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Cell Residence Time Calculation <br />Cell Residence Time (CRT) can also be determined by dividing the total pounds of MLVSS in the <br />aeration system by the number of pounds of biomass (sludge) wasted per day. If MLVSS is not <br />known, then use the MLSS in both the top and bottom of the equation. <br />The number of pounds wasted per day can be calculated by taking a grab sample of the WAS <br />calculate the dried sample weight, then multiply by 100 (assuming a starting sample volume of <br />10 ml) to determine the TSS in mg/I. <br />The volume wasted is the amount the air lift pumps (about 40 gpm) x (number of minutes the <br />pump is on). Verify the volume of sludge wasted by checking the starting and ending levels in the <br />solids handling tank in inches. Each inch of change in height in the solids tank is about 209 <br />gallons. <br />Waste Activated Sludge Wasted Per Day Calculation <br />(weight of dried sample mg) x (factor used to bring sample volume to 1 liter) = TSS WAS mg/I <br />Example: <br />Given dried sample weight of 83 mg from WAS return pipe outlet in solids tank <br />Given initial sample volume of 10 ml (10 ml x 100 = 1 liter) <br />❖ (83 mg) x (100) = 8,300 mg/I TSS of WAS <br />To convert to pounds: <br />(WAS return rate mgd) x (weight of water lbs/gallon) x (WAS TSS mg/1) = lbs/day of WAS <br />Example: <br />Given volume wasted is the WAS rate of 40 gpm for 15 minutes = 600 gal or 0.00060 <br />mgd <br />Given WAS TSS of 8,300 mg/I <br />❖ (0.00060 mgd) x (8.34 lbs/gal) x (8,300 mg/1) = 42 pounds/day WAS wasted <br />Cell Residence Time Calculation <br />(MLVSS lbs)/MLVSS wasted lbs/day) = CRT in days <br />or <br />(MLSS lbs)/MLSS wasted lbs/day) = CRT in days <br />