Directory UMM :Data Elmu:jurnal:E:Environmental Management and Health:Vol09.Issue5.1998:

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Enviro nme ntal Manage me nt and He alth

9 / 5 [1998] 2 0 9 –2 1 4 © MCB Unive rs ity Pre s s [ISSN 0956-6163]

Chemically assisted primary treatment of municipal

wastewater

S.S. Younis

Che mic al Engine e ring De partme nt, Fac ulty o f Engine e ring, Cairo Unive rs ity, Cairo , Egypt

N.M . Al M ansi

S.H. Fouad

The objective of this work is studying the use of chemicals to enhance primary treatment in municipal wastewater treatment plants, thereby reducing the cost of the secondary treatment stage. Jar tests using chemicals, polymers and sea water have been conducted to determine the best performing chemi-cals, alone and in combina-tions. The process perfor-mance parameters studied were pH, type and dosage of coagulant, mixing intensity, detention time, settling time and addition of coagulant aid. The efficiency of the process has been measured as per-centage removals in COD, TSS, turbidity, settleable matter and removals of heavy metals. The results proved the improvement of removal efficiencies.

1. Introduction

St a n d a r d s for d isp os a l of w a st ew a t er, w h et h er in t o a b ody of w a t er or t o la n d , va r y fr om on e cou n t r y t o a n ot h er. In E gy p t , Law 48 of 1982 n ecessit a t es secon d a r y t r ea t m en t of w a s t ew a t er p r ior t o d isp osa l in w a t er cou r ses. T h is r eq u ir es t h e a ch ievem en t of r ela t ively h igh levels of t r ea t m en t . T h ese t r ea t m en t t ech n ologies a r e u su a lly n ot a ffor d -a ble fr om t h e econ om ic-a l -a n d fi n -a n ci-a l p oin t of v iew. E s t im a t ed in ves t m en t r eq u ir em en t s for t h e yea r 2000 exceed 10 b illion L.E . (F IN N DAD Re p or t , 1991). In a d d it ion , s ec-on d a r y t r ea t m en t in va r ia bly u ses b iologica l t r ea t m en t p r ocess es w h ich r eq u ir e h igh op er a t ion a n d m a in t en a n ce cos t s. T h er e is a gr ow -in g t en d en cy n ow t o u s e ch em ica lly en h a n ced p r oces s es, w h er eby ch em ica ls a r e a d d ed t o w a s t ew a t er t o en h a n ce s olid s r em ova l t h r ou gh coa gu la t ion . It is con sid er ed a s a n in n ova t ive p h y sicoch em ica l t ech n ology in som e p a r t s of t h e wor ld w it h t h e ob ject ive of im p r ov in g t h e efficien cy of p r im a r y t r ea t -m en t p r oces s es a n d r ed u cin g t h e cos t of t h e secon d a r y t r ea t m en t st a ge eit h er by elim in a t -in g b iologica l t r ea t m en t , w h er e con d it ion s a llow, or by r ed u cin g t h e ca p a cit y of secon d a r y t r ea t m en t u n it s. H a r lem a n a n d Mor r is sey (1990) s t a t ed t h a t t h e ch em ica l-a s sist ed p r im l-a r y t r el-a t m en t p r ocess cl-a n ob t a in r em ova l efficien cies clos e t o b iologica l t r ea t m en t p r ocess.

Two a p p lica t ion s a r e p a r t icu la r ly r eleva n t for im p lem en t in g ch em ica l-a ssist ed p r im a r y t r ea t m en t . Th ese a r e sit u a t ion s wh er e h igh levels of BOD r em ova l a r e n ot r eq u ir ed , bu t h igh m icr ob ia l r ed u ct ion s a r e essen t ia l (su ch a s in t h e ca ses of r ecyclin g a n d r eu se of w a st e-w a t er for u n r est r ict ed u se in a gr icu lt u r e or a q u a cu lt u r e) a n d t h e d isp osa l of efflu en t in coa st a l w a t er s, wh er e t h er e a r e la r ge su r fa ce

