State of the art in the research area
The disinfection with chlorine-based compounds, is the most common treatment for the disinfection of drinking water. However, chlorinated hydrocarbons (such as trihalomethanes, chloroform, dichloromethane) and nitrosamines, which may be highly cancerous, are produced as by-products in the reaction of chlorine and organic compounds. The accumulation of these by-products, along with the accumulation of heavy metals in human body, increases the risk of cancer and cancer related diseases (Do et al., 2005; Mishra et al., 2014; Villanueva et al., 2015). The emissions from anthropogenic sources, especially industry, contribute to the presence of heavy metals in the environment. Their presence in human body is associated with the acute toxic and cancerous effects (Jergović, 2011). On the other hand, Escherichia coli (E.coli) is one of the main types of bacteria that lives in the lower digestive tract of the mammals, and it is necessary for the proper food digestion (Kuna, 2013). Normally, it is not pathogenic, but if it comes from the intestine to some other organs and tissues, it can cause infections (Ademović, 2010), and therefore, should not be present in drinking water.
According to the present research, some treatments, such as the advanced oxidation methods and electrochemical methods, show the possibility for efficient removal of different contaminates in water purification process. Advanced oxidation processes are increasingly considered as highly competitive technology for the water purification and for the removal of high chemical stability contaminants because they do not demand the addition of other substances (Al Mayyahi et al., 2018; Naddeo et al., 2014). Electrocoagulation (EC) method includes coagulation production by the use of electricity and sacrificing electrodes to extract, aggregate and deposit pollutants (suspended and dissolved solids) from water. In the EC reactor, the wastewater flows between electrodes while the direct current is applied to them. The choice of the electrode material and their arrangement depends on the type of the present contaminants and the required quality of treated water. Usually, aluminium (Al) is used for the drinking water treatment and iron (Fe) for the wastewater treatment. Aluminium and iron are relatively cheap and, compared to the other metals, such as titan and silver, easily available, non-toxic and proven effective (Gardić, 2007; Chen and Hung, 2007; Shammas et al., 2010; Kuokkanen et al., 2013; Hakizimana, 2017). Some of the advantages of the EC process are: effluent contains less total dissolved solids compared to the other chemical processes; easy maintenance of the device; more efficient and faster degradation of organic matter and larger and more stable flocs are formed than those produced by chemical coagulation; it is not necessary to control the pH of the water; no chemicals are required; generated sludge has better quality and smaller volume (50 – 70 % compared to chemical coagulation); can simultaneously remove several different types of pollutants; side reactions, such as generation of hydrogen or hypochlorite, can be useful for the disinfection; it can be used as decentralized system (Vepsäläinen, 2012; Kuokkanen, 2013; Marriaga-Cabarales and Machuca-Martínez, 2014; Hakizimana et al., 2017). However, some of the EC disadvantages are: the electricity costs may be significant; possible passivation of anode due to the oxygen presence and the deposition on the cathodes; the electrodes need to be regularly replaced; the high conductivity of the wastewater is required; depending on the electrode material, high ion concentrations need to be removed from the water; in some cases, the gelatinous hydroxides may be dissolved in water; it is not effective for the removal of the soluble substances such as ammonia, sugar, organic acids, solvents, phenols, alcohol and similar (Vepsäläinen, 2012, Kuokkanen et al., 2013, Marriaga-Cabarales and Machuca-Martínez, 2014, Hakizimana et al., 2017).
