Wet Air Oxidation of Desizing Wastewater From the Textile Industry

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    Wet Air Oxidation of Desizing Wastewater from the Textile Industry

    Lecheng Lei, Xijun Hu,* Guohua Chen, J ohn F. Porter, and Po Lock Yue

    Depart m ent of Chemi cal E ngi neeri ng, H ong K ong U ni versi t y of S ci ence and T echnol ogy,

    Cl ear Wat er B ay, K owl oon, Hong K ong

    We t a ir o xid atio n (WAO) wa s a p plie d to tre at th e d e siz in g wa ste wa te r f ro m n at u ral an d m an -

    ma de fiber processing in a 2-L a utoclave react or. The ra nge of operat ing tempera tur es exa minedwa s betw een 150 and 290 C . The part ial pressure of oxygen ra nged from 0.375 to 2.25 MPa a ta r eference tempera tur e of 25 C . Va ria tions in chemical oxygen deman d (COD ) a nd tota l orga niccarb on we re m o n itore d d u rin g e ach e xpe rim e n t an d u sed to e valu at e t h e p e rform a n ce of t h ereaction process. Experimenta l results showed th a t WAO is an efficient meth od for th e trea tmentof desizing w ast ewa ter. A higher COD remova l wa s a chieved under gentler rea ction conditionswith th e a id of a cat alyst . A tw o-p ara m e ter m a th e m at ical m od e l in volvin g a fast re action a n da slow reaction was used to describe the WAO reaction kinetics and to calculate the reactiona ctivat ion energies.

    Introduction

    Wast ewa ter discha rged from the desizing processesof the textile industry is chara cterized by its very high

    chemical oxygen demand (COD) and is one of the mainpollution sources in the textile industry. The desizingwa s t e wa t e r f rom n a t u ra l f ibe r p roce ss in g op era t io n sma in ly c o n t a in s s t a rc h , g lu c o s e , a n d y e a s t a n d h a s aCOD value of 10000-20000 mg/L a nd b iologica l oxygendema nd (B OD) from 5000 to 10 000 m g/L. Ma n-ma def iber de sizin g wa s t e wa t e rs t y p ica l ly h a v e p oly (v iny lalcohol) (PVA) as the major content, at COD levels ofbetw een 10 000 and 40 000 mg/L, a s w ell as a rela tivelysmall amount of BOD (500-1000 mg/L). The l ow B OD /CO D ra t io mea n s t h a t t h is k in d o f wa s t e wa t e r is v erydifficult to biodegra de. To comply wit h th e curr ent H ongKong discharge regulat ions, factories have to reducet h e ir CO D a n d B O D v a lu es t o 2000 a n d 800 mg /L ,respectively, before discharging to sewers.

    I n g en er a l , t h er e a r e f ou r m et h od s a v a i la b l e f orwastewater treatment: chemical oxidation, biochemicaloxidat ion, incinerat ion, an d wet a ir oxidat ion. Chemicaloxidat ion is to oxidize the pollutants in the w ast ewa terin t o c a rbon diox ide a n d wa t e r wit h s t ron g ox idizers ,such as ozone and hydrogen peroxide. The chemicaloxidation process is relatively simple to operate but isonly cost-effective for polluta nts a t low concentra tions.I t would become expensive for high COD wastewaters u ch a s t h a t disc ha rg e d f rom t h e de sizin g p roce ss .Biological oxidation is a commonly applied method inwa s t e wa t e r t re a t me n t . H o we ve r , i t is n o t s u i t a ble f orwa stewa ter with COD above 10 000 mg/L. The la rgequantity of sludge produced by this method also imposes

    further problems. Incineration may convert the pollut-ants effectively to innocuous end products at tempera-t u re s a bov e 700 C. I t is n o t e con omica l ly a t t ra c t iveunless the COD of the wa ste is a bove 100 000 mg/Lbe c a u s e a la rg e a mo u n t o f e n e rg y wil l be wa s t e d inbo il in g a n d h e a t in g t h e wa t e r ( s t e a m) i f t h e o rg a n icconcentra t ion is not high enough. Wet air oxidat ion(WAO), which mineralizes the pollutants at elevatedtempera tures (200-350 C ) a n d p re ss u re s, h a s t h e

