- Email: [email protected]
Reuse of filter backwash water using ultrafiltration technology
ro cessing drinking wat er from surfa ce wat er produces hug e am oun ts of filter backwash wat er. Thi s wat er results from flushing th e conventional filters and co ntains th e wh ole sum of solid particles from th e tr eat ed raw wat er. Legi slation in Germany nowadays is increasingly asking wat er utilities to redu ce production wastes. Mor eover, raw wat er resources for some of th e drinking wat er treatment plants are limited and a discharge of th e filter backwash wat er indu ces disposal costs . Thus, recy cli ng the filte r backwash wat er is of high int erest. Before recyclin g, th e filter backwash wat er is usually sto re d in a sed ime ntation tank . Until 1997 th e clear wat er fro m th e settling tank was th en directl y retu rn ed to th e raw water . In co mpliance with a recommendation of th e Drinking Water Co mmission of th e Fed eral Enviro nme nta l Agency (Llm wcltbundcs amt) th e reu se of th is wat er without further treatment had to be sto pped. Toda y th e reu se of filter backwash wat er requires th e separation of solids and moreover th e co m plete rem oval o f micro organism s and para sites I. Th e required sec ure retention of microbiological parameters cannot cost effectively be guaranteed by conv entional technologies. Th e discussion of reusing th e backwash wat er today oft en results in conside ring the use of membrane
CASE STUDY: THE
PROBLEM A dr inking water tr eatm en t plant in Hit feld run by tadtwerke Aachen AG ( TAWAG), Germany, processes 2. 7milli on m ' reservoir water per year. After flocculation and pH adjustme nt the raw wat er passes a recir culati on filter and a dual -m edia sand filter (Figu re 1) . Backwashing of these filter s co nsumes about 10 % of th e tr eat ed water. Since the processing capacity of th e tr eatment plant excee ds the annual co ntinge nt of raw wat er fro m the dam , standstills of the plant may occ ur in d ry years. In orde r to achieve a bette r plant
utili sation and to amid the costs o f th e wastewater, recycling of the filter backwa h wat er is highly desirable. Th er e for e on behal f of STAWAG a pilot -scale ultrafiltration plant was ope rate d by th e Institut fiir Verfahrenstechnik (IVT) in co-ope ratio n with Rochem UF- yste me GmbH , Hamburg (Roc he m). Th e pr omi sing resul ts o f th e pilot study result ed in th e decision of STAWAG to tr eat th e backwash water with a full -scale ult rafiltration plant. Her e , result s from th e pilot study, design and cost data as well as first expe r iences with th e full-scale plant are pr esented .
First filtration stage
Second flttrabon stage
Re-filtrabon ftoccU8tlon
M lib layer filtration
We hebach dam ...
...
... to dea n water vessel
Filler backftush Rn sing air bIowtn
j,
Permeate a lterna tive to
PotaS5ium penna nganat e
clean water vessel
F~ulabn g agen t
Membrane filtration
I
SeWing basin Filter backwash water
r - - , -....... Permeate
Water purification plant Concentrate
'----''--------- - - - - --------'
technolog/ .
22
JanuarylFebruary 2000
Filtration + separation
PILOT-SCALE EVALUATION
Membrane cus hio n ~
Research Objectives From th e middle of Jun e until th e middle of August 1998 , a pilot plant was set up in th e drinking wat er treatment plant at Hitfeld in orde r to investigate th e ope rational possibiliti es and th e efficiency of ultrafiltration for the treatment of filter backwash wat er. The pilot plant was bypassed to th e clear wat er disposal. After sedime ntation th e clear wat er is tr eat ed by the membrane plant to reuse it as a raw wat er source. Th e co nce ntrate o f th e membrane plant, resulting from module Ilushing, is disposed olT tog eth er with the sludge fro m th e sett ling tank. Only an operation und er real pr ocess co nd itions co uld give reliabl e result s regarding th e feasibility of tr eating filter backwash water with membrane technology. Of int er est is for exam ple the inllu en ce o f th e peri odi c downtimes du e to filter backwash, sett ling and sludge rem oval fro m the sett ling tank on lon g term membrane performance. After optimising th e plant ope ratio n the pilot study should provid e result s about th e efficiency of a me mbrane plant and design data for a full-scale plant. Th e foll owing qu estions had to be answe re d: Whi ch mem brane material is most suita ble for this application and can guara ntee the product ion of th e desired qua lity of filtra te in the long ter m ?Which Ilux can be ob ta ined on a lon g term basis? Wha t are the opti mum co nd itions for th e operation of the plant and wh at recm'ery rate can be obtaine d? How can th e mo d ule Ilushing be fitted to this applicatio n and how often are che mica ls require d for clea ning o r disinfectio n? How does th e periodic do wnti mes of th e plant influ en ce its ope ratio n?
