A treatment plant's effluent water quality would be best shown from a sample collected _____.

• A. On a major transmission main
• B. On a lateral or minor pipe at some of the furthest points in the distribution system
• C. Immediately past the customer's meter
• D. At a customer's tap, preferably a kitchen or bathroom sink

Answer: (A)

The main idea is to measure the water as it leaves the treatment plant and enters the distribution system. Sampling on a major transmission main captures the plant’s effluent before it has been significantly altered by long pipe runs, storage, or customer plumbing. This point provides a direct, representative measure of what the plant is delivering to the system.

Sampling downstream in the distribution network—such as on lateral lines far from the plant or at customers’ taps—can be affected by residence time, pipe material, stagnation, or usage, which can change chlorine levels, temperature, or other qualities. So those locations reflect not just the plant’s discharge but the cumulative effects of the distribution system and customer connections.

Therefore, a sample taken on a major transmission main best represents the plant’s effluent water quality.