    ability to convert most organic compounds into carbondiox ide a n d wa t e r .1,2 Co mpa re d wit h t ra di t ion a l t e ch -nologies, the wet air oxidat ion process has the advan-tage of high react ion rate, small quantity of secondarypolluta nts, a nd less spa ce requirement. Thus t he WAOprocess might be an economically feasible method fort h e t re a t me n t of de s izin g wa s t e wa t e r .

    D a t i ng b a ck t o t h e ea r ly 20t h cen t ur y , w et a i roxidat ion has been used for treat ing spent sulfite liquorfrom paper mills.3 And th ere a re over 200 Zimpro WAOunits in operation at more than 160 locations through-out the world.4 However, its application has been limiteddue to its severe operat ion condit ions; i .e. , i t usuallyruns under high pressure and temperature. The purposeo f t h is wo rk is t o in v e s t ig a t e t h e W A O t re a t me n t o fdesizing wa stewa ter, to lower t he react ion temperaturea n d p r e s s u r e b y a d d i n g a c a t a l y s t , a n d t o s t u d y t h ereaction kinetics.

    Experimental Section

    A schematic diagram of the wet air oxidation (WAO)s ys t em , w h ich h a s b een u sed t o t r ea t d y ei ng a n dprinting wastewater in our previous studies,5,6 is shownin Figure 1. The WAO system has a gas supply, a 2-Lre a c t o r , a wa s t e wa t e r c h a rg in g s y s t e m, a n d a l iq u idsampling point . The operat ion procedures of WAOp roce ss a re de scribed below. F irs t t h e re a ct o r wa spreheated to 60-70 C. The wa stewa ter (1.4 L) wa s thenc h a rg e d in t o t h e re a c t o r u s in g t h e wa t e r p u mp . T h espace occupied by gas inside the rea ctor wa s 600 mL.P u r e n i t r o g e n w a s u s e d t o p u r g e t h e a i r i n s i d e t h ere a c t o r a t a t o t a l p re s s u re o f 1 M P a f o r 2 min a f t e r

    which the system w as isolated. The reactor w as heatedto th e desired react ion tempera ture (betw een 150 a nd290 C ) w hich took between 1 a nd 2 h. Once the selectedre a c t io n t e mp e ra t u re wa s re a c h e d, p u re o x y g e n wa sin t ro du c e d in t o t h e re a c t o r a n d t h e re a c t io n wa s a s -sumed to start (time t ) 0). At designated time intervalsthereafter, l iquid samples were ta ken from t he reactorand analyzed for COD and total organic carbon (TOC)con t e n t s . Th e s y s t em p res s u re is k ep t con s t a n t bysupplying oxygen to ba lance the pressure drop causedby sa mpling. At t he end of the reaction time, the heat ingjacket w a s turned off and t he reactor wa s cooled for ha lf

    * To whom all correspondence should be addressed. Email:kexhu @ust .hk. F a x: (852) 23580054. Tel: (852) 2358 7134.

    2896 In d. Eng. Chem. Res. 2000, 39 , 2896-2901

    10.1021/ie990607s CCC: $19.00 2000 American C hemical SocietyP ubl ish ed on Web 06/20/2000

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    an hour. Ta p wa ter w as t hen passed through the coolingcoil t o f u rt h e r cool t h e re a ct o r t o 60-7 0 C . P u r enitrogen was passed through the reactor to discharget h e t re a t e d wa s t e wa t e r . T h e re a c t o r wa s f i l le d wit hdeionized water, and the st irrer was operated at 1000rpm for 5 min. This f inal step was repeated three orfour times until the dischar ged wa ter wa s visually clean.

