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Evaluation on the dewatering process of cyanobacteria-containing AlCl3 and PACl drinking water sludge
Separation and Purification Technology 150 (2015) 52–62
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Evaluation on the dewatering process of cyanobacteria-containing AlCl3 and PACl drinking water sludge Feng Sun a,b, Wenrong Hu a,c, Haiyan Pei a,c,⇑, Xiuqing Li a, Xiangchao Xu a, Chunxia Ma a a
School of Environmental Science and Engineering, Shandong University, 27 Shanda South Road, Jinan, Shandong 250100, PR China School of Environmental Science and Engineering, Yangzhou University, 196 Huayang West Road, Yangzhou, Jiangsu 225127, PR China c Shandong Provincial Engineering Centre on Environmental Science and Technology, 17923 Jingshi Road, Jinan, Shandong 250061, PR China b
a r t i c l e
i n f o
Article history: Received 15 October 2014 Received in revised form 16 June 2015 Accepted 22 June 2015 Available online 23 June 2015 Keywords: Cyanobacteria-containing sludge Dewatering process Filtration condition Mechanical action Sludge storage time
a b s t r a c t Most researches focused on the high efficiency removal of cyanobacteria from water phase during drinking water treatment, but did not address the potential danger of cyanobacterial cells being transferred into the sludge, especially during the dewatering process. In this study, the characteristics of cell lysis and microcystin (MC) release in the filtration of cyanobacteria-containing sludge were investigated. By evaluating filtration rates, filtrate microcystins (MCs) and turbidity levels, and SEM micrographs of the sludge in different operating conditions, the influences of mechanical actions and physical/chemical effects on the integrity of cyanobacterial cells were explicated, and the corresponding operation conditions were also optimized. The results showed that the sludge accumulation on unit filter area had significant influence on sludge dewatering characteristics, and with increase in the volume of sludge filtrated, more and more sludge accumulated on the surface of the filter media, resulting in reduced filtration rates and potential cyanobacteria lysis. More hydrophilic filter media with lower porosity should be chosen to enhance filtration efficiency. Compared with positive pressure filtration, vacuum filtration with low-destructibility should be a better choice considering the MC release. Under increased vacuum pressure, cyanobacteria-containing sludge had increasing filtration rate without causing damage of cyanobacterial cells. While the storage time of cyanobacteria-containing sludge should be severely restricted in actual water supply factories (4 d for AlCl3 sludge and 2 d for PACl sludge) to avoid the adverse influences of cyanobacteria lysis on sludge dewaterability and filtrate quality. This study produced deeper knowledge on the fate of cyanobacterial cells and intracellular MCs in the sludge dewatering process, and the results provided valuable scientific references for volume reduction of cyanobacteria-containing sludge in the actual treatment operations. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The increasing frequency and intensity of cyanobacterial blooms cause growing environmental and human health concern [1–3]. The most common toxins produced by cyanobacteria are microcystins (MCs), a class of hepatotoxic monocyclic heptapeptides [4,5]. MCs have caused several causes of poisoning of livestock and wildlife around the world, and they also pose a health hazard to human through drinking water supply. Orally ingested, MCs would be absorbed to hepatic cells and irreversibly inhibit phosphatase proteins, subsequently leading to disruption of cell structures, intrahepatic hemorrhage and death [4,6]. Therefore,
⇑ Corresponding author at: School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China. E-mail address: [email protected] (H. Pei). http://dx.doi.org/10.1016/j.seppur.2015.06.030 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
the control of MCs in drinking water treatment becomes a very important issue. However, most studies focused on the high efficiency removal of cyanobacteria from water phase during drinking water treatments, such as coagulation and flocculation [7–10], and did not address the potential danger of cyanobacterial cells being transferred into a solid phase, especially the drinking water sludge. In fact, during the production of drinking water, there will be large amounts of drinking water sludge, making up 4–7% of the total net of the water produced [11–13]. Drinking water sludge is not only rich in many water contaminants from raw water but also introduces a large number of treatment agents, which will cause serious environmental pollution and resource waste if the sludge is discharged directly without treatment [14,15]. Currently drinking water sludge treatment attracts more and more attention. The main method of sludge volume reduction is mechanical dewatering technology, and pressure filtration is one of the typical
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dewatering technologies widely used in the world [16–18]. However, various filtration conditions have different influences on different kinds of sludge. Besides, most researches on sludge treatment only focused on the dewaterability of sludge, few referred to the secondary pollution during the sludge formation and treatment processes [19–21]. For the drinking water treatment of cyanobacteria contaminated raw water, a typical feature is the high cyanobacteria contents in drinking water sludge (as most cyanobacterial cells of raw water transfer into coagulation flocs). Different from the cells in water bodies, the existing environment of cyanobacterial cells in coagulation flocs has obvious changes. The coexisting coagulants (e.g. polyaluminium chloride, aluminum sulfate, ferric chloride sulfate) and various actions (e.g. turbulence caused by flocculation) both could cause external stress on cyanobacterial cells and lead to cell damage [7,8,22–24]. In the treatment process of cyanobacteria-containing drinking water sludge, the damage to cyanobacterial cells will result in the released intracellular toxins getting into the reused water and increasing the load of toxin regulation. Researches showed that cells captured in the clarifiers could release all cyanotoxins within the first 48 h in clarifiers with solid retention time of over 48 h [24]. Cyanobacteria lysis retained on the filters could occur after 24–48 h of retention and good management of the filters maintenance process would be paramount [24,25]. Therefore, it is necessary to clarify the physical, chemical and mechanical effects on cyanobacterial cells in cyanobacteria-containing sludge during sludge treatment in order to avoid the cell damage and toxin release. In this study, we further investigated the cell stability and toxin release characteristics of cyanobacteria-containing AlCl3 and PACl sludge during the dewatering process after coagulation of cyanobacteria polluted raw water. Based on the filtration rates, MCs and turbidity levels in filtrates, and sludge scanning electron microscope (SEM) micrographs under different operating conditions, we optimized the filtration conditions and investigated the effects of mechanical action and storage time on the filtration characteristics during the sludge dewatering process. Limited to experiment scale, the amount of obtained cyanobacteria-containing sludge is too little to carry out plate pressure filtration, belt pressure filtration and centrifuge dewatering. By contrast, vacuum filtration is controllable for laboratory scale samples of sludge, and can be used to simulate and evaluate the potential influences of mechanical and physical/chemical effects on cyanobacterial cells in the sludge dewatering process [11]. 2. Materials and methods 2.1. Materials 2.1.1. Cyanobacteria Microcystis aeruginosa FACHB-905 that produced MCs was selected for experimental cyanobacteria and grown in BG11 medium with a light/dark cycle (14 h/10 h) and constant temperature (25 °C). The cultures were harvested at the late exponential phase of growth and had a final cell yield up to about 107 cells/mL [7,8]. A compound microscope (Olympus CX-31, Japan) was used to count the number of cyanobacterial cells per unit volume in a hemocytometer. 2.1.2. Raw water and bloom water The raw source water was taken from the Queshan reservoir (a drinking water source located in Jinan, Shandong province) and filtered through 0.45 lm Mili CA membrane to remove natural algae. The ranges of raw water quality parameters were: temperature 16.7 °C, pH 8.4, color 8 PCU, turbidity 4.2 NTU, CODMn 14.4 mg/L,
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DO 8.84 mg/L, NH3-N 0.19 mg/L, TN 2.2 mg/L, TP 0.03 mg/L, alkalinity 133.9 mg/L. To simulate the pollution of cyanobacterial bloom in a high algae-laden period according to the guidance value of WHO [26], raw water was spiked with cyanobacterial cultures to achieve a final cell density of about 106 cells/mL. 2.2. Coagulation/flocculation/sedimentation experiments Coagulation/flocculation/sedimentation experiments were performed at room temperature (25 ± 2 °C) in a six-paddle stirrer (ZR4-6, Zhongrun Water Industry Technology Development Co., Ltd., China). For the AlCl3 coagulation experiment, each water sample (1 L) was dosed with a predetermined concentration of coagulant AlCl3 from a stock solution (5 g/L, dissolving 2.5 g aluminum chloride in 500 mL ultrapure water to avoid the influence of other impurities on cyanobacterial cells) when the rapid mixing began. The optimum coagulation conditions for the effective removal of cyanobacterial cells (about 106 cells/mL) were a coagulant dose of 15 mg/L AlCl3, rapid mixing for 1 min at 250 r/min, and slow mixing for 20 min at 20 r/min [7]. For the PACl coagulation experiment, each water sample (1 L) was dosed with a predetermined concentration of coagulant PACl from a stock solution (2 g/L, dissolving 1.0 g aluminum polychloride in 500 mL ultrapure water) when the rapid mixing began. The optimum coagulation conditions for the effective removal of cyanobacterial cells (about 106 cells/mL) were a coagulant dose of 4 mg/L PACl, rapid mixing for 2 min at 150 r/min, and slow mixing for 30 min at 40 r/min [8]. After coagulation, the water sample was left to stand for 30 min to separate the supernatant and sludge. 2.3. Dewatering experiment of cyanobacteria-containing sludge The cyanobacteria-containing sludge left was 40 mL after removing the supernatant. Some of the sludge was mixed slightly and added into 2 mL syringe filters. In vacuum filtration, a solid phase extraction device with vacuum gauge (Tianjin Automatic Science Instrument Co., Ltd., China) was fixed with the filter. The filtrate was collected from the bottom of the collection tube for MCs and turbidity detections in different vacuum conditions and filtration time. In positive pressure filtration, the filter was applied manually with impetus based on the ideal gas state equation:
PV ¼ nRT
ð1Þ
where P is the pressure of the gas, V is the volume of the gas, n is the amount of substance of gas (also known as number of moles), R is the ideal gas constant and T is the temperature of the gas. According to this formula (1), it is easy to conclude that the pressure of the gas is inversely proportional to the volume of the gas in a closed container at a constant temperature. The sample and air were taken in a syringe and the pressure was calculated according to the change of air volume, and the filtrate was collected for MCs and turbidity detections. 2.4. Analytical methods 2.4.1. MCs MCs are the main component of noxious secondary metabolites produced during M. aeruginosa growth and reproduction [27,28]. The dissolved MCs in the filtrate were determined using the Beacon Microcystin Plate Kit that was kindly provided by Beacon Analytical Systems, Inc. (USA). All the following MCs results are presented as lg/L MCs because the Beacon Microcystin Plate Kit is an immunoassay test kit for the quantification of many kinds of MCs but does not differentiate the variants.
