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Effect of ozone and permanganate on algae coagulation removal – Pilot and bench scale tests
Chemosphere 74 (2009) 840–846
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Effect of ozone and permanganate on algae coagulation removal – Pilot and bench scale tests Jen-Jeng Chen a, Hsuan-Hsien Yeh b,*, I-Cheng Tseng c a
Department of Environmental Resources Management, Tajen University, Pingtung 907, Taiwan Department of Environmental Engineering, National Cheng Kung University, Tainan 701, Taiwan c Department of Life Sciences, National Cheng Kung University, Tainan 701, Taiwan b
a r t i c l e
i n f o
Article history: Received 17 May 2008 Received in revised form 1 October 2008 Accepted 3 October 2008 Available online 11 December 2008 Keywords: Ozone Permanganate Algae Coagulation
a b s t r a c t Both pilot and laboratory scale experiments are conducted to compare the effect of ozone and permanganate preoxidation on algae removal by alum coagulation. Both appropriately dosed preoxidants are shown to be beneficial to algae coagulation removal. This may be attributed to a decrease in cell stability; however, overdosing may cause cell lysis and the release of organics, which could interfere with algae cell coagulation. The presence of calcium further enhances the beneficial effect of preoxidation on algae coagulation; however, this phenomenon is more significant for using permanganate than ozone. It is speculated that this is due to the fact that the positively charged calcium ions can serve as bridges to hold the negatively charged MnO2 and algal cells together. Further, this behavior also explains the superior performance of permanganate preoxidation compared to that obtained using ozone for algae coagulation removal in pilot testing, as the source water contains high calcium content. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The presence of algae in source water can cause a variety of problems for drinking water treatment. Algae and its metabolites can have a significant impact on water quality, such as the production of unpleasant tastes and odors, formation of disinfection byproducts (DBPs), and toxins from cyanobacteria. Furthermore, algae are known to interfere with the water treatment process, causing short filter runs, increases in coagulant demand, and microbial regrowth in distribution systems (Suffet et al., 1995; Schmidt et al., 1998; Plummer and Edzwald, 2001). For algae removal, coagulation is the key step in conventional water treatment processes. In order to improve the efficiency of algae removal, control the growth of other nuisance microorganisms, and simultaneously oxidize reduced species such as iron and manganese, prechlorination may be used. Unfortunately, in addition to chlorinated DBP problems, chlorine can induce both physiological stress and membrane damage to cells, causing the release of extra- and intra-cellular organic matter (EOM and IOM), respectively, which may include taste- and odor-causing and toxic compounds (Peterson et al., 1995). Further, Sukenik et al. (1987) found that at lower dosage (2 mg L 1 Cl2), prechlorination did not result in a change in coagulant (alum) demand. However, at higher chlorine dosage (10 and 20 mg L 1 Cl2), the required alum dosage needed to be increased, compared to the same algae system without prechlorination. * Corresponding author. Tel.: +886 6 2757575x65823; fax: +886 6 2752790. E-mail address: [email protected] (H.-H. Yeh). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.10.009
Numerous researchers have demonstrated that other preoxidants, such as ozone, chlorine dioxide, and permanganate, can improve algae removal by coagulation and filtration processes in drinking water treatment plants (Fitzgerald, 1966; Sukenik et al., 1987; Petruševski et al., 1996; Steynberg et al., 1996). These preoxidants can improve algae coagulation by deactivating algal cells, reducing cell stability, or liberating EOM. The effects of the oxidant on algae removal during water treatment are dependent on oxidant type, oxidant dosage, and the algal species. For example, ozone was found to improve the coagulation of green algae (Scenedesmus quadricauda) but not that of diatoms (Cyclotella sp.) (Plummer and Edzwald, 2002). This is probably due to the difference in surface characteristics of the cells. Bernhardt and Clasen (1991) reported that EOM from cultures of green and blue–green algae and diatoms behaved like anionic and non-ionic polyelectrolytes. The amount and properties of EOM produced varied with algae species, oxidant type, and oxidant dosages. The EOM, depending on the concentration and molecular weight, can enhance or hinder flocculation. Low dosages of ozone can cause a release of EOM, which aids coagulation, but at higher doses, ozone affects EOM structure (e.g. by lowering molecular size), which may hinder subsequent coagulation (Paralkar and Edzwald, 1996). Petruševski et al. (1996) reported that a permanganate pretreatment, followed by coagulation and direct filtration for impounded surface water, distinctly improved particle and algae removal, in comparison to efficiencies commonly achieved without preoxidation. The in situ production of algae-derived coagulant aid and
J.-J. Chen et al. / Chemosphere 74 (2009) 840–846
deactivation of motile microorganisms were suggested to be the possible mechanisms induced by permanganate preoxidation. Knappe et al. (2004) conducted jar tests with algae-spiked surface water to study the effect of potassium permanganate preoxidation on algae removal, chlorine demand, and DBP formation. The results demonstrated that the removal of Microcystis aeruginosa, Anabaena flos-aquae, and Synura petersenii improved with an increase in KMnO4 dosage and contact time. Experiments with S. petersenii indicated that the addition of 3 mg L 1 KMnO4 increased coagulant demand, due to the release of IOM, the formation of MnO2, and the oxidation of natural organic matter. Nevertheless, KMnO4-induced cell lysis did not show an increased chlorine demand or DBP formation. Previous studies investigating the effect of ozone or permanganate oxidation on algae removal have only been conducted at a lab or pilot scale. Additionally, attention has not been given to the effect of background aqueous constituents, such as hardness. The purpose of this study is to examine and compare the effects oxidation on algae coagulation. In the first half of this investigation, a pilot study was conducted using eutrophic source water. In the second half of this investigation, a laboratory-scale study using pure-cultured algal suspensions was conducted to elucidate observations obtained from pilot testing. In the laboratory-scale study, the algal suspension was spiked with calcium ions to simulate a high hardness content of the eutrophic source water. Particular emphasis is paid to elucidate the potential mechanisms for enhancing algae coagulation by two different oxidants.
2. Materials and methods 2.1. Pilot-scale study The pilot plant was located in the Cheng Ching Lake Water Works, which is the major water supplier to about 2 million people in the greater Kaohsiung area in southern Taiwan. Raw water is supplied by the nearby eutrophic Cheng Ching Lake. The pilot experiments involved the operation of two parallel treatment trains, which included conventional coagulation, flocculation, sedimentation, filtration units, and preceded by ozone and potassium permanaganate oxidation, respectively. This research was mainly to compare the effect of these two preoxidants on algae removal by subsequent coagulation, flocculation and sedimentation processes. The flow rates for each train varied from 29.4 to 109 L min 1. Ozone was produced by an ozone generator (Type CFS-1, Ozat Ozone Generator, Ozonia, Duebendorf, Switzerland) employing pure oxygen as the feed gas. Ozone was bubbled upward through the ceramic diffuser on the bottom of the ozone contactor, with water flowing countercurrently. Potassium permanganate was directly added into a rapid mixing tank with a chemical dosing pump from a stock solution of KMnO4. The dosage for ozone and permanganate ranged 1–7 mg L 1, and 0.5– 1.75 mg L 1, respectively. Both trains used liquid alum (7.5% Al2O3) as a coagulant, with a dosage of approximately 70 mg L 1. The mean velocity gradient for flocculation was controlled to 40 s 1. Tube settlers were used for sedimentation with an overflow rate of 69.8 m3 m 2 d 1. The analysis of water quality parameters adhered to those of the Standard Methods (APHA, 1995).
