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Electrolytic treatment of swine wastewater: a review

Abstract

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Introduction

Since Industrial Revolution in the 1800s, burning fossil fuels for energy has been on the basis of human development. These fuels, so called fossil because they are not contemporaneous and take millions of years to be produced geochemically, have been so extensively used that the impacts of a sudden supply descontinuity would certainly have devastating consequences on the way of life of millions of people. Moreover, they are widely recognized not to be sustainable and to put a significant pressure on the natural world. Within this context, the search for alternative forms of energy has attracted more and more attention in recent times. Today, the combination of energy production and environmental protection through the treatment and recycling of agroindustrial wastes has been identified as one of the central topics of these novel developments, both in scientific circles and in practice.

The production and management of large quantities of agroindustrial wastes, especially wastewaters, is one of the main sources of environmental distress caused by the livestock sector throughout the world. As the industry has evolved in the last decades, farming activities have become more concentrated in small areas with a reduction in the number of producers and an increase in average farm size being the norm \cite{Sagastume_Guti_rrez_2016}. From this intensive farming, enormous pressure is put in the surrounding environment with the disposal of considerable amounts of wastewaters having the potential of exceeding soil assimilation capacity \cite{_i_kov__2012}. Wastewaters deriving from livestock production are therefore a problematic waste threatening ecological stability in many regions. Most agroindustries, including swine farms, generate wastewaters with high organic and nutrient loads which may contribute to water pollution if disposed without treatment. In general, wastewater discharges containing high N and P content are undesirable because they promote eutrophication in the receiving water system by accelerating the depletion of dissolved O2. The consequences are an increased toxicity to aquatic life and further problems may occur with certain forms of nitrogen (NH4+, NO2-, NO3-) affecting human health if present in drinkable water \cite{Cho_2010, Lahav_2013, Huang_2015}.

Although wastewater treatment processes are well established in terms of environmental science and engineering, the need to develop innovative and effective solutions for the degradation of wastewaters with high organic loads still exists. Generally, high strenght wastewaters, including swine wastewater, require the use of biological methods to be stabilzed, either aerobic (composting) or anaerobic (anaerobic digestion, lagooning). Anaerobic digestion, especially, have become increasingly popular lately due to the possibility of recycling agroindustrial wastes into an important source of bioenergy by producing biogas. However, exhaustive treatment of wastewaters via this technology is very time consuming because high retention times and large stabilization facilities are required; moreover, high efficiency losses may occur when dealing with especially dirty effluents. In this regard, the field of electrochemistry offers novel and interesting approaches for the treatment of high polluting wastewaters like swine wastewater. In particular, electrochemical techniques have the benefit of being naturally suited to overcome the limitations of biological treatments. Specifically, there are several favorable features of electrochemical processes which call for its application in wastewater remediation \cite{Ibanez_2014, Rajeshwar_1994,MOLLAH_2004}: a) electrons, the only facilitators of wastewater treatment in these techniques, are intrinsically clean reagents; (b) most of the reactions take place at mild conditions which reduces safety concerns; and c) electrochemistry is versatile and flexible enough to treat various organic, inorganic and biochemical pollutants. In addition, electrochemical processes are often identified as cost effective due to its simplicity and energy efficient depending on the application.

In the literature, the widespread interest in electrochemical techniques for wastewater treatment is well illustrated by the diversity of the work reported. Indeed, electrochemistry have been successfuly employed in the removal of several contaminants from wastewaters of agroindustrial (poultry \cite{Yetilmezsoy_2009, Wang_2016}, dairy \cite{Borb_n_2014, Ihara_2006, _engil_2006, Kushwaha_2010, Melchiors_2016, Davarnejad_2016}, aquaculture \cite{Mook_2012}, sugar \cite{Sahu_2015, Sahu_2015a}, and others \cite{Drogui_2008}) and industrial origin (textile \cite{Kobya_2014, Ozyonar_2015}, petroleum \cite{Yan_2014, Bhagawan_2014}, latex \cite{Vijayaraghavan_2008}, municipal \cite{Nielson_2005, P_rez_2012}, and others \cite{Garc_a_Garc_a_2015}). The main electrochemical methods studied for the remediation of these wastewaters include electrocoagulation (EC) and electrooxidation (EO). Recently, the electrochemical treatment of swine wastewater has drawn the interest of several authors with a growing emphasis in the post-treatment of effluents already stabilized anaerobically via biological methods. Within this context, the main focus of this review will be on the electrolytic treatment of high polluting effluents from swine farms. Specifically, we review the current status of electrochemistry-based approaches for remediation of swine wastewater and provide an in-depth discussion of fundamental principles, recent achievements in terms of pollutants removal and future developments associated with each technology. With this, we hope to shed light on…

