What is the approximate demand in ppm of chlorine when 6.1 ppm is applied and 0.6 is measured?

To determine the chlorine demand, you need to compare the amount of chlorine that was applied to what is actually measured after a certain period. When 6.1 ppm (parts per million) of chlorine is applied, and then later, the residual chlorine measured is 0.6 ppm, the demand can be calculated by subtracting the measured residual from the applied amount.

The calculation would look like this:

Chlorine demand = Applied chlorine - Measured chlorine = 6.1 ppm - 0.6 ppm = 5.5 ppm.

This result indicates how much of the chlorine has been consumed by reactions with organic matter, pathogens, or other substances in the water. A chlorine demand of 5.5 ppm is significant, as it highlights the amount of chlorine that has been reacted or used up, thus informing treatment processes and ensuring the remaining chlorine is sufficient for disinfection.

The other choices do not align with this calculation. A demand of 3.5 ppm is lower than the actual demand calculated, 6.0 ppm is too high considering the residual, and 6.7 ppm exceeds the initial application, which is not possible since demand cannot exceed the amount added. Hence, the accurate demand of 5.