Removal of total phosphorus from municipal wastewater using cylindrical aluminum electrode in continuous mode - Tran Tuan Viet

Journal of Science and Technology 54 (4B) (2016) 88-93 REMOVAL OF TOTAL PHOSPHORUS FROM MUNICIPAL WASTEWATER USING CYLINDRICAL ALUMINUM ELECTRODE IN CONTINUOUS MODE Tran Tuan Viet1, *, Yoon Yong Soo2, Tran Minh Chi1 1Institute for Tropicalization and Environment (ITE), 57A Truong Quoc Dung St., Phu Nhuan Dist., Ho Chi Minh City, Viet Nam 2Lab of Advance Environmental Technology, Dept. of Chemical Engineering, Dankook University, 448-701, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do, South Korea *Email: viet.vittep@gmail.com Received: 15th August 2016; Accepted for publication: 10th November 2016 ABSTRACT This study aimed to apply electro-coagulation method using cylindrical Aluminum electrode in continuous mode to remove total phosphorus (T-P) in municipal wastewater. The effects of NaCl concentration (or conductivity) and hydraulic retention time (HRT, or flow rate) on T-P removal efficiency were investigated. To determine the efficiency of this system the ratio Al consumption/T-P removal was also considered. The results showed that, with raw sewage as influent, HRT = 1.17 min and added NaCl = 0.04 % was found the best conditions for the highest T-P removal efficiency (92.80 %; T-P concentration remainder was 0.17 mg/L) and the ratio Al consumption/T-P removal was 5.0. In general, this method achieves a high efficiency of removing phosphorus from wastewater and it can be used in sewage treatment systems. Keywords: total phosphorus removal, cylindrical aluminum electrode, wastewater treatment, electro-coagulation. 1. INTRODUCTION The most common problem of phosphorus compounds in ecosystem is eutrophication. In 1998, Lawrence and Jackson defined eutrophication being the enrichment of bodies of fresh water by inorganic plant nutrients, such as nitrate, phosphate. It may occur naturally and can also be the result of human activity and is particularly evident in slow-moving rivers and shallow lakes, etc. Increased sediment deposition can eventually raise the level of the lake or river bed, allowing land plants to colonize the edges, and eventually converting the area to dry land. Total phosphorous in municipal wastewaters may be less than 20 mg/l. The individual contribution tends to increase, because phosphorous is one of the main constituent of synthetic detergents. The usual forms of phosphorous found in aqueous solutions include orthophosphates and polyphosphates. The technologies for phosphorus removal started to develop in the 1950s. Removal was initially achieved by chemical precipitation which remains the leading technology Removal of total Phosphorus from municipal wastewater using cylindrical 89 today. Lately, biological phosphorus removal has become strongly established, crystallization technology has also completed its progress towards commercialization and technologies extending chemical precipitation to assist nutrient removal are beyond the pilot stage. The extensive use of chemical precipitation for phosphorus removal in wastewater treatment started in Switzerland during the 1950s, in response to the growing problem of eutrophication. This simple technology is now firmly established in many countries around the world. The development of biological phosphorus removal was based on research in the late 1950s, which found that, under certain conditions, activated sludge could take up phosphorus in considerable excess to that required for normal biomass growth. The development of crystallization technology started in the 1970s, in response to more stringent phosphorus removal requirements combined with the desire to produce a more marketable end-product [1]. In recent years, the electrocoagulation technology for phosphorus removal has been researched by many scientists around the world. Electrocoagulation is a process consisting of creating metallic hydroxide flocks within the wastewater by electro-dissolution of soluble anodes, usually made of iron or aluminum [2]. The way ions are delivered is the difference between electrocoagulation and chemical coagulation. In electrocoagulation, coagulation and precipitation are not conducted by delivering coagulants to the system but via electrodes in the reactor. The most common electrode materials for electrocoagulation are aluminum and iron. They are cheap, readily available, and proven effective [3]. In others studies, the authors did experiments with batches of water samples, which means water sample stays in immovable state. In this study, T-P from water samples was removed while they were still moving inside aluminum electrodes. This research intends to apply electro-coagulation method to reduce T-P in wastewater under 0.2 mg/L (Korean limited value) using Aluminum electrode in continuous mode. To determine the optimal condition of this system, electric energy consumption and the ratio Al consumption/T-P removal were also considered. 