a r ea s p r ov id in g r ea er a t ion . Th e oxygen b a la n ce of t h ese r eceiv in g b od ies is n ot n or m a lly a r ea list ic en v ir on m en t a l con sid er a -t ion , -t h er efor e h igh COD r ed u c-t ion s a r e ir r eleva n t . Well d esign ed a n d op er a t ed a dva n ced p r im a r y ch em ica l coa gu la n t a n d sed im en t a t ion p la n t s ca n n or m a lly a ch ieve 70-75 p er cen t r ed u ct ion in BOD, wh ile p r od u c-in g a clea r efflu en t a lr ea dy of good m icr ob ia l q u a lit y, wh ich ca n b e effect ively a n d econ om i-ca lly d isin fect ed fu r t h er t o m eet even t h e st r ict est en v ir on m en t a l cr it er ia .

In a ddit ion , CAP T h a s t h e a d ded a dva n t a ge of pr ovidin g effect ive r em ova l of ph osph a t es a n d m a n y h eavy m et a ls n ot n or m a lly r em oved a t a ll by con ven t ion a l biologica l t r ea t m en t . Ma n y t ypes of ch em ica ls h ave been st u died in clu din g a lu m , FeCl₃a n d polym er s. T h e a im of t h is st u dy is t o pr ovide a r elia ble a n d r ea son a bly pr iced w a st ew a t er t r ea t m en t t ech -n iqu e t o be u sed i-n E gypt ia -n com m u -n it ies, t h u s cost / ben efi t is a ver y im por t a n t st e p.

Chemically assisted primary

treatment (CAPT) overview

T h e w a s t ew a t er t r ea t m en t ob ject ive is t h e r elia ble a n d efficien t r em ova l of ever in cr ea s-in g p ollu t a n t loa d s fr om m u n icip a l w a st e-w a t er. Am on g t h e e-w a s t ee-w a t er con t a m in a n t s a r e solid s, or ga n ic a n d / or in or ga n ic com p o-n eo-n t s (d issolved or u o-n d issolved , vola t ile or n on vola t ile), h eav y m et a ls, a d sor a ble or ga n ic h a logen com p ou n d s. T h ese con t a m in a n t s m ay b e ca r cin ogen ic, t oxic or m u t a gen ic. T h ey m ay b e b iod e gr a d a ble, d ifficu lt t o b iod e-gr a d e or n on b iod e e-gr a d a ble.

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S.S. Yo unis , N.M. Al Mans i and S.H. Fo uad

Che mic ally assiste d primary tre atme nt o f munic ipal waste wate r

9 / 5 [1 9 9 8 ] 2 0 9 –2 1 4

u n r es t r ict ed a gr icu lt u r a l u se or it ca n b e d is p os ed of in coa st a l w a t er.

Oed e ga a r d (1989) h a s t r ied t o d em on s t r a t e t h a t a ver y sign ifi ca n t p a r t of t h e con t a m n a n t s in w a s t ew a t er is a ss ocia t ed w it h p a r t i-cles, a n d t h a t con s eq u en t ly a s ign ifi ca n t r ed u ct ion in con t a m in a n t s m ay b e exp ect ed a s a r es u lt of d ir ect p a r t icle r em ova l.

Sin ce r ecor d ed h ist or y m a n h a s u s ed n a t -u r a l m a t er ia l t o cla r ify t -u r b id w a t er. M a t er i-a ls su ch i-a s cr u sh ed i-a lm on d s i-a n d i-a lu m w er e u sed . H igh s u s p en d ed solid s r em ova l w er e a ch ieved by t h e u se of lim e a n d ir on s a lt s a s coa gu la n t s. H ow ever, t h e la ck of s t a b ilit y, exp en s es a n d p r od u ct ion of la r ge q u a n t it ies of s lu d ge p r om p t ed t h e con ver s ion t o b iologi-ca l p r oces s es. Ru d olf et a l. (1992) p r es en t ed a n a r t icle d escr ib in g t h e b en efi t s of s m a ll con -cen t r a t ion s (5-15 m g/ l) of FeCl₃in w a st ew a t er. It w a s ob s er ved t h a t a sm a ll con cen t r a t ion of FeCl₃cou ld p r od u ce a m a r k ed in cr ea se in t h e set t lin g velocit y of p a r t icles.

