What is a Chlorine Demand in a pool?

Chlorine demand is defined as the inability to maintain a chlorine residual in a pool or spa even after repeated application of chlorinating product. There are an infinite number of substances that can contribute to chlorine demand. These include (but are not limited to) bacteria, algae, ammonia, urine, sweat, health and beauty products, bather waste and animal waste. These contaminants can enter the water a number of different ways such as source water, rain water, bathers, animals, fertilizers, plants/leaves and industrial pollution. Determining the cause of chlorine demand in a particular pool can seem like an insurmountable task. In many cases, the exact cause of the demand will be impossible to establish, and it is usually not truly relevant to the treatment needed.