The Pilot Plant Th e syste m chose n for use in th e backwash tr eatment was a FM module syste m develope d by Roch em in co -o peration with th e IVT. Th e module is sim ilar to th at descr ibe d in Th omas Pet er's membrane bioreact or article in th is month 's issue of F+S P: 18 . As described by Pet ers , th e principal clem ents of th e FM module ar e membrane cushions , eac h co nsisting o f two rectangul ar membranes, tw o int ernal permeat e space rs , and an incorporated stiffene r plat e. Th e membrane cushio n is sealed by ultrasoni c welding (Figure 2). A
Filtration + Separation
Perm eate Stiffener plate
1')==::::::::=1
Membrane
Perme ate spacer
Variable height of feed channel : 1·3 mm
.
Figure 2: Rochem FM • Module System. .
Removal of fouling layer
I
.
Discharge of sol ids
=
Detach ed fouling layer
". Cross-flow flush ing
Air-bubble flush ing
• Membrane
me mb rane element co nsists of abo ut 27 stac ked me mbra ne cushions . T he cushio ns are enclosed in tw o halves of a shell. In add it ion to fix ing the membrane stac ks , the gUide pins, in co mb inatio n wi th co r res po nd ing bore hol es in th e half-shell ele me nts, co llec t and tr ansport th e permeat e ". In th e pilot plant eight of th ese membrane cushion ele me nts are co nnec te d in se ries and install ed in a PMM A pr essure tub e. Th e pilot plant co nsists o f tw o modules eac h fitt ed with abo ut 7m 2 m embrane area. Both modules can be operate d separately and are installed vertically to allow for effective air Ilushing. In or de r to optim ise th e plant ope ration th e pilot plant can be run in manual or auto matic mod e. Th e ma in pr ocess paramet er s can be chose n separate ly for each module. Since solid matter co nce ntration in filter backwash water (es pecia lly afte r sedime ntatio n) is quite low, the dead -end
Membrane
mod e was chose n for thi s application . Thi s mea ns that during filtrati on the water flow s perpendicul arl y through th e me m bra ne . T he module is ope rate d as a tw o -end -m odule. Th e ret ained solid matt er forms a fouling layer on th e membrane surface and , g radually, builds up a Ilow resistan ce. To am id a decr easing permeat e Ilux, th e pr essur e d iffer ence has to be incr eased during th e filtrati on cycle. Wh en th e filtrati on cycle is co mplete the fouling layer has to be rem oved, i.c , the module has to be clean ed . Th e long term stability of a dead -end filtration process reli es on th e efficiency o f th e cleaning procedure . Th e configu ration of Ilat channels in th e FM module allow s a very efficient cleaning co nsisting o f a co mbination of feed sided air bubble flushin g and back washing of permeat e through th e membrane (Figu re 3). Th e air inject ed into th e raw wat er at th e base of th e vertically installed modules indu ces high shear stresses and removes th e fouling
JanuarylFebruary 2000
23
[l/m'h] 200 , - - -- - - - --
-
-
-
............. ... .....
150 l(
-
-
-
-
~~~
- - - - Membrane rep lacement
-
-
-
-
-
-
....................
[bar] --, 2,0
layer from the membrane surface. The subsequent short period of cross -flow flushing carries the detached particles out of the module. Since th e air injection technique proved to be very effective, cleaning chemicals arc not required,
1,5
:::l
u:::
100
50
...
1,0
Results of the Pilot Study
--+- Tra nsmembrane pressure difference [bar]
...
•
PA membrane 50kD 10
"
0,5
PAN membrane 50kD 40
30
20
June '98
Permeate flux [Um'h]
50
July '98
Figure 4: Membrane f)erformance in the f)ilot study
.
Figure 5: Membrane plant for the treatment of 20 mJIh filter backwash
[Um-h]
[bar]
Breakdown permeate backwash pump
• ••
200
J -'"
l(
:::l
-
;;: 150 Q)
III
Q)
E
•
100
Q)
.•
• ~ •
&
Q. 50
Chemical cleaning
"..."..,~
::
.~
Installation PAN 200kD 2,0
:"'P.;#
• •
•
•
•
...
Q)
,'/'"
~:.:.::-'.,.
-r-».
.\~ &
•
1,5
e Co
1,0
...
24
April
June
May
July
Q) Q)
Permeate flux [Vm'h) Feed pressure [bar]
August
0,5
~
JanuarylFebruary 2000
[d]
Sept.
Figure 6: Membrane performance of the full-scale plant. , ~
'0 LL
o L...-...--- --,,-- - -,--+- - -:--- - -,--i-- - ,..-- ...,....- =-:-- - .,-lo o 80 100 120 200 220 March
:::l III III
.
