Abstract-PACT. (Powdered Activated Carbon Treatment) process was developed for the treatment of wastewater containing compounds which are difficult to ...
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BIOMASS CONCENTRATION M. C. MARQUEZ*@ Chemical
Engineering
Wat. Res. Vol. 30, No. 9, pp. 2079-2085, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/96 $15.00 + 0.00
IN PACT PROCESS and C. COSTA
Sciences, University of Salamanca. Plaza de la Merced s/n, 37008 Salamanca, Spain Department,
Faculty
of Chemical
(First received July 1995; accepted in revised form January
1996)
Abstract-PACT (Powdered Activated Carbon Treatment) process was developed for the treatment of wastewater containing compounds which are difficult to oxidize. Enhancement properties of activated carbon added to activated sludge units are well known but not justified. In this work the process has been applied in lab plant for the treatment of a synthetic textile wastewater with an azodye used in the wool textile industry (Acid Orange 7, C.I. 15510). Microorganism growth on a carbon surface is only reached at low values of biomass concentration (low sludge age), and then a high percentage of dye removal is obtained (97.0 f. 1.6%). The influence of biomass concentration on COD and dye removal is discussed because this variable is essential especially in dye removal. The carbon surface is visualized by microscope techniques and it is shown that a strong dependence exists between pollutant removal and biomass concentration. Copyright 0 1996 Elsevier Science Ltd Key words-powdered
activated carbon, biomass, dye removal, activated sludge, wastewater treatment
INTRODUCTION
Over the last 20 years, Powdered Activated Carbon Treatment (PACT) for activated sludge processes has found special use in chemical and petrochemical wastewater treatment. Originally patented by Du Pont (Hutton and Robertaccio, 1975), the PACT process has been extended for the treatment of organic and inorganic compounds (Cormack et al., 1984); different yields, generally higher than conventional activated sludge systems, have been achieved. The advantages of this system as opposed to adsorption on activated carbon have been addressed by several authors (Sublette et al., 1982; Lankford, 1990), although the mechanism through which Powdered Activated Carbon (PAC) enhances the activated sludge process remains to be fully elucidated (Specchia et al., 1988). Application of this system for wastewater treatment has usually been focused on slowly biodegradable and adsorbable compounds (Flynn, 1975) in view of the mechanism suggested to explain pollutant removal. This mechanism consists of “stimulation of biological activity” and “bioregeneration” of activated carbon by microorganisms. The first phenomenon (stimulation of biological activity) is caused by a modification in the microbial medium consisting of the inclusion of a solid surface (activated carbon) able to adsorb toxic and inhibitory substances (Robertaccio, 1979). This acts as an oxygen reservoir (Lee, 1977) and concentrates the *Author to whom all correspondence should be addressed [Fax: (34) 923 294 5741.
substrate on its surface (Lee and Johnson, 1978). Substrate concentration on the surface of the carbon can favour microbial growth since it furnishes more nutrients and extracellular enzymes in the surrounding medium. This effect seems to be of special relevance for low substrate concentrations ( < 25 mg l-l), since in this situation the microorganisms are unable to use the pollutant as a source of carbon. Accordingly, the PACT process seems to be very advantageous in the treatment of dilute pollutants that are readily adsorbed onto the surface of carbon. The activated carbon “bioregeneration” theory in the PACT process has been formulated owing to the apparent loading observed. This parameter is defined as the amount of soluble organic matter (COD and TOC) removed in a PACT system minus the amount removed in a conventional system, both working under the same conditions of wastewater, temperature, hydraulic residence time and sludge age (Flynn et al., 1976). The apparent loadings of PACT systems are higher than those predicted by adsorption isotherms, this being explained by a synergistic effect between adsorption and biological degradation. If such an explanation is true, pollutant removal must depend on sludge age since the addition of PAC to an activated sludge system involves an increase in substrate-biomass contact time from 4-8 h (hydraulic residence time) to 5-50 d (sludge age). This has been reported by other authors (Sublette et al., 1982). The process can be applied for the treatment of textile dyeing wastewaters because they contain low concentrations of slowly biodegradable and adsorbable compounds: textile dyes. However, the results obtained by other authors do not show a good
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efficiency for removal of dyes (Specchia et al., 1988). This suggests the existence of a limiting variable which was not considered by other researchers and with an important role in dye removal. The purpose of this work is the study of significant variables affecting the PACT system when it is applied to the treatment of a synthetic textile wastewater with an azodye widely used in the wool dyeing industry. Since the PACT system is considered to be a process of adsorption4egradation, it is necessary to set up the optimum conditions of adsorption (carbon concentration, particle size, and stirring rate) before working in the continuous system mode. The process is optimized for COD and dye removal, considering biomass concentration as the variable; microbial population developing on the surface of the activated carbon is identified and visualized by microscopy. The theories on the PACT process formulated by other authors are explained and clarified by experimentally proven facts. MATERIALS E.uperimental
AND METHODS
Table
I. Composition of synthetic textile wasteWRter Concentration (ma I-‘)
Comvonent Peptone Meat extract Urea KzHPOI NaCl C&l> 2H10 MgS04 7H20 Acid Orange 7 (CL 15510)
160 110 30 28 7 4 2 20
COD = 250 + 30 mg 02 I-‘; COD/BOD = I .44; pH = 7.3.
