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Enhancing Rapid Sand Filtration by Backwashing with Alum

Enhancing Rapid Sand Filtration by Backwashing with Alum. Turbid Water. Alum. Rapid Sand Filtration. Rapid Sand Filtration: Filtration of water by passing it through a fine grained media to remove very small suspended particles

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Enhancing Rapid Sand Filtration by Backwashing with Alum

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  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: www.foodsubs.com/Misc.html

  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. Backwash effluent (to waste) - Solenoid valves Physical Apparatus 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 Manual needle valve Pressure regulating valve Peristaltic Pump 2

  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 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) 100 mg/L Control

  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

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