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17 Rapid filtration

Luiz di Bernardo



17 Rapid filtration

17.1 Introduction

As explained in the previous chapter , f iltration is the process whereby water is purified

by passin it throuh a porous !aterial "or medium#. In rapid f iltration sand is

co!!only used as the filter !ediu!1 but the process is $uite dif f erent f ro! slow sand

f iltration. %his is so because !uch coarser sand is used with an ef f ective rain si&e in

the rane 0.'(1.) !!, and the f iltration rate is !uch hiher , enerally between * and

1* !3+!

).h "1)0(360 !

3+!

).day#. ue to the coarse sand used , the pores of the f ilter

bed are relatively lare and the i!purities contained in the raw water penetrate deep

into the filter bed . %hus, the capacity of the filter bed to store deposited i!purities is

!uch !ore ef f ectively utilised and even very turbid river water can be treated with

rapid f iltration. %o clean a rapid filter bed it is not suf f icient to scrape off the top layer .

-leanin of rapid filters is ef f ected by bac washin . %his involves directin a hih(rate

f low of water bac throuh the filter bed whereby it expands and is scoured. %he

bacwash water carries away the deposited !aterial that was cloin the f ilter .

-leanin of a rapid filter can be carried out $uicly/ it need not tae !ore than about

half an hour . It can be done as f re$uently as re$uired, if necessary each day. owever ,

a li!it does have to be placed on the f re$uency of bacwashin because of the a!ount

of the wash water used . ash water has to co!e f ro! the f iltered water production.

If the f iltration runs fall below six hours, then there will be only a s!all net output of

f iltered water available f or drinin water supply. %hen so!e chane in operation will be

necessar y, f or exa!ple i!provin the pre(treat!ent in the sedi!entation stae. In

!onsoon periods coarse pre(filtration !ay be necessar y, as described in the previous

chapter f or protectin slow sand filters aainst hih turbidity loads .

Applications of rapid filtration

%here are several dif f erent applications of rapid f iltration in the treat!ent of water f or

drinin water supplies. In the treat!ent of roundwater , rapid f iltration is used f or the

re!oval of iron and !ananese. %o assist the f iltration process , aeration is f re$uently provided as a pre(treat!ent to f or! insoluble co!pounds of iron and !ananese

"2i . 17.)#. ee also chapter 13 on aeration.

1 Anthracite, crushed coconut shell, pu!ice, baed clay pellets, and other !aterials are also used especially in

!ultiple(layer filter beds where one or !ore layers of such !aterials are placed on top of a "shallow# sand

bed. 2or s!all co!!unity water supplies , !ore layers than two are not enerally reco!!ended as the

advantaes of !ultiple(layer filters are s!all co!pared with the additional desin and operational

co!plexities , in choosin the dif f erent !edia and !aintainin their relative layer positions durin

bac washin . Anthracites f ro! dif f erent parts of the world !ay have dif f erent densities . %his will af f ect the

bacwashin rei!e, and !ay ive rise to losses of anthracite over the wastewater weirs .



Chapter 17

Fig. 17 .1. Rapid filter (open-gravity type)

Fig. 17.2. Rapid filtration of pre-treated (aerated) water

2or water with a low turbidity, as f re$uently f ound in laes and so!eti!es in rivers,

rapid f iltration should be able to produce clear water , thouh it !ay still contain

pathoenic !ic ro(oranis!s. A f inal treat!ent such as chlorination is then necessary to

obtain bacterioloically safe water .



In the treat!ent of river water with hih turbidity, rapid f iltration !ay be used as a pre(

treat!ent to reduce the load on the f ollowin slow sand filters "2i . 17.3#, or it !ay be

applied f or treatin water that has been clarified by coaulation, f locculation and

sedi!entation "2i . 17.'#. In such cases aain a f inal chlorination is re$uired.

Fig. 17.3. Rapid filtration followed y !low !and filtration

Fig. 17.". Rapid filtration after #oagulation and flo##ulation $ and !ediment ation

17.) %ypes of rapid filters

Rapid filters are !ostly built open with the water passin down the filter bed by ravity

"2i . 17.1#. Rapid filters !ay be classif ied accordin to

4 hydraulic head5 ravity or pressurised

4 f low direction5 upflow or downflow

4 f iltration rate5 constant or declinin

%he influent to the filters !ay be 5

4 clarified water , when coaulation, f locculation and sedi!entation "or f lotation# are

per f or!ed before downflow f iltration/4 coaulated water , in upflow direct f iltration or in(line downflow direct f iltration/



Chapter 17

4 f locculated water , when coaulation and f locculation steps exist upstrea! of the

downflow direct f iltration/

4 pre(filtered water when upflow direct f iltration is used upstrea! of the downflow

f iltration.

Fig. 17.%. &re!!ure filter

Pressure filters "2i . 17.*# are of the sa!e construction as ravity(type filters but the

f ilter bed toether with the filter botto! is enclosed in a watertiht steel pressure vessel.

%he drivin f orce f or the f iltration process here is the water pressure applied on the f ilter

bed, which can be so hih that al!ost any desired lenth of filter run is obtainable.

ressure filters are co!!ercially available as co!plete units. %hey are not so easy to

install , operate and !aintain , part icularly as it is not readily possible to inspect the

condition of the !edia. 2or this reason they are not very well suited f or application in

s!all treat!ent plants in developin countries.

Upflow filters "2i . 17.6# provide f or a coarse(to(fine f iltration process . %he coarse

botto! layer of the filter bed filters out the !aor part of the sus(pended i!purities, even

f ro!

a turbid raw water , with no reat increase of the filter bed resistance, "'ead lo!!# due tothe lare pores. %he overlayin f ine layers have s!aller pores but here also the f ilter

resistance will increase only slowly as not !any i!purities are left to be f iltered out .