The pilot study started in early June ]998 . At the beginning, both modules wer e equipped with polyararnid (PA) membranes, At the cnd of July th e membranes in on e of the modules were replaced by polyacrylonitrile (PAN) membranes, Both membrane materials had a cut-off of 50 kl), The investment costs of dead -end membrane plants form the biggest part of the total specifi c tr eatment cos ts", Thus, incr easing the plant capacity by enhancing the permeate flux results in a substantial reduction for thc total treatment costs, Accordingly maximizing the flux rate obtainable at stabl e op eration cond itions was the most important aim of th c pilot study. Th e paramet ers to be optimised arc the filtration tim e and all paramet ers related to the modul e flushing pr ocedure. Th e star t up of th e pilot plant occurred at a relatively low flux rat e. Th e flux co uld be rai sed by g rad ually adjusting th e module flushin g paramet ers to this appli cation (Figure 4), Reason s for drops of th e transm embrane pr essuredifferen ce (T MP) at co nstant flux es were usuall y du e to extra m odule flushin gs with air and wat er. According to Figure 4 it was po ssible to incr ease th e performance of th e PA membrane to over 1501lm 2 h. After membrane repla cem ent th e flux had to be redu ced du e to a rapid incr ease ofT MP. Obviou sly the PA membrane material can onl y be used at a some what lower flux level. Applying a relatively high TMP a flux of about 115 IIm 2h co uld be obtained at stable cond itions with th e PA membrane. During dead -end filtration th e am ount of produced filtrat e equals th e co nte nt of feed. Only th e filtrat e used for hack washing and the feed used for th e crossflow flush reduce th e recovery rat e . After a shor t star t-up pha se th e plant reach ed a recovery rat e of about 96%. Recovery rat e as well as ene rgy co nsum pt ion and th e net permeat e flux dep end on th e filtration tim e . At th e end of th e pilot study the filtrati on tim e for th e PA membrane was 120 min and for the
Filtration + Separation
Operating parameter
Module 1
Module :2
Permeat e flow rat e
(I/h)
1280
900
Membrane mat erial
(- ) (0C)
PA
PAN
10
10
Durati on o f filtrati on cycle
(m in)
120
100
Durati on o f flush
(m in)
5. 1
5. 1
Temper ature of raw wat er (average)
Membran e ar ea per module
(m 2)
6 .7
7.2
Tr an sm embrane pr essure differ en ce
(ba r)
0.88
1.75
190
119
bar)
2 16
68
(%)
96.9
94 .7
(I/ m 2h)
18 1
114
(kWh/m l )
0 .15
0. 18
(l/m 2h)
Per meate flux Permeabili ty
(I/ m 2h
Recover y let permeate flux Energy co nsum ption
Table 1 Operating data of the pilot plant
PAN m embrane 100 min, resu ltin g in both cases in a high net flux and recovery rat e at low ene rgy co nsum ptio n (Table I ) . It mu st be m ention ed at thi s point, th at during th e three months of pil oting , no che m icals were used ne ith er for backwash enhance me nt nor for chemical cleaning. Th e high fluxes were ac hieved by o pti misa tio n of th e module flushin g pro cedure for thi s app lication. ince the m embran e filtered water is ut ilised in drinking wa te r p ro du ction th e per m eate q uality is important. T he integr ity of the membran es was conti nuo usly co nt ro lle d by par ti cle co unti ng and in in tervals th e pe r meate qu ality was de te r mi ne d by plat e co unti ng. Both m embran es alwa ys sho we d drinking wat er q ua lity and low particle co unts . cvc r thc lcss, th er e was a differen ce in per meate qu alit y between th e tw o m embran es. Th e ret ention o f CF U (R2A-agar) for PA membranes was ab out one log-sta ge better, and particle co unts (> I IJm) for PA were alwa ys below 0.5 particles per ml whil e th e co unts fo r th e PA membrane were between 0. 5 and 1. 5 particl es per ml. It was es tablishe d that thi s differen ce was d ue to th e different quality of th e ultrason ic welding of th e cushio n seams: th e PA membrane mat er ial co uld be weld ed with out probl em s - co nt rary to th e PA m embrane mat erial. Accordingly th e PAN membrane material was cho se n for th e full-scale plant .
Filtration + Separation
DESIGN AND COST CALCULATION FOR THE FULL-SCALE PLANT Design Data As a co nse que nce of th e positive pilot results STAWAG deci de d to insta ll th e membrane filtr ation techno logy for treatment of the filter backwash water. It was agreed that STAWAG would bu y th e treated filt rate fro m Roch em at a fixed price . Roch em wo uld build, own and o perate th e plant - with th e IVT as partner for ope ration and furthe r op tim isatio n. T he plant capacity was fixed to 10 000 m ' per month, i.e . abo ut 350 m ' per day. T he daily operating time was assumed to be abo ut 18 hour s. Duri ng plant design , the manufacturer deve loped new PA memb ranes with higher cut-off values. Thus, an opti mistic value for th e permeat e flux for th e plan t design see me d to be rea listic and it was assumed th at a flux of abo ut 100 II m ' h co uld be achieved (Table 2). As a res ult 20 0 m 2 membrane are a has been installed int o 20 modules.
Cost Calculation Before signing th e agreem ent th e speci fic treatm ent cos ts wer e esti mate d. Th e kev J design and cos t data are listed in Table 2. Spec ific investment costs of 190 0 DM 1m 2 (ca 98 0 US 1m 2 ) installed mem br ane area represent an average figure of co m parable plan ts from differ ent manu facturers. Th e cos ts of che m icals may be some what overes timate d since th e pilot plant was
op erated without che m ical cleaning. Th e total specific cos ts are about 0.6 DM /m 3 (ca 0.31 US 1m '), Th ese costs ar e relatively low for th e treatment of filter backwash wat er " and a co nseque nce of th e high assume d permeat e flux . According to thi s calculation th e d epreciati on co nt r ibutes about 70 % o f th e total cos ts . T his is characte r istic for membrane plants ope rate d in dead -end mod e .
