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Biological treatment of wastewater containing dimethyl sulphoxide from the semi-conductor industry
Process Biochemistry 36 (2001) 579 – 589 www.elsevier.com/locate/procbio
Biological treatment of wastewater containing dimethyl sulphoxide from the semi-conductor industry Se-Jin Park a, Tai-Il Yoon a,*, Jae-Ho Bae a, Hyung-Joon Seo a, Hyo-Jung Park b a
Department of En6ironmental. Engineering, Inha Uni6ersity c 253, Yonghyun-Dong, Namgu, Inchon, 402 -751, South Korea b Samsung Electronics Co. Ltd. cSan 24, Nongseo-Ri, Kihung-Up, Yong-in-Si, Kyung-gi, 449 -900, South Korea Received 28 May 2000; received in revised form 26 September 2000; accepted 3 October 2000
Abstract Wastewater containing dimethyl sulphoxide (DMSO), a widely used organic solvent in the semi-conductor industry, is usually classified as an industrial waste requiring high-cost treatment. This study was conducted to evaluate the feasibility of the biological treatment of DMSO wastewater with activated sludge (AS). The optimum conditions for Fenton treatment were also investigated. The optimum chemical dosage of H2O2: Fe2 + for Fenton treatment was 1000:1000 mg/l for wastewater containing 800 mg/l of DMSO. Although TOC and COD removal efficiencies by Fenton treatment were not satisfactory for most applications, the BOD/COD ratio was increased from 0.035 to 0.87, suggesting it as a very useful pretreatment method for biological treatment. Wastewater containing 800 mg/l of DMSO was treated successfully by AS without Fenton pretreatment, after 20 days acclimation period. Fenton pretreatment or pre-acclimation with easily biodegradable organics did not significantly reduce the acclimation period. Average removal efficiencies of TOC, SCOD, and SBOD by AS at an HRT of 24 h (loading rate of 0.8 kg DMSO/m3-day) were 90%, 87%, and 63%, respectively. Most of the sulphur in DMSO was oxidized to sulphate, eliminating the possibility of the production of sulphide-containing noxious intermediates. For 3500 mg/l of DPS-1300 wastewater containing 1925 mg/l of DMSO, satisfactory effluent qualities were obtained by AS at an HRT of 72 h (loading rate of 0.64 kg DMSO/m3-day). Control of pH was an important operating factor for AS operation as protons are produced as a final product of DMSO degradation. Results indicated that DMSO wastewater can be successfully treated with AS, which may significantly reduce the treatment cost compared to the chemical methods currently used. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dimethyl sulphoxide; Fenton treatment; Biological treatment; Acclimation; Activated sludge; Wastewater
1. Introduction Dimethyl sulphoxide (DMSO) is an organic solvent, which is colourless, highly hygroscopic, thermally and chemically stable, and has strong solvency for both Abbre6iations: AOP, advanced oxidation process; AS, activated sludge; BOD, 5 day biochemical oxygen demand; COD, chemical oxygen demand; DMS, dimethylsulphide (CH3)2S; DMSO, dimethylsulphoxide (CH3)2SO; HRT, hydraulic retention time; MLSS, mixed liquor suspended solids; MLVSS, mixed liquor volatile suspended solids; ThOD, theoretical oxygen demand; TOC, total organic carbon; SBOD, soluble 5 day biochemical oxygen demand; SCOD, soluble chemical oxygen demand; WWTP, wastewater treatment plant. * Corresponding author. Tel.: + 82-32-860500; fax: + 82-328679919. E-mail address: [email protected] (T.-I. Yoon).
organic and inorganic compounds. It has been widely used in the manufacture of electronics, polymers, dyes, membranes, etc. In nature, DMSO is an intermediate in the global sulphur cycle produced from photo-oxidation of dimethyl sulphide (DMS) [1]. It is also believed to play an important climatic role in the formation of marine tropospheric aerosol and cloud condensation nuclei [2]. Reported DMSO concentrations are 220 nM for seawater and 1–70 nM for fresh water [3]. DPS1300, which contains 55% DMSO, 30% ethyl digol (C6H14O3) and 15% monoethanolamine (C2H7NO) by
Fig. 1. Proposed biotic degradation pathway of DMSO.
0032-9592/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 5 2 - 1
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Table 1 Experimental condition for Fenton treatment to find the optimum conditions Wastewater
Variables examined
Standard conditions
DMSO (300 or 800 mg/l)
Reaction pH 16 Coagulation pH 311 Reagent dosage Fe2+ and H2O2 = 5002,000 mg/l Reagent mass ratio Fe2+/H2O2 = 0.54 Reaction time 3120 min. Reagent dosage Fe2+ and H2O2 =2502,000 mg/l
Fe2+:H2O2 =
1000 mg/l: 1000 mg/l
Reaction
pH: 3 Time: 12 hr at 200 rpm pH: 7 Time: 20 min. at 60 rpm Anionic polymer: 3 mg/l Time: 30 min.
DPS-1300 (1000 or 2000 mg/l)
mass, is also frequently used as an alternative chemical for DMSO because of its low cost. DMSO is not as toxic as chlorinated solvents. It is collected and handled separately from other wastewater due to its low treatability, resulting in higher treatment costs. Biological treatment processes have often experienced operational problems with intermittently produced DMSO waste. When H2O2, a common waste in the semi-conductor industry, is available, several advanced oxidation process (AOP) such as Fenton treatment, ozonation with H2O2, or UV combined with H2O2, can be applied for the treatment of DMSO [4]. Koito et al. [5] recently suggested Fenton treatment with UV irradiation as an efficient treatment method for DMSO. Although AOP generally can produce high quality effluent, the high costs of equipment, chemicals, electricity, and sludge disposal diminish the advantages of these processes. Furthermore, complete treatment of concentrated DMSO wastewater cannot usually be achieved by AOP alone. In this case, biological treatment should be considered first, if feasible, and AOP can be used as the pretreatment, or as post-treatment after the biological treatment. Studies on biological degradation of DMSO revealed that most microorganisms can use it as a carbon and energy source [6]. A proposed degradation pathway of DMSO is depicted in Fig. 1 [7]. DMSO reduction to DMS is mediated not only by microorganisms, but also by animals and plants [8]. The breakdown of DMS produces 2 mol of formaldehyde and 1 mol of sulphide. Intermediate products such as DMS, methyl mercaptan, and hydrogen sulphide cause odour problems, and especially in the case of hydrogen sulphide, are toxic to humans. Finally, formaldehyde is converted to CO2 or used for cell synthesis, and sulphide is oxidized to sulphate. The fact that the reduction of DMSO to DMS is required for the initiation of degradation led to the misconception that both anaerobic and aerobic biological processes are required for the complete treatment of DMSO [5]. Due to this belief, together with the possibility of bad odours often associated with anaerobic
Coagulation
Settling
treatment systems, biological treatment of DMSO containing wastewater has never been considered as a practical method. However, as indicated by several investigators [8–11], some aerobes can reduce DMSO. Zinder and Brock [8] speculated that the reduction of DMSO by aerobes may not provide any physiological benefit as DMSO is an electron acceptor. Also, Sklorz and Binert [10] suggested that the DMSO reduction method could be used for the estimation of the activity of activated sludge. This study was carried out to test the feasibility of aerobic biological treatment of DMSO containing wastewater generated from a semi-conductor manufacturing factory. First, analytical characteristics of DMSO with conventional water quality parameters were evaluated. Second, the optimum conditions for Fenton treatment of DMSO and DPS-1300 as a pretreatment for biological treatment were evaluated. Finally, aerobic biological treatabilities of DMSO and DPS-1300 were evaluated by the activated sludge process (AS). Here, the optimum operating conditions of AS were investigated to provide basic information for the design of the treatment process.
2. Materials and methods The optimum conditions for Fenton treatment as a pre-treatment for biological treatment or as a sole treatment process were investigated for both wastewaters containing DMSO and DPS-1300. A jar-test apparatus was used for Fenton treatment. The pH was adjusted to the desired value 9 0.02 with sulphuric acid or sodium hydroxide addition. Stock solutions of H2O2 (5%) and FeSO4·7H2O (5% as Fe2 + ) were used as reagents. After the addition of the desired amount of the reagents, samples were mixed rapidly at 200 rpm for 1–2 h. For the coagulation of the Fenton treated sample, the pH was adjusted to the desired value (normally 7), and then mixed slowly at 60 rpm for 20 min with the addition of 3 mg/l of anionic polymer (SA-307, Songwon Co.).
