width ratio of 40:1. If the basin depth is 10 feet, what is the approximate basin length?
(a) 185 feet
(b) 240 feet
(c) 320 feet
(d) 410 feet
Question 11
A municipal wastewater facility has an anaerobic digester with a volume of 125,000 ft³ receiving 18,000 gallons per day of primary and waste activated sludge at 5% solids content. What is the solids retention time in the digester?
(a) 18 days
(b) 22 days
(c) 28 days
(d) 35 days
Question 12
A civil engineer is evaluating an extended aeration activated sludge system treating 1.8 MGD with influent BOD₅ of 220 mg/L. The aeration basin volume is 0.95 MG and MLSS is 4,200 mg/L. What is the volumetric organic loading rate?
(a) 28 lb BOD₅/1,000 ft³-day
(b) 35 lb BOD₅/1,000 ft³-day
(c) 42 lb BOD₅/1,000 ft³-day
(d) 49 lb BOD₅/1,000 ft³-day
Question 13
A wastewater treatment plant uses an oxidation ditch with a volume of 1.5 MG treating 2.8 MGD. The influent BOD₅ is 185 mg/L and the effluent BOD₅ is 12 mg/L. If the MLVSS is 3,800 mg/L and represents 75% of MLSS, what is the observed yield coefficient if SRT is 18 days?
(a) 0.35 lb VSS/lb BOD removed
(b) 0.42 lb VSS/lb BOD removed
(c) 0.51 lb VSS/lb BOD removed
(d) 0.58 lb VSS/lb BOD removed
Question 14
A design engineer is sizing a belt filter press for dewatering waste activated sludge. The sludge feed rate is 50 gpm at 2.5% solids concentration. The cake solids concentration is 18% and the filtrate solids concentration is 400 mg/L. What is the cake production rate in lb/hr?
(a) 2,850 lb/hr
(b) 3,120 lb/hr
(c) 3,650 lb/hr
(d) 4,200 lb/hr
Question 15
A wastewater plant operator monitors a rotating biological contactor (RBC) system treating 1.2 MGD with influent BOD₅ of 160 mg/L. The total media surface area is 850,000 ft² and the effluent BOD₅ is 18 mg/L. What is the hydraulic loading rate on the RBC?
(a) 1.4 gpd/ft²
(b) 2.0 gpd/ft²
(c) 2.8 gpd/ft²
(d) 3.5 gpd/ft²
Question 16
A civil engineer is designing a sequencing batch reactor (SBR) for a small community. The design flow is 0.50 MGD with peak flow of 1.2 MGD. Using four reactors with a total cycle time of 6 hours (1 hr fill, 2 hr react, 1.5 hr settle, 0.5 hr decant, 1 hr idle), what minimum working volume is required per reactor?
(a) 52,000 gallons
(b) 75,000 gallons
(c) 95,000 gallons
(d) 125,000 gallons
Question 17
A wastewater treatment facility has a dissolved air flotation (DAF) unit for thickening waste activated sludge. The unit receives 0.18 MGD at 0.6% solids and produces thickened sludge at 3.2% solids with 92% capture efficiency. What is the float (thickened sludge) production rate?
(a) 24,500 gpd
(b) 30,800 gpd
(c) 34,200 gpd
(d) 41,600 gpd
Question 18
A design engineer is evaluating a membrane bioreactor (MBR) system treating 3.5 MGD. The system operates with MLSS of 8,500 mg/L, membrane flux of 18 gfd (gallons per square foot per day), and produces effluent with TSS < 5="" mg/l.="" what="" membrane="" area="" is="">
(a) 125,000 ft²
(b) 155,000 ft²
(c) 194,500 ft²
(d) 225,000 ft²
Question 19
A wastewater engineer is analyzing a two-stage nitrification system. The first stage oxidizes ammonia to nitrite and the second stage oxidizes nitrite to nitrate. The influent contains 35 mg/L NH₃-N and the flow is 7.5 MGD. Assuming complete nitrification, what is the theoretical oxygen requirement for the nitrification process?
(a) 7,300 lb O₂/day
(b) 8,950 lb O₂/day
(c) 10,200 lb O₂/day
(d) 11,650 lb O₂/day
Question 20
A civil engineer is designing a UV disinfection system for a wastewater treatment plant with a peak flow of 10.0 MGD. The target UV dose is 100 mJ/cm² to achieve a 3-log reduction of fecal coliform. The UV transmittance (UVT) at 254 nm is 65%. If each UV lamp delivers 40 watts and the system efficiency is 35%, approximately how many UV lamps are required?
(a) 145 lamps
(b) 185 lamps
(c) 225 lamps
(d) 280 lamps
