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enhancing rapid sand filtration by backwashing with alum

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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enhancing rapid sand filtration by backwashing with alum

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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?

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