Continuous Heterogeneous Fenton for Swine Wastewater Treatment: Converting an Industry Waste into a Wastewater Treatment Material
1. Introduction
The world’s population growth is contributing to a high demand in the food market [1,2,3]. Since pork meat is the second most consumed meat, the swine industry is growing quickly [4,5]. About 1 million pigs, corresponding to 102 million tons of meat, are produced per year. China leads the production with approximately 450 million pigs, followed by the European Union with 140 million, and the United States of America with 74 million heads in April 2022 [6,7,8].
The intensive meat production industry has critical consequences for public health and the environment due to the residues that are generated, namely animal wastes, contaminated wastewater, and the release of different kinds of gases [4]. Moreover, this livestock production results in high water consumption (an estimated of 6 m3/kg of pork meat in Brazil), with the swine industry being responsible for about 43% of the generated livestock wastewater [4,8,9]. The swine wastewater (SW) is complex, being characterized by high organic loads due to the existence of fatty acids, nitrogen, phosphorous, heavy metals, antibiotics, suspended solids, and fecal coliforms [1,4,6,9]. Therefore, this kind of wastewater can be dangerous due to the existence high loads of organic matter (OM), ammonium and ammonia, antibiotics, and pathogenic organisms [4,5,10,11,12].
Natural organic matter (NOM) occurs due to the residues from plants, animals, and humic substances, while synthetic organic matter (SOM) is usually complex and dangerous, and arising from the use of chemicals in industries or agriculture [13]. The high organic loads of swine wastewater are a threat to the environment if not properly treated. Phosphorous and nitrogen are macronutrients that can lead to the eutrophication phenomenon [14,15]. Ammonium is usually present at high concentrations in swine wastewater [16], and although it does not present toxic effects, it can cause odors and microbial development [17]. Ammonia can cause the acidification of water, eutrophication, or toxicity and other harmful effects on aquatic organisms [18]. Moreover, SW can present high levels of virus and protozoan agents, which can be sources of diseases [4], therefore, the disinfection of this wastewater should also be considered.
Antibiotics are used to treat diseases in animals and are usually poorly metabolized, with 30–90% being excreted as parent compounds or metabolites [19,20]. Urine and feces are usually treated and used as fertilizers in agriculture, which can be a source of soil contamination and water pollution due to the presence of pharmaceuticals [21]. In 2013, about 52% of the total consumption of antibiotics in China was attributed to animals, and about 50% entered the environment [21,22]. Several antibiotics have been found in rivers worldwide [11], with one contributing factor being the incapacity of industrial and urban wastewater treatment plants (WWTPs) to remove them before the (treated) wastewater discharge. There is also the risk of increasing antibiotic resistance due to the widespread and extensive use of antibiotics, which increases concerns for public health and the natural environment [23,24]. Therefore, it is important to ensure the proper swine wastewater treatment.
Water scarcity is also a problem that already affects the world, in which about 1.2 billion people already suffer from lack of water, with this number being expected to increase to about 1.8 billion people in 2025 [25,26]. In addition, about 2.7 billion people face water scarcity at least one month per year, and about 2.4 billion people are exposed to diseases (such as cholera or typhoid fever) due to inadequate sanitation [26]. Therefore, the correct water/wastewater management (with the correct water reuse) must be addressed, since it can mitigate the water scarcity and contamination problems [9,25]. Several countries have implemented policies to encourage proper SW management and treatment [8]. The SW treatment is usually done by biological systems (such as aerobic lagoons, anaerobic lagoons, anaerobic packed-bed reactors, anaerobic filters, anaerobic bioreactors), but these conventional methods may not be capable of reaching the legal limits for discharge [9]. In fact, some of these methods are extremely popular, presenting economic feasibility and energy recovery capability, but they also require high retention times and are very sensitive to the process conditions [4].
Advanced oxidation processes (AOPs) are complementary technologies for industrial wastewater treatment since they offer high reliability in the contaminant’s degradation. AOPs can be used either after or before biological processes, complementing the treatment quality. In this perspective, AOPs can be applied after the biological treatment to remove recalcitrant contaminants that the biological route was not capable of removing, while when used before, they can reduce the wastewater toxicity and improve the efficiency of the forthcoming biological treatment [27]. In this context, the Fenton reaction can be a suitable solution due to its simplicity and high efficiency in the removal of harmful pollutants from water and wastewater.
