Question

A conventional water treatment plant is fed from a reservoir 2 miles away through a 6 foot diameter pipe. Disinfection is provided from the supply reservoir to the plant influent at a free chlorine residual of \(0.2 \mathrm{mg} / \mathrm{L}\). The daily flow is a constant \(55 \mathrm{MGD}\). The water is \(10^{\circ} \mathrm{C}\) and has a pH of \(7.5\). 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 55 MGD to be 40 minutes. Does this plant meet compliance for \(\mathrm{CT}\) inactivation for viruses?

Short answer

Yes, the plant meets compliance for CT inactivation for viruses.

Step-by-step solution

- Define CT Inactivation Requirement

First, determine the regulatory requirement for CT inactivation for viruses. The EPA recommends a CT value (product of chlorine concentration C and contact time T) for virus inactivation at different water conditions including temperature and pH.

- Convert Daily Flow to Gallons per Minute

Convert the daily flow rate in million gallons per day (MGD) to gallons per minute (GPM). Use the conversion factor 1 MGD = 694.4 GPM.Given daily flow = 55 MGD,gpm = 55 MGD * 694.4 GPM/ MGDThus gpm = 38192 GPM.

- Determine Contact Time

Using the information that T10 (contact time) at rated capacity is 40 minutes, we see that the time is already provided. No additional calculations are needed here.

- Verify Chlorine Concentration

Given the chlorine concentration for disinfection is maintained at 0.2 mg/L, verify that this is the initial chlorine dose before contact time measurement.

- Calculate CT

Using the known values of chlorine concentration C = 0.2 mg/L and contact time T = 40 minutes, calculate the CT value.CT = C * T = 0.2 mg/L * 40 min = 8 mg*min/L.

- Determine Required CT Value

For water at 10°C and pH 7.5, check EPA guidelines for the required CT value for 4-log virus inactivation. According to EPA guidelines, the required CT value is 6 mg*min/L for 4-log virus inactivation under these conditions.

- Compare CT Values

Compare the calculated CT value (8 mg*min/L) with the required CT value (6 mg*min/L). Since 8 mg*min/L > 6 mg*min/L, the plant meets the EPA compliance requirement for virus inactivation.

Key concepts

Water Treatment Plant

A water treatment plant is essential for making water safe for human consumption and use. It processes raw water to remove harmful contaminants and microorganisms. The plant in our exercise is conventional, which means it uses a series of steps including coagulation, sedimentation, filtration, and disinfection. Disinfection, particularly, is crucial in inactivating harmful pathogens such as viruses, bacteria, and protozoa. The water treatment plant referenced is supplied from a reservoir 2 miles away via a 6-foot diameter pipe.

Chlorine Residual

Chlorine residual refers to the amount of chlorine remaining in the water after the disinfection process is complete. It is essential to maintain a certain level of free chlorine in the water to ensure continued protection against pathogens as the water moves through the distribution system. In the example given, the free chlorine residual is 0.2 mg/L at the influent of the water treatment plant. This initial dose is crucial for the subsequent calculation of CT value and ensures that the water meets safety standards before reaching consumers.

Contact Time (T10)

Contact time (T10) is a key parameter in assessing the effectiveness of the disinfection process. It represents the time that a given volume of water is in contact with the disinfectant, in this case, chlorine. T10 is often used instead of the actual contact time to provide a conservative estimate. Based on tracer studies, the T10 value at the rated capacity of our treatment plant is found to be 40 minutes. This value is used in conjunction with the chlorine concentration for calculating the CT value.

CT Value Calculation

CT value is the product of chlorine concentration (C) and contact time (T) and represents the level of disinfection achieved. For our example:

• Chlorine concentration (C): 0.2 mg/L
• Contact time (T): 40 minutes

The CT value calculation is straightforward:
CT = C * T
Substituting the given values: CT = 0.2 mg/L * 40 min = 8 mg*min/L.
This value is then compared to the required CT value specified by EPA guidelines to ensure compliance for virus inactivation.

EPA Guidelines

The Environmental Protection Agency (EPA) provides guidelines for the required CT values needed to achieve specific levels of pathogen inactivation, including viruses. These guidelines consider various factors like water temperature and pH. For the scenario in the exercise:

• Water temperature: 10°C
• pH: 7.5

According to EPA guidelines, the required CT value for 4-log (99.99%) virus inactivation under these conditions is 6 mg*min/L. Comparing this with our calculated CT value of 8 mg*min/L shows that the plant meets the compliance requirements as 8 > 6.