In upflow f ilters , sand is used as the sinle filter !ediu!. owever , there are hyienic

obections to usin the! as a f inal f iltration stae in drinin water purif ication. %he f irst

of these is that the f iltrate, bein above the sand bed , is liable to conta!ination by birds

"particularly water f owl who !ay lie to swi! in it# or f ro! airborne dust. -overin the

top, or operatin the filters in a buildin , is an answer to this obection, but involves

extra cost. %he other obection is that bacwashin the f ilter , which enerates very dirty

water , conta!inates the sa!e part of the filter unit as is used f or deliverin the clean

f iltered water . pecial pipe wor , such as a per f orated f iltrate pipe buried in the upper



part of the sand, can reduce this ha&ard to hy iene. %his aain would entail extra cost.

8pflow filters are f re$uently used f or the pre(treat!ent of water that is f ur ther purified

by ravity(type rapid filters or by slow sand f ilters . In such cases, they can ive excellent

results and !ay be well suited f or use in s!all treat!ent plants.

9ne drawbac is that the allowable resistance "head loss# over an upflow filter should

not be !ore than the sub!ered weiht of the filter bed . ith sand as the f ilter

!aterial , the available resistance head is about e$ual to the thicness of the bed . 2or

very turbid river water the lenth of the filter run and the allowable rate of f iltration are

thus very li!ited .

Fig. 17.. pflow filter

Upflow and downflow direct filtration

In direct f iltration the influent of the rapid filter is coaulated, f locculated or pre(filtered

water . 2or water with low turbidity "up to )* :%8 , with peas up to 100 :%8#,

downflow "2i . 17.)# or upflow "f i . 17.6# direct rapid f iltration should be able to

produce clear water . If raw water is of poor $uality "turbidity up to 100 with peas up to

)00 :%8#, double f iltration !ay be used to produce a ood ef f luent. ouble f iltration

!ay consist of upflow direct f iltration f ollowed by downflow rapid f iltration "2i . 17.'#.

%he double filter can also be constructed in one unit "2i . 17.*#. In all cases a f inal

treat!ent such as chlorination is then necessary to obtain bacterioloically safe water .



Chapter 17

Fig. 17 .7 . *ownflow rapid gravity filter re#eiving #oagulated water (downflow dire#t filtration)

Fig. 17.+. *oule filtration , downflow rapid gravity filter pre#eded y upflow #oar!e medium filter (a)a! a !eparate unit , () uilt in !ameunit



8sually 30(*0 ; of head loss due to solids retention occurs at the ravel layer , while the

re!ainin portion has been retained at the sand sub(layers next to the ravel(sand

inter f ace "thicness of about 0.'(0.6 !#. %he filter run can be extended by reular

down(flushes in the operation of constant rate f iltration. In this !ethod the inf luent at

the botto! is closed and the water in the filter is dischared by downf low. %hese down(f lushes result in partial re!oval of solids. <ravel layer and sand sub(layers next to the

ravel(sand inter f ace are partially cleaned and a sinificant recovery of hydraulic head is

achieved "2i . 17.=#.

Fig. 17.. pflow filter operation! wit' down-flu!'e! during filter run

Multiple-media filters "2i . 17.10# are ravity(type, downflow filters with the filter bed

co!posed of several dif f erent !aterials , which are placed coarse(to(fine in the direction

of f low. 2or s!all(si&ed rapid filters it is co!!on to use only two !aterials in

co!bination5 0.3(0.* ! of sand with an ef f ective si&e of 0.'(0.7 !! as the under layer ,

topped by 0.*(0.7 ! of anthracite "see also f ootnote on variability of anthracite, section

17.1#, pu!ice or crushed coconut huss with an ef f ective si&e of 1.0(1.6 !!. As a f inal

treat!ent, !ultiple(layer filters can ive excellent results and , when suitable !aterials

are available locally, application in s!all treat!ent plants is a possibility. owever , the

stability of !ultiple(layer filters is difficult to desin and !aintain , part icularly after

bac washin .



Chapter 17

Fig. 17 .1. *ual-media filter ed

17.3 %heoretical aspects

%he overall re!oval of i!purities f ro! the water in rapid f iltration is brouht about by a

co!bination of several dif f erent processes. %he !ost i!portant are strainin ,

sedi!entation, adsorption, and bacterial and bioche!ical processes. %hese are the sa!e

processes already described f or slow sand f iltration "in section 16.)#. In rapid f iltration,

however , the filter bed !aterial is !uch coarser and the f iltration rate !uch hiher "up

to *0 ti!es hiher than in slow sand f iltration#. %hese f actors co!pletely alter the

relative i!portance of the various purification processes.

%he strainin of i!purities is not an i!portant !echanis! in rapid sand f iltration due to

the relatively lare pores in the filter bed . edi!entation is a sinificant !echanis!, as

in downflow f iltration par ticles collect preferentially on the top of rains, f or!in caps/

this is also due to the la!inar f low re i!e. %hus, strainin and sedi!entation will retain

f ar f ewer i!purities than in a slow sand f ilter . %he upper filter bed layers in particular will be f ar less ef f ective and there will be a deep penetration of i!purities into the entire

bed of a rapid f ilter .

>y f ar the !ost i!portant purification ef f ect in rapid f iltration is the adsorption of

i!purities. Althouh the sur f ace of initially clean sand has a s!all neative electrostatic

chare, this neative chare is neutralised very close to the particle by dissolved !aterial

in the water , and deposited posit ively chared !aterial lie alu!iniu! or iron f locs.

%hereaf ter , positively chared f locs encounter positively chared f loc(covered sand

surf aces, and electrostatic repulsion !ay be evident. %he !anitude and ef f ect of this is

not only dependent on the electrostatic chares on the f loc particle and sand sur f ace,



but also the a!ount and nature of the salts dissolved in the water . %he !ore !inerals

the water contains , the less is the rane of ef f ect of the electrostatic f orces. ?ost

sinif icantly, f orces of @an der aals occur at very close approach "less than 1 !icro

!etre# of the sur f aces of the particle and the rain, due to the electronic nature of the

ato!s and !olecules of the two approachin sur f aces. uch f orces always attract andaccount f or the attach!ent "ad!orption# of f locs and other par ticles with an electric

chare to sand rains, and existin deposits with an opposite electric chare "see Ives,

1=67 f or f ur ther details#.