S.-J. Park et al. / Process Biochemistry 36 (2001) 579–589
Table 1 summarizes the experimental conditions for Fenton treatment of wastewaters containing DMSO or DPS-1300. Artificial wastewaters containing 300 or 800 mg/l of DMSO were used to find the optimum pH (for both Fenton reaction and coagulation), reagent dosage, and reaction time. All variables were set as standard conditions except for the tested variable. Effect of Fenton reaction pH was evaluated at pH between 1 and 6. The optimum reagent dosage was first determined by changing the amount of reagent from 500 to 2000 mg/l at a fixed Fe2 + /H2O2 mass ratio of 1. The mass ratio of Fe2 + /H2O2 changed from 0.5 to 4 at the fixed H2O2 concentration of 1000 mg/l. Changes in treatment efficiency during Fenton treatment were also evaluated to determine the optimum reaction time. For DPS-1300, only the optimum Fenton reagent dosage was determined for wastewater containing 1000 and 2000 mg/l of DPS at 1300 mg/l with the standard conditions listed in Table 1. All analysis was made with the supernatants after 30 min of sedimentation following coagulation. The aerobic biological treatability of 800 mg/l DMSO was tested with AS. A reactor with a 4.9 l aeration basin and a 2.9 l settling tank was seeded with the sludge from a domestic wastewater treatment plant, and was operated at 209 2°C. The dissolved oxygen concentration (DO) in AS was controlled at 2.5–3.5 mg/l. Mixed liquor volatile suspended solids (MLVSS) were maintained at 2300 mg/l, and the influent pH was kept between 7.5 and 8.2 with bicarbonate buffer. Operation of AS was divided into six phases, as summarized in Table 2. To evaluate the effect of Fenton pretreatment on biological treatment with AS, the effluent characteristics of AS were determined with Fenton pretreatment (Phase I and II) and without Fenton pretreatment (Phase III to VI). From Phase III to Phase VI, the DMSO loading rate was changed from 0.4 to 1.6 kg/m3-day to determine the maximum loading rate. The necessity of pre-acclimation for the direct treatment of DMSO by AS under two different conditions: with and without pre-acclimation of AS with easily degradable organics was investigated. One AS reactor
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received wastewater containing 400 mg/l of DMSO, 0.09 mM monosodium glutamate, 0.28 mM ammonium acetate and 0.028 mM glucose. After reaching steadystate, the influent containing only 800 mg/l of DMSO was fed in, and the effluent quality was measured. The other AS reactor was fed with wastewater containing only 800 mg/l of DMSO from the beginning of the operation. Both reactors were filled with fresh sludge from WWTP, and operated at a hydraulic retention time (HRT) of 48 h. As the BOD/COD ratio of DPS-1300 containing wastewater is known to be high, biological treatibility was tested without Fenton pretreatment. The AS operating conditions were divided into six phases according to the influent DPS-1300 concentration and HRT, as summarized in Table 3. DO concentration in AS was maintained at 2.5–3.5 mg/l. The treatability of DPS1300 was tested at different DMSO loading rates from 0.275 to 0.963 kg/m3-day, and the effect of an effluent recycle was also examined. Here, the MLVSS concentration was kept at 3400 mg/l. COD (closed reflux method), BOD, pH, alkalinity, VSS, H2O2 (iodometric method) were measured by procedures described in Standard Methods [12]. Soluble COD (SCOD) and soluble BOD (SBOD) were measured after filtration (0.45 mm filter paper), and used to evaluate the treatment efficiency of AS. TOC was measured with a Shimadzu TOC-5000 after filtration. DMSO concentration was measured with a GC (Hewlett Packard 5890, FID, capillary HP-624). However, the detection limit of DMSO was relatively high (10 mg/l) with this method. Sulphate concentration was measured with IC (Dionex 500, USA).
3. Results and discussion
3.1. Characteristics of DMSO containing wastewater To evaluate the analytical characteristics and the biological treatability, TOC, COD, and BOD were measured for DMSO and DPS-1300 at several concen-
Table 2 Operation conditions of activated sludge reactor for the treatment of DMSO Phase I Fenton (pretreated)
Phase II Fenton (pretreated)
Phase III DMSO (800 mg/l)
Phase IV DMSO (800 mg/l)
Phase V DMSO (800 mg/l)
Phase VI DMSO (800 mg/l)
COD
145
145
175
175
175
175
TOC pH
100 7.58.2 48 –
100 7.58.2 24 –
140 7.57.9 48 0.4
140 7.57.9 24 0.8
140 7.57.9 12 1.6
140 7.57.9 24 0.8
Influent type
Influent Conc. (mg/l)
HRT(hrs) DMSO loading (kg/m3 day)
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Table 3 Operation conditions of activated sludge reactor for the treatment of DPS-1300 Phase
Phase I
Phase II
Phase IIIa
Phase IVa
Phase V
Phase VI
Influent DPS-1300 Concentration (mg/l)
1000
1000
2000
3500
3500
3500
Influent Conc. (mg/l)
726 210 9.810.2 48 0.275
726 210 9.810.2 24 0.55
1440 212 10.110.5 48b 0.55
1440 360 10.6 72b 0.64
2760 570 10.6 48 0.963
2760 570 10.6 72 0.64
COD TOC pH
HRT (h) DMSO loading (kg DMSO/m3 - day) a b
Effluent was recycle at the recycle ratio of one, and therefore influent DPS-1300 conc. is about half of the value listed. HRT was calculated based on the wastewater flow rate to the treatment system, i.e. excluding the recycled flow rate.
trations, and compared to the theoretical oxygen demand (ThOD). As illustrated in Fig. 2, COD and TOC values were proportional to DMSO concentrations. However, the calculated COD/DMSO ratio of 0.22 represents only 12% of the ThOD/DMSO ratio of 1.85. This result indicates that COD is not a good parameter to represent DMSO concentration. Measured BOD values were very low, and the BOD/COD ratio decreased as the DMSO concentration increased. The highest BOD/COD ratio of 0.035 indicates that DMSO is non-biodegradable according to Simons’ classification [13]. TOC was the best parameter among the three parameters as the measured TOC/DMSO ratio was 60% of the theoretical TOC value of 0.3 g TOC/g DMSO. Measured values of TOC, COD, and BOD for wastewater containing DPS-1300 are illustrated in Fig. 3. The COD/DPS-1300 ratio of 0.78 represents 52% of the ThOD/DPS-1300 ratio of 1.49, and the observed BOD/COD ratio was 0.48. The high values of the COD/ThOD and BOD/COD ratios compared to the DMSO solution may have resulted from the easily biodegradable components of DPS-1300, such as ethyl digol and monoethanol amine. From the measured BOD/COD ratio, DPS-1300 can be characterized as biodegradable [13]. The measured TOC/DPS-1300 ratio of 0.18 is equivalent to 46% of the theoretical TOC value of 0.39 g TOC/g DPS-1300.
The optimum pH for coagulation was determined for 800 mg/l of DMSO solution with the Fenton reaction pH of 3, and results are illustrated in Fig. 5. The COD after Fenton treatment was higher than that before treatment when the coagulation pH was below 6. This increase may be explained by the remaining H2O2 with incomplete coagulation. For example, residual H2O2 concentrations were 306–567 mg/l at coagulation pH below 6. At coagulation pH higher than 7, removal efficiencies of COD and TOC were constant, and the values were 21% and 25%, respectively. As BOD (about 90 mg/l) was also not affected by coagulation pH, the BOD/COD ratio increased from 0.62 to 0.71 as the coagulation pH increased. This results suggests that a pH of 7 is recommended for the coagulation pH for Fenton treatment. The optimum dosage of Fenton reagent was obtained by changing both the total amount and the mass ratio of Fe2 + /H2O2 with 800 mg/l of DMSO at a reaction pH of 3 and a coagulation pH of 7. First, the amount of Fenton reagent was changed from 500 to 2000 mg/l at the standard Fe2 + /H2O2 mass ratio of one. As shown in Fig. 6a, the optimum amounts of both reagents were estimated as 1000 mg/l considering the removal efficiencies of TOC and COD and BOD/COD ratio after treatment. Next, the concentration of Fe2 +
3.2. Optimum conditions for Fenton treatment of DMSO and DPS-1300 The optimum pH for Fenton treatment of 300 mg/l DMSO was found to be three, based on TOC and COD removal efficiencies which were 37.1% and 57%, respectively, as illustrated in Fig. 4. The relatively large variation in COD removal as a function of pH compared to that of TOC might have resulted from the differing residual H2O2 concentrations, which can be measured as COD. The residual H2O2 concentrations was 3080 mg/l, and was the lowest at pH 3.
Fig. 2. TOC, COD and BOD values of DMSO solution at various concentrations.
S.-J. Park et al. / Process Biochemistry 36 (2001) 579–589
Fig. 3. TOC, COD, and BOD values of DPS-1300 solution at various concentrations.
was changed at the H2O2 dosage of 1000 mg/l with the standard conditions used before. Fig. 6b illustrates that the Fe2 + /H2O2 mass ratio between one and two is the optimum considering TOC and COD removal efficiencies. Therefore, the ratio of one would be the optimum considering the increased sludge production at higher ratios. The optimum Fenton reaction time was determined using the pre-determined optimum reaction pH (3), coagulation pH (7) and chemical dosage (both Fe2 + and H2O2 of 1,000 mg/l). As illustrated in Fig. 7, TOC and COD decreased for the first 5 and 15 min, respectively, and then both TOC and COD stayed constant. However, the BOD increased steadily for 120 min, and as a result, the BOD/COD ratio increased from 0.035 to 0.87. Concentrations of H2O2 and DMSO also decreased significantly within the first 5 min (Fig. 7b). The H2O2 concentration decreased from 1000 mg/l to 145 mg/l at t= 5 min, then decreased slowly thereafter. The DMSO concentration was 50.3 mg/l after 5 min, and
Fig. 4. Effect of reaction pH on the efficiency of Fenton treatment for 300 mg/l of DMSO.
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Fig. 5. Effect of coagulation pH on the efficiency of Fenton treatment for 800 mg/l of DMSO.
then finally decreased to 31.2 mg/l after 120 min. Therefore, at least 1 h of reaction time is necessary for Fenton treatment of DMSO to prevent any negative effects of residual H2O2.
