Chesapeake Section AWWA, Inc.

Pretreatment and Filter Maintenance,

Configuration, and Optimization Seminar

Filter Operations
April 23rd & 24th, 2013
Jeremy Hise, P.E.
Hazen and Sawyer P.C.

Overview
Learning objectives
Filtration Background
Filter Design Considerations
Why should operators optimize filtration?
How to measure the progress of optimization?
What are the important parameters?
What tools and techniques are needed?

Necessary skills to assess filter efficiency

Parameters used for comparison

History of Rapid Gravity Filters

First used at the turn of 20th
century
Used as roughing filters as pretreatment for slow sand filters e.g.
London, England
Used after sedimentation before
floc formation and settling was
well understood e.g. Cincinnati
and Detroit in the US, and Zurich,
Switzerland

1905 Richard Miller WTP, Cincinnati, OH

Filtration Where is it?

Purpose - remove particulate material from water
Conventional Treatment
Rapid Mix

Floc/Sed

Dual-Media
Filter

Storage

Distribution

Direct Filtration
Rapid Mix

Floc

Dual-Media
Filter

Storage

Distribution

Filtration What does it look like?

Whats missing???

History of Water Filtration

Filtered Water Turbidity Standards
12.0
10.0

10.0

Turbidity (NTU)

8.0
6.0

5.0

4.0
2.0
0.0

1.0

0.5

0.3

0.1

IESWTR Performance Standards

Turbidity Performance Requirements
Combined FE Turbidity must be < 0.3 NTU in 95%
of measurements, and never > 1 NTU

Individual Filter Requirements

Continuous monitoring required for each filter, and
exceptions reported

Filter Backwash Recycle Rule

FBRR applies to surface water and GWUDI
utilities that:
Recycle thickener supernatant
Recycle spent filter backwash water
Recycle dewatering system liquid streams

There is no size limitation for plants

Rule requires that these streams be returned to
a location prior to all conventional processes

Filtration Phases
Clean Bed
Removal

Ripening

Turbidity or Particle Count

Breakthrough

Steady state
Filtration

Headloss

Filter Run Time

What Else Do Filters Do??

We ask filters to do MORE than filter.
GAC taste & odor control
Mn treatment
Biological treatment
Especially for preozone

Filters and Particle Size

Particle Diameter
10-10
1

10-9
1 nm

10-8

10-7

10-6
1 m

10 nm 100 nm

10-5

10-4

10 m 100 m

10-3
1 mm

Cryptosporidium oocysts
Giardia cysts

Molecules
Colloids
Nano-particles
Virus

Suspended Particles
Bacteria

Micro-particles
Algae

RO
NF

Granular Media

UF
MF

1 nm

10 nm 100 nm

1 m

10 m 100 m

(After Stumm, ES&T, Vol. 11, p. 1066, 1977)

1 mm

10-2 m

10 m
(0.01 mm)

To address size & shape of particles

desired to be removed
Create environment in filter media for removal
Physical, but mostly
CHEMICAL

Chemical influences

Source water quality

Pretreatment
Softening
Recarbonation
Prefilter chemicals

The Prediction of Filtration Performance

Particle Removal and Filter Effluent

Quality; Head loss increase; A
Complex Function of:
Site Specific & Uncontrolled
Ionic composition
Temperature
Influent Particle sizes, surface
properties, shape and
concentration
Deposit morphology/porosity
Detachment
Designer Controlled
Filtration Rate
Media: size, depth, material,
surface characteristics, porosity
Coagulants/flocculants

What does this all mean?

