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Rapid Sand Filtration. Rapid Sand Filtration:Filtration of water by passing it through a fine grained media to remove very small suspended particlesUsually used after flocculation and sedimentation in water treatment plants for polishingRequires backwashing to clean filter. Filter Removal EfficiencyParticle BreakthroughRipening Time
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1. Enhancing Rapid Sand Filtration by Backwashing with Alum
3. Filter Removal Efficiency
Particle Breakthrough
Ripening Time– the time it takes for a filter to achieve the desired effluent turbidity
Minimum Turbidity Achieved Quantifying Filter Performance
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
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
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
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
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)
12. Results:Ripening Time Ripening Time estimated from effluent turbidity graphs
Virtually no ripening time with alum added
13. Results:Ripening Time Ripening time did not improve with increasing alum dose
14. Results:Ripening Time Minimum achieved turbidity did not correlate with alum dose either
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)
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?