Water Treatment Plant Design Calculations: Settling Tanks and Chemical Analysis

Water Quality Definitions

Alkalinity

Alkalinity: A measure of the water’s capability to absorb hydrogen ions without significant pH change (buffering capacity).

Problem 7-4: Settling Tank Design for Stillwater Water Plant Expansion

Design the settling tank(s) for the City of Stillwater’s water treatment plant expansion using the design overflow rate found in Example 7-3. The maximum day design flow is 0.5 m³/s. Assume a water temperature of 10° C.

Design Calculation Steps (Set 1)

1. Find the Surface Area

   Calculation: (0.5 m³/s) × (86,400 s/d) = 43,200 m³/d

2. Select the Number of Tanks

   

   = 221.66 or 222 m²/tank

3. Select a Trial Width for Calculation

   The maximum width for the chain-and-flight sludge collector is 6 m increments. Assume a width of 4 m.

4. Check Length-to-Width Ratio (L/W)

   L = = 55.5 m

   L/W = = 13.8 or 13.8:1

5. Select a Trial Depth

   2 m + 1 m + 0.6 m = 3.6 m

   Side Water Depth (SWD) = 3.0 m.

6. Check the Length-to-Depth Ratios (L/D)

   = = 27.75 or 28.1

   The L:D ratio is acceptable.

Water Quality Definitions (Repeated Entry)

Alkalinity

Alkalinity: A measure of the water’s capability to absorb hydrogen ions without significant pH change (buffering capacity).

Problem 7-4: Settling Tank Design (Set 2 Verification)

Design the settling tank(s) for the City of Stillwater’s water treatment plant expansion using the design overflow rate found in Example 7-3. The maximum day design flow is 0.5 m³/s. Assume a water temperature of 10° C.

Design Calculation Steps (Set 2)

1. Find the Surface Area

   Calculation: (0.5 m³/s) × (86,400 s/d) = 43,200 m³/d

2. Select the Number of Tanks

   

   = 221.66 or 222 m²/tank

3. Select a Trial Width for Calculation

   The maximum width for the chain-and-flight sludge collector is 6 m increments. Assume a width of 4 m.

4. Check Length-to-Width Ratio (L/W)

   L = = 55.5 m

   L/W = = 13.8 or 13.8:1

5. Select a Trial Depth

   2 m + 1 m + 0.6 m = 3.6 m

   Side Water Depth (SWD) = 3.0 m.

6. Check the Length-to-Depth Ratios (L/D)

   = = 27.75 or 28.1

   The L:D ratio is acceptable.

Additional Water Chemistry Problems

4. Alkalinity Calculation

   A sample water at pH 9 has 30 mg/L of CO₃^2- and 60 mg/L of HCO₃^-. Find the alkalinity as CaCO₃. (10 points)

5. Fluoride Equilibrium Concentration

   Find the equilibrium concentration of fluoride ions in pure water caused by the dissociation of BaF₂. Express the answer both in units of mol/L and mg/L. The K_sp at 25° C is 1.0 × 10^-6. (10 points)

6. Exponential Decay and Half-Life

   What is exponential decay? How is the half-life of any chemical compound expressed?

   • Exponential Decay Definition

     Exponential decay: A quantity is subject to exponential decay if it decreases at a rate proportional to its current value.

   • Half-life Definition

     Half-life (symbol t_1/2) is the time required for a quantity to reduce to half of its initial value.