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Reducing harmful algae in raw water by light-shading
Process Biochemistry 44 (2009) 357–360
Contents lists available at ScienceDirect
Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Short communication
Reducing harmful algae in raw water by light-shading Xue-Chu Chen a, Hai-Nan Kong a,*, Sheng-Bing He a, De-Yi Wu a, Chun-Jie Li a, Xiao-Chen Huang b a b
School of Environmental Science and Engineering, Shanghai JiaoTong University, Shanghai 200240, PR China Shanghai East Sea Marine Engineering Survey & Design Institute, Shanghai 200137, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 June 2008 Received in revised form 27 October 2008 Accepted 5 November 2008
The occurrence of harmful algal bloom in water source poses a serious water safety problem to local water supply systems. In order to ensure the raw water quality, the feasibility of an in situ light-shading measure was investigated through enclosure experiment and pilot-scale experiment. The results showed that harmful algal bloom could be controlled by light-shading lasting for 6–9 days, with water quality being partially improved. When aeration was added, the reduction of algal biomass could be enhanced, and water quality was further improved compared to that without aeration. These experimental results offered an attractive in situ algal control measure for lakes or reservoirs suffered from harmful algal bloom. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: Light-shading Raw water Harmful algae Water quality Aeration
1. Introduction Seasonal growth of harmful algae such as Microcystis aeruginosa issues severe challenge to traditional water treatment systems. It is recommended that the Chlorophyll-a concentration (Chla) in raw water should be below 50 mg/L [1], otherwise the conventional water treatment facilities would be ineffective to remove algae. In order to ensure the safety of local water supply systems, various pre-treatment measures have been investigated [2–5]. However, these measures may be unacceptable because of high operating cost or release of some toxic substances such as microcystines (MCs). In situ algal control is an alternative idea which tries to remove algae in water source area but not in water treatment plant [6,7]. Currently, a simple physical measure termed ‘‘partial lightshading’’ had been suggested [8]. Pilot-scale study revealed that great reduction of blue algal biomass could be achieved by lightshading of 30% of the water surface. However, for those lakes or reservoirs with large surface area, it is very difficult to carry out this measure of shading 30% water surface. Considering the core of the measure is to keep algae under prolonged darkness to reduce algal biomass, a new thought is initiated, that is, to build light-shading zone around water intake location. By forcing raw water to stay at dark area for a certain period, a significant reduction of those harmful algae is expected. The difference between this new thought and the previous ‘‘partial
* Corresponding author. Tel.: +86 2154744540; fax: +86 2154740825. E-mail address: [email protected] (H.-N. Kong). 1359-5113/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2008.11.002
light-shading’’ is that it focuses on algal control of daily raw waterconsumption, but not the whole water-body. Therefore the total light-shading area required for algal control can be reduced significantly. Accordingly, the light-shading period required for substantial algae biomass reduction, and the impact of lightshading on water quality should be determined. Nevertheless, considering that light-shading would induce DO depletion due to the inhibition of photosynthesis, the feasibility of keeping DO by aeration should be studied. In this study, enclosure experiment was conducted to illustrate the reduction of algal biomass and the variation of water quality parameters during light-shading period, and also the influence of aeration on this light-shading process. Furthermore, a pilot-scale experiment was carried out to verify the feasibility of the suggested light-shading measure. 2. Materials and methods The enclosure experiment was run in parallel with three enclosures in a lake situated in Shanghai JiaoTong University, east of China. The enclosure was about 1 m2 in area and about 1.5 m in depth. To maintain the enclosure in a eutrophic state, a certain quantity of NaNO3 and KH2PO4 were added at the beginning of the experiment. On August 10th, 2007, a M. aeruginosa bloom was observed on the surfaces of the three enclosures. In order to simulate light-shading condition, a black shade cloth with a light-shading ratio of 99% was covered on the surfaces of the two enclosures (No. 1 and No. 2). The third enclosure (No. 3) remained uncovered as control. Some air was introduced at the bottom of No. 1, and the air: water volume ratio throughout experiment was controlled at 3:1. During experiment, the average water temperature was 31 8C. Before sampling, each enclosure was aerated for 10 min to maintain water quality homogeneous. Pilot-scale experiment was conducted in a pond which supplied water for irrigation usage. Its effective water volume was about 560 m3. The water-body was in a serious eutrophic state in test period, and the algal bloom dominated by
X.-C. Chen et al. / Process Biochemistry 44 (2009) 357–360
358
Coelosphaeriu was observed. The average water temperature throughout the experiment was 22.9 8C. From the first day to the sixth day of experiment, two-layer shade sheet was covered and the light-shading ratio was 90%. After that, one more shade sheet was added to enhance the light-shading effect and the light-shading ratio rose to 98%. Water samples were taken from different depths, and thereafter mixed together to represent the average water quality. The Chla, pH, dissolved oxygen (DO), chemical oxygen demand (COD), and Dissolved phosphate (PO43–P) were monitored throughout the experiment using standard methods [9]. Microcystines (MCs) were detected by enzyme linked immune sorbent assay (ELISA) method as suggested in literatures [10,11].
