Pergamon PII: S0043-1354(96)00318-1
War. Res. Vol. 31, No. 4, pp. 868-876, 1997 © 1997ElsevierScienceLtd All rights reserved. Printedin Great Britain 0043-1354/97$17.00+ 0.00
T R E A T M E N T OF TEXTILE W A S T E W A T E R BY C H E M I C A L METHODS FOR REUSE SHENG H. LIN*@ and MING L. CHEN Department of Chemical Engineering, Yuan Ze Institute of Technology, Neili, Taoyuan 320, Taiwan, ROC (First received November 1995; accepted in revised form September 1996)
Abstract--Treatment of wastewater effluent from the secondary wastewater treatment plant of a dyeing and finishing mill is investigated for possible reuse. The treatment system employed in the present study consists of an electrochemical method, chemical coagulation and ion exchange. The electrochemical method and chemical coagulation are intended primarily to remove color, turbidity (NTU) and COD concentration of the wastewater effluent while ion exchange is employed to further lower the COD concentration and reduce Fe ion concentration, conductivity and total hardness of the wastewater. To enhance the efficiency of electrochemical method, addition of a small amount of hydrogen peroxide is found to be highly beneficial. Experimental results throughout the present study have indicated that the combined chemical treatment methods are very effective and are capable of elevating the water quality of the treated waste effluent to the reuse standard of the textile industry. © 1997 Elsevier Science Ltd. All rights reserved Key words--textile wastewater, reuse, electrochemical method, chemical coagulation, ion exchange
INTRODUCTION
effluent. The dyestuffs are highly structured polymers and are very difficult to decompose biologically. The textile industry is one of those industries that Hence there is relatively little change of these dyestuff consume considerable amounts of water in the molecules in an activated sludge process (Gurnham, manufacturing process. The water is primarily 1965).A strong color of the wastewater effluent, if not employed in the dyeing and finishing operations in removed, would cause disturbance to the ecological which the cloths are dyed and processed to finished system of the receiving waters. Color removal by products. In a typical dyeing and finishing mill, about activated carbon, hydrogen peroxide (H202), sodium 100 1 of water are consumed on the average for every hyperchlorite (NaCIO) and other chemical agents has ton of cloths processed (Internal Technical Report, been widely practised in the textile industries 1994). The water employed in the dyeing and (Gurnham, 1965; Snider and Porter, 1974; McKay, finishing processes eventually ends up as wastewater 1984, 1990; Paprowicz and Slodczyk, 1988; Kuo, which needs to be treated before final discharge. 1992; Lin and Peng, 1995). However, the cost of Normal textile dyeing and finishing operations are polishing operation using these chemicals is high. such that the dyestuffs used in a mill can vary from Ozonation is a new technique that has been suggested day to day and sometimes even several times a day in the recent literature as a potential alternative for mainly because of the batchwise nature of the dyeing decolorization purpose (Beszedits, 1980; Green and process. Frequent changes of dyestuff employed in Sokol, 1985; Gould and Groff, 1987; Lin and Lin, the dyeing process cause considerable variation in the 1993; Lin and Liu, 1994a, b). wastewater characteristics, particularly the pH, color Traditional activated sludge and chemical methods and wastewater COD concentration (Gurnham, (such as chemical coagulation and decolorization) 1965). Large pH swing is especially troublesome have been widely used for dealing with the textile because the pH tolerance of conventional biological wastewaters (Gurnham, 1965). These methods are and chemical treatment systems is very limited. Hence intended to treat the textile wastewater to a level that without proper pH adjustment, normal operation of meets the discharge standards required by the the treatment processes is essentially impossible, government. However, due to dwindling water Strong color is another important component of supplies and increasing demand of the textile the textile wastewater which is very difficult to deal industries, a better alternative is to attempt to further with. Combination of strong color and high dissolved elevate the water quality of wastewater effluent from solid content results in high turbidity of the waste a secondary wastewater treatment plant to a higher standard for reuse. Thus far, very little attention has *Author to whom all correspondence should be addressed, been paid to this aspect. The present study represents 868
Textile wastewater treatment for reuse
869
Table I. Water quality change after various treatment steps Original Electrochemical Chemical Ion Reuse wastewater process coagulation exchange standard COD (mg I-~) 111 31.2 30.2 5 l0 COD removal(%) -71.9 72.8 95.5 -NTU 9.3 -0.9 0.6 l Conductivity 4850 5200 6020 10 100 Color removal(%) --97.3 100 -Hardness (mg 1-~) --61.7 4.2 10 Alkalinity(mg 1-L) 360 -176 4 50 Fe Coac. (mg 1-') 0.28 -0.25 0.05 0.1 TDS (rag 1-') 3610 -4390 10 50 SS (mg I-~) 26.4 -0 0 00 Note: Conductivityin/Jmho cm-~.
