ANS:B - 15 - 20 years

The design period of sewage treatment works typically ranges from 15 to 20 years. This design period accounts for the expected lifespan of the infrastructure, the projected population growth, changes in water usage patterns, and regulatory requirements. It allows for the treatment works to adequately serve the community over a significant period before requiring major upgrades or expansion. While some projects may have longer design periods depending on specific factors, the range of 15 to 20 years is commonly used in the planning and design of sewage treatment works.