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Dye-sensitized-photo-oxidation—a new approach to the treatment of organic matter in sewage effluents
P,~ter Re~e~arch V o l I l, pp 55 ~ Io 582 Pergamon Press 1977 Printed m Great Britain
DYE-SENSITIZED-PHOTO-OXIDATION--A NEW APPROACH TO THE T R E A T M E N T O F O R G A N I C MATTER IN SEWAGE E F F L U E N T S * A. J. Ar,'HF,R'f Institute of Soils and Water. Division of Soil Residues Chemistry. The Volcani ('enter. Bet Dagan. Israel and I. ROSENTtIAI. Department of Organic Chemistry.. The Weizmann Institute of Science. Rehovot. Israel
(Receiced 25 .,lu~ust 1976, in rcrisedlbrm 19 .lanuary 19771 Abstract A new approach to the oxidation of organic matter in municipal v.astewaters by photosensitization is described. ,Samples of aerated sewage effluents containing an added d~e-sensitizer ~erc exposed to u.v.-lamp or solar radiation. The photochemical oxidation of the sewage caused decreases in the content of the fecal coliform, of the chemical oxygen demand and of the meth,,lenc hlue-acti\.e substal'lCCS b} 10G. 67 and 9(}?,> of their initial values, respectivel,~. The effluents obtained after the removal of the dye, by prccipitation with bentonite clay, were colorless, non-odorous and ~ilh the amount of suspended solids considerably reduced
INTR(.IDt
I(*I'ION
Due to the ever-increasing depletion of the natural resources of water, the re-use of treated municipal wastewater for crop irrigation, for artificial groundwater recharge, or even for Ix-~lable purposes, has become a real problem for the countries of the arid and semi-arid zones. Continuous efforts are being made to improve the existent purification methods and to develop new ones (Mahlman et aL, 1976: Idelovitch et ul., 1976). Several procedures employ the chemical oxidation of the organic matter as a key step in the wastewater treatment (Gomella & Guerree, 1973, Netzer et al., 1975). The limiting factor in the use of the chemical oxidation treatment of sewage el'fluents, containing large amounts of organic matter, is usually its high cost or undesirable by-products remaining in treated effluents (Christensen, 1974-B,'ungs, 1973). The aim of this stud} was to develop a new approach in treating the organic matter in sewage effluents, i.e. dye-sensitized photo-oxidation. The use of such a procedure appears a priori to be competitive with other oxidation methods, especially m arid and semi-arid areas, where thc climate (a large number of sunlight hours per day)is favorable for the promotion of the photo-oxidation reactions.
* Contribution from the Agricultural Research Organization. The Volcani Center. Bet Dagan, lsrael. 1976 Series. No. 232-E. -I-To whom correspondence should be addressed.
Dye-.sensitized photo-oxidations The term "'sensitized photo-oxidation'" also referred to as a "'photodynamic action" is synonymous with oxidations performed with smglet oxygen. These rcactions have been extensively studied with regard to their use in the syntheses or in the oxidative degradations of various naturall\ occurring and synthetic substances (Gollnick, 1968: Kearns, 1971 }. The sensitized photo-oxidation consists of the combined action of visible light and molecuktr oxygen upon organic matter, through the intermediary' of an appropriate photosensitizer [S). The S added to the aerated and light-exposed wastewater will absorb some of the light-radiated energy. This absorbed energy is then made available to the oxidation of organic materials (O:Vll b} either one or Both of the follov¢ing mechanisms, which can operate m aerobic photosensitized oxidation : 1. Primary interaction of the electronically excited sensitizer (S*} is with the OM to generate reactive. short-lived intermediates which subsequently react with atmospheric oxygen. [aj S + hv---~ S* (excited statel (b) S* + OM-.-* transient specia ,2~: oxidation products -F S. {free radicals, ion pairs, elc.I 2. Instead of interaction with OM (ll, the S* transfers its excitation energy' to the molecular oxygen. The addition of the excitation energy to the oxygen molecule changes its ground electronic state (triplet state. "~'YuO2) to the tirst excited singlet state 557
558
A . J . A('ttliR and I. ROSENTHAI
(IA~O2J which has a higher energy by' 22.5kcal mole '. When more energy is imparted to the oxygen molecule, a n o t h e r state is formed ('EoO2) which corresponds to a level of 37.5 k c a l m o l e ' above the 3E002. These p h e n o m e n a of different electronic configurations arc described by group theory and symmetry properties (Kaplan. 1971). which is beyond the .scope of this article. F r o m the properties of singlct oxygen (exceedingly, short lifetimes of ~E,jOz) it seems likely that only 'A+Oz is i m p o r t a n t in the solution oxidations. Therefore, the mechanism of the photo-oxidation of the OM bv the singlet oxygen can be written : (c) S* + "~EqO2 -+ S + 'A~() 2 (d) 'A+O_, + OA'/--~oxidation products. In both mechanisms the sensitizer is regenerated a n d undergoes hundreds of cyles so that only mint, te a m o u n t s of it are required. The efficiency of the bimolecular reaction of the excited sensitizer with either the substrate {OM) or molecular oxygen (3~-~-#O2}depends on how efficiently these processes compete with each other. In view of the diversity of c o m p o u n d s present in sewage water it is very difflcuh to decide which mechanism operates in the present process. It is. however, well known that singlet oxygen oxidizes unsaturated c o m p o u n d s (U(") to peroxides. The subsequent thermal or photochemical decomposition of these peroxides can further initiate free radical oxidation reactions of the saturated c o m p o n e n t s found in sewage water: 'AyO 2 + U C - - , ROOR---* RO' R O -r- R'H ~ R O H
7,- R
R" + 0 2 --* RO()'. etc. As a result of the above reactions, vital biological c o m p o n e n t s such as proteins, lipids and polysaccharides. undergo oxidations and degradations, and. consequently, any biological development in photo-oxidized effluent is inhibited. Micro-organisms such as viruses, algae, fungi, protozoa a n d multicellu 'lar plants a n d animals are affected (Spikes & Livingston. 1969: Spikes & MacKnight, 1970). A m o n g the sensitizers which induce reaction with visible light, the most efficient are tluorescein and phenothiazine derivatives, certain porphyrins and polycyclic a r o m a t i c h y d r o c a r b o n s (Foote. 1968). The following criteria have to be met by the sensitizer in order to be useful for photo-oxygenation of the sewage effluents: to induce reaction with visible light: to be chemically stable during the radiation (refractory to auto-oxidation); to be free of reactive functional groups which can react with chemicals present in sewage and inactivate it; to have a good light absorption capacity (to be effective at tow concentration); to be soluble in water but easy to remove. This paper describes the experiments carried out to define the optimal operating conditions for the photosensitized oxidation method for the treatment
of sewage effluents. These conditions entail determination of a suitable sensitizer, o p t i m u m photoreac,ion conditions and the best method for dye-sensitizer removal. EXPERIMEN'I+AI.
