What is a load duration curve and its use?

• A. A chart showing daily rainfall impacting chemical dosing.
• B. A graph of contaminant load over time used to design treatment capacity to meet demand.
• C. A table listing weekly maintenance tasks.
• D. A diagram of pump head vs flow rate.

Answer: (B)

A load duration curve is a tool for sizing treatment capacity by showing how the contaminant load changes over time. It’s created by calculating the load (flow times concentration) for different periods and plotting that load against time or against the percent of time the load is exceeded. This visualization helps engineers see how often high loads occur and how large those loads are, so they can design treatment capacity that will handle peak periods while still meeting demand and quality targets.

In practice, you gather historical flow and contaminant concentration data, compute load = Q × C, and then plot it. The curve reveals whether the plant’s capacity is sufficient during peak conditions and may indicate if some processes can be downsized during low-load periods or if additional capacity is needed for the high-load events.

The other concepts described in the options don’t fit this purpose: rainfall charts relate to dosing changes, weekly maintenance tables organize tasks, and the pump head vs flow graph is a pump curve, not a load duration curve.