841
CO2 was supplied to the cultures by syringe injection every day. Cultures were harvested during the log growth phase for use in the experiments. Stock cultures of Chodatella sp. cells were collected by centrifugation at 4000 rpm (3000g) for 5 min. The precipitated algal cells were then washed and resuspended in 0.015 M sodium perchlorate solution, simulating the ionic strength of the source water. The pH values ranged from 7.0 to 8.0. Algae cell concentration in the suspension solution was determined by measuring OD684 of the cells. Aliquot suspensions of constant algae cell concentration (4 107 cells mL 1) were prepared for experiments. In order to simulate the hardness of the source water, the algal suspensions were spiked with a CaCl2 solution to attain a hardness of 250 mg L 1 as CaCO3 in some of the experiments. 2.3. Preoxidation and settling tests Preozonation of algal suspensions was performed in a semibatch model. Ozone from a laboratory generator (Model T-816, Welsbach Ozone Systems Corp., San Jose, CA, USA) was applied to a sample in a 1 L gas washing bottle. A constant gas phase ozone concentration and flow rate were maintained and the required ozone dosages were obtained by varying the ozone injection time. For the permanganate preoxidation experiments, the stock solution of potassium permanganate (10 g L 1) was prepared by dissolving crystal potassium permanganate (Merck, Germany) in Milli-Q water and filtering through a 0.2 lm membrane filter (MFS, Japan). It was then standardized by titration with sodium oxalate and diluted to the concentration as needed for the experiments. Subsequently, a predetermined volume of KMnO4 solution was added to the algal suspension. After 60 min of contact time under slow mixing conditions using a magnetic stirrer, the preoxidized algal suspensions were poured into 1 L cone-shaped graduated plastic vessels (Imhoff cone) for a quiet settling. Immediately following, samples were collected for NPDOC (nonpurgeable dissolved organic carbon) measurements and for scanning electron microscopy (SEM). At different time intervals, samples were collected at a depth of 5 cm below the water surface, and residual algae concentrations analyzed by measuring OD684 with an ultraviolet/visible spectrophotometer (Model U-2001, Hitachi, Japan). The blank test without preoxidant dosing was also carried out simultaneously. 2.4. Coagulation of algal suspension by jar tests Jar tests were performed on algal suspensions, with and without preoxidation, using a six-paddle stirrer (Phipps and Bird, U.S.A.). Each 0.5 L sample was dosed with a predetermined concentration of alum from an alum stock solution (1000 mg L 1 Al2(SO4)3 18H2O). Each dosed sample was rapidly mixed for 3 min at 100 rpm, followed by a slow mixing for 15 min at 35 rpm, and then a quiet settling of 30 min. At the end of the settling period, the supernatant was taken at 3 cm below the water surface using a 50 mL wide bore pipette to determine the residual algae concentration. In some of the experiments, approximately 10 mL of the algal suspension was taken immediately after 1 min of rapid mixing to measure the zeta potential of algae floc.
2.2. Bench-scale testing
3. Analyses
2.2.1. Algal culturing and algal suspension preparation One of the dominant green algal species, Chodatella sp., isolated from Cheng Ching Lake, was cultured in a media according to Norris et al. (1955). The cultures were grown in 1 L bottles, and incubated at 28 °C under continuous illumination (10 000 ± 2000 lx).
3.1. NPDOC measurement For NPDOC measurement, the sample was first filtered through a 0.45 lm filter (MFS, Japan), acidified to a pH 2 by adding phosphoric acid (85%), purged by high purity N2 gas, and then injected
J.-J. Chen et al. / Chemosphere 74 (2009) 840–846
into a total organic carbon analyzer (Model TOC-5000, Shimadzu, Kyoto, Japan). 3.2. Zeta potential measurement The zeta potential measurements in this study utilized a zeta meter (Zetasizer 2000HSA, Malvern Instruments, UK), which measures the zeta potential using the principle of photo correlation spectroscopy. Samples with adequate cell concentrations were injected slowly into the zeta meter and measured at least 3 times to calculate an average value. 3.3. Scanning electron microscopy Algae samples for SEM were first filtered through a 0.45 lm nylon membrane filter (MFS, Japan). The membrane filters were placed in a phosphate buffer solution (pH = 7) and fixed with 2.5% glutaldehyde at 4 °C overnight. Next, the membrane filters were washed using a phosphate buffer solution, dehydrated with successively different concentrations of ethanol, and dried by critical point drying. Dried samples were mounted on copper stubs and sputter coated with gold-palladium. The specimens were observed and photographed using a JEOL JXA-840 at 25 kV or Hitachi S-3000 n SEM at 15 kV. 4. Results and discussion 4.1. Characteristics of raw water quality The raw water quality during the pilot-testing period is shown in Table 1. It is characterized by a high level of hardness, moderate turbidity, and a low dissolved organic content. The presence of nutrient in the lake water explains the algae bloom, with concentrations as high as 1.6 105 cells mL 1. The algae species not only Table 1 Characteristics of eutrophic raw water quality. Parameters
Range
Average
Temperature (°C) pH Turbidity (NTU) Alkalinity (mg L 1 as CaCO3) Dissolved oxygen (mg L 1) Total hardness (mg L 1 as CaCO3) NPDOC (mg L 1) UV-absorbance at 254 nm (m 1) Algae counts (cells mL 1) NH3–N (mg L 1) NO2 –N (mg L 1) NO3 –N (mg L 1) Total phosphate (mg P L 1)
16.6–31.3 7.9–8.7 5.8–113 103–218 5.7–11 190–273 0.5–2.1 1.2–4.4 2120–16 430 0.07–0.59 0.02–0.23 0.21–1.8 0.04–0.31
25.1 – 18.2 162 8.2 238 1.2 2.5 8140 0.21 0.09 0.94 0.13
Table 2 Effect of preoxidant doses on algae coagulation in the pilot study.* Preoxidant
Dose, mg L
1
Algae count, cells mL
1
Raw water
Settled
Removal, %
Without
0
7640
1220
84
Ozone
1 3 7
5790 9460 4070
540 640 2300
91 93 43
Permanganate
0.5 1.0 1.25 1.75
1290 1620 3220 2801
190 200 80 340
85 88 98 88
*
Alum: 70 mg L
1
(7.5% Al2O3).
frequently caused unpleasant tastes and odors, but also hindered coagulation processes. Based on the data collected during the pilot test period without preoxidation, it shows that the variation pattern of the settled water turbidity was almost synchronized with that of the raw water algae concentration. When the algae concentration exceeded 7000 cells mL 1, the settled water turbidity was rarely lower than 2 NTU. 4.2. Pilot-scale study Table 2 presents the effects of varying the ozone and permanganate doses on algae coagulation removal during the pilot study. During that time period, the dominant algae species in raw water were green algae, diatom, and blue–green algae. It can be seen that both preoxidants can improve the subsequent algae removal through coagulation, flocculation, and sedimentation. Without preoxidation, the algae removal was 84% at a 70 mg L 1 alum dosage. With 1 mg L 1 ozone preoxidation, the algae removal efficiency increased to 91% at the same coagulant dosage; however, higher ozone dosages did not further improve algae removal. When the preozonation dosage reached 7 mg L 1, the removal efficiency decreased to 43%, much lower than that without preoxidation. It is speculated that this may be due to the release of IOM after cell lysis under higher ozone dosage. The released organic matter may hinder the coagulation removal of turbidity (Chandrakanth et al., 1996). Potassium permanganate, as a preoxidant, also improved the algae coagulation removal. There also existed an optimum dosage; dosages exceeding this value decreased algae removal efficiency. However, Sukenik et al. (1987) did not notice any disruption in alum flocculation of algae (Scenedesmus sp.) with dosage for preozonation up to 8.1 mg L 1. The discrepancy between this study and the Sukenik’s could be due to the difference Table 3 Comparative simultaneously parallel algae removal between ozone and permanganate preoxidation in the pilot study. Preoxidant
Hardness, mg L as CaCO3
1 mg L 1 O3 0.75 mg L 1 KMnO4 3 mg L 1 O3 0.75 mg L 1 KMnO4 70 mg L
1
1
Algae count of raw water, cells mL 1
Removal, %
206
3150
73 84
218
9460
93 97
Alum (7.5% Al2O3).