Fig. 6. Effect of reagent dosage on the efficiency of Fenton treatment for 800 mg/l of DMSO; (a) at Fe2 + /H2O2 ratio of 1, and (b) at H2O2 dosage of 1000 mg/l.
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increasing reagent dosage was not significant. The TOC and COD removal efficiencies were maximized up to 25% and 41%, respectively, at the highest reagent dosage. Similar results were obtained for 2000 mg/l of DPS-1300 wastewater. Considering chemical costs, low TOC and COD removal efficiencies together with a relatively high BOD/COD ratio of DPS-1300 (0.48) without treatment, the use of Fenton treatment as a pretreatment for biological treatment may be uneconomical.
3.3. Biological treatability of DMSO with and without Fenton pre-treatment The biological treatability of wastewater containing DMSO (800 mg/l) was evaluated using AS with (Phase I and II) and without (Phase III to VI) Fenton pretreatment. As illustrated in Fig. 9a, a period of approximate 10 days was required for AS to acclimate to Fenton pre-treated DMSO wastewater. After the acclimation, the average effluent concentrations of TOC, SCOD, and SBOD were 12.8, 10.2, and 1.0 mg/l, respectively (Phase I). The corresponding removal effi-
Fig. 7. Trends in water quality parameters during Fenton treatment for 800 mg/l of DMSO; (a) TOC, COD, and BOD/COD ratio, and (b) H2O2 and DMSO.
The optimum conditions of Fenton treatment for wastewater containing 800 mg/l DMSO can be summarized as follows: reaction time= 1 – 2 h, reaction pH 3, coagulation pH 7, reagent dosage of both Fe(II) and H2O2 = 1000 mg/l. At these conditions, removal efficiencies of TOC and COD were 26% and 49% respectively, and BOD/COD ratio increased from 0.035 to 0.89. The low TOC and COD removal efficiencies indicate that the low molecular weight intermediate products produced during Fenton treatment was not complete. In sum, Fenton treatment alone cannot be used for the treatment of concentrated DMSO although it may be used as a pretreatment method to increase the biological treatment efficiency. Results of Fenton treatment for wastewater containing 1000 mg/l of DPS-1300 are illustrated in Fig. 8. The BOD value of the treated water increased with the increased reagent dosage, and as a result, the BOD/ COD ratio increased to 0.88. The DMSO concentration also decreased from 550 mg/l before treatment to 32 mg/l at Fe2 + and H2O2 dosages of 1500 mg/l. The COD removal efficiency increased linearly with reagent dosage. The change in TOC removal efficiency with
Fig. 8. Effect of Fenton reagent dosage on the treatment efficiency of DPS-1300; (a) 1000 mg/l of DPS-1300, and (b) 2000 mg/l of DPS1300.
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Fig. 9. Results of DMSO treatment by activated sludge with and without Fenton pre-treatment; (a) TOC, SCOD and SBOD, (b) pH and alkalinity, and (c) sulphur concentration.
ciencies of TOC, SCOD, and SBOD were 88, 93, and 99%, respectively. When the HRT was reduced from 48 to 24 h (Phase II), SCOD was increased from 1.0 to 6.8 mg/l, but no other significant changes in effluent characteristics were observed. Therefore, 800 mg/l of DMSO wastewater with Fenton pre-treatment was treated successfully by AS at an HRT of 24 h. During Phase III to VI, DMSO was fed directly to AS without Fenton pre-treatment. After 8 days of operation at HRT of 48 h in Phase III (loading rate of 0.4 kg DMSO/m3-day), the AS process appeared to reach steady-state. This short stabilization period might have been achieved because the microorganisms were already acclimated to low concentrations of DMSO during Phase I and II. After stabilization, the average concentrations of effluent SBOD and TOC reached the levels observed during Phase II. The slight increase in effluent SCOD might have been due to the increase in influent SCOD without Fenton pretreatment. During Phase IV, the DMSO loading rate was doubled (0.8 kg DMSO/m3-day) by reducing the HRT from 48 to 24 h. No significant changes in effluent characteristics were observed for 25 days after the change. During this period, average values of effluent TOC, SCOD, and SBOD were 14, 22, and 3 mg/l, respectively. The effluent TCOD and TBOD were slightly higher than SCOD and SBOD (data not shown). The deterioration
of effluent quality during the latter half of Phase IV may be related to pH shock, which will be discussed later. When the loading rate was further increased to 1.6 kg DMSO/m3-day (Phase V) by decreasing the HRT to 12 h, the effluent TOC, SCOD, and SBOD remained low for 10 days, but reached or exceeded the influent values after 15 days of operation, possibly due to the high DMSO loading rate. The fact that effluent SBOD far exceeded the influent value must have resulted from the production of intermediate products during incomplete degradation of DMSO. After the loading rate was reduced back to 0.8 kg DMSO/m3-day (Phase VI), effluent quality slowly improved over a period of 50 days to a level comparable to that of Phase I and II. However, this slow recovery indicates that shock loading or a higher loading rate should be avoided for proper AS operation. Detailed information on the operating characteristics of the AS process can be obtained from observed changes in the alkalinity and sulphate concentrations. As shown in Fig. 1, the final products of DMSO degradation were sulphate and CO2. Therefore, alkalinity is required to prevent the pH drop due to production of protons together with sulphate production. Theoretically, 1026 mg/l of alkalinity is required for neutralization of sulphate produced from 800 mg/l of
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DMSO if no sulphur is used for bacterial synthesis. In addition, the amount of sulphate produced may be used to determine the extent of DMSO degradation as DMSO concentration was not measured for this experiment. The effluent alkalinity data are illustrated in Fig. 9b. pH control in AS is very important because foaming could be a problem at low (B6) or high (\ 9) pH, diminishing the reactor performance. As the amount of sulphate production depends on the extent of degradation of DMSO, alkalinity was added to the influent to maintain the reactor pH between 7 and 8. At day 175 (Phase IV), the effluent pH dropped to 5.8 due to improper control of influent alkalinity and foam production in the reactor was observed. Also, the effluent alkalinity dropped to 10 mg/l as CaCO3. Therefore, the lower treatment efficiency during the latter period of Phase IV might have been caused by pH shock to the microorganisms. When the effluent TOC and SCOD concentrations approached to the influent values due to higher DMSO loading rate during Phase V, the amount of alkalinity consumption decreased to 50 mg/l as CaCO3. This low alkalinity consumption agrees with the low proton or sulphate production resulting from incomplete degradation of DMSO. During the early period of Phase VI, the amount of alkalinity consumption increased as the treatment efficiency increased. A sulphur mass balance can be used to determine the extent of DMSO degradation as sulphur was only introduced to AS as DMSO. Effluent sulphur concentrations are illustrated in Fig. 9c, together with the influent DMSO sulphur concentration. When the AS performance was good, more than 90% of the added sulphur was converted to sulphate. Therefore, most of the influent DMSO was completely oxidized to sulphate, eliminating the possibility of the production of sulphide-containing noxious intermediates. No bad odours related to sulphide or reduced sulphur compounds were noticed during the operation of the AS process. One interesting point is that sulphur recovery as sulphate did not decrease during the latter period of Phase IV when the effluent SCOD was high due to the pH drop. This would suggest that sulphur cleavage from DMSO and subsequent oxidation still continued at low pH, although complete oxidation of organic intermediate products was inhibited. When the effluent quality deteriorated with the increase in loading rate (Phase V), the efficiency of DMSO conversion into sulphate also decreased. During Phase VI, the efficiency of DMSO conversion into sulphate increased with the increasing of removal efficiencies in TOC and SCOD. Changes in the microbial ecology of the AS process were observed during the treatment of DMSO. When AS performance was good, the flagellate Bodo and Monaas plus Amoeba and Monostyla were the major species found with small numbers of Vorticcella, Aoelo-
soma hemprechi, and Hydracarina. Growth of the flagellate might be attributed to the toxicity of DMSO, and the occurrence of A. hemprechi might indicate good effluent quality [14]. However, when treatment efficiency was deteriorated due to improper pH control (latter period of Phase V), MLVSS decreased significantly and A. hemprechi and Vorticcella were not observed. When the treatment efficiency was recovering during Phase VI, Paramecium and large numbers of Hydracarina were found. During the latter period of Phase VI, large numbers of Vorticcella and A. hemprechi were also observed. In summary, biological treatment of a wastewater stream containing 800 mg/l of DMSO can be achieved either with or without Fenton pre-treatment. As Fenton pretreatment yielded slightly better effluent quality in terms of TOC, SCOD, and SBOD, it may be recommended for the treatment of AS effluent or as pretreatment only if H2O2 is available at no extra cost. Biological treatment of DMSO was found to be feasible up to a loading rate of 0.8 kg DMSO/m3-day, if proper control of pH and alkalinity are maintained. With these findings, the cost of DMSO wastewater can be significantly reduced.