Materials "Fhc sewage exposed to photooxtdation ,acre effluents of circulated oxidation ponds of rot, nit(pal water from the Haifa, Tel Aviv and Nazareth areas. The rest,Its given hcrc arc on the Nazareth effluent and arc similar hi those obtained with the other effluents. The effluents were stored in the dark at 5 'C. A typical analysis of a Na~'arcth effluent used m our experiments is given in Table 1. The sensitizers, rose bengal (BDH. No. 2617.21-11 and methylene blue (BDH. No. 26132 QI were used as a 0.5", solution in distilled water. Linear alkyl sulfonate (LAS, Lt No. 7181-7-75. f!nxlronmental Protection Agency, Ohio. U.SA.). ~as used as a standard anionic suffactant for preparation of methylene blue active substances (MBAS) calibration curve The solid supports used in preliminar~ experiments for linking the sensitizer were ion exchange resins {IR 400. O H - form. IR 120 and Dowex 50, tt" forml or natural clay minerals (kaolinite No. 5. A.P.I. 60 mesh: illite No. 36. A.P.I., 60 mesh: bentonite, Fisher Lab. Grade. powder. B-235). The bentonite clay employed for precipitation of metlDlone blue from sewage water was a 0.5", ~uspcnsiou m distilled water, shaken for 2 h before use.
.tnalytwal method,s and uppuratu~ The effectiveness of the photosensitized oxidanon of the organic matter from sewage water was evaluated by means of chemical oxygen demand [COD. mg O I-~1. MBAS img LAS I '), bacteriological enumeration (most probable number, MPN, methodl of viable fecal col(forms {Sumdurd Methods. 1971), and suspended solids C,, transmiuance at 530nm). The u.~. lamp used as a light source was a Hano,da 450W, high-pressure mercury lamp, equipped ,xith a water-cooled Pyrex filter. The average solar radiation mtensity {1900 _+ 200.uE m - ' s - ' ) in the 400. 7(Rlnm range. was measured by a quantum sensor {Lambda Instruments
Table 1. Composition of the circulated oxidanon pond effluent of municipal .sewage effluent from the Nazareth area Component Na' mgl' ('a:" mgl ' Mg-" mgl ' CI ,nequivl s HCOj mequivl- ~ N-NH+ mg I - ' N-NOsmgl ' N,ot~I mg I ' P,o,., mgl '
K - mg I ' pH Elect. conductivity mmho cm- ' COD mgl ~ MBAS mg LASI " ' Fecal coliform count ml- J Suspended matter %T Suspended matter mg I '
200.0 1250 52.11 ,, < 12 ~ 467 t+-I 6t) (I 5o
27.0 -: I ~
376.0 I 1.~ 5200.0 21~ 121).o
D',c-sensitizcd-photo-oxidation Corp.. Nebraska. U.S.A.) connected to a digital integrator (Typc TS 100A). A Varian Vis-UV Spectrophotometer, Techtron, model 635 was used for detcrmination of MBAS (625 nm) and of suspended solids (530 nm).
4OO
:500
Procedure Samples of sewage 050ml), containing methylene bluc (MB) were exposed at ambient temperature to radiation under continuous aeration. The air bubbled through the samples was supplied by a lal:x~ratory pump and cleaned by pas~tge through a v, atcr trap. The experiments using u.v. lamp radiation ~.cre performed in the laborator 3 in cylindrical glass vessels. The u.v. lamp, contained m a ~ater-cooled cylinder was immerscd in a larger outer vesscl containing the sample, so that there was ca. 1.5 cm sewage between the two vessels. A magnetic stirrer ensured adequatc mixing of the sewagc and the air entry into the sample was through a sintcrcd tilter on the wall of the outcr vessel. {he temperature of the sample was kept at 22- 24 C. The experiments run in sunlight (outdoors) wcrc carried out in 250 ml graduatcd cylinders, containing a 0.1 ml capillary pipettc through which the air was bubbled into the sample. ]'he temperature of thesc ~mplcs rose during the radmtion period to 32 35 ('. After exposure, the methylene bluc (MB) was removed from the etttuent b3 precipitation with bentonite clay (BI, employing a ratio of ~:1, B:MB. Following the addition of B, the stirring and aeration was continued for another 15 rain to permit good flocculation of B by MB. qhe u.v.radiated ,samples were transfcred from the experimental vessel into 250ml graduated cylinders and v, crc stored overnight (16 h) at 8 C. as were the solar-radiated samples. The clarified and colorless supcrnatants were analyzed for COD. MBAS, ".; transmittance, and fecal coliform content. Blank ~tmples (kept either in the dark, or without sensitizer in the light, or ~'ithout the addition of benlonitc), were also run simultaneously with other cxpcrimcnts and under the ~mac v,orking conditions.
RESUI.'i'S
AND
DIS(TSSION
Selectio~t o[ the sensitizer Two dye-scnsitizers wcre studied: methylene blue (MB, a phenothiazinc derivative, cationic type) and rose bengal (RB. a fluorescein derivativc, anionic type). The effectiveness of the sensitizcrs was checked by the determination of the survival of the fccal colifo,'ms in aerated sewage exposed to solar radiation (sunlight intensity was 1980pE m - 2 s - ~ ) . The complete dcstruclion of the fecal coliform population was achieved with 4 mg M B I ~ in 30 min, whercas using RB the same effect was reached with 1 0 m g l - t and 6('1min radiation. The cnumerations of the viablc fecal coliforms ( M P N ) in the blank experiments performed in the samc working conditions (without addition of bentonitc) but in the dark were a b o u t 5.0 x 105 fccal coliforms per 100 ml (the initial sewage content was 5.3 x 10 ~ fecal coliforms per 100 ml). The C O D determinations in the treated sewage, also indicated the advantage of M B : after 1 h u.v.-lamp radiation using 15 mgl t MB or RB, the C O D values wcre 140 and 280, respcctivcly, as c o m p a r e d with 330 of the blank experiment (in dark). Conscqucntly MB was selected to be used as the sensitizer in all subsequent expcriments,
559
Q.
