Water and Wastewater Treatment Flashcards

A water treatment plant uses rapid sand filtration. The design filtration rate is 4 gpm/ft². If the filter box is 20 ft × 30 ft, what is the design flow through this single filter?

Answer: 2,400 gpm

Flow = rate × area = 4 gpm/ft² × (20 × 30) ft² = 4 × 600 = 2,400 gpm.

Which disinfection byproduct (DBP) is primarily formed when chlorine reacts with natural organic matter (NOM) in drinking water?

Answer: Trihalomethanes (THMs)

Trihalomethanes (THMs) such as chloroform are the primary DBPs formed when free chlorine reacts with NOM.

In a conventional activated sludge system, the sludge volume index (SVI) is 120 mL/g and the MLSS is 3,000 mg/L. What is the settled sludge volume after 30 minutes?

Answer: 360 mL/L

SVI = (settled volume mL/L × 1,000) / MLSS mg/L, so settled volume = SVI × MLSS / 1,000 = 120 × 3,000 / 1,000 = 360 mL/L.

A clarifier has a surface overflow rate of 800 gpd/ft². If the flow is 1.5 MGD, what is the required surface area?

Answer: 1,875 ft²

Area = Q / SOR = 1,500,000 gpd / 800 gpd/ft² = 1,875 ft².

Which of the following best describes the purpose of alkalinity in coagulation during water treatment?

Answer: It provides bicarbonate/carbonate ions that react with coagulant to form floc

Alkalinity (bicarbonate/carbonate) reacts with alum or ferric coagulants to form hydroxide floc and release CO₂.

In wastewater treatment, what does a Return Activated Sludge (RAS) ratio of 0.5 mean?

Answer: 50% of the influent flow is returned from the clarifier

RAS ratio = RAS flow / influent flow, so 0.5 means the return flow is 50% of the incoming flow.

The CT concept in disinfection represents:

Answer: Disinfectant concentration times contact time

CT = disinfectant concentration (mg/L) × contact time (min), used to ensure adequate inactivation of pathogens per the Surface Water Treatment Rule.