In a slow sand filter the water stays in the filter bed f or several hours, but with rapid

f iltration the water passes in only a f ew !inutes. Accu!ulated oranic deposits are

f re$uently re!oved f ro! a rapid filter when the filter is cleaned by bac washin . %here

is very little ti!e and opportunity f or any bioderadation of oranic !atter to develop,

or f or illin of pathoenic !icro(oranis!s to tae place. %he li!ited deradation of

oranic !atter need not be a serious drawbac as the accu!ulated deposits will be

washed out of the filter durin bac washin . %he poor bacterioloical and bioche!ical

activity of a rapid filter will enerally be insuf f icient to produce bacterioloically safe

water . ence, f ur ther treat!ent such as slow sand f iltration or chlorination will be

necessary to produce water that is fit f or drinin and do!estic purposes.

17.' Rapid filter operation and control

%he operation of a rapid filter "ravity type# is shown sche!atically in f iure 17.11.

Fig. 17 .11. Rapid filter (gravity type)



Chapter 17

urin f iltration the water enters the filter throuh valve A, !oves down towards the

filter bed, f lows throuh the filter bed , passes the underdrainae syste! "filter botto!#

and f lows out throuh valve >. %he unit used to !easure f iltration rate is actually the

approach velocity, which is the inflow rate "!3+h# divided by the f iltration area "!

)#. %he

interstitial velocity in the bed is hiher , as it is the f iltration rate divided by the averae

porosity of the filter !ediu!.

ue to radual cloin of the pores the filter beds resistance aainst the downward

water f low will proressively increase. %his will reduce the filtration rate unless it is

co!pensated by a risin raw water level above the filter bed . In this exa!ple, the rapid

filters are desined to operate with a constant raw water level , which re$uires that the

filter is e$uipped with a filter rate control device in the influent or ef f luent line. %hese

filter rate controllers provide an adustable resistance to the water f low. %hey open

radually and auto!atically to co!pensate f or the filter beds increasin resistance and

so eep the operatin conditions of the rapid filter constant .

hen , after a period of operation, the filter rate controller is fully opened, f ur ther

cloin of the filter bed cannot be f ur ther co!pensated and the f iltration rate will f all .

%he filter is then taen out of service f or bac washin . 2or this, the valves A and > are

closed, and valve is opened to drain the re!ainin raw water out of the f ilter . A f ew

!inutes later valve B is opened to ad!it the wash water . %he bacwash rate should be

hih enouh to expand the filter bed by about )0; so that the filter rains can be

scoured, and the accu!ulated deposits carried away with the wash water . %he wash

water is collected in the wash water trouhs f ro! where it drains to waste. hen the

bacwashin is co!pleted, valves B and are closed and valve A is re(opened,

ad!ittin raw water to bein a new filter run.

2or f ine filter bed !aterial, the scourin action produced by the wash water durin

bacwashin !ay not be suf f icient to eep the filter bed clean in the lon run. It is then

desirable to provide an additional scour by usin air and water in co!bination f or bac(

washin . %his , however , is !uch !ore co!plex than bacwashin with water only, andis enerally not to be reco!!ended f or s!all water treat!ent plants.

Filter control

%able 17.1 ives an overview of the f our possible filter operation controls .

9ption ) with constant water level "available head#, constant total resistance and

constant f iltration rate has been discussed. 2iure 17.1) shows the head loss

division over bed and controller f or this case.



Table 17.1 2ilter operation control

Option Aailable head Total

resistance to

filtration

Filtration rate ! emar" s

1 -onstant @ariable eclinin ((((((((((((((((((((

) -onstant -onstant -onstant Inlet or outlet control devices

are necessary

3 @ariable @ariable -onstant 2ilters are individually f ed by

f ree dischare weirs and the

water level into each one

f ollows the increase of head

loss due to solids retention

' @ariable @ariable @ariable eclinin(rate f iltration syste!/

filter inlets are sub!ered/ the

cleanest filter in the batter y

wors at the hihest f iltration

rate and the dir tiest at the

!ini!u! f iltration rate

Fig. 17 .12. /ead lo!! in 0on!tant /ead 0on!tant Rate filter #ontrol

%he f iltration rate can be controlled ointly f or all filter units by the raw water f eedin

rate. It can be readily adusted to !eet the de!and f or f iltered water . In this

arrane!ent there will be considerable variations of the raw water level in the f ilters ,



Chapter 17

which !ay be aw ward. If so, another arrane(!ent as shown in f iure l7.1'c !ay be

pref erred. ere a f loat(controlled valve is used to eep the raw water level in each f ilter

constant. Individual rate controllers allow each filter unit to operate at its opti!al

f iltration rate "2i . 17.13#. %his advantae, however , is not very reat and such rate

controllers are enerally very expensive and not easy to !aintain.

Fig. 17 .13. Filter rate #ontrol

Filter control for option #$ ariable water leel % constant rate control

2ilter control arrane!ents usin an even distribution of the raw water " flow !plitting #

over the filter units or unifor! withdrawal of the f iltered water , are widely used in

Burope and :orth A!erica. @arious !ethods can be e!ployed. %he one shown in

fiure 17.1'b is probably the si!plest as there are no !ovin parts at all . In this type

the raw water enters the filter over a weir . %he weir crest is at the sa!e level f or all

f ilters . %he raw water conduit f eedin the filter units is enerously si&ed so that the water

will f low without any appreciable head loss. %he water level in it will be practically the

sa!e at each entrance weir . %hus, the over f low rate at each weir will be the sa!e , and

the

raw water f eed to the filter units will be e$ually s plit . %his !ethod of control is nown

as influent flow !plitting .