of water (CEE, 1982; Benedek et al., 1985). Oxygen was supplied by air flow through a submerged aerator at an air flow of 3.65.61 min-‘. The oxygen concentration of the bioreactor was controlled by a selective electrode working in the 5.9-8.8 mg 1-l range to avoid bulking (Gaudy and Srinivasaraghaven, 1974; Randall and Buth, 1984). The temperature was set at 20 k 3”C, which is standard in activated sludge processes (Sedlak and Booman, 1986; Chandra et al., 1987) and hence in PACT processes (Benedek et al., 1985; Specchia et al., 1988). Recirculation by air lift was used to attain the desired biomass concentrations.
equipment
The optimum conditions for dye adsorption onto activated carbon were determined in a batch system with magnetic stirring. 100 ml samples of synthetic wastewater with powdered activated carbon (PANREAC PR) were kept under stirred conditions for a contact time of 3.0 h; thereafter, the PAC was separated out by filtration with cellulose nitrate membranes (0.45 pm pore size) and the dye concentration was analysed in the bulk liquid. The different particle sizes of the activated carbon were chosen by sieving, using 74, 88, 149 and 297 pm mesh sieves. The carbon concentrations used were in the 0.5-3.0 g I-’ range. PACT experiments in the continuous mode were performed in an activated sludge lab plant (Fig. I), consisting of a completely mixed reactor of 3.0 I volume and a cylindrical settler (1.75 I) with distribution and recirculation tubes (0.75 1) made of methacrylate. The plant was fed via a volumetric pump with synthetic textile wastewater at a flow rate of 1.0 I h-l, corresponding to a hydraulic residence time of 3.75 h (3.75 I is the total volume of mixed zone: reactor, distribution and recirculation tubes), which is suitable for the treatment of this type
Synthetic
wastewater
The composition of the synthetic textile wastewater is shown in Table I. This composition is similar to textile dyeing wastewater (Crespi and Huertas, 1987; Escalas and Crespi, 1989), but has advantages over industrial wastewater as regards preparation, storage and hygiene. The dye chosen to simulate wastewater was Acid Orange 7 (C.I. 15510), an azodye widely used in the wool dyeing industry, obtained from textile factories. Analytical
methods
concentration, COD and biomass concentration were analysed for study of the PACT system behaviour. BOD and pH were determined for characterization of the wastewater and the estimation of dye biodegradability. Dye concentration was measured by spectrophotometry at a maximum absorbance wavelength of 484.0 nm (Fig. 2) after sample filtration on cellulose nitrate membranes with 0.45 pm pore diameter to avoid turbidity due to microorganisms. Dye
AIR
AIR
Acid Orange 7 (C.I. 15510)
350 0.0
Fig. I. Activated sludge lab plant.
I
450 ‘t
550
650
750
Wavelength
(nm)
850
s NO
’ 484.0
Fig. 2. Dye absorbance (IO mg I-‘) at different wavelengths.
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Biomass concentration in PACT process
Days
0.5
Carbon concentration
Fig. 5. Dye removal in continuous system that represents bacterial growth on carbon surface.
(g 1-I)
Fig. 3. Relationship between dye adsorption and carbon concentration in batch system (commercial carbon).