Different authors investigated the possibility of the removal of the microbiological contaminants and heavy metals by electrocoagulation. Ghernaout et al. (2008) used EC as electro disinfection of solution contaminated with non-pathogenic E. coli. Aluminium, iron and stainless steel electrodes are used in this research, and it showed that aluminium electrodes are the most efficient. Gosh et al. (2007) investigated the effect of EC on the removal of different iron concertation from drinking water. Aluminium electrodes were used in the research. It is noticed that the increase of the current density increases the removal efficiency, while the increase of the inter-electrode distance decreases the removal efficiency. Also, a thin film of metal hydroxides on the anodes, which generates the additional electrical resistance, increases by the increase of inter-electrode distance and thus removing contaminates more slowly. Vasuvedan et al. (2009) investigated the iron removal efficiency from the drinking water with magnesium anodes and iron cathodes. The maximal removal efficiency of 98.4 % was obtained after 60 min at the 0.06 A/dm2 of current density and pH 6. Wan et al. (2011) investigated the effect of different process parameters on the arsenic removal with iron electrodes. Ultimately, the optimal values resulted in 99.9 % removal of arsenic, with the removal process being slower at higher pH. Can et al. (2014) have also investigated the influence of the process parameters on the arsenic removal with iron electrodes. It is noticed that the initial pH very much affects the arsenic removal efficiency, and the highest efficiency of 99.5 % was obtained at pH 4. Also, the removal efficiency increases with the increase of the current density and decreases with the increase of the initial arsenic concentration. The effect of the mixing speed was also investigated, and the highest removal efficiency was obtained at 150 r/min. Mixing speeds higher than 150 r/min, prevented the generation of the iron hydroxides, and at the lower speeds, the solution was not homogenous. Flores et al. (2014) removed arsenic from the groundwater samples. They achieved 92.2 % of arsenic removal with aluminium electrodes, at current density of 6 mA/cm2, with the addition of 1 mg/l of hypochlorite. Ricordel et al. (2014) investigated the removal of E. coli, and the removal efficiency was 99.8 % and 97 %. Oreščanin et al. (2014) achieved complete removal of arsenic, phosphates, colour, turbidity, suspended solids and ammonia from the groundwater samples by the combination of electrochemical treatment with iron and aluminium electrodes at current of 6 A and ozonation. Gatsios et al. (2015) used cylindrical iron and aluminium electrodes. It was noticed that the decrease of the initial pH and the increase of the inter-electrode distance, negatively affects the removal efficiency and energy consumption. Optimal results were obtained at the initial current of 2 A and pH 6, and after 90 min of the treatment, the manganese removal efficiency was 89.6 %. Alferness et al. (2016) used steel electrodes for the removal of different arsenic concentrations from the water. It was noticed that the removal depends on the initial arsenic concentration and the current density. Hashim et al. (2017) used six perforated electrodes, and after 20 min, at pH 6, electrode distance of 5 mm and current density of 1.5 mA/cm2, achieved 98.5 % of iron removal efficiency. Xu et al. (2017) investigated the influence of the process parameters and the addition of the electrolytes on the heavy metal removal (cadmium, zinc and manganese) with aluminium and iron electrodes. It was noticed that the efficiency increases with the increase of the water pH and with the increase of the current density. Also, the anode material has a dominant effect on the heavy metals removal efficiency and the combination of Fe-Fe electrodes is more efficient than the other combinations (Al-Al, Al-Fe and Fe-Al). Same authors (Xu et al., 2018), in order to determine the optimal process conditions, investigated the influence of different power supply, current density, aeration intensity, flow and anions (SO42- and SO32-). It was concluded that the direct current gives better results than the alternating, and the increase of the current density from 10 mA/cm2 to 20 mA/cm2 increases the manganese removal efficiency from 34 % to 94.11 %.
The ultrasonic removal of contaminant from water has three basic, practically simultaneous mechanisms. Chemical mechanism is based on the chemical reactions between the generated free OH radicals and the contaminants present in water. Thermodynamical mechanism which, by implosion of gas (air) bubbles, developed in the low pressure phase, results in very high pressure (a few hundred [bars]) and temperature (a few thousand [°C]) thus creating the conditions that cause cellular decomposition of microorganisms. In the same process, free radicals are generated. These radicals cause fast oxidation of organic and inorganic matter, and the degradation of complex compounds in water (solution) (Ozturk et al., 2015). The third mechanism refers to hydromechanic action (it manifests as a sudden multidirectional local movement of the water) which results in strong local sheer stress and by breaking the bond between the molecules which is also caused by the implosion of the air bubbles (Doosti et al., 2012). The earliest oxidation effect by application of the ultrasound in water, was recorded in 1929 (Schmitt et al., 1929). Subsequently, the formation of the free OH radicals was postulated, and the procedure was investigated for the removal of organic matter from the water (Lur’e et al., 1962; Spurlock and Reifsneider, 1971; Witekowa, 1972). The main removal mechanisms were described by Henglein (1987). Subsequently, different authors investigated the effects of power and ultrasound frequency at the contaminants removal efficiency. However, there is a lack of methodological comparative research and the research on the joint action of electrochemical processes and ultrasound. Experimental results have also shown that this technique effectively decreases the water hardness (Hiratsuka et al., 2013; Vikulina et al., 2018). Gogate et al. (2004) have shown study result about the frequency ranges which are usually used at ultrasonic applications (20 – 200 kHz). It was concluded that the bubble collapse pressure increases with the increase of the frequency. It was also found that the optimum frequency for contaminant removal depends on a contaminant type and geometry of the system (Gogate et al., 2002). D’Agostino and Brennen (1983) have discovered that the number of the bubbles is proportional with the square of the frequency. Therefore, more bubbles are created at the higher frequencies, which is the result of more positive and negative pressure phase cycles (Fuchs et al., 2005). According to some authors, the high frequency will reduce cavitation effect because the phase of the negative pressure, due to the fast titration and fast phase change of the high and negative pressure, is not long enough to start the cavitation (Matei et al., 2005). For example, it has been shown that the high turbidity removal efficiency can be achieved at frequency range from 27 kHz to 40 kHz and power from 40 W to 60 W (Mutiarani et al., 2009; Liang et al., 2009; Stefan and Balan, 2011). The removal efficiency of total suspended solids at different operative conditions was investigated by Wang et al. (2008). It was determined that the highest efficiency is achieved after 30 minutes. The efficiency of the microbiological contamination removal was observed in different papers that investigated the effect of the ultrasound and the combination of other advanced oxidation processes. Mahvi and Dehghani, for the frequency of 42 kHz, determined that the percentage of algae removal from water depends on the treatment time, with the removal possibility of 100 % after 150 sec (Mahvi and Dehghani, 2005.). Other research have shown the cyanobacteria removal efficiency for the frequency range of 20 kHz to 1.7 MHz (Tang et al., 2003; Hao et al., 2004). Lower green algae removal efficiency was presented by D. Purcell (2009). The efficiency with corresponding treatment time for microcystin removal at 20 kHz was shown by Qiu et al. (2011). Also, other authors have investigated the removal efficiency of microbiological contamination for different operative parameters in single procedure and in co-operation with other processes (Bozhi et al., 2005; Zhang et al., 2009).
The application of ultrasound for the disinfection of drinking water was also investigated by some authors (Gomez-Lopez et al., 2009; Toor et al., 2007). The increased effect of ozone disinfection and hydrogen peroxide with simultaneous use of the ultrasound for 30 – 50 %, was established by Jyoti et al. (2003). Ayyildiz et al. (2011) obtained similar results but for the chlorine dioxide disinfection and with lower efficiency increase. The removal of Legionella after 30 min of treatment with ultrasound was 18 %, and the efficiency increases at 97 % with the addition of 1 g of titan dioxide (Shimizua et al., 2010). Also, the inactivation of E. coli was investigated by different authors with different frequencies and power. Generally, higher efficiency is achieved with higher energy consumption, and more bacteria demanded longer treatment time (Hulmans et al.). Other researches showed that the effect of ultrasonic degradation of organic contamination is not always improved with the increase of the ultrasonic frequency, i.e. there is optimal frequency area for the contaminant removal. Also, with the increase of the intensity, i.e. ultrasonic power, which fits the amplitude of the ultrasonic wave, cavitation bubbles collapse more violently, which develops higher temperature and higher pressure. However, the degradation of organic matter is also decreased if the intensity is higher than the optimal value (Wang et al., 2019). Wang (Wang et al., 2019) have shown that the combination of electrochemical reactions and ultrasound accelerates the complete aniline removal by four times compared to the single electrochemical treatment. These processes in the water treatment and in the water conditioning are intensively investigated in last 30 years, where the operative cost parameters, field research on pilot devices and case studies with the process optimization, as well as the effect on the increase of disinfection degree in regard to specific water composition, are missing. Most of the present research was conducted in laboratories on very small batch devices (reactors), where process parameters and obtained results (treatment efficiency and operative cost parameters), due to the different methodological approach of laboratory modelling (different settings and reactor sizes in combination with certain synthesized pollutants), are not directly comparable, nor applicable in real conditions, i.e. under the conditions of conditioning larger amounts of water. The huge disadvantage of the present research is related to the lack of the investigations of the combination of both processes, possibilities of developing a continuous process with increased purification capacity, as well as the lack of field investigations and case studies with the purification optimization (Wang et al., 2019).
References:
Adamovic, A. (2010) Preživljavanje bakterija Escherichia Coli i Acinetobacter Junii pri različitim koncentracijama natrijevog klorida, Sveučilište u Zagrebu, Prirodoslovno-matematički fakultet, Biološki odjel, diplomski rad.
Al Mayyahi, A., Ali Abed Al-Asadi, H. (2018) Advanced Oxidation Processes (AOPs) for Wastewater Treatment and Reuse: A Brief Review, Asian Journal of Applied Science and Technology, Volume 2, Issue 3, 18-30
Alferness, M.K., Casares, A.R., Chiesa, S.C. (2016) Evaluation of a Point-of-Use Electrocoagulation System for Arsenic Removal, International Journal for Service Learning in Engineering, Humanitarian Engineering and Social Entrepreneurship, vol. 11, br. 1, str. 51-66
Ayyildiz O, Sanik S, Ileri B. (2011). Effect of ultrasonic pretreatment on chlorine dioxide disinfection efficiency. Ultrasonics Sonochemistry, 18: 683-688
Bozhi, M., Yifang, C., Hongwei, H. (2005). Influence of ultrasonic field on microcystins produced by bloom-forming algae. Colloids and Surfaces B: Biointerfaces, 41: 197-201
Can B.Z., Boncukcuoğlu R., Yilmaz, A.E., Fil, B.A. (2014) Effect of some operational parameters on the Arsenic removal by electrocoagulation using iron electrodes, Journal of Environmental Health Science and Engineering, vol. 12, br. 95, str. 1-10
Chen, G., Hung, Y. (2007) Electrochemical Wastewater Treatment Process, Advanced Physicochemical Treatment Technologies, str. 57-106, DOI: 10.1007/978-1-59745-173-4_2
D’Agostino, L., Brennen, C. E (1983). On the acoustical dynamics of bubble cloud, in: American Society of Mechanical Engineers, New York: Fluids Engineering Division 2, 72–75.
Do, M. T., Birkett, N. J., Johnson, K. C., Krewski, D., Villeneuve, P., & Canadian Cancer Registries Epidemiology Research Group (2005). Chlorination disinfection by-products and pancreatic cancer risk. Environmental health perspectives, 113(4), 418–424. doi:10.1289/ehp.7403
Doosti, M.R., Kargar, R., Sayadi, M.H (2012) Water treatment using ultrasonic assistance: A review, Proceedings of the International Academy of Ecology and Environmental Sciences, 2(2):96-110
Flores, O.J., Nava, J.L., Carreno, G. (2014) Arsenic Removal from Groundwater by Electrocoagulation Process in a Filter-Press-Type FM01-LC Reactor, International Journal of Electrochemical Science, vol. 9, br. 1, str. 6658-6667
Gardić, V. (2007). Primena elektrohemijskih metoda za prečišćavanje otpadnih voda . Deo I – elektrodepozicija i elektrokoagulacija, Zaštita meterijala, vol. 48, br. 1, str. 49-58
Gatsios, E., Hahladakis, J.N., Gidarakos, E. (2015) Optimization of electrocoagulation (EC) process for the purification of a real industrial wastewater from toxic metals, Journal of Environmental Management, vol. 154, str. 117-127
Ghernaout, D., Badis, A., Kellil, A., Ghernaout, B. (2008) Application of electrocoagulation in Escherichia coli culture and two surface waters, Desalination, vol. 219, br. 1-3, str. 118-125
Ghosh, D., Solanski, H., Purkait, M.K. (2007) Removal of Fe(II) from tap water by electrocoagulation technique, Journal of Hazardous Materials, vol. 155, br. 1-2, str. 135-143
Gogate, P.R., Pandit, A. B. (2004) A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions, Advances in Environmental Research 8, 501–551.
Gomez-Lopez, M. D., Bayo, J., García-Cascales, M. S., (2009). Decision support in disinfection technologies for treated wastewater reuse. Journal of Cleaner Production, 17(16): 1504-1511
Hakizimana, J.N., Gourich, B., Chafi, M., Stiriba, Y., Vial, C., Drogui, P., Naja, J. (2017) Electrocoagulation process in water treatment: A review of electrocoagulation modeling approaches, Desalination, vol. 404, br. 1, str.1-21
Hao, H., Wu, M., Chen, Y. (2004). Cyanobacterial bloom control by ultrasonic irradiation at 20 kHz and 1.7 MHz. Journal of Environmental Science and Health – Part A Toxic/Hazardous Substances and Environmental Engineering, 39: 1435-1446
Hashim, K.S., Shaw, A., Al Khaddar, R., Pedrola, M.O., Phipps, D. (2017) Iron removal, energy consumption and operating cost of electrocoagulation of drinking water using a new flow column reactor, Journal of Environmental Management, vol. 189, br. 1, str. 98-108
Henglein, A., (1987). Sonochemistry: historical developments and modern aspects. Ultrasonics 25, 7–16.