%he water level in the filters of a ban !ust be dif f erent, otherwise the filters would all

end the run at the sa!e ti!e. %he necessary phasin can be acco!plished durin the

plant start(up by bacwashin each filter after so!e period of operation. 2or exa!ple,

with f our filters in a ban , filter 21 is bacwashed after ' hours of operation, filter 2)

after C hours, filter 23 after 1) hours and filter 2', after 16 hours. 9nce this situation is

reached , the operator will bacwash filter 21 only when the available head is consu!ed

"at the !axi!u! level f ixed by the desiner# and so on. ince the filters are f edindependently by usin f ree dischare weirs, the water level in each one will vary

independently f ro! the others, so that the end of the run is visible f or the operator . At



Fig. 17 .1". Filter #ontrol !y!tem!

the beinnin of the run, the !ediu! is clean. %o avoid the water level fallin below the

top of the sand bed , an outlet weir is reco!!ended with the crest ust above the level

of the sand bed . %his outlet weir can be placed in the individual box f or each filter or inthe co!!on outlet channel. -o!pared to constant(rate constant level control , the

influent f low splittin !ethod has the f ollowin advantaes5

4 2iltration rate is !aintained without the use of auto!atic controllin devices

4 %he influent volu!etric f low rate is e$ually split by !eans of a si!ple hydraulic

device, such as trianular or rectanular weirs

4 hen one filter is taen out of service f or bac washin , the f low rate is unifor!ly

distributed to the re!ainin filters in operation and the water level and the f iltration

rate in each one will radually rise

4 As soon as the clean filter is placed bac to wor , the decrease in f iltration rate is

also be radual in the re!ainin f ilters



Chapter 17

4 ead loss in any filter is visual and depends only on the operator identifyin the

!o!ent the dir tiest filter has to be bacwashed by observation of the water levels

in the units

4 %he position of the crest in the outlet weirs slihtly hiher than the top of the

!ediu! eli!inates the occurrence of neative pr

essure4 %he f low rate in each filter !ay be easily !easured at the inlet weirs

o!e disadvantaes of this syste! have been clai!ed, such as the excessive heiht of the

f ilter box and f loc brea(up due to the fall of water in the filter box. ithout any doubt,

the heiht of f ilter boxes is hiher , but it should be considered aainst the substantial cost

of ac$uisition of !echanical devices and the cost of operation and !aintenance when

constant(rate constant level control f ilters are used. In relation to the water f allin at the

inlet of the f ilters , it !ust be realised that a fall already exists in trouhs used f or settled

water collection . %he ef f ect on per f or!ance of water f allin upstrea! of the f ilters has

been carefully investiated and no da!ae to the f iltrate was observed.

Option &$ ariable water leel 'head( and declinin) rate control

hen no f iltration rate controllers are used, f iltration will tae place at a declinin(rate.

%he desin of declinin(rate filters is !uch si!pler than f or controlled(rate f ilters . i!ple

stop los or ates can be used f or filter control "2i . 17.1*#.

Fig. 17 .1%. *e#lining- rate filtration



All filters are in open connection with both raw and f iltered water conduits.

-onse$uently, all have approxi!ately the sa!e raw water level and f iltered water level,

so that all filters will operate under the sa!e head. %he f iltr ation rate f or the various

filter units, however , will be dif f erent5 hihest in the filter ust cleaned by bacwashin

and lowest f or the one lonest underway in its current filter run. 2or all filters 7ointly, the production will be deter!ined by the supply of raw water , which should be hih

enouh to !eet the de!and f or f iltered water . urin f iltration the filter beds radually

beco!e cloed and the raw water level in all filters will rise due to the increased

resistance aainst water f low. %he filter unit that has been in operation f or the lonest

period of ti!e will reach the !axi!u! allowable water level f irst, and then needs

cleanin by bac washin .

After cleanin, this f ilter will have the lowest resistance aainst f low so that a considerable

portion of the raw water supplied will pass throuh it . %he load on the other f ilters is

te!porarily reduced. %hese units will show a fall of the influent water level but later the

further cloin of the f ilter beds will cause the influent water level to rise aain . hen the

!axi!u! raw water level is reached in a second f ilter this one will be bacwashed and so

forth. %he water level in the co!!on inlet channel will vary dependin on the

operational stae of each of the filters/ the supernatant water level is close to this level

because the head loss at the inlet will be s!all. %his water level in the co!!on inlet

channel is lowest when the bacwashed f ilter is aain put into operation , and hihest ust

before the bacwashed f ilter is resu!in its operation . %his is shown in f iure 17.1*.

If no special !easures are ta en, the f iltration rate in a declinin(rate filter ust after

cleanin can be very hih, up to )* !3+!

).h , which is !uch hiher than the averae

rate of *(7 !3+!

).h. hen it is necessary to li!it the f iltration rate in order to safeuard

the f iltered water $uality, an extra f low resistance device "e . . an orifice# or a preset

butter f ly valve should be f itted in the ef f luent line. 2or pressure f ilters , declinin(rate

f iltration is co!!on practice. 2or ravity(type rapid f ilters , its application is radually

increasin in <reat >ritain , in Datin A!erica and also, to a li!ited extent , in :or th

A!erica. ue to its si!plicity, declinin(rate f iltration is certainly worth considerin f or

s!all water treat!ent plants in developin countries.

*e)atie head

o(called neative head can occur when the pressure in the filter bed falls locally below

at!ospheric pr essure. %his !ay occur towards the end of the filter run and leads to air

co!in out of solution and appearin as bubbles that bloc the pores in the filter !edia.

Brratic f low occurs , causin deposits to be scoured out of the filter pores and !ain the

f iltrate turbid. o!e bubbles rise to the sur f ace of the filter bed , creatin channels

throuh which unfiltered water !ay f low, aain causin a penetration of turbidity into

the f iltrate. ee also the re!ars about Eunder(pr essureF in chapter 16 section 16.).1 on

!ulti(stae f iltration.