- 200
C3 0 0
O
I00
l
5
1
,
I
i
I0 15 20 25 MB concentration, ppm
I
30
Fig. I. Effect of the MB concentration on the COD value of the cltlucnts. Sunlight radiation, e, h : . . . . u.v. radiation, lh.
Tlze ~J]~,ct qf MB colwemramm The effect of M B concentration on the photo-oxidation reaction was checked by dctermining the C O l ) and MBAS valucs in thc treated sewage (Figs. 1 and 2. respectively). The results after solar radiation of 6 h arc cxpressed as solid lines and those after u.v.lamp radiation of 1 h as broken lines. The data were not correctcd according to tbc results of ihc blank experiments. The minimurn C O D values. 120 in sunlight and 112 in u.~.-radiation were obtained at a concentration of 12mg M B I ~ in both cases (Fig. 1). Thc reason why concentrations below 1 0 m g M B I ~ were less effective may be that during thc photo-oxidation proccss, some of the dissolved MB is removed from the sewage by physical (adsorption on colloids a n d o r suspended solids) and chemical reactions {auto-oxidation. side reactions with chemicals prcscnt in scwage, e.g. strongl3 cationic exchangc-type st, bstanccsl. The lesscning of the effectiveness of the proccss, when the 12.0
,\ 9.0
Q.
o; 6.o
3.0 [.0
o
0
i 5
I I I I ~ 15 20 25 MB concentrotion, ppm
I 50
Fig. 2. El'feEt of the MB concentration o11 the MBAS value of the ctllucnts. Sunlight radiation. 6 h" . . . . u.v. radiation. 1 h.
560
A.J. A('IJ):Rand I. ROSENTI.~AI.
MB concentration increased above 15mgl ~, can coliforms was obtained within 30min under direct probably be explained by the decrease of the light sun radiation (1980,uEm-' s ~), whereas in the shade penetration into the darker medium. Experiments car- (681~Em-=s-') the same result was reached in ried out with water containing different concen- 150minutes (blank experiments in the dark or withtrations of MB indicated that the light penetration. out MB in the light did not show an) change in the coliform population, as compared with 5.3 × I(I5 colimeasured at 24 cm from the water surface, decreased by a ratio of 12, 2 and I at MB concentrations of forms per 10Oral of the initial sewage contentl. 6, 3. and 0mgl ~. respectively (the inner diameter Removal o/ the dve-,sen,sitirer lM B) of the experimental vessel was 18cm). The fact that In the preliminary experiments of this stud~.. this decrease in the photo-oxidation is less marked in the case of UV-light as compared to solar radiation attempts had been made to employ MB and RB is probably due to the apparatus design used in the adsorbed on to solid supports as heterogeneous scnsiexperinaent (magnetical stirring, the narrowness of the tizers. Strongly anionic and cationic exchange resins medium to be penetrated by the light and a strong and natural clay minerals, such as illite, kaolinite and and homogeneous light radiation). B"tank experiments bentonite were tried. The dyc-binded resins reacted performed with 12 mg MB 1- ~ in the dark; without with different ions from sewage water and released MB in the light: and without MB and bentonite in some linked dye that colors the effluent. Kaolinite the light showed decreases in the COD values. These and illite had poor adsorption capacities lbr RB and MB, and released them continuously into the ,,ewagc decreases, which were 16 18°,i of the initial COD value may be explained by the precipitation of some water. Bentonite clay. on the other hand. vet\ strongly complexed the MB in its interlayer spaces suspended ,solids containing organic substances which occurred as a rest.It of air mixing in sewage ,samples, (Hang & Brindle)'. 1970) and did not liberate dye into the effluent: this also prevents free approach b) other or, by their co-precipitation with bentonite--MB. The effect of the MB concentration on the photo- chemical species. These difficulties led us to the idea oxidation of the anionic detergents was measured b) of using dissolved MB in a homogeneous photo-oxiMBAS determinations in the effluents (Fig. 2). The dation reaction and then to adsorb the MB bv benbest results were obtained in the samples photo-oxi- tonite from the light-exposed effluent. Determination of the optimum amount of belldized with 12 mg MB I- ~, at which the MBAS values were reduced to less than one-tenth of their initial tonite necessary to precipitate the MB was accomconcentrations (from 11.8 to 0.8 mg I ~). The fact that plished by adding different quantities of bentonite the MBAS amount remained rather high (>2 mgl- ~) suspension to MB solutions and shaking until precipiat MB concentration of 20ppm or higher, as well tation occurred. Optimum flocculation was obtained as the fact that there was no change in the MBAS when silicate surfaces of bentonite were cfft~'tively value in the blank experiment performed in the dark, covered by MB ions, a state which caused the instabiboth indicated that a destructive photo-oxidation of lity of the colloid suspension b) .Tt'ta potential reducmethylene blue active substances took place and not tion. i.e. the quantity of MB close to the cation a mere chemical precipitation of the anionic surfac- exchange capacity of the bentonite (Hang & Brindle',.. 1970). The most efficient bentonite:MB (W:W) ratio tants by MB. for MB precipitation from distilled water i~ 6:1 The £~ect of the amount oJ enerqy supplied 13 min). and it is independent ol' MB concentration The amount of energy supplied to the photo-oxidalion reaction is dependent on the light intensity in 400 q12.O the range of the sensitizers" absorption i ().M, = 670 nm, 5.~, = 550 rim) and the time of radiation. The effect of the radiation time using the u.~.300 9.0 lamp on the photo-oxidation reaction was checked by COD and MBAS analyses. The results in Fig. 3 showed that COD values (110- 120) remained almost o.. 200 6.0d constant after 60rain radiation. The remaining 0 tr,, organic materials in the sewage were probably refrac° ' " - - o --COD ~)---,-~---o tory to further photo-oxidation. The methylene blue active substances were much more sensitive to the radiation time and continued to be degradated after ,,.. MBAS 60rain (the MBAS values were 0.9 and 0.3mg , , , I ' 1.0 LAS 1-' after 150 and 345 min. respectively, com0 0 30 6 0 90 120 150 " 3 4 5 pared with 11.6 of the initial sewage). Radiation t i m e , m i n u t e s in another set of experiments carried out in sunlight, with 4 mg MB 1 t , the survival of the fecal coli- Fig 3. Effect of the u.v. radiation time on the COD and forms in sewage treated with different amounts of MBAS values of the effluents. The concentration of the MB was 12ppm. (1 ppm • I mgl ~1. energy was followed up. The complete destruction of
