Question

A wastewater treatment plant consists of primary treatment followed by secondary treatment, and sludge digestion. The raw wastewater flow rate is 22,500 m^3/d with 300mg/L suspended solids and 250mg/LBOD5 . The primary clarifier removes 30%BOD 5 and 60% suspended solids from raw wastewater. The secondary underflow contains 1180 kg/d of biological solids. There are two options for transporting the primary and secondary sludges to the digester. They are:

Option \#1: Transport the primary and secondary sludges in separate pipelines to the digester The primary sludge has a solids concentration of 5%, and the secondary sludge has a solids concentration of 0.5%.
Option \#2: Mix the primary and secondary sludges in a mixing tank, before discharging to the digester. The mixed sludge has a solids concentration of 3.5%. Answer the following:
(a) Draw flow diagrams for the two treatment options.
(b) Calculate the volume required for option \#1, for treatment in a high-rate digester, which requires 15 days for complete digestion.
(c) Calculate the volume required for option \#2, for treatment in a high-rate digester, which requires 15 days for complete digestion.
(d) Comment on which one is the better option and why.

Answer 1

In option 1, the primary sludge is transported in a separate pipeline to the digester, while in option 2, the primary and secondary sludge are mixed before being sent to the digester. For option 1, the volume required for 15 days of complete digestion is 11,812.5 m³, and for option 2, it is 14,362.5 m³. Option 1 is the better option as it requires a smaller volume for digestion.

Step-by-step explanation:

(a) Flow Diagrams:

Option 1: Raw wastewater ➡ Primary clarifier ➡ Primary sludge pipeline ➡ Digester

Option 2: Raw wastewater ➡ Primary clarifier ➡ Secondary clarifier ➡ Secondary sludge pipeline ➡ Mixing Tank ➡ Digester

(b) Volume Calculation for Option 1:

The primary sludge concentration is 5% solids. The primary sludge flow rate can be calculated by multiplying the raw wastewater flow rate by the removal efficiency: 22,500 m³/d × (100% - 30% = 70%) × 5% = 787.5 m³/d. To calculate the volume required for 15 days of complete digestion, we use the formula: volume = flow rate × digestion time = 787.5 m³/d × 15 days = 11,812.5 m³.

(c) Volume Calculation for Option 2:

The mixed sludge concentration is 3.5% solids. The mixed sludge flow rate can be calculated by adding the primary and secondary sludge flow rates: 787.5 m³/d + 1180 kg/d = 957.5 m³/d. Using the same volume calculation formula, the volume required for 15 days of complete digestion is: 957.5 m³/d × 15 days = 14,362.5 m³.

(d) Comment on the Better Option:

Option 1 is the better option because it requires a smaller volume for digestion compared to Option 2. This means that Option 1 is more efficient in terms of space requirements and may be more cost-effective.