Chapter 17

17.* ,esin considerations

2or the desin of a rapid f ilter , f our para!eters need to be selected5 the rain si&e of the

filter !aterial/ the thicness of the filter bed / the depth of the supernatant water/ and

the rate of f iltration. %o the extent possible, these desin f actors should be based on

experience obtained in existin plants that treat the sa!e or co!parable raw water .

hen such experience does not exist, the desin should be based on the results

obtained with a pilot plant operatin experi!ental f ilters . %he $uality of the influent

water "usually pre(treated water# reatly influences the rane of the desin para!eters /

in table 17.), broad indicative ranes have been iven.

+ac"wash arran)ements

A rapid filter is cleaned by bac washin / that !eans directin a f low of clean water

upwards throuh the filter bed f or a period of a f ew !inutes. 2iltered water

accu!ulated by pu!pin to an elevated tan can be used, or the ef f luent f ro! the

other "operatin # filter units of the f iltration plant can be used directly " !elf-wa!'

arrangement!#. %he velocity of the upward water f low should be hih enouh to

produce a li!ited expansion of the filter bed , so that the accu!ulated deposits can be

loosened and carried away with the wash water but without carryin sand or other f ilter

!edia over the weir "2i . 17.16#. It is i!portant to chec !edia expansion if convertin

a sand filter to an anthracite+sand dual(!edia bed .

Fig. 17 .1. Ba#wa!'ing of rapid filter

2or a filter bed of sand "s pecif ic weiht5 ).6* +c!3# typical bacwash rates ivin about

)0 ; expansion are listed in table 17.3.



Table17., Indicative desin ranes f or rapid f iltration

-esi)n parameter -ownflow rapid

sand filter usin)conentional sand

-ownflow rapid

sand filter usin)uniform sand

Upflow direct

filtration

-ouble filtration$

1. upflow usin))rael

',. as downflow

sand filter(

-ouble filtration$

1. upflow usin)coarse sand

',. as downflow

sand filter(

<rain si&e of f ilter <rain si&e5 <rain si&e5 <rain si&e5 2our sub(layers <rain si&e5

!ediu! 0.'1().0 !! 0.C'(1.6C !! 0.*=().0 !! "each '0 c!# 1.0(3.) !!

Bffective si&e5 Bffective si&e5 Bffective si&e5 ranin f ro! Bffective si&e5

0.'*(0.** !! 1.0(1.) !! 0.7*(0.C* !! 3) !!('.C !! 1.1=(1.'1 !!

2ilter bed 0.6(0.C ! 0.C(1.) ! 1.6().0 ! 1.*(1.C ! 1.)(1.6 !

thicness

upernatant water 0.6().0 ! G 0.C().* ! G :ot applicable

depth

2iltration rate *(7.* !+h 7.*(1).* !+h *(10 !+h ).*(* !+h *(10 !+h

upport layer 0,'(0,6 ! GG 0.3(0.* ! GG 0.6(0.C ! :ot applicable 0.6(0.C !

thicness

G epends on the !ethod of filter control and the f iltration rate

GG epends on the drainae syste!



Chapter 17

Table 17.# %ypical bacwash rates

d "!!# 0.' 0.* 0.6 07 0.C 0= 1.0 1.1 1.)

t "H-# >ac wash rate "!3+!

).h#

10H- 1) 17 )) )C 3' '0 '7 *' 6)

)0 1' )0 )6 33 '0 'C *6 6' 73

30 16 )3 30 3C '7 *6 6* 7* C6

d averae rain si&e of filter sand "!!#

t bacwash water te!perature "H-#

v bacwash rate "!3+!

).h#

If the wash water is supplied by pu!pin , three "in very s!all installations two# pu!ps

are nor!ally used, one of which serves as the reserve unit. 2or hih bacwash rates and

lare filter bed areas, these pu!ps need to be of hih capacity and their installation and

operation is rather expensive . A wash water reservoir such as the one shown in f iure

17.17 is then pref erable/ s!aller pu!ps will be ade$uate to fill the reservoir durin

the intervals between successive bac washins. %he reservoir enerally should have

a capacity between 3 and 6 !3 per !

)of filter bed area and it should be placed about

'(6 ! above the water level in the f ilter .

Aain , three pu!ps are usually provided , one of which is the reserve unit . %he total

capacity of the two operatin pu!ps should be about 10()0; of the wash water supplyrate. A special wash water tan or reservoir is not necessary when the re$uired was h

water is taen f ro! the f iltered water reser voir . owever , this !ay cause undesirable

pressure f luctuations in the distribution syste!, due to the interrupted supply of water .

A si!pler solution is to increase the depth of the water standin over the f ilter beds and to

li!it the !axi!u! f ilter resistance. %he f iltered water will then be available at a head of

so!e 1.*() ! above the f ilter beds and that should be suf f icient. %he operatin units of the

f iltration plant !ust supply enouh water f or the re$uired bacwash rate. 2or this reason

a rapid f iltration plant usin this bacwash arrane!ent should have at least six f ilter units.

%he wash water is ad!itted at the underside of the f ilter bed throuh the underdrain

syste! " filter ottom#. %o divide the wash water evenly over the entire f ilter bed area, the

underdrain syste! !ust provide a suf f icient resistance aainst the passae of wash water

"enerally 0.6(1.0 ! head of water#. A f re$uently used underdrain syste! consists of

laterals placed about 0.) ! apart and connected to a !anifold "2i . 17.1=#. %he

laterals have holes at the underside , with a dia!eter of about 10 !!. Riid plastic

pipes are enerally used in this underdrain syste!.



Fig. 17 .17 . a!' water tan arrangement

Fig. 17 .1+. 4elf-wa!' arrangement! for (a) !mall plant! and () medium-!ized and large plant!



Chapter 17

Fig. 17 .1. Lateral underdrain !y!tem!

