Gerais, Belo Horizonte, Minas Gerais, Brazil. Topic: Utilities Asset Management Sub-topic: Water Treatment Technologies

Transcription

1 Assessing the coagulation effectiveness of high eutrophic water sources in batch scale Marcelo Libânio 1, Sílvia M. Gandolfi, Débora Moraes and Alaine Guerra Corresponding Author: Marcelo Libânio 1 1 Department of Hydraulic Engineering and Water Resources, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil. Topic: Utilities Asset Management Sub-topic: Water Treatment Technologies Keywords: Eutrophic waters, cyanobacteria, coagulation, water treatment. Introduction and Relevance Frequently the remarkable presence of algae and the cyanobacteria (blue-green algae) arises as one of most important consequences of the diffuse or punctual pollution related to several human activities. Usually in developing regions, mainly in low income countries, urban lakes have been suffering the increase of pollution as consequence of the discharge of wastewaters and storm waters. In this context, Pampulha Lake ( S and W) mirrors a typical situation of urban lakes located in populous cities. The high eutrophication level translates the water quality degradation limiting its use in several ways. This artificial lake was constructed in the mid 1940s and its proposal as one of the water sources for Belo Horizonte (Minas Gerais state capital) lasted until the end of the 1980s, when the Pampulha water treatment plant (average flow rate about 13,000 m 3.day -1 ) was inactivated due to the remarkable silting up of the lake and the high concentration (density) of cyanobacteria. Cyanobacteria became a relevant water quality parameter mainly due to the history of several intoxications caused by cianotoxins and, particularly in Brazil, to the fatal event in Caruaru (1996), when sixty chronic renal patients of a hemodialysis clinic died as a consequence of microcystin presence in the water. In such way, cyanobacteria were included in the Brazilian Drinking-water Standards under the Regulation 518 (Healthy Brazil, 2004). The same norms were established by World Health Organisation on this topic some months later and were kept in the recent edition (WHO, 2011). The conventional water treatment method based on chemical coagulation is usually inefficient for cyanotoxins removal probably because of the inefficacy of the coagulants in the destabilization and the precipitation of these organic compounds, hampering their removal in the subsequent steps of the treatment (sedimentation or dissolved air flotation). In this context, cyanobacteria removal opens two different perspectives in terms of the conventional water treatment technology. On a first point of view, the usual alternative converges on the prior oxidation (chlorine, ozone, hydrogen peroxide, potassium permanganate) and the subsequent adsorption by activated carbon (powdered or granular). On the other hand, the intact cells removal has been prioritized aiming to avoid the cell wall rupture. Based on the last premise, a research was carried out using eutrophic waters from Taihu Lake located in one of the most Page 1 of 5

2 developed regions in China. This study evaluated distinct rates between the primary coagulant (aluminium sulfate with 15.8% of Al2O3) and a synthetic cationic polymer as coagulant aid. The tests were performed with low flocculation time (2.0 min) and high sedimentation time (30 min). The best results indicated residual turbidity lower than 2.0 ntu (raw water of ntu) and 1.0 log removal for an average cyanobacteria concentration of 10 4 cells.ml -1. The most relevant conclusion of this study pointed out the reduction of prior chlorination (and even occasional suppression) in the water treatment plant (Zhao et al., 2010). In similar premise, the present research was focused on the intact cyanobacteria cells removal in batch scale, using different coagulant aids and jar-test parameters, as described in Methods. Objective Based on the above concepts, the main objective of this research was to evaluate the efficiency of aluminum sulfate (alum), polialuminum chlorite (PAC) and a natural polymer, with or without coagulant aids, on cyanobacteria removal in eutrophic waters. Methods Experimental apparatus The program-controlled jar-test equipment, shown in Figure 1, was able to allow simultaneous dispersion of the coagulants (primary coagulant and polymer), simultaneous sampling of settled water and to provide velocity gradients from 10 to 2,000 s -1. The settled water samples were collected 7 cm below the surface. Fig. 1. The jar-test equipment used in the research. Raw water sampling The raw water samples were always collected 20 cm below the surface in the border of Pampulha Lake lake during the spring and the summer (from October 2010 to April 2011). Before each jar-test, 200-mL volume of the sample was separated for determination of water quality parameters, such as temperature, ph, apparent color, true color (as a predictor of natural organic matter) and turbidity. The same volume of raw water was diluted in distilled water, from 5% to 100% (raw water sample), to evaluate Chlorophyll-a concentration as a predictor of the concentration of cyanobacteria. The proposal was to identify a correlation among Chlorophyll-a concentration and those water quality parameters easier to monitor during jar-tests or in the field. Despite some limitation as a predictor of cyanobacteria (Kasprzak et al., 2008), Chlorophyll-a concentration has been used in monitor program of water quality in lakes and reservoirs as an indicator of primary productivity. Page 2 of 5 Experimental tests The 83 jar-tests were conducted using the most widely used coagulant (alum) with

