Elsevier

Chemical Engineering Journal

Volume 336, 15 March 2018, Pages 133-140
Chemical Engineering Journal

Electrochemical treatment of real wastewater. Part 1: Effluents with low conductivity

https://doi.org/10.1016/j.cej.2017.11.046Get rights and content

Highlights

  • Effective electrochemical treatment of real wastewater with low conductivity.

  • The addition of sodium sulphate can decrease the TOC abatement.

  • Best performances are obtained in a micro cell.

Abstract

The treatment of a real wastewater characterized by low conductivity was performed by anodic oxidation at boron doped diamond (BDD) in both conventional and microfluidic cells. The electrolyses carried out in conventional cells without supporting electrolyte were characterized by very high TOC removals but excessively high energetic consumptions and operating costs. The addition of sodium sulphate, as supporting electrolyte, allowed to strongly reduce the cell potentials and consequently the energetic consumptions and the operating costs. However, under various operating conditions, the addition of Na2SO4 caused a lower removal of the TOC. The best results in terms of both TOC removal, energetic consumptions and operating costs (about 1 €/m3) were obtained using a cell with a very low inter-electrode distance (50 µm) with no addition of a supporting electrolyte.

Introduction

In the last years, many efforts have been devoted to the development of electrochemical processes for the effective treatment of wastewater contaminated by organic pollutants resistant to conventional biological processes and/or toxic for microorganisms [1], [2], [3], [4], [5]. It was shown that some electrochemical approaches, including the direct anodic oxidation at suitable anodes such as boron doped diamond (BDD) and/or electro-Fenton (EF) at suitable operating conditions and cells [1], [2], [3], [4], [5], [6] can allow to treat effectively a very large number of organic pollutants. Electrochemical processes present several advantages with respect to other advanced oxidation processes such as: very mild operative conditions, no transport or storage of oxidants, limited operative and capital costs, wide versatility and very high removal of various kinds of pollutants [1], [7], [8]. Furthermore, their attractiveness is enhanced by three other relevant factors:

  • the possibility to use both cathodic and anodic processes to enhance the effectiveness of the treatment [1], [9], [10];

  • the possibility to couple the wastewater treatment performed in one compartment of the cell with another process carried out in the other compartment, such as, for example, the carbon dioxide conversion to formic acid, thus improving dramatically the economics [8];

  • the possibility to use the amount of intermittent electric energy generated by renewable sources in excess with respect to the needs of the grid.

As a result of all these considerations, a plethora of studies were dedicated to the evaluation of various electrochemical approaches and the selections of proper operating conditions and cells [1], [2], [3], [4], [5], [6]. However, most of the investigations were performed using synthetic wastewater [1]. Hence, it is now mandatory to study the problems connected to the passage from synthetic wastewater to the real ones. In this context, in the last years various researchers started to investigate the applicability of some electrochemical approaches to real wastewater [11] coming often from textile [12], [13], [14], pharmaceutical [15] and chemistry [16] companies, urban wastewaters plants after secondary treatment [17], [18], [19] and landfill leachates [20], [21], [22], [23], [24]. The performances of electrochemical treatment are expected to strongly depend on many factors including the conductivity of the real wastewater. This problem is not encountered with synthetic wastewater since in these studies a supporting electrolyte is added to the solvent. On the other hand, the addition of a supporting electrolyte to a real wastewater is not easy for real applications for economic and in some cases safety reasons. Real wastewater are characterized by a very large variety of salt content and conductivity. Hence, we have decided to study in detail the electrochemical treatment of wastewater characterized by very low or very high salt contents. In particular, in this work we have focused on real wastewater with low conductivity, in order to evaluate if these effluents can be treated with electrochemical methods. Conversely, in a successive work, we will focus on the electrochemical treatment of real wastewater with high salt content.