    The COD value is assayed with an HACH-DR2000COD an alyzer. The an alyt ical agent , w hich consists of0.05 g of HgSO 4, 2.5 mL of 98% H 2S O4, 0 . 5 mL o f 1mol /L K 2C r 2O5, i s m i x e d w i t h 2 m L o f w a s t e w a t e rs a mp le a n d in cu ba t e d a t 150 C f or 2 h . C O D is t h e na s s a y e d a f t er cool in g w it h t h e CO D a n a ly zer . All t h echemicals used are in analyt ical purity grade.

    The desizing wastewaters investigated in this studywer e provided by t w o Hong Kong textile companies. Thefirst w as from a cotton desizing process a nd containedprimar ily sta rch an d glucose. The COD, B OD, a nd TOCvalues for the cotton d esizing w ast ewa ter w ere 10280,3754, an d 3558 mg/L, r espectively . The second w a s froma ma n -ma de f iber de sizin g p roce s s, a n d p oly (v iny lalcohol) (P VA) wa s its ma jor component with COD,

    B OD, a nd TOC values of 12600, 620, a nd 3824 mg/L,respectively.

    Results and Discussion

    COD and TOC Removals Caused by Heating.Since heating the reactor to the desired react ion tem-per a t u re t ook f rom 1 t o 2 h , t h e C O D a n d TO Cvaria t ions during the hea ting period in t he a bsence ofoxygen were examined first . Figure 2 shows the CODand TOC reductions of the cotton desizing wa stewa terwh e n i t wa s h e a t e d f ro m a mbien t t e mpe ra t u re t o 290C. The reduction or removal is defined as the differencebetween the init ial COD or TOC va lue and its va lue ata given t ime divided by its init ial amount. I t was seen

    that there was lit t le change in COD or TOC when thetemperature was below 240 C. When the wastewaterwa s f u rt h e r h e a t e d f ro m 240 t o 290 C, t h e re wa s asignificant COD reduction. A COD reduction of 24%anda TO C re mov a l of 8% cou ld be a c h iev ed by s implethermal decomposit ion at 290 C.7 This is the reasonthe C OD or TOC reduction a t zero react ion t ime ha s afinite number instead of zero in the following figures.The reduction of COD is more prominent t han tha t ofTOC during thermal decomposition. This is due to the

    pyrolysis producing hydrogen gas that might be lostfrom t he solution at high temperatur e, which does notaffect the TOC value.

    Effect of Oxygen on the WAO Process. However,by keeping the reactor a t 290 C for a longer t imesupto 150 minslittle further r eduction in COD or TOC w asobserved in the absence of oxygen, as shown in Figure3. Figure 3 also shows t ha t a significant increase in bothCOD a nd TOC r emova l (over 75%) w a s observed in th epresence of oxygen at a partial pressure of 1.5 MPa (ata re fe ren ce t e mpe ra t u re o f 25 C )swh ic h is j u s t t h erequired st oichiometric am ount to completely minera lizethe organic carbon in the cotton desizing wastewater.

    Five different levels of initia l oxygen pa rtia l pressure,0.375, 0.75, 1.125, 1.5, and 2.25 MPa, all at a referencetempera tur e of 25 C, w hich correspond to 25%, 50%,75%, 100%, a nd 150%of th e th eoretica l req uirem ent ofoxygen, were tested t o study th e effect of oxygen part ialpressure on the performa nce of th e WAO process. Thestoichiometric (theoretical) amount is defined as theam ount of oxygen needed to chemically mineralize a llthe pollutant s in th e wa stewa ter. The WAO trea tmento f n a t u ra l f ibe r de s izin g wa s t e wa t e r u n de r di f f e re n tinit ial oxygen part ial pressures is shown in Figure 4.The COD a nd TOC removal ra tes were not surprisinglyfound to increase with oxygen part ial pressure as theconcentra tion of dissolved oxygen in t he liquid pha se isproportional to the corresponding oxygen partial pres-sure.

    Figure 1. Schematic diagram of the wet a ir oxidat ion system.

