Correct Answer - Option 1 : storm water
Concept:
A combined sewerage system provides only one sewer to carry sewage as well as rain water. The following are some of its characteristics:
1. High initial cost but less maintenance cost.
2. Due to its larger size, less chances of choking.
3. Not suitable for region where heavy and concentrated rainfall occurs for very short period of time because the remaining period where rainfall occurs very less or not at all, sewage flow may not able to achieve minimum velocity required for self cleansing of sewage which, otherwise, leads to silting in sewerage. Hence, it is more appropriate for developed areas where rainfall occurs almost uniformly throughout the year.
4. The storm water dilutes the sewage so strength of sewage is reduced but cost of treating this sewage will be more because of increase of sewage discharge.
