Enhancing Rapid Sand Filtration by Backwashing with Alum

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Enhancing Rapid Sand Filtration by Backwashing
with Alum

Turbid Water
Alum
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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
  particles
• Usually used after flocculation and sedimentation
  in water treatment plants for polishing
• Requires backwashing to clean filter

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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

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How to Improve Filter Efficiency?

• ALUM
• Al2(SO4)314.3H2O
• Alum decreases repellant inter-particles forces
• Commonly used in coagulation in WTPs
• If alum is in a filter it should mediate
  particle-media attachement

Picture from www.foodsubs.com/Misc.html
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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
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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 filters turbidity removal.
  efficiency and ripening time

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

• 5 states used in experimentation
• Backwash
• Backwash With Alum
• Settle After Backwash
• Filter
• Settle After Filter

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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

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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)
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ResultsParticle 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
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ResultsRipening 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
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ResultsRipening Time

• Ripening time did not improve with increasing
  alum dose

Summary of observed ripening times for all 4 hour
runs.
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ResultsRipening Time

• Minimum achieved turbidity did not correlate with
  alum dose either

Summary of observed minimum turbidities for all 4
hour runs.
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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 . . .

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Difficulties

• Assumption that filter influent was consistently
  25 NTU
• Stamp box response to state changes (hence the
  settling states in Process Controller)

YAAGH!!!!
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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

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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

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Questions?
Turbid Water
Alum