Reduction of natural organic matter by an electrolytic process, conventional coagulation and direct descending filtration in a pilot-scale water treatment system

The population explosion associated with rapid urbanisation and industrial and technological development in recent years has increased the demand for water. Furthermore, the effluent generated from various urban activities negatively impacts water reserves, leading to a deterioration of the quality of surface and groundwater and often precluding its use. To meet the needs of human consumption and the activities that require high quality water, it is necessary to provide endowed supply systems for water treatment plants that meet potability standards established by the Ordinance enforced in the country (Lima 2011). In the state of Ceará, Ponte et al. (2013) note that high algal content and high colour and turbidity are major problems in reservoirs. Sales (2005) highlights the need to enter a stage of pre-oxidation in a water treatment plant to remove natural organic matter that originates from humic substances (humic and fulvic acids). These substances form halogens, mainly trihalomethanes. The electrolytic process has emerged as an efficient technology applied in environmental sanitation. Electrolysis is a non-spontaneous process that requires the use of an electric current, which usually adds oxidation, flotation and coagulation (Atkins & Jones 2001). Electrolysis is based on the use of electrodes (sacrificed or not) with reversible polarity, and coagulant ions can be generated from the anode and hydrogen bubbles at the cathode, which causes the flotation of flakes (Rubio et al. 2002). It is a simple, low-cost and versatile automated process, requiring only a small area of the treatment plant. In addition to the benefits to public health from not using potentially toxic chemicals, it is a particularly clean process because the electron is the main reagent (Gusmão et al. 2010; Ribeiro et al. 2013). The electroflotation process acts as a solid/liquid separator based on the suspension of particles by gas bubbles (hydrogen and oxygen) that are generated on the surface of the electrodes (Araya-Farias et al. 2008). In conventional flotation processes, the diameter of the bubbles produced varies between 600 and 1,000 micrometres. Because the diameters are too large, these bubbles are not effective for the flotation of particles with diameters less than 20 micrometres. However, electroflotation can produce bubbles with a diameter between 15 and 80 micrometres, which are efficient for the flotation of small particles (Sarkar et al. 2011). Jeong et al. (2006) and Curteanu et al. (2011) argue that the use of an inert electrode, besides active chlorine, can also be used for the formation of reactive oxygen compounds, such as ^•OH, O₃, H₂O₂ and ^•O₂^–. However, the electrodes may generate an iron coagulant effect and cause increased flakes. According to Phalakornkule et al. (2010), the coagulant produced by the electrolytic oxidation of iron in the electrodes may be in the form of hydroxides or highly charged ions, such as Fe(H₂O)₆³⁺, Fe(H₂O)₅(OH)²⁺, Fe(H₂O)₄(OH)₂⁺, Fe₂(H₂O)₈(OH)₂⁴⁺ and Fe₂(H₂O)₆(OH)₄⁴⁺. The electrolytic process has been applied in several studies in recent years. Abdala Neto & Aquino (2012) compared a photoreactor (electro-oxidation and heterogeneous photocatalysis) with other pre-oxidants in a water treatment system. The photoreactor reached a higher removal efficiency for turbidity (99.5%), chemical oxygen demand (65.50%) and chlorophyll ‘a’ (98.97%). In addition, it achieved a greater reduction in the amount of trihalomethanes (77.20 mg L^–1) compared with the other pre-oxidants. Qi-yan et al. (2007) investigated various process variables in the use of aluminium electrodes to remove humic acid from water. Within 60 minutes, this process removed 97.8% of the humic acid with a current density of 4.76 mA cm^–2 and a 1.0 cm spacing between the electrodes in a pH acid. Benhadji et al. (2011) investigated the application of electrocoagulation with aluminium electrodes in tannery wastewater. With a current density of 7.5 mA cm^–2 for 45 minutes, there was a reduction of over 90% for biochemical oxygen demand, chemical oxygen demand, turbidity, chromium, iron and nitrate. Some studies have demonstrated the application of the electrolytic process in the removal of contaminants in drinking water. Kobya et al. (2011) obtained an arsenic removal efficiency of 95.7% with aluminum electrodes and 93.5% with iron electrodes. Using aluminum electrodes, Vasudevan et al. (2011) removed 98.2% of chromium present in the water. This process can also remove anions and cations. In addition to removing fluoride ions with the use of aluminum electrodes (Zuo et al. 2008; Bennajah et al. 2009; Zhao et al. 2009; Un et al. 2013), Kumar & Goel (2010) removed 84% of nitrate in water with mild steel electrodes. Malakootian et al. (2010) removed 98.2 and 97.4% of calcium and total hardness, respectively. The objective of this research was to investigate the removal of natural organic matter by electrolytic processes (electroflotation, electro-oxidation and electrocoagulation) operating on alternating current and using electrodes made of carbon steel and stainless steel for treating water that is used for human consumption. Furthermore, energy consumption, the parameters of water clarification (turbidity and colour), and the variation of pH in the process were evaluated following the guidelines related to the maximum values established by Ordinance No. 2914 (Brasil 2011), which sets forth procedures for the control and surveillance of water quality for human consumption and its potability standards.

