Chlorine demand of water is measured by
**Core Concept**
Chlorine demand of water refers to the amount of chlorine required to achieve a certain level of disinfection in water. This concept is crucial in water treatment and public health, as it ensures the water is safe for consumption. Chlorine demand is influenced by various factors, including the presence of organic matter, pH, and temperature.
**Why the Correct Answer is Right**
The correct answer is related to the concept of chlorine demand being influenced by the amount of organic matter present in the water, which is typically measured by the total organic carbon (TOC) or biological oxygen demand (BOD). When organic matter is present, it reacts with chlorine, consuming it and reducing its effectiveness as a disinfectant. Therefore, measuring the chlorine demand of water helps to determine the required dose of chlorine to achieve adequate disinfection. This process involves the reaction of chlorine with organic compounds, such as amino acids, proteins, and other organic molecules.
**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because chlorine demand is not directly related to the total hardness of water, although it may indirectly affect the chlorine demand by influencing the pH and alkalinity of the water.
**Option B:**
This option is incorrect because chlorine demand is not measured by the residual chlorine level in water. While residual chlorine is an important parameter in water treatment, it is not a measure of the chlorine demand.
**Option C:**
This option is incorrect because chlorine demand is not directly related to the pH of water, although pH can affect the chlorine demand by influencing the reactivity of chlorine with organic matter.
**Clinical Pearl / High-Yield Fact**
A high chlorine demand in water typically indicates the presence of organic matter, which can be due to contamination from agricultural runoff, sewage, or industrial waste. This highlights the importance of regular water quality monitoring and effective treatment processes to ensure safe drinking water.
**Correct Answer:** D. Total Organic Carbon (TOC) measurement.