FIRST EXPERIENCES WITH THE FULL-SCALE PLANT In Februar y 1999 th e full-sca le plant was install ed and ope ration was sta r ted . The plant co nsists of two blocks with ten mod ules eac h (Fig ure 5). At star t- up th e plant was eq uip pe d w ith the PA 50 kD membran e , i.c, t he PAN m embran e of the pil ot plant. The perm eabilit y o f thi s membran e for th e firs t four m onths o f o pe ratio n is sho wn in Figure 6. A br eakd own of th e permeat e backwash pump after 24 days resulted in a shar p declin e in m embran e permeability. Without backwash, m odule flushin g proved to be inefficient - a thi ck fouling layer d evel op ed on th e membranes. This co uld be obse r ved "o n lin e" since one of th e m odule housings was made o f PMMA . Attempts to recover th e former m embrane permeability by e xt ra module flushing failed . Even furth er improvem ent o f th e modul e flushin g by adapting th e pr ocedure to th e co nd itio ns o f th e full -scal e plant co uld not recover th e membrane permeability to th e former level.
JanuarylFebruary 2000
25
Design data (mIl d)
Daily treatment duty Operating hours per day Capacity umber of modules Total membrane ar ea Expec te d membrane life tim e Permeat e flux Filtration cyc le Rccovcrv Ene rg y co nsum ption O pe rating pressure
(hi d) (m l/ h) (- ) (m 2) (a) (1/ m 2h) (mi n) (%) (kWh /rn ' ) (bar)
350 18 19 .4 20 200 4 97 90 95 0 .2 I - 1.5
Investment cos t T im e of depreciati on Int er est rat e Ca pital facto r Load factor
(DM) (a) (%) (%) (%) (m l /a) (DM /k W h)
380 000 10 7 14. 24 75 127 000 200 0.2
(DMlm 3 )
0.610
(D M/ m l ) (D M /m l )
0.426 0 .040 0 .035 0 .030 0 .079
(D M / m 2 )
Specific cost Depre ciat ion Energy Che m icals Maintena nce and staff Memb rane replacem ent
( D ~lI ml)
(D M/ m l ) (D M/ m l )
Table 2 Design and estimated cost data for the full-scale UF- plant
Thus, after abo ut 3 months in op eration a chemica l cleaning was car r ied out. Preced ing tests at the IVT led to th e decisio n to usc an alka line cleaner and aOC I as clea ni ng age nts. Per meabi lity afte r che mical clean ing was almost eq ua l to th e sta r t -up value . Applyi ng th e improved module flush ing it was possible to achi eve a stable permeability o f 50 II m' h bar for t he PAN 50 kD membrane . In par allel to th e full -scale plant th e pilot plant was ope rate d with th e before m ention ed new PAl membrane with a cut-off of 200 kD . As expecte d thi s membrane showe d a \·ery high permeability. Its ret ention for particles (> 1 urn ) and CFU was identical to th e ret ention o f th e 50 kD membrane . Th er efore, in blo ck 2 o f th e full -scale plant the 50 kD membranes wer e repla ced by 200 kD m embranes. Th ese m embranes ar c ope rate d at a flux of about liS II m -h. With thi s membrane th e design capacity of th e plant has been achi eved .
26
REFERENCES I . Bund esgesundheitsblatt 12/97, .484
Cost data
Ca paci ty per yea r Cos t of membran e replacem ent Elec tr icity cos t
injection and permeate backwash can reduce the consumption of ch emicals signifi cantly - a chemical cleaning e\"er)' 3-4 months is sufficient. Based on this positive experience STAWAG d ecided to double the capacity of th e ultra filtration plant. Moreover, the results o f thi s plant optimisation arc thc basi s for th e d esign and op eration of a 130 m ' lh plant in a wat er co m pany in Aschaffenburg. This plant will be co m m issione d in early 200 0.
JanuaryIFebruary 2000
SUMMARY A safe re te n tio n of m icro o rgani sm s in co m binatio n with h igh fluxes arc esse nt ial for a successful app lication of U F in th c t reatment of filte r backw ash water. In th e reported case it has been proved t hat high fluxes can be maintain ed fo r lon g ti mc pe r iods by tailoring th e m odul e flus hing pro cedure to the ind ividual ap plicat io n . It has been dcmon strat ed , that th e co m bina tio n o f fee ds ide air
ff. 2. P. Upp et aI., Desalination 119 ( 199 8) 13 3-142 3. T. Pet ers, Llmw clt Tcchnologie Aktu cll Int ernational Ed ition , 4 ( 1998) 4. R. Rautenbach, K. Vollenkaul , Pr oceedings "2. Aach en erTagung Sied lungs wasse r-wir tsc haft und Verfahr en stechnik : Membrantechnik in der 6 ffentl iche n Wassc raufbcrcitung und Abwasserbehandlung", Aach en , 1998,A1 8
ACKNOWLEDGEMENTS Thi s pap er was based on work car r ied o ut by And reas Briigger, K. Vossenkaul , T. Mclin , and R . Rautenbach at th e Inst itut fiir Verfahren stechnik, Rheini sch Westfal isch Technisch e Hoch schul e Aach en , TurmstraBe 46,52062 Aache n, Ger ma ny and B. Go lling, U. Jacob s, and I~ O hle nfo rst at Stadtwc rkc Aache n AG, Post fach 500 155 , 52085 Aache n, Germany.