Hiratsuka, A. and Pathak, D. (2013) Application of Ultrasonic Waves for the Improvement of Water Treatment, Journal of Water Resource and Protection, Vol. 5 No. 6, pp. 604-610.
Hrvatski zavod za javno zdravstvo (HZJZ) (2019) Izvještaj o zdravstvenoj ispravnosti vode za ljudsku potrošnju u Republici Hrvatskoj za 2018. godinu, preuzeto s: https://www.hzjz.hr/sluzba-zdravstvena-ekologija/izvjestaj-o-zdravstvenoj-ispravnosti-vode-za-ljudsku-potrosnju-u-republici-hrvatskoj-za-2018-godinu/: veljača 2020.
Hulsmans, A., Joris, K., Lambert, N. (2010). Evaluation of process parameters of ultrasonic treatment of bacterial suspensions in a pilot scale water disinfection system. Ultrasonics Sonochemistry, 17: 1004-1009
Jergović, M. Prisutnost metala i drugih rijetkih elemenata i utjecaj na zdravlje stanovništva Istočne Hrvatske [Presence of metals and other rare elements and health influence on eastern Croatian population]. Doktorska disertacija, Sveučilište u Zagrebu. 2011.
Jyoti, K. K., Pandit ,A. B., (2003) Hybrid cavitation methods for water disinfection: simultaneous use of chemicals with cavitation. Ultrasonics Sonochemistry, 10: 255-264
Kuna, E. (2013): Dinamika koliformnih bakterija u poplavnom području Kopački rit, Veučilište Josipa Jurja Strossmayera u Osijeku, Odjel za biologiju, diplomski rad
Kuokkanen, V., Kuokkanen T., Rämö, J., Lassi, U. (2013) Recent Applications of Electrocoagulation in Treatment of Water and Wastewater – A Review, Green and Sustainable Chemistry, vol. 81, br. 1, str. 89-121
Liang H, Nan J, He W, et al. (2009). Algae removal by ultrasonic irradiation-coagulation. Desalination, 239: 191-197
Lur’e, Y. Y., Kandzas, P.F., Mokina, A.A. (1962) Oxidation of phenol in an ultrasonic field, Russ. J. Phys. Chem. 36, 1422 – 1425
Mahvi A.H., Dehghani M.H. (2005). Evaluation of ultrasonic technology in removal of algae from surface waters. Pakistan Journal of Biological Science, 8: 1457-1459
Marriaga-Cabarales, N. and Machuca- Martínez, F. (2014) Fundamentals of electrocoagulation, Evaluation of Electrochemical Reactors as a New Way to Environmental Protection, Editors: Peralta-Hernández, J.M., Rodrigo-Rodrigo M.A. and Martínez-Huitle, C.A., str. 1-16
Matei, N., Scarpete, D. (2015). Treatment of Wastewater by Ultrasound Intensity and Frequency Effect: A Review; ARPN Journal of Science and Technology, vol. 5, no. 11, 591 – 597
Mishra, B. K., Gupta, S. K., & Sinha, A. (2014). Human health risk analysis from disinfection by-products (DBPs) in drinking and bathing water of some Indian cities. Journal of environmental health science & engineering, 12, 73. doi:10.1186/2052-336X-12-73
Naddeo, V., Cesaro, A., Mantzavinos, D., Fatta-Kassinos, D., & Belgiorno, V. (2014). Water and wastewater disinfection by ultrasound irradiation-a critical review, Global NEST Journal, Vol 16, No 3, 2014., pp 561-577
Oreščanin, V., Kollar, R., Nađ, K., Halkijević, I., Kuspilić, M., Findri Gustek, Š. (2014) Removal of arsenic, phosphates and ammonia from well water using electrochemical/chemical methods and advanced oxidation: a pilot plant approach, J Environ Sci Health A Tox Hazard Subst Environ Eng., vol.49, br. 9, str. 1007-1014
Ozturk E., Bal N. (2015) Evaluation of ammonia–nitrogen removal efficiency from aqueous solutions by ultrasonic irradiation in short sonication periods, Ultrasonics Sonochemistry 26, 422–427.
P.R. Gogate, (2002) Cavitations: an auxiliary technique in wastewater treatment schemes, Advances in Environ. Research 6, 335–358.
