ANS:B - 40, 000 to 50, 000 litres/sqm/day

A range of 40,000 to 50,000 liters per square meter per day (liters/sqm/day) refers to the overflow rate in plain primary sedimentation tanks. This parameter is crucial in wastewater treatment as it determines the hydraulic loading on the sedimentation tank and influences the efficiency of solid-liquid separation. Here's an explanation of the significance of this overflow rate range:

1. Overflow Rate: The overflow rate represents the volume of wastewater (in liters) passing through each square meter of the sedimentation tank's surface area per day. It is calculated by dividing the total flow rate of wastewater entering the tank by the surface area of the tank. This rate helps in assessing the capacity of the tank to handle the incoming flow while allowing sufficient time for sedimentation to occur.
2. 40,000 to 50,000 liters/sqm/day Range: This range indicates the typical values for overflow rates in plain primary sedimentation tanks. In other words, for effective sedimentation to occur in such tanks, the flow rate should ideally fall within this range. If the overflow rate is too low, it may result in inadequate detention time for settling of solids, leading to poor sedimentation efficiency. Conversely, if the overflow rate is too high, it can cause turbulence and disturbance in the sedimentation process, hindering the settling of solids.
3. Significance: Maintaining the overflow rate within the range of 40,000 to 50,000 liters/sqm/day ensures optimal performance of the primary sedimentation process. This range allows sufficient time for suspended solids to settle at the bottom of the tank, facilitating the removal of a significant portion of the organic and inorganic solids from the wastewater. It helps in reducing the organic load and turbidity of the effluent, which is essential for subsequent treatment processes in the wastewater treatment plant.