ANS:C - both (a) and (b)

The use of coarse screens for the disposal of sewage can be dispensed with by both (a) and (b), which are a comminutor and a shredder. Comminutors and shredders are mechanical devices used in wastewater treatment plants to break down large solid materials into smaller particles. They perform a similar function to coarse screens but are more effective at reducing the size of solids in sewage. Here's how they work:

• Comminutor: A comminutor is a mechanical device equipped with rotating blades or hammers that grind and shred solid particles in sewage, reducing them to smaller sizes. This process helps to prevent clogging of pipes, pumps, and other equipment downstream.
• Shredder: A shredder operates similarly to a comminutor, using sharp blades or teeth to tear apart solid materials in sewage, breaking them down into smaller fragments. Shredders are particularly effective at reducing the size of bulky or fibrous solids.

By using comminutors or shredders in wastewater treatment processes, the need for coarse screens for sewage disposal can be eliminated, as these devices effectively break down solids to sizes that can be handled by downstream treatment processes without causing blockages or operational issues. Therefore, both comminutors and shredders offer viable alternatives to coarse screens for sewage disposal.