Filter media
Provides pore spaces to collect particles

Collected particles
Accumulate
Need to be removed
Taken away and dealt with

Performance depends on
Particle conditions shape and chemistry
Media / underdrain conditions
Operational techniques

Hydraulic Considerations

Example Filter Cross Section Layout

Design Velocity Guidelines
Inlet Velocities to Prevent Floc Damage
Inlet Velocities to Prevent Media Scouring
Distribution of Backwash Water
Distribution of Air Scour

Simplified Cross-Section of Filter Normal

Flow Path
To prevent media scouring ensure
inlet gate is not opposite a trough
Air Scour Header
Filter Inlet
Channel
and
Overflow

End
Gullet

Filter Gallery
Troughs

Waste
Washwater
Outlet
Channel

Media

Fill

Underdrain and Plenum

5 ft

Backwash
Filtered
Header
Water
Channel
Filtered
Water Outlet
FM

Filter to Waste
with air break

Design Velocity Guidelines

Inlet Channel ~3 ft/s excluding gate
width
Inlet Gate or Valve ~2 ft/s

>10 ft

Waste Outlet Gate or Valve ~6

ft/s

Backwash Header ~6 ft/s

Keep flooded to prevent air
ingress
Air Scour Header,
valve and drop
pipe ~80 ft/s
Filter Outlet
Channel ~4 ft/s
Filtered Water
Outlet ~6 ft/s,
Filter to Waste 8
ft/s
Discharge Weir above floor

Backwash Distributers ~4
ft/s several needed
0

5 ft

Waste Channel free

discharge to tank
velocity can be high

Air Scour 3.0 scfm/sq ft

Water surface 6 inches above media
at start, but increases during low rate
backwash - affects blower back
pressure

Note: SCFM not

ACFM
Air Header
above Top
Water Level
cannot flood

Underdrain to give
+/- 5% air
distribution

5 ft

Air Scour Distributer

Header design
varies to suit
underdrain type

Backwash 6 to 25 gpm/sq ft
Media expansion
15 to 30%, 20%
normal

Allow > 1 ft under

trough during
backwash

Underdrain to give
better than +/- 5%
distribution during
backwash
Backwash
distribution directed
to floor note
closed end

Free discharge
from troughs
and gullet
0

5 ft

Multi- and Mono-Media Filters

Dual-Media - Anthracite and Sand (> 3 ft) most common
Multi-Media Lower layer of garnet or ilmenite (FeTiO3)
usually not beneficial mixes with sand layer
Dual-Media - Sand with GAC cap for taste and odor
removal not organics removal
Mono-Medium Coarse Sand Used for tertiary filtration
and some overseas water applications for simplicity
Mono-Medium Coarse Anthracite (5 to 6 ft) not
recommended for potable water, especially if high rate
Mono-Medium GAC Can be up to 10 ft deep for high
EBCT for organics removal - 12 inches sand underneath
to prevent biomass sloughing into filtrate

Media Recommendations
Use AWWA B100-01 Granular Filter Media as basis for
specifications - Read and apply it!!!
Enforce full QA/QC procedures from suppliers premises
to after backwashing and skimming
Do NOT use bulk tanker delivery severe media attrition
will result plus possible contamination
Use semi-bulk containers / bags of woven material
Protect bags from weather sun (UV), rain and freezing
Do not stack bulk bags they will burst
Avoid hydraulic placement if possible attrition can be
severe leading to more backwashes to remove fines

Filter Media Key Issues

Filters are the most important part of water treatment
process
Media must be sourced from experienced vendors (e.g.
Unifilt and F.B. Leopold)
Tight QA/QC must be maintained
Media is heavy (4,000 lb) check structural floor loads,
including fork lift trucks and hoppers
Anthracite is particularly vulnerable to attrition and
incorrect supply (SG, ES, UC etc.)
Effective backwash procedures are crucial for good
media performance

Filter Backwashing
Filters get cleaned by
Using correct amount of wash water
At the correct flow rate
For the correct amount of time

Most common problems

Not following procedures

Inconsistent schedules
Poorly designed filters
Poorly designed support facilities
Backwashing based on run time and not
adjusting for water quality

Constant Rate Control - Recommended

L

Mode of Operation
Inlet water channel level provides flow set point.
Valve position controlled by flow signal.
As flow to inlet channel increases, water level increases,
allowing more flow through all filters. Flow feedback loop
maintains constant flow though individual filter. As
headloss builds up, the flow drops, causing valve to open
to maintain constant flow rate. All filters operate at same
flow rate. Avoid control loop hunting as this leads to
turbidity breakthrough.