3. Results 3.1. Reduction of algal biomass by light-shading in enclosure experiment Fig. 1 shows the reduction of algal biomass by light-shading. During the first 2 days, in both light-shading enclosures (No. 1 and No. 2), a rapid reduction in algal biomass was observed. In No. 2 enclosure, the Chla decreased from 196.1 mg/L to 125.7 mg/L, with a reduction rate of 35.3 mg/(L day), whereas in No. 1 enclosure, the Chla from 184.7 mg/L to 80.6 mg/L, with a much higher reduction rate of 52.1 mg/(L day). Thereafter, in both enclosures, the algal biomass continually decreased. On the sixth day, in No. 2 enclosure, the Chla was about 33.5 mg/L, whereas in No. 1 enclosure, the Chla reduced to 20.0 mg/L. On the eighth day, no significant decrease in algal biomass was observed in both enclosures. In contrast, the Chla in control enclosure (No. 3) increased from 89.7 mg/L to 138.6 mg/L on the first day, and then varied slightly during the next 8 days.
Fig. 2. Variation of DO and pH under light-shading condition. (&) DO, control; (&) DO, light-shading; (~) DO, light-shading plus aeration; (^) pH, control; (&) pH, light-shading; (&) pH, light-shading plus aeration.
3.2. DO and pH influenced by light-shading in enclosure experiment As shown in Fig. 2 under light-shading condition with the reduction of algal biomass, DO and pH were much lower than the values under normal condition. In No. 2 enclosure, on the first day DO declined significantly from 9.7 mg/L to as low as 0.3 mg/L, afterwards DO remained at such low level. In aeration enclosure (No. 1), on the first day DO declined from 9.2 mg/L to 2.1 mg/L, afterwards DO varied slightly. For pH, in enclosure without aeration (No. 2), it declined from 9.0 to 7.0, whereas in aeration enclosure (No. 1) pH decreased less, from 8.4 to 7.5. In contrast, in control enclosure (No. 3) DO and pH varied fluctuantly, and no significant decline was observed.
Fig. 3. Variation of MCs in light-shading water enclosure.
The variations of MCs, COD and PO43–P under light-shading condition are shown in Figs. 3–5. In no aeration enclosure (No. 2),
MCs, COD and PO43–P all increased on the first day, from 0.85 mg/ L to 1.8 mg/L, 44.8 mg/L to 47.3 mg/L and 0.50 mg/L to 0.79 mg/L, respectively. From the second day to the third day, a significant decrease was observed, with MCs decreased to 0.25 mg/L, COD to 26.9 mg/L, and PO43–P to 0.55 mg/L, respectively. From the third day to the eighth day, MCs and PO43–P increased slightly whereas COD continuously decreased. In aeration enclosure (No. 1), the variation trend of MCs was not significant difference compared to no aeration enclosure (No. 2) (T test, P > 0.1), and on the eighth day MCs was only 0.15 mg/L. During the experiment no increase of appeared, and on the eighth day, COD was 13.9 mg/L, reduced by 67.1% compared to initial COD. PO43–P decreased gradually until the fourth day, then increased slightly, and on the eighth day,
Fig. 1. Reduction of algal biomass in light-shading water enclosure.
Fig. 4. Variation of COD in light-shading water enclosure.
3.3. MCs, COD, and PO43–P influenced by light-shading in enclosure experiment
X.-C. Chen et al. / Process Biochemistry 44 (2009) 357–360
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Table 2 Rate constants k calculated under different experiment conditions. No aeration
Enclosure experiment Pilot-scale experiment
Fig. 5. Variation of PO43–P in light-shading water enclosure.