i
,oo
\%
--~Z-~
Dye-sensitized-photo-oxidation For sewage, the ratio is increased to 8:1-10:1 and the time of shaking required until the flocculation occurs to 10 15 rain. The use of ratios of bentonite to M B other than the optimum ones, leads to relatively stable suspension of bentonite from which MB cannot bc completely removed {at higher MB concentrations, charge reversal resulting from MB sorbed in excess of the clays" exchange capacity causes redispcrsion of the st, spensionl. The etli'ct ol the dw" n'moral on the suspended solids
The above effect was checked by the determination of per cent transmittance of the treated effluents at 530 nm and is shown in Fig. 4. The aforemcntioncd flocculation of bentonite by MB enhanced the co-precipitation of other colloids of suspended materials from sewage, and improved the light transmittance. An increased dcgree of oxidation of the organic materials from the effluent was observed to increase the instabilit.,, of the suspension to bentonite flocculation (the maximum per cent transmittance- 48°0 overlapped the most effective concentration of MB in photo-oxidation reaction). The ratio of bentonite to MB (S:l) being constant, the absolute amount of the clay addcd to the exposed efflucnt was greater m the experiments where a highcr conccntration of M B was used. Nevertheless. the per cent transmittance ,creamed practically unchanged when concentrations greater than 120rag bentonitcl ~ ( 1 5 m g M B I ~) ~ere used. At this concentration the transmittance measurements indicated a more than twofold reduction m the turbidity. The blank experiment (without MB and bentonite) showed a transmittance of 23",. as compared to 21". for the initial scwage. l-flcct ol the photO-OlV.idatioll o1~ the surviral qf fecal colilm'ms m sewa~le
Bacteriological experiments and determinations {before addition of the bentonite) were also performed. The results showed, as expected, that the
561
microorganisms are much morc sensitive to the singlet oxygen oxidations than the organic compounds. Poor photo-oxidation reaction conditions i3 5rag M B I t and sunlight intensity of 6 8 1 , E m -'s t). which are not effective for the oxidation of organic compounds succeeded in disinfection of the sewage of fecal coliforms. These results demonstrated the efl'cctivencss of the photo-oxidation method for the disinfection of the sewage effluents. A more detailed study of the bacteriologi~tl effects of the photosensilized oxidation was performed and the results will bc published separately (Acher & Juven, 1977). ( ' O N CLI.ISIONN
The new approach of the oxidation of organic materials contained in sewage proposes the use of methylene blue as a photosensitizer, air as the oxygen source, sunlight as the energ~ source to induce photo-oxidation and bentonite clay for dye and other species precipitation. Laboratory-scale experiments, applied to recirculalion oxidation pond effluent of municipal storage. produced a decrease in the C O D to one-third of its initial wdue. and MBAS content of less than I ppm, a reduction in the turbidity caused bx colloids and suspended materials of more than twofold, and an effluent completely frcc of fecal coliform. This approach is especially attractive in arid and semi-arid zones, where the climate is fa~,orablc tbr the promotion of photo-oxidation. The procedure is probably suitable either for small settlements where this operation can bc used for linal puritication of the effluent obtained from the rccirculatcd oxidation pond of the domestic wastewatcr, or as a purilication operation step in an advanced municipal se~agc treatment process. Pilot phmt experiments arc being planned in order to afire engineering problcnas and check the economical feasibility of this approach. REFERE:NCEs
60
-
~x
o
1 5
1 1 1 I I0 15 20 25 MB concentration, ppm
I ~K)
Fig. 4. Effect of the dye removal on the suspended ,solids of the cfl]ucnts. Sunlight radiation, 6 h : . . . . u . v . radiation. 1 h. I ppm MB = 8 ppm bentonite.
Acher A. J. & Juven B. 1. (1977) Destruction of the fecal coliform in sewage water by dye-sensitized photo-oxidalion. Appl. Environ. Microbiol. 3,a,{5), in press. Brungs W. A (1973) Effects of residual chlorine on aquatic life. J. Wat. Pollut. Control Fed. 45, 2180 2193. Christensen G L. (1974) Use of ozone and oxygen in advanced wastewater treatment. J. |~l~jt. Pollut. ('o,trol Fed. 46, 2054 2055. Foote C, S. (1968) Mechanisms of photosensili/ed oxidation. Science 162, 963,970. Gollnick K. {1968)Type II pl~oto-ox,,genation reactions in solution. Adv. Photochem. 6, 1-122. Gomella C. & Guerree H. (1973) l,e 7?aitcme, t des l-'au,; de Distribution. pp. 133--152. Editions Eyrolles. Paris. Hang P, T. & Brindley G. W. 11970) Methylene blue adsorption by clay minerals. Clav.s Clay Mira,,'. 18, 203 212. Idelovitch E., Roth T., Michail M. & ('ohen A. (1976) Advanced treatment and re-use of municipal v,astewater. Mekorot Water Co. and Tahal. TeI-Aviv. Israel, I-V-I 7. Kaplan M. L. (1971) Singlet oxygen. ('hem. Tech. 23, 621-626.
562
A.J. ACI-IER and I. RONENTHAI.,
Kearns D. R. (19711 Physical and chemical properties of singlet molecular oxygen. Chem. Rev. 71, 395-427. Mahlman H. A., Sisson W. G. Kraus K. A & Hohnson J. S., Jr. (1976) Cross-flow filtration in physical-chemical treatment of municipal sewage effluents, pp. 4-116. U.S, EPA-600/2-76-025 Municipal Environ Rcs. Lab., Cincinatti, Ohio. Netzer A., Kilkinson P., Beszedits P. & Miyamoto H. K. 119751 Treatment of the dye water by ozonation. Prcsented at 2nd Int. Ozone Syrup.. Montreal. 1975.
Spikes J. D. & Livingston R. i1969) The molecular blolog) of photo-dynamic action: sensitized photo-auto-oxidations in biological systems. Adt'. Biol. 3, 29. 121. Spikes J. D. & MacKnight M. L. (19701 Dye-sensitized photo-oxidation of proteins. Am1..\:.~ Acad St.i. 171, 149-162. Standard MethodsJor the Examination of Water and Wastewater (1971) 13th edn, pp. 339-342. 495--499. 669-672. American Public Health Assoc.. Washingtox~ I)(.
