Optimización de Filtros de Medio Granular

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Evaluation & Optimization of

Granular Media Filters

Molly Page, PE



Water Workshop

OTCO

March 27, 2012

• Approaches to FilterOptimization

• What defines “good”design/performance ?

– Water Quality Goals

– Operational Goals

• Common Causes of Sub-ParPerformance

• Evaluation & OptimizationTechniques

Presentation Outline



Water Workshop

OTCO

March 27, 2012

• Mostly common sense…not a“black art”

• Be organized, stick to the facts

• Systematic gathering ofinformationabout plant performance:

– Data trending & analysis – Check plant design criteria

against “actual”

– Track chemical dosing vsperformance

– Field measurements

• Visual Observations

Approaches to Filter Evaluation & Optimization



Water Workshop

OTCO

March 27, 2012

Typical Filter Run

Filter-to-Waste will

improve overall

filtered water quality



Water Workshop

OTCO

March 27, 2012

• Consistently less than 0.3 NTU

• Particle counts < 50particles/mL

• > 2-log removal of Giardia andCryptosporidium sized particles

• Long and predictable filter runs(24+ hours) – Same for eachfilter

• Minimal premature particlebreakthrough

What constitutes “good” filter performance ?



Common Filter Operating

Problems & Causes



Water Workshop

OTCO

March 27, 2012

Common Filter Problems

Filter Hydraulic Shock



Water Workshop

OTCO

March 27, 2012


Valve “Hunting”



Water Workshop

OTCO

March 27, 2012

• Backwashing of other filters increases flow to remaining filters

• Short term hydraulic shock dislodges particles


Spiking during Backwash



Water Workshop

OTCO

March 27, 2012


Effluent Turbidity “Creep”



Water Workshop

OTCO

March 27, 2012

Particle Count Data is particularly sensitive for filter monitoring

1

10

100

1,000

10,000

0 10 20 30 40 50 60

Filter Run Time (h)

N u m b e r

o f P a r t i c l e s

> 2 u m ( C

o u n t s / m L )

Filter#2 Raw

1-log removal

2-log removal

2.5-log removal

Filters are at 1.5 log removal

for latter part of filter run



Water Workshop

OTCO

March 27, 2012

• Poor pre-treatment performance

• Excessive operational loading rates• Lack of filter-to-waste capability

• Hydraulic “shock”

• Filter media loss or upset

• Filter underdrain damage or failure

• Poor cleaning effectiveness

– Mudballing

– Short circuiting

– Intermixing of media

Common Causes for Poor Filter Performance



Water Workshop

OTCO

March 27, 2012

• All media should be selected toshare a common fluidization velocity

– Minimize intermixing of layers

• Severe intermixing causes shortfilter runs by reducing void volumein upper layer of filter

• Note: Media characteristics canchange over time:

– Encrustations

– Deposition

– Physical degradation of media grains(wear)

Good Filter Design Practice



Water Workshop

OTCO

March 27, 2012

Appropriate Backwashing Rates

Note: Rates are at 20C, and must

be adjusted for other temperatures



Filter Evaluation Techniques



Water Workshop

OTCO

March 27, 2012

• Visual Inspection

• In-Filter Techniques

– Filter Core Sampling

– Backwash Waste Characterization

– Floc Retention Profiling

– Bed Fluidization

– Mudball Analysis

– Gravel Profiling

• SAFETY FIRST

Granular Media Filters Evaluation Techniques



Water Workshop

OTCO

March 27, 2012

• Never walk directly on filter media

• Ensure filter is FULLY drainedbefore entering filter box

• Beware of filter appurtenances – Wear a hard hat

• Use a safety harness whereapplicable, particularly during bedfluidization testing

Filter Evaluation Safety Issues

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Water Workshop

OTCO

March 27, 2012

• The first line of defense in filtermonitoring and evaluation

• Look for easy to recognize issues:

– Media boiling during wash

– Uneven wash distribution

– Uneven overflow into BW troughs

– Cratering in media surface

– Visible mudballs

• Create a map of the filter and trackobservations for future mitigation

Visual Observation of Filters



Water Workshop

OTCO

March 27, 2012

• Boiling is usually clearly visible during the earlystages of backwash

Visual Inspection Filter “Boiling”



Water Workshop

OTCO

March 27, 2012

• “Mounding” of filter mediasurface – Suggests possible

disturbance in gravel layer – High localized flow

Visual Inspection Media “Mounding”



Water Workshop

OTCO

March 27, 2012

• Suggests possible damage tounderdrain

Visual Inspection Media “Cratering”



Water Workshop

OTCO

March 27, 2012

• Insufficient Wash Velocities toremove heavier mud and silts

• Effectively blocks off filter areaand increases local loadingrates

Visual Inspection Mudballs



Water Workshop

OTCO

March 27, 2012

• “Cracking” at media surface

• Sand separation at filter walls

• Visible algae growth

• Filter media in troughs

• Has scaling or fouling changed

the backwash characteristics ofthe media ?

• Depth of Media – Is it uniform ?

• Are the washwater troughslevel ?

• Freeboard – Top of media tounderside of trough

• Does surface wash effectivelyreach the corners ?