Electrocoagulation

Fundamentals

EC is a simple equipment method for the treatment of many types of wastewater, including swine derived effluents. The basic principle of EC comes from electrolysis which uses an electric current as the driving force of non-spontaneous chemical reactions. The process occurs in an electrolyte or aqueous medium that allows the possibility of ion transfer between two metal plates called electrodes; at these electrodes, cations are reduced and anions are oxidized (Sahu 2013). As a treatment technology, EC uses a direct current source passing between two electrodes immersed in the effluent to be treated in order to promote the removal of pollutants via coagulation. The process is called electrocoagulation because coagulation of the pollutants is achieved by electrolytic means. Chemically, EC can be described as the in situ generation of highly charged metal cations resulting from the electrodissolution of the anode in an aqueous medium which act as effective coagulation agents. Simultaneously water is reduced at the cathode and hydrogen is evolved according to Faraday’s law. Despite simple in principle, EC is regarded to be complex in nature due to the chemical and physical phenomena working synergistically (Sahu 2013). Considering a simple EC reactor with metal M = anode material and n = number of electrons involved in the oxidation/reduction, the following electrochemical reactions are induced by the application of a direct current electric field (Sahu 2013, Liu 2009, Mollah 2004, i 2012):

At the anode,

$$M(s)\rightarrow M_{(}aq)^{n+}+ne^{-}\\$$

At the cathode,

$$nH_{2}O(l)+ne^{-}\rightarrow\frac{n}{2}H_{2}(g)+nOH^{-}\\$$

Overall,

$$M^{n}+nOH^{-}(aq)\rightarrow M(OH)_{n}(s)\\$$

Metal anode M is oxidized generating corresponding cations M+ by reaction (1). These highly charged cations hydrolize to form polymeric hydroxides M(OH)n in the vicinity of the anode through reaction (3). As electrolysis progresses, the ionic strenght of the medium increases and negatively charged particles in colloidal suspension are carried towards the anode by electrophoresis. Ultimately, electrostatic interparticle repulsion is reduced to the extent that van der Waals forces predominate, and aggregation occurs when the electrogenerated hydrated cations neutralize the charge of the colloidal particles present by mutual collision. The process of overcoming the repulsive barrier and allowing particle aggregation is called coagulation. The agglomerates formed as a result of coagulation usually have high adsorption properties, thus bonding to pollutants (bridge coagulation), or entrapping them (sweep coagulation) before removal by either sedimentation or flotation. Simultaneously, hydrogen evolution occurs at the cathode by reaction (2) producing bubbles which may contribute to flotation of the agglomerated particles to the surface of the wastewater (Sahu 2013, Liu 2009, Mollah 2004, i 2012). Depending on reaction conditions, operating parameters, pollutant types, and electrode materials the dominant mechanism may shift throughout the EC process and other electrochemical and chemical processes may also take place. In summary, during EC there are generally four main consecutive steps(Thirugnanasambandham 2015): a) electrochemical reactions at electrode surfaces and formation of coagulating species, (b) destabilization of pollutants, e.g. colloids, emulsions, suspensions, (c) aggreation of destabilized phases and flocculation and (d) removal of flocs by sedimentation or flotation.

EC benefits from most of the advantages mentioned for electrochemical methods and offers reduced sludge production with simple design reactors due to rapid reactions without the need of chemicals (coagulants, oxidants) and microorganisms. On the other hand, the following drawbacks can also be added (Mollah 2004, i 2012) : the electrodes are consumed as a result of oxidation and need to be replaced periodically; operation costs may be high due to electrical energy consumption and electrode replacement; electrode passivation may halt reactor performance over time; a minimum conductivity of the wastewater suspension is required; and wastewater pretreatment steps (pH adjustment, equalization, etc.) may be needed.

Electrode materials

The most common electrode materials in EC are aluminium (Al), iron (Fe), mild steel and stainless steel (SS) since they are easily available and cost effective. In general, these metal electrodes are used because they have high coagulating power and are anodically soluble (Sahu 2013). In fact, multivalent ions of higher valence such as AL and Fe affect the stability (i.e, the tendency of the particles to aggregate) of colloids/suspended solids in wastewater streams more effectively by lowering the minimum concentration of counterions to induce coagulation (critical coagulation concentration) (Oncsik 2014, Zhang 2015).When Al and Fe are used as electrodes in EC, metal ions are generated from the electrolytic dissolution of the anode and many ionic monomeric-polymeric hydrolysis species are formed. The speciation of these Al and Fe hydroxides is variable and dependent on the pH of the wastewater.

Various reactions take place at the electrodes when Al electrodes are used (acidic conditions):