## **Core Concept**
The strength of sewage refers to the measure of its organic content, which indicates the amount of organic matter present that needs to be broken down. This concept is crucial in wastewater treatment as it helps in determining the treatment required. The strength of sewage is often quantified by measuring the amount of oxygen required to break down the organic matter.

## **Why the Correct Answer is Right**
Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen required by microorganisms to break down organic material in a given volume of water over a specific period, usually 5 days at 20°C. It is a direct indicator of the organic pollution in sewage. Therefore, BOD is considered the best expression of the strength of sewage because it directly relates to the organic load that needs to be processed.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Chemical Oxygen Demand (COD) measures the total amount of oxygen required to break down organic matter in water, providing a quicker but broader measure than BOD. While COD gives an immediate indication of organic content, it does not differentiate between biodegradable and non-biodegradable organic matter.
- **Option B:** Total Suspended Solids (TSS) measures the total amount of suspended solids in water, which can include both organic and inorganic matter. TSS does not directly indicate the organic strength or the oxygen demand of sewage.
- **Option C:** pH is a measure of the acidity or alkalinity of water and does not directly relate to the organic strength of sewage.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that BOD5 (5-day BOD) is commonly used as a standard measure for sewage strength. High BOD levels indicate a high level of organic pollution, which can lead to significant environmental and health issues if not properly treated.

## **Correct Answer:** .