Tod ay ch em ica l t r ea t m en t is m ost ly u sed in t er t ia r y t r ea t m en t p r ocess es for t h e r em ova l of n u t r ien t s a n d sp ecifi ca lly p h osp h or u s (Bow k er a n d St en s el, 1990; F a r ooq a n d Ba n , 1986; Kr eisl a n d Wes t r ick , 1972; USE PA, 1976; Web er, 1972). Sed im en t a t ion is on ly effect ive for t h e r em ova l of p a r t icles la r ger t h a n a b ou t 50µm on a p a r t icle b a sis. Ba s ed on St ok e’s law a n d a p a r t icle over fl ow r a t e on t h e r a n ge of 1-2m / h r, t h e m in im u m p a r t icle s ize t h a t wou ld b e r em oved by sed im en t a t ion is in t h e r a n ge of 50-70µm , a ss u m in g a n aver a ge d en s it y of p a r t icles of 1-2g/ m3_{a n d a t em p er a t u r e of} 15°C. How ever, a m a jor p a r t of t h e p a r t icles la r ger t h a n 0. 1 µm m ay b e s e p a r a t ed by sed im en t a t ion s u b s eq u en t t o coa gu la t ion / fl occu -la t ion (Lev in e et a l., 1985). H yd r oly zin g m et a l s a lt s, s u ch a s a lu m in u m a n d fer r ic coa gu -la n t s, a r e w id ely u sed a s p r im a r y coa gu -la n t s t o p r om ot e t h e for m a t ion of a ggr e ga t es. For op t im a l p er for m a n ce of su b s eq u en t solid -liq u id s e p a r a t ion p r ocess es, it is im p er a t ive t h a t a ggr e ga t es of a cer t a in s ize, st r en gt h a n d d en s it y b e for m ed in t h e coa gu la t ion p r ocess (O’M elia , 1970, Ra m a ley et a l., 1985).

St a n d a r d s r eq u ir ed by t h e E gy p t ia n Gov -er n m en t n eed s u se of m or e effect ive

p r oces s es s u ch a s coa gu la t ion r a t h er t h a n t h e exist in g t r ea t m en t of m u n icip a l w a t er.

Ma n y a u t h or s (H a r lem a n , 1992; H a r lem a n et a l., 1992; Mor r is sey a n d H a r lem a n , 1984; M u r cot t a n d H a r lem a n , 1994a ; P ir n ie a n d M a lcolm , 1994) h ave cover ed t h e a p p lica t ion s a n d exp er ien ces of ch em ica l a s s ist ed p r im a r y t r ea t m en t . H a r lem a n et a l. (1992) a n d M u r cot t (1993) em p h a sized t h a t t h e CAP T t ech n ology is a cost effect ive t ech n ology a n d ca n b e u s ed t o u p gr a d e t h e exis t in g fa cilit ies of con ven -t ion a l -t ech n ologies.

2. Experimental work

To st u dy t h e u se of ch em ica ls t o en h a n ce p r im a r y t r ea t m en t in m u n icip a l w a st ew a t er p la n t s, gr a b sa m p les of t h e in fl u en t t o t h e p r im a r y s et t ler s for m u n icip a l w a st ew a t er t r ea t m en t p la n t of Zen ein d is t r ict h ave b een collect ed . F r esh sa m p les w er e exa m in ed in t h e la b or a t or y u sin g a st a n d a r d ja r t est a p p a -r a t u s (P h ip p s a n d Bi-r d 400 se-r ies six-p a d d le st ir r er ). T h e a n a ly sis of sa m p les b efor e a n d a ft er t r ea t m en t h a s b een d on e u sin g t h e eq u ip m en t sh ow n in Ta ble I.

P a r a m et er s st u d ied con t r ollin g t h e p r ocess p er for m a n ce a r e a s follow s: op t im a l p H , t y p e a n d d osa ge of coa gu la n t , m ixin g in t en sit y, d et en t ion t im e, set t lin g t im e a n d u se of coa g-u la n t a id or sea w a t er.