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2.4.2. Turbidity The turbidity of the filtrate was tested by spectrophotometry according to the national water quality standard GB13200-91, and modified to allow for the small volume of the sample. The turbidity standard solution of 40 NTU was prepared with 5 mL hydrazine sulfate stock solution (10 g/L) and hexamethylenetetramine stock solution (100 g/L) and volume to 1 L. Other turbidity standard solutions were diluted in proportion. The absorbances of turbidity standard solutions were tested in a 30 mm cuvette (Yixing Jingke Optical Instrument Co., Ltd., China) at 680 nm and plotted into a standard curve. The turbidities of actual samples were tested and calculated according to the standard curve. 2.4.3. Scanning electron microscope (SEM) SEM is a visual method that images a sample producing signals that contain information about the sample’s surface topography, composition, and other properties [29]. Filtrated cyanobacteria-containing sludge under different vacuum pressures and after different storage time were taken for SEM analysis in this study. Samples were fixed with 3% glutaraldehyde overnight, and washed with deionized water three times. Then the samples were subsequently fixed with 1% osmium tetroxide for 2 h, and washed with deionized water for three times. After that, samples were consecutively treated with 30%, 50%, 70%, 85%, 95%, 100%, 100% ethanol (15 min each). Dehydrated algae cells were fully dried with a critical point dryer, and then mounted on copper stubs and sputter-coated with gold–palladium by a sputter coater. The specimens were observed and photographed using a scanning electron microscope (Carl Zeiss Jena EVOLS15, Germany) at 10 kV. 2.4.4. Capillary suction time (CST) CST test is a commonly used method to measure the filterability and the easy of removing moisture from slurry and sludge in numerous environmental and industrial applications [30–33]. Considering the potential changes of sludge characteristics under different sludge storage conditions, the cyanobacteria-containing AlCl3 sludge and PACl sludge after different storage time were taken for CST analysis. The CST analysis was carried out according to the standard method [34] with multipurpose filtration equipment (Triton-W.R.C. Multipurpose Filtration 319) and standard filtration paper (Whatman International Ltd., Maidstone England).
cyanobacteria-containing AlCl3 sludge remained stable initially but decreased obviously when the volume of sludge was equal to or more than 1.0 mL. However, as can be seen in Fig. 1B, the filtration rate of cyanobacteria-containing PACl sludge remained stable when the volume of sludge was less than 1.25 mL and decreased when the volume of sludge was equal to or more than 1.25 mL. It seems that with less sludge volume, the retained sludge was not enough to form a stable sludge filter layer and thus the filtration rate was constant, while with the continuous accumulation of the retained sludge on the surface of filter membrane, a new filter layer was formed to separate the sludge and filtrate, and the new resistance resulted in a lower average filtration rate and higher overall energy consumption. The optimized volume difference between cyanobacteria-containing AlCl3 sludge (1.0 mL) and PACl sludge (1.25 mL) could be due to the different floc characteristics of the two sludges: the average size of AlCl3 sludge is smaller than that of PACl sludge and the AlCl3 floc is more compact than PACl floc, which make the filtration resistance of AlCl3 sludge greater than that of PACl sludge. Besides, the higher dosage of AlCl3 coagulant is perhaps also a contributory factor leading to the more compact structure and lower filtration rate of AlCl3 sludge. In the same system, the changes of MCs in the filtrate during the sludge filtration process were investigated (Fig. 2). Results showed that the MC concentrations in filtrate (both Fig. 2A and B) under different vacuum pressures were all higher than that in treated water (coagulation supernatant), and increased slightly with increasing sludge volume. Combined with the results and analysis in Fig. 1, it could be suspected that the depth filtration produced by the new filter layer of retained sludge could cause some pressure on flocs and result in damage of cyanobacterial cells. Therefore, the excessive accumulation of sludge layer should be avoided in actual operation to improve filtration rate and reduce toxins release. However, considering the amount of obtained cyanobacteria-containing sludge is small, being limited to experiment scale, a relatively higher filtration load could be selected to simulate the real system. As mentioned above, when the sludge volume increased to 1.0 mL and 1.25 mL for AlCl3 and PACl sludge respectively, the filtration rates of these two sludges began to decrease to an acceptable level, but with no obvious release of MCs. It was concluded that 1.0 mL and 1.25 mL of sludge volume were effective for the filtration of cyanobacteria-containing AlCl3 sludge and PACl sludge, respectively, which could be selected for later experiments.
3. Results and discussion 3.1. Optimization and selection of basic filtration conditions The volume of sludge and filter media are important factors that influence the filtration of cyanobacteria-containing sludge and could probably induce the damage of cyanobacterial cells in sludge [23,24]. Therefore, it is necessary to optimize the basic filtration conditions before evaluating the stability of cyanobacterial cells in the sludge filtration process. 3.1.1. Optimization of sludge volume Under the provided effective filter area, the accumulation of sludge is a key factor determining the filtration type and filtration efficiency, and thus influencing the mechanical effect of cells in the sludge. The filtration rate response to the volume of cyanobacteria-containing AlCl3 sludge and PACl sludge was plotted in Fig. 1. It can be seen that with the studied volume of sludge, the filtration of cyanobacteria-containing sludge (Fig. 1A, AlCl3 sludge; Fig. 1B, PACl sludge) at lower vacuum pressure (0.1 bar) was much slower than at higher vacuum pressure (0.9 bar). But both in lower and higher vacuum pressures, the filtration rate decreased with the increase of sludge volume. In Fig. 1A, the filtration rate of
3.1.2. Selection of filter media The filtration rates of cyanobacteria-containing sludge through different filter media were plotted in Fig. 3. It can be seen from Fig. 3A and B that AlCl3 sludge and PACl sludge had similar filtration characteristics. In the four kinds of filter media, the filtration rates were listed as polyether sulfone membrane (PES) > mixed cellulose ester membrane (MCE) > nylon membrane (Nylon) > polytetrafluoroethylene membrane (PTFE). These differences were caused by the different hydrophilic properties of filter media [35,36]: MCE and PES membranes are more hydrophilic than Nylon and PTFE membranes, which give them a higher filtration rate. The effects of filter porosity on filtration rate had no obvious difference, which is probably due to the considerably larger sizes of cyanobaterial cells and flocs (5 lm and 50 lm, respectively) relative to filter porosity, making the effect of filter porosity negligible [37,38]. On the basis of the evaluation of the filtration rate, the MC concentration and turbidity in cyanobacteria-containing sludge filtrate through different filter media were further investigated (Fig. 4). It can be shown in Fig. 4A-a and B-a, the MC concentration in cyanobacteria-containing sludge filtrate was higher than that in treated water (coagulation supernatant). Comparing the systems
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Fig. 1. The filtration rate response to the volume of cyanobacteria-containing sludge ((A) AlCl3 sludge, (B) PACl sludge, filter media: 0.22 lm MCE).
Fig. 2. The MCs in filtrate response to the volume of cyanobacteria-containing sludge ((A) AlCl3 sludge, (B) PACl sludge, filter media: 0.22 lm MCE).
Fig. 3. The filtration rate of cyanobacteria-containing sludge through different filter media ((A) AlCl3 sludge, (B) PACl sludge).
with different filter media, filter porosities and vacuum pressures, the MC differences between filtrate and treated water were consistent, which indicated these different filtration pressures had similar effects on MCs in the filtrates. However, for the turbidity in cyanobacteria-containing sludge filtrate, there existed slightly different effect trends of filter media, filter porosity and vacuum pressure (Fig. 4A-b and B-b). The filtrate turbidity of PES was slightly higher than that of MCE, Nylon and PTFE. For all the four kinds of filter media, reducing the filter porosity could decrease the filtrate
turbidity, while the two kinds of vacuum pressures had similar effects on the filtrate turbidity. Combined with the results of filtration rate, filtrate MCs and turbidity, it is better to choose more hydrophilic filter media with lower filter porosity on the premise of keeping filtration effectiveness and filtrate water quality. In this study, 0.22 lm MCE membrane was selected for later experiments considering its low cost and wide use in research applications.
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Fig. 4. The MCs and turbidity in cyanobacteria-containing sludge filtrate through different filter media ((A) AlCl3 sludge, (B) PACl sludge; (a) MCs, (b) turbidity).
3.2. Effects of mechanical strength and mode of action on the dewatering characteristics of cyanobacteria-containing sludge In the mechanical dewatering process of drinking water sludge, the pressure difference across the both sides of membrane is the driving force for separation, and also determines the damage extent of mechanical action to the cyanobacterial cells in sludge [11]. In view of this, it is necessary to evaluate the effects of mechanical strength and mode of action on the dewatering characteristics of cyanobacteria-containing sludge.
