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SVI of 250 mg/I or higher.At this elevated SVI.The sludge settles very slowly and compacts <br /> poorly_ The MLSS looks light and fluffy, not very dense. <br /> SVI is calculated using two numbers. The first number comes from the 30-minute settleability <br /> test. The volume of the settled sludge is measured at 30 minutes in ml/I. The second number is <br /> the MLSS in mg/I. <br /> Sludge Volume Index Calculation - SVI <br /> (Settleability ml/1) x (1,000 mg/g) /(MLSS mg/1) =the SVI ml/g <br /> Example: <br /> Given a 30-minute settleability of 250 ml/I <br /> Given MLSS of 3,600 mg/1 <br /> ❖ (250 ml/I) x (1,000 mg/g)/(3,600 mg/1) = 69 mg/1 <br /> Food to Microorganism Ratio <br /> The food to microorganism ratio (F:M) is one of the primary controls used. The ratio helps the <br /> operator maintain a balance between the food available with the number of microorganisms <br /> needing food. <br /> The food available is the BOD of the wastewater entering the aeration tank and the <br /> microorganisms are the MLVSS in the tank. If the MLVSS are not know use the MLSS. Best <br /> treatment is typically in the F:M range of 0.05 to 0.15. <br /> Food to Microorganism Calculation - F•M <br /> (BOD lbs/day)/(MLVSS lbs/day) = F:M ratio <br /> Example: <br /> Given BOD of 120 rng/I which equals 54 lbs/day at influent rate of 54,000 gpd <br /> Given MLVSS of 2,690 mg/I which equals 1,211 lbs/day in influent rate of 54,000 gpd <br /> (54 lbs/day)/(1,211 lbs/day) = 0.04 <br /> Since the operator has no control over the number of pounds of BOD entering the plant, the F:M <br /> ratio is adjusted by increasing or decreasing the wasting time. <br /> F:M is most useful when the operator sets a target F:M based on the average BOD loading, and <br /> then calculates the pounds of MLVSS needed in the system to achieve the target ratio. <br />