PO43–P was about 0.36 mg/L. Furthermore, compared to that in no aeration enclosure (No. 2), all values of those three water quality parameters in aeration enclosure (No. 1) were lower during the experiment, which indicated that aeration had a positive effect on water quality under light-shading condition. 3.4. Pilot-scale experiment A pilot-scale experiment was conducted to investigate the effect of light-shading on water quality. Throughout pilot-scale experiment, aeration was not applied because the average water temperature was as low as 22.9 8C, and the algal biomass was much lower than that in enclosure experiment. The results indicated a removal of 81.0% was obtained by light-shading for 9 days, and the algal bloom was well controlled. Meanwhile, the water quality was partially improved, as shown in Table 1. The results showed that turbidity and COD were reduced obviously, with a removal efficiency of 73.1%, and 94.5%, respectively. The decrease of turbidity and COD might relate to the reduction of algal biomass, considering that Chla had a very close correlation with turbidity (R = 0.99, P < 0.001) and COD (R = 0.98, P < 0.001). It indicated that algae contributed most of COD and turbidity when water bloom occurred. It could be seen that TP remained almost constant whereas TN increased very slightly, therefore it could be concluded that although light-shading controlled algae effectively, it had little influence on nutrient concentration. Also, before and after light-shading, NH4+–N kept almost unchanged. For DO, a rapid decrease was also observed. At the beginning, DO was as high as 12 mg/L, which was in hyper-saturated state due to the algae photosynthesis. After light-shading for 9 days, the value stayed at 3.2 mg/L. This relatively high DO level suggested that aeration was not necessary. 4. Discussion Previous studies had reported that in batch test with optimum conditions, M. aeruginosa decreased markedly with the length of the darkness period and was reduced to only 1% of the initial cell number after 20 days of darkness [12]. Our study confirmed that light-shading could bring significant adverse effect on M.
Aeration
k (d1)
R2
k (day1)
R2
0.24 0.16
0.944 0.903
0.31
0.959
aeruginosa, causing the phenomenon noted as ‘‘light-limited growth’’. The reduction of algal biomass under light-shading condition might mainly relate to the dramatic decrease in photosynthetic rate of algae. Under light-shading condition, when incident light was lower than light compensation point, the photosynthesis of algae was inhibited, and the respiration was in dominance, forcing the algae to continuously consume its own components and oxygen in water, and then became extinct gradually [13]. Assuming that decrease rate constant was proportional to algal biomass, a dynamic model for the reduction process is presented as follows [14]: M ¼ M0kt The loss of algal biomass is fitted through a least square regression of algal density or Chla versus time to yield rate constants (k) by using a simple first-order equation, where the algal density or Chla at a point in time (M) is the product of its original values (M0) and the rate constant at that time. As shown in Table 2, regression coefficients obtained throughout enclose experiment and pilot-scale experiment were all above 0.9, indicating that this simple first-order equation could be used to describe the reduction process of algal biomass under lightshading condition. Furthermore, a significant DO depletion was observed after light-shading was implemented (T test, P < 0.01) compared to the value in control enclosure. It could be attributed to the respiration of algae and the decomposition of algal biomass. However, if DO in water declined to a low level, the reduction process might have been inhibited due to the insufficient oxygen supply for respiration. Previous studies illustrated that at the water-sediment interface, oxic condition would promote the algal decay processes [15]. Our experiment confirmed that oxygen had a positive effect on algal control, considering that when aeration was added, the k value was much higher (Table 2). In addition, enclosure experiment showed that PO43–P increased under light-shading condition, while when aeration was added, the k value decreased. In water column, PO43–P was significantly influenced by DO [16]. Under light-shading condition, the DO deceased to extremely low level (lower than 0.5 mg/L). Therefore, the sediment tended to release PO43–P, and PO43–P in water increased. On the other hand, when aeration was added, the DO was always above 2 mg/L. Therefore, the release of PO43–P was avoided. For pH, it is noted that with the photosynthesis of algae, the pH increases due to the photosynthesis consuming CO2 in water. Therefore, it could be inferred that the decrease of pH observed in our experiment was because the inhabitation of algae photosynthesis under lightlimited condition.
