At a drinking-water plant, chemicals are added so tiny suspended particles clump together before settling. Which stage is being used?

After floc forms in drinking water, the water is held still so the heavy clumps sink. What stage removes those clumps?

A water utility passes clarified water through sand and other media to remove clays, silts, and remaining microorganisms. What treatment stage is this?

Before treated drinking water enters distribution pipes, chlorine is added to kill potentially dangerous microbes. Which stage is this?

A plant has removed floc by settling but still needs to remove smaller particles before disinfecting the water. Which stage should come next?

Testing finds arsenic in source water that is not adequately removed by filtration or sedimentation. Which treatment process can remove this inorganic contaminant?

Raw water has an objectionable odor and contains organic compounds. Which treatment process is best suited to remove such contaminants?

Wastewater first passes through rows of metal bars that catch rags, cans, and sticks before they can clog pumps. What is this removal step called?

After screening, sewage flows slowly through a unit where sand, cinders, and small stones settle out. Which unit is this?

In a wastewater plant, flow slows in a tank and suspended solids sink to the bottom before any biological treatment. Which process is removing the solids?

In a wastewater tank, grease and scum with trapped air rise to the top of the water. How is this material removed?

Biomass sloughs off a trickling filter after microbes consume organic matter. Which unit removes this suspended biomass from the effluent?

A wastewater facility must reduce disease-causing microorganisms in its final effluent before discharge. Which treatment step addresses that need?

A plant adds alum so a nutrient that feeds algal blooms clumps into heavier masses that can settle out. Which wastewater-treatment approach is being used?

Primary-treated sewage enters an aeration tank where microbes feed on the waste. What is the main result of this secondary treatment?

Why is air mixed into an activated-sludge aeration tank?

A trickling filter sends wastewater over rocks or plastic media exposed to air. What is the main function of the media?

Why does secondary biological treatment lower biochemical oxygen demand in wastewater?

In an activated-sludge system, microbes multiply while using sewage as food. Why must excess microbial growth later be removed?

Untreated sewage with high biochemical oxygen demand enters a stream. As decomposers break down the organic matter, what is most likely to happen to dissolved oxygen?

A treatment plant relies on microbial processes after primary settling. Which pollutant is least likely to be removed mainly by that biological stage?

At a conventional septic system, where does liquid effluent receive final treatment after leaving the septic tank?

What happens to heavy solids and oils in a properly functioning septic tank?

A household sends far more wastewater to its drainfield than the soil can absorb. What failure is most likely?

Why must a conventional drainfield be installed in unsaturated soil?