This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Wet Air Oxidation & Electrocoagulation”.
1. Wet air oxidation is a form of hydrothermal treatment.
a) True
b) False
View Answer
Explanation: Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidizer. It is referred to as “Wet Air Oxidation” (WAO) when air is used.
2. The oxidation process occurs above 374˚C.
a) True
b) False
View Answer
Explanation: In superheated water at a temperature above the normal boiling point of water (100 °C), but below the critical point (374 °C), the oxidation reactions occur.
3. In a bubble column reactor, fresh water enters from the _________of the column.
a) Bottom
b) Top
c) Side
d) Top and side
View Answer
Explanation: Commercial systems typically use a bubble column reactor, where the air is bubbled through a vertical column that is liquid full of the hot and pressurized wastewater. Fresh wastewater enters the bottom of the column and oxidized wastewater exits the top.
4. The heat released during the oxidation is_______
a) Released into the atmosphere
b) Cooled using cooling systems
c) Used to maintain an operating temperature
d) Used as energy to boil water
View Answer
Explanation: The heat released during the oxidation is used to maintain the operating temperature. WAO is a liquid phase reaction using dissolved oxygen in the water to oxidize wastewater contaminants. The dissolved oxygen is typically supplied using pressurized air, but pure oxygen can also be used.
5. Which form of oxygen is used in the process of wet air oxidation?
a) Molecular oxygen
b) Dissolved oxygen
c) Pure oxygen
d) Atomic oxygen
View Answer
Explanation: WAO is a liquid phase reaction using dissolved oxygen in the water to oxidize wastewater contaminants. The dissolved oxygen is typically supplied using pressurized air, but pure oxygen can also be used.
6. What is the minimum temperature required for WAO?
a) 30˚C
b) 50˚C
c) 90˚C
d) 150˚C
View Answer
Explanation: The oxidation reaction generally occurs at moderate temperatures of 150°-320 °C and at pressures from 10 to 220 bars. The process converts organic contaminants to carbon dioxide, water and biodegradable short chain organic acids.
7. What is the percentage of dry solid slurry that can be processed in a WAO system?
a) 1%
b) 2%
c) 3%
d) 4%
View Answer
Explanation: The thermal conditioning occurs at temperatures of 210 – 240 °C. 4% dry solid slurry can be processed in a WAO system where it is disinfected and the treated effluent can be dewatered to 55% dry solids using a filter press.
8. What is the maximum pressure below which oxidation reactions occur?
a) 110 bar
b) 150 bar
c) 220 bar
d) 250 bar
View Answer
Explanation: The oxidation reactions occur at temperatures of 150°C to 320°C and at pressures from 10 bars to 220 bars. Higher temperatures require higher pressure to maintain a liquid phase in the system.
9. What is the minimum feed flow rate for industrial applications of WAO?
a) 1 m3/h
b) 5 m3/h
c) 10 m3/h
d) 50 m3/h
View Answer
Explanation: Higher temperatures require higher pressure to maintain a liquid phase in the system. The feed flow rate must be 1m3/h to 50m3/h for industrial applications for the WAO.
10. Electro-coagulation process removes bacteria.
a) True
b) False
View Answer
Explanation: Electrocoagulation process removes emulsified oils, total suspended solids (TSS), heavy metals, emulsified oils, bacteria and other contaminants from water.
11. The cathode material corrodes during the process.
a) True
b) False
View Answer
Explanation: An electrocoagulation reactor is made up of an electrolytic cell with one anode and one cathode. The anode material will electrochemically corrode due to oxidation, while the cathode will be subjected to passivation.
12. What is the minimum number of electrodes required for the process?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: The minimum number of electrodes required for the process of electro-coagulation is 2. Minimum one anode and a cathode are required for the reaction to begin. A series of electrodes are connected for better results.
13. Which among the following corrodes due to oxidation?
a) Anode
b) Cathode
c) Anode and cathode
d) Only cathode
View Answer
Explanation: The anode material will electrochemically corrode due to oxidation, while the cathode will be subjected to passivation when it is connected to an external source.
14. The conductive metal plates are placed in ________
a) Parallel
b) Perpendicular
c) Either parallel or perpendicular
d) Neither parallel or perpendicular
View Answer
Explanation: An EC system essentially consists of pairs of conductive metal plates in parallel, which act as monopolar electrodes. It furthermore requires a direct current power source, a resistance box to regulate the current density and a multimeter to read the current values.
15. _________ results from the oxygen and hydrogen ions that bond into the water receptor sites of emulsified oil molecules.
a) Seeding
b) Emulsion breaking
c) Bleaching
d) Complexing
View Answer
Explanation: The oxygen and hydrogen ions that bond into the water receptor sites of emulsified oil molecules creating a water-insoluble complex separating water from oil, driller’s mud, dyes, inks and fatty acids etc. causes emulsion breaking.
16. What is the common method used for mixing of wastewater in electrocoagulation process?
a) Mechanical mixing
b) Natural mixing
c) Electrical stirrer
d) Magnetic bar stirrer
View Answer
Explanation: The common method used for mixing of wastewater in electrocoagulation process is through the use of magnetic bar stirrer. This is placed in the bottom of the jar at the centre.
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