2. MATERIALS AND METHODS 2.1. Sampling methods Figure 1. Aluminum electrode dimensions. The electrodes were made by aluminum with the dimensions as shown in Fig. 1. The cathode is bigger than anode and located outside. Cathode and anode were attached with special base which was made by acrylic. That base is called distributed tank cause it stores the sample before they move in the electrodes. d1 = 95 mm; d2 = 48 mm; H1 = 441.6 mm; H2 = 500 mm; Cathode thickness = 3 mm; Anode thickness = 1 mm; L = 22.5 mm Tran Tuan Viet, Yoon Yong Soo, Tran Minh Chi 90 The HRT in the electrode was calculated: HRT = V/q (1) V = π H2 (r12 – r22) (2) where V is the volume of space between cathode and anode (L); q is flow rate (l/m). q (l/m) 1 2 3 4 5 HRT (min) 2.33 1.17 0.78 0.58 0.47 Figure 2. The electrocoagulation continuous system. The experiment system was setup as shown in Fig. 2. The flow followed the arrow and was controlled by flow meter (UNICELL-Canada and Dwyer-USA). Sample tank was 200 liters in volume. DC power supply is Model SC15-30A from Sun Chang Electric company with maximum current is 30 A. The distributed tank and electrodes can be separated easily when the system need to be cleaned. 2.2. Preparation of samples The synthetic wastewater was made by dissolving Potassium phosphate KH2PO4 (Duksan, 99.0 %) into tap water. The conductivity was controlled by adding Sodium chloride NaCl (Samchun, 99.0 %). The pH was adjusted to 7±1 with NaOH or HCl. The T-P concentration was controlled from 1 to 5 mg/L and NaCl was 0.03 % (see Table 1). Table 1. Experimental conditions for synthetic and real wastewater. Synthetic wastewater Real wastewater Potential (V) 10 10 pH 6-8 6-8 Initial concentration of T-P (mg/L) 1; 2; 3; 4; 5 Real condition Temperature Room temperature Room temperature Electrode Aluminum Aluminum NaCl (%) 0.03 0.00; 0.01; 0.02; 0.03; 0.04; 0.05 Flow rate (l/min) 1; 2; 3; 4; 5 1; 2; 3; 4; 5 The real wastewater in this study was municipal raw wastewater from wastewater treatment plant locate at Suji-gu, Yongin-si, South Korea. Its characteristics are shown in Table 2. Removal of total Phosphorus from municipal wastewater using cylindrical 91 According these data, T-P concentration was in range 2.34 ± 1 mg/L. The percentage of added NaCl was adjusted in range 0.00 to 0.05 % with other conditions (Table 1). Table 2. Characteristics of raw wastewater in Suji-gu, Yongin-si, South Korea [4]. Parameters Units Concentration pH - 7.72 ± 0.23 SS mg/L 282.89 ± 170 BOD5 mg/L 99.11 ± 33 CODMn mg/L 41.85 ± 15 T-N mg/L 26.56 ± 8 T-P mg/L 2.34 ± 1 2.3. Analytical method The mechanism of generating ions by electrocoagulation in case of Al electrode is shown below: Anode: Al  Al(aq)3+ + 3e- (1) Cathode: 3H2O+ + 3e-  3/2H2(g) + 3OH- (2) Overall: Al3+ + 3H2O  Al(OH)3 + 3H+ (3) When phosphorous in form of PO43- in solution: Al3+ + PO43-  AlPO4↓ (4) In the reactions (1 to 3), electrocoagulation is combination of oxidation, flocculation and flotation. Electrocoagulation has occurred in three steps. In the first step, coagulation has formed because of oxidation of anode. In the second step, pollutants have destabilized. In the last step, destabilized matters have united [5]. The pH, temperature and conductivity were measured by pH meter (OAKTON, Singapore) and conductivity meter (ORION model 130, USA). Total suspended solids (TSS) were determined following Standard method 2540D with 0.45 μm glass fiber filter (47 mm Φ, Whatman GF/C) and dry oven (Daesung, Korea). T-P was analyzed according HUMAS method (Program 3000) using Spectrometer (HUMAS, Think En HS 3300, UV-type, Korea). The amount of aluminum released into solution by electrolytic oxidation of anode was estimated from Faraday’s law: ݓ ൌ ூ௧ெಲ೗௓ி (5) where w is the aluminum dissolved (g); I is the current (A); t is the electrocoagulation time (s); MAl is the molecular weight of Aluminum (26.98 g/mol); Z is the number of electrons involved in the redox reaction; F is the Faraday’s constant (96,500 C per mole of electrons). 3. RESULTS AND DISCUSSION 3.1. Synthetic wastewater The first experiment was done with initial T-P concentration from 1 to 5 mg/L and 0.03 % of NaCl. According Fig.3a, in all HRT, with initial T-P concentration was 1 mg/L, all the results met the limit 0.2 mg/L (Korean limit value for T-P). With the same conditions, the increase of Tran Tuan Viet, Yoon Yong Soo, Tran Minh Chi 92 initial T-P concentrations made the increase of T-P remainder. When HRT was 2.33 min, all the samples were less than T-P standard 0.2 mg/L (Fig. 3a). (a) (b) Figure 3. (a) T-P concentration remainders and (b) T-P removal efficiency after various HRT in different initial T-P concentrations; Synthetic wastewater – NaCl: 0.03 % and T-P: 1 to 5 mg/l. Figure 3b showed T-P removal efficiency after various HRT in different initial T-P concentrations from 1 to 5 mg/L in case of synthetic wastewater with 0.03 % NaCl. Over 90 % T-P were removed after 2.33 min HRT in all samples. The T-P removal efficiencies were high in case of small initial T-P concentration. Over 90 % of T-P was removed in all samples when HRT was 2.33 minutes. However, the Al consumption was very high, around 10 times higher than the amount of T-P removal. In case of HRT was 1.17 minutes, the ratio Al consumption/T- P removal were around 3. On the other hand, it took over 10 minutes to reach at least 90% of the phosphorus removal from the solution content 10 mg/L [6] or 50 mg/L [7] phosphorus using aluminum plate electrodes. 