13.2 Filtration, Disinfection, and CT Logic

Filtration as the Barrier After Settling

After sedimentation, a conventional drinking-water plant still needs filtration because small floc, cyst-sized particles, and fine turbidity remain. Granular media filtration removes particles by straining, attachment, interception, and depth filtration through sand, anthracite, garnet, or dual-media beds. The PE Civil WRE exam usually does not ask for microscopic filtration theory. It asks whether you can check filter rate, interpret headloss and turbidity trends, and connect upstream treatment to filter performance.

A filter's hydraulic loading rate is Q/A, commonly in gpm/ft^2. Use the active filter area, not the installed area if one unit is out of service. Filters also have a run cycle. A clean bed starts with low headloss, then headloss increases as solids accumulate. A run ends when headloss reaches the terminal limit, effluent turbidity breaks through, or operating policy requires backwash.

Filtration Checks

High filter effluent turbidity is often an upstream problem. If coagulant dose, pH, flocculation energy, or sedimentation performance is wrong, the filter becomes the first visible failure point. Increasing chlorine dose does not fix particle breakthrough. Backwashing removes accumulated solids, but if every run is short, the root cause may be coagulation or hydraulic overloading.

Disinfection Options and Tradeoffs

Free chlorine provides a durable distribution residual and supports CT calculations, but it can form regulated disinfection byproducts when natural organic matter is present. Chloramines are weaker primary disinfectants but maintain long distribution residuals with fewer trihalomethanes. Ozone and ultraviolet light can provide strong primary disinfection, but they do not provide a lasting residual by themselves. The exam may ask for the operational consequence, not just the strongest oxidant.

CT Logic

CT means disinfectant concentration times contact time. In PE problems, C is the disinfectant residual concentration at the relevant point, usually in mg/L. T is effective contact time, usually in minutes. The product has units of mg-min/L.

Do not use chemical dose as C unless the problem explicitly says residual equals dose. Chlorine demand, decay, short-circuiting, and basin hydraulics reduce the effective value. Do not use theoretical detention time blindly. A contact basin may require a baffling factor or T10 value, which represents the time by which 10 percent of water has exited.

CT Calculation Workflow

1. Identify the target organism, required log inactivation, disinfectant, pH, and temperature from the prompt or table.
2. Find the required CT value for those conditions.
3. Compute theoretical detention time as basin volume divided by flow.
4. Convert days or hours to minutes.
5. Apply the baffling factor if T10 is not given directly.
6. Use the measured residual concentration at the end of the contact segment.
7. Compute actual CT = C x T and compare it with required CT.
8. If actual CT is low, solve for required residual, volume, flow reduction, or improved baffling, depending on what the question asks.

Exam Traps

CT problems are unit traps. A clearwell volume in MG divided by flow in MGD gives days. Multiply by 1,440 min/day before multiplying by mg/L. Also watch whether the plant is operating at average day, maximum day, or peak hour flow. Contact time shrinks as flow increases, so a design that passes at average flow may fail at maximum day flow.

For concept questions, keep the barrier concept in mind. Filtration controls particles and turbidity. Disinfection inactivates pathogens. Good plant operation uses both barriers without creating avoidable disinfection byproduct or residual problems.