an attempt to utilize electrochemical methods in conjunction with chemical coagulation and ion exchange to treat wastewater effluent from the secondary wastewater treatment plant of a large dyeing and finishing mill. The water quality is enhanced to a level that meets the reuse standards of the industry. Experiments were conducted to examine the effects of various operating conditions on the water quality after treatment. To test the efficiency of each treatment unit employed in the present study, sample wastewater was obtained from a large dyeing and finishing mill in northern Taiwan. Currently, the mill purchases 60% of water it needs daily from the city water supply and the rest is obtained from groundwater. The groundwater is softened first using slacked lime, followed by filtering through a fine sand filter. It is then mixed with the city water supply. The water is finally passed through an anionic ion exchange bed to lower inorganic ion concentrations before it is used in the dyeing and finishing processes. According to the current practices of these industries (Internal
2
1
Technical Report, 1994), water employed in the processes should be totally free from any color and suspended solids (SS). Its conductivity must be kept below 100 #mho cm -~ and C O D concentration below 10 mg l-k The Fe ion concentration is maintained below 0.05 mg l -~ and the total hardness less than 10 mg l -~. The water quality requirements are listed in the last column in Table 1 (Internal Technical Report, 1994). MATERIALS AND METHODS The present experimental investigation consisted of three major parts: electrochemical treatment, chemical coagulation and ion exchange. The three treatment units can be operated independently in a batchwise fashion or in combined sequence. In the combined treatment mode, the wastewater effluent was first fed to the electrochemical unit. The effluent was then chemically treated using appropriate amounts of polyaluminum chloride (PAC) and polymer. After sedimentation, the wastewater effluent was finally treated by ion exchange. The electrochemical experimental apparatus is shown in Fig. I. Two pairs of anodic and cathodic electrodes (steel plates) are situated approximately 1.5 cm apart to each other and are dipped in the textile
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Fig. 1. Experimental setup of electrochemical process.
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S.H. Lin and M. L. Chen
wastewater. The total effective surface area of electrodes was 22.6 cm-'. The current input was controlled by an ammeter and the total power consumption was integrated and registered by a power integrator. In each run, approximately 11 of sample wastewater was placed in the electrolytic cell. In many test runs, a small amount of hydrogen peroxide (H_,O_,) was added to the electrolytic cell before the electrochemical treatment was started. Addition of H20~ was found to be highly beneficial for the present electrochemical process, as will be elaborated in detail later. During the electrochemical treatment of wastewater effluent, a small amount of iron was dissolved into the wastewater from the steel electrodes. The dissolved iron when combined with OH- or CI- would form many small fiocs during the electrochemical process which are very difficult to settle out of wastewater because of their small sizes. This phenomenon was observed very frequently in the present study. The presence of small fiocs in wastewater would interfere with the measurements of COD concentration, NTU and color removal. To overcome the floc settling problem, all samples were centrifuged for 5 min before the measurement was undertaken. In many test runs, chemical coagulation was adopted after the electrochemical treatment to determine its effect on the treatment efficiency. Chemical coagulation employing 100 mg I ~ of polyaluminum chloride (PAC) and 1 mg l-' of polymer was found to be rather effective and the effect of chemical coagulation will be further elaborated later, As mentioned earlier, the sample wastewater for the present study was obtained from the discharge effluent of the wastewater treatment plant of a large dyeing and finishing