1. ENG421 (9ab) – Filtration Filtration Types of Filters Filter Media
Filtration – design considerations
Filtration – design examples

2. General Water Treatment Technologies (Week 4)
Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

3. Filtration (1 of 11) Widely used unit process in water treatment
water passes through a porous medium
→ suspended solid matter in water removed
Filters remove wide size range of impurities in water :
microstrainers remove millimetre size range
reverse osmosis removes fraction of nanometre size range
Filter after water has been pre-treated
Removes :
clay and silt particles colloidal organic substances
algae micro-organisms
inorganic precipitates of iron, manganese, aluminium, and calcium
Filter bed of packed granular media
usually sand
can also use :
anthracite coal
garnet
diatomaceous earth
aka diatomite, or kieselgur
fossilised remains of diatoms (ancient algae)
powdered activated carbon
Granular media filters operate continuously
filter stopped for cleaning and regeneration
when filtered water quality drops
when filter bed pressure drop is an issue

4. Filtration (2 of 11)

5. Filtration (3 of 11)
common water treatment practice (conventional treatment)
coagulation and flocculation ↓
sedimentation
filtration

6. Filtration (4 of 11)
common water treatment practice (direct filtration : water with low turbidity)
coagulation and flocculation ↓
filtration

7. Filtration (5 of 11)
common water treatment practice (in-line filtration, aka contact filtration)
coagulation ↓
filtration (may be two stage process)

8. Filtration (6 of 11)
advanced water treatment practice

9. Filtration (7 of 11)
advanced water treatment practice (cont)

10. Filtration (8 of 11)
advanced water treatment practice (cont)

11. Filtration (9 of 11)
advanced water treatment practice (cont)

12. Filtration (10 of 11)
advanced water treatment practice (cont)

13. Filtration (11 of 11)
advanced water treatment practice (cont)

14. Types of Filters (1 of 9) classified by : 1. driving force
causes flow through filter
- gravity filter
- pressure filter
- vacuum filter
2. flow direction
flow through filter bed
- up-flow filter
- down-flow filter
- bi-flow filter
- horizontal flow filter
3. solid removal mechanisms
how collected suspended solids are removed from water
- cake filtration
- depth filtration
4. flow rate
how fast water flows to filter
- slow sand filter
- rapid sand filter
- high-rate filter
5. filtration rate
how fast water is filtered
- declining rate filter
- constant rate filter

15. Types of Filters (2 of 9) - gravity filter open to atmosphere
flow through differential head pressure due to gravity
very common

16. Types of Filters (3 of 9) - slow sand filter
fine sand bed supported by porous gravel layer
biofilm (schumutzdecke) forms on top surface
of filter bed
physical and biological mechanisms remove
suspended particles
dissolved matter
occurs at top layer of filter (cake filtration)
within 0.5 – 3 cm of top layer
→ scrape off thin top layer
filter is cleaned
filter restored to original capacity
filter run time is interval between cleanings
1 month – 6 months
resanding : filter media replenished
when depth reduced to 0.4 – 0.5 m

17. Types of Filters (4 of 9) - slow sand filter (cont)
fine sand – 0.3 mm
fine sand bed depth 1 – 1.5 m
supporting gravel 5 – 50 mm
gravel layer depth ~ 30 cm
perforated underpipes collect filtered water
placed 2.5 – 3.5 m centre-to-centre
surface area 100 – 200 m2
may use more than one filter
flow rates :
0.06 – 0.3 m/h
1.5 – 7 m3/m2.day
advantages :
easy to build
easy to operate (unskilled operators)
minimum power requirements
produces good quality water
disadvantages :
higher land requirement
clogging caused by algae and fine clay particles
lengthy start up procedure
build up biofilm

18. Types of Filters (5 of 9) - rapid sand filter
sand 0.6 – 1.2 mm (c.f. slow sand filter : 0.15 – 0.3 mm)
particles in water are usually less than pores (spaces) in rapid sand filter media
→ particles pass through
→ eventually deposit in a pore in the bed
requires chemical coagulation
→ particles attach to filter media on contact
accumulated particles eventually saturate the pores
→ reduces removal efficiency
concentration of impurities may approach water quality limit (breakthrough)
→ increases headloss
headloss may approach maximum headloss provided in plant (available headloss)
at predecided headloss (terminal headloss) or filtered water concentrations
filter is backwashed
water is sent from bottom of filter through filter
filter medium expands and accumulated deposits flushed out
filter cycle is the time between backwashes

19. Types of Filters (6 of 9) - rapid sand filter (cont) flow rates :
5 – 15 m/h (c.f. slow sand filter : 0.06 – 0.3 m/h)
m3/m2.day (c.f. slow sand filter : 1.5 – 7 m3/m2.day)
filter depth usually 3 – 5 m (but can be 1 – 6 m)

20. Types of Filters (7 of 9) - rapid sand filter (cont)
sand may be graded or ungraded
maintained in bed in various configurations (see below)
coarse-to-fine
water contacts coarse sand grains first
→ larger suspended matter is retained there
as water continues through fine sand grains → more fine particles retained
this means better utilisation of bed
→ better water quality, lower headloss, longer filter run time