Table 1 Water quality of the test pond before and after light-shading. Parameter
Before light- shading After light-shading Removal efficiency
Chla (mg/L)
COD (mg/L)
Turbidity (NTU)
DO (mg/L)
TP (mg/L)
TN (mg/L)
NH4+–N (mg/L)
130.5 24.8 81%
30.9 1.7 94.5%
20.0 5.4 73.1%
12.0 3.2 73.3%
0.067 0.063 6.0%
1.2 1.4 –
0.05 0.05 –
360
X.-C. Chen et al. / Process Biochemistry 44 (2009) 357–360
Based on the limit-factor theory, controlling aquatic nutrients especially phosphorus was regarded as the most chief measure in eutrophication prevention [17,18]. However, some long-term researches and engineering practices discovered that, for those water sources with comparatively high background phosphorus concentration, the expenses of source control were extremely high, and usually the harmful algal bloom would not be controlled until ten years later or more [19,20]. On the other hand, the raw water quality should be ensured although the eutrophication problem was not yet solved [21,22]. Light was also a limiting factor related to algae growth, and the results of Kojima validated the applicability of algae control by ‘‘partial light-shading’’ [8]. However, this measure needed to be improved for the required shading area is too large. Considering this problem, we suggested building a light-shading zone around the water intake location, therefore by forcing the water to stay within that area for a certain period, the high pollution load caused by bloom in raw water could be significantly reduced. Regarding this new idea, the primary question was to determine the relationship between light-shading duration and algae distinction. Our experiment verified that algal biomass could be dramatically reduced after approximately 6–9 days of surface light-shading, and the Chla remained in raw water was only 20–30 mg/L, therefore it could be handled by the conventional water treatment process [1]. The results indicated the feasibility of this measure for practical application. However, it should be noted that light-shading could lead to DO depletion in raw water. According to our study, lightshading accompanied by aeration were suggested to solve this problem. 5. Conclusion Results of this study concluded that light-shading could be an attractive measure for raw water protection from harmful algal bloom. In enclosure experiment, algal biomass decreased under light-shading condition, meantime, the significant decrease of pH, COD and MCs was also observed. However, mainly due to the respiration of algae, DO declined dramatically. When light-shading was accompanied by aeration, the reduction process of algal biomass could be enhanced, and the water quality was improved compared to that under no aeration condition. In addition, a firstorder equation was used to describe the reduction process of algal biomass, and it was found that aeration could increase the rate constants k. In pilot-scale experiment, after 9 days of light-shading, the Chla in water declined from 130.5 mg/L to 24.8 mg/L, with turbidity and COD greatly improved, which confirmed that the measure was effective.
Reference [1] Bartram J, Burch M, Falconer I, Jones G, Kuiper-Godman T. Situation assessment, planning and management. In: World Health Organization, editor. Toxic cyanobacteria in water. A guide to their public health consequences, monitoring and management. London: E & FN SPON; 1999. [2] Ahn CY, Park MH, Joung SH, Kim HS, Jang KY, Oh HM. Growth inhibition of cyanobacteria by ultrasonic radiation: laboratory and enclosure studies. Environ Sci Technol 2003;37:3031–7. [3] Takaara T, Sano D, Konno H, Omura T. Cellular proteins of M. aeruginosa inhibiting coagulation with polyaluminum chloride. Water Res 2007;41: 1653–8. [4] Teixeira MR, Rosa MJ. Integration of dissolved gas flotation and nanofiltration for M. aeruginosa and associated microcystins removal. Water Res 2006;40:3612–20. [5] Rodrı´guez E, Majado ME, Meriluoto J. Oxidation of microcystin by permanganate: Reaction kinetics and implications for water treatment. Water Res 2007;41:102–10. [6] Pan G, Zou H, Chen H, Yuan X. Removal of harmful cyanobacterial blooms in Taihu Lake using local soils. III. Factors affecting the removal efficiency and an in situ field experiment using chitosan-modified local soils. Environ Pollut 2006;141:206–12. [7] Asaeda T, Priyantha DGN, Saitoh S, Gotoh K. A new technique for controlling algal blooms in the withdrawal zone of reservoirs using vertical curtains. Ecol Eng 1996;7:95–104. [8] Kojima S. Corroborating study on algal control by partial shading of lake surface. Raw Waste Water 2000;42:5–12 (in Japanese). [9] APHA/AWWA/WEF. Standard methods for the examination