Concacc : A. BruBBcr lnstitut fli r l'cifahrcnsrcchnik, RWTH Aachcn, Turmscraftc 46, 520 6 2 Aachcn, Germany. Tel.: +49 241 80-3996; E-mai l: brucBBcr@il" ulI.chaachen .de, I"osscnka u/@i l"l. rll'ch-aa chcn .dc
Filtration + Separation
CASE STUDY: THE
PROBLEM A dr inking water tr eatm en t plant in Hit feld run by tadtwerke Aachen AG ( TAWAG), Germany, processes 2. 7milli on m ' reservoir water per year. After flocculation and pH adjustme nt the raw wat er passes a recir culati on filter and a dual -m edia sand filter (Figu re 1) . Backwashing of these filter s co nsumes about 10 % of th e tr eat ed water. Since the processing capacity of th e tr eatment plant excee ds the annual co ntinge nt of raw wat er fro m the dam , standstills of the plant may occ ur in d ry years. In orde r to achieve a bette r plant
utili sation and to amid the costs o f th e wastewater, recycling of the filter backwa h wat er is highly desirable. Th er e for e on behal f of STAWAG a pilot -scale ultrafiltration plant was ope rate d by th e Institut fiir Verfahrenstechnik (IVT) in co-ope ratio n with Rochem UF- yste me GmbH , Hamburg (Roc he m). Th e pr omi sing resul ts o f th e pilot study result ed in th e decision of STAWAG to tr eat th e backwash water with a full -scale ult rafiltration plant. Her e , result s from th e pilot study, design and cost data as well as first expe r iences with th e full-scale plant are pr esented .
First filtration stage
Second flttrabon stage
Re-filtrabon ftoccU8tlon
M lib layer filtration
We hebach dam ...
...
... to dea n water vessel
Filler backftush Rn sing air bIowtn
j,
Permeate a lterna tive to
PotaS5ium penna nganat e
clean water vessel
F~ulabn g agen t
Membrane filtration
I
SeWing basin Filter backwash water
r - - , -....... Permeate
Water purification plant Concentrate
'----''--------- - - - - --------'
technolog/ .
22
JanuarylFebruary 2000
Filtration + separation
PILOT-SCALE EVALUATION
Membrane cus hio n ~
Research Objectives From th e middle of Jun e until th e middle of August 1998 , a pilot plant was set up in th e drinking wat er treatment plant at Hitfeld in orde r to investigate th e ope rational possibiliti es and th e efficiency of ultrafiltration for the treatment of filter backwash wat er. The pilot plant was bypassed to th e clear wat er disposal. After sedime ntation th e clear wat er is tr eat ed by the membrane plant to reuse it as a raw wat er source. Th e co nce ntrate o f th e membrane plant, resulting from module Ilushing, is disposed olT tog eth er with the sludge fro m th e sett ling tank. Only an operation und er real pr ocess co nd itions co uld give reliabl e result s regarding th e feasibility of tr eating filter backwash water with membrane technology. Of int er est is for exam ple the inllu en ce o f th e peri odi c downtimes du e to filter backwash, sett ling and sludge rem oval fro m the sett ling tank on lon g term membrane performance. After optimising th e plant ope ratio n the pilot study should provid e result s about th e efficiency of a me mbrane plant and design data for a full-scale plant. Th e foll owing qu estions had to be answe re d: Whi ch mem brane material is most suita ble for this application and can guara ntee the product ion of th e desired qua lity of filtra te in the long ter m ?Which Ilux can be ob ta ined on a lon g term basis? Wha t are the opti mum co nd itions for th e operation of the plant and wh at recm'ery rate can be obtaine d? How can th e mo d ule Ilushing be fitted to this applicatio n and how often are che mica ls require d for clea ning o r disinfectio n? How does th e periodic do wnti mes of th e plant influ en ce its ope ratio n?
The Pilot Plant Th e syste m chose n for use in th e backwash tr eatment was a FM module syste m develope d by Roch em in co -o peration with th e IVT. Th e module is sim ilar to th at descr ibe d in Th omas Pet er's membrane bioreact or article in th is month 's issue of F+S P: 18 . As described by Pet ers , th e principal clem ents of th e FM module ar e membrane cushions , eac h co nsisting o f two rectangul ar membranes, tw o int ernal permeat e space rs , and an incorporated stiffene r plat e. Th e membrane cushio n is sealed by ultrasoni c welding (Figure 2). A
Filtration + Separation
Perm eate Stiffener plate
1')==::::::::=1
Membrane
Perme ate spacer
Variable height of feed channel : 1·3 mm
.
Figure 2: Rochem FM • Module System. .
Removal of fouling layer
I
.