DYE-SENSITIZED-PHOTO-OXIDATION--A NEW APPROACH TO THE T R E A T M E N T O F O R G A N I C MATTER IN SEWAGE E F F L U E N T S * A. J. Ar,'HF,R'f Institute of Soils and Water. Division of Soil Residues Chemistry. The Volcani ('enter. Bet Dagan. Israel and I. ROSENTtIAI. Department of Organic Chemistry.. The Weizmann Institute of Science. Rehovot. Israel
(Receiced 25 .,lu~ust 1976, in rcrisedlbrm 19 .lanuary 19771 Abstract A new approach to the oxidation of organic matter in municipal v.astewaters by photosensitization is described. ,Samples of aerated sewage effluents containing an added d~e-sensitizer ~erc exposed to u.v.-lamp or solar radiation. The photochemical oxidation of the sewage caused decreases in the content of the fecal coliform, of the chemical oxygen demand and of the meth,,lenc hlue-acti\.e substal'lCCS b} 10G. 67 and 9(}?,> of their initial values, respectivel,~. The effluents obtained after the removal of the dye, by prccipitation with bentonite clay, were colorless, non-odorous and ~ilh the amount of suspended solids considerably reduced
INTR(.IDt
I(*I'ION
Due to the ever-increasing depletion of the natural resources of water, the re-use of treated municipal wastewater for crop irrigation, for artificial groundwater recharge, or even for Ix-~lable purposes, has become a real problem for the countries of the arid and semi-arid zones. Continuous efforts are being made to improve the existent purification methods and to develop new ones (Mahlman et aL, 1976: Idelovitch et ul., 1976). Several procedures employ the chemical oxidation of the organic matter as a key step in the wastewater treatment (Gomella & Guerree, 1973, Netzer et al., 1975). The limiting factor in the use of the chemical oxidation treatment of sewage el'fluents, containing large amounts of organic matter, is usually its high cost or undesirable by-products remaining in treated effluents (Christensen, 1974-B,'ungs, 1973). The aim of this stud} was to develop a new approach in treating the organic matter in sewage effluents, i.e. dye-sensitized photo-oxidation. The use of such a procedure appears a priori to be competitive with other oxidation methods, especially m arid and semi-arid areas, where thc climate (a large number of sunlight hours per day)is favorable for the promotion of the photo-oxidation reactions.
* Contribution from the Agricultural Research Organization. The Volcani Center. Bet Dagan, lsrael. 1976 Series. No. 232-E. -I-To whom correspondence should be addressed.
Dye-.sensitized photo-oxidations The term "'sensitized photo-oxidation'" also referred to as a "'photodynamic action" is synonymous with oxidations performed with smglet oxygen. These rcactions have been extensively studied with regard to their use in the syntheses or in the oxidative degradations of various naturall\ occurring and synthetic substances (Gollnick, 1968: Kearns, 1971 }. The sensitized photo-oxidation consists of the combined action of visible light and molecuktr oxygen upon organic matter, through the intermediary' of an appropriate photosensitizer [S). The S added to the aerated and light-exposed wastewater will absorb some of the light-radiated energy. This absorbed energy is then made available to the oxidation of organic materials (O:Vll b} either one or Both of the follov¢ing mechanisms, which can operate m aerobic photosensitized oxidation : 1. Primary interaction of the electronically excited sensitizer (S*} is with the OM to generate reactive. short-lived intermediates which subsequently react with atmospheric oxygen. [aj S + hv---~ S* (excited statel (b) S* + OM-.-* transient specia ,2~: oxidation products -F S. {free radicals, ion pairs, elc.I 2. Instead of interaction with OM (ll, the S* transfers its excitation energy' to the molecular oxygen. The addition of the excitation energy to the oxygen molecule changes its ground electronic state (triplet state. "~'YuO2) to the tirst excited singlet state 557
558
A . J . A('ttliR and I. ROSENTHAI
(IA~O2J which has a higher energy by' 22.5kcal mole '. When more energy is imparted to the oxygen molecule, a n o t h e r state is formed ('EoO2) which corresponds to a level of 37.5 k c a l m o l e ' above the 3E002. These p h e n o m e n a of different electronic configurations arc described by group theory and symmetry properties (Kaplan. 1971). which is beyond the .scope of this article. F r o m the properties of singlct oxygen (exceedingly, short lifetimes of ~E,jOz) it seems likely that only 'A+Oz is i m p o r t a n t in the solution oxidations. Therefore, the mechanism of the photo-oxidation of the OM bv the singlet oxygen can be written : (c) S* + "~EqO2 -+ S + 'A~() 2 (d) 'A+O_, + OA'/--~oxidation products. In both mechanisms the sensitizer is regenerated a n d undergoes hundreds of cyles so that only mint, te a m o u n t s of it are required. The efficiency of the bimolecular reaction of the excited sensitizer with either the substrate {OM) or molecular oxygen (3~-~-#O2}depends on how efficiently these processes compete with each other. In view of the diversity of c o m p o u n d s present in sewage water it is very difflcuh to decide which mechanism operates in the present process. It is. however, well known that singlet oxygen oxidizes unsaturated c o m p o u n d s (U(") to peroxides. The subsequent thermal or photochemical decomposition of these peroxides can further initiate free radical oxidation reactions of the saturated c o m p o n e n t s found in sewage water: 'AyO 2 + U C - - , ROOR---* RO' R O -r- R'H ~ R O H
7,- R
R" + 0 2 --* RO()'. etc. As a result of the above reactions, vital biological c o m p o n e n t s such as proteins, lipids and polysaccharides. undergo oxidations and degradations, and. consequently, any biological development in photo-oxidized effluent is inhibited. Micro-organisms such as viruses, algae, fungi, protozoa a n d multicellu 'lar plants a n d animals are affected (Spikes & Livingston. 1969: Spikes & MacKnight, 1970). A m o n g the sensitizers which induce reaction with visible light, the most efficient are tluorescein and phenothiazine derivatives, certain porphyrins and polycyclic a r o m a t i c h y d r o c a r b o n s (Foote. 1968). The following criteria have to be met by the sensitizer in order to be useful for photo-oxygenation of the sewage effluents: to induce reaction with visible light: to be chemically stable during the radiation (refractory to auto-oxidation); to be free of reactive functional groups which can react with chemicals present in sewage and inactivate it; to have a good light absorption capacity (to be effective at tow concentration); to be soluble in water but easy to remove. This paper describes the experiments carried out to define the optimal operating conditions for the photosensitized oxidation method for the treatment
of sewage effluents. These conditions entail determination of a suitable sensitizer, o p t i m u m photoreac,ion conditions and the best method for dye-sensitizer removal. EXPERIMEN'I+AI.
