Wastewater Treatment Processes Questions and Answers

Answer: C

The unit that needs a large area is the Oxidation pond.

An oxidation (stabilisation) pond treats sewage in a shallow open basin, typically 1 to 1.5 m deep, where algae supply oxygen through photosynthesis and bacteria break down the organic matter. Because it depends on sunlight and a long detention time, it needs a large land area, which is why it suits small towns where land is cheap.

Biological (trickling) filters and septic tanks are compact units that treat sewage in a much smaller footprint.

Answer: B

Pre-treatment removes grit and large suspended material.

The preliminary stage of a sewage treatment plant protects the pumps and later units. Bar screens remove large floating and suspended objects such as rags, plastics and wood, and grit chambers remove sand, gravel and other heavy inorganic particles that would abrade equipment and settle in tanks.

Large suspended material alone leaves out grit removal, which is the other essential preliminary unit. Settleable (printed 'Settlable') organic solids are removed in the primary sedimentation tank, the next stage. Oil and grease call for skimming tanks, which only some plants provide, so they are not the standard content of pre-treatment.

Answer: C

The system is the rotating biological contactor (printed 'contractors').

A rotating biological contactor consists of closely spaced discs of corrugated plastic, often 2 to 4 metres in diameter, mounted on a horizontal shaft and about 40 per cent submerged in the wastewater. As the shaft turns slowly, the biofilm on the discs is dipped in the wastewater to take up organic matter and then lifted into the air to take up oxygen.

A trickling filter is a fixed bed over which wastewater is sprayed, a sequencing batch reactor is a suspended-growth activated sludge system run in batches, and an in-vessel submerged system has its media fully immersed, not on a rotating shaft.