Wastewater Treatment Methods Questions and Answers

Answer: A

Settleable solids and floating matter are removed in primary classifiers (primary clarifiers).

In the primary sedimentation tank, sewage flows slowly enough for settleable organic solids to sink as primary sludge, while grease, oil and other floating matter rise and are skimmed off as scum. It typically removes about 50-70% of suspended solids and a quarter to a third of the BOD.

The bar screen comes first and catches large objects such as rags and plastics. The grit chamber removes heavy inorganic grit and sand, not organic settleable solids. An equalization tank evens out variations in flow and strength; it does not separate solids.

Answer: D

Of the steps listed, sedimentation comes first.

Raw surface water first passes through screens and then into settling basins, where plain sedimentation lets the heavy suspended matter (sand, silt) settle under gravity. That reduces the load on everything downstream. Only then is a coagulant such as alum added, followed by flocculation and further settling, then filtration through sand beds, and finally disinfection by chlorination.

So filtration and chlorination are late stages, and coagulation follows the initial settling. The usual textbook sequence is screening, sedimentation, coagulation, filtration and disinfection, which puts sedimentation first among the four.

Answer: C

An oxidation pond works through bacteria and algae together.

An oxidation (stabilisation) pond depends on a symbiosis. Aerobic bacteria break down the organic matter of the sewage into carbon dioxide, water and nutrients such as ammonia and phosphate. Algae use that carbon dioxide and those nutrients for photosynthesis and release oxygen, which the bacteria need to keep oxidising the waste.

Bacteria alone would soon use up the oxygen, and algae alone do not break down the organic load, so neither single option is right. 'All of the above' adds nothing beyond the bacteria and algae of option C, which names the two partners together.

Answer: B

Biological treatment removes dissolved organic solids.

Secondary (biological) treatment, such as the activated sludge process or a trickling filter, uses micro-organisms that feed on the dissolved and colloidal organic matter that settling cannot remove. The organisms oxidise part of it to carbon dioxide and water and turn the rest into new cells, which settle as sludge. This is what lowers the BOD of the effluent.

Suspended organic solids are mostly removed earlier by primary sedimentation. Dissolved inorganic solids, such as salts, pass through biological treatment largely unchanged and need tertiary processes like ion exchange or reverse osmosis, so 'all of the above' is also wrong.

Answer: C

The attached growth process is the rotating biological contactor.

In attached growth (fixed film) processes, micro-organisms grow as a film on a solid surface and the wastewater passes over them. A rotating biological contactor is a set of closely spaced discs on a shaft, partly submerged and slowly rotating, so the biofilm on the discs takes up organic matter while submerged and oxygen while in the air. Trickling filters are the other classic example.

The activated sludge process, the anaerobic contact process and the upflow anaerobic sludge blanket are suspended growth processes: the organisms are held in suspension as flocs or granules in the liquid, not fixed to a medium.

Answer: B

The correct matching is a-iii, b-i, c-iv, d-ii.

• (a) Stripping removes volatile compounds by passing air or steam through the waste: ethanol.
• (b) Precipitation converts dissolved toxic metals into insoluble hydroxides or sulphides that settle out.
• (c) Sorption on activated carbon removes dissolved organic solvents such as xylene.
• (d) Solvent extraction transfers oils and organochlorine compounds into an immiscible solvent.

The fastest check is precipitation, the standard method for heavy metals: only option B pairs precipitation with toxic metals. Option A pairs precipitation with oils, option C with ethanol and option D with xylene, none of which is removed by precipitation.

Answer: C

Facultative ponds involve aerobic and anaerobic processes.

A facultative pond is a waste stabilization pond about 1 to 2 m deep with three zones. In the upper layer, algae release oxygen by photosynthesis and aerobic bacteria oxidise organic matter. At the bottom, settled sludge is broken down anaerobically. In between, facultative bacteria work with or without oxygen. Both kinds of process therefore operate together.

A purely aerobic or purely anaerobic process describes only part of the pond, and 'all of the above' would wrongly count each of those partial descriptions as correct.

Answer: D

Anaerobic treatment has all of the above advantages.

Anaerobic digestion breaks down organic matter without oxygen. It reduces the COD and BOD of strong effluents such as those from distilleries and dairies. It generates methane-rich biogas that can be used as fuel. And because no aeration is needed, its power requirement is much lower than that of aerobic treatment; it also produces less sludge.

Each listed advantage is real, so choosing only one of them leaves out the others.

Answer: D

The correct sequence is screening → grit removal → clarification → biological treatment.

Treatment starts with preliminary steps that protect later units. Screens remove large floating objects such as rags and plastics first. Grit chambers then settle sand and gravel that would wear pumps. Primary clarification (sedimentation) removes settleable organic solids, and the clarified sewage then goes to biological (secondary) treatment such as activated sludge or trickling filters.

Grit removal before screening would let large objects damage the grit chamber, and biological treatment cannot come first. Putting biological treatment straight after grit removal leaves out primary clarification, which the standard sequence places before it.

Answer: B

Chlorination is done to remove Bacteria.

Chlorine is a disinfectant: as hypochlorous acid it penetrates and kills pathogenic bacteria and inactivates many viruses, and a small residual protects the water in the distribution pipes. It is usually the last step of treatment.

The other targets are dealt with by other steps. Hardness (calcium and magnesium) is removed by lime-soda softening or ion exchange. Sediments and suspended solids are removed by plain sedimentation, coagulation with alum and filtration, all of which come before disinfection.