Filtration by Douglas Rittmann, Ph.D., P.E.

Definition Types of Filters * Filtration is a solid-liquid separation by passing through a porous medium to remove fine suspended solids. Types of Filters 1. Single –medium Filters: Sand or crushed anthracite coal 2. Dual Media filters: Crushed anthracite coal and sand. 3. Multimedia Filters: 3 types , anthracite coal, sand & garnet

Advantages of Dual media coal-sand filter Conventional Sand Filter vs. Conventional Sand Filter 1. Longer filter runs 2. Higher filter rates

Why Chemical Coagulation before Filtration? * Reduce Turbidity Load prior to filter * Residual coagulant removes microscopic matter

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

Qo = Fine grained media and low filtration rates Q1 = Filter action is by depth removal Q2 = Filter action is by surface and depth removal

Filter Design Effective Size = Sieve size in mm that will pass 10%(wt.) of sand Uniformity Coefficient = Sieve size passing 60% of the sand ÷ by size passing 10% Sand Filter Standard Rate = 2 gpm/square foot Maximum Standard Rate for Filtration = 5 gpm/square foot

Optimum Filter Operation Occurs When : 1. Maximum Allowable Head Loss is 8 to 10 feet 2. Maximum allowable effluent turbidity < 0.3 ntu 3. Backwash Water Use is <1% of Filtered Water

Filter Operational Goals 1. Effluent Turbidity < maximum permissible limit or <0.3 ntu 2. Backwash Flow Rate = 15 to 20 gpm/square foot 3. Filter Bed Expansion = 20% to 50% 4. Backwash Water Amount = < 1%

Rate of Flow Controller Vs. Declining-Rate Controller Rate of Flow Controller Maintains Constant Flow during Filter Run. Declining-Rate Controller Decreases flow during filter run a. Less attention by Operator b. Avoid Harmful rate changes during filter run

Filter Backwash Procedure Step 1 – Close Influent Valve Step 2 – Close Effluent Valve Step 3 – Surface Washers are turned on. Step 4 – Backwash Valve is opened partially Step 5 – Backwash Valve opened to expand bed 20 to 50% Step 6 – Backwash period = to 5 to 15 minutes Step 7 – Surface wash turned off 1 minute before end of BW Step 8 – Wash Water Valve closed Step 9 – Filter Bed is rested for 30 to 60 minutes Step 10 – Filter Effluent Valve is opened Step 11 – Filter to Waste is opened Step 12 – Filter to Waste ends when Eff. Turbidity is OK Step 13 – Filter to Waste Valve is closed Step 14 – Filter Effluent Valve is opened

Filter Problems Mudball Formation – inadequate backwash and surface wash Filter Bed Shrinkage – Bed compresses and shrinks Gravel Displacement – Backwash valve is opened too quickly Excessive Media Loss – Caused by excessive backwashing rate Air Binding – negative head operation and low water levels.

QUESTIONS???