The correct answer is: A. 20% of the design discharge.
The drainage to sewage ratio is the ratio of the volume of runoff to the volume of sewage. A drainage to sewage ratio of 20 means that for every 20 gallons of runoff, there is 1 gallon of sewage.
The peak dry weather flow is the maximum flow that a sewer system can handle during a dry weather period. It is usually calculated as a percentage of the design discharge, which is the maximum flow that the sewer system is designed to handle.
For a drainage to sewage ratio of 20, the peak dry weather flow is 20% of the design discharge. This is because the volume of runoff is 20 times the volume of sewage, so the peak dry weather flow will be 20 times the peak sewage flow.
Option B is incorrect because the peak dry weather flow is not slightly less than 5% of the design discharge. The peak dry weather flow is 20% of the design discharge, which is much greater than 5%.
Option C is incorrect because the peak dry weather flow is not slightly more than 5% of the design discharge. The peak dry weather flow is 20% of the design discharge, which is much greater than 5%.
Option D is incorrect because the peak dry weather flow is not none of these. The peak dry weather flow is 20% of the design discharge.