This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Methods of Sludge Processing and Disposal – 1”.
1. Primary sludge includes __________
a) Total suspended solids
b) Suspended solids
c) Removable solids
d) Settleable solids
View Answer
Explanation: Sludge is mostly water with lesser amounts of solid material removed from liquid sewage. Primary sludge includes settleable solids removed during primary treatment in primary clarifiers.
2. Secondary sludge includes treated sewage sludge.
a) True
b) False
View Answer
Explanation: Primary sludge includes settleable solids removed during primary treatment in primary clarifiers. Secondary sludge separated in secondary clarifiers includes treated sewage sludge from secondary treatment bioreactors.
3. Sludge treatment helps is reducing treatment costs.
a) True
b) False
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Explanation: Reducing sludge weight and volume to reduce disposal costs is the aim of sludge treatment. The choice of a sludge treatment method depends on the volume of sludge generated, and comparison of treatment costs required for available disposal options.
4. Which of the following methods are used in rural communities?
a) Aerobic digestion
b) Mechanical dewatering
c) Dewatering
d) Composting
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Explanation: Air-drying and composting may be attractive to rural communities, while limited land availability may make aerobic digestion and mechanical dewatering preferable for cities, and economies of scale may encourage energy recovery alternatives in metropolitan areas.
5. From which of the following methods can toxic chemicals be removed?
a) Sorption
b) Adsorption
c) Absorption
d) Dewatering
View Answer
Explanation: Coarse primary solids and secondary sewage sludge may include toxic chemicals removed from liquid sewage by sorption onto solid particles in clarifier sludge. Reducing sludge volume may increase the concentration of some of these toxic chemicals in the sludge.
6. What is the term used for reuse of sewage sludge?
a) Compost
b) Solids
c) Biosolids
d) Sludge
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Explanation: “Biosolids” is a term often used in 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.
7. In which year was the term Biosolids introduced?
a) 1990
b) 1995
c) 1998
d) 2000
View Answer
Explanation: The term “biosolids” was introduced by the Water Environment Federation in the U.S. in 1998. However, some people argue that the 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.
8. What is the first step in the sewage treatment process?
a) Dewatering
b) Thickening
c) Phosphorus recovery
d) Digestion?
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Explanation: Thickening is often the first step in a sludge treatment process. Sludge from primary or secondary clarifiers may be stirred (often after addition of clarifying agents) to form larger, more rapidly settling aggregates.
9. What is the amount of solids released into wastewater by human beings?
a) 30 gpcd
b) 40 gpcd
c) 60 gpcd
d) 70 gpcd
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Explanation: Humans deposit about 70 g per capita per day of solids into wastewater. With ‘garbage grinders’, this can reach 100g per day. The impurities present in the wastewater must either be transformed into innocuous end-products or be effectively separated from the effluent stream.
10. What is the percentage of aerobically digested sludge?
a) 0.2-0.5
b) 0.3-0.6
c) 1.0-2.0
d) 1.5-3.0
View Answer
Explanation: The solids concentration in thickened, aerobically digested sludge is generally in the range 1.0% – 2.0% as determined by digester design and operation.
11. Ws = Wsp + Wss
What does Wsp stand for in the above equation?
a) Total dry solids
b) Raw primary solids
c) Suspended solids
d) Polluted solids
View Answer
Explanation: Waste solids production in primary and secondary processing can be estimated using the calculation above. Ws stand for total dry solids. Wsp stands for raw primary solids. Wss stands for secondary biological solids.
12. Sludge from the primary clarifier is ______
a) Brown and flocculant like appearance
b) Dark in colour
c) Gray and slimy
d) Dark brown in colour
View Answer
Explanation: Sludge from the primary clarifier is gray in colour. It is also slimy in appearance. Also it has an offensive odour.
13. How does the chemical sludge appear?
a) Gray in colour and not slimy
b) Brown and flocculant like appearance
c) Red/grayish brown
d) Black in colour
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Explanation: In case the sludge contains iron it is red in colour. In case it is lime sludge, it is grayish brown in colour. This sludge also has an objectionable odour.
14. How does the activated sludge appear?
a) Gray
b) Red
c) Brown and flocculant like appearance
d) Greasy
View Answer
Explanation: The activated sludge is brown in colour. It also has flocculant like appearance. If the colour is dark, then the sludge has approached septic condition.
15. What is the % of dry solids assumed for a primary sludge from a settling tank at the outlet?
a) 12%
b) 5-7%
c) 8%
d) 9-11%
View Answer
Explanation: The percentage of dry solids for a primary sludge is assumed as 5-7% at the outlet. In most cases, it is considered as 6%. In the case of sludge is from the flocculator or clarifier the percentage of solids is considered as 2-4%.
16. What is the percentage of dry solids assumed for the sludge at the outlet of a centrifuge?
a) 10-15%
b) 1-3%
c) 4-8%
d) 9-12%
View Answer
Explanation: The percentage of dry solids assumed for the sludge at the outlet of a centrifuge is 4-8%. In most cases, this is considered as 5%. Even for a Gravity belt thickener with the addition of polyelectrolyte the percentage of dry solids is considered as 4-8%.
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