Fig. 5. The filtration rate of cyanobacteria-containing sludge under different vacuumpressure.
3.2.1. Effect of mechanical strength on the dewatering characteristics of cyanobacteria-containing sludge The effect of mechanical strength on the filtration rate of cyanobacteria-containing sludge was shown in Fig. 5. With the increase of vacuum pressure, the pressure difference between the two sides of the membrane became obvious and the corresponding impetus increased, significantly increasing the filtration rate. Compared to AlCl3 sludge, the filtration rate of PACl sludge was slightly higher. It indicated that the PACl sludge had less resistance during the dewatering process, which was probably due to the looser structure of PACl sludge causing greater water permeability of the filter layer. For the cyanobacteria-containing sludge, the mechanical strength could perhaps cause a certain amount of damage to cyanobacterial cells and release of MCs, therefore the MC concentrations and turbidities in filtrate (Fig. 6) and the SEM micrographs of filtrated cyanobacteria-containing sludge (Fig. 7) under different vacuum pressures were further investigated. Fig. 6 showed that the MC concentrations in the filtrates were higher than that in the treated water (coagulation supernatant), but the upward trends under different vacuum pressures were consistent. It indicated the effect of studied vacuum pressure on filtrate MCs was probably based on the filtration procedure rather than the magnitude of pressure. In the same systems, the filtrate turbidity had no obvious difference under different vacuum pressures, which led to the conclusion that the effect of studied mechanical strength on the filtrate turbidity of cyanobacteria-containing sludge was negligible. Directly observed from Fig. 7A and B, the cyanobacterial cells were embraced in AlCl3 sludge and PACl sludge and still remained intact without cell damage under different vacuum pressures. The different number of embraced cells under the three vacuum pressures was because of the different image captures of these three samples. It can therefore be concluded that the studied mechanical strength
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Fig. 6. The MCs and turbidity in cyanobacteria-containing sludge filtrate under different vacuum pressure ((A) AlCl3 sludge, (B) PACl sludge).
Fig. 7. SEM micrographs of cyanobacteria-containing sludge under different vacuum pressure ((A) AlCl3 sludge, (B) PACl sludge; the sludge samples obtained after coagulation and sedimentation were filtrated immediately with vacuum pressure of (a) 0.1 bar, (b) 0.5 bar, (c) 0.9 bar).
would not damage the cyanobacterial cells in cyanobacteria-containing sludge. In view of this, the increased MC concentration in filtrate relative to treated water (in Fig. 6) might be attributed to either the deflocculation of coagulant absorbed MCs or the exfiltration of intracellular MCs caused by the filtration procedure. Combining all the above results, it was found that higher vacuum pressure could improve the filtration rate without causing cell damage in sludge, therefore it should be possible to use higher vacuum pressure in the dewatering process of cyanobacteria-containing sludge. 3.2.2. Analysis of filtration differences between positive pressure filtration and vacuum filtration Positive pressure filtration can also be used for dewatering of cyanobacteria-containing sludge. But compared with vacuum filtration, positive pressure filtration has greater impetus range and poorer controllability, which will make its mechanical strength on cyanobacteria-containing sludge unstable and thus could lead to the damage of cyanobacterial cells. Therefore, the effect of positive pressure filtration on the dewatering characteristics of cyanobacteria-containing sludge was tested and the filtration differences between positive pressure filtration and vacuum filtration
were analyzed. The filtration time, filtration rate, filtrate MCs and filtrate turbidity of cyanobacteria-containing AlCl3 sludge and PACl sludge under different positive filtration pressures were listed in Tables 1 and 2, respectively. Results showed that with the increasing pressure, the filtration time was shortened and the filtration rate was improved, which was almost consistent with the
Table 1 The dewatering characteristics of cyanobacteria-containing AlCl3 sludge with positive pressure filtration.
DP (bar)a
Time (min)
Positive pressure filtration 0.1 20.40 ± 1.62b 0.5 7.87 ± 0.62 0.9 2.86 ± 0.22 1.5 1.47 ± 0.17 2.0 1.03 ± 0.19 3.0 0.67 ± 0.07 Vacuum filtration 0.9 3.23 ± 0.20 a b
Filtration rate (mL/ min)
MCs (lg/ L)
Turbidity (NTU)
0.049 ± 0.004 0.127 ± 0.013 0.350 ± 0.030 0.690 ± 0.079 1.008 ± 0.182 1.49 ± 0.140
23.1 ± 1.1 24.0 ± 1.2 26.1 ± 1.4 27.7 ± 1.3 28.1 ± 0.9 28.4 ± 1.2
27.60 ± 0.75 27.90 ± 0.65 28.71 ± 0.90 29.81 ± 0.94 29.82 ± 1.05 30.21 ± 0.92
0.31 ± 0.02
23.3 ± 1.2
27.75 ± 0.83
DP = Actual pressure Standard atmospheric pressure. Each data were tested for three times.
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Table 2 The dewatering characteristics of cyanobacteria-containing PACl sludge with positive pressure filtration. Filtration rate (mL/ min)
MCs (lg/ L)
Turbidity (NTU)
Positive pressure filtration 0.1 23.58 ± 2.03b 0.5 8.99 ± 0.66 0.9 3.21 ± 0.31 1.5 1.97 ± 0.19 2.0 1.49 ± 0.10 3.0 0.77 ± 0.05
0.053 ± 0.005 0.139 ± 0.006 0.390 ± 0.035 0.635 ± 0.053 0.83 ± 0.052 1.61 ± 0.091
23.3 ± 0.9 24.3 ± 1.0 26.6 ± 1.1 28.0 ± 1.1 28.4 ± 1.5 28.7 ± 1.2
27.71 ± 0.68 27.92 ± 0.81 28.91 ± 0.87 29.69 ± 0.90 29.95 ± 0.93 30.45 ± 0.78
Vacuum filtration 0.9 3.84 ± 0.27
0.327 ± 0.023
23.7 ± 0.9
28.19 ± 0.76
DP (bar)a
a b
Time (min)
DP = Actual pressure Standard atmospheric pressure. Each data were tested for three times.
changing trend of filtration in the vacuum filtration process. But the filtration rate in positive pressure filtration was a little higher than that in vacuum filtration. Taking the results of 0.9 bar in vacuum filtration and 0.9 bar in positive pressure filtration for example, the filtration rate of cyanobacteria-containing AlCl3 sludge was 0.31 mL/min in vacuum filtration but 0.35 mL/min in positive pressure filtration; and the filtration rate of cyanobacteria-containing PACl sludge was 0.33 mL/min in vacuum filtration but 0.39 mL/min in positive pressure filtration. However, the results of MCs and turbidity showed that the positive pressure filtration probably caused damage of sludge flocs and partial cyanobacterial cells, and increased the MC concentration and turbidity in the filtrate. Also take the results of 0.9 bar in vacuum filtration and 0.9 bar in positive pressure filtration for example, the filtrate MCs and turbidity of cyanobacteria-containing AlCl3 sludge were 23.3 lg/L and 27.75 NTU in vacuum filtration but 26.1 lg/L and 28.71 NTU in positive pressure filtration; and the filtrate MCs and turbidity of cyanobacteria-containing PACl sludge were 23.7 lg/L and 28.19 NTU in vacuum filtration but 26.6 lg/L and 28.91 NTU in positive pressure filtration. When the pressure was added above 0.9 bar, the MCs and turbidity increased continually and the results were consistent with increased damage to flocs and cells. Compared with the results of cyanobacteria-containing AlCl3 sludge, the filtration rate of cyanobacteria-containing PACl sludge in positive pressure filtration was improved to some extent, but at the same time the filtrate MCs and turbidity showed a small increase, which indicated that the mechanical strength of positive pressure filtration caused slightly greater damage to PACl sludge than to AlCl3 sludge (though the difference was not significant). Overall, whether for AlCl3 sludge or PACl sludge, it is better to use lower-destructive vacuum filtration to minimize the cell damage and MCs release during the drinking water sludge dewatering process. 3.3. Effect of sludge storage time on the dewatering characteristics of cyanobacteria-containing sludge Besides filter conditions and mechanical actions, the cyanobacteria-containing sludge could also be influenced during the sludge storage process. With the storage time increased, the cyanobacteria-containing sludge would be influenced by the storage environment. Therefore it is necessary to evaluate the effect of sludge storage time on the dewatering characteristics of cyanobacteria-containing sludge. The filtration rate of cyanobacteria-containing sludge at different storage time was plotted in Fig. 8. The two kinds of vacuum pressures (0.9 bar and 0.1 bar) were chosen for filtration and they showed a consistent changing trend of filtration rate. For the cyanobacteria-containing AlCl3 sludge (Fig. 8A-a and A-b), the