100
95
Residual algae (%)
842
90 -1
0 mg L
-1
O3
-1
KMnO4
0.9 mg L
85
1.0 mg L
80 0
1
2
3
4
5
Settling time (hr) Fig. 1. Effect of ozone and permanganate preoxidation on cell settling (Batch test, synthetic water with initial algae concentration: 4 107 cells mL 1, no Ca++ added).
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in the algae cell concentration in the test waters. In Sukenik’s study, high concentration (2 106 cells mL 1) of pure-cultured cell suspensions was used. While in our pilot study, the algae cell concentration in the lake water was all lower than 10 000 cells mL 1. Therefore, the amount of oxidant per cell received in our study would be much higher than that of the Sukenik’s. Table 3 shows the results of algae removal from the simultaneously parallel operation of two coagulation and sedimentation treatment trains in the pilot testing. The only difference in these two trains was the preoxidants and their dosages, one with ozone and the other with permanganate. The results show that the train with permanganate had a higher algae removal, even at a lower dosage than that of ozone. 4.3. Bench-scale study 4.3.1. Effects of preoxidation on algae settling and cell morphology Fig. 1 shows the effect of preoxidation on cell settling. Although both preoxidants improve algae settling, the efficiency of perman-
ganate was more significant than that of ozone. Even by naked eye observation, cell aggregations were observed on the wall of the Imhoff cones when permanganate was used as a preoxidant. No cell aggregations were observed using ozone or in the control system. Fig. 2 shows the SEM of Chodatella sp., taken before and after preoxidation by either ozone or permanganate. Before preoxidation, the fine markings on the cell wall and bristles can be clearly seen (Fig. 2a). The SEM (Fig. 2b) demonstrates extensive cell destruction (cell lysis), where bristles broke off at an ozone dose of 1.3 mg L 1. At a permanganate preoxidation dose of 1.5 mg L 1, no significant cell lysis was observed. Furthermore, the surface of the cell seems to be coated with a thick layer of materials, which were probably the aggregations of EOM and MnO2 (Fig. 2c). The better performance of the permanganate compared to the ozone was conjectured to be due to the incorporation of algae flocs onto the reduced product, MnO2, thereby increasing its specific gravity and its settling velocity. Further, it is also speculated that permanganate could induce the release of EOM from algae cell, which may further enhance the aggregation of algal cells and MnO2 (Chen and Yeh, 2005). Fig. 3 shows the increase in the NPDOC value of the algal suspension under various doses of preoxidant, and demonstrates an increasing NPDOC when the preoxidants doses were higher than 1.2 mg L 1. The increase in NPDOC is certainly due to the release of EOM or IOM. This phenomenon is more significant for ozone than permanganate, due to the higher oxidation power of the former. The fact that release of NPDOC and associated cell lysis, as demonstrated by SEM, at higher ozone doses explains the phenomenon of poorer algae removal under high preoxidation doses in the previous pilot plant testing.
1.4
-1
NPDOC released from cells (mg L )
4.3.2. Effects of preoxidation on coagulation of cultured algal suspension Fig. 4a shows the jar test results of algal suspension with alum under various preozonation dosages. Generally speaking, preozonation is helpful for algae coagulation removal. The major mechanism is considered to be through the destabilization of algae floc, as shown by the decreasing absolute value of the zeta potential of algae floc with increasing preozonation dosage (Fig. 4b). The results also imply that there probably exists an optimum dosage for preozonation. For example, at a low alum dosage of 10 mg L 1,
O3 KMnO4
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
1
2
3
4
Preoxidant dose (mg L-1 ) Fig. 2. SEM micrographs of cell (Chodatella sp.) surface morphology with and without preoxidation. (a) Without preoxidation, 8 k, (b) 1.3 mg L 1 O3, 15 k, and (c) 1.5 mg L 1 KMnO4, 19 k.
Fig. 3. Effect of preoxidant doses on the NPDOC (method detention limit: 0.11 mg L 1) (batch test, synthetic water with initial algae concentration: 4 107 cells mL 1, no Ca++ added).
J.-J. Chen et al. / Chemosphere 74 (2009) 840–846
a
100
Algae removal (%)
844
80 60
O3 (mg L )
0 0.5 1.2 1.7 2.1
KMnO4 (mg L-1 )
0 0.5 1.2 1.7 2.1
-1
40 20 0
Zeta potential (mV)
b
-21 -24 -27 -30
c
100
Algae removal (%)
-33
80 60 40 20 0
Zeta potential (mV)
d
-26 -28 -30 -32 -34 0
10
20
30
40
50
-1
Alum dose (mg L ) Fig. 4. Effect of preoxidants doses on algae coagulation removal and zeta potential of algal floc. (a and b for ozone, c and d for permanganate, batch test, synthetic water with initial algae concentration: 4 107 cells mL 1, no Ca++ added).
preozonation at doses higher than 1.2 mg L 1 does not seem helpful to subsequent algae removal via alum coagulation (Fig. 4a). The finding is consistent with the zeta potential data. Fig. 4b also shows that the absolute values of the algae floc zeta potential did not further decrease when the ozone dosage exceeded 1.2 mg L 1. As we have previously discussed, ozone overdose may cause cell lysis and the release of EOM or IOM, which can compete with the suspended and colloidal solids for alum coagulant. Previous studies, concerned with the effect of preoxidation on algae coagulation removal, also showed that preoxidants can damage algae cell surface architecture, and bring an efflux of intracellular contents to the aqueous phase (Sukenik et al., 1987; Peterson et al., 1995). Whether the preoxidation will enhance or inhibit algae coagulation removal depends on many factors, such
as the algae species, type of oxidants, and the ratio of oxidant dose to cell number. Plummer and Edzwald (2002) showed that ozone or chlorine preoxidation could yield an improvement in coagulation removal of green alga (Scenedesmus sp.) by polyaluminium chloride, but not for diatom (Cyclotella sp.). Sukenik et al. (1987) studied the effect of chlorine, ozone, and chlorine dioxide on Scenedesmus sp. cultures, and reported that prechlorination at high dosage increased the required alum dosage, believing it was due to the leak of macromolecular organics from the cells, which interfered with the interaction between alum and algae cells and thus inhibited flocculation. Using a pretreatment with stronger oxidants, namely chlorine dioxide and ozone, the organics released were further oxidized. The improvement of algal flocculation with alum by ClO2 or O3 preoxidation was thought to be through cell surface
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alteration, which resulted in colloidal destabilization, therefore improving flocculation. Fig. 4a also shows that the decrease in algae coagulation removal caused by ozone overdose can be compensated by higher alum dosage. Fig. 4c shows the permanganate dosage results are similar to those of Fig. 4a. Permanganate preoxidation was also found to enhance alum coagulation of algae. There also existed an optimum preoxidant dosage, 1.7 mg L 1 KMnO4 in this case, as higher dosage did not further improve coagulation. The effect of permanganate preoxidation on the algae floc zeta potential was less significant than that of preozonation, probably due to the weak oxidation power of permanganate. Further, the relationship between the zeta potential and the algae removal efficiency was also not as clear as that of preozonation. This may be due to the difference in the destabilization mechanism. For permanganate preoxidation, as has been mentioned before, it is speculated that algae removal could involve the adsorption of MnO2 into the algae floc, rather than simple charge neutralization.
