In this work, electrocoagulation (EC) followed by ozonation process were applied in industrial wastewater plant which treats wastewater from 300 industries located in the industrial corridor Toluca - Lerma. Mexico. Electrocoagulation technology is effectively used for treatment of wastewater. Electrocoagulation occurs via serial steps: (i) Electrolytic reactions at electrode surfaces (ii) In-situ oxidation of metal ions and subsequent precipitation of metal hydroxides in aqueous phase, and (iii) Adsorption of soluble or colloidal pollutants on coagulants surface. Electrocoagulation is well known in wastewater treatments. One main advantage of electrocoagulation is that this treatment does not use chemicals which remain in the treated water.

Ozone is a powerful oxidizing agent so that have been developed for treatment of wastewater. Ozone once dissolved into aqueous solution, part of ozone molecules are considered to decompose through a series of chain reactions, into highly-oxidative hydroxyl radicals HO• (E_{0 =} +2.80V), allowing the occurrence of advanced oxidation process known as AOPs.

In this study we applied response surface methodology (RSM); RSM is an experimental design methodology that optimize the process in multifactor experiments. This methodology utilizes mathematical and statistical techniques for: (i) developing second-order polynominal model, (ii) understanding the effects of different variables (factors) and their interactions on response, (iii) determining comparative significance of several effective factors and (iv) optimizing the process.

 A sample consisting of 60 L was taken from entrance to wastewater treatment plant and storing at 4 ° C according APHA.

Electrochemical cell

The electrochemical cell was run in a batch mode at 1.5 L effluent per batch; seven rectangular commercial aluminum plates served as anodes (3), cathodes (3) and 1 sacrificial electrode; the anodic and cathodic active surface area were 2,522.5 cm². A DC power source supplied the system with electric current obtaining 0.0028, 0.0064, 0.01, 0.0136 y 0.0172 mA cm⁻² current density. The electrocoagulation was performed without additional electrolyte (water conductivity 2,346 µS). Experiments were adjusted at 4.0, 5.5, 7.0, 8.5 y 10.0 of pH by adding NaOH 5N y H₂SO₄ 1N. 25 mL of sample were taken every 195 s during the electrocoagulation process. The samples were allowed to settle for 1 h, and then analyzed. The electrodes were sanded before each set of experiments.

 Ozonation process

Ozone was generated using a laboratory-scale ozone generator using air (20.95% O₂) as feed gas flowing at 0.3 – 0.4 Standard Cubic Feet per Minute (SCFM) rate. The ozonation process was performed in a reactor comprising a 60 cm long acrylic column of 20 cm inner diameter (ID) fitted with a stone diffuser at the bottom to aid uniform distribution and good mixing of the ozone gas and the effluent. The reactor was run in a batch mode at 1.5 L effluent per batch. Ozone was used as a pretreatment at different times (3, 6, 9 and 12 min) before EC process; subsequently continued with elctrocoagulaciòn process previously described. The excess ozone flowed from the top of the reactor into the ozone trap containing a 0.1 M KI solution.

 RSM methodology

The Central Composite Design, which is a widely used form of RSM, was selected for the optimization of EC process. Variables, experimental ranges and levels are presented in Table 1.

Table 1. Experimental range and levels of the test variables in EC process.

 Results

In order to study the combined effect of factors showed in Table 1, experiments were performed for different combinations of the parameters using statistically designed experiments. The coefficients of the response function, the t and P values for turbidity removal efficiency are presented in Table 2.

Table 2. Estimated regression coefficients for turbidity removal efficiency (%) in coded units.

The second order polynomial equations for turbidity removal efficiency in terms of coded factors are given by

Equation (1):

The best variables amount to achieve maximum turbidity removal efficiency are pH 7, 0.0172 A/cm² for the current density and 26 min for the reaction time. Figure 1 indicated the turbidity removal efficiency was sensitive to alterations current density, and reaction time. 

Figure 1. Surface plot and their corresponding contour plot as a function of current density and reaction time at initial pH of 7.

The optimization results of the process variables (7 pH, 0.0172 A/cm² current density and 26 min reaction time) are shown in Table 3.

Table 3. Optimum values of turbidity removal efficiency.

Figure 2 show the raw (a) and treated (b) wastewater respectively.

Figure 2. a) raw wastewater and b) treated wastewater.