Controlled Declining Rate Avoid in

US
L

If weir gate is used then

constant rate possible. Weirs
split flow equally to all filters.

Mode of Operation
Water level in the filter controls valve position directly.
As flow to inlet channel increases, water level increases,
allowing more flow through all filters. The flow meter is
passive and monitors flow only. As headloss builds up, the
level increases in the filter, causing valve to open to
maintain the same water level for all filters. Filters operate
at higher flows when clean and lower flows as the
headloss builds up. Filters are usually backwashed on a
regular time schedule.

Underdrains and Backwashing

Techniques
Large variety of underdrain designs lateral and plenum
are two main groups
Backwashing Techniques
Separate air scour + high rate backwash (good)
Combined air scour + low rate backwash, followed by high rate
backwash (best)
High rate backwash with surface sweeps (adequate)

Waste backwash water removal:

Troughs in USA,
Weirs in Europe

Backwashing fluidizes the media

Effect of Water Temperature and

Viscosity, on Backwash Rate
Nearly 30% more

25% less
Absolute Viscosity of Water (cp)
vs. Temperature (deg C)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.0

10.0

20.0

30.0

40.0

Media Selection and Fluidization

Sand
SG 2.6 to 2.70
ES 0.45 to 0.55 mm
UC <1.4
UC < 1.4 (critical)

Anthracite
UC 1.7
19.2 gpm/ft2

Sand (Average)
SG 2.65 ES 0.50 mm
ES x UC = 0.70
Backwash = 16.5 gpm/ft2

Anthracite
SG 1.55
14.0
gpm/ft2

Anthracite
SG 1.6 to 1.7 (critical)
ES 0.8 to 0.9
UC < 1.4 (critical)
Anthracite (Average)
SG 1.65 ES 0.85
ES x UC = 1.19
Backwash = 15.8 gpm/ft2

Susuma Kawamura ISBN 0-471-35093-1

Note 20 C 68 F

European anthracite
has low SG of 1.4

Media Expansion, Backwash and

Temperature Effects
Example
Aim for 20% to 30%
expansion for
sand/anthracite

50%
40%
30%
20%
10%

At low
temperature and
high backwash
the result is too
high expansion of
50% media
loss

Backwash
Operating
Band

Troughs Stainless Steel

Trough support up
and down thrust
restraint adjustable
Lateral brace to
prevent vibration
Trough stiffener to
prevent vibration
Trough side
stiffener (retro)

Media Loss
Common Causes

Uncontrolled air

How Air Causes Media

Media Loss
No
No Media
Loss

Media
Loss
Loss

Most common

Poor air water distribution

Possible, watch for spouts

Too high backwash rate

Air
Air Scour and
Risin
g Wash LLimit
imit
Rising

Possible, watch for churning

Gas bubbles on media

Unlikely, but dry-bed causes
severe foaming
Biogrowth leading to bubbles on
grains

Foam
after dry
bed

Air Scour Only

Only Limit

Anthr acite

Sand

Anthracite

Sand

Filter Underdrains - Lateral and Plenum

Lateral (Orifice Based with
and without diffusion caps)

F B Leopold (Shown)
AWI
Johnson
Roberts
Severn Trent

Plenum Concrete
Monolithic Floor with
Nozzles
Eimco (Shown)
IDI
Orthos

Lateral vs. Plenum Floor

Pros
Quicker to install
Easier to retro-fit
Pressure contained in
pipe lateral leaks less
likely
Factory built tends to be
more consistent
Be careful applying the
capped versions of
lateral underdrains,
especially for bio-filters,
due to blockage risks

Cons
Requires skilled
installation
Deep flume disrupts base
slab, makes deeper
excavation
Distribution efficiency
depends on lateral length
No access to clean
underdrain if blocked
Plastic laterals less
successful than SS
34

Filter Aid (Polymer)

Lowers effluent turbidity
Proper dose
Reduce ripening time
Stabilize turbidity
Stabilize rate of headloss

Overdose
May increases rate of headloss development
Take longer to clean

Why Optimize Filters?