2 .1 M aterials

2 .1 .1 Waste wate r

Sa m p les of in fl u en t t o p r im a r y set t ler s (a ft er gr it r em ova l) h ave b een t a k en in t h e in t er va l fr om Se p t em b er 1995 t o M ay 1996 a t 10.30 a .m . Sa m p les w er e of d iffer en t in it ia l con d it ion s. Ta ble II su m m a r izes t h e w a st ew a t er ’s ch a r a c-t er isc-t ics.

2 .1 .2 Primary c o agulants

• Alu m Al₂(SO₄)₃. 16H₂O w it h 97 p er cen t p u r it y a n d m olecu la r w eigh t of 630.38. Su p p lied fr om E l N a sr P h a r m a ceu t ica l Ch em ica l Com p a n y.

• Fer r ic ch lor id e FeCl₃w it h 99 p er cen t p u r it y, m olecu la r w eigh t of 162.21. Ma n u fa ct u r ed by Ried el d e H a en .

• Lim e Ca (OH )₂w it h 90 p er cen t p u r it y, m ole-cu la r w eigh t of 74.09. M a n u fa ct u r ed by s.d . fi N E -CH E M LTd .

Table I

Equipme nt us e d fo r analys e s

Parameter Equipment

Temperature The Cole Parmer Ec onomy

pH/ mV/ °C Benc htop Meter

pH The Cole Parmer Ec onomy

pH/ mV/ °C Benc htop Meter

Turbidity Hac h COD Reac tor Model

4 5 6 0 0 and model DR2 0 0 0 Spec trophotometer

COD Hac h COD Reac tor Model

TSS Hac h COD Reac tor Model

Heavy metals Atomic Absorption Spec trophotometer

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2 .1 .3 Co agulant aid

Ma gn a fl oc 155, M a gn a fl oc 1011 a n d p olya cr y -la m id e (BDM ). Sea w a t er w a s ob t a in ed off t h e coa s t of E in Su k h n a , Red Sea .

2 .2 Experimental technique

Aliq u ot s of fr es h ly collect ed m u n icip a l w a s t e-w a t er e-w er e d ist r ibu t ed a m on g t h e s ix ja r s a ft er t h or ou gh m ixin g. Coa gu la n t d osa ge w a s t h en a d d ed in va r y in g p r op or t ion s (follow in g t h e exp er im en t a l p la n ) t o give a t ot a l volu m e of on e lit er in ea ch ja r follow ed im m ed ia t ely by t h e in it ia t ion of fl a sh m ixin g (150 r p m ). Aft er on e m in u t e, m ixin g w a s r ed u ced t o 30r p m a n d h eld a t t h is level for 10 m in . F in a lly a q u iescen t s et t lin g p er iod of 20 m in u t es w a s a llow ed . At t h e en d of t h e set t lin g p er iod , a s a m p le of t h e s u p er n a t a n t w a s a n a ly zed for t h e va r iou s p a r a m et er s.

3. Results and discussions

Sa m p les of w a st ew a t er b efor e t r ea t m en t w er e a n a ly zed a n d t h eir in it ia l con d it ion s a r e t a bu la t ed in Ta ble II.

3 .1 Effect of pH

T h e effect of p H on t h e efficien cy of t h e p r oces s h a s b een eva lu a t ed for ea ch coa gu -la n t a s p er cen t a ge r em ova l of COD, t u r b id it y a n d TSS. T h e efficien cy of t h e p r ocess is a ffect ed by t h e va r ia t ion in p H . E a ch coa gu -la n t h a s a n op t im u m p H a t w h ich a m a xim u m r em ova l is a t t a in ed . T h e r es u lt s a r e s h ow n in F igu r e 1. It is r ea d ily n ot ed fr om t h e r es u lt s t h a t t h e op t im a l va lu es of p H , a s ob t a in ed fr om t h e r es u lt s a r e 6, 4, 11.2 for a lu m , fer r ic ch lor id e a n d lim e r esp ect ively. N ea r ly sim ila r t r en d s w er e n ot ed in r em ova ls of COD, t u r b id -it y a n d TSS.