3.2. Real wastewater In real wastewater experiment, the initial T-P concentrations in raw wastewater were smaller than 2.5 mg/L; SS were around 165 mg/L; and EC were approximate 550 µS/cm. Without adding NaCl, maximum T-P removal efficiency was only 86.98 %. With added NaCl from 0.01 % to 0.05 % almost T-P removal efficiencies over 90 % at HRT greater than 1.17 min (only 1 sample with T-P removal efficiency was 88.31 % with 0.02 % added NaCl at HRT = 1.17 min). In case of flow rate was 5 l/m (or HRT was 0.47 min), all samples had T-P removal efficiencies under 50 %. Figure 4. T-P removal efficiency after various HRT in different concentrations of added NaCl – Raw wastewater. Figure 5. T-P removal efficiency and electric energy consumption in various concentrations of added NaCl; Raw wastewater; HRT = 1.17 min or Flow rate = 2 l/min. HRT = 1.17 min and added NaCl = 0.04 % was the best condition for the best result, T-P removal efficiency was 92.80 % (T-P concentration remainder was 0.17 mg/L), electric energy consumption was 0.2833 kWh/m3, aluminum consumption was 8.21 × 10-4 mol (with the ratio Al consumption/T-P removal was 5.0) (Fig. 4 and 5). Removal of total Phosphorus from municipal wastewater using cylindrical 93 In this experiment, TSS concentration was considered because it affected on T-P efficiency. TSS in raw wastewater was very higher than them in synthetic wastewater so that T-P efficiency in first experiment was higher. But the ability to separate in 2 phases of samples was high when HRT increased in raw wastewater experiment. This ability is very important in wastewater treatment project; it could decrease the treating cost and increase the treating efficiency. 4. CONCLUSIONS This study showed that the applying of Aluminum electrode in continuous mode to remove T-P from municipal waste water is very efficient. According to this results, the samples with low TSS concentrations were removed T-P more efficient than other. With high TSS concentrations, the ability to separate into 2 phases of samples was high when HRT increased. In case of low TSS, 0.01 % NaCl is recommend to add to sample. The HRT could be controlled by changing flow rate in any time. In case of high TSS, 0.04 % of NaCl was strong recommended to remove T-P until reached the limit 0.2 mg/L. With raw wastewater (T-P initial = 2.36 mg/L), HRT up to 1.17 min and added NaCl = 0.04 % was the optimal condition for the best result, T-P removal efficiency was 92.80 % (T-P concentration remainder was 0.17 mg/L); Aluminum consumption was 8.21 × 10-4 mol; T-P removal was 1.65 × 104 mol (with the ratio Al consumption/T-P removal was 5.0). This system can install in wastewater treatment plant to reduce T-P. However, a study about the optimal position to set up it is still required. Acknowledgements. We would like to thank Dankook University for hospitality and other support for this study. We also thank ITE staffs for giving us advices and general support. REFERENCES 1. Morse G.K., Brett S.W., Guy J.A., Lester J.N. - Review: Phosphorus removal and recovery technologies, Sci. Total Environ., 212 (1998) 69-81. 2. Dermentzis K., Valsamidou E. and Lazaridou A. - Nickel removal from wastewater by electrocoagulation with aluminum electrodes, Jo. Eng. Sci. Technol. Re. 4 (2) (2011) 188- 192. 3. Xueming Chen, Guohua Chen and Po Lock Yue - Separation of pollutants from restaurant wastewater by electrocoagulation, Sep. Purif. Technol. 19 (2000) 65–76. 4. Nguyen Dinh Duc - Applied research of flat-plate submerged membrane bioreactor in real municipal wastewater treatment with pilot large-scale 50m3/day, Dissertation, Dankook University (2010). 5. Sahset Irdemez, Nuhi Demircioglu, Yalcin Sevki Yıldız, Zuleyha Bingul - The effects of current density and phosphate concentration on phosphate removal from wastewater by electrocoagulation using aluminum and iron plate electrodes, Sep. Purif. Technol. 52 (2006) 218-223. 6. Nihal Bektas, Hilal Akbulut, Hatice Inan and Anatoly Dimoglo – Removal of phosphate from aqueous solutions by electro-coagulation, Journal of Hazardous materials 106B (2004) 101-105. 7. Sahset Irdemez, Yalcin Sevki Yıldız and Vahdettin Tocunoglu – Optimization of phosphate removal from wastewater by electrocoagulation with aluminum plate electrodes, Sep. Purif. Technol. 52 (2006) 394-401.