CT, sedimentation and filtration on the PE Civil WRE exam

A conventional water plant removes particles and pathogens in three units: the settling basin, the filter and the contact tank. Drinking Water Distribution and Treatment carries 6–9 of the 80 questions on the WRE exam, spread across ten subtopics. These three give quick points once you have the definitions right.

What you actually need to own

• CT: the disinfectant residual C (mg/L) times the contact time T (min). The time used is t₁₀, the time for 10% of the water to pass through, not the average detention time.
  • Without tracer data, estimate t₁₀ = θ × BF, where θ = V/Q and BF is the baffling factor.
  • BF runs from about 0.1 (unbaffled, mixed) to 1.0 (plug flow, like a pipeline).
  • Compute θ at peak hourly flow. By convention, V is the lowest operating volume.
• Log credit: look up the tabulated CT for the organism and disinfectant. The Giardia table for free chlorine is set at 3-log inactivation and is indexed by temperature, pH and residual.
  • For Giardia by free chlorine, log inactivation ≈ 3 × (CT_calc / CT_table), using the 3-log table value.
  • Filtration takes removal credit first, so a conventional plant owes only the rest in inactivation. The Handbook tabulates both the credits and the requirements.
• Ideal settling:
  • Overflow rate v₀ = Q/A_surface. Any particle with settling velocity v_t ≥ v₀ is fully removed.
  • A slower particle is removed in the fraction v_t/v₀.
  • Discrete particles settle at the Stokes velocity, v_t = g(ρ_s − ρ)d²/(18μ). Check that the particle Reynolds number is in the laminar range.
• Why depth drops out: a deeper basin gives the water more time, but the particle also has to fall farther. The two cancel, so removal depends only on Q/A. Real basins still get depth and detention criteria for sludge storage, scour and short-circuiting. The ideal removal calculation just doesn't use them.
• Filter loading: loading rate = Q/A, usually in gpm/ft². Rapid granular filters run at a few gpm/ft², and slow sand filters at a small fraction of that. Size the area with one filter out of service (for backwash), and get backwash volume from the backwash rate × area × duration.

Where people lose points

• Mean detention time instead of t₁₀, or no baffling factor at all. If the stem says unbaffled, use the unbaffled factor, even when the tank looks long.
• Wrong CT table cell. Use the stated temperature, pH and residual. When a value falls between rows, the conservative read is the colder temperature, higher pH and higher residual, unless the problem says to interpolate.
• Overflow rate on the wrong area. Count only the settling zone. A circular clarifier's area is πD²/4, less any center well the problem gives.
• "A longer detention time improves removal." In the ideal model it doesn't. Only Q/A matters, and doubling the depth at the same plan area changes nothing.
• Unit slips.
  • Overflow rates come in gpd/ft², and settling velocities in ft/s or mm/s. 1 gpd/ft² ≈ 0.134 ft/day.
  • Filter rates come in gpm/ft²; 1 gpm/ft² ≈ 2.44 m/h.
  • The CT tables use milligrams per liter and minutes.
• Losing a filter. Filter count and area are checked with one unit out of service. Dividing by the full count understates the loading.

How to study it

Treat the three units as one train. Take a design flow, size the basin by overflow rate and check a target particle's removal. Then size the filters with one unit offline, and finish with the contact tank: θ, t₁₀, CT and the log credit left over after filtration. Before any calculation, say which quantity sets the answer (Q/A for settling and filtering, t₁₀ for CT). In the Handbook, CT lives under Chlorine Contact Chambers, and ideal settling under Settling and Sedimentation. Chemical doses run on the 8.34 conversion.

Part of the Drinking Water Distribution and Treatment area of the PE Civil WRE exam. → Start practicing free — the whole practice bank, free with an account.