COD and BOD were analyzed by standard methods (A.P.H.A. et al., 1986) and biomass concentration was determined by filtering .lOOml of samples with a 0.45 pm Millipore membrane, drying (24 h at 105°C) and weighing the dried filter. This nrocedure (Rodier. 1981) seems to be more suitable than the standard method (determination of volatile suspended solids, VSS) to determine the concentration of biological solids since it does not measure organic compounds smaller than 0.45 ym also present in wastewater. pH and temperature were determined with a calibrated electrode and thermometer, respectively. RESULTS
AND DISCUSSION
Optimum conditions of adsorption Optimal dye adsorption conditions on PAC (carbon concentration and particle size) were established in the batch mode for a stirring rate of 30 r.p.m., the minimum value shown to maintain all the carbon suspended. Experiments were carried out
50
1
0
I
t
loo
200
Particle size (pm) Fig. 4. Relationship between dye adsorption and particle size in batch system (0.5 g I-’ of carbon concentration).
tinuous mode: 20°C and 3.0 h contact time, the latter equivalent to the hydraulic residence time in the reactor. For different carbon concentrations (no selected particle size), experiments showed >90% of adsorbed dye (Fig. 3). For the following tests the selected carbon concentration was the minimum due to economic reasons: 0.5 g 1-r. Experimental values reflect that the particle size strongly affects dye adsorption (Fig. 4). Of the different particle sizes assayed, 81 pm (74-88 Frn mesh) proved to be the most suitable for obtaining a high percentage of dye adsorption: 86.9%. The smallest size (< 74 pm mesh) was discarded because it includes all sizes smaller than 74 pm and hence the particle size is not controlled. Experiments in the lab plant
I
300
81
with commercial activated carbon under conditions similar to those required by the process in the con-
The optimal adsorption conditions established in the batch mode (0.5 g I-’ carbon concentration and 81 pm particle size) were used in the continuous mode for the activated sludge lab plant. To keep a constant carbon concentration, PAC was added to compensate for the lost PAC in the sludge drain. In the lab plant, carbon loss at the effluent did not occur. Tests were run at different biomass concentrations, X (obtained from different cell residence times &.), because this variable strongly affected dye removal. This phenomenon has been reported by other authors and has been attributed to the floe characteristics of activated sludge, since at low biomass concentrations (low cell residence time) the floe is more compact than at high biomass concentrations and this could favour substrate availability for the microorganisms (De Walle et al., 1977). The microorganism population was obtained by natural selection from the system itself. The operation time of the lab plant was 60 days for each biomass concentration. Different values of biomass
M. C. Marquez and C. Costa
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concentr ,ation were obtained from different sludge ages (6, 12 and 25 days). For a cell residence time of 6 days (Fig. 5) a biomass concentration of 1000 + 200 mg I-’ was achieved . The relationship between dye removal and the days the plant was running can be interpreted in terms of dye saturation over the first 3 days, followed by a peri lad during which dye removal is insignificant (3-32 da ys) and then by a spectacular increase in dye
Fig. 6. SEM microphotographs
removal, values of 97.0 & 1.1% being reached. After the adsorbent has become saturated, the percentage of dye removal is similar to the values reported for conventional activated sludge processes because the carbon only exerts a mechanical effect (improving sludge settleability). The strong increase observed in pollutant removal can only be accounted for by a rapid colonization of the activated carbon by microorganisms able to degrade the dye. The evolution of
of bacteria present on carbon surface: (a) x 5000; (b) x 10,000
Biomass
0’ 0
concentration
I ‘Oca
2000
Biomass concentration
3OM)
4OCKl
(mg 1-l)
Fig. 7. COD removal in continuous system at different values of biomass concentration.
dye removal for a cell residence time of 6 days can be explained by a bacterial growth curve, with a lag phase (3-32 days), an exponential growth phase (32-38 days) and a stationary phase (after 38 days). Microorganisms colonizing the surface of the carbon and adapted to dye removal were photographed with a scanning electron microscope @EM) (Fig. 6) and identified (patent-protected). Samples of biomass with carbon obtained from the reactor were visualized directly, without any procedure that could modify the bacterial population. Photographs show the presence of microorganisms (spherical shapes for about l-l.5 pm size) in the opening of carbon pores; this is because the dye concentrates there, favouring microbial growth. Thus, the dye concentration can increase from 20 mg l-l, the concentration in the bulk liquid, to a much higher value on the surface of the carbon due to PAC adsorption. Another aspect explains the microbial growth on the surface of PAC; namely. the increase in contact time between the biomass and the substrate from the hydraulic residence time (3.75 h) to a much higher value, close to the cell residence time. The increase in biomass-substrate contact time permits the degra-
Biomass concentration
(mg I-‘)
Fig. 8. Dye removal in continuous system at different values of biomass concentration.