important role in algae coagulation removal in the absence of calcium. In the presence of calcium, Fig. 5d shows a much lower absolute value of the zeta potential for the entire system, compared with that of Fig. 5b. This is due to charge neutralization in the presence of calcium. The higher absolute value of the zeta potential for permanganate preoxidation than that with preozonation is contrary to the higher algae removal using permanganate, as shown in Fig. 5c. It probably also supports our previous speculation that, for the case of permanganate, the algae removal could be more due to the adsorption of MnO2, the reducing product of permanganate, into the algae floc. In the pH range of neutral water, the surfaces of both MnO2 and the algae cell are negatively charged. Calcium ions may serve as bridges to hold the two negatively charged surfaces together. Therefore, in the presence of calcium, the beneficial effect of permanganate preoxidation on algae coagulation removal was significantly enhanced. These bench-scale testing results also provide explanation to our pilot-scale testing results, which showed preoxidation with permanganate induced better algae removal by coagulation-sedimentation than preozonation, as the raw water of the pilot plant contained a hardness of about 240 mg L 1 as CaCO3.
4.3.3. Effect of calcium on algae removal by preoxidation–coagulation process Fig. 5a and c compare the effect of the presence or absence of calcium on algae removal by the preoxidation-coagulation process. Under the testing conditions, preoxidations using either ozone or permanganate at the dosage of 1.3 mg L 1 were beneficial to alum coagulation of algae. Adding calcium to a system greatly improved algae coagulation removal either with or without preoxidation. However, in the absence of calcium, preozonation generally induced higher algae removal, especially at a higher alum dosage (Fig. 5a). In the presence of calcium (250 mg L 1 as CaCO3), the performance of the system with permanganate was better than that with preozonation, as good coagulation occurred for low alum doses. Fig. 5b shows the corresponding zeta potential of the coagulated algae floc of Fig. 5a. Without calcium, the system with preozonation showed much lower absolute value of the zeta potential, which corresponds to a lower colloidal stability. This finding is consistent with the higher algae removal using preozonation. This also indicates that charge neutralization probably plays an
Algae removal (%)
a
5. Conclusions Both pilot and laboratory experiments were conducted for comparing the effect of ozone and permanganate preoxidation on algae removal by alum coagulation. Both experiments show preoxidation under an appropriate dose was beneficial to algae coagulation removal. Preoxidation for improving algae coagulation may be attributed to a decrease in cell stability; however, overdosing may cause cell lysis and the release of organics, which could interfere with algae cell coagulation. In the presence of calcium, both preoxidants further improved algae coagulation; and this phenomenon was more significant for preoxidation using permanganate than ozone. It is speculated that this is due to the fact that the positively charged calcium ions can serve as bridges to hold the
100
c
80
With Ca 2+ (250 mg L-1 as CaCO3)
60
0 mg L-1
40
1.3 mg L-1 O3
2+
Without Ca
20
1.3 mg L-1 KMnO4
0
Zeta potential (mV)
b
d
0 -10 -20 -30 -40
0
10
20
30
40 -1
Alum dose (mg L )
50
0
10
20
30
40
50
-1
Alum dose (mg L )
Fig. 5. Comparison of algae coagulation removal and the zeta potential of algal floc, with and without preoxidation in the absence (a, b) and presence (c, d) of calcium. (Batch test, synthetic water with initial algae concentration: 4 107 cells mL 1).
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negatively charged MnO2 and algae cells together. The laboratory results also explain the observations from the pilot testing, which demonstrate preoxidation using permanganate has a better effect on algae removal than ozone. This is due to the high hardness content of the raw water in the pilot plant. Acknowledgments The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract No. NSC 92-2211-E-006-027. The authors also acknowledge the assistance of Mr. Yu-ren Chen in parts of the pilot-scale testing. References APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington, DC. Bernhardt, H., Clasen, J., 1991. Flocculation of micro-organisms. J. Water Supply: Res. Technol. – AQUA. 40 (2), 76–87. Chandrakanth, M.S., Krishnan, S., Amy, G.L., 1996. Interactions between ozone, AOM, and particles in water treatment. J. Environ. Eng. – ASCE. 122, 459–468. Chen, J.J., Yeh, H.H., 2005. The mechanisms of potassium permanganate on algae removal. Water Res. 39, 4420–4428. Fitzgerald, G.P., 1966. Use of potassium permanganate for control of problem algae. J. Am. Water Works Ass. 58 (5), 609–614.
Knappe, R., Detlef, R.U., Belk, C., Briley, D.S., Grandy, S.R., Rastogi, N., Rike, A.H., 2004. Algae Detection and Removal Strategies for Drinking Water Treatment Plants. AWWA Research Foundation, Denver, CO, USA. Norris, L., Norris, R.E., Calvin, M., 1955. A survey of the rates and products of shortterm photosynthesis in plants of nine Phyla. J. Exp. Bot. 6, 64–74. Paralkar, A., Edzwald, J.K., 1996. Effect of ozone on EOM and coagulation. J. Am. Water Works Ass. 88 (4), 143–154. Peterson, H.G., Hrudey, S.E., Cantin, I.A., Perley, T.R., Kenefick, S.L., 1995. Physiological toxicity, cell membrane damage and the release of dissolved organic carbon and geosmin by Aphanizomenon flos-aquae after exposure to water treatment chemicals. Water Res. 29, 1515–1523. Petruševski, B., van Breemen, A.N., Alaerts, G., 1996. Effect of permanganate pretreatment and coagulation with dual coagulants on algae removal in direct filtration. J. Water Supply Res. Technol. – AQUA. 45 (6), 316–326. Plummer, J.D., Edzwald, J.K., 2001. Effect of ozone on algae as precursors for trihalomethane and haloacetic acid. Environ. Sci. Technol. 35, 3661–3668. Plummer, J.D., Edzwald, J.K., 2002. Effects of chlorine and ozone on algal cell properties and removal of algae by coagulation. J. Water Supply Res. Technol. – AQUA. 51 (6), 307–318. Schmidt, W., Hambsch, B., Petzoldt, H., 1998. Classification of algogenic organic matter concerning its contribution to the bacterial regrowth potential and byproducts formation. Water Sci. Technol. 37 (2), 91–96. Steynberg, M.C., Gugleilm, M.M., Geldenhuys, J.C., Pieterse, A.J.H., 1996. Chlorine and chlorine dioxide: pre-oxidants used as algocide in potable water plants. J. Water Supply Res. Technol. – AQUA. 45 (4), 162–170. Suffet, I.H., Mallevialle, J., Kawczynski, E., 1995. Advances in Taste-and-Odor Treatment and Control. Cooperative Research Reports, AWWA Research Foundation, Denver, CO, USA. Sukenik, A., Teltch, B., Wachs, A.W., Shelef, G., Nir, I., Levanon, D., 1987. Effect of oxidants on microalgal flocculation. Water Res. 21, 533–539.
