Water and Wastewater Microbiology (Exam Questions)

Everything you need to know about water and wastewater microbiology. Some of the frequently asked exam questions are as follows:-

Q.1. Give the steps of typical sewage treatment plant are as given below

Ans: The steps adopted in a typical sewage treatment plant are as given below:

(i) Primary treatment which comprises two parts:

(a) Screening, skimming and grinding of sewage.

(b) Settling of solid matter.

(ii) Secondary treatment or biological oxidation:

In it the primary effluent undergoes aeration where microorganisms oxidize organic matter. In secondary treatment either a trickling filter is used or an activated sludge (aeration tank) is used but not both in a particular system.

(iii) Disinfection and release:

The effluent is disinfected by chlorination and released.

(iv) Sludge digestion:

The sludge received from the settling tank of step:

(1) And the settling tank of secondary treatment of step

(2) Is digested in anaerobic sludge digesters. The sludge digestion is followed by drying of sludge in drying beds and then the final step is to remove the dried sludge either to a landfill or to agricultural land. The methane produced in a sludge digester is either burnt or is used for power heaters.

Q.2. What is biochemical oxygen demand (BOD)?

Ans: The BOD is a measure of biologically degradable organic matter in any water. It is measured by the amount of oxygen required by bacteria to metabolize the organic matter present in the water sample.

Q.3. How is BOD measured?

Ans: The BOD bottles with airtight stoppers are filled with test water samples or dilutions of test water samples. The water is initially aerated to provide a relatively high level of dissolved oxygen and is seeded with bacteria if needed. The filled bottles are incubated at 20°C for 5 days and the decrease in dissolved oxygen is measured using a chemical or electronic testing method. The more the amount of oxygen used by the bacteria to degrade the organic matter of the sample, the higher will be the BOD which is expressed in milligrams (mg) of oxygen per litre of water.

Q.4. The trickling filters remove 80% to 85% of the BOD while activated sludge systems remove 75% to 95% of BOD and hence are comparatively less efficient. Why are trickling filters still used?

Ans: They are more convenient to operate and have fewer problems from overloads of toxic sewage.

Q.5. What is a trickling filter?

Ans: In a trickling filter the sewage is sprayed over a bed of rocks or moulded plastic honeycomb like structure.

Q.6. Name the common bacterial species which form the flocculent masses (floes) in activated sludge systems, in which the bacterial cells are embedded in a gelatinous matter.

Ans: Zoogloea.

Q.7. What is it that causes bulking of sludge?

Ans: When aeration is stopped in a sludge digester the sludge floats without settling out.

Q.8. What is the permissible number of coliforms for discharge of sewage after treatment, in a small water body and in the ocean, respectively?

Ans: 23 coliforms, per 100 ml for a small water body and 240 coliforms per 100 ml for discharge in the ocean.

Q.9. What is a lagoon or an oxidation pond?

Ans: Some small communities and industries use the oxidation ponds or stabilization ponds also called lagoons for the treatment of water from sewage. The system comprises two stages. The first is analogous to primary treatment and the second stage corresponds to secondary treatment.

Q.10. It is difficult to aerate lagoons which are large. What is the alternative to maintain aerobic conditions to lower CO₂ generation during bacterial action of decomposing organic matter?

Ans: By growing algae.

Q.11. What is tertiary treatment of sewage?

Ans: When primary and secondary treatments are not able to remove all the biologically degradable material and particularly while the water is discharged in small streams or recreational lakes, it is then that tertiary treatment plants are developed.

Q.12. What is done for the removal of nitrogen and phosphorus in tertiary treatment?

Ans: The nitrogen is converted to ammonia and is released using denitrifying bacteria; and phosphorus is precipitated by combining with chemicals like alum, lime and ferric chloride.

Q.13. What is the constituent of water on which the concentration of bacteria depends?

Ans: Concentration of bacteria is proportional to the amount of organic matter present in the water.

Q.14. What is the significance of waves in a water mass?

Ans: The wave action increases the amount of dissolved oxygen (DO).

Q.15. What is sewage?

Ans: It is domestic wastewater.

Q.16. What is the main objective of primary sewage treatment?

Ans: Primary sewage treatment is meant to remove the solid matter known as sludge.

Q.17. What is secondary treatment of sewage?

Ans: It is the biological degradation of organic matter in sewage and follows primary treatment.

Q.18. What is a septic tank?

Ans: Septic tanks are designed for primary treatment of sewage in rural areas. They need large leaching fields for the effluent.