3 14% of Al 2 O 3, polialuminum chlorite (PAC) with 11% of Al 2 O 3 and a natural polymer as primary coagulants. In addiction, the best results with these previous coagulants were repeated with a synthetic cationic or nonionic high-molecular-weight polymer as coagulant aids. Based on the usual design parameters of conventional water treatment plants established by technical literature (ASCE, 2005) and the Brazilian Technical Standards Association (ABNT, 1992), the tests were carried out with rapid mix velocity gradient and rapid mix detention time of 600 s -1 e 15 s, respectively. For flocculation, the adopted detention time and velocity gradient were, respectively, 20 min and 600 s -1. The settled water was collected after 5 min of quiescent sedimentation, resulting in settling velocity approximately of 1.4 cm.min -1. This settling velocity is in accord with the typical sedimentation surface load rate for rectangular sedimentation basins dealing with high eutrophic waters and using alum as primary coagulant (20.2 m 3.m 2.day -1 ). Results and Discussion Raw water characteristics The 16 water samplings and 83 jar-tests produced approximately 500 results and the main characteristics and their descriptive statistics of raw water are shown in Table 1. Table 1. Descriptive statistics of raw water characteristics. Water Quality Parameter Mean Coefficient of Variation (%) Median ph Temperature Apparent color True color Turbidity These raw water characteristics showed the same pattern compared with a previous study, carried out in October 2009 which presented cyanobacteria concentration from 10 4 to 10 5 cells.ml -1, with remarkable prevalence of the species C. raciborskii or S. brasiliense (Campos, 2010). Lower variations were detected for ph, temperature (as the coefficients of variation depict), hardness and total alkalinity, whose values were always from 100 to 110 mg.l -1 CaCO3. All analyses, including Chlorophyll-a concentration, were carried out based on the premises established by APHA (2005). For identifying the mentioned identification of the correlation among Chlorophyll-a and some water quality parameters, 22 Chlorophyll-a concentrations were determined (maximum concentration for raw water was µg.l -1 ). As expected, high coefficients of determination for turbidity (R 2 = 0.845), apparent color (R 2 = 0.795) and true color (R 2 = 0.825) were obtained, confirming the suitable use of these parameters for evaluation of the jar-tests results. Jar-test results First of all, the optimum coagulation conditions were determined applying just the mentioned primary coagulants focusing on the removal of turbidity, apparent color and true color. The best results of the first phase of this research were synthesized in Table 2. Table 2. Optimum conditions of coagulation without coagulant aid. Alum (Al + ) PAC (Al + ) Natural polymer Optimum dosage (mg.l -1 ) Coagulation ph Turbidity removal (%) Apparent color removal (%) Page 3 of 5