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Effect of ozone and permanganate on algae coagulation removal – Pilot and bench scale tests Jen-Jeng Chen a, Hsuan-Hsien Yeh b,*, I-Cheng Tseng c a
Department of Environmental Resources Management, Tajen University, Pingtung 907, Taiwan Department of Environmental Engineering, National Cheng Kung University, Tainan 701, Taiwan c Department of Life Sciences, National Cheng Kung University, Tainan 701, Taiwan b
a r t i c l e
i n f o
Article history: Received 17 May 2008 Received in revised form 1 October 2008 Accepted 3 October 2008 Available online 11 December 2008 Keywords: Ozone Permanganate Algae Coagulation
a b s t r a c t Both pilot and laboratory scale experiments are conducted to compare the effect of ozone and permanganate preoxidation on algae removal by alum coagulation. Both appropriately dosed preoxidants are shown to be beneficial to algae coagulation removal. This may be attributed to a decrease in cell stability; however, overdosing may cause cell lysis and the release of organics, which could interfere with algae cell coagulation. The presence of calcium further enhances the beneficial effect of preoxidation on algae coagulation; however, this phenomenon is more significant for using permanganate than ozone. It is speculated that this is due to the fact that the positively charged calcium ions can serve as bridges to hold the negatively charged MnO2 and algal cells together. Further, this behavior also explains the superior performance of permanganate preoxidation compared to that obtained using ozone for algae coagulation removal in pilot testing, as the source water contains high calcium content. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The presence of algae in source water can cause a variety of problems for drinking water treatment. Algae and its metabolites can have a significant impact on water quality, such as the production of unpleasant tastes and odors, formation of disinfection byproducts (DBPs), and toxins from cyanobacteria. Furthermore, algae are known to interfere with the water treatment process, causing short filter runs, increases in coagulant demand, and microbial regrowth in distribution systems (Suffet et al., 1995; Schmidt et al., 1998; Plummer and Edzwald, 2001). For algae removal, coagulation is the key step in conventional water treatment processes. In order to improve the efficiency of algae removal, control the growth of other nuisance microorganisms, and simultaneously oxidize reduced species such as iron and manganese, prechlorination may be used. Unfortunately, in addition to chlorinated DBP problems, chlorine can induce both physiological stress and membrane damage to cells, causing the release of extra- and intra-cellular organic matter (EOM and IOM), respectively, which may include taste- and odor-causing and toxic compounds (Peterson et al., 1995). Further, Sukenik et al. (1987) found that at lower dosage (2 mg L 1 Cl2), prechlorination did not result in a change in coagulant (alum) demand. However, at higher chlorine dosage (10 and 20 mg L 1 Cl2), the required alum dosage needed to be increased, compared to the same algae system without prechlorination. * Corresponding author. Tel.: +886 6 2757575x65823; fax: +886 6 2752790. E-mail address: [email protected] (H.-H. Yeh). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.10.009
Numerous researchers have demonstrated that other preoxidants, such as ozone, chlorine dioxide, and permanganate, can improve algae removal by coagulation and filtration processes in drinking water treatment plants (Fitzgerald, 1966; Sukenik et al., 1987; Petruševski et al., 1996; Steynberg et al., 1996). These preoxidants can improve algae coagulation by deactivating algal cells, reducing cell stability, or liberating EOM. The effects of the oxidant on algae removal during water treatment are dependent on oxidant type, oxidant dosage, and the algal species. For example, ozone was found to improve the coagulation of green algae (Scenedesmus quadricauda) but not that of diatoms (Cyclotella sp.) (Plummer and Edzwald, 2002). This is probably due to the difference in surface characteristics of the cells. Bernhardt and Clasen (1991) reported that EOM from cultures of green and blue–green algae and diatoms behaved like anionic and non-ionic polyelectrolytes. The amount and properties of EOM produced varied with algae species, oxidant type, and oxidant dosages. The EOM, depending on the concentration and molecular weight, can enhance or hinder flocculation. Low dosages of ozone can cause a release of EOM, which aids coagulation, but at higher doses, ozone affects EOM structure (e.g. by lowering molecular size), which may hinder subsequent coagulation (Paralkar and Edzwald, 1996). Petruševski et al. (1996) reported that a permanganate pretreatment, followed by coagulation and direct filtration for impounded surface water, distinctly improved particle and algae removal, in comparison to efficiencies commonly achieved without preoxidation. The in situ production of algae-derived coagulant aid and
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deactivation of motile microorganisms were suggested to be the possible mechanisms induced by permanganate preoxidation. Knappe et al. (2004) conducted jar tests with algae-spiked surface water to study the effect of potassium permanganate preoxidation on algae removal, chlorine demand, and DBP formation. The results demonstrated that the removal of Microcystis aeruginosa, Anabaena flos-aquae, and Synura petersenii improved with an increase in KMnO4 dosage and contact time. Experiments with S. petersenii indicated that the addition of 3 mg L 1 KMnO4 increased coagulant demand, due to the release of IOM, the formation of MnO2, and the oxidation of natural organic matter. Nevertheless, KMnO4-induced cell lysis did not show an increased chlorine demand or DBP formation. Previous studies investigating the effect of ozone or permanganate oxidation on algae removal have only been conducted at a lab or pilot scale. Additionally, attention has not been given to the effect of background aqueous constituents, such as hardness. The purpose of this study is to examine and compare the effects oxidation on algae coagulation. In the first half of this investigation, a pilot study was conducted using eutrophic source water. In the second half of this investigation, a laboratory-scale study using pure-cultured algal suspensions was conducted to elucidate observations obtained from pilot testing. In the laboratory-scale study, the algal suspension was spiked with calcium ions to simulate a high hardness content of the eutrophic source water. Particular emphasis is paid to elucidate the potential mechanisms for enhancing algae coagulation by two different oxidants.
2. Materials and methods 2.1. Pilot-scale study The pilot plant was located in the Cheng Ching Lake Water Works, which is the major water supplier to about 2 million people in the greater Kaohsiung area in southern Taiwan. Raw water is supplied by the nearby eutrophic Cheng Ching Lake. The pilot experiments involved the operation of two parallel treatment trains, which included conventional coagulation, flocculation, sedimentation, filtration units, and preceded by ozone and potassium permanaganate oxidation, respectively. This research was mainly to compare the effect of these two preoxidants on algae removal by subsequent coagulation, flocculation and sedimentation processes. The flow rates for each train varied from 29.4 to 109 L min 1. Ozone was produced by an ozone generator (Type CFS-1, Ozat Ozone Generator, Ozonia, Duebendorf, Switzerland) employing pure oxygen as the feed gas. Ozone was bubbled upward through the ceramic diffuser on the bottom of the ozone contactor, with water flowing countercurrently. Potassium permanganate was directly added into a rapid mixing tank with a chemical dosing pump from a stock solution of KMnO4. The dosage for ozone and permanganate ranged 1–7 mg L 1, and 0.5– 1.75 mg L 1, respectively. Both trains used liquid alum (7.5% Al2O3) as a coagulant, with a dosage of approximately 70 mg L 1. The mean velocity gradient for flocculation was controlled to 40 s 1. Tube settlers were used for sedimentation with an overflow rate of 69.8 m3 m 2 d 1. The analysis of water quality parameters adhered to those of the Standard Methods (APHA, 1995).