of water and wastewater, Washington DC; 1998. [10] Lei LM, Wu YS, Gan NQ, Song LR. An ELISA-like time-resolved fluorescence immunoassay for microcystin detection. Clin Chim Acta 2004;348:177–80. [11] Mcdermott CM, Feola R, Plude J. Detection of cyanobacterial toxins (microcystins) in waters of northeastern Wisconsin by a new immunoassay technique. Toxicon 1995;11:1433–42. [12] Furusato E, Asaeda T, Manatunge J. Tolerance for prolonged darkness of three phytoplankton species, M.aeruginosa (Cyanophyceae), Scenedesmus quadricauda (Chlorophyceae), and Melosira ambigua (Bacillariophyceae). Hydrobiologia 2004;527:153–62. [13] Kirk JTO. Light and photosynthesis in aquatic ecosystem. Cambridge, U.K.: Cambridge University Press; 1994. [14] Reynolds CS. The ecology of freshwater phytoplankton. Cambridge, U.K.: Cambridge University Press; 1984. [15] Harvey HR, John TJ, Tyler B. Kinetics of phytoplankton decay during simulated sedimentation: Changes in biochemical composition and microbial activity under oxic and anoxic conditions. Geochim Cosmochim Acta 1995;59:3367–77. [16] Jiang X, Jin XC, Yao Y, Li LH, Wu FC. Effects of biological activity, light, temperature and oxygen on phosphorus release processes at the sediment and water interface of Taihu Lake, China. Water Res 2008;42:2251–9. [17] Martin TD, Katrin T. Cyanobacterial dominance in lakes. Hydrobiologia 2000;438:1–12. [18] Jing XC. Analysis of eutrophication state and trend for lakes in China. J Limnol 2003;62:60–6 (in Chinese). [19] Van LL, Parma S, Gulati RD. Working group water quality research Loosdreht Lakes: its history, structure, research programmer, and some results. Hydrobiologia 1992;233:1–9. [20] Welch EB, Barbiero RP, Bouchard D, Jones CA. Lake trophic state change and constant algal composition following dilution and diversion. Ecol Eng 1992;1:173–97. [21] Barrett PR, Littlejohn JW, Curnow J. Long-term algal control in a reservoir using barley straw. Hydrobiologia 1999;415:309–13. [22] Kim HK, Kim JM, Lee YJ, Kim BI, Lee BC, Chang NI. Vertical profile of algal distribution during aeration prior to intake tower for safe drinking water. Water Sci Technol 2007;55:321–7.
Contents lists available at ScienceDirect
Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Short communication
Reducing harmful algae in raw water by light-shading Xue-Chu Chen a, Hai-Nan Kong a,*, Sheng-Bing He a, De-Yi Wu a, Chun-Jie Li a, Xiao-Chen Huang b a b
School of Environmental Science and Engineering, Shanghai JiaoTong University, Shanghai 200240, PR China Shanghai East Sea Marine Engineering Survey & Design Institute, Shanghai 200137, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 June 2008 Received in revised form 27 October 2008 Accepted 5 November 2008
The occurrence of harmful algal bloom in water source poses a serious water safety problem to local water supply systems. In order to ensure the raw water quality, the feasibility of an in situ light-shading measure was investigated through enclosure experiment and pilot-scale experiment. The results showed that harmful algal bloom could be controlled by light-shading lasting for 6–9 days, with water quality being partially improved. When aeration was added, the reduction of algal biomass could be enhanced, and water quality was further improved compared to that without aeration. These experimental results offered an attractive in situ algal control measure for lakes or reservoirs suffered from harmful algal bloom. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: Light-shading Raw water Harmful algae Water quality Aeration
1. Introduction Seasonal growth of harmful algae such as Microcystis aeruginosa issues severe challenge to traditional water treatment systems. It is recommended that the Chlorophyll-a concentration (Chla) in raw water should be below 50 mg/L [1], otherwise the conventional water treatment facilities would be ineffective to remove algae. In order to ensure the safety of local water supply systems, various pre-treatment measures have been investigated [2–5]. However, these measures may be unacceptable because of high operating cost or release of some toxic substances such as microcystines (MCs). In situ algal control is an alternative idea which tries to remove algae in water source area but not in water treatment plant [6,7]. Currently, a simple physical measure termed ‘‘partial lightshading’’ had been suggested [8]. Pilot-scale study revealed that great reduction of blue algal biomass could be achieved by lightshading of 30% of the water surface. However, for those lakes or reservoirs with large surface area, it is very difficult to carry out this measure of shading 30% water surface. Considering the core of the measure is to keep algae under prolonged darkness to reduce algal biomass, a new thought is initiated, that is, to build light-shading zone around water intake location. By forcing raw water to stay at dark area for a certain period, a significant reduction of those harmful algae is expected. The difference between this new thought and the previous ‘‘partial