mill. The sample wastewater was first filtered using a screen filter to remove large suspended solids before the wastewater was used for the subsequent experiment. The initial pH of wastewater effluent in the electrolytic cell was adjusted to around 3 except for those cases noted. The present electrochemical treatment proceeded very rapidly because of low COD concentration (below 200 mg 1-') of the wastewater effluent, hence the treatment duration was limited to within l0 min. The COD concentration, NTU and total hardness of wastewater samples were determined by the standard methods (APHA, 1992). Its color was measured using a GBC 916 UV-Vis spectrophotometer (GBC Scientific Equipment, Ltd, Victoria, Australia) and the conductivity measured by Suntex conductivity meter (Suntex SC-12, Suntex Industrial Co., Taiwan). The UV absorbances of wastewater sample before and after each treatment were registered for determining the color removal, The Fe ion concentration was measured using a GBC 932 atomic absorption spectrophotometer (GBC Scientific Equipment, Ltd, Victoria, Australia). Other inorganic ions were determined by an ion chromatography (Dionex 2000, Dionex Corp., California, USA). It should be noted that except for the cases with consequent chemical coagulation, all samples for the electrochemical treatment were centrifuged before they were analyzed. This was a precautionary step to prevent the interference of small suspended flocs with the water quality measurements. The ion exchange resins employed here were the H-type Ambersep 132 and OH-type Ambersep 900 (both obtained from Rohm and Haas, Inc., Pennsylvania, USA). They were, respectively, strong acid cationic and strong base anionic ion exchange resins. The ion exchange resins needed to be pretreated before they were used in the experiments. They were first washed several times using deionized water, The resins were then immersed in ketone (normal type) to remove chemical impurities. They were washed using n-hexane. The resins were finally dried in an oven for over l h at 70°C and were put in a desiccator, The ion exchange experiments were conducted in a batchwise fashion. After the electrochemical treatment was completed, the pH of wastewater was adjusted to about 7 and the wastewater filtered using a glass-wool filter to
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remove all suspended solids so as to not cause interference with the ion exchange treatment process. If chemical coagulation was employed after electrochemical treatment, natural sedimentation was sufficient for removing flocs from wastewater. One liter of this wastewater effluent after sedimentation was put in a beaker and 20 g l-~ of the H-type and 40 g l-~ of the OH-type Ambersep ion exchange resins were added. The content in the beaker was maintained at a constant temperature and well mixed by a mixer at 300 rpm to minimize the external mass transfer resistance. Samples were taken periodically for measurements of COD concentration, NTU, color, conductivity and Fe ion concentration. The secondary wastewater treatment plant of the dyeing and finishing mill, from where the wastewater sample for the present study was obtained, employed pH equalization, chemical coagulation, activated sludge, decolorization and post-biofiltration steps in the treatment process. The COD concentration of the wastewater effluent from the wastewater treatment plant was consistently below 200 mg I-' and the transparency was above 20 cm with SS below l0 mg I ~. These water quality parameters were significantly better than the discharge standards required by the government for the textile industry. However, the wastewater effluent was still faintly colored and the conductivity rather high, being over 4500/~mho cm-'. This high conductivity was caused by the fact that NaOH, other inorganic compounds and surfactants were used in various steps of the dyeing and finishing process. Although the majority of sodium hydroxide was recovered for reuse, a significant amount of NaOH and other inorganic compounds still remained in the wastewater. The conductivity of treated wastewater needs to be reduced by ion exchange to meet the reuse requirement.
