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

2. 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

6. Advantages of Dual media coal-sand filter Conventional Sand Filter
vs.
Conventional Sand Filter
1. Longer filter runs
2. 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 removal
Q2 = Filter action is by surface and depth removal

10. 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

11. 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

12. 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%

13. 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

14. 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

15. 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.

16. QUESTIONS???