3.4. Effect of pre-acclimation on the acclimation period of AS The effect of pre-acclimation with easily degradable organics on the acclimation period was tested for bacterial acclimation of the AS process to DMSO with/without Fenton pretreatment. First, 400 mg/l of DMSO mixed with easily biodegradable organics (0.09 mM monosodium glutamate, 0.28 mM ammonium acetate and 0.028 mM glucose) was fed to AS with a starter culture of fresh sludge from the municipal WWTP. After 20 days of operation at an HRT of 48 h, the AS process reached steady-state, and the removal efficiencies of TOC, SCOD, and SBOD were 94%, 93%, and 85%, respectively. Nine days after reaching steady-state, wastewater containing only 800 mg/l of DMSO was fed, and the results are illustrated in Fig. 10a. Effluent TOC and SCOD concentrations peaked 10–12 days after the direct introduction of 800 mg/l DMSO, and then decreased after that. With pre-acclimation on 400 mg/l of DMSO and easily biodegradable organics, about 20 days was required for the microorganisms to acclimate to DMSO. In the case of direct acclimation to 800 mg/l of DMSO without pre-acclimation (Fig. 10b), the trends of TOC and SCOD were similar to those found with pre-acclimation. Comparison of the two cases revealed that pre-acclimation with easily biodegradable organics did not result in a significant reduction in the acclimation period. However, peak values of effluent TOC and SCOD during acclimation were significantly higher without the pre-acclimation.
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Although the reason for this result is not clear, it may be explained by the difference in total acclimation period: total 50 days for the case of pre-acclimation vs 30 days for the case without pre-acclimation.
3.5. Biological treatment of DPS-1300 -containing wastewater Results of direct treatment of DPS-1300-containing wastewater with AS are illustrated in Fig. 11. The acclimation time required for 1000 mg/l of DPS-1300 was about 11 days (early period of Phase I), which is shorter than that required for DMSO with or without Fenton pretreatment. No significant changes in effluent quality were observed when the loading rate was doubled (0.55 kg DMSO/m3-day) by reduction of the HRT (Phase II). During Phase III and IV, AS effluent was recycled at the recycle ratio (influent flow rate/recycle flow rate) of one. For phase III, influent was made with 2000 mg/l of DPS-1300, but the influent flow rate was halved. Therefore, the DMSO loading rate during phase III was kept equal to that during Phase II. No changes in effluent quality were observed between
Fig. 10. Results of bacterial pre-acclimation to 800 mg/l of DMSO without Fenton pre-treatment; (a) pre-acclimation with easily biodegradable organics and (b) direct acclimation to DMSO.
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Phase II and III. DMSO loading rate was then further increased to 0.64 kg DMSO/m3-day during Phase IV by increasing DPS-1300 concentration from 2000 to 3500 mg/l. At the same time, the HRT was also increased from 24 to 36 h, with effluent recycle at a ratio of one. Again, no significant changes in effluent quality were observed. The average effluent concentrations of TOC, SCOD, and SBOD were 11, 39, and 2 mg/l, respectively. The Phase V experiment was conducted with a higher loading rate (0.96 kg DMSO/m3-day) without effluent recycle. Here the influent DPS-1300 concentration was 3500 mg/l and the HRT was 48 h. After 10 days of operation, effluent quality started to deteriorate, and then reached another steady-state with the elevated effluent TOC, SCOD, SBOD concentrations of 258, 573, and 270 mg/l, respectively. To improve the effluent quality, the loading rate was decreased to the previous level of 0.64 kg DMSO/m3-day and without effluent recycle (Phase VI). After this change, effluent quality slowly improved, and then reached another steadystate. At this new steady-state, effluent SCOD and SBOD were 156 and 48 mg/l, respectively, which were significantly higher than those in Phase IV. The effluent TOC was 88 mg/l, which is eight times higher than that during Phase IV. Thus, the effluent quality of Phase VI was worse than that of Phase IV although the loading rate was the same. Although the effects of effluent recycle or toxicity caused by high influent DMSO concentration during Phase VI without effluent recycle may be possible explanations, further investigation is needed. The trends in alkalinity changes and sulphur mass balance are illustrated in Fig. 11b and c, respectively. For DPS-1300, alkalinity may be consumed not only by production of proton together with sulphate, but also by nitrification. Theoretically, alkalinity consumption is 963 mg (705 mg by proton production and 258 mg by nitrification ammonia produced from monoethanolamine) for the complete degradation of 1g of DPS-1300. However, nitrification of the ammonia produced from monoethanolamine did not occur during this experiment i.e., most of nitrogen was present as − − NH+ 4 -N and the concentrations of NO3 -N and NO2 N remained less than 1 mg/l. Therefore, alkalinity consumption by nitrification was negligible. When the effluent quality was good (Phase I, II, III, and IV), the amount of alkalinity consumption was proportional to the influent DPS-1300 concentration, and was close to the theoretical value for proton production. For example, the average alkalinity consumption represented 99% of the theoretical value of proton production during Phase IV. Also, sulphur recovered as sulphate accounted for more than 90% of sulphur added as DMSO. When the effluent water quality deteriorated with high DMSO loading rate (Phase V), the amount of
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Fig. 11. Results of DPS-1300 treatment by activated sludge; (a) TOC, SCOD and SBOD, (b) pH and alkalinity, and (c) sulphur concentration.
alkalinity consumption and the efficiency of DMSO conversion into sulphate decreased significantly. As effluent quality improved during Phase VI, the amount of alkalinity consumption and sulphur conversion efficiency recovered to their previous levels. These results indicated that alkalinity consumption was mainly related to the degradation of DMSO. However, in many cases, excess alkalinity would be necessary to prevent the pH drop caused by the possible nitrification of ammonia produced from monoethanolamine The microbial ecology of the AS process underwent changes similar to those found in the DMSO experiments. A. hemprechi, flagellate and Amoeba were the major of microorgnisms found. A. hemprech and Vorticella were also found in large numbers when treatment efficiency was good. Paramecium was found when treatment efficiency was recovering during Phase VI. In addition, small numbers of Lepadella were found, which are often responsible for nitrification.
In summary, DPS-1300 wastewater can also be treated by AS wastewater with direct acclimation. However, to achieve similar effluent qualities, the DMSO loading rate should be kept lower for DPS-1300 than for DMSO only. Since DPS-1300 contains both ethyl digol and monoethanolamine in addition to DMSO, the organic loading rate was significantly higher for DPS-1300 wastewater, which probably led to the effluent quality problems. Furthermore, ammonia produced from monoethanolamine reached about 100 mg/l (data not shown), which could have an inhibitory effect. Nevertheless, DPS-1300 wastewater can be treated by AS when the design is based on an appropriate loading rate. 4. Conclusions 1. Direct biological treatment of DMSO wastewater was found to be feasible, and thus significant sav-
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2.
3.
4.
5.
ings of wastewater treatment costs can be anticipated for the semi-conductor industry. For 800 mg/l of DMSO, effluent TOC was 14 mg/l after treatment with activated sludge at an HRT of 24 hrs, achieving 90% removal efficiency. Effluent SCOD and SBOD were 22 and 3 mg/l, respectively. For wastewater containing 3500 mg/l of DPS-1300, effluent TOC, SCOD, and SBOD were 11, 39, and 2 mg/l, respectively, at an HRT of 36 h with effluent recycle. The maximum loading rate for activated sludge were 0.80 and 0.64 kg DMSO/m3-day for DMSO and DPS1300, respectively. The effluent quality of the activated sludge process treating DMSO-containing wastewater yielded slightly better effluent quality with Fenton pretreatment than without it. As direct biological treatment of DMSO is feasible, it is desirable to apply Fenton treatment as a pre-treatment method for highly concentrated DMSO, or as a post-treatment to polish the effluent of biological treatment when wasted H2O2 is available. As protons together with sulphate are produced as final products of DMSO degradation, proper control of pH such as buffer addition may be necessary to achieve stable operation of the activated sludge process. TOC was a better parameter than COD and BOD as a surrogate measure of the concentration of DMSO. The measured TOC was about 60% of the theoretical TOC of DMSO. The measured COD was only 12% of the theoretical COD. With the measured BOD/ COD ratio of 0.035, DMSO can be classified as non-biodegradable. The optimum conditions for Fenton treatment of 800 mg/l of DMSO were as follows: Fe2 + : H2O2 dosages of 1000: 1000 mg/l at reaction pH 3, coagulation pH 7. Fenton treatment achieved a TOC removal efficiency of 26%, and the BOD/COD ratio of the wastewater was increased to 0.87. For DPS-1300-containing wastewater, optimum Fe2 + :H2O2 dosages were 1500:1500 and 2000:2000 mg/l for DPS-1300 concentrations of 1000 and 2000 mg/l, respectively.
.
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Acknowledgements Financial support provided by Samsung Electronics Co. Ltd. is greatly appreciated.