21. Types of Filters (8 of 9) - high-rate filter
consists of coarse monomedium or a multimedia bed
usually down flow, but can be upflow
influent contacts coarse media first
→ entire bed is effectively used
media material :
sand
anthracite coal
ilmenite (iron titanium ore associated with iron oxides – hematite
and magnetite, sp.gr. 4.2 – 4.6)
garnet (silicates of iron, aluminium, and calcium, sp.gr. 3.6 – 4.2)

22. Types of Filters (9 of 9) - pressure filter
filter media placed inside a pressure vessel
tanks are available at standard sizes
installed with vertical or horizontal axis
concept and operation similar to high-rate filters
suitable for smaller communities
more economical in smaller plants c.f. sand filters in domestic swimming pools

23. Filtration – design considerations (1 of 15)
factors to consider :
influent water characteristics
filtrate quality requirements
type of filter
filter media type
size and size range of filter media
filtration rate
underdrain for filter
filter regeneration
headloss in filter
other units in process train
water storage facilities
provision for future expansion
local conditions
weather changes
discharge regulations (for filtration unit waste)
site topography

24. Filtration – design considerations (2 of 15)
Filter influent
conventional treatment system
filter influent has small particles
(large particles removed in sedimentation)
→ rapid sand filter
(good with water with small suspended particles
in low concentrations)
direct filtration and contact filtration treatment systems
no sedimentation stage
→ all particles to be removed in filters
→ particles have broader size range
→ stratified multi-media down flow filter or
graded upflow filter

25. Filtration – design considerations (3 of 15)
Filter media and bed
availability and cost of filter media material main aspects in selection
filter is chosen on :
space requirement
flow rate
high-rate filter
stratified/mixed or
monomedium/multimedia
decision based on :
size distribution of suspended particles in influent
concentration of suspended particles in influent
backwash arrangements

26. Filtration – design considerations (4 of 15)

27. Filtration – design considerations (5 of 15)
U.C. = uniformity coefficient

28. Filtration – design considerations (6 of 15)
Filter media and bed (cont)
- size classification
effective size (d10 or dg)
size for which 10% of grains are smaller by weight
uniformity coefficient (UC)
ratio of d60/d10

29. Filtration – design considerations (7 of 15)
Filter media and bed (cont)
- filter efficiency/performance
effected by :
filter bed and media
coagulation and flocculation set-up
sedimentation (where installed)
effect of coagulants on raw water particles
variation in influent characteristics

30. Filtration – design considerations (8 of 15)
Filter media and bed (cont)
- filter grain size
must select appropriate grain size
especially high rate filters
grains must not intermix after backwash
(must have same settling velocity)
to determine size and density of grains :

31. Filtration – design considerations (9 of 15)
Filtration rate
size of treatment plant (throughput) determines :
number of filters
filter size
filter flow rate
flow control methods
when filter taken out of service for regeneration
other filters handle increased load at that time
throughput (Q)
< 100 L/s small plant, at least two filters
> 100 L/s at least four filters
Rule of Thumb :

32. Filtration – design considerations (10 of 15)
Headloss
occurs when water flows through a filter
for packed granular medium with uniform size, near spherical grains :

33. Filtration – design considerations (11 of 15)
Headloss (cont)
but filter media grains are not spherical
characterised by equivalent diameter and shape factor
from equation, headloss a function of :
flow rate
grain size
porosity
viscosity
for stratified beds
headloss is determined for each layer
then summed for entire bed
gravity filters
design available headloss for filtration 2.5 – 3.5 m

34. Filtration – design considerations (12 of 15)
Filter underdrain systems
support the filter media
collects the filtered water
distributes backwash water and air for scouring
prevents loss of filter media through filtered water outlet
orifices (5 -15 mm diameter) in underdrain pipes larger than filter grains
→ layer of graded gravel placed over underdrain pipes
(strains the media grains)

35. Filtration – design considerations (13 of 15)
Filter backwashing
accomplished by fluidising filter media
pump water or air (or both) at a rate to partially expand the bed
individual grains are suspended and abrade by contact with other grains
2 – 3 % of water produced in treatment plant used for backwashing
backwashing duration depends on :
filter
influent characteristics
backwashing method
avoid operating problems during backwashing :
loss of media
mudball effect
sand boil
back wash rate 30 – 45 m/h (6 – 9 times filtration rate)
filter bed expansion 15 – 30% (can be up to 80%)

36. Filtration – design considerations (14 of 15)
Filter backwashing (cont)

37. Filtration – design considerations (15 of 15)
Filter backwashing (cont)
Rule of Thumb to determine height and spacing of backwash troughs

38. Filtration – design criteria (1 of 3)
Filter media grain size – filter depth relationship

39. Filtration – design criteria (2 of 3)
Slow sand filters

40. Filtration – design criteria (3 of 3)
Rapid sand and high-rate filters

41. References
Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D ), pages 416 – 456
Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H ) , pages , , 715 – 762
Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K ), pages 194 – 291
MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W ), pages , 955 – 1034
Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E ), pages
Parsons, S.A. and Jefferson, B., 2006, Introduction to Potable Water Treatment Systems, Blackwell, Oxford (TD430 .P ), pages 72 – 94
Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages