AES BI OFLUX
Ad va n ce s in En vir on m e n t a l Scie n ce s -
I n t e r n a t ion a l Jou r n a l of t h e Biof lu x Socie t y
Re m ov a l of or ga n ic loa d a n d su spe n de d solids
fr om w a t e r by e le ct r ocoa gu la t ion m e t h od
1
I lie Pisoi,
2Crist ian Danielescu,
3Florica Manea,
4Sm aranda Masu,
2
Cecilia Savii, and
3Georget a Burt ic
ă
1
SECOM Com pany , Dr obet a- Tur nu Sev er in, Rom ania; 2I nst it ut e of Chem ist r y Tim isoar a of Rom anian Academ y , Tim isoar a, Rom ania; 3” Polit ehnica” Univ er sit y of Tim isoara, Tim isoar a, Rom ania; 4Nat ional Resear ch and Dev elopm ent I nst it ut e for I ndust r ial
Ecology - ECOI ND, Tim isoar a, Rom ania. Cor r esponding aut hor : F. Manea, florica. m anea@chim . upt . r o
Abst r a ct . Elect r ocoagulat ion ( EC) is an elect r ochem ical t echnique inv olv ing in- sit u gener at ion of coagulat ion agent s fr om sacr ificial anodes by t y pe of alum inium and ir on. I n t he pr esent st udy EC has been ev aluat ed as a t r eat m ent t echnology for or ganic load and suspended solids r em ov al fr om a r eal wast ewat er pr oceeded fr om pulp and p aper indust r y , which was char act er ized by high cont ent of or ganic load and suspended solids. The efficiency of t he elect r ocoagulat ion pr ocess was assessed by m onit or ing Chem ical Ox y gen Dem and ( COD) and suspended solids ( s. s.) par am et er s. Labor at or y- scale ex per im ent s wer e conduct ed wit h alum inum anode t o assess it s efficiency under galv anost at ic r egim e. To est ablish t he opt im um oper at ional par am et er select ed as cur r ent densit y , t he elect r oly sis was carr ied out at 50, 100 and 20 0 Am- 2. The elect r ocoagulat ion pr ocess per for m ance was assessed based on t he efficiencies of
COD and s. s. r em ov al corr elat ed wit h t he specific elect r ical ener gy consum pt ion. Applying EC pr ocess allowed t o achiev e t he COD r em ov al ef ficiencies r anged bet ween 83. 8 0 and 94. 0 0 % and s. s. r em ov al efficiencies bet ween 87. 77 and 9 5. 47 % .
Ke y W or ds: wast ewat er , pulp and paper indust r y , elect r ocoagulat ion, alum inium an odes.
I n t r odu ct ion. Pulp and paper indust r y is sev erely pollut ing t he env ir onm ent , especial
because of lar ge v olum es of w ast ew at er dischar ged int o t he aquat ic environm ent s. This k ind of pulp and paper w ast ew at er , cont aining m ainly t oxic or ganic subst ances, is char act er ized by a high lev el of chem ical ox y gen dem and ( COD) and suspended solids ( s. s. ) . Thus, t he w ast ew at er t r eat m ent t o r educe any possible im pact s on t he aquat ic env ir onm ent is r equir ed ( Gav rilescu & Puit el 2007; Gav r ilescu & Bobu 2 00 9; Pat el & Sur esh 2 008 ; Wang et al 2006ab) . I n gener al, t he w ast ew at er t r eat m ent is a r eal challenge for m any of t he indust r ies t o com ply wit h applicable effluent st andar ds, w hich ar e est ablished in r elat ion w it h t he r eceiv er envir onm ent .
Ther e ar e sev er al conv ent ional w ast ew at er t r eat m ent m et hods, including phy sical, chem ical and biological m et hods ( Babu et al 200 7; Budiy ono et al 2010; Car neir o et al 200 3; I nan et al 200 4; Mouli et al 2004; Raghu & Basha 2 007 ) . End- of- pipe t r eat m ent of w ast ew at ers can be accom plished by int egr at ion of t r adit ional biological t r eat m ent pr ocesses wit h phy sical- chem ical applicat ions. The biological m et hods ar e v er y effect iv e for t he t r eat m ent of w ast ew at er w it h a high v alue of biological ox y gen dem and ( BOD) , but t hey ar e ineffect iv e if r ecalcit r ant or ganic com pounds ar e pr esent because t hey m ust be suppor t ed by a phy sical- chem ical t r eat m ent . Coagulat ion is one of t he m ost com m only used t echniques. I n coagulat ion oper at ions, a coagulat ion agent is added t o a colloidal suspension t o cause it s dest abilizat ion and flocculat ion. How ev er , t his t echnology usually needs addit ional chem icals w hich pr oduce a hu ge v olum e of sludge ( Raghu & Basha 200 7) .
Coagulant s ar e pr oduced by t he elect r oly t ic oxidat ion of appr opr iat e anode m at erials, such as st ainless st eel and alum inum elect r odes, w hich r esult in for m at ion of highly char ged poly m eric m et al hy dr ox yl species. These species neut r alize t he elect r ost at ic char ges on t he suspended solids and facilit at e agglom er at ion r esult ing in separ at ion fr om t he aqueous phase ( Cior ba et al 2000; Cior ba et al 2002) .
I n com par ison w it h t he t r adit ional conv ent ional coagulat ion pr ocess, v arious adv ant ages of elect r ocoagulat ion pr ocess hav e been r epor t ed ( Mollah et al 2001; Raj eshw ar & I banez 19 96) . Thus, t he elect r ocoagulat ion pr ocess can be r egar ded as a gr een t echnology in r elat ion wit h bot h t he clean r eagent t hat is t he elect r on and no adding any chem icals t o pr oduce secondar y pollut ion. Anot her im por t ant adv ant age is given by t he sim plicit y and com pact ness of t he t r eat m ent facilit y and t her e is a possibilit y of com plet e aut om at ion. Elect r ocoagulat ion pr ocess has t he adv ant age of r em ov ing t he sm allest colloidal par t icles, t he flocs for m ed dur ing t he elect r o coagulat ion pr ocess t ends t o be m uch lar ger, t her efor e can be separ at ed by filt r at ion. I t is a low sludge pr oducing pr ocess, and t he sludge for m ed dur ing t he pr ocess t ends t o be r eadily set t able and easy t o dew at er .
I n t his paper ar e present ed som e r esult s r egar ding t he assessm ent of t he elect r ocoagulat ion applicat ion on a r eal w ast ewat er pr oceeded fr om pulp and paper indust r y. The opt im um oper at ing condit ions under galv anost at ic r egim e w er e est ablished based on t he cur r ent densit y, elect r oly sis t im e and t he specific elect rical ener gy consum pt ion. The elect r ocoagulat ion per for m ance assessed in r elat ion wit h chem ical ox y gen dem and ( COD) and suspended solids ( s.s. ) r em ov al w as com par ed wit h sim ple conv ent ional coagulat ion using t he opt im um dose of alum inium sulphat e.
M a t e r ia l a n d M e t h od. A r eal r aw w ast ew at er proceeded fr om a pulp and paper fact or y
w as used for t he labor at or y ex perim ent s. The qualit y of t he r aw and t r eat ed w ast ew at er s w as m onit or ed in t er m s of COD ( for or ganic load) and s. s. ( suspended solids) par am et er s. The quant it at iv e det er m inat ions of COD and s. s. par am et er s w er e m ade in accor ding wit h Am er ican St andar d Met hods ( Fr anson 19 95) .
The r em ov al of pollut ant s by elect r oflot ocoagulat ion and separ at ion w as car ried out in t he elect r olysis cell at galv anost at ic condit ion. The schem at ic pr esent at ion of t he ex perim ent al set - up w as pr esent ed in our pr evious st udy ( Bebeselea et al 2006) . The char act er ist ics of r aw w ast ew at er ar e gat her ed in Table 1. The elect r oflot ocoagulat ion pr ocess w as oper at ed at differ ent cur r ent densit ies t o det er m ine t he opt im um oper at ing v ar iables in t er m s of cur r ent densit y and t he elect r oly sis t im e. The assessm ent of t he elect r ocoagulat ion pr ocess w as based on bot h t he r em ov al pr ocess efficiency and t he ener gy consum pt ion.
Table 1 The char act er ist ics of r aw w ast ew at er pr oceeded
fr om pulp and paper indust r y
Param et er pH COD ( m gO2L- 1) s.s. ( m gL- 1) Cl- ( m gL- 1) SO42- ( m gL- 1)
Value 6.73 2285 3600 350 525
The r em ov al pr ocess efficiency in r elat ion wit h suspended solids and chem ical oxy gen dem and aft er elect r ocoagulat ion t r eat m ent w as det er m ined based on t he equat ion ( 1) : Rem ov al efficiency ( % ) = [ ( C0 – C) / C0] x 100 ( 1) The specific ener gy consum pt ion, Wsp, w as calculat ed wit h t he r elat ion ( 2) :
( k Wh dm- 3) ( 2 )
1000 * 1
U V Q
Wsp
Al2( SO4)3* 18H2O r eagent pr ov ided by Chim opar Bucur est i w as used for t he coagulat ion
ex perim ent s. Coagulat ion ex perim ent s w er e car r ied out using Degr em ont Jar Test m et hod in bat ches of 1L. Aft er t he addit ion of t he coagulant s, t he sam ples w ere subj ect ed t o r apid m ixing for t w o m inut es at 250 r pm , t hen for 1 0 m inut es at 25 r pm and last set t ling for 30 m inut es, wit hout pH cor r ect ion. The super nat ant w as analy zed. Opt im al dose for coagulant agent w as det er m ined also accor ding t o COD and s. s.
Re su lt s a n d D iscu ssion. Taking int o account t he char act er ist ics of r aw w at er in r elat ion
w it h t he pr esence of chlor ide and sulphat e, t he elect r oly sis experim ent s wer e car ried out w it hout any adding of supplem ent ar y salt s, w hich are usually necessar y for elect r ochem ical t r eat m ent . Based on our pr ev ious st udy ( Bebeselea et al 2006) t he ex perim ent s wer e conduct ed at t hr ee cur r ent densit ies, i. e., 50, 100 and r espect iv e, 200 Am- 2.
Table 2 pr esent s w or k ing condit ion and t he assessm ent of pr ocess cont r ol par am et er s, i. e., COD and s. s. r em ov al efficiency and specific elect rical ener gy consum pt ion. The used quant it y of elect ricit y ( char ge consum ed) is ex pr essed as Ah/ dm3 for bat ches of 50 0 cm3.
Table 2 Ev olut ion of t he elect r oflot ocoagulat ion pr ocess
under cur r ent densit y of 50 A/ m2
Q/ V ( Ah/ dm3)
t im e ( m inut es)
U ( V)
Wsp
( k Wh/ m3)
pHf COD
( m gO2L- 1)
s. s. ( m gL- 1)
0. 1 15 8. 7 0. 87 7. 74 177. 0 440
0. 4 60 11. 7 4. 68 7. 77 151. 0 240
0. 8 120 14. 6 11. 68 7. 90 146. 0 230
1. 0 150 15. 0 15. 00 8. 11 142. 0 220
1. 2 180 15. 3 18. 36 8. 18 137. 0 163
The m ain elect r ochem ical r eact ions t hat occur dur ing t he elect r oly sis at galv anost at ic condit ions can be described by act iv e dissolut ion of anode ( alum inium ) , cat hodic hy dr ogen ev olut ion, leading t o t he pH incr easing and alum inium hy dr oxicom plex es species for m at ion. I ncr easing local pH fav our ed t he gener at ion of t he alum inium hy dr ox icom plex es, w hich can be adsor bed on t he colloids and cr eat es bridges bet w een t he par t icles. I n addit ion, t he am or phous solid alum inium hy dr ox ide flocs t hat for m s at elev at ed pH, set t le dow n causing sw eep flocculat ion. The elect r ocoagulat ion pr ocess inv olv es t hr ee successiv e st ages ( Mollah et al 2001) , e. g. , for m at ion of coagulant s by elect r oly t ic oxidat ion of t he sacr ificial anode elect r odes, dest abilizat ion of t he cont am inant s, par t iculat e suspension, and t he aggr egat ion of t he dest abilized phases t o for m flocs.
The m ain r eact ions t hat occur during t he elect r oly sis under galv anost at ic condit ions can be described by act iv e dissolut ion of anode ( alum inium ) , cat hodic hy dr ogen ev olut ion, a pH incr ease and alum inium hy dr ox icom plex es species for m at ion, as follow s:
Al anode r eact ions:
Al Al3 ++ 3e- ( 3)
H2 + OH-2H2O + 2e- ( 4)
4OH-O2+ 2H2O + 4e- ( 5)
St ainless st eel cat hode r eact ions:
2H2O + 2e-H2+ 2OH- ( 6)
O2+ 2H2O + 4 e- 4 OH- ( 7)
Bulk solut ion:
Al3 + + 3H2O Al( OH)3 + 3H+ ( 8)
pH affect ed t he elect r ocoagulat ion m echanism , especial in bulk solut ion. At low pH v alues of 2–3, cat ionic m onom er ic species Al3 + and Al( OH)2+ pr edom inat e. When pH is
bet w een 4–9, t he Al3 + and OH- ions gener at ed by t he elect r odes r eact t o for m v ar ious
Al6( OH)1 53 +, Al7( OH)1 74 + and Al1 3( OH)3 45 + t hat finally t r ansfor m int o insoluble am or phous
Al( OH)3( s) t hr ough com plex poly m erizat ion/ pr ecipit at ion kinet ics [ 26] . When pH is higher
t han 10, t he m on om er ic Al( OH)4- anion concent r at ion incr eases t o Al( OH)3( s) det rim ent .
Tw o m ain m echanism s ar e gener ally consider ed for elect r ocoagulat ion in t he bulk solut ion: pr ecipit at ion for pH low er t han 4 and adsor pt ion for higher pH. Adsor pt ion m ay pr oceed on Al( OH)3 or on t he m onom er ic Al( OH)4- anion depending on t he pollut ant
chem ical st r uct ur e. The for m at ion of Al( OH)3( s) is opt im al in t he 4–9 pH r ange, and t his
pH w as used in t his w or k.
The m ain elect r ode pr ocesses can be par t ially act iv at es or inhibit s on t he pr esence of or ganic and suspended m at t er by adsor pt ion on elect r ode and inv olv em ent in sur face film for m at ion. Also, t he pr esence of Cl-, SO42 - anions influences t he alum inium
dissolut ion, wit h fur t her inv olv em ent in t he elect r ocoagulat ion pr ocess.
The t heor et ical doses of elect r ochem ical dissolv ed alum inium w er e calculat ed based on t he Far aday ’s law pr esent ed in equat ion ( 9) :
w = it M/ ZF ( 9)
w her e: w = alum inium dissolving ( g) , i = cur r ent ( A) ,
t = t im e ( s) ,
M = m olecular weight of Al
Z = num ber of elect r ons inv olv ed in t he r edox r eact ions and F= Far aday ’s const ant = 96500 C/ m ol
Also, t he ex perim ent al dissolv ed alum inium doses w er e det er m ined by elect r ode w eighing befor e and aft er elect r oly sis. Under st udied condit ions, t he ov er- Far aday s dissolut ion behaviour of alum inium anode w as achieved, w hich r epr esent s t he com m on aspect for alum inium . This infor m s t hat no inhibit ion of t he alum inium dissolut ion occur r ed under t he condit ions of 350 m gL- 1 Cl- and 525 m gL- 1 SO42 -.
Figur es 1 and 2 pr esent t he t echnical r esult s of t he elect r ocoagulat ion applicat ion on t he r eal w ast ew at er pr oceeded fr om pulp and paper indust ry at t he t hr ee cur r ent densit ies Under all cur r ent densit ies, t he qualit y of t he t r eat ed w ast ew at er is suit able for dischar ging int o t he sew er age and not int o t he n at ur al r eceiv er. How ev er , t he v er y good efficiencies higher t han 90 % w er e r eached for t he cur r ent densit y of 50 Am- 2. For com par ison, t he classical coagulat ion using alum inium sulphat e w as st udied by Jar- t est m et hod and t he opt im um dose of alum inium sulphat e w as 22. 5 m g Al L- 1 ( t he r esult s ar e not show n her e) . For t his opt im um dose, t he coagulat ion efficiency w as w or se t han t he elect r ocoagulat ion one.
Figur e 1. Ev olut ion of COD r em ov al efficiency v er sus char ge consum ed dur ing elect r ocoagulat ion pr ocess at differ ent cur r ent densit ies.
0.1 0.2 0.3 0 .4 0.5 0.6 0 .7 0.8 0.9 1.0 1 .1 1.2 1.3 8 4
8 6 8 8 9 0 9 2 9 4 9 6 9 8 10 0
C
O
D
r
e
m
o
va
l
e
ff
ici
e
n
cy
(%
)
Q (Ahdm-3)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 85
90 95 100
s
.s
.
re
m
o
v
a
l
e
ff
ic
ie
n
c
y
(
%
)
Q(Ahdm-3) i=50 Am-2 i= 100 Am-2 i=200 Am-2
Figur e 2. Ev olut ion of s. s. r em ov al efficiency v er sus char ge consum ed dur ing elect r ocoagulat ion pr ocess at differ ent cur r ent densit ies.