%o prevent the filter !aterial f ro! enterin the laterals throuh the holes, the filter bed

should be supported by a layer of coarse !aterial "e . . ravel# that will not be disloded

by the bacwash water ett in f ro! the underdrain holes. 2or exa!ple, filter sand of 0.7(1.0 !! ef f ective si&e would re$uire f our ravel layers / f ro! top to botto!5

0.1* ! x )().C !!, 0.1 ! x *.6(C !!, 0.1 ! x 16()3 !! and 0.) ! x 3C(*' !!.

%he total ravel pac would be 0.** ! deep. o!e other filter underdrain syste!s

developed by the water industry are shown in the section E-onstructionF.

After passin the filter bed , the wash water carryin the washed out i!purities is

collected and drained off via wash(water trouhs. %he distance the wash water will have

to travel hori&ontally to a trouh should be li!ited to about 1.*().* !. %he trouhs are

set with their top at 0.*(0.6 ! above the unexpanded sand bed , and their cross(

sectional area is derived f ro! the consideration that at the dischare end of the trouh

the water depth will be the f ree dischare "EcriticalF# depth "2i . 17.)0#.

%able 17.' ives wash water f low rates "J# f or co!binations of depth of wash

water f low "# and width of wash water trouh "b#.

%he wash water trouhs can be placed in several ways . 2iure 17.)1 shows

typical arrane!ents.



Fig. 17.2. Flow #ondition in wa!' water troug'

Table 17.& ash water carryin capacity of trouhs "l+s#

/ -epth of wash 0idth of trou)h

water flow in trou)h 0.)* ! 0.3* ! 0.'* !

0.)* ! '3 60 7C

0.3* ! 7) 100 1)=

0.'* ! 10' 1'6 1CC

Fig. 17 .21. 5ypi#al arrangement! of wa!' water troug'!

articularly when f ine sand is used with a rain si&e less than about 0.C !!, the

scourin f orce of the risin wash water !ay be inade$uate to eep the filter rains clean

in the lon run. After so!e ti!e they could beco!e covered with a sticy layer of

oranic !atter . %his !ay cause proble!s such as !ud balls and filter cracs .



Chapter 17

%hese can be prevented by providin an additional air(wash scour . 2ilter cleanin now

starts by bacwashin with air at a 30(*0 !+h rate, usually co!bined with a water wash

at a 10(1* !+h rate. %his should re!ove the coatins f ro! the filter rains, and the

loosened !aterial is carried away by the f ollowin water wash. 2or bacwashin with air

a separate pipe syste! is nece

ssar y. An exa!ple is shown in f iure 17.)). It should be

noted that air(and(water bacwashin enerally is too co!plex an arrane!ent f or

s!all water treat!ent plants.

Fig. 17.22. Ba#wa!'ing wit' air and water

Fig. 17.23. 6ir-and-water a#wa!' arrangement 4our#e7 4en $ R.8 . 9ndian 9n!titute of 5e#'nology :'aragpur ( 9nd ia )

(applied at ;etropolitan ater Board London $ #a. 1-3.)



An interestin arrane!ent f or f eedin air and water f or bacwashin is shown in f iure

17.)'. >ac washin s tarts by allowin water f ro! cha!ber 1 to f low into cha!ber ).

%he air in cha!ber ) is pressuri&ed and ad!itted f or scourin the f ilter . %hen the water

collected in cha!ber ) is used to bacwash the f ilter .

!apid filtration plant laout

A rapid f iltration plant consists of a nu!ber of filter units "!ini!u! )#, each with an

area A. hen one filter is out of operation f or cleanin , the re!ainin units !ust be

able to provide the re$uired capacity J at the selected rate of f iltration r . %his is

expressed in the f or!ula5 J "n ( 1# A.r

2or s!all plants there is little choice reardin suitable co!binations of n and A, but f or

larer plants the choice should be such that the cost of construction is !ini!i&ed. As

a tentative desin step, the unit filter bed area "A# expressed in s$uare !etres !ay be

taen as about 3.* ti!es the nu!ber of filter units n.

2or econo!y in construction and operation, the filter units should be set in a co!pact

roup with the influent and ef f luent lines and any che!ical f eed lines as short as

possible. %he sitin of the various units of a rapid f iltration plant is a !atter that

warrants the closest attention of the desin en( ineer . Allowance should be !ade f or

a f uture expansion of the plant . An exa!ple is shown in f iure 17.)'. -o!!on f acilities

such as wash water pu!ps and tan s, and che!ical solution f eeders are best placed in

a service buildin , which also should contain the of f ice, laboratory and storeroo!s , and

che!ical handlin , storae, and sanitary f acilities.

Fig. 17.2". Rapid filtration plant layout



Chapter 17

?any desins place the service buildin in the centre, while in the wins the various

filter units are arraned on one or two sides of a two level corridor , the upper level

bein the operatin f loor and the lower level the pipe aller y.

17.6 -onstruction

As explained in the precedin sections , a rapid filter consists of a tan containin the

underdrain syste!, the filter bed and the supernatant water . %he filter tan is enerally

!ade of reinforced concrete, rectanular and with vertical walls. %he desin of the

concrete structure f ollows co!!on rules with the added difficulty that the water

retainin structures !ust be water tiht . An a!ple concrete cover should be provided to

protect the reinforce!ent bars aainst corrosion.

All bars should be placed f ar enouh apart to allow the concrete to surround the!

co!pletely. Doadin stresses should be ept to a !ini!u! . Any stresses developin in

the concrete due to dr yin , shrin ae, te!perature chanes and dif f erences in soil

subsidence should be li!ited as f ar as possible by sub(dividin the buildin into

a nu!ber of independent sections connected with water tiht expansion 7oin ts. %he

concrete !ixAs ce!ent content and the placin of the !ix should ai! at full water

tihtness and as little dryin shrinae durin hardenin as possible. A plaster f inish

should never be used. A ood f inish can be obtained by usin s!ooth shutterin , f or

instance !ade of la!inated wood. %o prevent short(circuitin of the water f low alon

the walls of the filter box , the inside shutterin next to the filter bed should be !ade of

unplaned plans placed hori&ontally. henever possible, the filters should be set above

the hihest roundwater table, if necessary on elevated land.

