This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Sludge Characteristics, Thickening and Concentration – 1”.
1. Sludge is composed of liquid components alone.
a) True
b) False
View Answer
Explanation: Sewage sludge treatment describes the processes used to manage and dispose of sewage sludge produced during sewage treatment. Sludge is mostly water with lesser amounts of solid material removed from liquid sewage.
2. How many types of sludge are present?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: There are two types of sludge present, primary and secondary sludge. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers.
3. _______ is a term used for reuse of sewage sludge.
a) Solids
b) Biosolids
c) Potential solids
d) Manure
View Answer
Explanation: “Biosolids” is a term often used in wastewater engineering publications and public relations efforts by local water authorities when they want to put the focus on reuse of sewage sludge after the sludge has undergone suitable treatment processes.
4. Biosolids are the inorganic wastewater solids.
a) True
b) False
View Answer
Explanation: Biosolids are defined as organic wastewater solids that can be reused after stabilization processes such as anaerobic digestion and composting. The term “biosolids” was introduced by the Water Environment Federation in the U.S. in 1998.
5. Who introduced the term biosolids?
a) UNEPA
b) USEPA
c) EPA
d) WEF
View Answer
Explanation: The term “biosolids” was introduced by the Water Environment Federation (WEF). Some people argue that a term is a form of “propaganda” with the aim to hide the fact that sewage sludge may also contain substances that could be harmful to the environment when the treated sludge is applied to land.
6. Which year was the term biosolids introduced?
a) 1990
b) 1993
c) 1995
d) 1998
View Answer
Explanation: The term biosolids was introduced in the year 1998 by the Water Environment Federation (WEF).
7. Which of the following processes is not used for water reduction?
a) Centrifugation
b) Filtration
c) Heating
d) Evaporation
View Answer
Explanation: Water content of sludge may be reduced by centrifugation, filtration and by evaporation to reduce transportation costs of disposal or to improve suitability for composting. Centrifugation may be a preliminary step to reduce sludge volume for subsequent filtration or evaporation.
8. Sand drying bed is used for filtration.
a) True
b) False
View Answer
Explanation: Centrifugation may be a preliminary step to reduce sludge volume for subsequent filtration or evaporation. In a sand drying bed or as a separate mechanical process filtration may occur through underdrain in a belt filter press.
9. How many products are obtained after dewatering?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: Sludge treatment technologies that are used for thickening or dewatering of sludge have two products and those are the thickened or dewatered sludge and a liquid fraction.
10. The liquid has high contents of phosphorus and nitrogen, if the sludge has been______
a) Dewatered
b) Aerobically digested
c) Anaerobically digested
d) Heated
View Answer
Explanation: Liquids require treatment as it is high in nitrogen and phosphorus, particularly if the sludge has been anaerobically digested. The treatment can take place in the sewage treatment plant itself or as a separate process.
11. Sludge treatment reduces the formation of struvite scales in pipes.
a) True
b) False
View Answer
Explanation: For phosphorus recovery, sewage treatment plant operators of treating sludge dewatering streams is that it reduces the formation of obstructive struvite scale in pipes, pumps and valves.
12. Which of these is not a method of digestion?
a) Composting
b) Aerobic digestion
c) Evaporation
d) Anaerobic digestion
View Answer
Explanation: Sludges are treated using a variety of digestion techniques, the purpose of which is to reduce the amount of organic matter and the number of disease-causing micro-organisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion and composting.
13. Anaerobic digestion is carried out in ____
a) Presence of oxygen
b) Presence of carbon-dioxide
c) Absence of oxygen
d) Absence of carbon-dioxide
View Answer
Explanation: Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. Sludge is fermented in tanks at a temperature of 55 °C in thermophilic digestion and in mesophilic digestion it is fermented at a temperature of around 36 °C.
14. What is the temperature to be maintained for thermophilic digestion?
a) 44˚C
b) 55˚C
c) 66˚C
d) 77˚C
View Answer
Explanation: The temperature to be maintained for a thermophilic digestion is 55˚C and the temperature to be maintained for a mesophilic digestion is 36˚C. Thermophilic digestion is more expensive in terms of energy consumption for heating the sludge.
15. What is the duration of sludge in tanks for mesophilic anaerobic digestion?
a) 10 days
b) 12 days
c) 15 days
d) 18 days
View Answer
Explanation: For treating sludge produced at sewage treatment plants mesophilic anaerobic digestion is a common method. The sludge is fed into large tanks and held for a minimum of 12 days to allow the digestion process to perform the four stages necessary to digest the sludge.
16. Which of the following is not a product of anaerobic digestion?
a) Water
b) Oxygen
c) Carbon dioxide
d) Methane
View Answer
Explanation: These are hydrolysis, acidogenesis, acetogenesis and methanogenesis. In this process, the complex proteins and sugars are broken down to form more simple compounds such as water, carbon dioxide and methane.
17. Which of the following is an advantage of anaerobic digestion?
a) Capital cost
b) Time
c) Methane generation
d) Space
View Answer
Explanation: Methane generation is a key advantage of the anaerobic process. Its key disadvantage is the long time required for the process (up to 30 days) and the high capital cost.
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