American Public Health Association formula for aeration time (T) in hours is (where, La = BOD of the aeration tank sewage influent (mg/litre) to be removed)

Explanation:

American Public Health Association Formula for Aeration Time (T):

The formula for calculating the aeration time (T) in hours is given as:

T = (La / 20) - 1

Where:

• La = BOD (Biochemical Oxygen Demand) of the aeration tank sewage influent (mg/L) to be removed.
• T = Aeration time in hours.

This formula determines the time required for the aeration process in a sewage treatment plant, where the influent water is treated to remove the desired amount of BOD. Biochemical Oxygen Demand is a critical parameter in wastewater treatment, as it indicates the amount of organic matter present in the water. Efficient removal of BOD ensures that the treated water meets environmental discharge standards and reduces the load on receiving water bodies.

Derivation and Application:

The formula T = (La / 20) - 1 is derived based on empirical relationships and operational data from sewage treatment plants. It accounts for the time required for microbial activity in the aeration tank to break down organic matter, as indicated by the BOD value of the influent sewage. The "20" in the denominator represents the average rate of BOD removal efficiency (mg/L per hour) under standard conditions in a typical aeration tank.

Let’s break this down step by step:

1. The BOD of the sewage influent (La) represents the amount of oxygen that microorganisms will consume to degrade organic matter.
2. The rate of BOD removal is assumed to be 20 mg/L per hour in this formula, which is an average value based on experimental data and operational conditions of aeration tanks.
3. Dividing La by 20 gives the base time required to remove the specified amount of BOD.
4. Subtracting 1 from the result accounts for additional factors, such as initial conditions and system efficiency adjustments.

Thus, the formula provides a quick and reliable estimate of the aeration time required for a given influent sewage BOD level under typical operating conditions.