Visual Inspection Other Issues



Water Workshop

OTCO

March 27, 2012

• A number of techniques can beused to diagnose filter

performance issues – Filtration Rate Checking

– Backwash Rise Rate Checking

– Floc Retention Profiling

– Backwash WasteCharacterization

– Gravel profiling

– Sieve testing of media size

– Media bed depth checking

– Bed Fluidization Checking

In-Filter Evaluation Techniques

• Tests are relatively easy, andcan use

home-made testing equipment



Water Workshop

OTCO

March 27, 2012

Homemade Device for Walking on Filter Media



Water Workshop

OTCO

March 27, 2012

• Source: Filter Maintenance &Operations Guidance Manual,

AWWARF, 2002

Filter Core Sampling Coring Device



Water Workshop

OTCO

March 27, 2012

Filter Core Sampling Procedure



Water Workshop

OTCO

March 27, 2012

• Perform timed sampling of backwashwaste to determine solids content

• It is possible to over-wash a filter

– Backwash waste characterizationcan help assess the “right”duration

– Use data to assess when toterminate washing

• May allow reduction in waterwastage, and residuals volumes

Backwash Waste Characterization



Water Workshop

OTCO

March 27, 2012

Backwash Waste Characterization Typical Results

0.1

1

10

100

1000

0 2 4 6 8 10 12 14 16 18 20

W a s t e T u r b i d i t y , N T U

Backwash Duration, Minutes

Backwash Waste Turbidity, NTU

Target Turbidity, NTU

Backwash should

be cut-off here



Water Workshop

OTCO

March 27, 2012

• Take a core sample

• Sub-divide the core into depthfractions

• Rinse each fraction using a knownvolume of water, to clean solids offthe media

• Measure turbidity in each fraction,before and after washing of filter

• Measures how well solids arebeing removed from the bed

Floc Retention Profiling



Water Workshop

OTCO

March 27, 2012


AWWARF, 2002

Floc Retention Profiling Sampling Equipment



Water Workshop

OTCO

March 27, 2012

Floc Retention Profiling Typical Results

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150

D e p t h f r o m T o p o f M e

d i a , i n c h e s

Turbidity, NTU

After Backwash

Before Backwash



Water Workshop

OTCO

March 27, 2012

Turbidity, NTU Filter Media Condition

0 - 30 NTU Clean - Unripened filter – Long ripening time

30 - 60 NTU Clean - Ripened Filter

60 - 120 NTU Slightly dirty, Still OK

120 - 300 NTU Dirty - Re-Evaluate Backwashing

> 300 NTU Mudball Problems

Floc Retention Profiling Evaluation of Results



Water Workshop

OTCO

March 27, 2012


AWWARF, 2002

Filter Bed Fluidization Testing Equipment



Water Workshop

OTCO

March 27, 2012

Filter Bed Fluidization Test Protocol



Water Workshop

OTCO

March 27, 2012

Backwash Flow Rate Bed Expansion Notes

Below required < 20%

Increase backwash rate and

repeat test

Below required 20 - 30%Increase backwash rate and

repeat test

Below required > 30% Check water temperature

Correct Flow < 20%

Check for Polymer Buildup on

Filter

Correct Flow 20 - 30% No Action Required

Correct Flow > 30% Check Media Specs.

Above Required < 20%Check for Polymer Buildup on

Filter

Above Required 20 - 30%Check for Polymer Buildup on

Filter

Above Required > 30%Reduce Backwash Rate and

Repeat Test

Filter Bed Fluidization Evaluation of Results

M db ll A l i S li P l



Water Workshop

OTCO

March 27, 2012

Mudball Analysis Sampling Protocol

M db ll A l i E l ti f R lt



Water Workshop

OTCO

March 27, 2012

Percent Mudballs Filter Condition

0 - 0.1% Excellent

0.1 - 0.2% Very good

0.2 - 0.5% Good

0.5 - 1.0% Fair

1.0 - 2.5% Fairly Bad

2.5 - 5% Bad

Over 5% Very Bad

• Used to physically determinethe extent of filter mudballing

due to chronic underwashing• Collect 6” Core Samples

• Gently sieve the samples toseparate mudballs from media

• Place mudballs into a 250 mLgraduated cylinder

Mudball Analysis Evaluation of Results

G l P fili



Water Workshop

OTCO

March 27, 2012

• Manual measurement of graveldepth at various locations in the

filter

Gravel Profiling

• Variation should be no morethan± 25 mm

G l P fil E l



Water Workshop

OTCO

March 27, 2012

Gravel Profile Examples

GoodSevere

P ibl S l ti f P Filt P f



Water Workshop

OTCO

March 27, 2012

• Optimization of FilterBackwashing

– Even distribution of flow – Selection of Appropriate Wash

Rates

– Levelling of Wash Trough Crests

– Air Scour

– Surface Wash

• Addition to Filter-to-Waste

• Use of filter aid polymers

• Addition of coagulant or other

chemicals to backwash water

Possible Solutions for Poor Filter Performance

E ll t R



Water Workshop

OTCO

March 27, 2012

• AWWA Std. M37 – “Operational Control of Coagulation and Filtration Processes”, 2000

• AWWARF,“Filter Maintenance and Operations Guidance Manual”, 2002

• AWWARF,“Self Assessment Guide for Surface WTP Optimization”, 1997

• AWWA,“Filter Evaluation Procedures for Granular Media”, 2002

• AWWARF,“Design & Operation Guidelines for Optimization of the High-RateFiltration Process – Plant Survey Results”, 1989

• AWWA,“Filter Troubleshooting & Design Handbook”, 2005

Excellent Resources



Questions ?

Optimización de Filtros de Medio Granular

Documents

filter optimization

short filter

typical filter

predictable filter

filter appurtenances

filter box

good filter performance

backwash water workshop