:u!erous underdrain syste!s "popularly nown as filter botto!s# have been developed

in the past. >ut unfortunately !any are either too expensive or unable to ensure an

even distribution of the wash water over the full underside of the filter bed . %he si!ple

syste! that was described earlier , usin per f orated laterals, can be so constructed that

a ood wash water distribution is obtained. It has the added advantae that it !ay be!ade of locally available !aterials usin local s ills. Another ood solution is the f alse

botto! and strainer underdrain syste! . It consists of prefabricated concrete slabs, about

0.6 x 0.6 !), placed on and anchored to short concrete colu!ns.

%he slabs are provided with holes, about 60 per s$uare !etre, in which the strainers are

set. %he slits in these strainers are narrow, about 0.* !!, ivin a suf f icient resistance

aainst the passae of the wash water f or an even distribution of the water . %his

underdrain syste! allows the filter sand to be placed directly on the filter botto! with

the strainers, and no supportin ravel layers are needed.



Chapter 17

ensure the s!ooth operation of any f ollowin slow sand f ilters . %here should be f ew

obections aainst such an application of rapid f ilters . %he use of rapid f iltration f or the

re!oval of iron and !ananese f ro! roundwater also presents f ew proble!s , as the

health ha&ard of possible conta!ination of the treated water will be s!all.

Assu!in a water use of '0 litres+person.day, the re$uired water f iltration capacity f or

10,000 people would be '00 !3+day or '0 !

3+h f or a 10(hour daily operatin period .

ith a f iltration rate of * !+h this calls f or C !)

filter bed area, which !ay be provided

in three circular filters of ) ! dia!eter each "one filter as reserve#. %he underdrain

syste! would probably best be !ade of per f orated laterals "see section 17.'#, covered

with raded layers of ravel, broen stones or hard brics chipped to the desired si&e.

hen coarse sand is available it should be raded usin suitable sieves. <radin li!its

would be 0.C(l.) !! f or pre(f ilters / 1.0(1.* !! f or iron and !ananese re!ovin f ilters .

2or pre(filters the sand bed thicness should be taen at 1.0 ! and f or iron and

!ananese re!ovin filters at 1.* !. In the event that sand cannot be obtained, si!ilar

!aterials !ay be used , such as crushed stones, brics , crystalline calciu! carbonate,

dolo!ite, etc. %hese should be raded to a si&e about '0; larer than the si&es

!entioned above. In so!e instances burned rice huss and crushed coconut shells have

iven acceptable results. >ef ore the filter is co!!issioned it should be bacwashed f or

about half an hour to clean the filter !aterial. %he depth of supernatant water !ay be

fixed between 1.* and ) !. %he filter box will then have a total depth of 3.*(' !.

%he reatest difficulty encountered in villae(scale rapid f iltration is the bacwashin

process . It is unecono!ical to use a wash water pu!p . In the exa!ple presented earlier

a capacity of 100()00 !3+h would be needed, in duplicate, to allow f or !echanical

f ailures. -o!pared with the plant capacity of '0 !3+h, this is an enor!ous pu!p

involvin a considerable invest!ent and hih operatin costs. ith an elevated wash

water reservoir of )0 !3

volu!e the pu!p capacity can be reduced to 10 !3+h , but the

costs of the tan should be taen into account. 2or villaes with low buildins, the

pressure in the distribution syste! enerally does not need to be !ore than 6 !. In

these cases, a ood solution will be to use an elevated service reservoir f or bacwashin

the f ilters . :o separate pu!ps would then be needed.

Fig. 17.2. <eneral layout of a rapid filtration plant



%he layout of the rapid f iltration plant as described above is shown in f iure 17.)=. %he

raw water enters the filter throuh valve A and falls into the wash water trouh to

disperse the f low enery. %he branch pipes into which valves A are set have a s!all

dia!eter , ivin suf f icient f low resistance "e. . 0.* ! of head# to assure an even

distribution of the raw water over the individual filter units. %he f iltered water isdischared throuh valve and passes over a weir placed in the weir cha!ber . %he top

of the weir is set so hih that the lowest raw water level in the filter tan will be at least

0.) ! above the filter bed . ue to cloin , the level of the supernatant water will rise

until at reaches the water pressure level in the supply pipe/ no !ore water will enter the

f ilter . %he filter should then be cleaned by f eedin the wash water throuh it and

discharin it throuh valve >. %he dirty wash water should be clarified by sedi!entation

after which it !ay be dischared bac into the river , so!e distance downstrea! of the

raw water inta e.

17.C Rouhin filtration

Rouhin f iltration is often a desirable process to be used when the turbidity of the

inflow to slow sand filters exceeds )*!+l "approxi!ately )0 :%8#. uch pre(treat!ent

protects the slow sand filters f ro! beco!in cloed in a f ew days, which is possible,

part icularly in !onsoon periods. A bypass pipe can isolate rouhin filters when they

are not needed, so that they can be switched into service only when necessar y.

o!eti!es a !ore li!ited treat!ent than rapid f iltration usin a sand bed can be

ade$uate f or treatin the raw water . %his can be obtained by usin ravel or plant f ibres

as filter !aterial. In the chapter on !ulti(stae f iltration theor y, desin and construction

details have been iven f or rouhin f iltration usin hori&ontal f low , downflow and

upflow !ethods .