The Fenton reaction uses Fe2+ to initiate and catalyze H2O2 decomposition, forming hydroxyl radicals (•OH) and Fe3+, and, due to the action of H2O2, at the same time the Fe3+ is reduced to Fe2+, forming the hydroperoxyl radical (•HO2), allowing the chain reaction to continue [28,29]. Both Fe2+ and the H2O2 can also be •OH scavengers. In excess, the Fe2+ can react with the •OH radicals and form Fe3+ and OH− [29,30], while the H2O2 can produce •HO2 or self-decompose in water and oxygen. When the amount of H2O2 is low, it can cause low radical generation, decreasing the Fenton efficiency [31,32]. The Fenton key parameters are the concentration of Fe and H2O2, pH, temperature, and the concentration of organic and inorganic species, and to obtain the maximum performance, it is also necessary to understand the relation between the [Fe2+]/[H2O2] ratio and •OH production and consumption [33].
Typically, the Fenton reaction occurs as a homogeneous system [28], and often at low pH, with the best reported pH in literature being 3–4 [33,34]. A neutral pH can also be used, but the efficiency is lower [35], and at a basic pH, the Fenton reaction is limited, since the iron precipitates in the Fe(OH)3 form [36]. Additionally, at a high pH, the hydrogen peroxide can decompose in oxygen and water. Other metals, such as Cu, Ce, Mn, Cr Co, Ru, or Al can be used in processes known as Fenton-like processes [29], and although Fenton-like reactions can be used in neutral or basic pH, they can present disadvantages, such as high-costs, metal leaching, complex mechanisms, and low catalyst reuse [29]. Iron sludge production is a common drawback of homogeneous Fenton, but the use of heterogeneous materials could overcome this issue. Using zero valent iron as a heterogeneous iron source can be advantageous, since it is cheap and widely available (iron powders, filings, wires, nails, wool, or nanoparticles), simple and easy to handle [37]. The Fe0 can be oxidized to Fe2+ by different mechanisms (Equations (1)–(4)), generating •OH or other radicals in the classic Fenton reaction mechanism [37,38]. However, the use of Fe0 has the unique advantage of being capable of regenerating the Fe3+ into Fe2+ at the iron surface (Equation (5)), being a cost-saving process when compared to the typical regeneration of Fe3+ by H2O2 action [39,40].
Fenton can present several advantages when compared to other AOPs. It is characterized by low-cost, simplicity and low toxicity of the required materials [41]. When coupled with radiation (photo-Fenton) it can increase the efficiency and decrease the required amount of Fe2+ and H2O2 [34,42,43,44,45,46]. The presence of radiation favors the reduction of Fe3+ into Fe2+ also forming •OH (Equation (6)) [37]. Using the sun as a radiation source can be an interesting alternative to promote photo-Fenton without increasing energy costs [36,46,47,48].
Regarding the other technologies, ozonation can be efficient in removing color, odor, taste, and pollutants from wastewater, but the action of ozone molecules is selective, being associated with low gas/liquid transference rates and poor COD and TOC removal. Moreover, it can present a short lifetime of ozone and a high energy demand as drawbacks [36,49]. Combining ozone with hydrogen peroxide (the peroxone process) can enhance the reaction kinetics, improving the •OH generation and reducing the amount of required ozone, but the reaction initiation step is slow [29,30]. Combining radiation with H2O2 can also be used for •OH generation (Equation (7)), but this reaction has a low quantum yield and requires UVC radiation [30,37].
Photocatalysis requires radiation and a photocatalyst, and it is usually associated with low reaction kinetics and high recombination rates [34,50,51]. Adding H2O2 to photocatalysis allows the generation of additional •OH radicals due to the better trapping of conduction band electrons or the reaction with superoxide radicals [37]. Unfortunately, the traditional materials used in photocatalysis are in powder form, which implies difficult operations for recovering the photocatalyst, posing an obstacle to industrial application [52]. Therefore, Fenton can be a suitable technology over other advanced oxidation processes, and by using industrial wastes as the iron source, it increases the economic feasibility of Fenton while maintaining high efficiency, simplicity, and easy separation of the treated liquid from the iron source.
Using homogenous Fenton with FeSO4, Lee et al. [53] removed 86% of COD from a SW with 5000–5700 mgO2/L using [Fe2+] = 4312 mg/L, [H2O2] = 5250 mg/L, time = 30 min, and pH = 4, while Riaño et al. [54] achieved a removal of 78% in 30 min using [Fe2+] = 100 mg/L, [H2O2] = 8000 mg/L, and pH = 3 for the treatment of a pre-treated SW with 769 mg/L of initial COD. Using other AOPs for the treatment of SW, Garcia et al. [4] obtained COD removals of 45–85.6% by photocatalysis with TiO2 P25, depending on the photocatalyst concentration, irradiation time, and wastewater concentration. Moreover, using another approach, Chen et al. [1] obtained 50.6% COD removal in 35 min by electrocoagulation, while Gomes et al. [55] obtained a COD decrease of 40% by coagulation and a 51% decrease in 4 h by biofiltration with the Asian clam Corbicula fluminea. They also showed that the COD could be naturally removed just by bubbling the wastewater with air during a large period of time, due to the aerobic digestion of organic matter by the organisms present in the wastewater.
