Question

With respect to particle settling in wastewater treatment systems; the correct statement(s) is/are

• A. Settling in grit chamber and primary sedimentation tanks are examples of Type-I settling
• B. Settling in primary sedimentation tank and secondary sedimentation tank are examples of Type-II settling
• C. Settling in grit chamber is an example of Type-I settling, whereas settling in primary sedimentation tank is an example of Type-II settling
• D. Settling in secondary sedimentation tank is an example of Type-III settling, whereas settling in primary sedimentation tank is an example of Type-II settling

Answer 1

The Correct Option is C, D

Step 1: Define settling types in wastewater treatment. 
Type-I settling (Discrete settling): Occurs when particles settle independently without interaction. Common in grit chambers. 
Type-II settling (Flocculent settling): Occurs when particles flocculate and settle as groups. Seen in primary sedimentation tanks. 
Type-III settling (Hindered settling): Occurs when particles settle as a layer due to high concentrations. Found in secondary sedimentation tanks. 
Type-IV settling (Compression settling): Occurs when particles settle under the influence of compression at very high concentrations. 


Step 2: Analyze the options. 
(1) Settling in grit chamber and primary sedimentation tanks are examples of Type-I settling: Incorrect. Grit chambers involve Type-I settling, but primary sedimentation involves Type-II settling. 
(2) Settling in primary sedimentation tank and secondary sedimentation tank are examples of Type-II settling: Incorrect. Secondary sedimentation involves Type-III settling, not Type-II. 
(3) Settling in grit chamber is an example of Type-I settling, whereas settling in primary sedimentation tank is an example of Type-II settling: Correct. (
4) Settling in secondary sedimentation tank is an example of Type-III settling, whereas settling in primary sedimentation tank is an example of Type-II settling: Correct.