Materials "Fhc sewage exposed to photooxtdation ,acre effluents of circulated oxidation ponds of rot, nit(pal water from the Haifa, Tel Aviv and Nazareth areas. The rest,Its given hcrc arc on the Nazareth effluent and arc similar hi those obtained with the other effluents. The effluents were stored in the dark at 5 'C. A typical analysis of a Na~'arcth effluent used m our experiments is given in Table 1. The sensitizers, rose bengal (BDH. No. 2617.21-11 and methylene blue (BDH. No. 26132 QI were used as a 0.5", solution in distilled water. Linear alkyl sulfonate (LAS, Lt No. 7181-7-75. f!nxlronmental Protection Agency, Ohio. U.SA.). ~as used as a standard anionic suffactant for preparation of methylene blue active substances (MBAS) calibration curve The solid supports used in preliminar~ experiments for linking the sensitizer were ion exchange resins {IR 400. O H - form. IR 120 and Dowex 50, tt" forml or natural clay minerals (kaolinite No. 5. A.P.I. 60 mesh: illite No. 36. A.P.I., 60 mesh: bentonite, Fisher Lab. Grade. powder. B-235). The bentonite clay employed for precipitation of metlDlone blue from sewage water was a 0.5", ~uspcnsiou m distilled water, shaken for 2 h before use.
.tnalytwal method,s and uppuratu~ The effectiveness of the photosensitized oxidanon of the organic matter from sewage water was evaluated by means of chemical oxygen demand [COD. mg O I-~1. MBAS img LAS I '), bacteriological enumeration (most probable number, MPN, methodl of viable fecal col(forms {Sumdurd Methods. 1971), and suspended solids C,, transmiuance at 530nm). The u.~. lamp used as a light source was a Hano,da 450W, high-pressure mercury lamp, equipped ,xith a water-cooled Pyrex filter. The average solar radiation mtensity {1900 _+ 200.uE m - ' s - ' ) in the 400. 7(Rlnm range. was measured by a quantum sensor {Lambda Instruments
Table 1. Composition of the circulated oxidanon pond effluent of municipal .sewage effluent from the Nazareth area Component Na' mgl' ('a:" mgl ' Mg-" mgl ' CI ,nequivl s HCOj mequivl- ~ N-NH+ mg I - ' N-NOsmgl ' N,ot~I mg I ' P,o,., mgl '
K - mg I ' pH Elect. conductivity mmho cm- ' COD mgl ~ MBAS mg LASI " ' Fecal coliform count ml- J Suspended matter %T Suspended matter mg I '
200.0 1250 52.11 ,, < 12 ~ 467 t+-I 6t) (I 5o
27.0 -: I ~
376.0 I 1.~ 5200.0 21~ 121).o
D',c-sensitizcd-photo-oxidation Corp.. Nebraska. U.S.A.) connected to a digital integrator (Typc TS 100A). A Varian Vis-UV Spectrophotometer, Techtron, model 635 was used for detcrmination of MBAS (625 nm) and of suspended solids (530 nm).
4OO
:500
Procedure Samples of sewage 050ml), containing methylene bluc (MB) were exposed at ambient temperature to radiation under continuous aeration. The air bubbled through the samples was supplied by a lal:x~ratory pump and cleaned by pas~tge through a v, atcr trap. The experiments using u.v. lamp radiation ~.cre performed in the laborator 3 in cylindrical glass vessels. The u.v. lamp, contained m a ~ater-cooled cylinder was immerscd in a larger outer vesscl containing the sample, so that there was ca. 1.5 cm sewage between the two vessels. A magnetic stirrer ensured adequatc mixing of the sewagc and the air entry into the sample was through a sintcrcd tilter on the wall of the outcr vessel. {he temperature of the sample was kept at 22- 24 C. The experiments run in sunlight (outdoors) wcrc carried out in 250 ml graduatcd cylinders, containing a 0.1 ml capillary pipettc through which the air was bubbled into the sample. ]'he temperature of thesc ~mplcs rose during the radmtion period to 32 35 ('. After exposure, the methylene bluc (MB) was removed from the etttuent b3 precipitation with bentonite clay (BI, employing a ratio of ~:1, B:MB. Following the addition of B, the stirring and aeration was continued for another 15 rain to permit good flocculation of B by MB. qhe u.v.radiated ,samples were transfcred from the experimental vessel into 250ml graduated cylinders and v, crc stored overnight (16 h) at 8 C. as were the solar-radiated samples. The clarified and colorless supcrnatants were analyzed for COD. MBAS, ".; transmittance, and fecal coliform content. Blank ~tmples (kept either in the dark, or without sensitizer in the light, or ~'ithout the addition of benlonitc), were also run simultaneously with other cxpcrimcnts and under the ~mac v,orking conditions.
RESUI.'i'S
AND
DIS(TSSION
Selectio~t o[ the sensitizer Two dye-scnsitizers wcre studied: methylene blue (MB, a phenothiazinc derivative, cationic type) and rose bengal (RB. a fluorescein derivativc, anionic type). The effectiveness of the sensitizcrs was checked by the determination of the survival of the fccal colifo,'ms in aerated sewage exposed to solar radiation (sunlight intensity was 1980pE m - 2 s - ~ ) . The complete dcstruclion of the fecal coliform population was achieved with 4 mg M B I ~ in 30 min, whercas using RB the same effect was reached with 1 0 m g l - t and 6('1min radiation. The cnumerations of the viablc fecal coliforms ( M P N ) in the blank experiments performed in the samc working conditions (without addition of bentonitc) but in the dark were a b o u t 5.0 x 105 fccal coliforms per 100 ml (the initial sewage content was 5.3 x 10 ~ fecal coliforms per 100 ml). The C O D determinations in the treated sewage, also indicated the advantage of M B : after 1 h u.v.-lamp radiation using 15 mgl t MB or RB, the C O D values wcre 140 and 280, respcctivcly, as c o m p a r e d with 330 of the blank experiment (in dark). Conscqucntly MB was selected to be used as the sensitizer in all subsequent expcriments,
559
Q.
