1. Enhancing Rapid Sand Filtration by Backwashing with Alum
Turbid Water
Alum

2. Rapid Sand Filtration Rapid Sand Filtration:
Filtration of water by passing it through a fine grained media to remove very small suspended particles
Usually used after flocculation and sedimentation in water treatment plants for polishing
Requires backwashing to clean filter

3. Quantifying Filter Performance
Filter Removal Efficiency
Particle Breakthrough
Ripening Time– the time it takes for a filter to achieve the desired effluent turbidity
Minimum Turbidity Achieved

4. How to Improve Filter Efficiency?
ALUM
Al2(SO4)3*14.3H2O
Alum decreases repellant inter-particles forces
Commonly used in coagulation in WTP’s
If alum is in a filter it should mediate particle-media attachement
Picture from:

5. Our Approach Add alum in a backwash state
Mixing should evenly coat the media throughout the filter Avoid creating a cap of flocculated particles on top of the filter column that creates high head loss
Flocculated “Cap”
Alum

6. Objectives To create a method to add alum to a filter in backwash mode
To characterize the effects of varying the alum dose on the filter’s turbidity removal. efficiency and ripening time

7. Physical Apparatus Backwash effluent (to waste) - Solenoid valves
Concentrated clay suspension (~ 4 g/L) 4
Stir plate
Peristaltic Pump 1
Sand filter column
Effluent (to waste)
Flow Accumulator
Turbidimeter
Overhead water influent 1 2
20 g/L alum stock
Head loss device and 7 kPa pressure sensor 3
Pressure regulating valve
Manual needle valve
Peristaltic Pump 2
Physical Apparatus

8. Process Control 5 states used in experimentation: Backwash
Backwash With Alum
Settle After Backwash
Filter
Settle After Filter

9. Experimentation
Tested five different initial alum doses to the filter and one control filter
Control, 25, 50, 100, 200, and 590 mg/L
4 hour filter runs (extended from 2 hour runs)
Alum dose changed by increasing/decreasing peristaltic pump speed
Calibrated the influent turbidity before each experiment to 25 NTU by manually adjusting the turbidity pump speed

10. Results: Overall Filter Performance
Increased overall removal efficiency
Positive correlation between increased alum dose and increased particle removal
Alum Dose
Percent Removal
control 53
25 mg/L 76
50mg/L 78
100 mg/L 82
590 mg/L
90%
Filter performance for 590 mg/L and 50 mg/L alum dosage (4 hrs run)

11. Results: Particle Breakthroughs
Reduction in magnitude of particle breakthroughs
Elimination of breakthroughs during approx. first hour of filtration.
Reduced particle breakthroughs at high alum doses (590 mg/L)
Control
100 mg/L

12. Results: Ripening Time
Ripening Time estimated from effluent turbidity graphs
Virtually no ripening time with alum added
Control ripening time
~ 1200 seconds
25 mg/L alum ripening time
~300 seconds

13. Results: Ripening Time
Ripening time did not improve with increasing alum dose
Summary of observed ripening times for all 4 hour runs.

14. Results: Ripening Time
Minimum achieved turbidity did not correlate with alum dose either
Summary of observed minimum turbidities for all 4 hour runs.

15. Capacity Analogy
The results show that alum dose to the filter can be thought of as a “capacity” for particle removal
Improved overall efficiency for 4 hour run
No correlation between alum dose and ripening time or minimum turbidity
Filling two glasses analogy . . .

16. Difficulties
Assumption that filter influent was consistently 25 NTU
Stamp box response to state changes (hence the settling states in Process Controller)
YAAGH!!!!

17. Future Research
Use of two turbidimeters – one above and one below the filter – to make constant NTU assumption moot
Measure head loss through pre-coated filter
Longer filter runs
Vary influent turbidity and alum dosage

18. Summary & Conclusions
Unique method of alum addition to a rapid sand filter
Substantial impact on filter performance under low turbidity conditions:
Improved overall performance with alum addition
Reduced particle breakthrough
Virtual elimination of the ripening time
No correlation between increased alum dose and shortened ripening time or minimum turbidity

19. Questions?
Turbid Water
Alum