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 Usually used after flocculation and sedimentation in water treatment plants for polishing Requires backwashing to clean filter

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

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

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

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

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

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

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

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)

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

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

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

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

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 . . .

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

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

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

Questions? Turbid Water Alum