841
CO2 was supplied to the cultures by syringe injection every day. Cultures were harvested during the log growth phase for use in the experiments. Stock cultures of Chodatella sp. cells were collected by centrifugation at 4000 rpm (3000g) for 5 min. The precipitated algal cells were then washed and resuspended in 0.015 M sodium perchlorate solution, simulating the ionic strength of the source water. The pH values ranged from 7.0 to 8.0. Algae cell concentration in the suspension solution was determined by measuring OD684 of the cells. Aliquot suspensions of constant algae cell concentration (4 107 cells mL 1) were prepared for experiments. In order to simulate the hardness of the source water, the algal suspensions were spiked with a CaCl2 solution to attain a hardness of 250 mg L 1 as CaCO3 in some of the experiments. 2.3. Preoxidation and settling tests Preozonation of algal suspensions was performed in a semibatch model. Ozone from a laboratory generator (Model T-816, Welsbach Ozone Systems Corp., San Jose, CA, USA) was applied to a sample in a 1 L gas washing bottle. A constant gas phase ozone concentration and flow rate were maintained and the required ozone dosages were obtained by varying the ozone injection time. For the permanganate preoxidation experiments, the stock solution of potassium permanganate (10 g L 1) was prepared by dissolving crystal potassium permanganate (Merck, Germany) in Milli-Q water and filtering through a 0.2 lm membrane filter (MFS, Japan). It was then standardized by titration with sodium oxalate and diluted to the concentration as needed for the experiments. Subsequently, a predetermined volume of KMnO4 solution was added to the algal suspension. After 60 min of contact time under slow mixing conditions using a magnetic stirrer, the preoxidized algal suspensions were poured into 1 L cone-shaped graduated plastic vessels (Imhoff cone) for a quiet settling. Immediately following, samples were collected for NPDOC (nonpurgeable dissolved organic carbon) measurements and for scanning electron microscopy (SEM). At different time intervals, samples were collected at a depth of 5 cm below the water surface, and residual algae concentrations analyzed by measuring OD684 with an ultraviolet/visible spectrophotometer (Model U-2001, Hitachi, Japan). The blank test without preoxidant dosing was also carried out simultaneously. 2.4. Coagulation of algal suspension by jar tests Jar tests were performed on algal suspensions, with and without preoxidation, using a six-paddle stirrer (Phipps and Bird, U.S.A.). Each 0.5 L sample was dosed with a predetermined concentration of alum from an alum stock solution (1000 mg L 1 Al2(SO4)3 18H2O). Each dosed sample was rapidly mixed for 3 min at 100 rpm, followed by a slow mixing for 15 min at 35 rpm, and then a quiet settling of 30 min. At the end of the settling period, the supernatant was taken at 3 cm below the water surface using a 50 mL wide bore pipette to determine the residual algae concentration. In some of the experiments, approximately 10 mL of the algal suspension was taken immediately after 1 min of rapid mixing to measure the zeta potential of algae floc.
2.2. Bench-scale testing
3. Analyses
2.2.1. Algal culturing and algal suspension preparation One of the dominant green algal species, Chodatella sp., isolated from Cheng Ching Lake, was cultured in a media according to Norris et al. (1955). The cultures were grown in 1 L bottles, and incubated at 28 °C under continuous illumination (10 000 ± 2000 lx).
3.1. NPDOC measurement For NPDOC measurement, the sample was first filtered through a 0.45 lm filter (MFS, Japan), acidified to a pH 2 by adding phosphoric acid (85%), purged by high purity N2 gas, and then injected
J.-J. Chen et al. / Chemosphere 74 (2009) 840–846
into a total organic carbon analyzer (Model TOC-5000, Shimadzu, Kyoto, Japan). 3.2. Zeta potential measurement The zeta potential measurements in this study utilized a zeta meter (Zetasizer 2000HSA, Malvern Instruments, UK), which measures the zeta potential using the principle of photo correlation spectroscopy. Samples with adequate cell concentrations were injected slowly into the zeta meter and measured at least 3 times to calculate an average value. 3.3. Scanning electron microscopy Algae samples for SEM were first filtered through a 0.45 lm nylon membrane filter (MFS, Japan). The membrane filters were placed in a phosphate buffer solution (pH = 7) and fixed with 2.5% glutaldehyde at 4 °C overnight. Next, the membrane filters were washed using a phosphate buffer solution, dehydrated with successively different concentrations of ethanol, and dried by critical point drying. Dried samples were mounted on copper stubs and sputter coated with gold-palladium. The specimens were observed and photographed using a JEOL JXA-840 at 25 kV or Hitachi S-3000 n SEM at 15 kV. 4. Results and discussion 4.1. Characteristics of raw water quality The raw water quality during the pilot-testing period is shown in Table 1. It is characterized by a high level of hardness, moderate turbidity, and a low dissolved organic content. The presence of nutrient in the lake water explains the algae bloom, with concentrations as high as 1.6 105 cells mL 1. The algae species not only Table 1 Characteristics of eutrophic raw water quality. Parameters
Range
Average
Temperature (°C) pH Turbidity (NTU) Alkalinity (mg L 1 as CaCO3) Dissolved oxygen (mg L 1) Total hardness (mg L 1 as CaCO3) NPDOC (mg L 1) UV-absorbance at 254 nm (m 1) Algae counts (cells mL 1) NH3–N (mg L 1) NO2 –N (mg L 1) NO3 –N (mg L 1) Total phosphate (mg P L 1)
16.6–31.3 7.9–8.7 5.8–113 103–218 5.7–11 190–273 0.5–2.1 1.2–4.4 2120–16 430 0.07–0.59 0.02–0.23 0.21–1.8 0.04–0.31
25.1 – 18.2 162 8.2 238 1.2 2.5 8140 0.21 0.09 0.94 0.13
Table 2 Effect of preoxidant doses on algae coagulation in the pilot study.* Preoxidant
Dose, mg L
1
Algae count, cells mL
1
Raw water
Settled
Removal, %
Without
0
7640
1220
84
Ozone
1 3 7
5790 9460 4070
540 640 2300
91 93 43
Permanganate
0.5 1.0 1.25 1.75
1290 1620 3220 2801
190 200 80 340
85 88 98 88
*
Alum: 70 mg L
1
(7.5% Al2O3).
frequently caused unpleasant tastes and odors, but also hindered coagulation processes. Based on the data collected during the pilot test period without preoxidation, it shows that the variation pattern of the settled water turbidity was almost synchronized with that of the raw water algae concentration. When the algae concentration exceeded 7000 cells mL 1, the settled water turbidity was rarely lower than 2 NTU. 4.2. Pilot-scale study Table 2 presents the effects of varying the ozone and permanganate doses on algae coagulation removal during the pilot study. During that time period, the dominant algae species in raw water were green algae, diatom, and blue–green algae. It can be seen that both preoxidants can improve the subsequent algae removal through coagulation, flocculation, and sedimentation. Without preoxidation, the algae removal was 84% at a 70 mg L 1 alum dosage. With 1 mg L 1 ozone preoxidation, the algae removal efficiency increased to 91% at the same coagulant dosage; however, higher ozone dosages did not further improve algae removal. When the preozonation dosage reached 7 mg L 1, the removal efficiency decreased to 43%, much lower than that without preoxidation. It is speculated that this may be due to the release of IOM after cell lysis under higher ozone dosage. The released organic matter may hinder the coagulation removal of turbidity (Chandrakanth et al., 1996). Potassium permanganate, as a preoxidant, also improved the algae coagulation removal. There also existed an optimum dosage; dosages exceeding this value decreased algae removal efficiency. However, Sukenik et al. (1987) did not notice any disruption in alum flocculation of algae (Scenedesmus sp.) with dosage for preozonation up to 8.1 mg L 1. The discrepancy between this study and the Sukenik’s could be due to the difference Table 3 Comparative simultaneously parallel algae removal between ozone and permanganate preoxidation in the pilot study. Preoxidant
Hardness, mg L as CaCO3
1 mg L 1 O3 0.75 mg L 1 KMnO4 3 mg L 1 O3 0.75 mg L 1 KMnO4 70 mg L
1
1
Algae count of raw water, cells mL 1
Removal, %
206
3150
73 84
218
9460
93 97
Alum (7.5% Al2O3).