Major barrier against pathogen passage
Maximize production efficiency
Minimize spent filter backwash water
Duration
Frequency

Increasing reliance on other WQ goals

T&O control
Mn control
Bacteriological stability

How is Progress Measured?

DATA / TRENDS
Individual filter turbidity
Filter run times
Amount of filter backwash water required
Production efficiency
What comes in vs. what goes out

Uniform filter run volume (UFRV)

Filters Provide Flexibility, but.

More Filters - greater chance that:
Bad filter goes unnoticed
Other processes can control filter ops

Less Filters greater chance that:

Other filters stressed when one is out of service
Operations less flexible
Plants may operate only part of day

Operator Perspective of Filter Theory

Filter is particle storage device not just particle
removal device
During storage phase gentle handling needed
During removal phase vigorous handling needed

Filters often designed as dual-media units

Provides deeper bed filtration
Longer runs

Good filtration depends on good pretreatment

Remember multiple barriers
Short run times = poor efficiency, lots of spent
backwash

Key to Good Filter Operational Techniques

Continuous operation
At startup
Bring filter rate up slowly
Dont start a dirty filter

During filter run

Ensure filter applied water is stable

Avoid or minimize hydraulic shock
Monitor headloss, NTU, run time
Use filter aid if appropriate for conditions

After backwash
Rest the filter before returning to service, or
Filter to waste

Key to Good Filter Backwash Techniques

Prior to backwash
Record filter run information
Verify backwash program parameters

During backwash

Choose a temperature dependent high flow wash rate

Avoid washes that are too short or long
Hose down the side walls and pipes/gutters
OBSERVE THE BACKWASH

Observations at each backwash

Surface or air wash effectiveness
View surface for boils or hot spots
Look for uneven wash areas or uneven troughs

42

Backwash Program
Drain
Make sure level is low enough to maximize
energy and minimize media loss

Surface wash or air wash

3 to 4 minutes is usually sufficient

Low rate initiates expansion

High rate expands media, temp dependent
Low rate - restratification

Key to Good Filter Maintenance Techniques

Once per quarter (per season)

Adjust high flow rate for temperature

Check media expansion make adjustments
Review unit filter run volume data
Check media depth
Review all filter profiles

Once per year

Core the filter solids retention

Send media to lab for sieve analysis
Add media if necessary
BUT know why its being lost

Probing Media Depth to the Gravel Layer

What Parameters are Important?

Parameters to examine

Media depth and percent expansion

L/de ratio
Unit Filter Run Volume (UFRV)
Solids retention of media
Backwash use / turbidity / temperature
Filter profile
Sieve analyses ES & UC, loss of mass

What do these parameters tell us?

Filter health
Process modifications
Backwash procedure modifications

Filter Inspection Techniques

Visual observation of filter surface and
components
Probing media
Solids retention analyses
Core sampling
Sieve analyses / media assessments

47

Tools and Techniques for Inspection

HEALTH & SAFETY REQUIREMENTS
Review AWWA Standard B100
Measurement Tools
Shovel, level, 3/8 inch steel rod, tape measure

Coring Tool
11/2 inch electrical conduit, 5 foot length, baggies

Expansion Tool
One-inch interval tubes or cups

Laboratory Instruments and Tests

Turbidimeter, glassware, balance, sample bottles, baggies

COMMUNICATIONS

Filter Inspection Tools

Media Assessment L/D ratio

Bed Depth Measurement (Drained Bed)
Know original specs
Effective size - Uniformity Coefficient - Depth - L/D ratio

Use a 3/8 inch steel rod to poke into media, or dig

into it to measure depth
If filter is dual or mixed bed, note depth of each strata,
and depth of mixed interface

Check to see if troughs are level, then measure

distance from trough to bed - check for mounding
Calculate L/D ratio - should be >1100 for low NTU
production

Example L/D
Dual Media - originally 36 inches 1 mm
anthracite and 6 inches 0.5 mm sand
Measurement shows 32 inches of anthracite, 6
inches of mixed layer, and 3 inches of sand = 41
total inches
Rough L/D calculation (send media for analysis)