A m aj or aspect r egar ding t he assessm ent of t he elect r ochem ical per for m ance is t he specific elect rical ener gy consum pt ion, w hich has t o be cor r elat ed wit h t he best t r eat m ent per for m ance. The r esult s r elat ed t o elect rical ener gy consum pt ion for elect r ocoagulat ion pr ocess ar e show ed in Figur e 3. As w e expect ed, t he lowest specific elect rical ener gy consum pt ion w as r eached under t he condit ions of t he cur r ent densit y of 50 Am- 2. To det er m ine t he dur at ion of t he elect r ocoagulat ion applicat ion, t he dependence of t he elect r oly sis t im e at each cur r ent densit y ver sus t he char ge consum ed dur ing t he elect r oly sis is show n in Figur e 4. Based on t he t echnical- econom ical point of v iew, t he assessm ent of t he elect r ocoagulat ion pr ocess on t he w ast ew at er wit h t he specific char act er ist ics, t he oper at ing condit ions w as est ablished as t he cur r ent densit y of 50Am- 2 for t he elect r oly sis t im e of 60 m inut es wit h t he specific elect rical ener gy consum pt ion of 4 k Whm- 3.
Figur e 3. Ev olut ion of specific elect rical ener gy consum pt ion v er sus char ge consum ed dur ing elect r ocoagulat ion pr ocess at differ ent cur r ent densit ies.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0
4 8 12 16 20 24 28 32
W
s
p
(k
W
h
m
-3 )
Q (Ah/dm3)
Figur e 4. Dependence of t he elect r oly sis t im e v er sus char ge consum ed dur ing elect r ocoagulat ion pr ocess at differ ent cur r ent densit ies.
Con clusion s. The applicat ion of elect r ocoagulat ion pr ocess in t he r em ov al of or ganic
load and suspended solids fr om a r eal w ast ewat er pr oceeded fr om pulp and paper indust r y giv en good r esult s, t he r em ov al efficiency of COD r eached 94 % and t he r em ov al efficiency of suspended solids r eached 95. 5 % for t he cur r ent densit y of 50 Am- 2 aft er 180 m inut es of t he elect r oly sis t im e. The elect r ocoagulat ion r esult s w er e bet t er v er sus sim ple coagulat ion using alum inium sulphat e at opt im um dose. The r eal w ast ew at er com posit ion allow ed t he applicat ion of elect r ocoagulat ion pr ocess wit hout a supplem ent ary adding of suppor t ing elect r oly t e ( chlor ide and sulphat e) . The opt im um w or k ing condit ions should be select ed based on a cer t ain r em ov al efficiency t aking int o account t he dependence bet w een t he specific elect r ical ener gy consum pt ion, t he cur r ent densit y and t he elect r oly sis t im e.
Ack n ow le dge m e n t s. This w or k w as suppor t ed by Rom anian Nat ional Resear ch Pr ogr am s – PNI I no. 3 2- 1 25/ 20 08- STEDI WAT and PNI I no. 72- 15 6/ 2 00 8- NANO-ZEOREZI D.
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0.0 0.2 0.4 0.6 0.8 1.0 1.2
0 20 40 60 80 100 120 140 160 180 200
50 Am-2 100 Am-2 200 Am-2
e
le
c
tr
o
ly
s
is
t
ime
/mi
n
u
te
s
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Receiv ed: 12 Mar ch 20 11. Accept ed: 24 July 2011. Published online: 25 July 20 11. Aut hor s:
I lie Pisoi, SECOM Com pany , st r eet Car ol I , n o. 53, Dr obet a- Tur nu Sev er in, Rom ania.
Cr ist ian Danielescu, I nst it ut e of Chem ist r y Tim isoar a of Rom an ian Academ y , 24 Mihai Vit eazu Av e. , 30 022, Tim isoar a, Rom ania.
Flor ica Manea, ” Polit ehnica” Univ er sit y of Tim isoar a, P- t a Vict oriei, no. 2, 300 006, Tim isoar a, Rom ania; e- m ail: flor ica. m anea@chim . upt . r o
Sm ar anda Masu,Nat ional Resear ch and Dev elopm ent I nst it ut e for I ndust r ial Ecology - ECOI ND, Tim isoar a, Rom ania.
Cecilia Sav ii,I nst it ut e of Chem ist r y Tim isoar a of Rom anian Academ y , 24 Mihai Vit eazu Av e. , 300 22, Tim isoar a, Rom ania.
Geor get a Bur t ică,” Polit ehnica” Univ er sit y of Tim isoar a, P- t a Vict or iei, no. 2, 3000 06, Tim isoar a, Rom ania. How t o cit e t his ar t icle: