2 Definition Types of Filters * Filtration is a solid-liquid separation by passing through a porous medium to remove fine suspended solids.Types of Filters1. Single –medium Filters: Sand or crushed anthracite coal2. Dual Media filters: Crushed anthracite coal and sand.3. Multimedia Filters: 3 types , anthracite coal, sand & garnet
6 Advantages of Dual media coal-sand filter Conventional Sand Filter vs.Conventional Sand Filter1. Longer filter runs2. Higher filter rates
7 Why Chemical Coagulation before Filtration? * Reduce Turbidity Load prior to filter* Residual coagulant removes microscopic matter
8 Limitations of Direct Filtration? * Color units less than 40 units* Turbidity less than 5 NTUs* Iron < 0.3 mg/L and Manganese < 0.05 mg/l
9 Qo = Fine grained media and low filtration rates Q1 = Filter action is by depth removalQ2 = Filter action is by surface and depth removal
10 Filter Design Effective Size = Sieve size in mm that will pass 10%(wt.) of sandUniformity Coefficient =Sieve size passing 60% of the sand ÷ by size passing 10%Sand Filter Standard Rate = 2 gpm/square footMaximum Standard Rate for Filtration = 5 gpm/square foot
11 Optimum Filter Operation Occurs When :1. Maximum Allowable Head Loss is 8 to 10 feet2. Maximum allowable effluent turbidity < 0.3 ntu3. Backwash Water Use is <1% of Filtered Water
12 Filter Operational Goals 1. Effluent Turbidity < maximum permissible limit or <0.3 ntu2. Backwash Flow Rate = 15 to 20 gpm/square foot3. Filter Bed Expansion = 20% to 50%4. Backwash Water Amount = < 1%
13 Rate of Flow Controller Vs. Declining-Rate Controller Rate of Flow Controller Maintains Constant Flow duringFilter Run.Declining-Rate Controller Decreases flow during filter runa. Less attention by Operatorb. Avoid Harmful rate changes during filter run
14 Filter Backwash Procedure Step 1 – Close Influent ValveStep 2 – Close Effluent ValveStep 3 – Surface Washers are turned on.Step 4 – Backwash Valve is opened partiallyStep 5 – Backwash Valve opened to expand bed 20 to 50%Step 6 – Backwash period = to 5 to 15 minutesStep 7 – Surface wash turned off 1 minute before end of BWStep 8 – Wash Water Valve closedStep 9 – Filter Bed is rested for 30 to 60 minutesStep 10 – Filter Effluent Valve is openedStep 11 – Filter to Waste is openedStep 12 – Filter to Waste ends when Eff. Turbidity is OKStep 13 – Filter to Waste Valve is closedStep 14 – Filter Effluent Valve is opened
15 Filter ProblemsMudball Formation – inadequate backwash and surface washFilter Bed Shrinkage – Bed compresses and shrinksGravel Displacement – Backwash valve is opened too quicklyExcessive Media Loss – Caused by excessive backwashing rateAir Binding – negative head operation and low water levels.
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