filtration rate increased slowly in 0–4 d, significantly more rapidly in 4–8 d, and then remained stable in 8–12 d; while for the cyanobacteria-containing PACl sludge (Fig. 8B-a and B-b), the filtration rate increased slowly in 0–2 d, significantly more rapidly in 2–6 d, and then remained stable in 6–10 d. In the operating conditions provided, the filtration rate of sludge could only be relevant to the sludge structure. From the above results, it could be found that the morphology structure of cyanobacteria-containing AlCl3 sludge might cause changes after storage for 4 d. However, the changes of cyanobacteria-containing PACl sludge was observed earlier than in AlCl3 sludge (after 2 d), which could be due to the looser structure of cyanobacteria-containing PACl sludge. A similar difference was also noted in our previous studies [7,8], which indicated that sludge storage time had advanced influence on the PACl sludge and affected the stability of cyanobacterial cells more significantly. In addition, when the morphology structure of cyanobacteria-containing sludge was changed, its dewaterability would also be influenced. The CST (which is used to evaluate sludge dewaterability [30–33]) values of cyanobacteria-containing sludge after different storage time were shown in Fig. 9. It can be seen that the CST of AlCl3 sludge and PACl sludge had a similar changing trend (decreased at first and then increased), but the lowest value occurred at different sludge storage time (6 d for AlCl3 sludge, 3 d for PACl sludge). This phenomenon is consistent with the results in Fig. 8, which also indicated that the morphology structure of cyanobacteria-containing AlCl3 and PACl sludge would change after storage for 4 d and 2 d, respectively. The effects of sludge storage time on MCs and turbidity in the filtrate of cyanobacteria-containing sludge were shown in Fig. 10. It can be seen from Fig. 10A-a and B-a that the MC concentrations in the filtrates of both cyanobacteria-containing AlCl3 sludge and cyanobacteria-containing PACl sludge increased gradually with the sludge storage time: the MCs of cyanobacteria-containing AlCl3 sludge increased gradually in 0–4 d, significantly more rapidly in 4–10 d, then slowed down in 10–12 d (Fig. 10A-a); while the MCs of cyanobacteria-containing PACl sludge increased gradually in 0–2 d, significantly more rapidly in 2–8 d, then slowed down in 8–10 d (Fig. 10B-a). The increase of MCs was probably due to the breakage of cyanobacterial cells, which suggested that the cyanobacterial cells in sludge became gradually unstable with the storage time. The difference between cyanobacteria-containing AlCl3 sludge and PACl sludge was the breakage time of cyanobacterial cells (4 d was an important point for AlCl3 sludge, while it was 2 d for PACl sludge), which was probably also due to the different structures of the two kinds of sludge. Compared with the sludge storage time, the vacuum pressure had less influence on the MCs in filtrate: the MCs under 0.9 bar and 0.1 bar had no difference in 0 d, and had slightly lower value at lower vacuum pressure after 2 d, and had more obvious difference after 6 d. This indicated that increasing the storage time would weaken the endurance of cyanobacterial cells to mechanical strength. From the results in Fig. 10A-b and B-b, the turbidity in the filtrate had similar changing trends with the filtration rate: the turbidity of cyanobacteria-containing AlCl3 sludge increased slowly in 0–4 d, significantly more rapidly in 4–8 d, and then remained stable in 8–12 d (Fig. 10A-b); while the turbidity of cyanobacteria-containing PACl sludge increased slowly in 0–2 d, significantly more rapidly in 2–6 d, and then remained stable in 6–10 d (Fig. 10B-b). This indicated that the sludge storage process could cause damage of cyanobacterial cells (inferred by increasing MCs) and structural changes of cyanobacteria-containing sludge (inferred by increasing filtration rate and turbidity). In order to observe the sludge changes directly, SEM micrographs of filtrated (vacuum pressure: 0.9 bar) cyanobacteria-containing sludge after different storage time were shown in Fig. 11. It can be seen from
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Fig. 8. The filtration rate of cyanobacteria-containing sludge after different storage time ((A) AlCl3 sludge, (B) PACl sludge; (a) vacuum pressure 0.9 bar, (b) vacuum pressure 0.1 bar).
Fig. 9. The CST of cyanobacteria-containing sludge after different storage time ((A) AlCl3 sludge, (B) PACl sludge).
Fig. 11A-a that the cyanobacterial cells embraced in sludge and kept intact at 0 d. Fig. 11A-b showed a deflocculation phenomenon of cyanobacterial cells from AlCl3 sludge occurred with the storage. In Fig. 11A-c, cell lysis occurred until the AlCl3 sludge storage for 6 d. And the lysis would be more obvious with the storage time increased (Fig. 11A-d). For PACl sludge (Fig. 11B), the similar changes could also be observed, but the cell lysis occurred at 4 d (Fig. 11B-c). Combined with the evidence of SEM, the changing trend of CST in Fig. 9 could lead to the conclusion that the deflocculation of cyanobacteria-containing sludge decreased the filtration resistance of the sludge but the lysis of cyanobacteria cells
and increasing cell debris would add another resistance to increase the CST. Combining all the above results, we see that prolonging the sludge storage time could improve the filtration efficiency to some extent, but obviously caused the damage of cyanobacterial cells in sludge, and lowered the quality of recycled water. Therefore, it is important to shorten the sludge storage time and complete the volume reduction process as soon as possible in actual operation (the storage time for AlCl3 and PACl sludge should not be more than 2 d and 4 d, respectively).
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Fig. 10. The MCs and turbidity in cyanobacteria-containing sludge filtrate after different storage time ((A) AlCl3 sludge, (B) PACl sludge; (a) MCs, (b) turbidity).
Fig. 11. SEM micrographs of cyanobacteria-containing sludge after different storage time ((A) AlCl3 sludge, (a) 0 day, (b) 4 day, (c) 6 day, (d) 8 day; (B) PACl sludge, (a) 0 day, (b) 2 day, (c) 4 day, (d) 6 day; all the samples were filtrated with vacuum pressure of 0.9 bar).
4. Conclusions This study focused on the dewatering characteristics of cyanobacteria-containing AlCl3 sludge and PACl sludge, optimized the filtration conditions and investigated the effect of mechanical action and storage time on the filtration efficiency and filtrate quality during the sludge volume reduction process.
(1) Sludge accumulation: Under the studied effective area, the accumulation of sludge is a key factor determining the filtration characteristics. Sludge volumes below 1.0 mL were not enough to form a stable sludge filter layer to reduce the filtration rate, while the continuous accumulation of retained sludge on the membrane surface formed a new filter layer with more filtration resistance and resulted in lower average filtration rate and higher MC concentration in the filtrate. In
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practice, excessive accumulation of sludge layer should be avoided to improve the filtration rate and reduce toxins release. (2) Filter media: Aiming at keeping the filtration efficiency, hydrophilic filter media with lower filter porosity should be chosen for filtering cyanobacteria-containing sludge. (3) Mechanical action: High vacuum pressure could improve the filtration rate without causing cell damage and increasing the filtrate MCs and turbidity. Therefore on the basis of keeping filtration efficiency, it is better to choose stringer vacuum pressure in vacuum filtration process of cyanobacteria-containing sludge. Compared with vacuum filtration, positive pressure filtration improved the filtration rate but also increased the MCs and turbidity in filtrate. Overall, it is better to use lower-destructive vacuum filtration to decrease the cell damage and MCs release during drinking water sludge dewatering process. (4) Storage time: Prolonging the sludge storage time improved the filtration efficiency to some extent, but caused obvious cell damage in the sludge. Therefore, it is important to shorten the sludge storage time and complete the volume reduction process as soon as possible in actual operation to maintain the quality of recycled water. The sludge amount studied in the lab is much less than that produced in a drinking water plant, thus the pressure values are correspondingly lower. However, the influences of sludge accumulation per unit area on filtration rate and cyanobacteria lysis would still exist whenever the sludge amount is less or more. And the filter media chosen in this study are all organic polymer materials which are similar to those used in real full-scale systems. Besides, the assessment and evaluation indices and strategies used in this study could be equally applied to other processing methods under sufficient sample volume conditions. Therefore, the corresponding simulation results of this study provided scientific references for the volume reduction of cyanobacteria-containing drinking water sludge in the actual treatment. Acknowledgments The authors acknowledge financial supports from National Science Fund for Excellent Young Scholars (51322811), International Science & Technology Cooperation Program of China (2010DFA91150), International Cooperation Research of Shandong Province (2011176), Science and Technology Development Project of Shandong Province (2012GHZ30020), the Program of the Ministry of Education of China for New Century Excellent Talents in University (NCET-12-0341), Graduate Innovation Foundation of Shandong University (yyx10035), National Natural Science Foundation (51178408) and Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors thank Dr. Findlay Nicol of Shandong University of Finance and Economics for revising the English in the manuscript. References [1] H.W. Paerl, H. Xu, M.J. McCarthy, G.W. Zhu, B.Q. Qin, Y.P. Li, W.S. Gardner, Controlling harmful cyanobacterial blooms in a hyper-eutrophic lake (Lake Taihu, China): the need for a dual nutrient (N & P) management strategy, Water Res. 45 (2011) 1973–1983. [2] A.J. Lewitus, L.M. Brock, M.K. Burke, K.A. DeMattio, S.B. Wilde, Lagoonal stormwater detention ponds as promoters of harmful algal blooms and eutrophication along the South Carolina coast, Harmful Algae 8 (2008) 60–65. [3] G. Pan, M.M. Zhang, H. Chen, H. Zou, H. Yan, Removal of cyanobacterial blooms in Taihu Lake using local soils. I. Equilibrium and kinetic screening on the flocculation of Microcystis aeruginosa using commercially available clays and minerals, Environ. Pollut. 141 (2006) 195–200.