- 200
C3 0 0
O
I00
l
5
1
,
I
i
I0 15 20 25 MB concentration, ppm
I
30
Fig. I. Effect of the MB concentration on the COD value of the cltlucnts. Sunlight radiation, e, h : . . . . u.v. radiation, lh.
Tlze ~J]~,ct qf MB colwemramm The effect of M B concentration on the photo-oxidation reaction was checked by dctermining the C O l ) and MBAS valucs in thc treated sewage (Figs. 1 and 2. respectively). The results after solar radiation of 6 h arc cxpressed as solid lines and those after u.v.lamp radiation of 1 h as broken lines. The data were not correctcd according to tbc results of ihc blank experiments. The minimurn C O D values. 120 in sunlight and 112 in u.~.-radiation were obtained at a concentration of 12mg M B I ~ in both cases (Fig. 1). Thc reason why concentrations below 1 0 m g M B I ~ were less effective may be that during thc photo-oxidation proccss, some of the dissolved MB is removed from the sewage by physical (adsorption on colloids a n d o r suspended solids) and chemical reactions {auto-oxidation. side reactions with chemicals prcscnt in scwage, e.g. strongl3 cationic exchangc-type st, bstanccsl. The lesscning of the effectiveness of the proccss, when the 12.0
,\ 9.0
Q.
o; 6.o
3.0 [.0
o
0
i 5
I I I I ~ 15 20 25 MB concentrotion, ppm
I 50
Fig. 2. El'feEt of the MB concentration o11 the MBAS value of the ctllucnts. Sunlight radiation. 6 h" . . . . u.v. radiation. 1 h.
560
A.J. A('IJ):Rand I. ROSENTI.~AI.
MB concentration increased above 15mgl ~, can coliforms was obtained within 30min under direct probably be explained by the decrease of the light sun radiation (1980,uEm-' s ~), whereas in the shade penetration into the darker medium. Experiments car- (681~Em-=s-') the same result was reached in ried out with water containing different concen- 150minutes (blank experiments in the dark or withtrations of MB indicated that the light penetration. out MB in the light did not show an) change in the coliform population, as compared with 5.3 × I(I5 colimeasured at 24 cm from the water surface, decreased by a ratio of 12, 2 and I at MB concentrations of forms per 10Oral of the initial sewage contentl. 6, 3. and 0mgl ~. respectively (the inner diameter Removal o/ the dve-,sen,sitirer lM B) of the experimental vessel was 18cm). The fact that In the preliminary experiments of this stud~.. this decrease in the photo-oxidation is less marked in the case of UV-light as compared to solar radiation attempts had been made to employ MB and RB is probably due to the apparatus design used in the adsorbed on to solid supports as heterogeneous scnsiexperinaent (magnetical stirring, the narrowness of the tizers. Strongly anionic and cationic exchange resins medium to be penetrated by the light and a strong and natural clay minerals, such as illite, kaolinite and and homogeneous light radiation). B"tank experiments bentonite were tried. The dyc-binded resins reacted performed with 12 mg MB 1- ~ in the dark; without with different ions from sewage water and released MB in the light: and without MB and bentonite in some linked dye that colors the effluent. Kaolinite the light showed decreases in the COD values. These and illite had poor adsorption capacities lbr RB and MB, and released them continuously into the ,,ewagc decreases, which were 16 18°,i of the initial COD value may be explained by the precipitation of some water. Bentonite clay. on the other hand. vet\ strongly complexed the MB in its interlayer spaces suspended ,solids containing organic substances which occurred as a rest.It of air mixing in sewage ,samples, (Hang & Brindle)'. 1970) and did not liberate dye into the effluent: this also prevents free approach b) other or, by their co-precipitation with bentonite--MB. The effect of the MB concentration on the photo- chemical species. These difficulties led us to the idea oxidation of the anionic detergents was measured b) of using dissolved MB in a homogeneous photo-oxiMBAS determinations in the effluents (Fig. 2). The dation reaction and then to adsorb the MB bv benbest results were obtained in the samples photo-oxi- tonite from the light-exposed effluent. Determination of the optimum amount of belldized with 12 mg MB I- ~, at which the MBAS values were reduced to less than one-tenth of their initial tonite necessary to precipitate the MB was accomconcentrations (from 11.8 to 0.8 mg I ~). The fact that plished by adding different quantities of bentonite the MBAS amount remained rather high (>2 mgl- ~) suspension to MB solutions and shaking until precipiat MB concentration of 20ppm or higher, as well tation occurred. Optimum flocculation was obtained as the fact that there was no change in the MBAS when silicate surfaces of bentonite were cfft~'tively value in the blank experiment performed in the dark, covered by MB ions, a state which caused the instabiboth indicated that a destructive photo-oxidation of lity of the colloid suspension b) .Tt'ta potential reducmethylene blue active substances took place and not tion. i.e. the quantity of MB close to the cation a mere chemical precipitation of the anionic surfac- exchange capacity of the bentonite (Hang & Brindle',.. 1970). The most efficient bentonite:MB (W:W) ratio tants by MB. for MB precipitation from distilled water i~ 6:1 The £~ect of the amount oJ enerqy supplied 13 min). and it is independent ol' MB concentration The amount of energy supplied to the photo-oxidalion reaction is dependent on the light intensity in 400 q12.O the range of the sensitizers" absorption i ().M, = 670 nm, 5.~, = 550 rim) and the time of radiation. The effect of the radiation time using the u.~.300 9.0 lamp on the photo-oxidation reaction was checked by COD and MBAS analyses. The results in Fig. 3 showed that COD values (110- 120) remained almost o.. 200 6.0d constant after 60rain radiation. The remaining 0 tr,, organic materials in the sewage were probably refrac° ' " - - o --COD ~)---,-~---o tory to further photo-oxidation. The methylene blue active substances were much more sensitive to the radiation time and continued to be degradated after ,,.. MBAS 60rain (the MBAS values were 0.9 and 0.3mg , , , I ' 1.0 LAS 1-' after 150 and 345 min. respectively, com0 0 30 6 0 90 120 150 " 3 4 5 pared with 11.6 of the initial sewage). Radiation t i m e , m i n u t e s in another set of experiments carried out in sunlight, with 4 mg MB 1 t , the survival of the fecal coli- Fig 3. Effect of the u.v. radiation time on the COD and forms in sewage treated with different amounts of MBAS values of the effluents. The concentration of the MB was 12ppm. (1 ppm • I mgl ~1. energy was followed up. The complete destruction of
