Recirculation ratio refers to the ratio of the volume of recirculated sewage to the volume of raw sewage. It is commonly used in wastewater treatment systems to assess the efficiency of the treatment process and the amount of treatment required.

Recirculation ratio plays a crucial role in determining the overall performance of a wastewater treatment system. It helps in evaluating the effectiveness of the treatment process and understanding the degree of treatment required to achieve the desired effluent quality.

Here is a detailed explanation of the concept:

1. Definition of recirculation ratio:
The recirculation ratio is defined as the ratio of the volume of recirculated sewage to the volume of raw sewage. It is expressed as a dimensionless number or a percentage.

2. Importance of recirculation ratio:
The recirculation ratio provides insights into the amount of recirculation required to achieve the desired treatment efficiency. It helps in determining the degree of dilution or concentration of pollutants in the wastewater stream.

3. Calculation of recirculation ratio:
The recirculation ratio can be calculated by dividing the volume of recirculated sewage (Qr) by the volume of raw sewage (Qs):

Recirculation ratio = Qr / Qs

4. Role in wastewater treatment:
a) Efficiency assessment: The recirculation ratio helps in evaluating the efficiency of the treatment process. A higher recirculation ratio indicates a higher degree of treatment and better removal of pollutants.
b) Process optimization: By adjusting the recirculation ratio, the treatment process can be optimized to meet the desired effluent quality. Increasing the recirculation ratio can enhance the treatment efficiency, while decreasing it can reduce costs.
c) Nutrient removal: In wastewater treatment systems that incorporate biological processes, such as activated sludge or sequencing batch reactors, the recirculation ratio is important for nutrient removal. It affects the concentration of nutrients in the mixed liquor and influences the growth of microorganisms responsible for nutrient removal.
d) Solids retention: The recirculation ratio also affects the solids retention in the treatment system. By adjusting the recirculation ratio, the concentration of solids in the system can be controlled, leading to better sludge settling and overall process stability.

In conclusion, the recirculation ratio is an important parameter in wastewater treatment systems as it helps in evaluating the efficiency of the treatment process and determining the degree of treatment required. It plays a crucial role in optimizing the treatment system and achieving the desired effluent quality.