(32in X 25.4)/1mm 813

(6in X 25.4)/0.75 mm 203
(3in X 25.4)/0.5mm 152
Therefore L/D = 1168

Core Sampling for Solids Retention

Solids retention analysis best way
to determine backwash
effectiveness
Use core sampling tool and
baggies to obtain depth samples
Take samples at 0-2 inches, 2-6, 6-12,
12-18, 18-24, etc., until all bed strata
are sampled
Sample before and after backwash
Wash 50 grams of each sample with 5
successive 100 mL washes of lab
water
Measure turbidity of each X 2- plot on
graph as NTU/100 grams media

Inserting Core Sampling Tubes

53

Examining Media Core Samples

54

Lab Setup for Core Samples

Turbidimeter
Pan balance
Baggies before
and after
Glassware
Lab water
Weigh boats or
other plastic cups

Guidelines After Backwash

< 30 NTU
Bed is too clean - examine wash rate and length - this
bed will not ripen quickly

30 - 60 NTU
Well cleaned and ripened bed - no need for action

60 - 120 NTU
Slightly dirty bed - reschedule retention analysis soon

> 120 NTU

Dirty bed - evaluate filter wash system and procedures

> 300 NTU

Mudball problem - rehab bed

Solids Retention
160
140

N T U /1 0 0 g r a m s o f m e d ia

Measures the
effectiveness of
backwash
Can show too little
or too much
backwash
Change in historical
solids retention is
cause for concern
Graph results for
database

120
100
Before
After

80
60
40
20
0
0-2 in

2-6 in 6-12 in 12-18 in 18-24 in 24-30 in

Spent Backwash Turbidity Analyses

Too little / too much washing is a common problem
After the first coring, and before the bed expansion
measurement and second coring, the washwater
turbidity should be measured for duration of wash
Sample at 1 minute intervals and analyze
Graph results as NTU vs. time
Record all data
Volume of backwash, rates,
Ramping intervals, operator habits

Washwater Turbidity Plot

Turbidity vs. Time
Helps prevent
Excessive washing

300

Wastes washwater
Strips ripening

200

AWWA goal of 10 NTU

This filter washed too
long

NTU of Wash

AWWA Standard

250

150
100
50
0
1 min 2 min 3 min 4 min 5 min 6 min 7 min 8 min 9 min

Operators Sampling Backwash Water

Bed Expansion Measurement with

Expansion Tool
Check high flow wash rate
(seasonally adjusted)
Desire 20 30% expansion
Position and tie down the
expansion tool so that it rests
on top of the bed
Wash bed under normal
conditions and observe
amount of expansion

Use of Expansion Tool

Backwash Rate Temp Correction

Bed Expansion Measurement Rate Requirement

Temp (Deg C)
30
25
20
15
10
5

Multiply 25 Deg value by

1.09
1.00
0.91
0.83
0.75
0.68

Example Bed Expansion

Bed Expansion Measurement with Expansion Tool
(Example for 30 inch bed)
dual media bed - need for a ramping rate, and a final rate
initial ramp might be 5 to 10 percent, or about 2-3 inches
observe expansion tool and adjust

Bed depth measured at 30 inches

Bed expansion tool captured 9 inches
Bed Expansion Measurement calculations
9 inches divided by 30 inches = 30% approx

Calculation of ES & UC
from sieve analysis
Sieve pans used for
media size analysis
ES = D10

90% larger, 10% smaller

UC = D60 / D10
Example sieve analysis
for anthracite
ES= 1.2
UC = 1.2
1.2 better than 1.4

Unit Filter Run Volume

UFRV - amount of water that is filtered during
the filter run time
should be determined for every filter run
Goal - UFRV of 5,000 gallons per square foot per run
Same at low rate or high rate

Excessive UFRVs are risky

Change in historical UFRV cause for concern
Example:
600,000 gals per run / 120 square feet = 5,000 UFRV

Sometimes, you can just tell.

Thank you!
Questions?