3 .2 Effect of coagulant type and dose F igu r es 2, 3 a n d 4 illu s t r a t e t h e effect of coa g-u la n t d os a ge on COD, t g-u r b id it y a n d TSS r em ova ls.

1 Ch em ica l ox ygen d em a n d. E xp er im en t s w it h a lu m a n d fer r ic ch lor id e sh owed h igh er COD r em ova ls t h a n t h ose w it h lim e. A r em ova l p er cen t a ge a s h igh a s 80 p er cen t w a s r ecor d ed for a lu m wh en t h e d ose w a s 80m g/ l a n d for fer r ic ch lor id e a t a d ose of 40m g/ l. Wit h lim e, r em ova l r ea ch ed 62 p er cen t on t h e aver a ge a t 600m g/ l. It sh ou ld b e n ot ed , h owever, t h a t a ll exp er m en t s d o n ot h ave t h e sa m e in it ia l con d i-t ion s. Also h igh er d osa ges of FeCl₃a bove 40 m g/ l led t o a yellow color in su p er n a t a n t layer, d u e t o t h e exist en ce of ir on ion s wh ich h a s b een ver ifi ed by a n a ly sis w it h a t om ic a b sor p t ion sp ect r op h ot om et er, i.e. d osa ges a b ove 40m g/ l of fer r ic ch lor id e lea d s t o a p p ea r a n ce of Fe+ + +_{in w a t er.} 2 T u r b id ity rem ov a l. For t h e t h r ee coa gu

-la n t s u sed , F igu r es 2, 3 a n d 4 in d ica t e t h a t fer r ic ch lor id e a n d a lu m a r e h igh ly effec-t ive coa gu la n effec-t s for effec-t u r b id ieffec-t y r em ova l, w h ich r ea ch ed a b ove 90 p er cen t . Lim e is less effect ive w it h a n aver a ge r em ova l of 80 p er cen t a t a lim e d ose of 600m g/ l. Also t h er e w a s n o a p p r ecia ble effect of in cr ea s-in g lim e d oses on t u r b id it y r em ova l. 3 T ota l su sp en d ed solid s rem ov a l. H igh r em ova l r a t es > 96 p er cen t of t ot a l su sp en d ed solid s w er e r ecor d ed w it h t h e u se of a lu m . Fer r ic ch lor id e a ls o r em oved > 90 p er cen t of TSS w h ile lim e gave a TSS r em ova l of a lm ost 80 p er cen t . T h e aver a ge p er cen t a ge r em ova l d id n ot ch a n ge w it h a d d it ion a l in cr ea s e in lim e d ose.

It is n ot ewor t h y t o say t h a t t h e lim e d ose w a s n ea r ly eq u a l t o 1.5 t im es t h e t ot a l a lk a lin it y of t h e w a st ew a t er, w h ich w a s ca lcu la t ed a s 385 m g/ l. Sin ce r e gu la t ion s for w a st ew a t er d is-p osa l in E gy is-p t r eq u ir es t h e is-p H t o b e in t h e r a n ge of 6-9, a n ot h er set of exp er im en t s w er e ca r r ied ou t w it h ou t p H a d ju st m en t w h ich sh ow t h e p er cen t r em ova ls w h en a lu m a n d fer r ic ch lor id e w er e u sed . Alt h ou gh sligh t ly low er r em ova ls w er e ob t a in ed , p H w a s in t h e r ecom m en d ed levels (p H ~ 7). E xp er im en t s w it h lim e s h ow ed t h a t t h e lim e d ose r a ised t h e p H t o 11.2.

Table II

Co mpo s itio n o f untre ate d munic ipal was te wate r

Grab samples

Average M aximum M inimum n

COD, mg/ l 4 3 0 8 8 2 1 9 3 3 6

Turbidity, FTU 2 6 0 4 1 8 1 6 7 3 6

TSS, mg/ l 3 7 7 8 2 5 2 3 7 3 6

pH 7 8 .2 4 7 .1 3 3 6

Temperature, °C 2 2 2 7 1 7 .1 3 5

Settleable

matter, ml/ l 1 4 2 6 2 1 9

Figure 1

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3 .3 Effect of slow-mixing intensity St u dy in g t h e effect of slow m ixin g in t en sit y r evea ls t h a t t h e m ixin g in t en s it y h a s a n op t i-m u i-m va lu e b et w een 30-50r p i-m u s in g t h e t h r ee coa gu la n t s.

