The term "Detention Time" is frequently heard in wastewater treatment plants. But, before we calculate the Detention Time of any tank or basin, we should first understand what it is and why it is important.
What is Detention time in wastewater treatment and How to Calculate?
Detention time is the amount of time water is held in a vessel or basin, or the time it takes for water to flow from one end of a settling basin to the other. When we calculate unit process detention times, we are calculating how long it takes for the water to flow through that unit process.
Values of Detention time
The values you receive when calculating detention time are only theoretical. This is because, it is impossible to predict when a single drop of water will flow through a tank or basin. These theoretical values are close enough for operational calculations in most cases.
In reality, some particles may be retained in a basin for a much longer period of time, while others may pass through with little or no time spent in it. Short-circuiting occurs when water flows directly through a tank, like this.
Dead spaces at the basin's corners, stratification due to temperature differences, insufficient baffling, and surface effects from wind, all affect how any two drops of water flow through a basin. These are the same factors that cause a basin to short-circuit.
Detention times are typically calculated for the basins or tanks listed below:
• Flocculation basins
• Flash mix chambers in (seconds) or (minutes)
• Clarifiers or sedimentation tanks in (hours)
• Sewerage ponds in (days)
Now consider a simple example!
To calculate the detention period of a tank, the tank's volume must first be determined.
The volume (V) of a tank 70 feet long (L), 25 feet wide (W), and 12 feet deep (D) would be:
V = L x W x D
V = 70 x 25 x 12
V = 21,000 ft3
Gallons = V x 7.48 [1ft3=7.48gallons]
So, total tank volume in gallons = 21,000 x 7.48 = 157,080 gallons
If we assume that the plant filters 300 gpm, we have 157,080 300 = 524 minutes of detention time, or roughly 9 hours. In other words, the detention time is the amount of time it takes, for the coagulated water to flow through the tank / basin.
How do you determine detention time?
Here's how to calculate detention time:
Detention Time = Volume/Flow rate
Detention time is calculated by dividing the tank or basin volume by the flow rate. It's fairly straightforward. Making sure the units at the top and bottom of the formula cancel out, is the most difficult part of calculating detention time.
Challenges in calculating detention time!
The most difficult aspect of detention time problems is ensuring that your units remain consistent, throughout the problem. Many problems provide dimensions in feet as well as flow rates, in gallons per day or gallons per hour.
After calculating the tank's volume in cubic feet, you must convert that value to gallons. Multiply the volume (in cubic feet) by 7.48 gallons per cubic foot. This results in gallons on the top half of the equation, which cancels out with gallons in the flow rate on the bottom half. This leaves you with only time units.
The calculated detention time will be in the flow rate units you specify. If the flow rate is gallons per hour, the detention time will be expressed in hours.
Clarifiers typically have a detention time of 2 to 3 hours for both water and wastewater treatment. The flow spends a long time in the clarifier to allow the solids to settle to the bottom. Otherwise, the clarifier's effluent will contain more solids.
How can we assist?
If you have any questions about wastewater treatment plants, or if you would like to discuss your specific needs, please contact the Netsol Water team right away. Our team is an expert in water and wastewater treatment and can help you tailor a solution, to your specific requirements.
We collaborate with our clients to design a customized wastewater treatment solution, which meets your specific requirements, as well as an on-going service plan to completely maintain your system.
Netsol Water is Greater Noida-based leading water & wastewater treatment plant manufacturer. We are industry's most demanding company based on client review and work quality. We are known as best commercial RO plant manufacturers, industrial RO plant manufacturer, sewage treatment plant manufacturer, Water Softener Plant Manufacturers and effluent treatment plant manufacturers. Apart from this 24x7 customer support is our USP. Call on +91-9650608473, or write us at enquiry@netsolwater.com for any support, inquiry or product-purchase related query.
