Water Resources and Environmental Engineering Flashcards

Which coagulant is most commonly used in conventional US drinking water treatment?

Answer: Aluminum sulfate (alum)

Alum is the most widely used coagulant because its hydrolysis destabilizes colloidal particles.

In drinking water disinfection, the 'CT' value is the product of:

Answer: Residual disinfectant concentration and contact time

CT = residual concentration (mg/L) × contact time (min) and is used to confirm pathogen inactivation.

Water hardness is mainly caused by which dissolved ions?

Answer: Calcium and magnesium

Hardness is mostly the sum of divalent cations, mainly Ca²⁺ and Mg²⁺.

In lime-soda ash softening, soda ash (Na₂CO₃) is added mainly to remove:

Answer: Noncarbonate hardness

Lime removes carbonate hardness, and soda ash supplies the carbonate needed to precipitate noncarbonate hardness.

A sedimentation basin treats 1 MGD and has a surface area of 1,000 ft². What is its overflow rate?

Answer: 1,000 gpd/ft²

Overflow rate = Q/A = 1,000,000 gpd ÷ 1,000 ft² = 1,000 gpd/ft².

In breakpoint chlorination, the breakpoint is the point at which:

Answer: Chloramines are destroyed and free chlorine residual starts to build

Past the breakpoint, added chlorine shows up as free available chlorine residual.

Slow sand filters remove contaminants mainly through:

Answer: A biological layer called the schmutzdecke

The schmutzdecke is a biologically active surface layer that strains and biodegrades contaminants.