Discharge of sol ids
=
Detach ed fouling layer
". Cross-flow flush ing
Air-bubble flush ing
• Membrane
me mb rane element co nsists of abo ut 27 stac ked me mbra ne cushions . T he cushio ns are enclosed in tw o halves of a shell. In add it ion to fix ing the membrane stac ks , the gUide pins, in co mb inatio n wi th co r res po nd ing bore hol es in th e half-shell ele me nts, co llec t and tr ansport th e permeat e ". In th e pilot plant eight of th ese membrane cushion ele me nts are co nnec te d in se ries and install ed in a PMM A pr essure tub e. Th e pilot plant co nsists o f tw o modules eac h fitt ed with abo ut 7m 2 m embrane area. Both modules can be operate d separately and are installed vertically to allow for effective air Ilushing. In or de r to optim ise th e plant ope ration th e pilot plant can be run in manual or auto matic mod e. Th e ma in pr ocess paramet er s can be chose n separate ly for each module. Since solid matter co nce ntration in filter backwash water (es pecia lly afte r sedime ntatio n) is quite low, the dead -end
Membrane
mod e was chose n for thi s application . Thi s mea ns that during filtrati on the water flow s perpendicul arl y through th e me m bra ne . T he module is ope rate d as a tw o -end -m odule. Th e ret ained solid matt er forms a fouling layer on th e membrane surface and , g radually, builds up a Ilow resistan ce. To am id a decr easing permeat e Ilux, th e pr essur e d iffer ence has to be incr eased during th e filtrati on cycle. Wh en th e filtrati on cycle is co mplete the fouling layer has to be rem oved, i.c , the module has to be clean ed . Th e long term stability of a dead -end filtration process reli es on th e efficiency o f th e cleaning procedure . Th e configu ration of Ilat channels in th e FM module allow s a very efficient cleaning co nsisting o f a co mbination of feed sided air bubble flushin g and back washing of permeat e through th e membrane (Figu re 3). Th e air inject ed into th e raw wat er at th e base of th e vertically installed modules indu ces high shear stresses and removes th e fouling
JanuarylFebruary 2000
23
[l/m'h] 200 , - - -- - - - --
-
-
-
............. ... .....
150 l(
-
-
-
-
~~~
- - - - Membrane rep lacement
-
-
-
-
-
-
....................
[bar] --, 2,0
layer from the membrane surface. The subsequent short period of cross -flow flushing carries the detached particles out of the module. Since th e air injection technique proved to be very effective, cleaning chemicals arc not required,
1,5
:::l
u:::
100
50
...
1,0
Results of the Pilot Study
--+- Tra nsmembrane pressure difference [bar]
...
•
PA membrane 50kD 10
"
0,5
PAN membrane 50kD 40
30
20
June '98
Permeate flux [Um'h]
50
July '98
Figure 4: Membrane f)erformance in the f)ilot study
.
Figure 5: Membrane plant for the treatment of 20 mJIh filter backwash
[Um-h]
[bar]
Breakdown permeate backwash pump
• ••
200
J -'"
l(
:::l
-
;;: 150 Q)
III
Q)
E
•
100
Q)
.•
• ~ •
&
Q. 50
Chemical cleaning
"..."..,~
::
.~
Installation PAN 200kD 2,0
:"'P.;#
• •
•
•
•
...
Q)
,'/'"
~:.:.::-'.,.
-r-».
.\~ &
•
1,5
e Co
1,0
...
24
April
June
May
July
Q) Q)
Permeate flux [Vm'h) Feed pressure [bar]
August
0,5
~
JanuarylFebruary 2000
[d]
Sept.
Figure 6: Membrane performance of the full-scale plant. , ~
'0 LL
o L...-...--- --,,-- - -,--+- - -:--- - -,--i-- - ,..-- ...,....- =-:-- - .,-lo o 80 100 120 200 220 March
:::l III III
.