References [1] Bentley MD, Douglass IB, Lacadie JA, Whittier DR. The photolysis of dimethylsulphide in the air. J Air Pollut Control Assoc 1972;22:359 – 63. [2] Charlson RJ, Lovelock JE, Andreae MO, Warren SG. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 1987;326:655 – 61. [3] Andreae MO. Dimethylsulphoixde in marine and freshwaters. Limol Oceanogr 1972;25:1054 – 63. [4] Vogelpohl A, Geissen SU. Oxidation technologies for water and wastewater treatment. In: The International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 12 – 15 May, 1996, 1996. [5] Koito T, Tekawa M, Toyoda A. A novel treatment technique for DMSO wastewater. IEEE Transactions on Semiconductor Manufacturing 1998;1(11):3 – 8. [6] Suylen GMH, Stefess D. Chemolithotrophic potential of a Hyphomicrobium species, capable of growth on methylated sulphur compounds. Arch Microbiol 1986;146:192 – 8. [7] de Bont JAM, Dijken JP, Harber W. Dimethyle sulphoxide and demethyl sulphide as a carbon, sulphur compounds. J Gen Microbiol 1981;127:315 – 23. [8] Zinder SH, Brock TD. Dimethyl sulphoxide reduction by microorganisms. J Gen Microbiol 1978;105:342 – 55. [9] Alef K, Kleiner D. Rapid and sensitive determination of microbial activity in soils and in soil aggregates by dimethylsulphoxide reduction. Biol Fertil Soils 1989;8:349 – 55. [10] Sklorz M, Binert J. Determination of microbial activity in activated sewage by dimethylsulphoxide reduction. Environ Sci Pollut Res 1994;1:140 – 5. [11] Griebler C. Dimethylsulphoxide (DMSO) reduction: a new approach to determine microbial activity in freshwater sediments. J Microbiol Methods 1997;29:31 – 40. [12] APHA, Standard methods for the examination of water and wastewater, 17th ed. Washington DC: American Public Health Association, 1992. [13] Simons JM. A Procedure for determination of the biological treatability of industrial wastes. J WPCF 1960;2(8):841– 51. [14] US EPA, The causes and control of activated sludge bulking and foaming. Ohio, 1987.
Biological treatment of wastewater containing dimethyl sulphoxide from the semi-conductor industry Se-Jin Park a, Tai-Il Yoon a,*, Jae-Ho Bae a, Hyung-Joon Seo a, Hyo-Jung Park b a
Department of En6ironmental. Engineering, Inha Uni6ersity c 253, Yonghyun-Dong, Namgu, Inchon, 402 -751, South Korea b Samsung Electronics Co. Ltd. cSan 24, Nongseo-Ri, Kihung-Up, Yong-in-Si, Kyung-gi, 449 -900, South Korea Received 28 May 2000; received in revised form 26 September 2000; accepted 3 October 2000
Abstract Wastewater containing dimethyl sulphoxide (DMSO), a widely used organic solvent in the semi-conductor industry, is usually classified as an industrial waste requiring high-cost treatment. This study was conducted to evaluate the feasibility of the biological treatment of DMSO wastewater with activated sludge (AS). The optimum conditions for Fenton treatment were also investigated. The optimum chemical dosage of H2O2: Fe2 + for Fenton treatment was 1000:1000 mg/l for wastewater containing 800 mg/l of DMSO. Although TOC and COD removal efficiencies by Fenton treatment were not satisfactory for most applications, the BOD/COD ratio was increased from 0.035 to 0.87, suggesting it as a very useful pretreatment method for biological treatment. Wastewater containing 800 mg/l of DMSO was treated successfully by AS without Fenton pretreatment, after 20 days acclimation period. Fenton pretreatment or pre-acclimation with easily biodegradable organics did not significantly reduce the acclimation period. Average removal efficiencies of TOC, SCOD, and SBOD by AS at an HRT of 24 h (loading rate of 0.8 kg DMSO/m3-day) were 90%, 87%, and 63%, respectively. Most of the sulphur in DMSO was oxidized to sulphate, eliminating the possibility of the production of sulphide-containing noxious intermediates. For 3500 mg/l of DPS-1300 wastewater containing 1925 mg/l of DMSO, satisfactory effluent qualities were obtained by AS at an HRT of 72 h (loading rate of 0.64 kg DMSO/m3-day). Control of pH was an important operating factor for AS operation as protons are produced as a final product of DMSO degradation. Results indicated that DMSO wastewater can be successfully treated with AS, which may significantly reduce the treatment cost compared to the chemical methods currently used. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dimethyl sulphoxide; Fenton treatment; Biological treatment; Acclimation; Activated sludge; Wastewater
1. Introduction Dimethyl sulphoxide (DMSO) is an organic solvent, which is colourless, highly hygroscopic, thermally and chemically stable, and has strong solvency for both Abbre6iations: AOP, advanced oxidation process; AS, activated sludge; BOD, 5 day biochemical oxygen demand; COD, chemical oxygen demand; DMS, dimethylsulphide (CH3)2S; DMSO, dimethylsulphoxide (CH3)2SO; HRT, hydraulic retention time; MLSS, mixed liquor suspended solids; MLVSS, mixed liquor volatile suspended solids; ThOD, theoretical oxygen demand; TOC, total organic carbon; SBOD, soluble 5 day biochemical oxygen demand; SCOD, soluble chemical oxygen demand; WWTP, wastewater treatment plant. * Corresponding author. Tel.: + 82-32-860500; fax: + 82-328679919. E-mail address: [email protected] (T.-I. Yoon).
organic and inorganic compounds. It has been widely used in the manufacture of electronics, polymers, dyes, membranes, etc. In nature, DMSO is an intermediate in the global sulphur cycle produced from photo-oxidation of dimethyl sulphide (DMS) [1]. It is also believed to play an important climatic role in the formation of marine tropospheric aerosol and cloud condensation nuclei [2]. Reported DMSO concentrations are 220 nM for seawater and 1–70 nM for fresh water [3]. DPS1300, which contains 55% DMSO, 30% ethyl digol (C6H14O3) and 15% monoethanolamine (C2H7NO) by
Fig. 1. Proposed biotic degradation pathway of DMSO.
0032-9592/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 5 2 - 1
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Table 1 Experimental condition for Fenton treatment to find the optimum conditions Wastewater
Variables examined
Standard conditions
DMSO (300 or 800 mg/l)
Reaction pH 16 Coagulation pH 311 Reagent dosage Fe2+ and H2O2 = 5002,000 mg/l Reagent mass ratio Fe2+/H2O2 = 0.54 Reaction time 3120 min. Reagent dosage Fe2+ and H2O2 =2502,000 mg/l
Fe2+:H2O2 =
1000 mg/l: 1000 mg/l
Reaction
pH: 3 Time: 12 hr at 200 rpm pH: 7 Time: 20 min. at 60 rpm Anionic polymer: 3 mg/l Time: 30 min.
DPS-1300 (1000 or 2000 mg/l)
mass, is also frequently used as an alternative chemical for DMSO because of its low cost. DMSO is not as toxic as chlorinated solvents. It is collected and handled separately from other wastewater due to its low treatability, resulting in higher treatment costs. Biological treatment processes have often experienced operational problems with intermittently produced DMSO waste. When H2O2, a common waste in the semi-conductor industry, is available, several advanced oxidation process (AOP) such as Fenton treatment, ozonation with H2O2, or UV combined with H2O2, can be applied for the treatment of DMSO [4]. Koito et al. [5] recently suggested Fenton treatment with UV irradiation as an efficient treatment method for DMSO. Although AOP generally can produce high quality effluent, the high costs of equipment, chemicals, electricity, and sludge disposal diminish the advantages of these processes. Furthermore, complete treatment of concentrated DMSO wastewater cannot usually be achieved by AOP alone. In this case, biological treatment should be considered first, if feasible, and AOP can be used as the pretreatment, or as post-treatment after the biological treatment. Studies on biological degradation of DMSO revealed that most microorganisms can use it as a carbon and energy source [6]. A proposed degradation pathway of DMSO is depicted in Fig. 1 [7]. DMSO reduction to DMS is mediated not only by microorganisms, but also by animals and plants [8]. The breakdown of DMS produces 2 mol of formaldehyde and 1 mol of sulphide. Intermediate products such as DMS, methyl mercaptan, and hydrogen sulphide cause odour problems, and especially in the case of hydrogen sulphide, are toxic to humans. Finally, formaldehyde is converted to CO2 or used for cell synthesis, and sulphide is oxidized to sulphate. The fact that the reduction of DMSO to DMS is required for the initiation of degradation led to the misconception that both anaerobic and aerobic biological processes are required for the complete treatment of DMSO [5]. Due to this belief, together with the possibility of bad odours often associated with anaerobic
Coagulation
Settling
treatment systems, biological treatment of DMSO containing wastewater has never been considered as a practical method. However, as indicated by several investigators [8–11], some aerobes can reduce DMSO. Zinder and Brock [8] speculated that the reduction of DMSO by aerobes may not provide any physiological benefit as DMSO is an electron acceptor. Also, Sklorz and Binert [10] suggested that the DMSO reduction method could be used for the estimation of the activity of activated sludge. This study was carried out to test the feasibility of aerobic biological treatment of DMSO containing wastewater generated from a semi-conductor manufacturing factory. First, analytical characteristics of DMSO with conventional water quality parameters were evaluated. Second, the optimum conditions for Fenton treatment of DMSO and DPS-1300 as a pretreatment for biological treatment were evaluated. Finally, aerobic biological treatabilities of DMSO and DPS-1300 were evaluated by the activated sludge process (AS). Here, the optimum operating conditions of AS were investigated to provide basic information for the design of the treatment process.
2. Materials and methods The optimum conditions for Fenton treatment as a pre-treatment for biological treatment or as a sole treatment process were investigated for both wastewaters containing DMSO and DPS-1300. A jar-test apparatus was used for Fenton treatment. The pH was adjusted to the desired value 9 0.02 with sulphuric acid or sodium hydroxide addition. Stock solutions of H2O2 (5%) and FeSO4·7H2O (5% as Fe2 + ) were used as reagents. After the addition of the desired amount of the reagents, samples were mixed rapidly at 200 rpm for 1–2 h. For the coagulation of the Fenton treated sample, the pH was adjusted to the desired value (normally 7), and then mixed slowly at 60 rpm for 20 min with the addition of 3 mg/l of anionic polymer (SA-307, Songwon Co.).
