To calculate the solids loading rate on the secondary clarifier, we need to follow these steps:

Given:

• Diameter of the clarifier = 75 ft
• Flow rate (Q) = 3.9 MGD (Million Gallons per Day)
• MLSS (Mixed Liquor Suspended Solids) = 2940 mg/L

Steps:

1. Convert flow rate to gallons per minute (GPM): $Q = 3.9 \, \text{MGD} = 3.9 \times 10^6 \, \text{gallons per day}$ $\text{Flow rate in GPM} = \frac{3.9 \times 10^6}{1440} = 2708.33 \, \text{GPM}$ (since there are 1440 minutes in a day).

2. Calculate the surface area of the clarifier (assuming it’s circular): $A = \pi \times \left(\frac{D}{2}\right)^2$ Where $D = 75 \, \text{ft}$: $A = \pi \times \left(\frac{75}{2}\right)^2 = \pi \times (37.5)^2 = 4417.86 \, \text{sq ft}$

3. Convert MLSS to pounds per gallon (lbs/gal): 1 mg/L = 8.34 × 10⁻⁶ lbs/gal, so: $\text{MLSS in lbs/gal} = 2940 \, \text{mg/L} \times 8.34 \times 10^{-6} = 0.0245 \, \text{lbs/gal}$

4. Calculate the total solids flow rate (lbs/day): $\text{Total solids flow} = Q \times \text{MLSS in lbs/gal}$ $\text{Total solids flow} = 2708.33 \, \text{GPM} \times 0.0245 \, \text{lbs/gal}$ $\text{Total solids flow} = 66.46 \, \text{lbs/min}$ Convert to lbs/day: $\text{Total solids flow} = 66.46 \times 1440 = 95,561.04 \, \text{lbs/day}$

5. Calculate the solids loading rate (lbs/day/sq ft): $\text{Solids loading rate} = \frac{\text{Total solids flow (lbs/day)}}{\text{Clarifier area (sq ft)}}$ $\text{Solids loading rate} = \frac{95,561.04}{4417.86} \approx 21.6 \, \text{lbs/day/sq ft}$

Final Answer:

The solids loading rate on the secondary clarifier is approximately 21.6 lbs/day/sq ft.

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Would you like further details on any of the calculations or concepts used here?

Here are some related questions for you:

1. How would you calculate the surface overflow rate on a clarifier, and why is it important?
2. What effect does increasing the MLSS concentration have on the solids loading rate?
3. How can the clarifier's performance be improved if the solids loading rate is too high?
4. What factors influence the clarification process in secondary clarifiers?
5. How do you calculate the sludge volume index (SVI) in a secondary clarifier?

Tip: When performing calculations involving flow rates and concentrations, always ensure that units are consistent throughout to avoid errors.