* Corresponding author. Tel.: +86 2154744540; fax: +86 2154740825. E-mail address: [email protected] (H.-N. Kong). 1359-5113/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2008.11.002
light-shading’’ is that it focuses on algal control of daily raw waterconsumption, but not the whole water-body. Therefore the total light-shading area required for algal control can be reduced significantly. Accordingly, the light-shading period required for substantial algae biomass reduction, and the impact of lightshading on water quality should be determined. Nevertheless, considering that light-shading would induce DO depletion due to the inhibition of photosynthesis, the feasibility of keeping DO by aeration should be studied. In this study, enclosure experiment was conducted to illustrate the reduction of algal biomass and the variation of water quality parameters during light-shading period, and also the influence of aeration on this light-shading process. Furthermore, a pilot-scale experiment was carried out to verify the feasibility of the suggested light-shading measure. 2. Materials and methods The enclosure experiment was run in parallel with three enclosures in a lake situated in Shanghai JiaoTong University, east of China. The enclosure was about 1 m2 in area and about 1.5 m in depth. To maintain the enclosure in a eutrophic state, a certain quantity of NaNO3 and KH2PO4 were added at the beginning of the experiment. On August 10th, 2007, a M. aeruginosa bloom was observed on the surfaces of the three enclosures. In order to simulate light-shading condition, a black shade cloth with a light-shading ratio of 99% was covered on the surfaces of the two enclosures (No. 1 and No. 2). The third enclosure (No. 3) remained uncovered as control. Some air was introduced at the bottom of No. 1, and the air: water volume ratio throughout experiment was controlled at 3:1. During experiment, the average water temperature was 31 8C. Before sampling, each enclosure was aerated for 10 min to maintain water quality homogeneous. Pilot-scale experiment was conducted in a pond which supplied water for irrigation usage. Its effective water volume was about 560 m3. The water-body was in a serious eutrophic state in test period, and the algal bloom dominated by
X.-C. Chen et al. / Process Biochemistry 44 (2009) 357–360
358
Coelosphaeriu was observed. The average water temperature throughout the experiment was 22.9 8C. From the first day to the sixth day of experiment, two-layer shade sheet was covered and the light-shading ratio was 90%. After that, one more shade sheet was added to enhance the light-shading effect and the light-shading ratio rose to 98%. Water samples were taken from different depths, and thereafter mixed together to represent the average water quality. The Chla, pH, dissolved oxygen (DO), chemical oxygen demand (COD), and Dissolved phosphate (PO43–P) were monitored throughout the experiment using standard methods [9]. Microcystines (MCs) were detected by enzyme linked immune sorbent assay (ELISA) method as suggested in literatures [10,11].
3. Results 3.1. Reduction of algal biomass by light-shading in enclosure experiment Fig. 1 shows the reduction of algal biomass by light-shading. During the first 2 days, in both light-shading enclosures (No. 1 and No. 2), a rapid reduction in algal biomass was observed. In No. 2 enclosure, the Chla decreased from 196.1 mg/L to 125.7 mg/L, with a reduction rate of 35.3 mg/(L day), whereas in No. 1 enclosure, the Chla from 184.7 mg/L to 80.6 mg/L, with a much higher reduction rate of 52.1 mg/(L day). Thereafter, in both enclosures, the algal biomass continually decreased. On the sixth day, in No. 2 enclosure, the Chla was about 33.5 mg/L, whereas in No. 1 enclosure, the Chla reduced to 20.0 mg/L. On the eighth day, no significant decrease in algal biomass was observed in both enclosures. In contrast, the Chla in control enclosure (No. 3) increased from 89.7 mg/L to 138.6 mg/L on the first day, and then varied slightly during the next 8 days.
Fig. 2. Variation of DO and pH under light-shading condition. (&) DO, control; (&) DO, light-shading; (~) DO, light-shading plus aeration; (^) pH, control; (&) pH, light-shading; (&) pH, light-shading plus aeration.
3.2. DO and pH influenced by light-shading in enclosure experiment As shown in Fig. 2 under light-shading condition with the reduction of algal biomass, DO and pH were much lower than the values under normal condition. In No. 2 enclosure, on the first day DO declined significantly from 9.7 mg/L to as low as 0.3 mg/L, afterwards DO remained at such low level. In aeration enclosure (No. 1), on the first day DO declined from 9.2 mg/L to 2.1 mg/L, afterwards DO varied slightly. For pH, in enclosure without aeration (No. 2), it declined from 9.0 to 7.0, whereas in aeration enclosure (No. 1) pH decreased less, from 8.4 to 7.5. In contrast, in control enclosure (No. 3) DO and pH varied fluctuantly, and no significant decline was observed.
Fig. 3. Variation of MCs in light-shading water enclosure.
The variations of MCs, COD and PO43–P under light-shading condition are shown in Figs. 3–5. In no aeration enclosure (No. 2),
MCs, COD and PO43–P all increased on the first day, from 0.85 mg/ L to 1.8 mg/L, 44.8 mg/L to 47.3 mg/L and 0.50 mg/L to 0.79 mg/L, respectively. From the second day to the third day, a significant decrease was observed, with MCs decreased to 0.25 mg/L, COD to 26.9 mg/L, and PO43–P to 0.55 mg/L, respectively. From the third day to the eighth day, MCs and PO43–P increased slightly whereas COD continuously decreased. In aeration enclosure (No. 1), the variation trend of MCs was not significant difference compared to no aeration enclosure (No. 2) (T test, P > 0.1), and on the eighth day MCs was only 0.15 mg/L. During the experiment no increase of appeared, and on the eighth day, COD was 13.9 mg/L, reduced by 67.1% compared to initial COD. PO43–P decreased gradually until the fourth day, then increased slightly, and on the eighth day,
Fig. 1. Reduction of algal biomass in light-shading water enclosure.
Fig. 4. Variation of COD in light-shading water enclosure.
3.3. MCs, COD, and PO43–P influenced by light-shading in enclosure experiment
X.-C. Chen et al. / Process Biochemistry 44 (2009) 357–360
359
Table 2 Rate constants k calculated under different experiment conditions. No aeration
Enclosure experiment Pilot-scale experiment
Fig. 5. Variation of PO43–P in light-shading water enclosure.
PO43–P was about 0.36 mg/L. Furthermore, compared to that in no aeration enclosure (No. 2), all values of those three water quality parameters in aeration enclosure (No. 1) were lower during the experiment, which indicated that aeration had a positive effect on water quality under light-shading condition. 3.4. Pilot-scale experiment A pilot-scale experiment was conducted to investigate the effect of light-shading on water quality. Throughout pilot-scale experiment, aeration was not applied because the average water temperature was as low as 22.9 8C, and the algal biomass was much lower than that in enclosure experiment. The results indicated a removal of 81.0% was obtained by light-shading for 9 days, and the algal bloom was well controlled. Meanwhile, the water quality was partially improved, as shown in Table 1. The results showed that turbidity and COD were reduced obviously, with a removal efficiency of 73.1%, and 94.5%, respectively. The decrease of turbidity and COD might relate to the reduction of algal biomass, considering that Chla had a very close correlation with turbidity (R = 0.99, P < 0.001) and COD (R = 0.98, P < 0.001). It indicated that algae contributed most of COD and turbidity when water bloom occurred. It could be seen that TP remained almost constant whereas TN increased very slightly, therefore it could be concluded that although light-shading controlled algae effectively, it had little influence on nutrient concentration. Also, before and after light-shading, NH4+–N kept almost unchanged. For DO, a rapid decrease was also observed. At the beginning, DO was as high as 12 mg/L, which was in hyper-saturated state due to the algae photosynthesis. After light-shading for 9 days, the value stayed at 3.2 mg/L. This relatively high DO level suggested that aeration was not necessary. 4. Discussion Previous studies had reported that in batch test with optimum conditions, M. aeruginosa decreased markedly with the length of the darkness period and was reduced to only 1% of the initial cell number after 20 days of darkness [12]. Our study confirmed that light-shading could bring significant adverse effect on M.