    Figure 2. COD and TOC reductions during the reactor heat ingperiod for cot ton desizing w astew ater.

    Figure 3. E f f ec t of ox y g en on t h e WAO of c ot t on d e siz in gw a s t e w a t e r a t 2 9 0 C.

    Ind. Eng . C hem. Res. , Vol. 39, No. 8, 2000 2897

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    In Figure 4 we noted that the COD removal exceededthe oxygen supplied at low oxygen part ial pressure. Forexample, 48%of COD removal is achieved with 25%ofoxygen supplied. The rea son for this phenomenon is t ha tthe removal of COD is the overall result of oxidat ionre a ct ion a n d t h e rmoly s is .7-9 An ot h e r re a s on is t h ere gu la r a ddit ion s of ox y g en t o ma in t a in t h e re a ct ionpressure aft er sampling so tha t in fact th e COD remova lis not in excess of the actual oxygen supply; i .e. , theactua l oxygen supply is m ore tha n 25%. This phenom-

    enon begins t o disappear when the oxygen supplied ismore than 75%of the theoretical amount.

    Effect of Reaction Temperature on the WAOProcess. The WAO trea tment of natu ra l fiber desizingwa stewat er wa s further examined at f ive temperat ures,with the supply of oxygen fixed at 1.5 MPa, which isequivalent to the stoichiometric amount of 100%CODremoval. The rates of WAO reaction and COD removalin c re a s e wit h t h e re a c t io n t e mp e ra t u re , a s s h o wn inFigure 5. There is no significant COD or TOC removalat a t empera ture of 150 C. A COD r emova l of near 80%is obta ined after 100 min of react ion at 290 C.

    Catalytic WAO Process. Although the WAO r eac-tion is very successful in the treatment of natural fiberde sizin g w a s t e wa t e r , i t mu s t be op era t e d u n der re la -tively severe conditions in order to achieve a high C ODremoval r at e. I t is necessary to improve the process toreduce the reaction temperature and pressure. Addingsome catalyst in the WAO process is one of the mostpopular options.5,10-12 I n t h is s t u dy , t ra n s i t ion me t a lsin t h e f o rm o f s u l f a t e s a n d n i t ra t e s we re u s e d a s t h ecata lysts. The am ount of cat alyst added int o the reactionsystem wa s 100 mg/L in t erms of meta l ion concentra -tion. Again, t he oxygen supply w as fixed a t t he theoreti-cal requirement, 1.5 MPa at a reference temperatureof 25 C. The react ion temperature was set at 240 C.The catalyt ic wet air oxidat ion (CWAO) treatment ofcotton desizing wastewater is shown in Figures 6 and7. The a ddit ion of cat alysts significant ly enha nces t he

    removal ra tes of COD a nd TOC. The cata lyt ic act ivityof nitrates is slightly higher than that of sulfates, andCu(NO3)2is the best cata lyst a chieving a COD r emovalof 90%.

    WAO of Desizing Wastewater from a Man-MadeFiber Process. Ha ving shown WAO to be a useful t ooli n t h e t r ea t m e n t of cot t on d es iz in g w a s t e w a t e r , i t sa p p lica bi l it y t o t h e t re a t me n t of de sizin g wa s t e wa t e rfrom a m an -ma de fiber processing operation w as exam-in ed. Th e in i t ia l C O D , B O D , a n d TO C v a lu es of t h is

    Figure 4. Effect of oxygen part ial pressur e on the WAO of cottond e siz in g w a s t e w a t e r a t 2 40 C.

    Figure 5. Effect of reaction temperature on the WAO of cottondesizing wastewater a t 1.5 MPa part ia l oxygen pressure.

    Figure 6. E ffect of meta l sulfat es on the WAO of cott on desizingw a s t e w a t e r a t 2 40 C a n d 1 .5 M P a p a r t ia l ox y ge n p r es s u r e.

    2898 Ind. E ng. C hem. Res. , Vol. 39, No. 8, 2000

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    wa stew at er w ere 12 600, 620,...

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