The effectiveness of the electrochemical treatment depends on many factors including the electrode material. Several anodic materials have been tested, but most of them presented important drawbacks such as loss of activity (graphite), release of toxic ions (PbO2), low service life (SnO2), not complete oxidation (IrO2) [2], [3]. On the contrary, synthetic boron diamond (BDD), with its high anodic stability and wide potential window, is considered to be an effective material for the direct combustion of organics in wastewater. Hence, the experiments were performed with BDD anode in an undivided cell, to avoid the cost of the membrane and the corresponding ohmic losses.

The problem of electrochemical treatment of real wastewater with low conductivity was rarely investigated in the literature, in spite of the fact that many domestic and industrial wastewater present quite low conductivity. Indeed, in most of cases wastewater with high conductivity were selected [11], [12], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. In few cases real wastewater with low conductivity were used [25], [26], [27], [28]; in some occasions, the researchers solved the problem of the low conductivity, by the addition of a supporting electrolyte such as sodium sulphate. However, from an applicative point of view, the addition of chemicals to the solution leads to a strong increase of the operating costs and a more difficult authorization procedure since it can cause the formation of secondary pollutants. As a further example, Tsantaki et al. [13] studied the electrochemical treatment of a textile wastewater adding perchloric acid (HClO4) as supporting electrolyte to increase effluent conductivity and decrease the electrical consumption, a critical approach in view of both the cost and the toxicity of ClO4.

In this work, the problem of wastewater with low conductivity will be investigated in detail, using a real wastewater containing organics (initial TOC of 210 mg l−1) and characterized by quite low conductivity (1.4 mS cm−1), coming from the separation of oil and water phases performed from one company devoted to the treatment of wastewater located in the south of Sicily. In the last years, it has been shown that synthetic wastewater with low conductivity can be effectively treated using filter-press flow cell with a very small inter-electrode distance (“micro cell”) [29]. Furthermore, such cells improve the removal of organic pollutants performed by both direct anodic oxidation [29], [30], [31], [32], cathodic reduction [33], [34], electro-Fenton [34], [35] and coupled processes [35], [36]. In addition, in micro-cells it is possible to achieve high conversions of the pollutants for single passage, thus allowing to operate in a continuous mode. This opened to the possibility to use a multi stage system involving cells operating in series with different operating conditions/electrodes, thus allowing to maximize the current efficiency and to minimize the treatment time [37]. However, the utilization of micro cells for the treatment of real wastewater was not investigated up to now. Hence, in this work, two different cells were used: a conventional undivided batch cell (“conventional cell”), often used in the labs, operated both in the absence and in the presence of a supporting electrolyte, and a micro cell, with the main aim to evaluate if real wastewater with low conductivity can be properly treated by electrochemical oxidation at BDD anode and to select the more promising cell and route.

Section snippets

Experimental

Electrolyses at the macro-scale were performed in batch mode in a cylindrical undivided tank glass cell (Fig. 1A) equipped with a nickel cathode and a BDD anode (working area A = 3.75 cm2) and containing 50 mL of solution, under relatively vigorous stirring performed by a magnetic stirrer (400 rpm min−1) in order to speed-up the mass transfer of the organics to the anode surface. The inter-electrode gap was about 2 cm. Experiments in the micro reactor were performed in a continuous mode with a

Electrolyses performed in conventional cell in the absence of supporting electrolyte

First electrolyses were carried out in a conventional cell without the addition of a supporting electrolyte at a boron doped diamond (BDD) anode and a Nickel cathode at a current intensity of 60 mA (corresponding to a current density of 16 mA cm−2) for 7 h. As shown in Fig. 2A, a quite good but not total abatement of TOC of about 60% was achieved at the end of the electrolysis. To improve the abatement of TOC, a series of electrolysis was carried out at different current intensities (in the

Conclusions

In this work we have studied the anodic treatment at a BDD anode of a real wastewater contaminated by organic pollutants and characterized by low conductivity. The process was performed under three different conditions: (i) conventional cell in the absence of supporting electrolyte; (ii) conventional cell with the addition of Na2SO4 or NaClO4 as supporting electrolyte; (iii) microfluidic cell without supporting electrolyte.

It was found that the performances of the process drastically depend on

Acknowledgments

The work was supported by Università di Palermo.

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