RESULTS AND DISCUSSION The initial p H o f wastewater effluent is an important operating factor influencing the performance characteristics o f electrochemical process. To d e m o n s t r a t e its effect o n the treatment efficiency, the p H o f wastewater effluent was adjusted to between 2 and 9. Figure 2 shows the final percent C O D and color removal and the N T U change o f wastewater after electrochemical treatment for 5 m i n as a function o f the initial pH. It is apparent that there exists an optimal p H around 3. Deviation from p H
Textile wastewater treatment for reuse 3 incurs a significant reduction in the COD and color removal and a sharp increase in the wastewater NTU. The optimal pH is coincidentally in line with that of textile wastewater treatment by Fenton's reagent (Kuo, 1992; Lin and Pen, 1995). Such a similarity of optimal pH is understandable because the Fe ion (dissolved from the steel plate electrodes) and H202 provided by the present electrochemical process have the same oxidation power as that provided by Fenton's reagent. Hence, all subsequent electrochemical experimental runs were conducted at this optimal pH except for those cases as noted. The initial COD concentration for the particular case shown in Fig. 2 was 94 mg I-L After 5 min of electrochemical treatment at pH 3, the COD concentration was lowered to 2 mg 1-~ and the wastewater was completely colorless. The general water quality of the treated wastewater effluent was excellent except for the conductivity which remained around 47 #mho cm-'. The fact that an optimal initial pH is present in the electrochemical process as demonstrated above, is also reflected by the amount of residual H202 that remained in the wastewater and the power consumption as shown in Fig. 3. At the optimal initial pH 3, essentially all H202 was consumed in just 5 min of electrochemical treatment. As the initial pH deviated from the optimal value, a small amount of residual H202 started to shew up at the end of electrochemical treatment, it is noted in this figure that the power consumption varies only slightly with the initial pH. However, the final pH of the treated wastewater effluent was seen to increase with an increase in the initial pH and the difference between the final and initial pHs varies between 0.5 and 1.5. Hydrogen pero:dde was found to significantly enhance the efficiency of the electrochemical treatment process in terms of COD and color removal, as demonstrated in Figs 4 and 5, respectively. The COD ~o
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removal is seen in Fig. 4 to be drastically improved from 33% without H202 to 78% with an addition of 100mg 1-~ H202. In other words, the COD concentration of wastewater effluent was reduced from the initial 190 mg 1-' to 127 mg 1-~ for the case without H202 while with an addition of 100 mg 1-~ H202 the final COD concentration was lowered to 42 mg I- ' after electrochemical treatment. Certainly both effluent COD concentrations are not up to the reuse standard and further treatment of wastewater effluent is necessary. It is also observed here that the case without H202 requires slightly less than 3 min to reach the steady state COD concentration which was about one half the time for the case with 100 mg 1-~ H202 addition to reach the final steady state COD. The effect of H202 addition on color removal is seen in Fig. 5 to be much milder than that on COD removal. Without H202, the color removal of electrochemical process reached 83%, but with H:O2, the color removal was elevated to 92%, a less than 10% enhancement. In fact, at this level of color removal, the difference in wastewater color for both
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cases was not detectable by the naked eyes. However, as seen in Fig. 5, the time to reach a steady state color removal for the case with H202 addition is shortened in comparison with that without H202 which is contrary to the observed effect of H202 addition on the time to reach a steady state C O D . One possible explanation is that discolorization is a more rapid process than oxidation of organic compounds, The efficiency enhancement of electrochemical process by addition of a small amount of H2Oz was because of presence of iron dissolved from the electrodes, as observed above. The combined effects of H202 and Fe ion are similar to that of Fenton's reagent. However, an effective Fenton process requires a certain ratio of H20: and Fe ion concentrations (Kuo, 1992; Lin and Peng, 1995). A question raised here is whether the Fe ion was present in sufficient quantity in the electrochemical process to meet the H202/Fe ratio requirement of Fenton's reagent. To address this issue, test runs of electrochemical experiment were performed by using both H202 and H202/FeSO4 (Fenton's reagent). The results are demonstrated in Figs 6 and 7. It is apparent that the effect of addition of Fenton's reagent on color removal and N T U in Fig. 6 and the pH and power consumption in Fig. 7 is not much different from that using H202 alone. However, the difference of the H202 and H20:/FeSO4 effects on C O D removal is rather appreciable, especially at low reaction time. Such a difference could be related to the H202 consumption as shown in Fig. 7. In the presence of FeSO4, H202 consumption sped up because significantly more H202 was converted to O H - which is a strong oxidant for the C O D removal. Figure 8 illustrates the transient profiles of C O D removal and N T U for a given H20: concentration of 1 5 0 m g l - ' . It is noted that the C O D removal increases rather rapidly within 3 min of electrochemicai treatment and the wastewater N T U stays very low. After that period, the C O D removal levels
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off and surprisingly there is a rapid increase in N T U . The transition occurs in concurrence with complete consumption of H202 in the electrochemical process. In other words, when only 150 m g l - ' of H202 is employed, the electrochemical treatment cannot last more than 3 min because of rapid worsening of water quality after that time duration. Figure 9 displays the color and C O D removal and N T U of the treated wastewater versus the amount of H202. The improvement on C O D and color removal and N T U for H20: beyond 200 mg 1-' is seen to be rather limited. However, in light of low C O D concentration and relatively stable water quality of wastewater effluent from the secondary wastewater treatment plant, an amount of HEO2---no more than 200 mg l - ' - - i s deemed more than sufficient to assure good treatment results of electrochemical process. The effect of current input on the C O D removal is illustrated in Fig. 10. Figure 1 0 s h o w s a rapid increase 100
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Fig. 11. The NTU change as a function of free settling time with 2.5 A current, 200 mg i-~ H202, pH 3 and 5 min of electrochemical treatment.