The pilot study started in early June ]998 . At the beginning, both modules wer e equipped with polyararnid (PA) membranes, At the cnd of July th e membranes in on e of the modules were replaced by polyacrylonitrile (PAN) membranes, Both membrane materials had a cut-off of 50 kl), The investment costs of dead -end membrane plants form the biggest part of the total specifi c tr eatment cos ts", Thus, incr easing the plant capacity by enhancing the permeate flux results in a substantial reduction for thc total treatment costs, Accordingly maximizing the flux rate obtainable at stabl e op eration cond itions was the most important aim of th c pilot study. Th e paramet ers to be optimised arc the filtration tim e and all paramet ers related to the modul e flushing pr ocedure. Th e star t up of th e pilot plant occurred at a relatively low flux rat e. Th e flux co uld be rai sed by g rad ually adjusting th e module flushin g paramet ers to this appli cation (Figure 4), Reason s for drops of th e transm embrane pr essuredifferen ce (T MP) at co nstant flux es were usuall y du e to extra m odule flushin gs with air and wat er. According to Figure 4 it was po ssible to incr ease th e performance of th e PA membrane to over 1501lm 2 h. After membrane repla cem ent th e flux had to be redu ced du e to a rapid incr ease ofT MP. Obviou sly the PA membrane material can onl y be used at a some what lower flux level. Applying a relatively high TMP a flux of about 115 IIm 2h co uld be obtained at stable cond itions with th e PA membrane. During dead -end filtration th e am ount of produced filtrat e equals th e co nte nt of feed. Only th e filtrat e used for hack washing and the feed used for th e crossflow flush reduce th e recovery rat e . After a shor t star t-up pha se th e plant reach ed a recovery rat e of about 96%. Recovery rat e as well as ene rgy co nsum pt ion and th e net permeat e flux dep end on th e filtration tim e . At th e end of th e pilot study the filtrati on tim e for th e PA membrane was 120 min and for the
Filtration + Separation
Operating parameter
Module 1
Module :2
Permeat e flow rat e
(I/h)
1280
900
Membrane mat erial
(- ) (0C)
PA
PAN
10
10
Durati on o f filtrati on cycle
(m in)
120
100
Durati on o f flush
(m in)
5. 1
5. 1
Temper ature of raw wat er (average)
Membran e ar ea per module
(m 2)
6 .7
7.2
Tr an sm embrane pr essure differ en ce
(ba r)
0.88
1.75
190
119
bar)
2 16
68
(%)
96.9
94 .7
(I/ m 2h)
18 1
114
(kWh/m l )
0 .15
0. 18
(l/m 2h)
Per meate flux Permeabili ty
(I/ m 2h
Recover y let permeate flux Energy co nsum ption
Table 1 Operating data of the pilot plant
PAN m embrane 100 min, resu ltin g in both cases in a high net flux and recovery rat e at low ene rgy co nsum ptio n (Table I ) . It mu st be m ention ed at thi s point, th at during th e three months of pil oting , no che m icals were used ne ith er for backwash enhance me nt nor for chemical cleaning. Th e high fluxes were ac hieved by o pti misa tio n of th e module flushin g pro cedure for thi s app lication. ince the m embran e filtered water is ut ilised in drinking wa te r p ro du ction th e per m eate q uality is important. T he integr ity of the membran es was conti nuo usly co nt ro lle d by par ti cle co unti ng and in in tervals th e pe r meate qu ality was de te r mi ne d by plat e co unti ng. Both m embran es alwa ys sho we d drinking wat er q ua lity and low particle co unts . cvc r thc lcss, th er e was a differen ce in per meate qu alit y between th e tw o m embran es. Th e ret ention o f CF U (R2A-agar) for PA membranes was ab out one log-sta ge better, and particle co unts (> I IJm) for PA were alwa ys below 0.5 particles per ml whil e th e co unts fo r th e PA membrane were between 0. 5 and 1. 5 particl es per ml. It was es tablishe d that thi s differen ce was d ue to th e different quality of th e ultrason ic welding of th e cushio n seams: th e PA membrane mat er ial co uld be weld ed with out probl em s - co nt rary to th e PA m embrane mat erial. Accordingly th e PAN membrane material was cho se n for th e full-scale plant .
Filtration + Separation
DESIGN AND COST CALCULATION FOR THE FULL-SCALE PLANT Design Data As a co nse que nce of th e positive pilot results STAWAG deci de d to insta ll th e membrane filtr ation techno logy for treatment of the filter backwash water. It was agreed that STAWAG would bu y th e treated filt rate fro m Roch em at a fixed price . Roch em wo uld build, own and o perate th e plant - with th e IVT as partner for ope ration and furthe r op tim isatio n. T he plant capacity was fixed to 10 000 m ' per month, i.e . abo ut 350 m ' per day. T he daily operating time was assumed to be abo ut 18 hour s. Duri ng plant design , the manufacturer deve loped new PA memb ranes with higher cut-off values. Thus, an opti mistic value for th e permeat e flux for th e plan t design see me d to be rea listic and it was assumed th at a flux of abo ut 100 II m ' h co uld be achieved (Table 2). As a res ult 20 0 m 2 membrane are a has been installed int o 20 modules.
Cost Calculation Before signing th e agreem ent th e speci fic treatm ent cos ts wer e esti mate d. Th e kev J design and cos t data are listed in Table 2. Spec ific investment costs of 190 0 DM 1m 2 (ca 98 0 US 1m 2 ) installed mem br ane area represent an average figure of co m parable plan ts from differ ent manu facturers. Th e cos ts of che m icals may be some what overes timate d since th e pilot plant was
op erated without che m ical cleaning. Th e total specific cos ts are about 0.6 DM /m 3 (ca 0.31 US 1m '), Th ese costs ar e relatively low for th e treatment of filter backwash wat er " and a co nseque nce of th e high assume d permeat e flux . According to thi s calculation th e d epreciati on co nt r ibutes about 70 % o f th e total cos ts . T his is characte r istic for membrane plants ope rate d in dead -end mod e .
FIRST EXPERIENCES WITH THE FULL-SCALE PLANT In Februar y 1999 th e full-sca le plant was install ed and ope ration was sta r ted . The plant co nsists of two blocks with ten mod ules eac h (Fig ure 5). At star t- up th e plant was eq uip pe d w ith the PA 50 kD membran e , i.c, t he PAN m embran e of the pil ot plant. The perm eabilit y o f thi s membran e for th e firs t four m onths o f o pe ratio n is sho wn in Figure 6. A br eakd own of th e permeat e backwash pump after 24 days resulted in a shar p declin e in m embran e permeability. Without backwash, m odule flushin g proved to be inefficient - a thi ck fouling layer d evel op ed on th e membranes. This co uld be obse r ved "o n lin e" since one of th e m odule housings was made o f PMMA . Attempts to recover th e former m embrane permeability by e xt ra module flushing failed . Even furth er improvem ent o f th e modul e flushin g by adapting th e pr ocedure to th e co nd itio ns o f th e full -scal e plant co uld not recover th e membrane permeability to th e former level.