S.-J. Park et al. / Process Biochemistry 36 (2001) 579–589
Table 1 summarizes the experimental conditions for Fenton treatment of wastewaters containing DMSO or DPS-1300. Artificial wastewaters containing 300 or 800 mg/l of DMSO were used to find the optimum pH (for both Fenton reaction and coagulation), reagent dosage, and reaction time. All variables were set as standard conditions except for the tested variable. Effect of Fenton reaction pH was evaluated at pH between 1 and 6. The optimum reagent dosage was first determined by changing the amount of reagent from 500 to 2000 mg/l at a fixed Fe2 + /H2O2 mass ratio of 1. The mass ratio of Fe2 + /H2O2 changed from 0.5 to 4 at the fixed H2O2 concentration of 1000 mg/l. Changes in treatment efficiency during Fenton treatment were also evaluated to determine the optimum reaction time. For DPS-1300, only the optimum Fenton reagent dosage was determined for wastewater containing 1000 and 2000 mg/l of DPS at 1300 mg/l with the standard conditions listed in Table 1. All analysis was made with the supernatants after 30 min of sedimentation following coagulation. The aerobic biological treatability of 800 mg/l DMSO was tested with AS. A reactor with a 4.9 l aeration basin and a 2.9 l settling tank was seeded with the sludge from a domestic wastewater treatment plant, and was operated at 209 2°C. The dissolved oxygen concentration (DO) in AS was controlled at 2.5–3.5 mg/l. Mixed liquor volatile suspended solids (MLVSS) were maintained at 2300 mg/l, and the influent pH was kept between 7.5 and 8.2 with bicarbonate buffer. Operation of AS was divided into six phases, as summarized in Table 2. To evaluate the effect of Fenton pretreatment on biological treatment with AS, the effluent characteristics of AS were determined with Fenton pretreatment (Phase I and II) and without Fenton pretreatment (Phase III to VI). From Phase III to Phase VI, the DMSO loading rate was changed from 0.4 to 1.6 kg/m3-day to determine the maximum loading rate. The necessity of pre-acclimation for the direct treatment of DMSO by AS under two different conditions: with and without pre-acclimation of AS with easily degradable organics was investigated. One AS reactor
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received wastewater containing 400 mg/l of DMSO, 0.09 mM monosodium glutamate, 0.28 mM ammonium acetate and 0.028 mM glucose. After reaching steadystate, the influent containing only 800 mg/l of DMSO was fed in, and the effluent quality was measured. The other AS reactor was fed with wastewater containing only 800 mg/l of DMSO from the beginning of the operation. Both reactors were filled with fresh sludge from WWTP, and operated at a hydraulic retention time (HRT) of 48 h. As the BOD/COD ratio of DPS-1300 containing wastewater is known to be high, biological treatibility was tested without Fenton pretreatment. The AS operating conditions were divided into six phases according to the influent DPS-1300 concentration and HRT, as summarized in Table 3. DO concentration in AS was maintained at 2.5–3.5 mg/l. The treatability of DPS1300 was tested at different DMSO loading rates from 0.275 to 0.963 kg/m3-day, and the effect of an effluent recycle was also examined. Here, the MLVSS concentration was kept at 3400 mg/l. COD (closed reflux method), BOD, pH, alkalinity, VSS, H2O2 (iodometric method) were measured by procedures described in Standard Methods [12]. Soluble COD (SCOD) and soluble BOD (SBOD) were measured after filtration (0.45 mm filter paper), and used to evaluate the treatment efficiency of AS. TOC was measured with a Shimadzu TOC-5000 after filtration. DMSO concentration was measured with a GC (Hewlett Packard 5890, FID, capillary HP-624). However, the detection limit of DMSO was relatively high (10 mg/l) with this method. Sulphate concentration was measured with IC (Dionex 500, USA).
3. Results and discussion
3.1. Characteristics of DMSO containing wastewater To evaluate the analytical characteristics and the biological treatability, TOC, COD, and BOD were measured for DMSO and DPS-1300 at several concen-
Table 2 Operation conditions of activated sludge reactor for the treatment of DMSO Phase I Fenton (pretreated)
Phase II Fenton (pretreated)
Phase III DMSO (800 mg/l)
Phase IV DMSO (800 mg/l)
Phase V DMSO (800 mg/l)
Phase VI DMSO (800 mg/l)
COD
145
145
175
175
175
175
TOC pH
100 7.58.2 48 –
100 7.58.2 24 –
140 7.57.9 48 0.4
140 7.57.9 24 0.8
140 7.57.9 12 1.6
140 7.57.9 24 0.8
Influent type
Influent Conc. (mg/l)
HRT(hrs) DMSO loading (kg/m3 day)
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Table 3 Operation conditions of activated sludge reactor for the treatment of DPS-1300 Phase
Phase I
Phase II
Phase IIIa
Phase IVa
Phase V
Phase VI
Influent DPS-1300 Concentration (mg/l)
1000
1000
2000
3500
3500
3500
Influent Conc. (mg/l)
726 210 9.810.2 48 0.275
726 210 9.810.2 24 0.55
1440 212 10.110.5 48b 0.55
1440 360 10.6 72b 0.64
2760 570 10.6 48 0.963
2760 570 10.6 72 0.64
COD TOC pH
HRT (h) DMSO loading (kg DMSO/m3 - day) a b
Effluent was recycle at the recycle ratio of one, and therefore influent DPS-1300 conc. is about half of the value listed. HRT was calculated based on the wastewater flow rate to the treatment system, i.e. excluding the recycled flow rate.
trations, and compared to the theoretical oxygen demand (ThOD). As illustrated in Fig. 2, COD and TOC values were proportional to DMSO concentrations. However, the calculated COD/DMSO ratio of 0.22 represents only 12% of the ThOD/DMSO ratio of 1.85. This result indicates that COD is not a good parameter to represent DMSO concentration. Measured BOD values were very low, and the BOD/COD ratio decreased as the DMSO concentration increased. The highest BOD/COD ratio of 0.035 indicates that DMSO is non-biodegradable according to Simons’ classification [13]. TOC was the best parameter among the three parameters as the measured TOC/DMSO ratio was 60% of the theoretical TOC value of 0.3 g TOC/g DMSO. Measured values of TOC, COD, and BOD for wastewater containing DPS-1300 are illustrated in Fig. 3. The COD/DPS-1300 ratio of 0.78 represents 52% of the ThOD/DPS-1300 ratio of 1.49, and the observed BOD/COD ratio was 0.48. The high values of the COD/ThOD and BOD/COD ratios compared to the DMSO solution may have resulted from the easily biodegradable components of DPS-1300, such as ethyl digol and monoethanol amine. From the measured BOD/COD ratio, DPS-1300 can be characterized as biodegradable [13]. The measured TOC/DPS-1300 ratio of 0.18 is equivalent to 46% of the theoretical TOC value of 0.39 g TOC/g DPS-1300.
The optimum pH for coagulation was determined for 800 mg/l of DMSO solution with the Fenton reaction pH of 3, and results are illustrated in Fig. 5. The COD after Fenton treatment was higher than that before treatment when the coagulation pH was below 6. This increase may be explained by the remaining H2O2 with incomplete coagulation. For example, residual H2O2 concentrations were 306–567 mg/l at coagulation pH below 6. At coagulation pH higher than 7, removal efficiencies of COD and TOC were constant, and the values were 21% and 25%, respectively. As BOD (about 90 mg/l) was also not affected by coagulation pH, the BOD/COD ratio increased from 0.62 to 0.71 as the coagulation pH increased. This results suggests that a pH of 7 is recommended for the coagulation pH for Fenton treatment. The optimum dosage of Fenton reagent was obtained by changing both the total amount and the mass ratio of Fe2 + /H2O2 with 800 mg/l of DMSO at a reaction pH of 3 and a coagulation pH of 7. First, the amount of Fenton reagent was changed from 500 to 2000 mg/l at the standard Fe2 + /H2O2 mass ratio of one. As shown in Fig. 6a, the optimum amounts of both reagents were estimated as 1000 mg/l considering the removal efficiencies of TOC and COD and BOD/COD ratio after treatment. Next, the concentration of Fe2 +
3.2. Optimum conditions for Fenton treatment of DMSO and DPS-1300 The optimum pH for Fenton treatment of 300 mg/l DMSO was found to be three, based on TOC and COD removal efficiencies which were 37.1% and 57%, respectively, as illustrated in Fig. 4. The relatively large variation in COD removal as a function of pH compared to that of TOC might have resulted from the differing residual H2O2 concentrations, which can be measured as COD. The residual H2O2 concentrations was 3080 mg/l, and was the lowest at pH 3.
Fig. 2. TOC, COD and BOD values of DMSO solution at various concentrations.