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Prévost, Toxic cyanobacterial breakthrough and accumulation in a drinking water plant: a monitoring and treatment challenge, Water Res. 46 (2012) 1511– 1523. [24] J. Pietsch, K. Bornmann, W. Schmidt, Relevance of intra- and extracellular cyanotoxins for drinking water treatment, Acta Hydrochim. Hydrobiol. 30 (2002) 7–15. [25] L. Lepisto, K. Lahti, J. Niemi, M. Fardig, Removal of cyanobacteria and other phytoplankton in four Finnish waterworks, Hydrobiology 75 (1994) 167–181. [26] World Health Organization (WHO), Toxic Cyanobacteria in Water: A Guide to Their Public Health Consequences Monitoring and Management, Safe Level and Safe Practices, E&FN Spon, London, 1999. pp. 161–179 (Chapter 5). [27] C. Martin, L. Oberer, T. Ino, W.A. König, M. Busch, J. Weckesser, Cyanopeptolins, new depsipeptides derived from the cyanobacterium Microcystis aeruginosa PCC 7806, J. Antibiot. 46 (1993) 1550–1556. [28] C. Ross, L. Santiago-Vázquez, V. 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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Evaluation on the dewatering process of cyanobacteria-containing AlCl3 and PACl drinking water sludge Feng Sun a,b, Wenrong Hu a,c, Haiyan Pei a,c,⇑, Xiuqing Li a, Xiangchao Xu a, Chunxia Ma a a
School of Environmental Science and Engineering, Shandong University, 27 Shanda South Road, Jinan, Shandong 250100, PR China School of Environmental Science and Engineering, Yangzhou University, 196 Huayang West Road, Yangzhou, Jiangsu 225127, PR China c Shandong Provincial Engineering Centre on Environmental Science and Technology, 17923 Jingshi Road, Jinan, Shandong 250061, PR China b
a r t i c l e
i n f o
Article history: Received 15 October 2014 Received in revised form 16 June 2015 Accepted 22 June 2015 Available online 23 June 2015 Keywords: Cyanobacteria-containing sludge Dewatering process Filtration condition Mechanical action Sludge storage time
a b s t r a c t Most researches focused on the high efficiency removal of cyanobacteria from water phase during drinking water treatment, but did not address the potential danger of cyanobacterial cells being transferred into the sludge, especially during the dewatering process. In this study, the characteristics of cell lysis and microcystin (MC) release in the filtration of cyanobacteria-containing sludge were investigated. By evaluating filtration rates, filtrate microcystins (MCs) and turbidity levels, and SEM micrographs of the sludge in different operating conditions, the influences of mechanical actions and physical/chemical effects on the integrity of cyanobacterial cells were explicated, and the corresponding operation conditions were also optimized. The results showed that the sludge accumulation on unit filter area had significant influence on sludge dewatering characteristics, and with increase in the volume of sludge filtrated, more and more sludge accumulated on the surface of the filter media, resulting in reduced filtration rates and potential cyanobacteria lysis. More hydrophilic filter media with lower porosity should be chosen to enhance filtration efficiency. Compared with positive pressure filtration, vacuum filtration with low-destructibility should be a better choice considering the MC release. Under increased vacuum pressure, cyanobacteria-containing sludge had increasing filtration rate without causing damage of cyanobacterial cells. While the storage time of cyanobacteria-containing sludge should be severely restricted in actual water supply factories (4 d for AlCl3 sludge and 2 d for PACl sludge) to avoid the adverse influences of cyanobacteria lysis on sludge dewaterability and filtrate quality. This study produced deeper knowledge on the fate of cyanobacterial cells and intracellular MCs in the sludge dewatering process, and the results provided valuable scientific references for volume reduction of cyanobacteria-containing sludge in the actual treatment operations. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The increasing frequency and intensity of cyanobacterial blooms cause growing environmental and human health concern [1–3]. The most common toxins produced by cyanobacteria are microcystins (MCs), a class of hepatotoxic monocyclic heptapeptides [4,5]. MCs have caused several causes of poisoning of livestock and wildlife around the world, and they also pose a health hazard to human through drinking water supply. Orally ingested, MCs would be absorbed to hepatic cells and irreversibly inhibit phosphatase proteins, subsequently leading to disruption of cell structures, intrahepatic hemorrhage and death [4,6]. Therefore,
⇑ Corresponding author at: School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China. E-mail address: [email protected] (H. Pei). http://dx.doi.org/10.1016/j.seppur.2015.06.030 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
the control of MCs in drinking water treatment becomes a very important issue. However, most studies focused on the high efficiency removal of cyanobacteria from water phase during drinking water treatments, such as coagulation and flocculation [7–10], and did not address the potential danger of cyanobacterial cells being transferred into a solid phase, especially the drinking water sludge. In fact, during the production of drinking water, there will be large amounts of drinking water sludge, making up 4–7% of the total net of the water produced [11–13]. Drinking water sludge is not only rich in many water contaminants from raw water but also introduces a large number of treatment agents, which will cause serious environmental pollution and resource waste if the sludge is discharged directly without treatment [14,15]. Currently drinking water sludge treatment attracts more and more attention. The main method of sludge volume reduction is mechanical dewatering technology, and pressure filtration is one of the typical
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dewatering technologies widely used in the world [16–18]. However, various filtration conditions have different influences on different kinds of sludge. Besides, most researches on sludge treatment only focused on the dewaterability of sludge, few referred to the secondary pollution during the sludge formation and treatment processes [19–21]. For the drinking water treatment of cyanobacteria contaminated raw water, a typical feature is the high cyanobacteria contents in drinking water sludge (as most cyanobacterial cells of raw water transfer into coagulation flocs). Different from the cells in water bodies, the existing environment of cyanobacterial cells in coagulation flocs has obvious changes. The coexisting coagulants (e.g. polyaluminium chloride, aluminum sulfate, ferric chloride sulfate) and various actions (e.g. turbulence caused by flocculation) both could cause external stress on cyanobacterial cells and lead to cell damage [7,8,22–24]. In the treatment process of cyanobacteria-containing drinking water sludge, the damage to cyanobacterial cells will result in the released intracellular toxins getting into the reused water and increasing the load of toxin regulation. Researches showed that cells captured in the clarifiers could release all cyanotoxins within the first 48 h in clarifiers with solid retention time of over 48 h [24]. Cyanobacteria lysis retained on the filters could occur after 24–48 h of retention and good management of the filters maintenance process would be paramount [24,25]. Therefore, it is necessary to clarify the physical, chemical and mechanical effects on cyanobacterial cells in cyanobacteria-containing sludge during sludge treatment in order to avoid the cell damage and toxin release. In this study, we further investigated the cell stability and toxin release characteristics of cyanobacteria-containing AlCl3 and PACl sludge during the dewatering process after coagulation of cyanobacteria polluted raw water. Based on the filtration rates, MCs and turbidity levels in filtrates, and sludge scanning electron microscope (SEM) micrographs under different operating conditions, we optimized the filtration conditions and investigated the effects of mechanical action and storage time on the filtration characteristics during the sludge dewatering process. Limited to experiment scale, the amount of obtained cyanobacteria-containing sludge is too little to carry out plate pressure filtration, belt pressure filtration and centrifuge dewatering. By contrast, vacuum filtration is controllable for laboratory scale samples of sludge, and can be used to simulate and evaluate the potential influences of mechanical and physical/chemical effects on cyanobacterial cells in the sludge dewatering process [11]. 2. Materials and methods 2.1. Materials 2.1.1. Cyanobacteria Microcystis aeruginosa FACHB-905 that produced MCs was selected for experimental cyanobacteria and grown in BG11 medium with a light/dark cycle (14 h/10 h) and constant temperature (25 °C). The cultures were harvested at the late exponential phase of growth and had a final cell yield up to about 107 cells/mL [7,8]. A compound microscope (Olympus CX-31, Japan) was used to count the number of cyanobacterial cells per unit volume in a hemocytometer. 2.1.2. Raw water and bloom water The raw source water was taken from the Queshan reservoir (a drinking water source located in Jinan, Shandong province) and filtered through 0.45 lm Mili CA membrane to remove natural algae. The ranges of raw water quality parameters were: temperature 16.7 °C, pH 8.4, color 8 PCU, turbidity 4.2 NTU, CODMn 14.4 mg/L,
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DO 8.84 mg/L, NH3-N 0.19 mg/L, TN 2.2 mg/L, TP 0.03 mg/L, alkalinity 133.9 mg/L. To simulate the pollution of cyanobacterial bloom in a high algae-laden period according to the guidance value of WHO [26], raw water was spiked with cyanobacterial cultures to achieve a final cell density of about 106 cells/mL. 2.2. Coagulation/flocculation/sedimentation experiments Coagulation/flocculation/sedimentation experiments were performed at room temperature (25 ± 2 °C) in a six-paddle stirrer (ZR4-6, Zhongrun Water Industry Technology Development Co., Ltd., China). For the AlCl3 coagulation experiment, each water sample (1 L) was dosed with a predetermined concentration of coagulant AlCl3 from a stock solution (5 g/L, dissolving 2.5 g aluminum chloride in 500 mL ultrapure water to avoid the influence of other impurities on cyanobacterial cells) when the rapid mixing began. The optimum coagulation conditions for the effective removal of cyanobacterial cells (about 106 cells/mL) were a coagulant dose of 15 mg/L AlCl3, rapid mixing for 1 min at 250 r/min, and slow mixing for 20 min at 20 r/min [7]. For the PACl coagulation experiment, each water sample (1 L) was dosed with a predetermined concentration of coagulant PACl from a stock solution (2 g/L, dissolving 1.0 g aluminum polychloride in 500 mL ultrapure water) when the rapid mixing began. The optimum coagulation conditions for the effective removal of cyanobacterial cells (about 106 cells/mL) were a coagulant dose of 4 mg/L PACl, rapid mixing for 2 min at 150 r/min, and slow mixing for 30 min at 40 r/min [8]. After coagulation, the water sample was left to stand for 30 min to separate the supernatant and sludge. 2.3. Dewatering experiment of cyanobacteria-containing sludge The cyanobacteria-containing sludge left was 40 mL after removing the supernatant. Some of the sludge was mixed slightly and added into 2 mL syringe filters. In vacuum filtration, a solid phase extraction device with vacuum gauge (Tianjin Automatic Science Instrument Co., Ltd., China) was fixed with the filter. The filtrate was collected from the bottom of the collection tube for MCs and turbidity detections in different vacuum conditions and filtration time. In positive pressure filtration, the filter was applied manually with impetus based on the ideal gas state equation:
PV ¼ nRT
ð1Þ
where P is the pressure of the gas, V is the volume of the gas, n is the amount of substance of gas (also known as number of moles), R is the ideal gas constant and T is the temperature of the gas. According to this formula (1), it is easy to conclude that the pressure of the gas is inversely proportional to the volume of the gas in a closed container at a constant temperature. The sample and air were taken in a syringe and the pressure was calculated according to the change of air volume, and the filtrate was collected for MCs and turbidity detections. 2.4. Analytical methods 2.4.1. MCs MCs are the main component of noxious secondary metabolites produced during M. aeruginosa growth and reproduction [27,28]. The dissolved MCs in the filtrate were determined using the Beacon Microcystin Plate Kit that was kindly provided by Beacon Analytical Systems, Inc. (USA). All the following MCs results are presented as lg/L MCs because the Beacon Microcystin Plate Kit is an immunoassay test kit for the quantification of many kinds of MCs but does not differentiate the variants.
