Question

A conventional water treatment plant is fed from a reservoir 3 miles away through a 5 foot diameter pipe. Disinfection is provided from the supply reservoir to the plant influent at a free chlorine residual of \(0.3 \mathrm{mg} / \mathrm{L}\). The daily flow is a constant \(50 \mathrm{MGD}\). And the water is \(10^{\circ} \mathrm{C}\) and has a pH of \(8.0\). The treatment plant maintains a chloramines residual of \(1.0 \mathrm{mg} / \mathrm{L}\). Tracer studies have shown the contact time \(\left(\mathrm{T}_{10}\right)\) for the treatment plant at the rated capacity of 50 MGD to be 30 minutes. Does this plant meet compliance for CT inactivation for Giardia?

Short answer

If the calculated CT value (9 mg·min/L) is greater than or equal to the required CT value for 10°C and pH 8.0, the plant meets compliance for Giardia inactivation.

Step-by-step solution

Determine the required CT value for Giardia inactivation at 10°C and pH 8.0

Refer to regulatory guidelines or reference tables for the CT inactivation values for Giardia at the given temperature (10°C) and pH (8.0) for free chlorine. For this step, find the CT value required for 99.9% inactivation (3-log reduction).

Calculate the actual CT value

Use the given chlorine residual and the verified contact time to find the actual CT value. CT is calculated as the chlorine concentration (mg/L) multiplied by the contact time (minutes). Here, the contact time, \(\text{T}_{10}\), is 30 minutes and the free chlorine residual is 0.3 mg/L, so:\[ \text{CT} = 0.3 \frac{\text{mg}}{\text{L}} \times 30 \text{ minutes} \]

Plug in the values and solve for actual CT

Calculate the value from the equation in Step 2:\[ \text{CT} = 0.3 \times 30 = 9 \frac{\text{mg} \times \text{minutes}}{\text{L}} \]

Compare the actual CT to the required CT

Compare the actual CT value (9 mg·min/L) with the required CT value obtained in Step 1. If the actual CT value is greater than or equal to the required CT value, the plant meets compliance for Giardia inactivation.

Key concepts

Water Treatment

Water treatment is a crucial process to ensure safe drinking water. It includes several steps like coagulation, flocculation, sedimentation, filtration, and disinfection. Each step removes different types of contaminants.
The main goal of disinfection is to inactivate pathogens like bacteria, viruses, and protozoa, including Giardia. Disinfection is often achieved using chemicals such as chlorine. Chlorine is popular due to its effectiveness and ease of application.

CT Value Calculation

CT value stands for Concentration and Contact Time. It is a measure used to determine the effectiveness of a disinfectant in inactivating pathogens. The CT value is calculated as:
\( \text{CT} = \text{Concentration of disinfectant (mg/L)} \times \text{Contact Time (minutes)} \)
For example, if the concentration of free chlorine is 0.3 mg/L and the contact time is 30 minutes, the CT value is:
\( \text{CT} = 0.3 \frac{\text{mg}}{\text{L}} \times 30 \text{ minutes} = 9 \frac{\text{mg·minutes}}{\text{L}} \).
This value is then compared against regulatory guidelines to ensure compliance.

Chlorine Residual

Chlorine residual refers to the amount of chlorine remaining in water after a specified contact time. It's crucial for maintaining the effectiveness of disinfection throughout the distribution system.
In the exercise, a free chlorine residual of 0.3 mg/L is used. This means that after some time has elapsed, 0.3 mg/L of free chlorine is still present in the water. This residual ensures that pathogens are continuously being inactivated, reducing the risk of waterborne diseases.

Contact Time

Contact time, denoted as \( T_{10} \), is the time water spends in contact with the disinfectant before reaching the first consumer. It’s a critical factor in effective disinfection as it determines how long pathogens are exposed to the disinfectant.
In our example, the contact time is 30 minutes. This is determined through tracer studies which measure how water flows through a treatment plant. Knowing this duration helps in computing the CT value and ensuring the treatment is sufficient for pathogen inactivation.

Regulatory Guidelines

Regulatory guidelines provide the required CT values necessary for inactivating specific pathogens under various conditions like temperature and pH. These guidelines are developed by health organizations to ensure water safety.
For Giardia inactivation at 10°C and pH 8.0, the CT value for a 99.9% inactivation (3-log reduction) is specified. If the actual CT value calculated in the treatment process meets or exceeds this requirement, the treatment is compliant.
In the exercise, the calculated CT value is 9 mg·min/L. By comparing it with the required CT value from the guidelines, we can determine if the plant meets compliance for Giardia inactivation.