The experiment was conducted at the Research Center for Companhia de Água e Esgoto do Ceará, located in the annexed area of the Gavião Water Treatment Plant (WTP Gavião), which is in the municipality of Pacatuba. The reactor was made of acrylic with a thickness of 10 mm and was divided into three chambers, as shown in Figure 1(a). Between each chamber, four holes were made with a diameter of 1.28 cm for possible contact with substances generated in the electrolytic process with water. The dimensions of the working volume were 22.5 cm high, 24.0 cm wide, and 30.0 cm long (giving a useful volume of 16.2 litres). In the first chamber, iron electrodes (carbon steel 1080) were used; in the second and third chambers, electrodes of stainless steel (316L stainless steel) were used. All electrodes measured 20.0 cm long, 5.0 cm tall and 0.2 cm thick. The average current density was 1.83 mA cm^–2 for the iron electrodes and 4.10 mA cm^–2 for the stainless steel electrodes.

Table 1 shows the values of the pressure drop on the filter along the filtration career and the free residual chlorine in the treated water.

Table 3 shows the values of DOC, absorbance of ultraviolet light at 254 nm and SUVA. The Tukey test with a 95% confidence interval for the three parameters was applied.

For all parameters, a significant difference was observed between raw water, which belongs to group A, and treated water, which belongs to group B or C. The results show that the removal efficiency of SUVA was 19.51, 20.47, and 26.45% for filtration careers 1, 2 and 3, respectively. The significant reduction in the presence of organic matter in natural water can decrease the formation of disinfection by-products. According to Jiang et al. (2002), in the electrocoagulation process, iron electrodes are dissolved in electrolysis and form a variety of species of metal hydroxides and coagulants. These species destabilise and aggregate the suspended particles, and the dissolved iron adsorbs contaminants such as dissolved organic matter. Moreover, according to Mohora et al. (2012), the hydrogen bubbles produced at the cathode are responsible for the removal of natural organic matter due to the flotation of the particles present in water. The energy used for the electrolytic pretreatment was monitored based on the power measured in each of the filtration careers. Table 4 shows the data that was used to calculate electricity consumption, which includes the average consumption of electricity (power) of the three filtration careers, average time of each filtration career, average volume of treated water produced, and price of the local electricity per kilowatt-hour. This information can be used to calculate the cost of electricity in step electrolytic pretreatment.

Since the price of electricity per kilowatt-hour is $0.08, the cost of electricity for the electrolytic pretreatment step was $0.10 for every m³ of treated water.

The results from this study demonstrate that the electrolytic process with the use of an alternating current in conjunction with chemical coagulation and direct filtration significantly removed the natural organic matter in the water. The reduction of the SUVA reached an efficiency of 19.51–26.45%, with a hydraulic retention time of 1.62 minutes and a current density of 1.83 mA cm^–2 for the iron electrodes and 4.10 mA cm^–2 for the stainless steel electrodes. Furthermore, the electrolytic treatment process resulted in average values of turbidity and colour of the water that were in accordance with the maximum allowable values for Brasil (2011). The conclusion from this study is that the electrolytic process employed did not significantly alter the pH and that the cost of the energy consumed per m³ of treated water was $0.10 at this stage of treatment. Thus, the technique of electrocoagulation can be used in water treatment stations to meet the standards established by the applicable Ordinance and to achieve low-cost water production.