Water Resources and Environmental Engineering Flashcards

A wastewater has BOD₅ = 200 mg/L and k = 0.23/day (base e). What is the approximate ultimate BOD?

Answer: 293 mg/L

L₀ = BOD₅/(1 − e^(−0.23×5)) = 200/0.683 ≈ 293 mg/L.

In activated sludge, the solids retention time (SRT) is defined as:

Answer: Mass of solids in the system divided by mass of solids wasted per day

SRT is the average time biomass stays in the system, equal to solids inventory divided by solids leaving per day.

In activated sludge design, a high food-to-microorganism (F/M) ratio usually leads to:

Answer: Poorer settling and lower treatment efficiency

A high F/M means more substrate per unit of biomass, which favors dispersed growth and poor flocculation.

Nitrification converts:

Answer: Ammonia to nitrite and then to nitrate

Nitrosomonas oxidizes NH₄⁺ to NO₂⁻, and Nitrobacter oxidizes NO₂⁻ to NO₃⁻ under aerobic conditions.

Biological denitrification requires:

Answer: Anoxic conditions and an organic carbon source

Denitrifying bacteria use nitrate as the electron acceptor when no oxygen is present, and they need a carbon source such as methanol.

A mixed liquor sample settles to 300 mL/L in 30 minutes, and MLSS is 3,000 mg/L. What is the sludge volume index (SVI)?

Answer: 100 mL/g

SVI = (300 × 1,000)/3,000 = 100 mL/g, which indicates good settling.

Which chemicals are commonly used to remove phosphorus from wastewater by precipitation?

Answer: Alum or ferric chloride

Aluminum and iron salts react with orthophosphate to form insoluble precipitates.