in COD removal and a rapid decrease in reaction time as the current is increased from 2 to 2.5 A. The reaction time is defined here as the time for complete H20: depletion in electrochemical process. The rapid improvement in COD removal is seen to be achieved by only a slow increase in power consumption. This is the reason that 2.5 A current input was adopted in most of the present electrochemical treatment runs. As shown in Fig. 7, complete H202 depletion was accompanied by ~: rapid rise in the final pH of wastewater. Hence an approximate reaction time for complete H202 depletion could be relatively easily detected in the experimental runs. Such a reaction time serves the practical purpose of terminating the electrochemical treatment, After the electrochemical treatment, a large number of small flocs were generated which took rather long to settle out of wastewater because of their small size. Hence centrifugation was applied to
the samples to prevent those small flocs from interfering with the water quality measurements. To demonstrate how long it takes for the small flocs to settle out, samples were taken at various times after the electrochemical treatment was finished. The samples were employed to determine their respective NTU without centrifugation. Figure 11 shows the NTU ofwastewater without centrifugation versus the sampling (or settling) time. It took approximately 12 h for the NTU to drop below 2. A parallel test run was also performed with which the samples taken at various sampling time were centrifuged for 5 min first and then their NTUs were determined. Surprisingly enough, a similar decrease trend of NTU versus the sampling time was observed. The only difference in the NTU results between these two runs was that the NTU level of the latter run was lower, verifying rather long settling time of small flocs. To overcome the floe settling problem of the electrochemically treated wastewater, chemical coagulation using proper amounts of polyaluminum chloride (PAC) and polymer might help. Figure 12 demonstrates the effect of various amounts of PAC on the NTU of the treated wastewater. The effect of chemical coagulation is observed to be quite drastic. After chemical coagulation, the wastewater NTU was reduced from an initial value above 50 to below 2 in just 10 min. The best results are obtained by using 100 mg 1-I
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PAC. For practical purposes, a settling timecoagu.Of 30 min would be sufficient for all chemical lation runs with 100 mg 1-~ PAC. But for those runs with a lower PAC concentration below 100 mg 1-', it takes significantly longer to achieve a similar NTU result. To put the effect of PAC concentration on the water quality in better perspective, the COD and color removal and NTU of wastewater after chemical coagulation treatment and settling for 60 min is displayed in Figure 13. The improvement in the water quality with an increase in the PAC concentration is seen to be marginal. But in light of its short settling
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Fig. 14. Effect of the amount of cationic ion exchange resin on the wastewater conductivity with 2.5 A current, 200 m g I-' H:O2, p H 3, 5 rain of electrochemical treatment and 40 g l- ~ of anionic ion exchange resin.
time, an optimal PAC concentration of 100 mg !-' can be recommended for practical purposes, In spite of the fact that the water quality of wastewater is considerably improved by the electrochemical process and chemical coagulation, the COD concentration, hardness, conductivity and Fe ion concentration usually fall far short of the reuse standard. The conductivity of wastewater after electrochemical treatment was ooserved to be consistently above 4500/~mho cm-~ which exceeds the reuse standard of 500/~mho c m - ' by a wide margin. Such a high conductivity of wastewater after electrochemical treatment indicates that the wastewater still contains a significant amount of inorganic ions. To remove the salt ions and other impurities, both cationic (Ambersep 132) and anionic (Ambersep 900) ion exchange resins were employed. As the ion exchange experiments were conducted in a batch fashion, it is important to determine the amount of
ion exchange resins for best results in terms of low conductivity, Fe ion and COD concentrations of the wastewater. Figure 14 shows the conductivity of treated wastewater versus the amount of Ambersep 132 for a fixed amount (40 g l -t) of Ambersep 900. To meet the reuse standard, a minimum amount of 20 g l - ' cationic ion exchange resin and an ion exchange treatment of 4 min would be required. As the amount of cationic ion exchange resin was increased to 25 g 1-~, the ion exchange treatment time was shortened to less than 3 min. For a fixed ratio of anionic to cationic ion exchange resins of 2:1, Fig. 15 demonstrates the effect of the amount of Ambersep 900 on the conductivity of the aqueous solution. The reuse standard can not be met for Ambersep 900 less than 40 g !-'. It should be noted that the optimal ratio of anionic to cationic ion exchange resins of 2:l
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an ion chromatography and are listed in Table 2. Three major inorganic ions, i.e. CI-, NO;- and SO;, were detected in the treated wastewater. Also listed in this table for comparison are the inorganic ion concentrations in the deionized water. The inorganic ion concentrations measured in the treated wastewater after ion exchange are amazingly close to those in the deionized water. They are seen to be several orders of magnitude lower than those before ion exchange, demonstrating the high efficiency of the last treatment step in removing the inorganic compounds.