In a previous study, Domingues et al. [35] conducted an optimization to obtain the best conditions for SW pollutant removal by adsorption using red mud (RM—a waste product from the alumina production industry) and the heterogeneous Fenton at batch conditions with iron filings (IF—a waste from steel production). The single RM adsorption removed up to 55% of COD, while the coagulation with the PDADMAC decreased the COD by 30%. The batch Fenton with the IF reduced the initial COD value by 66% when applied after coagulation, and 86% when applied after the RM adsorption. The best conditions for the adsorption with the RM were using 2.5 g/L of RM during 5 min at pH = 3, while for the Fenton experiments an IF load of 15 g/L and [H2O2] = 50 mg/L led to the best results. This shows a decrease in the amount of hydrogen peroxide when compared to the studies of Lee et al. [53] and Riaño et al. [54] with similar COD decreases (comparing to the 86% obtained by RM adsorption and Fenton). Therefore, the adsorption with the RM followed by the heterogeneous Fenton method with IF can be an interesting option for the treatment of SW wastewater. Moreover, this can also benefit from the fact that the iron sludge formation is usually more problematic in the homogeneous Fenton than in the heterogeneous form, due to the higher concentration of dissolved Fe2+.
In this work, the main objective was to investigate the use of waste materials in the treatment of real swine wastewater, targeting a circular economy approach. For this, adsorption with red mud was evaluated as a potential substitute for PDADMAC coagulation as a pre-treatment. Then, the efficiency of iron fillings waste as the iron source in the Fenton process was investigated. The Fenton process was effectively applied in continuous operation mode using a very low concentration of H2O2, which should motivate the industrial application of such technology. To the best of our knowledge, no other work reports the treatment of real swine wastewater in continuous operation using RM and IF as raw materials.
4. Conclusions
This work shows a potential usage of waste materials in the wastewater treatment process, which improves sustainable development, following the circular economy approach.
Different approaches to improve the Fenton reaction using coagulated SW were considered. Increasing the H2O2 concentration to 500 mg/L resulted in the same COD removal using either acidic or neutral pH, which is probably related to the generation of less oxidative radicals (such as •HO2). Moreover, conducting a partial addition of H2O2 after a 60 min reaction did not improve the COD reduction. This was expected, since before the H2O2 addition, there were still traces of H2O2, suggesting that the COD removal was not limited by the total consumption of this reagent.
Considering other SW treatment methods, the adsorption with RM revealed a potential alternative to the coagulation with PDADMAC, being capable of achieving higher COD removal and causing lower toxicity. Moreover, the IF was also revealed to be an attractive source of iron catalyst for the Fenton process in continuous operation. Nevertheless, despite the good COD removal observed with the studied pre-treatments (86% and 50% for the RM adsorption and PDADMAC coagulation, respectively), the COD discharge limits imposed by law could only be respected through the subsequent application of the Fenton reaction. After the overall treatment, the toxicity evaluated by the L. sativum GI changed from a “strong inhibition” to a “non-inhibition”. Considering other discharge aspects imposed by Portuguese decrees of law, the overall treatment complies with some of the imposed limits, and the application of subsequent treatments (namely, biological and iron removal processes) should allow for compliance with other mandatory values imposed by the regulations regarding wastewater discharge or water reuse of irrigation.
Therefore, the advantages of this methodology are the efficient use of wastes as raw materials (in which the RM adsorption can efficiently replace the PDADMAC coagulation), the reduction of treatment costs and the contribution to the circular economy (due to the reuse of wastes for a new purpose), as well as the provision of high efficiency for the treatment of real wastewater. As drawbacks, the iron filings must be replaced after some operational time, and other processes will be required to complete the treatment (e.g., remove iron and ammonium/ammonia), for which biological treatments could be a suitable solution for this purpose.
For future work, different types of real wastewater should be tested with this methodology, and scale-up studies should be carried out. The use of photo-Fenton, instead of Fenton, namely using solar radiation, could also be an interesting approach and should not be dismissed. In addition, toxicity studies involving other trophic level species should be carried out.