3 .4 Effect of time of slow mix

Wh en t h e t im e of slow m ix h a s b een ch a n ged u sin g t h e coa gu la n t s, it is ob s er ved t h a t r ea s on a ble r em ova ls h ave b een a t t a in ed s t a r t in g w it h 10 m in u t es of slow m ix a n d t h er e is n o a p p r ecia ble ch a n ge in r em ova ls w it h ch a n ge in t im e of s low m ix.

3 .5 Effect of settling time

Wh en t h e effect of set t lin g t im e h a s b een s t u d ied u s in g t h e t h r ee coa gu la n t s, it w a s

ob ser ved t h a t a set t lin g t im e of 10-20 m in u t es w a s su it a ble a n d fu r t h er in cr ea se in set t lin g t im e d id n ot y ield a n y in cr ea s e in p er cen t r em ova ls.

T h ey d o n ot on ly im p r ove r em ova ls bu t a lso d ecr ea se gr ea t ly t h e t im e of s et t lin g t o b e 20 m in . r a t h er t h a n 2h r a t lea st in t h e con ven -t ion a l p r im a r y -t r ea -t m en -t p r ocess. T h is m ea n s les s ca p a cit y set t lin g t a n k s or a d d i-t ion a l a m ou n i-t s of w a si-t ew a i-t er i-t o b e i-t r ea i-t ed in t h e sa m e ca p a cit y set t ler s.

3 .6 Effect of coagulants aids

Ma gn a fl oc 155, M a gn a fl oc 1011 a n d p olya cr y -la m id e h ave b een u sed a s coa gu -la n t s a id s w it h a lu m , (F igu r es 5 a n d 6). M a gn a fl oc 155 a n d M a gn a fl oc 1011 h ave a lso b een t est ed w it h fer r ic ch lor id e (F igu r e 7). T h e r esu lt s in d ica t e t h a t a d d it ion of a id im p r oves t h e r em ova ls t o a ver y s ligh t ext en t , w h ich d oes n ot exceed 2 p er cen t a t m ost a n d u sin g a n a id gen er a lly d oes n ot im p r ove t h e p er cen t r em ova ls of COD, t u r b id it y or TSS. It w a s n ot ed t h a t t h e p oly m er s u s ed h ave p r om ot ed fl occu la t ion a n d for m ed la r ge, st r on g fl oc w it h in cr ea sed set t lin g r a t e a n d low er set t lin g t im e. Also t h e s et t lea ble m a t t er m ea -su r ed w er e m u ch low er w h en u sin g a n a id d u e t o fl oc st r en gt h en in g a s w ell.

Figure 2

Effe c t o f alum do s e o n pe r c e nt re mo val

Figure 3

Effe c t o f fe rric c hlo ride do s e o n pe r c e nt re mo val

Figure 4

Effe c t o f lime do s e o n pe r c e nt re mo val Figure 5

Effe c t o f me tal s alt (alum) + c o agulant aid (Magna flo c 1 5 5 ) additio n o n pe r c e nt re mo val

Figure 6

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3 .7 Effect of using coagulant mixtures Mixt u r es of coa gu la n t s w er e t es t ed in d iffer -en t p r op or t ion s in or d er t o ob t a in b et t er r esu lt s w it h r efer en ce t o ch em ica l cos t a n d t o t r y t o m od ify t h e p H of w a st ew a t er efflu en t s. It w a s ob s er ved t h a t m ixt u r es w it h lim e in gen er a l d id n ot give b et t er p er cen t a ge r em ova ls t h a n u s in g lim e a lon e. T h is is b eca u s e even if t h e lim e d os a ge w a s t oo sm a ll, t h e effect on in cr ea sin g t h e p H w a s ver y gr ea t . At h igh p H levels, b ot h a lu m a n d fer r ic ch lor id e d o n ot a ct effect ively, a s h a s b een d is cu ss ed in it em 3.1.