JanuarylFebruary 2000
25
Design data (mIl d)
Daily treatment duty Operating hours per day Capacity umber of modules Total membrane ar ea Expec te d membrane life tim e Permeat e flux Filtration cyc le Rccovcrv Ene rg y co nsum ption O pe rating pressure
(hi d) (m l/ h) (- ) (m 2) (a) (1/ m 2h) (mi n) (%) (kWh /rn ' ) (bar)
350 18 19 .4 20 200 4 97 90 95 0 .2 I - 1.5
Investment cos t T im e of depreciati on Int er est rat e Ca pital facto r Load factor
(DM) (a) (%) (%) (%) (m l /a) (DM /k W h)
380 000 10 7 14. 24 75 127 000 200 0.2
(DMlm 3 )
0.610
(D M/ m l ) (D M /m l )
0.426 0 .040 0 .035 0 .030 0 .079
(D M / m 2 )
Specific cost Depre ciat ion Energy Che m icals Maintena nce and staff Memb rane replacem ent
( D ~lI ml)
(D M/ m l ) (D M/ m l )
Table 2 Design and estimated cost data for the full-scale UF- plant
Thus, after abo ut 3 months in op eration a chemica l cleaning was car r ied out. Preced ing tests at the IVT led to th e decisio n to usc an alka line cleaner and aOC I as clea ni ng age nts. Per meabi lity afte r che mical clean ing was almost eq ua l to th e sta r t -up value . Applyi ng th e improved module flush ing it was possible to achi eve a stable permeability o f 50 II m' h bar for t he PAN 50 kD membrane . In par allel to th e full -scale plant th e pilot plant was ope rate d with th e before m ention ed new PAl membrane with a cut-off of 200 kD . As expecte d thi s membrane showe d a \·ery high permeability. Its ret ention for particles (> 1 urn ) and CFU was identical to th e ret ention o f th e 50 kD membrane . Th er efore, in blo ck 2 o f th e full -scale plant the 50 kD membranes wer e repla ced by 200 kD m embranes. Th ese m embranes ar c ope rate d at a flux of about liS II m -h. With thi s membrane th e design capacity of th e plant has been achi eved .
26
REFERENCES I . Bund esgesundheitsblatt 12/97, .484
Cost data
Ca paci ty per yea r Cos t of membran e replacem ent Elec tr icity cos t
injection and permeate backwash can reduce the consumption of ch emicals signifi cantly - a chemical cleaning e\"er)' 3-4 months is sufficient. Based on this positive experience STAWAG d ecided to double the capacity of th e ultra filtration plant. Moreover, the results o f thi s plant optimisation arc thc basi s for th e d esign and op eration of a 130 m ' lh plant in a wat er co m pany in Aschaffenburg. This plant will be co m m issione d in early 200 0.
JanuaryIFebruary 2000
SUMMARY A safe re te n tio n of m icro o rgani sm s in co m binatio n with h igh fluxes arc esse nt ial for a successful app lication of U F in th c t reatment of filte r backw ash water. In th e reported case it has been proved t hat high fluxes can be maintain ed fo r lon g ti mc pe r iods by tailoring th e m odul e flus hing pro cedure to the ind ividual ap plicat io n . It has been dcmon strat ed , that th e co m bina tio n o f fee ds ide air
ff. 2. P. Upp et aI., Desalination 119 ( 199 8) 13 3-142 3. T. Pet ers, Llmw clt Tcchnologie Aktu cll Int ernational Ed ition , 4 ( 1998) 4. R. Rautenbach, K. Vollenkaul , Pr oceedings "2. Aach en erTagung Sied lungs wasse r-wir tsc haft und Verfahr en stechnik : Membrantechnik in der 6 ffentl iche n Wassc raufbcrcitung und Abwasserbehandlung", Aach en , 1998,A1 8
ACKNOWLEDGEMENTS Thi s pap er was based on work car r ied o ut by And reas Briigger, K. Vossenkaul , T. Mclin , and R . Rautenbach at th e Inst itut fiir Verfahren stechnik, Rheini sch Westfal isch Technisch e Hoch schul e Aach en , TurmstraBe 46,52062 Aache n, Ger ma ny and B. Go lling, U. Jacob s, and I~ O hle nfo rst at Stadtwc rkc Aache n AG, Post fach 500 155 , 52085 Aache n, Germany.
Concacc : A. BruBBcr lnstitut fli r l'cifahrcnsrcchnik, RWTH Aachcn, Turmscraftc 46, 520 6 2 Aachcn, Germany. Tel.: +49 241 80-3996; E-mai l: brucBBcr@il" ulI.chaachen .de, I"osscnka u/@i l"l. rll'ch-aa chcn .dc
Filtration + Separation