# Energy Engineering Questions and Answers – Problem involved in Biogas Production

This set of Energy Engineering Problems focuses on “Problem involved in Biogas Production”.

1. For minimum of how many days within starting of fermentation, initiates anaerobic bacterial action?
a) 2
b) 10
c) 14
d) 20

Explanation: The storage of raw materials in a damp, confined space for over ten days initiates anaerobic bacterial action that, though causing some gas loss, reduces the time for the digester to become operational.

2. What should be the ratio of raw material to water?
a) 1:1
b) 4:1
c) 5:2
d) 3:6

Explanation: Experience has shown the raw material (domestic and poultry wastes and manure) ratio to water should be 1:1, i.e., 100kg of excrete to 100 kg of water. In the slurry, this corresponds to a total solids concentration of 8-11 per cent by weight.

3. Higher loading rates have been used when the ambient temperature is ____________
a) High
b) Low
c) At 0o
d) At medium temperature

Explanation: The size of the digester depends upon the loading, which is determined by the influent solids content, retention time and the digester temperature. Optimum loading rates vary with their different digesters and sites of location. Higher loading rates have been used when the ambient temperature is high.

4. What is the full form of TVS?
a) Tempered volatile solid
b) Total variant solid
c) Total volatile solid
d) Termed variant solid

Explanation: In general, the literature is filled with a variety of conflicting loading rates. In practice, the loading rate should be an expression of either (a) Weight of total volatile solids (TVS) added per day per unit volume of digest, or (b) the weight of TVS added per day per unit weight of TVS in the digester.

5. ______________ is a major factor in determining the methane yield and methane production rates from the digestion of biomass.
a) Seeding
b) Contamination
c) Substrate composition
d) Moisture content

Explanation: Substrate composition is a major factor in determining the methane yield and methane production rates from the digestion of biomass. Techniques to determine the compositional characteristics of the feedstock are available, while parameters such as solids, elemental, and organic analyses are important for digester design and operation.
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6. What is anaerobic digestion PH range?
a) 6.0 – 8.0
b) 1.3 – 4.2
c) 2.5 – 6.5
d) 7.2 – 10.0

Explanation: Low pH inhibits the growth of the methanogenic bacteria and gas generation and is often the result of overloading. A successful pH range for anaerobic digestion is 6.0 – 8.0; efficient digestion occurs at a pH near neutrality.

7. With a mesophilic flora digestion precedes best at what temperature?
a) 30 – 40oC
b) 65 – 75oC
c) 10 – 30oC
d) 10 -15oC

Explanation: With a mesophilic flora, digestion precedes best at 30 – 40oC; with thermophiles, the optimum range is 50 – 60oC. The choice of the temperature to be used is influenced by climatic considerations.

8. Which of the following two are most important nutrients?
a) Carbon and nitrogen
b) Methane and carbon
c) Methane and oxygen
d) Methane and nitrogen

Explanation: The maintenance of optimum microbiology activity in the digester is crucial to gas generation and consequently is related to nutrient availability. Two of the most important nutrients are carbon and nitrogen.

9. What does excess availability of nitrogen lead?
a) NO3
b) NO2
c) NH3
d) NH4

Explanation: Excess availability of nitrogen leads to the formation of NH3, the concentration of which inhibits further growth. Ammonia toxicity can be remedied by low loading or by dilution. So in practice it is necessary to maintain the levels of nitrogen.

10. How is potential toxicity, due to ammonia is corrected?
a) By dilution
d) By adding extra carbon rich content

Explanation: Wastes and biodegradable residue are often accompanied by a variety of pollutants that could inhibit anaerobic digestion. Potential toxicity can be corrected by remedying the C/N ratio of manure by dilution.

11. Which of the following salt group is stimulatory or toxic in action?
a) Sodium, potassium, calcium and magnesium
b) Copper, zinc, sodium and magnesium
c) Copper, calcium, magnesium and potassium
d) Sodium, magnesium, zinc, and potassium

Explanation: Salts of sodium, potassium, calcium and magnesium may be stimulatory or toxic in action, both manifestations being associated with the cat-ion rather than the anionic portion of the salt. Pesticides and synthetic detergents may also be troublesome to the process.

12. Agitation is performed ___________
a) Mechanically
b) Manually
c) By help of cattle
d) Feedstock

Explanation: Agitation can be done either mechanically or with a plunger or by means of rotational spraying of fresh influent. Agitation is normally required for both digesters, ensures exposure of new surfaces to bacterial action prevents viscid satisfaction and slow down of bacterial activity.

13. Which is the initial issue of the biogas plant?
a) Feedstock
b) Contaminants
c) Moisture contents
d) Nutrients

Explanation: The most important initial issue when considering the application of anaerobic digestion systems is the feedstock to the process. Almost any organic material can be processed with anaerobic digestion; however, if biogas production is the aim, the level of digestion is the key factor in its successful application.

14. The anaerobic digestion can be inhibited.
a) True
b) False

Explanation: The anaerobic digestion process can be inhibited by several compounds, affecting one or more of the bacterial groups responsible for the different organic matter degradation steps. The degree of the inhibition depends, among other factors, on the concentration of the inhibitor in the digester. Potential inhibitors are ammonia, sulfide, light metal ions and some organics.

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