in PACT process
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dation of compounds which are difficult to oxidize. such as the dye used in this work. Relationships between COD (Fig. 7) and dye removal (Fig. 8) versus biomass concentrations were obtained in the lab plant running at different cell residence times. As may be seen in Fig. 7. COD removal displays typical behaviour for activated sludge wastewater treatment plants, with a maximum around 2000 mg 1-l. The importance of this figure is that it shows the decrease in organic matter removal for a high biomass concentration (around 3000 mg 1-l). Other authors have explained this as an increase in the inert fraction of the sludge (Sublette et al., 1982), which produces a decrease in the active biomass (microorganisms) due to the reduction of substrate availability. Dye removal is strongly dependent upon biomass concentration (Fig. 8). Whereas the value remains at 97% for a broad range of biomass concentrations (700-2500 mg l-l), it decreases dramatically above 2500 mg 1-l and is almost abolished above 3000 mg 1-l. The behaviour of this curve may be interpreted as being due to a radical change in the environmental conditions of the microorganisms; initially, such conditions permit microbial growth with a high value of dye removal but then cause massive cell death and hence low dye removal. This is supported by the micrographs with light microscopy (x 500) of activated sludge with PAC (Fig. 9). At biomass concentrations of 1000 and 2000mg 1-l (Fig. 9(a) and (b)), activated carbon particles, black colour in photographs, form part of the floe and their surface is in contact with the bulk liquid; the surface of the adsorbent is clean and permits dye adsorption and hence microbial growth. For a higher biomass concentration (Fig. 9(c): 3000 mg l-l), the carbon particles are trapped in the floe matrix and lose their properties of adsorption. Under these conditions, microbial growth is not favoured and dye removal is practically abolished. In SEM observation for a high value of biomass concentration (3000 mg l-l), a very few number of microorganisms were observed on the carbon surface, because in these conditions carbon pores are practically closed by the inert biomass. Inert biomass forms a gelatinous structure on the carbon surface and when its value is high, it involves the surface of the adsorbent and closes the carbon pores. In this situation, dye adsorption is obstructed because active sites of the adsorbent are less available for the dye. This decrease in dye removal due to the increase in biomass concentration has not been described by other authors, who consider azodye degradation to be proportional to the biomass concentration employed (Ganesh et al., 1994). However, this effect is probably not seen in sludges from activated sludge processes, which do not contain PAC particles and which, additionally, only cause low dye degradation.
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M. C. Mdrquez and C. Costa
(a)
W
* I
t .
Fig. 9. Microphotographs from activated sludge with PAC taken with light microscope (x 500) at different values of biomass concentration: (a) 1000 mg I-‘; (b) 2000 mg I-‘; (c) 3000 mg I-‘.
CONCLUSIONS
Although other authors applied PACT process for the treatment of wastewaters which are difficult to oxidize, the influence of biomass concentration on the process has not been reported. This is probably the reason for unsuccessful attempts in several works, for example, in textile effluents (Specchia et al., 1988). In the present work a great efficiency of dye removal was obtained. For the design and operation of PACT systems, it is first necessary to establish the optimum adsorption conditions in the batch mode. In this work, for the degradation of an azodye widely used in the wool textile industry, a carbon concentration of 0.5 g 1-l and a 81 pm particle size were selected. With these parameters, obtained in the batch mode, the PACT process was run in continuous mode and microbial growth on the surface of the carbon was achieved at a biomass concentration of 1000 k 200 mg 1-l. The microorganisms adapted to degradation of the azodye attained a removal of 97.0 + 1.6% for an influent dye concentration of 20 +_ 3 mg 1-l (COD = 250 + 30 mg O2 1-l). A strong relationship was seen between dye removal and biomass concentration. This was explained by light microscope techniques. At a
biomass concentration below 2500 mg I-‘, the surface of the carbon is in contact with the bulk liquid, permitting dye adsorption and favouring microbial growth. At a higher biomass concentration, the carbon particles become trapped within the floe matrix, their pores are closed and they lose their properties of adsorption. In this situation, microbial growth cannot proceed and dye removal is very low. The PACT process is an adsorption-degradation process in which the possibilities of removing compounds classified by BOD tests as slowly or non-biodegradable are considerably improved. This group of compounds includes textile dyes because these display a strong affinity for activated carbon and their large molecules can only be degraded by a few microorganisms. Application of this process to the treatment of textile wastewaters is an important economic improvement which, in a single step, allows the removal of COD and colour from textile wastewater with no additional physicochemical treatment.
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