Hello everyone! I've asked questions to the forum in the past with a lot of success so I was hoping I could get some help again  . I am currently calculating the retention time of our equalization tank. This tank is filled at a rate of 167 gpm until it reaches 95" at which point the tank is pumped down to 65". To calculate retention time, would I use the entire fill volume of the tank or the volume between our HWL(95") and LWL(65"). The tank is thoroughly mixed. Because of this, I would think I would use the entire volume of fill (11,021 gal+21,158 gal) but I wanted to confirm. Below is a link to diagram that may help demonstrate it's operation.

http://dl.dropbox.com/u/12362113/WWTP/Equalization%20Pit.JPG

To increase detention time, would it be advisable to increase the settings for our HWL or decrease the LWL?

grrun, thanks for the great response! As you suggested, we will take grab samples at the influent and effluent of the equalization tank to measure our COD levels. We can do this at a variety of different HWL and LWL settings to see what works best for our system.

I believe we should offset the time between grab samples by the retention time of the equalization pit. Should I only use the volume of water between the HWL and LWL to calculate our retention time? Given the definition of retention time you provided (time between cycles), this would seem to be the case.

i.e. volume of water between HWL and LWL=11,021 gallons and we have an average influent flow of 167 gallons/min. Therefore the time it takes to fill that volume is about 66 minutes. We have a pump that operates at 300gpm to empty the water from the tank once it reaches the HWL. Water still flows into the tank while it is being emptied so the tank will be emptied at 300-176=133gpm. It would take this pump 83 minutes to empty the volume of water between HWL and LWL. Therefore our retention time would be 66(fill time)+83(transfer time)=149 minutes. Meaning we should offset our grab samples by 149 minutes on average. Would you agree? Of course, this is a very rough estimate because the flow is not always constant and retention time will change between each cycle but will ultimately average about 149minutes. Another simpler idea may be to take a grab sample at the influent/effluent of the tank after each emptying cycle is complete.  

I agree with your thoughts regarding the HWL and LWL. We have a fair amount of play to increase our HWL because the equalization pit precedes our EAF which can operate well past our maximum flow so overflow is not an issue. The LWL will be a bit trickier to optimize because our slug loads are unpredictable.