S.-J. Park et al. / Process Biochemistry 36 (2001) 579–589
Fig. 3. TOC, COD, and BOD values of DPS-1300 solution at various concentrations.
was changed at the H2O2 dosage of 1000 mg/l with the standard conditions used before. Fig. 6b illustrates that the Fe2 + /H2O2 mass ratio between one and two is the optimum considering TOC and COD removal efficiencies. Therefore, the ratio of one would be the optimum considering the increased sludge production at higher ratios. The optimum Fenton reaction time was determined using the pre-determined optimum reaction pH (3), coagulation pH (7) and chemical dosage (both Fe2 + and H2O2 of 1,000 mg/l). As illustrated in Fig. 7, TOC and COD decreased for the first 5 and 15 min, respectively, and then both TOC and COD stayed constant. However, the BOD increased steadily for 120 min, and as a result, the BOD/COD ratio increased from 0.035 to 0.87. Concentrations of H2O2 and DMSO also decreased significantly within the first 5 min (Fig. 7b). The H2O2 concentration decreased from 1000 mg/l to 145 mg/l at t= 5 min, then decreased slowly thereafter. The DMSO concentration was 50.3 mg/l after 5 min, and
Fig. 4. Effect of reaction pH on the efficiency of Fenton treatment for 300 mg/l of DMSO.
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Fig. 5. Effect of coagulation pH on the efficiency of Fenton treatment for 800 mg/l of DMSO.
then finally decreased to 31.2 mg/l after 120 min. Therefore, at least 1 h of reaction time is necessary for Fenton treatment of DMSO to prevent any negative effects of residual H2O2.
Fig. 6. Effect of reagent dosage on the efficiency of Fenton treatment for 800 mg/l of DMSO; (a) at Fe2 + /H2O2 ratio of 1, and (b) at H2O2 dosage of 1000 mg/l.
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increasing reagent dosage was not significant. The TOC and COD removal efficiencies were maximized up to 25% and 41%, respectively, at the highest reagent dosage. Similar results were obtained for 2000 mg/l of DPS-1300 wastewater. Considering chemical costs, low TOC and COD removal efficiencies together with a relatively high BOD/COD ratio of DPS-1300 (0.48) without treatment, the use of Fenton treatment as a pretreatment for biological treatment may be uneconomical.
3.3. Biological treatability of DMSO with and without Fenton pre-treatment The biological treatability of wastewater containing DMSO (800 mg/l) was evaluated using AS with (Phase I and II) and without (Phase III to VI) Fenton pretreatment. As illustrated in Fig. 9a, a period of approximate 10 days was required for AS to acclimate to Fenton pre-treated DMSO wastewater. After the acclimation, the average effluent concentrations of TOC, SCOD, and SBOD were 12.8, 10.2, and 1.0 mg/l, respectively (Phase I). The corresponding removal effi-
Fig. 7. Trends in water quality parameters during Fenton treatment for 800 mg/l of DMSO; (a) TOC, COD, and BOD/COD ratio, and (b) H2O2 and DMSO.
The optimum conditions of Fenton treatment for wastewater containing 800 mg/l DMSO can be summarized as follows: reaction time= 1 – 2 h, reaction pH 3, coagulation pH 7, reagent dosage of both Fe(II) and H2O2 = 1000 mg/l. At these conditions, removal efficiencies of TOC and COD were 26% and 49% respectively, and BOD/COD ratio increased from 0.035 to 0.89. The low TOC and COD removal efficiencies indicate that the low molecular weight intermediate products produced during Fenton treatment was not complete. In sum, Fenton treatment alone cannot be used for the treatment of concentrated DMSO although it may be used as a pretreatment method to increase the biological treatment efficiency. Results of Fenton treatment for wastewater containing 1000 mg/l of DPS-1300 are illustrated in Fig. 8. The BOD value of the treated water increased with the increased reagent dosage, and as a result, the BOD/ COD ratio increased to 0.88. The DMSO concentration also decreased from 550 mg/l before treatment to 32 mg/l at Fe2 + and H2O2 dosages of 1500 mg/l. The COD removal efficiency increased linearly with reagent dosage. The change in TOC removal efficiency with
Fig. 8. Effect of Fenton reagent dosage on the treatment efficiency of DPS-1300; (a) 1000 mg/l of DPS-1300, and (b) 2000 mg/l of DPS1300.
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Fig. 9. Results of DMSO treatment by activated sludge with and without Fenton pre-treatment; (a) TOC, SCOD and SBOD, (b) pH and alkalinity, and (c) sulphur concentration.
ciencies of TOC, SCOD, and SBOD were 88, 93, and 99%, respectively. When the HRT was reduced from 48 to 24 h (Phase II), SCOD was increased from 1.0 to 6.8 mg/l, but no other significant changes in effluent characteristics were observed. Therefore, 800 mg/l of DMSO wastewater with Fenton pre-treatment was treated successfully by AS at an HRT of 24 h. During Phase III to VI, DMSO was fed directly to AS without Fenton pre-treatment. After 8 days of operation at HRT of 48 h in Phase III (loading rate of 0.4 kg DMSO/m3-day), the AS process appeared to reach steady-state. This short stabilization period might have been achieved because the microorganisms were already acclimated to low concentrations of DMSO during Phase I and II. After stabilization, the average concentrations of effluent SBOD and TOC reached the levels observed during Phase II. The slight increase in effluent SCOD might have been due to the increase in influent SCOD without Fenton pretreatment. During Phase IV, the DMSO loading rate was doubled (0.8 kg DMSO/m3-day) by reducing the HRT from 48 to 24 h. No significant changes in effluent characteristics were observed for 25 days after the change. During this period, average values of effluent TOC, SCOD, and SBOD were 14, 22, and 3 mg/l, respectively. The effluent TCOD and TBOD were slightly higher than SCOD and SBOD (data not shown). The deterioration
of effluent quality during the latter half of Phase IV may be related to pH shock, which will be discussed later. When the loading rate was further increased to 1.6 kg DMSO/m3-day (Phase V) by decreasing the HRT to 12 h, the effluent TOC, SCOD, and SBOD remained low for 10 days, but reached or exceeded the influent values after 15 days of operation, possibly due to the high DMSO loading rate. The fact that effluent SBOD far exceeded the influent value must have resulted from the production of intermediate products during incomplete degradation of DMSO. After the loading rate was reduced back to 0.8 kg DMSO/m3-day (Phase VI), effluent quality slowly improved over a period of 50 days to a level comparable to that of Phase I and II. However, this slow recovery indicates that shock loading or a higher loading rate should be avoided for proper AS operation. Detailed information on the operating characteristics of the AS process can be obtained from observed changes in the alkalinity and sulphate concentrations. As shown in Fig. 1, the final products of DMSO degradation were sulphate and CO2. Therefore, alkalinity is required to prevent the pH drop due to production of protons together with sulphate production. Theoretically, 1026 mg/l of alkalinity is required for neutralization of sulphate produced from 800 mg/l of
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DMSO if no sulphur is used for bacterial synthesis. In addition, the amount of sulphate produced may be used to determine the extent of DMSO degradation as DMSO concentration was not measured for this experiment. The effluent alkalinity data are illustrated in Fig. 9b. pH control in AS is very important because foaming could be a problem at low (B6) or high (\ 9) pH, diminishing the reactor performance. As the amount of sulphate production depends on the extent of degradation of DMSO, alkalinity was added to the influent to maintain the reactor pH between 7 and 8. At day 175 (Phase IV), the effluent pH dropped to 5.8 due to improper control of influent alkalinity and foam production in the reactor was observed. Also, the effluent alkalinity dropped to 10 mg/l as CaCO3. Therefore, the lower treatment efficiency during the latter period of Phase IV might have been caused by pH shock to the microorganisms. When the effluent TOC and SCOD concentrations approached to the influent values due to higher DMSO loading rate during Phase V, the amount of alkalinity consumption decreased to 50 mg/l as CaCO3. This low alkalinity consumption agrees with the low proton or sulphate production resulting from incomplete degradation of DMSO. During the early period of Phase VI, the amount of alkalinity consumption increased as the treatment efficiency increased. A sulphur mass balance can be used to determine the extent of DMSO degradation as sulphur was only introduced to AS as DMSO. Effluent sulphur concentrations are illustrated in Fig. 9c, together with the influent DMSO sulphur concentration. When the AS performance was good, more than 90% of the added sulphur was converted to sulphate. Therefore, most of the influent DMSO was completely oxidized to sulphate, eliminating the possibility of the production of sulphide-containing noxious intermediates. No bad odours related to sulphide or reduced sulphur compounds were noticed during the operation of the AS process. One interesting point is that sulphur recovery as sulphate did not decrease during the latter period of Phase IV when the effluent SCOD was high due to the pH drop. This would suggest that sulphur cleavage from DMSO and subsequent oxidation still continued at low pH, although complete oxidation of organic intermediate products was inhibited. When the effluent quality deteriorated with the increase in loading rate (Phase V), the efficiency of DMSO conversion into sulphate also decreased. During Phase VI, the efficiency of DMSO conversion into sulphate increased with the increasing of removal efficiencies in TOC and SCOD. Changes in the microbial ecology of the AS process were observed during the treatment of DMSO. When AS performance was good, the flagellate Bodo and Monaas plus Amoeba and Monostyla were the major species found with small numbers of Vorticcella, Aoelo-
soma hemprechi, and Hydracarina. Growth of the flagellate might be attributed to the toxicity of DMSO, and the occurrence of A. hemprechi might indicate good effluent quality [14]. However, when treatment efficiency was deteriorated due to improper pH control (latter period of Phase V), MLVSS decreased significantly and A. hemprechi and Vorticcella were not observed. When the treatment efficiency was recovering during Phase VI, Paramecium and large numbers of Hydracarina were found. During the latter period of Phase VI, large numbers of Vorticcella and A. hemprechi were also observed. In summary, biological treatment of a wastewater stream containing 800 mg/l of DMSO can be achieved either with or without Fenton pre-treatment. As Fenton pretreatment yielded slightly better effluent quality in terms of TOC, SCOD, and SBOD, it may be recommended for the treatment of AS effluent or as pretreatment only if H2O2 is available at no extra cost. Biological treatment of DMSO was found to be feasible up to a loading rate of 0.8 kg DMSO/m3-day, if proper control of pH and alkalinity are maintained. With these findings, the cost of DMSO wastewater can be significantly reduced.