Q.19. What is the purpose of oxidation tanks? What is their limitation?

Ans: Small communities can use oxidation tanks also called lagoons for the secondary treatment. Their drawback is that they need large areas for artificial lakes or ponds.

Q.20. What are the main objectives of tertiary treatment?

Ans: The main objectives of tertiary treatment are:

(1) Physical filtration and chemical precipitation to remove all the BOD, nitrogen and phosphorus.

(2) Tertiary treatment provides drinkable water while secondary treatment provides water which may be used for irrigation only.

Q.21. Why do municipal landfills prevent decomposition of solid wastes?

Ans: Because they are dry and anaerobic.

Q.22. What can be done to promote biodegradation of organic matter in a municipal landfill?

Ans: Composting and continuous flow digesters can be used.

Q.23. What is a micros-trainer and an immedium filter?

Ans: In tertiary sewage treatment the micros-trainer is a hollow cylinder of fine mesh stainless steel fabric which rotates on a horizontal axis while an immedium filter is a sand filter.

24. How do heavy metals and their compounds lead to pollution?

Ans: There is no doubt that traces of certain heavy metal ions are essential for the growth of microorganisms. However, a higher concentration may exhibit antimicrobial Activity. Heavy metal ions bind to certain groups, particularly the thiol group and exert antimicrobial activity largely by inactivating proteins, nucleic acids and other microbial constituents. Typically they have little or no activity against bacterial spores.

Antimicrobial activity of mercury ions may be reversed by the presence of substances like cysteine, glutathione, etc. Antimicrobial activity of metals can also be antagonized by other substances, e.g., the inhibition of Lactobacillus species by zinc can be reversed by addition of manganese. Some ions appear to have antimicrobial activity only when they are in a particular redox state.

25. How do some bacteria show resistance to heavy metals?

Ans: In bacteria resistance to heavy metals and their compounds may be plasmid borne. Thereby some bacteria can synthesize an inducible plasmid encoded NADPH linked mercuric ions.

26. What is leaching and pollution of ground water? Elaborate.

Ans: Leaching is basically the process of water or a liquid slowly filtering through the perforation in soil. Any material on the surface or in the soil may leach into and lead to pollution of ground water, e.g., many wells become contaminated with toxic nitrogen compounds that come from fertilizers used in surrounding agricultural fields.

Leaching from municipal landfills, i.e., city trash buried in ground often contaminates ground water, particularly while chemical waste materials from industry are disposed of in landfills or deep wells without taking proper precautions.

Thus the ground water gets polluted with leached chemical more and more. Mining activity may also add to ground water pollution as surface mining disturbs large amounts of earth in the process of extracting desired materials. In mining raw rock surfaces are exposed to water draining, e.g., the sulphate (SO₄^—) present in rock and soil minerals may be leached out as sulphuric acid. Acid drains from coal mines lead to spoilage of surface as well as ground water.

27. Give some of antimicrobial applications of heavy metals that may also lead to pollution of water.

Ans: Heavy metals and their compounds have been used as agricultural antifungal agents, as antiseptics as chemotherapeutic agents for the treatment of diseases of bacterial and protozoal (causation), as preservatives for textiles and wool and in disinfection of water.

28. How do Some Bacteria Detoxify Heavy Metals?

Ans: Reductase that can reduce Hg⁺₂ to relatively nontoxic metallic mercury which is readily lost from the cell. Such cells may also synthesize an organomercural lyase that cleave mercury bond in organomercurals such as phenyl mercuric acetate. Pseudomonas aeruginosa, a Gram-negative bacteria can reduce Hg²⁺ to HgO which possesses genes for mercury resistance on a plasmid. These genes are known as mer genes that are arranged in an operon and controlled by the regulatory protein Mer R.

It is interesting to note that Mer R functions as both repressor and an activator. The mercuric reductase is produced by Mer A gene and Mer D, which is the product of mer D also plays a regulatory role while mer P encodes a periplasmic Hg²⁺ binding protein called Mer P.The Mer P binds Hg²⁺ and transfers it to a membrane protein called Mer T. The Mer T is a product of mer T and Transports Hg²⁺ into the cell for reduction by the enzyme mercuric reductase.

In this way Hg²⁺ is reduced to HgO which being volatile gets released from the cell. In Staphylococcus aureus plasmid encoded resistance to cadmium may involve pnf dependent efflux of cadmium ions via a cadmium proton antiporter which prevents intracellular accumulation of Cd²⁺.