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Fig. 16. The wastewater pH change as a function of the amount of cationic ion exchange resin and treatment time with 2.5 A current, 200 mg 1-' H202, pH 3, 5 min of electrochemical treatment and 40 g 1-~ of anionic ion exchange resin,
is much in line with that recommended by Jiang (1990). Pertaining to Fig. 14, the pH change of the aqueous solution is demonstrated in Fig. 16. For a fixed amount of 40 g 1-' Ambersep 900, the final pH of the aqueous solution increases significantly with a decrease in the amount of Ambersep 132. To maintain the final treated wastewater as neutral as possible, at least 20 g I-L of Ambersep 132 is seen to be necessary, The total hardness and Fe ion concentration are two important parameters of water quality requirement for reuse. The Fe ion concentration of wastewater effluent from the secondary wastewater treatment plant was observed to be between 0.2 and 0.4 mg 1-~. The Fe ion concentration was elevated to near 1 mg 1-1 after electrochemical treatment, as seen in Table 1 for a typical run. The increase in Fe ion concentration was d~ueto iron dissolved from the steel electrodes during electrochemical process. But chemical coagulation and ion exchange were able to lower the Fe ion concentration to below the reuse standard of 0.1 mg 1-'. The l:otal hardness of wastewater was measured to fluctuate between 50 and 75 mg 1-' and it did not appear to change much through the electrochemical treatment and chemical coagulation, However, after ion exchange treatment, the total hardness was effectively reduced to lower than the 10 mg 1-t reuse standard. Other inorganic ions in the treated wastewater before and after ion exchange were measured using Table 2. Inorganicion concentrationsin the treated wastewater CI- NO~- SOz Treated wastewater befc,reion exchange 834 54 2385 Treated wastewaterafter ion exchange 0.98 0.24 1.15 Deionizedwater 0.85 0.25 0.93 Note: All concentrationsin mg I-'.
Wastewater effluent from the secondary wastewater treatment plant of a dyeing and finishing mill is chemically treated for possible reuse. The treatment system consists of electrochemical, chemical coagulation and ion exchange processes. The electrochemical and chemical coagulation methods are found to effectively reduce the color, turbidity (NTU) and COD concentration of wastewater effluent while the ion exchange is capable of further lowering the COD concentration, conductivity, Fe concentration and total hardness to the reuse standard. Addition of a small amount of hydrogen peroxide in the magnitude of about 200 mg 1-~ has been found to elevate the efficiency of electrochemical treatment process by as much as 100%. The electrochemical treatment was observed to generate a large number of small flocs which could cause settling problem. Such a problem was effectively resolved by chemical coagulation using 100 mgl -~ PAC and 1 m g l - ' polymer. To further elevate the water quality to the reuse standard, ion exchange using cationic and anionic ion exchange resins was found necessary. Experimental results clearly indicate that a ratio of cationic to anionic ion exchange resins of 2 would be optimal for such an operation. Furthermore, 40 g 1-' of cationic ion exchange resin would be required to bring the water quality of waste effluent to the reuse level. The water quality of the treated waste effluent was excellent, comparable to that of deionized water. Acknowledgements--The authors are grateful to the National Science Council, Taiwan, ROC for the financial support (under the grant NSC85-2621-P155-001) of this project.
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