100
95
Residual algae (%)
842
90 -1
0 mg L
-1
O3
-1
KMnO4
0.9 mg L
85
1.0 mg L
80 0
1
2
3
4
5
Settling time (hr) Fig. 1. Effect of ozone and permanganate preoxidation on cell settling (Batch test, synthetic water with initial algae concentration: 4 107 cells mL 1, no Ca++ added).
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in the algae cell concentration in the test waters. In Sukenik’s study, high concentration (2 106 cells mL 1) of pure-cultured cell suspensions was used. While in our pilot study, the algae cell concentration in the lake water was all lower than 10 000 cells mL 1. Therefore, the amount of oxidant per cell received in our study would be much higher than that of the Sukenik’s. Table 3 shows the results of algae removal from the simultaneously parallel operation of two coagulation and sedimentation treatment trains in the pilot testing. The only difference in these two trains was the preoxidants and their dosages, one with ozone and the other with permanganate. The results show that the train with permanganate had a higher algae removal, even at a lower dosage than that of ozone. 4.3. Bench-scale study 4.3.1. Effects of preoxidation on algae settling and cell morphology Fig. 1 shows the effect of preoxidation on cell settling. Although both preoxidants improve algae settling, the efficiency of perman-
ganate was more significant than that of ozone. Even by naked eye observation, cell aggregations were observed on the wall of the Imhoff cones when permanganate was used as a preoxidant. No cell aggregations were observed using ozone or in the control system. Fig. 2 shows the SEM of Chodatella sp., taken before and after preoxidation by either ozone or permanganate. Before preoxidation, the fine markings on the cell wall and bristles can be clearly seen (Fig. 2a). The SEM (Fig. 2b) demonstrates extensive cell destruction (cell lysis), where bristles broke off at an ozone dose of 1.3 mg L 1. At a permanganate preoxidation dose of 1.5 mg L 1, no significant cell lysis was observed. Furthermore, the surface of the cell seems to be coated with a thick layer of materials, which were probably the aggregations of EOM and MnO2 (Fig. 2c). The better performance of the permanganate compared to the ozone was conjectured to be due to the incorporation of algae flocs onto the reduced product, MnO2, thereby increasing its specific gravity and its settling velocity. Further, it is also speculated that permanganate could induce the release of EOM from algae cell, which may further enhance the aggregation of algal cells and MnO2 (Chen and Yeh, 2005). Fig. 3 shows the increase in the NPDOC value of the algal suspension under various doses of preoxidant, and demonstrates an increasing NPDOC when the preoxidants doses were higher than 1.2 mg L 1. The increase in NPDOC is certainly due to the release of EOM or IOM. This phenomenon is more significant for ozone than permanganate, due to the higher oxidation power of the former. The fact that release of NPDOC and associated cell lysis, as demonstrated by SEM, at higher ozone doses explains the phenomenon of poorer algae removal under high preoxidation doses in the previous pilot plant testing.
1.4
-1
NPDOC released from cells (mg L )
4.3.2. Effects of preoxidation on coagulation of cultured algal suspension Fig. 4a shows the jar test results of algal suspension with alum under various preozonation dosages. Generally speaking, preozonation is helpful for algae coagulation removal. The major mechanism is considered to be through the destabilization of algae floc, as shown by the decreasing absolute value of the zeta potential of algae floc with increasing preozonation dosage (Fig. 4b). The results also imply that there probably exists an optimum dosage for preozonation. For example, at a low alum dosage of 10 mg L 1,
O3 KMnO4
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
1
2
3
4
Preoxidant dose (mg L-1 ) Fig. 2. SEM micrographs of cell (Chodatella sp.) surface morphology with and without preoxidation. (a) Without preoxidation, 8 k, (b) 1.3 mg L 1 O3, 15 k, and (c) 1.5 mg L 1 KMnO4, 19 k.
Fig. 3. Effect of preoxidant doses on the NPDOC (method detention limit: 0.11 mg L 1) (batch test, synthetic water with initial algae concentration: 4 107 cells mL 1, no Ca++ added).
J.-J. Chen et al. / Chemosphere 74 (2009) 840–846
a
100
Algae removal (%)
844
80 60
O3 (mg L )
0 0.5 1.2 1.7 2.1
KMnO4 (mg L-1 )
0 0.5 1.2 1.7 2.1
-1
40 20 0
Zeta potential (mV)
b
-21 -24 -27 -30
c
100
Algae removal (%)
-33
80 60 40 20 0
Zeta potential (mV)
d
-26 -28 -30 -32 -34 0
10
20
30
40
50
-1
Alum dose (mg L ) Fig. 4. Effect of preoxidants doses on algae coagulation removal and zeta potential of algal floc. (a and b for ozone, c and d for permanganate, batch test, synthetic water with initial algae concentration: 4 107 cells mL 1, no Ca++ added).
preozonation at doses higher than 1.2 mg L 1 does not seem helpful to subsequent algae removal via alum coagulation (Fig. 4a). The finding is consistent with the zeta potential data. Fig. 4b also shows that the absolute values of the algae floc zeta potential did not further decrease when the ozone dosage exceeded 1.2 mg L 1. As we have previously discussed, ozone overdose may cause cell lysis and the release of EOM or IOM, which can compete with the suspended and colloidal solids for alum coagulant. Previous studies, concerned with the effect of preoxidation on algae coagulation removal, also showed that preoxidants can damage algae cell surface architecture, and bring an efflux of intracellular contents to the aqueous phase (Sukenik et al., 1987; Peterson et al., 1995). Whether the preoxidation will enhance or inhibit algae coagulation removal depends on many factors, such
as the algae species, type of oxidants, and the ratio of oxidant dose to cell number. Plummer and Edzwald (2002) showed that ozone or chlorine preoxidation could yield an improvement in coagulation removal of green alga (Scenedesmus sp.) by polyaluminium chloride, but not for diatom (Cyclotella sp.). Sukenik et al. (1987) studied the effect of chlorine, ozone, and chlorine dioxide on Scenedesmus sp. cultures, and reported that prechlorination at high dosage increased the required alum dosage, believing it was due to the leak of macromolecular organics from the cells, which interfered with the interaction between alum and algae cells and thus inhibited flocculation. Using a pretreatment with stronger oxidants, namely chlorine dioxide and ozone, the organics released were further oxidized. The improvement of algal flocculation with alum by ClO2 or O3 preoxidation was thought to be through cell surface
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alteration, which resulted in colloidal destabilization, therefore improving flocculation. Fig. 4a also shows that the decrease in algae coagulation removal caused by ozone overdose can be compensated by higher alum dosage. Fig. 4c shows the permanganate dosage results are similar to those of Fig. 4a. Permanganate preoxidation was also found to enhance alum coagulation of algae. There also existed an optimum preoxidant dosage, 1.7 mg L 1 KMnO4 in this case, as higher dosage did not further improve coagulation. The effect of permanganate preoxidation on the algae floc zeta potential was less significant than that of preozonation, probably due to the weak oxidation power of permanganate. Further, the relationship between the zeta potential and the algae removal efficiency was also not as clear as that of preozonation. This may be due to the difference in the destabilization mechanism. For permanganate preoxidation, as has been mentioned before, it is speculated that algae removal could involve the adsorption of MnO2 into the algae floc, rather than simple charge neutralization.
