Given the following data for an activated sludge process, calculate the F/M (food to mass) ratio: The aeration tank has a volume of 1.5 million gallons, and the daily influent flow (Q) is 0.99 mgd. The influent total suspended solids (TSS) concentration is 240 mg/L, and the effluent TSS is 20 mg/L. The influent biochemical oxygen demand (BOD) is 320 mg/L, with an effluent BOD of 10 mg/L. The mixed liquor suspended solids (MLSS) concentration in the aeration tank is 2,100 mg/L, and the return activated sludge (RAS) MLSS is 3,240 mg/L. A settleometer reading taken after 30 minutes is 420 mL.
The answer to this question is C, .10. This can be solved using the F/M ratio formula below. To solve this problem, you need to determine the pounds of BOD coming into the plant by multiplying the influent flow (.99 million gallons) by the influent BOD (320 mg/L) then by the constant (8.34). Then determine the pounds of MLSS in the system by multiplying the MLSS (2,100 mg/L) by the volume (1.5 million gallons) then by the constant (8.34). Now divide the pounds of BOD by the pounds of MLSS. F/M ratios are useful for determining the BOD available to the MLSS in the aeration system. Notice the extra data provided in the question, this is done on many examinations as a way of confusing the examinee. This is a pretty simple math problem found on many certification exams but is an essential factor in wastewater treatment.
The F/M formula is as follows:
What disinfectant is less likely to develop trihalomethanes (THMs)?
The correct answer is D, chlorine dioxide. Regarding disinfection, the use of hypochlorite and chlorine gas are similar. The most notable difference between hypochlorite and chlorine gas are the side chemical reactions of the end products. Chlorine gas tends to lower the pH and hypochlorite tends to raise the pH of the treated water. Chlorine dioxide does not react with organics like chlorine gas and/or hypochlorite, so there is less concern with developing THMs. Chlorine dioxide is also an effective disinfectant at elevated pH levels.