-oconut f ibres have been used f or filter !aterial in filter units si!ilar to sand f ilters . %he

filter bed is only 0.3(0.* ! thic and the depth of supernatant water about 1 !. %he f ilter

is operated at rates of 0.*(1 !+h , which ives a lenth of filter run of several wee s. %oclean the filter it is first drained, after which the coconut f ibres are taen out and

discarded. %he filter is repaced with new !aterial that has previously been soaed in

water f or )' hours to re!ove as !uch oranic !atter as possible. -oconut f ibre f ilters

appear to be able to cope with considerable f luctuations in their loadin while producin

an ef f luent of al!ost constant $uality. %here is a re!arably constant behaviour of the

coconut f ibre f ilters . %he overall turbidity re!oval varies between 60 and C0;.

Another alternative is the pebble !atrix f ilter , which consists of a deep bed "about 1(1.) !#

of pebbles, about *0 !! in si&e. About 0.C ! of the lower depth of the pebble bed is

filled in with sand. %he turbid water enterin the top first encounters a 0.)(0.' ! depth of



Chapter 17

pebbles and then passes throuh the sand within the pores of the pebbles. %his partially

clarifies the water , and as any sand(filled pore beco!es cloed the f low diverts to passin

over the adacent pebble sur f ace. %he water transfers f ro! pebble to pebble down throuh

the f ilter , enterin sand in a lower , less cloed pore. ilot experi!ents have shown that

turbid water containin up to *,000 !+l clay suspension can be clarified to less than

)* !+l at f low rates of 0.7 !+h over a )0 hour run with a head loss of 1.* !. -leanin is

acco!plished by repeated drainae, and refillin f ro! below with raw water , usually about

three ti!es se$uentially. After about a wee of such drainae cleanin, a thorouh

bacwash is needed usin drinin water supply. %his should a itate and f luidise the sand

in the pebble pores, with so!e expansion up into the pebble only layer . It is i!portant that

this expansion does not f orce the sand over the top of the pebble layer , as it is then

dif f icult to put it bac into the pebble pores below.



>iblioraphy

Cleasb2 3.4.5 0illiamson2 *.M . and +aumann2 6.!. '18#(. KBffect of f iltration rate

chanes on $ualityA. In5 =ournal 6meri#an ater or! 6!!o#iation $ vol. **, no . 7, p. C6=(

C7*.

Cornwell2 -.A .5 +ishop 2 M.M . and unn2 .3. '19&(. Keclinin(rate filters5 reulatory

aspects and operatin results.A In5 =ournal 6meri#an ater or! 6!!o#iation, vol . 76,

no. 1), p. **.

-i +ernardo2 4. and Cleasb2 3.4. 's.a.(. Keclinin(rate versus constant(rate f iltration.

In5 =ournal of t'e 6meri#an 4o#iety of 0ivil >ngineer! $ vol . 106, no . BB6, p. 10)3.

-i +ernardo2 4. and :saac2 !.4. ',;;1(. K8pflow direct f iltration5 a review. In5

&ro#eeding ! $ 0 9 > ; 9nternational 0onferen#e on 6dvan#e! in Rapid <ranular Filtration

in ater 5reatment $ '(6 April, Dondon, Bnland.

-i +ernardo2 4. ',;;1(. K-onvertin f iltration control f ro! constant(rate to declinin(rate

in a conventional water treat!ent plant A. In5 &ro#eeding ! $ 0 9 > ; 9nternational

0onferen#e on 6dvan#e! in Rapid <ranular Filtration in ater 5reatment $ '(6 April,

Dondon, Bnland.

-i +ernardo2 4. '11(. Kater(supply proble!s and treat!ent technoloies in

developin countries of outh A!erica. In5 6?ua7 @ournal of water !upply re!ear#' and

te#'nology, vol . '0, no. 3, p.1*6.

Fair2 <.M .5 <eer2 3.C. and O" un 2 -.A . '189(. Kater and wastewater enineerinA.

In5 ater purifi#ation and wa!tewater treatment and di!po!al . @ol . ). :ew Lor , :L,

83A, Mohn iley.

ilmoe2 -.3. and Cleasb2 3.4. '198(.K-o!parin constant(rate and declinin(rate directf iltration of a sur f ace water. In5 =ournal 6meri#an ater or! 6!!o#iation $ vol . 7C, no.

1), p. )6.

:es2 = .3. '187(. K>asic concepts of filtration. In5 &ro#. 4o#. ater 5reatm. >Aam ., vol. 16,

p. 1'7(16=.

Miller2 -. <. '17,(. KBxperiences with upflow filtration. In5 4ympo!ium on new met'od!

of water treatment $ Asuncion, arauay.



Chapter 17

Min>2 -. '18,(. Ko!e results of research into drinin water purification

and disinfection. In5 Bulletin of t'e /$ vol . )6, p. **3.

Purchas 2 -.+ . and 0a" eman2 !.3. '198(. 4olidCli?uid !eparation !#ale-up .

)nd ed. -roydon

, 8N

, 8plands ress

.

! a?apa" se2 !.3. and :es 2 = .3. '1;(. Kre(filtration of very hihly turbid waters

usin pebble !atrix filtration. In5 =ournal of 9n!titution of ater and >nvironmental

;anagement $ vol . ', no. ), p. 1'0(1'7.

!obec" 2 <.< . '17,(. K?odern concepts in water f iltration In5 &6/ !ympo!ium on

modern water treatment met'od! $ 6!un#ion "arauay#, 1' Auust, 1=7).

"?anual no . 1'#. Di!a , eru, -BI3.

@hull2 = .6 '18A(. KBxperiences with !ultiple(bed f ilters A. In5 =ournal of t'e 6meri#an

ater or! 6!!o#iation $ vol . *7, no. 3, p. )30(31'.

Twort2 A.-.5 ! atnaa" a2 -.-. and +randt2 M.3. ',;;;(. ater 4upply. *th ed. Dondon, 8N ,

Arnold ublishers and IA ublishin . -hapter 7, part II, p. 317(36=.

0a)ner2 6.<. and udson 3r2 .6. '19,(. KDow(dosae hih(rate direct f iltration.

In5 =ournal of t'e 6meri#an ater or! 6!!o#iation, vol . 76, no. *, p. )*6.

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