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2.4.2. Turbidity The turbidity of the filtrate was tested by spectrophotometry according to the national water quality standard GB13200-91, and modified to allow for the small volume of the sample. The turbidity standard solution of 40 NTU was prepared with 5 mL hydrazine sulfate stock solution (10 g/L) and hexamethylenetetramine stock solution (100 g/L) and volume to 1 L. Other turbidity standard solutions were diluted in proportion. The absorbances of turbidity standard solutions were tested in a 30 mm cuvette (Yixing Jingke Optical Instrument Co., Ltd., China) at 680 nm and plotted into a standard curve. The turbidities of actual samples were tested and calculated according to the standard curve. 2.4.3. Scanning electron microscope (SEM) SEM is a visual method that images a sample producing signals that contain information about the sample’s surface topography, composition, and other properties [29]. Filtrated cyanobacteria-containing sludge under different vacuum pressures and after different storage time were taken for SEM analysis in this study. Samples were fixed with 3% glutaraldehyde overnight, and washed with deionized water three times. Then the samples were subsequently fixed with 1% osmium tetroxide for 2 h, and washed with deionized water for three times. After that, samples were consecutively treated with 30%, 50%, 70%, 85%, 95%, 100%, 100% ethanol (15 min each). Dehydrated algae cells were fully dried with a critical point dryer, and then mounted on copper stubs and sputter-coated with gold–palladium by a sputter coater. The specimens were observed and photographed using a scanning electron microscope (Carl Zeiss Jena EVOLS15, Germany) at 10 kV. 2.4.4. Capillary suction time (CST) CST test is a commonly used method to measure the filterability and the easy of removing moisture from slurry and sludge in numerous environmental and industrial applications [30–33]. Considering the potential changes of sludge characteristics under different sludge storage conditions, the cyanobacteria-containing AlCl3 sludge and PACl sludge after different storage time were taken for CST analysis. The CST analysis was carried out according to the standard method [34] with multipurpose filtration equipment (Triton-W.R.C. Multipurpose Filtration 319) and standard filtration paper (Whatman International Ltd., Maidstone England).
cyanobacteria-containing AlCl3 sludge remained stable initially but decreased obviously when the volume of sludge was equal to or more than 1.0 mL. However, as can be seen in Fig. 1B, the filtration rate of cyanobacteria-containing PACl sludge remained stable when the volume of sludge was less than 1.25 mL and decreased when the volume of sludge was equal to or more than 1.25 mL. It seems that with less sludge volume, the retained sludge was not enough to form a stable sludge filter layer and thus the filtration rate was constant, while with the continuous accumulation of the retained sludge on the surface of filter membrane, a new filter layer was formed to separate the sludge and filtrate, and the new resistance resulted in a lower average filtration rate and higher overall energy consumption. The optimized volume difference between cyanobacteria-containing AlCl3 sludge (1.0 mL) and PACl sludge (1.25 mL) could be due to the different floc characteristics of the two sludges: the average size of AlCl3 sludge is smaller than that of PACl sludge and the AlCl3 floc is more compact than PACl floc, which make the filtration resistance of AlCl3 sludge greater than that of PACl sludge. Besides, the higher dosage of AlCl3 coagulant is perhaps also a contributory factor leading to the more compact structure and lower filtration rate of AlCl3 sludge. In the same system, the changes of MCs in the filtrate during the sludge filtration process were investigated (Fig. 2). Results showed that the MC concentrations in filtrate (both Fig. 2A and B) under different vacuum pressures were all higher than that in treated water (coagulation supernatant), and increased slightly with increasing sludge volume. Combined with the results and analysis in Fig. 1, it could be suspected that the depth filtration produced by the new filter layer of retained sludge could cause some pressure on flocs and result in damage of cyanobacterial cells. Therefore, the excessive accumulation of sludge layer should be avoided in actual operation to improve filtration rate and reduce toxins release. However, considering the amount of obtained cyanobacteria-containing sludge is small, being limited to experiment scale, a relatively higher filtration load could be selected to simulate the real system. As mentioned above, when the sludge volume increased to 1.0 mL and 1.25 mL for AlCl3 and PACl sludge respectively, the filtration rates of these two sludges began to decrease to an acceptable level, but with no obvious release of MCs. It was concluded that 1.0 mL and 1.25 mL of sludge volume were effective for the filtration of cyanobacteria-containing AlCl3 sludge and PACl sludge, respectively, which could be selected for later experiments.
3. Results and discussion 3.1. Optimization and selection of basic filtration conditions The volume of sludge and filter media are important factors that influence the filtration of cyanobacteria-containing sludge and could probably induce the damage of cyanobacterial cells in sludge [23,24]. Therefore, it is necessary to optimize the basic filtration conditions before evaluating the stability of cyanobacterial cells in the sludge filtration process. 3.1.1. Optimization of sludge volume Under the provided effective filter area, the accumulation of sludge is a key factor determining the filtration type and filtration efficiency, and thus influencing the mechanical effect of cells in the sludge. The filtration rate response to the volume of cyanobacteria-containing AlCl3 sludge and PACl sludge was plotted in Fig. 1. It can be seen that with the studied volume of sludge, the filtration of cyanobacteria-containing sludge (Fig. 1A, AlCl3 sludge; Fig. 1B, PACl sludge) at lower vacuum pressure (0.1 bar) was much slower than at higher vacuum pressure (0.9 bar). But both in lower and higher vacuum pressures, the filtration rate decreased with the increase of sludge volume. In Fig. 1A, the filtration rate of
3.1.2. Selection of filter media The filtration rates of cyanobacteria-containing sludge through different filter media were plotted in Fig. 3. It can be seen from Fig. 3A and B that AlCl3 sludge and PACl sludge had similar filtration characteristics. In the four kinds of filter media, the filtration rates were listed as polyether sulfone membrane (PES) > mixed cellulose ester membrane (MCE) > nylon membrane (Nylon) > polytetrafluoroethylene membrane (PTFE). These differences were caused by the different hydrophilic properties of filter media [35,36]: MCE and PES membranes are more hydrophilic than Nylon and PTFE membranes, which give them a higher filtration rate. The effects of filter porosity on filtration rate had no obvious difference, which is probably due to the considerably larger sizes of cyanobaterial cells and flocs (5 lm and 50 lm, respectively) relative to filter porosity, making the effect of filter porosity negligible [37,38]. On the basis of the evaluation of the filtration rate, the MC concentration and turbidity in cyanobacteria-containing sludge filtrate through different filter media were further investigated (Fig. 4). It can be shown in Fig. 4A-a and B-a, the MC concentration in cyanobacteria-containing sludge filtrate was higher than that in treated water (coagulation supernatant). Comparing the systems
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Fig. 1. The filtration rate response to the volume of cyanobacteria-containing sludge ((A) AlCl3 sludge, (B) PACl sludge, filter media: 0.22 lm MCE).
Fig. 2. The MCs in filtrate response to the volume of cyanobacteria-containing sludge ((A) AlCl3 sludge, (B) PACl sludge, filter media: 0.22 lm MCE).
Fig. 3. The filtration rate of cyanobacteria-containing sludge through different filter media ((A) AlCl3 sludge, (B) PACl sludge).
with different filter media, filter porosities and vacuum pressures, the MC differences between filtrate and treated water were consistent, which indicated these different filtration pressures had similar effects on MCs in the filtrates. However, for the turbidity in cyanobacteria-containing sludge filtrate, there existed slightly different effect trends of filter media, filter porosity and vacuum pressure (Fig. 4A-b and B-b). The filtrate turbidity of PES was slightly higher than that of MCE, Nylon and PTFE. For all the four kinds of filter media, reducing the filter porosity could decrease the filtrate
turbidity, while the two kinds of vacuum pressures had similar effects on the filtrate turbidity. Combined with the results of filtration rate, filtrate MCs and turbidity, it is better to choose more hydrophilic filter media with lower filter porosity on the premise of keeping filtration effectiveness and filtrate water quality. In this study, 0.22 lm MCE membrane was selected for later experiments considering its low cost and wide use in research applications.
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Fig. 4. The MCs and turbidity in cyanobacteria-containing sludge filtrate through different filter media ((A) AlCl3 sludge, (B) PACl sludge; (a) MCs, (b) turbidity).
3.2. Effects of mechanical strength and mode of action on the dewatering characteristics of cyanobacteria-containing sludge In the mechanical dewatering process of drinking water sludge, the pressure difference across the both sides of membrane is the driving force for separation, and also determines the damage extent of mechanical action to the cyanobacterial cells in sludge [11]. In view of this, it is necessary to evaluate the effects of mechanical strength and mode of action on the dewatering characteristics of cyanobacteria-containing sludge.
Fig. 5. The filtration rate of cyanobacteria-containing sludge under different vacuumpressure.
3.2.1. Effect of mechanical strength on the dewatering characteristics of cyanobacteria-containing sludge The effect of mechanical strength on the filtration rate of cyanobacteria-containing sludge was shown in Fig. 5. With the increase of vacuum pressure, the pressure difference between the two sides of the membrane became obvious and the corresponding impetus increased, significantly increasing the filtration rate. Compared to AlCl3 sludge, the filtration rate of PACl sludge was slightly higher. It indicated that the PACl sludge had less resistance during the dewatering process, which was probably due to the looser structure of PACl sludge causing greater water permeability of the filter layer. For the cyanobacteria-containing sludge, the mechanical strength could perhaps cause a certain amount of damage to cyanobacterial cells and release of MCs, therefore the MC concentrations and turbidities in filtrate (Fig. 6) and the SEM micrographs of filtrated cyanobacteria-containing sludge (Fig. 7) under different vacuum pressures were further investigated. Fig. 6 showed that the MC concentrations in the filtrates were higher than that in the treated water (coagulation supernatant), but the upward trends under different vacuum pressures were consistent. It indicated the effect of studied vacuum pressure on filtrate MCs was probably based on the filtration procedure rather than the magnitude of pressure. In the same systems, the filtrate turbidity had no obvious difference under different vacuum pressures, which led to the conclusion that the effect of studied mechanical strength on the filtrate turbidity of cyanobacteria-containing sludge was negligible. Directly observed from Fig. 7A and B, the cyanobacterial cells were embraced in AlCl3 sludge and PACl sludge and still remained intact without cell damage under different vacuum pressures. The different number of embraced cells under the three vacuum pressures was because of the different image captures of these three samples. It can therefore be concluded that the studied mechanical strength
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Fig. 6. The MCs and turbidity in cyanobacteria-containing sludge filtrate under different vacuum pressure ((A) AlCl3 sludge, (B) PACl sludge).
Fig. 7. SEM micrographs of cyanobacteria-containing sludge under different vacuum pressure ((A) AlCl3 sludge, (B) PACl sludge; the sludge samples obtained after coagulation and sedimentation were filtrated immediately with vacuum pressure of (a) 0.1 bar, (b) 0.5 bar, (c) 0.9 bar).
would not damage the cyanobacterial cells in cyanobacteria-containing sludge. In view of this, the increased MC concentration in filtrate relative to treated water (in Fig. 6) might be attributed to either the deflocculation of coagulant absorbed MCs or the exfiltration of intracellular MCs caused by the filtration procedure. Combining all the above results, it was found that higher vacuum pressure could improve the filtration rate without causing cell damage in sludge, therefore it should be possible to use higher vacuum pressure in the dewatering process of cyanobacteria-containing sludge. 3.2.2. Analysis of filtration differences between positive pressure filtration and vacuum filtration Positive pressure filtration can also be used for dewatering of cyanobacteria-containing sludge. But compared with vacuum filtration, positive pressure filtration has greater impetus range and poorer controllability, which will make its mechanical strength on cyanobacteria-containing sludge unstable and thus could lead to the damage of cyanobacterial cells. Therefore, the effect of positive pressure filtration on the dewatering characteristics of cyanobacteria-containing sludge was tested and the filtration differences between positive pressure filtration and vacuum filtration
were analyzed. The filtration time, filtration rate, filtrate MCs and filtrate turbidity of cyanobacteria-containing AlCl3 sludge and PACl sludge under different positive filtration pressures were listed in Tables 1 and 2, respectively. Results showed that with the increasing pressure, the filtration time was shortened and the filtration rate was improved, which was almost consistent with the
Table 1 The dewatering characteristics of cyanobacteria-containing AlCl3 sludge with positive pressure filtration.
DP (bar)a
Time (min)
Positive pressure filtration 0.1 20.40 ± 1.62b 0.5 7.87 ± 0.62 0.9 2.86 ± 0.22 1.5 1.47 ± 0.17 2.0 1.03 ± 0.19 3.0 0.67 ± 0.07 Vacuum filtration 0.9 3.23 ± 0.20 a b
Filtration rate (mL/ min)
MCs (lg/ L)
Turbidity (NTU)
0.049 ± 0.004 0.127 ± 0.013 0.350 ± 0.030 0.690 ± 0.079 1.008 ± 0.182 1.49 ± 0.140
23.1 ± 1.1 24.0 ± 1.2 26.1 ± 1.4 27.7 ± 1.3 28.1 ± 0.9 28.4 ± 1.2
27.60 ± 0.75 27.90 ± 0.65 28.71 ± 0.90 29.81 ± 0.94 29.82 ± 1.05 30.21 ± 0.92
0.31 ± 0.02
23.3 ± 1.2
27.75 ± 0.83
DP = Actual pressure Standard atmospheric pressure. Each data were tested for three times.
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Table 2 The dewatering characteristics of cyanobacteria-containing PACl sludge with positive pressure filtration. Filtration rate (mL/ min)
MCs (lg/ L)
Turbidity (NTU)
Positive pressure filtration 0.1 23.58 ± 2.03b 0.5 8.99 ± 0.66 0.9 3.21 ± 0.31 1.5 1.97 ± 0.19 2.0 1.49 ± 0.10 3.0 0.77 ± 0.05
0.053 ± 0.005 0.139 ± 0.006 0.390 ± 0.035 0.635 ± 0.053 0.83 ± 0.052 1.61 ± 0.091
23.3 ± 0.9 24.3 ± 1.0 26.6 ± 1.1 28.0 ± 1.1 28.4 ± 1.5 28.7 ± 1.2
27.71 ± 0.68 27.92 ± 0.81 28.91 ± 0.87 29.69 ± 0.90 29.95 ± 0.93 30.45 ± 0.78
Vacuum filtration 0.9 3.84 ± 0.27
0.327 ± 0.023
23.7 ± 0.9
28.19 ± 0.76
DP (bar)a
a b
Time (min)
DP = Actual pressure Standard atmospheric pressure. Each data were tested for three times.
changing trend of filtration in the vacuum filtration process. But the filtration rate in positive pressure filtration was a little higher than that in vacuum filtration. Taking the results of 0.9 bar in vacuum filtration and 0.9 bar in positive pressure filtration for example, the filtration rate of cyanobacteria-containing AlCl3 sludge was 0.31 mL/min in vacuum filtration but 0.35 mL/min in positive pressure filtration; and the filtration rate of cyanobacteria-containing PACl sludge was 0.33 mL/min in vacuum filtration but 0.39 mL/min in positive pressure filtration. However, the results of MCs and turbidity showed that the positive pressure filtration probably caused damage of sludge flocs and partial cyanobacterial cells, and increased the MC concentration and turbidity in the filtrate. Also take the results of 0.9 bar in vacuum filtration and 0.9 bar in positive pressure filtration for example, the filtrate MCs and turbidity of cyanobacteria-containing AlCl3 sludge were 23.3 lg/L and 27.75 NTU in vacuum filtration but 26.1 lg/L and 28.71 NTU in positive pressure filtration; and the filtrate MCs and turbidity of cyanobacteria-containing PACl sludge were 23.7 lg/L and 28.19 NTU in vacuum filtration but 26.6 lg/L and 28.91 NTU in positive pressure filtration. When the pressure was added above 0.9 bar, the MCs and turbidity increased continually and the results were consistent with increased damage to flocs and cells. Compared with the results of cyanobacteria-containing AlCl3 sludge, the filtration rate of cyanobacteria-containing PACl sludge in positive pressure filtration was improved to some extent, but at the same time the filtrate MCs and turbidity showed a small increase, which indicated that the mechanical strength of positive pressure filtration caused slightly greater damage to PACl sludge than to AlCl3 sludge (though the difference was not significant). Overall, whether for AlCl3 sludge or PACl sludge, it is better to use lower-destructive vacuum filtration to minimize the cell damage and MCs release during the drinking water sludge dewatering process. 3.3. Effect of sludge storage time on the dewatering characteristics of cyanobacteria-containing sludge Besides filter conditions and mechanical actions, the cyanobacteria-containing sludge could also be influenced during the sludge storage process. With the storage time increased, the cyanobacteria-containing sludge would be influenced by the storage environment. Therefore it is necessary to evaluate the effect of sludge storage time on the dewatering characteristics of cyanobacteria-containing sludge. The filtration rate of cyanobacteria-containing sludge at different storage time was plotted in Fig. 8. The two kinds of vacuum pressures (0.9 bar and 0.1 bar) were chosen for filtration and they showed a consistent changing trend of filtration rate. For the cyanobacteria-containing AlCl3 sludge (Fig. 8A-a and A-b), the
filtration rate increased slowly in 0–4 d, significantly more rapidly in 4–8 d, and then remained stable in 8–12 d; while for the cyanobacteria-containing PACl sludge (Fig. 8B-a and B-b), the filtration rate increased slowly in 0–2 d, significantly more rapidly in 2–6 d, and then remained stable in 6–10 d. In the operating conditions provided, the filtration rate of sludge could only be relevant to the sludge structure. From the above results, it could be found that the morphology structure of cyanobacteria-containing AlCl3 sludge might cause changes after storage for 4 d. However, the changes of cyanobacteria-containing PACl sludge was observed earlier than in AlCl3 sludge (after 2 d), which could be due to the looser structure of cyanobacteria-containing PACl sludge. A similar difference was also noted in our previous studies [7,8], which indicated that sludge storage time had advanced influence on the PACl sludge and affected the stability of cyanobacterial cells more significantly. In addition, when the morphology structure of cyanobacteria-containing sludge was changed, its dewaterability would also be influenced. The CST (which is used to evaluate sludge dewaterability [30–33]) values of cyanobacteria-containing sludge after different storage time were shown in Fig. 9. It can be seen that the CST of AlCl3 sludge and PACl sludge had a similar changing trend (decreased at first and then increased), but the lowest value occurred at different sludge storage time (6 d for AlCl3 sludge, 3 d for PACl sludge). This phenomenon is consistent with the results in Fig. 8, which also indicated that the morphology structure of cyanobacteria-containing AlCl3 and PACl sludge would change after storage for 4 d and 2 d, respectively. The effects of sludge storage time on MCs and turbidity in the filtrate of cyanobacteria-containing sludge were shown in Fig. 10. It can be seen from Fig. 10A-a and B-a that the MC concentrations in the filtrates of both cyanobacteria-containing AlCl3 sludge and cyanobacteria-containing PACl sludge increased gradually with the sludge storage time: the MCs of cyanobacteria-containing AlCl3 sludge increased gradually in 0–4 d, significantly more rapidly in 4–10 d, then slowed down in 10–12 d (Fig. 10A-a); while the MCs of cyanobacteria-containing PACl sludge increased gradually in 0–2 d, significantly more rapidly in 2–8 d, then slowed down in 8–10 d (Fig. 10B-a). The increase of MCs was probably due to the breakage of cyanobacterial cells, which suggested that the cyanobacterial cells in sludge became gradually unstable with the storage time. The difference between cyanobacteria-containing AlCl3 sludge and PACl sludge was the breakage time of cyanobacterial cells (4 d was an important point for AlCl3 sludge, while it was 2 d for PACl sludge), which was probably also due to the different structures of the two kinds of sludge. Compared with the sludge storage time, the vacuum pressure had less influence on the MCs in filtrate: the MCs under 0.9 bar and 0.1 bar had no difference in 0 d, and had slightly lower value at lower vacuum pressure after 2 d, and had more obvious difference after 6 d. This indicated that increasing the storage time would weaken the endurance of cyanobacterial cells to mechanical strength. From the results in Fig. 10A-b and B-b, the turbidity in the filtrate had similar changing trends with the filtration rate: the turbidity of cyanobacteria-containing AlCl3 sludge increased slowly in 0–4 d, significantly more rapidly in 4–8 d, and then remained stable in 8–12 d (Fig. 10A-b); while the turbidity of cyanobacteria-containing PACl sludge increased slowly in 0–2 d, significantly more rapidly in 2–6 d, and then remained stable in 6–10 d (Fig. 10B-b). This indicated that the sludge storage process could cause damage of cyanobacterial cells (inferred by increasing MCs) and structural changes of cyanobacteria-containing sludge (inferred by increasing filtration rate and turbidity). In order to observe the sludge changes directly, SEM micrographs of filtrated (vacuum pressure: 0.9 bar) cyanobacteria-containing sludge after different storage time were shown in Fig. 11. It can be seen from
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Fig. 8. The filtration rate of cyanobacteria-containing sludge after different storage time ((A) AlCl3 sludge, (B) PACl sludge; (a) vacuum pressure 0.9 bar, (b) vacuum pressure 0.1 bar).
Fig. 9. The CST of cyanobacteria-containing sludge after different storage time ((A) AlCl3 sludge, (B) PACl sludge).
Fig. 11A-a that the cyanobacterial cells embraced in sludge and kept intact at 0 d. Fig. 11A-b showed a deflocculation phenomenon of cyanobacterial cells from AlCl3 sludge occurred with the storage. In Fig. 11A-c, cell lysis occurred until the AlCl3 sludge storage for 6 d. And the lysis would be more obvious with the storage time increased (Fig. 11A-d). For PACl sludge (Fig. 11B), the similar changes could also be observed, but the cell lysis occurred at 4 d (Fig. 11B-c). Combined with the evidence of SEM, the changing trend of CST in Fig. 9 could lead to the conclusion that the deflocculation of cyanobacteria-containing sludge decreased the filtration resistance of the sludge but the lysis of cyanobacteria cells
and increasing cell debris would add another resistance to increase the CST. Combining all the above results, we see that prolonging the sludge storage time could improve the filtration efficiency to some extent, but obviously caused the damage of cyanobacterial cells in sludge, and lowered the quality of recycled water. Therefore, it is important to shorten the sludge storage time and complete the volume reduction process as soon as possible in actual operation (the storage time for AlCl3 and PACl sludge should not be more than 2 d and 4 d, respectively).
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Fig. 10. The MCs and turbidity in cyanobacteria-containing sludge filtrate after different storage time ((A) AlCl3 sludge, (B) PACl sludge; (a) MCs, (b) turbidity).
Fig. 11. SEM micrographs of cyanobacteria-containing sludge after different storage time ((A) AlCl3 sludge, (a) 0 day, (b) 4 day, (c) 6 day, (d) 8 day; (B) PACl sludge, (a) 0 day, (b) 2 day, (c) 4 day, (d) 6 day; all the samples were filtrated with vacuum pressure of 0.9 bar).
4. Conclusions This study focused on the dewatering characteristics of cyanobacteria-containing AlCl3 sludge and PACl sludge, optimized the filtration conditions and investigated the effect of mechanical action and storage time on the filtration efficiency and filtrate quality during the sludge volume reduction process.
(1) Sludge accumulation: Under the studied effective area, the accumulation of sludge is a key factor determining the filtration characteristics. Sludge volumes below 1.0 mL were not enough to form a stable sludge filter layer to reduce the filtration rate, while the continuous accumulation of retained sludge on the membrane surface formed a new filter layer with more filtration resistance and resulted in lower average filtration rate and higher MC concentration in the filtrate. In
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practice, excessive accumulation of sludge layer should be avoided to improve the filtration rate and reduce toxins release. (2) Filter media: Aiming at keeping the filtration efficiency, hydrophilic filter media with lower filter porosity should be chosen for filtering cyanobacteria-containing sludge. (3) Mechanical action: High vacuum pressure could improve the filtration rate without causing cell damage and increasing the filtrate MCs and turbidity. Therefore on the basis of keeping filtration efficiency, it is better to choose stringer vacuum pressure in vacuum filtration process of cyanobacteria-containing sludge. Compared with vacuum filtration, positive pressure filtration improved the filtration rate but also increased the MCs and turbidity in filtrate. Overall, it is better to use lower-destructive vacuum filtration to decrease the cell damage and MCs release during drinking water sludge dewatering process. (4) Storage time: Prolonging the sludge storage time improved the filtration efficiency to some extent, but caused obvious cell damage in the sludge. Therefore, it is important to shorten the sludge storage time and complete the volume reduction process as soon as possible in actual operation to maintain the quality of recycled water. The sludge amount studied in the lab is much less than that produced in a drinking water plant, thus the pressure values are correspondingly lower. However, the influences of sludge accumulation per unit area on filtration rate and cyanobacteria lysis would still exist whenever the sludge amount is less or more. And the filter media chosen in this study are all organic polymer materials which are similar to those used in real full-scale systems. Besides, the assessment and evaluation indices and strategies used in this study could be equally applied to other processing methods under sufficient sample volume conditions. Therefore, the corresponding simulation results of this study provided scientific references for the volume reduction of cyanobacteria-containing drinking water sludge in the actual treatment. Acknowledgments The authors acknowledge financial supports from National Science Fund for Excellent Young Scholars (51322811), International Science & Technology Cooperation Program of China (2010DFA91150), International Cooperation Research of Shandong Province (2011176), Science and Technology Development Project of Shandong Province (2012GHZ30020), the Program of the Ministry of Education of China for New Century Excellent Talents in University (NCET-12-0341), Graduate Innovation Foundation of Shandong University (yyx10035), National Natural Science Foundation (51178408) and Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors thank Dr. Findlay Nicol of Shandong University of Finance and Economics for revising the English in the manuscript. References [1] H.W. Paerl, H. Xu, M.J. McCarthy, G.W. Zhu, B.Q. Qin, Y.P. Li, W.S. Gardner, Controlling harmful cyanobacterial blooms in a hyper-eutrophic lake (Lake Taihu, China): the need for a dual nutrient (N & P) management strategy, Water Res. 45 (2011) 1973–1983. [2] A.J. Lewitus, L.M. Brock, M.K. Burke, K.A. DeMattio, S.B. Wilde, Lagoonal stormwater detention ponds as promoters of harmful algal blooms and eutrophication along the South Carolina coast, Harmful Algae 8 (2008) 60–65. [3] G. Pan, M.M. Zhang, H. Chen, H. Zou, H. Yan, Removal of cyanobacterial blooms in Taihu Lake using local soils. I. Equilibrium and kinetic screening on the flocculation of Microcystis aeruginosa using commercially available clays and minerals, Environ. Pollut. 141 (2006) 195–200.
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