3 .8 Effect of sea water addition

Sea w a t er w a s u s ed a s a n i n ex p en s ive s ou r ce of m a gn es i u m a n d ca lci u m , t o t es t i f i t wou ld b e effect ive, a s w ell a s econ om i ca lly v i a b le, i n t h e t r ea t m en t of w a s t ew a t er. It i s clea r t h a t s ea w a t er a lon e h a s n o s i gn i fi ca n t effect on r em ov a ls. Ad d i t i on of s ea w a t er t o li m e i m p r oves t h e r em ov a ls a t a con cen t r a t i on of 24 p er cen t (volu m e p er cen t ). T h e b es t com -b i n a t i on -b et w een li m e a n d s ea w a t er, w a s 500 m g/ l li m e p lu s 4 p er cen t s ea w a t er w h i ch gave t h e h i gh es t r em ov a ls. Wi t h a lu m , s om e i m p r ovem en t i n r em ov a ls h a s b een ob s er ved w i t h s ea w a t er con cen t r a t i on s of 2 p er cen t .

3 .9 Effect of CAPT on t he removal of heavy met als

St u dy i n g t h e effect of CAP T on t h e r em ov a l of h eav y m et a ls r evea ls t h a t t h e u s e of CAP T cou ld b e effect ive i n r em ov i n g h eav y m et a ls i f i t w er e t o ex i s t . Sa m p les of w a s t ew a t er con t a i n i n g 0.5m g/ l N i h a s b een t r ea t ed w i t h a lu m a n d a r em ov a l of 40 p er cen t of N i h a s b een a ch i eved . Wi t h s a m p les con t a i n i n g 0.6 m g/ l N i a n d 0.001m g/ l Cd , r em ov a ls of 17 p er cen t N i a n d 100 p er cen t Cd h ave b een r ea ch ed u s i n g li m e. Gen er a lly m os t of w a s t ew a t er s a m p les w er e fr ee fr om h eav y m et a ls.

3 .1 0 Effect of initial load composition By com p a r in g ou r r esu lt s w it h t h ose ob t a in ed by H a r lem a n (1992) w e ca n d ed u ce t h a t t h e com p osit ion of t h e in it ia l loa d of w a s t ew a t er h a s a m ea su r a ble effect on p er cen t a ge r em ova ls. For loa d s of 200 COD, H a r lem a n r ea ch ed r em ova ls of 56 p er cen t a n d 62 p er cen t u s in g 60m g/ l of a lu m a n d fer r ic ch lor id e r esp ect ively. Ou r r es u lt s h ave in d ica t ed h igh er r em ova ls for n ea r ly t h e s a m e loa d s. For in it ia l loa d of 193m g/ l COD, r em ova l of 64 p er cen t u sin g 60m g/ l of a lu m a n d fer r ic ch lo-r id e lo-r es p ect ively h a s b een a ch ieved . Ou lo-r r esu lt s h ave in d ica t ed h igh er r em ova ls for n ea r ly t h e s a m e loa d . For a n in it ia l loa d of 193 m g/ l COD, r em ova l of 64 p er cen t w a s

a ch ieved u sin g 60m g/ l of a lu m . Wit h loa d s of 228m g/ l COD, r em ova l of 73 p er cen t u sin g 50m g/ l fer r ic ch lor id e h a s b een a t t a in ed . Also w it h loa d s of 385m g/ l COD, H a r lem a n

r ea ch ed r em ova ls of n ea r ly 52 p er cen t a n d 57.5 p er cen t u sin g 60m g/ l a lu m a n d fer r ic ch lor id e r esp ect ively. In t h is wor k , r em ova ls of 69 p er cen t u sin g 60m g/ l a lu m w it h in it ia l loa d s of 394m g/ l COD w h ile r em ova ls of 79 p er cen t w it h loa d s of 346m g/ l COD u sin g 70m g/ l fer r ic ch lor id e h ave b een r ea ch ed . Also r em ova ls by con ven t ion a l m et h od (p r im a r y set t lin g w it h zer o coa gu la n t d ose) sh ow ed h igh er r em ova ls in ou r wor k .