Aeration
k (d1)
R2
k (day1)
R2
0.24 0.16
0.944 0.903
0.31
0.959
aeruginosa, causing the phenomenon noted as ‘‘light-limited growth’’. The reduction of algal biomass under light-shading condition might mainly relate to the dramatic decrease in photosynthetic rate of algae. Under light-shading condition, when incident light was lower than light compensation point, the photosynthesis of algae was inhibited, and the respiration was in dominance, forcing the algae to continuously consume its own components and oxygen in water, and then became extinct gradually [13]. Assuming that decrease rate constant was proportional to algal biomass, a dynamic model for the reduction process is presented as follows [14]: M ¼ M0kt The loss of algal biomass is fitted through a least square regression of algal density or Chla versus time to yield rate constants (k) by using a simple first-order equation, where the algal density or Chla at a point in time (M) is the product of its original values (M0) and the rate constant at that time. As shown in Table 2, regression coefficients obtained throughout enclose experiment and pilot-scale experiment were all above 0.9, indicating that this simple first-order equation could be used to describe the reduction process of algal biomass under lightshading condition. Furthermore, a significant DO depletion was observed after light-shading was implemented (T test, P < 0.01) compared to the value in control enclosure. It could be attributed to the respiration of algae and the decomposition of algal biomass. However, if DO in water declined to a low level, the reduction process might have been inhibited due to the insufficient oxygen supply for respiration. Previous studies illustrated that at the water-sediment interface, oxic condition would promote the algal decay processes [15]. Our experiment confirmed that oxygen had a positive effect on algal control, considering that when aeration was added, the k value was much higher (Table 2). In addition, enclosure experiment showed that PO43–P increased under light-shading condition, while when aeration was added, the k value decreased. In water column, PO43–P was significantly influenced by DO [16]. Under light-shading condition, the DO deceased to extremely low level (lower than 0.5 mg/L). Therefore, the sediment tended to release PO43–P, and PO43–P in water increased. On the other hand, when aeration was added, the DO was always above 2 mg/L. Therefore, the release of PO43–P was avoided. For pH, it is noted that with the photosynthesis of algae, the pH increases due to the photosynthesis consuming CO2 in water. Therefore, it could be inferred that the decrease of pH observed in our experiment was because the inhabitation of algae photosynthesis under lightlimited condition.
Table 1 Water quality of the test pond before and after light-shading. Parameter
Before light- shading After light-shading Removal efficiency
Chla (mg/L)
COD (mg/L)
Turbidity (NTU)
DO (mg/L)
TP (mg/L)
TN (mg/L)
NH4+–N (mg/L)
130.5 24.8 81%
30.9 1.7 94.5%
20.0 5.4 73.1%
12.0 3.2 73.3%
0.067 0.063 6.0%
1.2 1.4 –
0.05 0.05 –
360
X.-C. Chen et al. / Process Biochemistry 44 (2009) 357–360
Based on the limit-factor theory, controlling aquatic nutrients especially phosphorus was regarded as the most chief measure in eutrophication prevention [17,18]. However, some long-term researches and engineering practices discovered that, for those water sources with comparatively high background phosphorus concentration, the expenses of source control were extremely high, and usually the harmful algal bloom would not be controlled until ten years later or more [19,20]. On the other hand, the raw water quality should be ensured although the eutrophication problem was not yet solved [21,22]. Light was also a limiting factor related to algae growth, and the results of Kojima validated the applicability of algae control by ‘‘partial light-shading’’ [8]. However, this measure needed to be improved for the required shading area is too large. Considering this problem, we suggested building a light-shading zone around the water intake location, therefore by forcing the water to stay within that area for a certain period, the high pollution load caused by bloom in raw water could be significantly reduced. Regarding this new idea, the primary question was to determine the relationship between light-shading duration and algae distinction. Our experiment verified that algal biomass could be dramatically reduced after approximately 6–9 days of surface light-shading, and the Chla remained in raw water was only 20–30 mg/L, therefore it could be handled by the conventional water treatment process [1]. The results indicated the feasibility of this measure for practical application. However, it should be noted that light-shading could lead to DO depletion in raw water. According to our study, lightshading accompanied by aeration were suggested to solve this problem. 5. Conclusion Results of this study concluded that light-shading could be an attractive measure for raw water protection from harmful algal bloom. In enclosure experiment, algal biomass decreased under light-shading condition, meantime, the significant decrease of pH, COD and MCs was also observed. However, mainly due to the respiration of algae, DO declined dramatically. When light-shading was accompanied by aeration, the reduction process of algal biomass could be enhanced, and the water quality was improved compared to that under no aeration condition. In addition, a firstorder equation was used to describe the reduction process of algal biomass, and it was found that aeration could increase the rate constants k. In pilot-scale experiment, after 9 days of light-shading, the Chla in water declined from 130.5 mg/L to 24.8 mg/L, with turbidity and COD greatly improved, which confirmed that the measure was effective.
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