3.4. Effect of pre-acclimation on the acclimation period of AS The effect of pre-acclimation with easily degradable organics on the acclimation period was tested for bacterial acclimation of the AS process to DMSO with/without Fenton pretreatment. First, 400 mg/l of DMSO mixed with easily biodegradable organics (0.09 mM monosodium glutamate, 0.28 mM ammonium acetate and 0.028 mM glucose) was fed to AS with a starter culture of fresh sludge from the municipal WWTP. After 20 days of operation at an HRT of 48 h, the AS process reached steady-state, and the removal efficiencies of TOC, SCOD, and SBOD were 94%, 93%, and 85%, respectively. Nine days after reaching steady-state, wastewater containing only 800 mg/l of DMSO was fed, and the results are illustrated in Fig. 10a. Effluent TOC and SCOD concentrations peaked 10–12 days after the direct introduction of 800 mg/l DMSO, and then decreased after that. With pre-acclimation on 400 mg/l of DMSO and easily biodegradable organics, about 20 days was required for the microorganisms to acclimate to DMSO. In the case of direct acclimation to 800 mg/l of DMSO without pre-acclimation (Fig. 10b), the trends of TOC and SCOD were similar to those found with pre-acclimation. Comparison of the two cases revealed that pre-acclimation with easily biodegradable organics did not result in a significant reduction in the acclimation period. However, peak values of effluent TOC and SCOD during acclimation were significantly higher without the pre-acclimation.
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Although the reason for this result is not clear, it may be explained by the difference in total acclimation period: total 50 days for the case of pre-acclimation vs 30 days for the case without pre-acclimation.
3.5. Biological treatment of DPS-1300 -containing wastewater Results of direct treatment of DPS-1300-containing wastewater with AS are illustrated in Fig. 11. The acclimation time required for 1000 mg/l of DPS-1300 was about 11 days (early period of Phase I), which is shorter than that required for DMSO with or without Fenton pretreatment. No significant changes in effluent quality were observed when the loading rate was doubled (0.55 kg DMSO/m3-day) by reduction of the HRT (Phase II). During Phase III and IV, AS effluent was recycled at the recycle ratio (influent flow rate/recycle flow rate) of one. For phase III, influent was made with 2000 mg/l of DPS-1300, but the influent flow rate was halved. Therefore, the DMSO loading rate during phase III was kept equal to that during Phase II. No changes in effluent quality were observed between
Fig. 10. Results of bacterial pre-acclimation to 800 mg/l of DMSO without Fenton pre-treatment; (a) pre-acclimation with easily biodegradable organics and (b) direct acclimation to DMSO.
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Phase II and III. DMSO loading rate was then further increased to 0.64 kg DMSO/m3-day during Phase IV by increasing DPS-1300 concentration from 2000 to 3500 mg/l. At the same time, the HRT was also increased from 24 to 36 h, with effluent recycle at a ratio of one. Again, no significant changes in effluent quality were observed. The average effluent concentrations of TOC, SCOD, and SBOD were 11, 39, and 2 mg/l, respectively. The Phase V experiment was conducted with a higher loading rate (0.96 kg DMSO/m3-day) without effluent recycle. Here the influent DPS-1300 concentration was 3500 mg/l and the HRT was 48 h. After 10 days of operation, effluent quality started to deteriorate, and then reached another steady-state with the elevated effluent TOC, SCOD, SBOD concentrations of 258, 573, and 270 mg/l, respectively. To improve the effluent quality, the loading rate was decreased to the previous level of 0.64 kg DMSO/m3-day and without effluent recycle (Phase VI). After this change, effluent quality slowly improved, and then reached another steadystate. At this new steady-state, effluent SCOD and SBOD were 156 and 48 mg/l, respectively, which were significantly higher than those in Phase IV. The effluent TOC was 88 mg/l, which is eight times higher than that during Phase IV. Thus, the effluent quality of Phase VI was worse than that of Phase IV although the loading rate was the same. Although the effects of effluent recycle or toxicity caused by high influent DMSO concentration during Phase VI without effluent recycle may be possible explanations, further investigation is needed. The trends in alkalinity changes and sulphur mass balance are illustrated in Fig. 11b and c, respectively. For DPS-1300, alkalinity may be consumed not only by production of proton together with sulphate, but also by nitrification. Theoretically, alkalinity consumption is 963 mg (705 mg by proton production and 258 mg by nitrification ammonia produced from monoethanolamine) for the complete degradation of 1g of DPS-1300. However, nitrification of the ammonia produced from monoethanolamine did not occur during this experiment i.e., most of nitrogen was present as − − NH+ 4 -N and the concentrations of NO3 -N and NO2 N remained less than 1 mg/l. Therefore, alkalinity consumption by nitrification was negligible. When the effluent quality was good (Phase I, II, III, and IV), the amount of alkalinity consumption was proportional to the influent DPS-1300 concentration, and was close to the theoretical value for proton production. For example, the average alkalinity consumption represented 99% of the theoretical value of proton production during Phase IV. Also, sulphur recovered as sulphate accounted for more than 90% of sulphur added as DMSO. When the effluent water quality deteriorated with high DMSO loading rate (Phase V), the amount of
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Fig. 11. Results of DPS-1300 treatment by activated sludge; (a) TOC, SCOD and SBOD, (b) pH and alkalinity, and (c) sulphur concentration.
alkalinity consumption and the efficiency of DMSO conversion into sulphate decreased significantly. As effluent quality improved during Phase VI, the amount of alkalinity consumption and sulphur conversion efficiency recovered to their previous levels. These results indicated that alkalinity consumption was mainly related to the degradation of DMSO. However, in many cases, excess alkalinity would be necessary to prevent the pH drop caused by the possible nitrification of ammonia produced from monoethanolamine The microbial ecology of the AS process underwent changes similar to those found in the DMSO experiments. A. hemprechi, flagellate and Amoeba were the major of microorgnisms found. A. hemprech and Vorticella were also found in large numbers when treatment efficiency was good. Paramecium was found when treatment efficiency was recovering during Phase VI. In addition, small numbers of Lepadella were found, which are often responsible for nitrification.
In summary, DPS-1300 wastewater can also be treated by AS wastewater with direct acclimation. However, to achieve similar effluent qualities, the DMSO loading rate should be kept lower for DPS-1300 than for DMSO only. Since DPS-1300 contains both ethyl digol and monoethanolamine in addition to DMSO, the organic loading rate was significantly higher for DPS-1300 wastewater, which probably led to the effluent quality problems. Furthermore, ammonia produced from monoethanolamine reached about 100 mg/l (data not shown), which could have an inhibitory effect. Nevertheless, DPS-1300 wastewater can be treated by AS when the design is based on an appropriate loading rate. 4. Conclusions 1. Direct biological treatment of DMSO wastewater was found to be feasible, and thus significant sav-
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2.
3.
4.
5.
ings of wastewater treatment costs can be anticipated for the semi-conductor industry. For 800 mg/l of DMSO, effluent TOC was 14 mg/l after treatment with activated sludge at an HRT of 24 hrs, achieving 90% removal efficiency. Effluent SCOD and SBOD were 22 and 3 mg/l, respectively. For wastewater containing 3500 mg/l of DPS-1300, effluent TOC, SCOD, and SBOD were 11, 39, and 2 mg/l, respectively, at an HRT of 36 h with effluent recycle. The maximum loading rate for activated sludge were 0.80 and 0.64 kg DMSO/m3-day for DMSO and DPS1300, respectively. The effluent quality of the activated sludge process treating DMSO-containing wastewater yielded slightly better effluent quality with Fenton pretreatment than without it. As direct biological treatment of DMSO is feasible, it is desirable to apply Fenton treatment as a pre-treatment method for highly concentrated DMSO, or as a post-treatment to polish the effluent of biological treatment when wasted H2O2 is available. As protons together with sulphate are produced as final products of DMSO degradation, proper control of pH such as buffer addition may be necessary to achieve stable operation of the activated sludge process. TOC was a better parameter than COD and BOD as a surrogate measure of the concentration of DMSO. The measured TOC was about 60% of the theoretical TOC of DMSO. The measured COD was only 12% of the theoretical COD. With the measured BOD/ COD ratio of 0.035, DMSO can be classified as non-biodegradable. The optimum conditions for Fenton treatment of 800 mg/l of DMSO were as follows: Fe2 + : H2O2 dosages of 1000: 1000 mg/l at reaction pH 3, coagulation pH 7. Fenton treatment achieved a TOC removal efficiency of 26%, and the BOD/COD ratio of the wastewater was increased to 0.87. For DPS-1300-containing wastewater, optimum Fe2 + :H2O2 dosages were 1500:1500 and 2000:2000 mg/l for DPS-1300 concentrations of 1000 and 2000 mg/l, respectively.
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Acknowledgements Financial support provided by Samsung Electronics Co. Ltd. is greatly appreciated.
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