4 Despite the significant turbidity removal, the unusual application of the natural polymer as primary coagulant seemed unfeasible due to the persistent residual true color, probably as a consequence of the black acacia that is the main constituent of this polymer. After the definition of optimum coagulation conditions depicted in Table 2, dosages from 0.4 to 2.4 mg.l -1 of synthetic cationic and synthetic nonionic polymers as coagulant aids were tested. For all primary coagulants the removal was increased with distinct polymers as coagulant aids and the best results were obtained with PAC as primary coagulant. The results with cationic polymer as a coagulant aid for the optimum coagulation conditions with PAC are shown by Figure 2. Removal (%) Turbidity Log. (Apparent color) 30 0,0 0,4 0,8 1,2 1,6 2,0 2,4 Apparent color Log. (Turbidity) R 2 = 0,9791 R 2 = 0,9669 Dosages of cationic polymer (mg/l) Fig. 2. Results of optimum dosage of PAC (2,56 mgal +3.L -1 ) with cationic polymer as coagulant aid. The highest removal (approximately 86% for both parameters) was obtained for dosage of 2.4 mg.l -1 of cationic polymer. All tests showed a logarithmic rate between turbidity removal and polymer dosage. Modest results were obtained with natural polymer as coagulant aid and optimum coagulation conditions for PAC as primary coagulant. With nonionic polymer as coagulant aid, the more expressive results were depicted in Figure 3, keeping the same mentioned logarithmic rate. Removal (%) 90,00 75,00 60,00 45,00 Turbidity Log. (Turbidity) Apparent color Linear (Apparent color) R 2 = 0,8546 R 2 = 0, ,00 0,0 0,5 1,0 1,5 2,0 2,5 Dosage of nonionic polymer (mg/l) Fig. 3. Results of optimum dosage of PAC (2,56 mgal +3.L -1 ) with nonionic polymer as coagulant aid. Based on the best results obtained for PAC (dosage of 40 mg.l -1 ) and cationic polymer as coagulant aid (dosages of 2.2 and 2.4 mg.l -1 ), that indicated residual turbidity of 3.4 ntu and residual apparent color of 9 Hu, it is possible to infer cyanobacteria removal higher than 90%. Conclusions The results of 83 jar tests allow to conclude: i) For the adopted settling velocity it is possible to confirm the unfeasibility of the evaluated primary coagulants without coagulant aid for eutrophic waters. This performance becomes more evident when C. racinborskii is the prevalent cyanobacteria species. These species present specific air vesicles that confer buoyancy and provide lower density to the flocs; ii) It is important to point out the best results with PAC as primary coagulant for practically the same dosage (in terms of Al +3 ) in comparison with alum. These results Page 4 of 5

5 indicated the tendency relating to the superiority of this coagulant for eutrophic waters, minimizing the aluminum concentration in effluent of the plant. In a similar context, high dosages of natural polymer as primary coagulant provided good results in terms of turbidity removal but favored a significant increase of true color; iii) The cationic polymer presented the best results as coagulant aid in practically all tests, probably due to the negative electrophoretic mobility of algae and cyanobacteria suspensions that usually increases with ph increasing; iv) Finally, despite the recognized limitation of Chlorophyll-a as a predictor, it is possible to estimate cyanobacteria removal as higher than 90% in optimum coagulation conditions with PAC and cationic polymer as coagulant aid. This fact assumes more relevance due to the significant higher settling velocity utilized in these jar tests in comparison with similar researches, such as the mentioned study conducted with raw water from Taihu Lake in China. In such way, this assertive opens the perspective of Pampulha lake to becomes again one of the water sources of the city in future. Pampulha Lake, Master Thesis, Federal University of Minas Gerais, 101 p. Health Brazil (2004) - Brazilian Drinkingwater Standards, Regulation 518, 15 p. Kasprzak, P. et al. (2008) - Chlorophyll a concentration across a trophic gradient lakes: An estimator of phytoplankton biomass?. Limnologica, 38(1), World Health Organisation - WHO (2011) - Chemical fact sheets In: Guidelines for Drinking-water Quality, Geneva, p Zhao, X. et al. (2010) Algae removal efficiencies of AS/PDMDAAC coagulants, Journal American Water Works Association, 102(7), Acknowledgments This research was supported by CNPq (proc /2009-9) and Capes (Procad 218/2007). References ABNT - Brazilian Technical Standards Association (1992) - Water treatment plants design, NBR 12216, Rio de Janeiro, 19 p. (in Portuguese). ASCE American Society of Civil Engineers (2005) Clarification in: Water Treatment Plant Design, 4 th Ed., McGraw- Hill, Inc, 912 p. APHA - American Public Health Association (2005) - Standard Methods for the Examination of Water and Wastewater, 21 st Edition, Washington DC. Campos, M. O. (2010) Intervening factors on spatial distribution of phytoplankton in Page 5 of 5