important role in algae coagulation removal in the absence of calcium. In the presence of calcium, Fig. 5d shows a much lower absolute value of the zeta potential for the entire system, compared with that of Fig. 5b. This is due to charge neutralization in the presence of calcium. The higher absolute value of the zeta potential for permanganate preoxidation than that with preozonation is contrary to the higher algae removal using permanganate, as shown in Fig. 5c. It probably also supports our previous speculation that, for the case of permanganate, the algae removal could be more due to the adsorption of MnO2, the reducing product of permanganate, into the algae floc. In the pH range of neutral water, the surfaces of both MnO2 and the algae cell are negatively charged. Calcium ions may serve as bridges to hold the two negatively charged surfaces together. Therefore, in the presence of calcium, the beneficial effect of permanganate preoxidation on algae coagulation removal was significantly enhanced. These bench-scale testing results also provide explanation to our pilot-scale testing results, which showed preoxidation with permanganate induced better algae removal by coagulation-sedimentation than preozonation, as the raw water of the pilot plant contained a hardness of about 240 mg L 1 as CaCO3.
4.3.3. Effect of calcium on algae removal by preoxidation–coagulation process Fig. 5a and c compare the effect of the presence or absence of calcium on algae removal by the preoxidation-coagulation process. Under the testing conditions, preoxidations using either ozone or permanganate at the dosage of 1.3 mg L 1 were beneficial to alum coagulation of algae. Adding calcium to a system greatly improved algae coagulation removal either with or without preoxidation. However, in the absence of calcium, preozonation generally induced higher algae removal, especially at a higher alum dosage (Fig. 5a). In the presence of calcium (250 mg L 1 as CaCO3), the performance of the system with permanganate was better than that with preozonation, as good coagulation occurred for low alum doses. Fig. 5b shows the corresponding zeta potential of the coagulated algae floc of Fig. 5a. Without calcium, the system with preozonation showed much lower absolute value of the zeta potential, which corresponds to a lower colloidal stability. This finding is consistent with the higher algae removal using preozonation. This also indicates that charge neutralization probably plays an
Algae removal (%)
a
5. Conclusions Both pilot and laboratory experiments were conducted for comparing the effect of ozone and permanganate preoxidation on algae removal by alum coagulation. Both experiments show preoxidation under an appropriate dose was beneficial to algae coagulation removal. Preoxidation for improving algae coagulation may be attributed to a decrease in cell stability; however, overdosing may cause cell lysis and the release of organics, which could interfere with algae cell coagulation. In the presence of calcium, both preoxidants further improved algae coagulation; and this phenomenon was more significant for preoxidation using permanganate than ozone. It is speculated that this is due to the fact that the positively charged calcium ions can serve as bridges to hold the
100
c
80
With Ca 2+ (250 mg L-1 as CaCO3)
60
0 mg L-1
40
1.3 mg L-1 O3
2+
Without Ca
20
1.3 mg L-1 KMnO4
0
Zeta potential (mV)
b
d
0 -10 -20 -30 -40
0
10
20
30
40 -1
Alum dose (mg L )
50
0
10
20
30
40
50
-1
Alum dose (mg L )
Fig. 5. Comparison of algae coagulation removal and the zeta potential of algal floc, with and without preoxidation in the absence (a, b) and presence (c, d) of calcium. (Batch test, synthetic water with initial algae concentration: 4 107 cells mL 1).
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negatively charged MnO2 and algae cells together. The laboratory results also explain the observations from the pilot testing, which demonstrate preoxidation using permanganate has a better effect on algae removal than ozone. This is due to the high hardness content of the raw water in the pilot plant. Acknowledgments The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract No. NSC 92-2211-E-006-027. The authors also acknowledge the assistance of Mr. Yu-ren Chen in parts of the pilot-scale testing. References APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association, Washington, DC. Bernhardt, H., Clasen, J., 1991. Flocculation of micro-organisms. J. Water Supply: Res. Technol. – AQUA. 40 (2), 76–87. Chandrakanth, M.S., Krishnan, S., Amy, G.L., 1996. Interactions between ozone, AOM, and particles in water treatment. J. Environ. Eng. – ASCE. 122, 459–468. Chen, J.J., Yeh, H.H., 2005. The mechanisms of potassium permanganate on algae removal. Water Res. 39, 4420–4428. Fitzgerald, G.P., 1966. Use of potassium permanganate for control of problem algae. J. Am. Water Works Ass. 58 (5), 609–614.
Knappe, R., Detlef, R.U., Belk, C., Briley, D.S., Grandy, S.R., Rastogi, N., Rike, A.H., 2004. Algae Detection and Removal Strategies for Drinking Water Treatment Plants. AWWA Research Foundation, Denver, CO, USA. Norris, L., Norris, R.E., Calvin, M., 1955. A survey of the rates and products of shortterm photosynthesis in plants of nine Phyla. J. Exp. Bot. 6, 64–74. Paralkar, A., Edzwald, J.K., 1996. Effect of ozone on EOM and coagulation. J. Am. Water Works Ass. 88 (4), 143–154. Peterson, H.G., Hrudey, S.E., Cantin, I.A., Perley, T.R., Kenefick, S.L., 1995. Physiological toxicity, cell membrane damage and the release of dissolved organic carbon and geosmin by Aphanizomenon flos-aquae after exposure to water treatment chemicals. Water Res. 29, 1515–1523. Petruševski, B., van Breemen, A.N., Alaerts, G., 1996. Effect of permanganate pretreatment and coagulation with dual coagulants on algae removal in direct filtration. J. Water Supply Res. Technol. – AQUA. 45 (6), 316–326. Plummer, J.D., Edzwald, J.K., 2001. Effect of ozone on algae as precursors for trihalomethane and haloacetic acid. Environ. Sci. Technol. 35, 3661–3668. Plummer, J.D., Edzwald, J.K., 2002. Effects of chlorine and ozone on algal cell properties and removal of algae by coagulation. J. Water Supply Res. Technol. – AQUA. 51 (6), 307–318. Schmidt, W., Hambsch, B., Petzoldt, H., 1998. Classification of algogenic organic matter concerning its contribution to the bacterial regrowth potential and byproducts formation. Water Sci. Technol. 37 (2), 91–96. Steynberg, M.C., Gugleilm, M.M., Geldenhuys, J.C., Pieterse, A.J.H., 1996. Chlorine and chlorine dioxide: pre-oxidants used as algocide in potable water plants. J. Water Supply Res. Technol. – AQUA. 45 (4), 162–170. Suffet, I.H., Mallevialle, J., Kawczynski, E., 1995. Advances in Taste-and-Odor Treatment and Control. Cooperative Research Reports, AWWA Research Foundation, Denver, CO, USA. Sukenik, A., Teltch, B., Wachs, A.W., Shelef, G., Nir, I., Levanon, D., 1987. Effect of oxidants on microalgal flocculation. Water Res. 21, 533–539.