Purcell, D., Control of Algal Growth in Reservoirs with Ultrasound. PhD Thesis. Cranfield University, UK, 2009.
Qiu, Y.J., Yang, F., Rong, F., (2011). Degradation of Microcystins by UV in the Presence of Low Frequency and Power Ultrasonic Irradiation. Measuring Technology and Mechatronics Automation (ICMTMA), 2011 Third International Conference
Ricordel, C., Miramon, C., Hadijev, D., Darchen, A. (2014) Investigations of the mechanism and efficiency of bacteria abatement during electrocoagulation using aluminum electrode, Desalination and Water Treatment, vol. 52, str. 5380-5389
Schmitt, F.O, Johnson, C.H. and Olson, H.R. (1929) Oxidations promoted by ultrasonic radiation. J. Chem. Soc. 51, 370-375.
Shammas, N.K., Pouet, M., Grasmick, A. (2010) Wastewater Treatment by Electrocoagulation–Flotation, Flotation Technology, Wang, L., Eds.; Springer: New York, NY, SAD, str. 199-220
Shimizua N, Ninomiya N, Ogino C, (2010). Potential uses of titanium dioxide in conjunction with ultrasound for improved disinfection. Biochemical Engineering Journal, 48: 416-423
Spurlock, L., Reifsnei. SB. (1969). Some effects of ultrasonic irradiation on simple organic molecules. In Abstracts of Papers of the American Chemical Society, Series 67, 27 – 34
Stefan A, Balan G. (2011). The Chemistry of the Raw Water Treated By Air-Jet Ultrasound Generator. Rev. Roum. Sci. Tech. Mec. Appl., 56(1): 85-92
Tang, J., Wu, Q., Hao, H., (2003). Growth inhibition of the cyanobacterium Spirulina (arthrospira) platensis by 1.7 MHz ultrasonic irradiation. Journal of Applied Phycology, 15: 37-43
Toor R., Mohseni M. (2007). UV-H2O2 based AOP and its integration with biological activated carbon treatment for DBP reduction in drinking water. Chemosphere, 66(11): 2087-2095
treatment of organic pollutants in water by ultrasonic technology, Ultrasonics Sonochemistry, doi: https://doi.org/10.1016/j.ultsonch.2019.01.017
Vasudevan, S., Lakshmi, J., Sozhan, G. (2009) Studies on the Removal of iron from Drinking water by electrocoagulation, Clean, vol. 37, br. 1, str. 45-51
Vepsäläinen, M. (2012) Electrocoagulation in the treatment of industrial waters and wastewaters, VTT Technical Research Centre of Finland, doctoral thesis
Vikulina, V. and Vikulin, P. (2018). The effect of ultrasound on the process of water softening. MATEC Web of Conferences. 251., 1 – 8, https://doi.org/10.1051/matecconf/201825103003
Villanueva, C.M., Cordier, S., Font-Ribera, L. et al. (2015). Overview of Disinfection By-products and Associated Health Effects, Curr Envir Health Rpt 2: 107-15. https://doi.org/10.1007/s40572-014-0032-x
Wan, W., Pepping, T.J., Banerji, T., Chaudhari, S., Giammar, D.E. (2011) Effects of water chemistry on arsenic removal from drinking water by electrocoagulation, Water Research, vol. 45, br. 1, str. 384-392
Wang Li-ping, et al. (2008). Ultrasonic/modified clay process for removal of blue algae from artificial waters. China Water and Wastewater, 19: 44-46
Wang, J., Wang, Z., Vieira, C.L.Z., Wolfson, J.M., Pingtian, G., Huang, S., (2019) Review on the
Witekowa, S. (1972). Chemical effects of ultrasonic waves. XIII: investigations of some chemical oxidation and reduction reactions. Acta Chem. 17, 97–104
Xu, L., Cao, G., Xu, X., Liu, S., Duan, Z., He, C., Wang, Y., Huang, Q. (2017) Simultaneous removal of cadmium, zinc and manganese using electrocoagulation: Influence of operating parameters and electrolyte nature, Journal of Environmental Management, vol. 204, br. 1, str. 394-403
Xu, L., Xu, X., Cao, G., Liu, S., Duan, Z., Song, S., Song, M., Zhang, M. (2018) Optimization and assessment of Fe electrocoagulation for the removal of potentially toxic metals from real smelting wastewater, Journal of Environmental Management, vol. 218, br. 1, str. 129-138