F r om t h e p r ev iou s r esu lt s a lu m h a s b een ch osen t o b e t h e m ost su it a ble coa gu la n t for t r ea t m en t of w a st ew a t er in E gy p t d u e t o m a n y fa ct or s; p H r a n ge, b est r em ova ls ob t a in ed a n d n o cor r osion p r oblem s.

A sim p le m od el for p r ed ict in g t h e a p p r op r i-a t e d osi-a ge of i-a lu m h i-a s b een ob t i-a in ed by p lot t in g t h e in it ia l loa d a n d op t im u m d osa ge on d iffer en t coor d in a t es. T h e op t im u m d osa ge h a s b een ch os en w it h r esp ect t o b est COD r em ova ls. T h e b es t fi t h a s b een ob t a in ed on sem i-log coor d in a t es w it h t h e follow in g r ela t ion :

y = 41.621 e 0.018x w h er e

y = in it ia l COD in m g/ l x = op t im u m d ose in m g/ l Also, t r ia ls t o p r ed ict t h e p er cen t a ge r em ova ls w h ich cou ld b e a ch ieved by a lu m a n d lim e for a n y in it ia l loa d h ave b een p r o-p osed by t h e follow in g eq u a t ion s :

For a lu m :

R = 0.011 y + k w h er e

R = p er cen t a ge r em ova l y = in it ia l loa d in m g/ l

k = con st a n t va r ies w it h t y p e of loa d

for COD, k 1 = –0.2336 for t u r b id it y, k 2 = –0.0046 for TSS, k 3 = 0.0021 Figure 7

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9 / 5 [1 9 9 8 ] 2 0 9 –2 1 4

For lim e:

R= 0.0014 y + k ' for COD, k ´₁= –0.052 for t u r b id it y, k ´₂= 0.0033 for TSS, k ´₃= 0.1616

4. Conclusion

CAP T is a ver y efficien t p r ocess a s it im p r oves t h e r em ova ls of COD, TSS a n d t u r b id it y in p r im a r y t r ea t m en t t o r ea ch a b ove 90 p er cen t r em ova ls in TSS a n d t u r b id -it y, w h ile a lm ost 80 p er cen t r em ova ls in COD ca n b e a t t a in ed . CAP T gr ea t ly r ed u ces t h e t im e of s et t lin g t o b e n ea r ly 20 m in u t es r a t h er t h a n 2.5 h ou r s in t h e con ven t ion a l p r ocess, t h u s m a k es it p oss ible t o t r ea t a d d it ion a l a m ou n t s of w a st ew a t er or t o u se s m a ller s ize p r im a r y t a n k s. T h er e is a n op t im u m d os e for ea ch coa gu la n t cor r esp on d in g t o ea ch in it ia l loa d . Bot h t u r b id it y a n d TSS lim it s a ft er CAP T a r e low er t h a n t h e r ecom m en d ed lev els, bu t t h e a b solu t e va lu e of COD a ft er t r ea t -m en t is s t ill a b ove t h e r eco-m -m en d ed levels ow in g t o t h e exis t en ce of solu ble com p ou n d s. It is b et t er t o u se ea ch coa gu la n t a lon e ow in g t o in t er fer en ce b et w een coa gu la n t m ixt u r es w h ich lea d t o u n d u e a d ju st m en t in p H .

Alu m h a s p r oved t o b e t h e m ost r elia ble. T h e a d d it ion of p oly m er s im p r oves t h e r em ova ls in TSS t o r ea ch a b ou t 100 p er cen t u sin g 0.3m g/ l p oly m er d ose. Ad d it ion of p oly -m er s a lso in cr ea ses t h e set t lin g r a t e, r ed u ces t h e set t lea ble m a t t er s a n d im p r oves h eav y m et a ls r em ova ls if it wer e t o exist . Ad d it ion of sea w a t er im p r oves r em ova ls sligh t ly w it h con cen t r a t ion of 2 p er cen t by volu m e.

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