i
,oo
\%
--~Z-~
Dye-sensitized-photo-oxidation For sewage, the ratio is increased to 8:1-10:1 and the time of shaking required until the flocculation occurs to 10 15 rain. The use of ratios of bentonite to M B other than the optimum ones, leads to relatively stable suspension of bentonite from which MB cannot bc completely removed {at higher MB concentrations, charge reversal resulting from MB sorbed in excess of the clays" exchange capacity causes redispcrsion of the st, spensionl. The etli'ct ol the dw" n'moral on the suspended solids
The above effect was checked by the determination of per cent transmittance of the treated effluents at 530 nm and is shown in Fig. 4. The aforemcntioncd flocculation of bentonite by MB enhanced the co-precipitation of other colloids of suspended materials from sewage, and improved the light transmittance. An increased dcgree of oxidation of the organic materials from the effluent was observed to increase the instabilit.,, of the suspension to bentonite flocculation (the maximum per cent transmittance- 48°0 overlapped the most effective concentration of MB in photo-oxidation reaction). The ratio of bentonite to MB (S:l) being constant, the absolute amount of the clay addcd to the exposed efflucnt was greater m the experiments where a highcr conccntration of M B was used. Nevertheless. the per cent transmittance ,creamed practically unchanged when concentrations greater than 120rag bentonitcl ~ ( 1 5 m g M B I ~) ~ere used. At this concentration the transmittance measurements indicated a more than twofold reduction m the turbidity. The blank experiment (without MB and bentonite) showed a transmittance of 23",. as compared to 21". for the initial scwage. l-flcct ol the photO-OlV.idatioll o1~ the surviral qf fecal colilm'ms m sewa~le
Bacteriological experiments and determinations {before addition of the bentonite) were also performed. The results showed, as expected, that the
561
microorganisms are much morc sensitive to the singlet oxygen oxidations than the organic compounds. Poor photo-oxidation reaction conditions i3 5rag M B I t and sunlight intensity of 6 8 1 , E m -'s t). which are not effective for the oxidation of organic compounds succeeded in disinfection of the sewage of fecal coliforms. These results demonstrated the efl'cctivencss of the photo-oxidation method for the disinfection of the sewage effluents. A more detailed study of the bacteriologi~tl effects of the photosensilized oxidation was performed and the results will bc published separately (Acher & Juven, 1977). ( ' O N CLI.ISIONN
The new approach of the oxidation of organic materials contained in sewage proposes the use of methylene blue as a photosensitizer, air as the oxygen source, sunlight as the energ~ source to induce photo-oxidation and bentonite clay for dye and other species precipitation. Laboratory-scale experiments, applied to recirculalion oxidation pond effluent of municipal storage. produced a decrease in the C O D to one-third of its initial wdue. and MBAS content of less than I ppm, a reduction in the turbidity caused bx colloids and suspended materials of more than twofold, and an effluent completely frcc of fecal coliform. This approach is especially attractive in arid and semi-arid zones, where the climate is fa~,orablc tbr the promotion of photo-oxidation. The procedure is probably suitable either for small settlements where this operation can bc used for linal puritication of the effluent obtained from the rccirculatcd oxidation pond of the domestic wastewatcr, or as a purilication operation step in an advanced municipal se~agc treatment process. Pilot phmt experiments arc being planned in order to afire engineering problcnas and check the economical feasibility of this approach. REFERE:NCEs
60
-
~x
o
1 5
1 1 1 I I0 15 20 25 MB concentration, ppm
I ~K)
Fig. 4. Effect of the dye removal on the suspended ,solids of the cfl]ucnts. Sunlight radiation, 6 h : . . . . u . v . radiation. 1 h. I ppm MB = 8 ppm bentonite.
Acher A. J. & Juven B. 1. (1977) Destruction of the fecal coliform in sewage water by dye-sensitized photo-oxidalion. Appl. Environ. Microbiol. 3,a,{5), in press. Brungs W. A (1973) Effects of residual chlorine on aquatic life. J. Wat. Pollut. Control Fed. 45, 2180 2193. Christensen G L. (1974) Use of ozone and oxygen in advanced wastewater treatment. J. |~l~jt. Pollut. ('o,trol Fed. 46, 2054 2055. Foote C, S. (1968) Mechanisms of photosensili/ed oxidation. Science 162, 963,970. Gollnick K. {1968)Type II pl~oto-ox,,genation reactions in solution. Adv. Photochem. 6, 1-122. Gomella C. & Guerree H. (1973) l,e 7?aitcme, t des l-'au,; de Distribution. pp. 133--152. Editions Eyrolles. Paris. Hang P, T. & Brindley G. W. 11970) Methylene blue adsorption by clay minerals. Clav.s Clay Mira,,'. 18, 203 212. Idelovitch E., Roth T., Michail M. & ('ohen A. (1976) Advanced treatment and re-use of municipal v,astewater. Mekorot Water Co. and Tahal. TeI-Aviv. Israel, I-V-I 7. Kaplan M. L. (1971) Singlet oxygen. ('hem. Tech. 23, 621-626.
562
A.J. ACI-IER and I. RONENTHAI.,
Kearns D. R. (19711 Physical and chemical properties of singlet molecular oxygen. Chem. Rev. 71, 395-427. Mahlman H. A., Sisson W. G. Kraus K. A & Hohnson J. S., Jr. (1976) Cross-flow filtration in physical-chemical treatment of municipal sewage effluents, pp. 4-116. U.S, EPA-600/2-76-025 Municipal Environ Rcs. Lab., Cincinatti, Ohio. Netzer A., Kilkinson P., Beszedits P. & Miyamoto H. K. 119751 Treatment of the dye water by ozonation. Prcsented at 2nd Int. Ozone Syrup.. Montreal. 1975.
Spikes J. D. & Livingston R. i1969) The molecular blolog) of photo-dynamic action: sensitized photo-auto-oxidations in biological systems. Adt'. Biol. 3, 29. 121. Spikes J. D. & MacKnight M. L. (19701 Dye-sensitized photo-oxidation of proteins. Am1..\:.~ Acad St.i. 171, 149-162. Standard MethodsJor the Examination of Water and Wastewater (1971) 13th edn, pp. 339-342. 495--499. 669-672. American Public Health Assoc.. Washingtox~ I)(.