Abstract
Water is an essential resource for life, the environment and the industrial development; hence the treatment of wastewater has become crucial to ensure a good quality effluent before disposal the municipal sewer systems. The slaughterhouse industry produces a large volume of wastewater due to the slaughtering of animals, meat processing and the cleaning of the slaughterhouse facilities [1]. Treatment and adequate disposal of slaughterhouse wastewater (SW) is a worldwide economy and public health necessity which is regulated by International Standard [2]. SW contains elevated amounts of organic matter and salts. Typical parametrical analyses include pH, chemical oxygen demand (COD), biochemical oxygen demand (BOD), total nitrogen (TN), total phosphorous (TP), total organic carbon (TOC) and total suspended solids (TSS) [3]. After preliminary treatment, where most large particles are removed, the coagulation-flocculation technology is the most used primary treatment. However, it produces secondary pollution. Nowadays, electrochemical processes such anodic oxidation (AO) have been considered an alternative technology for the treatment of SW due to its mild operation conditions, quite compact and easy operation and not secondary waste streams among others. AO may oxidize the organic pollutants via two different ways: i) directly oxidized, where electron transfer occurs at the anode surface (M) and/or ii) indirectly oxidized, where the organic pollutants are oxidized through the mediation of some species electro-generated in situ at the anode surface and/or agents in the bulk solution.
In this study, a real beef slaughterhouse wastewater presented the following characteristics: TOC (1150 mg L–1), COD (4320 mg L–1), TP (25 mg L–1), TN (72.28 mg L–1), TSS (1433 mg L–1) at pH 7.18 and conductivity of 2.79 mS cm–1. Bright red color was observed at 416 nm (1.24 A.U.) and the presence of coliform bacteria was confirmed (> 1600 MPN). AO tests were carried out in a single open cell compartment in batch operation mode. 0.1 L of SW was introduced into the electrolytic cell with constant stirring to ensure mass transport of the oxidant specie towards/from the anode to the bulk. AO was assessed using two different dimensionally stable anodes (DSA) type anodes: i) Ti/IrO2/Ta2O5 coating (DSA-O2) ii) Ti/Ru0,3Ti0,7O2 (DSA-Cl2). AISI 304 stainless steel plate was used as cathode. The geometrical bare area of both, cathode and anode, was about 5 cm2, having a 1 cm of interelectrode gap. AO experiments were analyzed at different applied current densities (2, 10 and 20 mA cm–2) and supporting electrolyte (0.05 M Na2SO4; 0.05 M NaCl) in order to find the best operating conditions.
This study focuses on TOC and COD removal as a function of the current density and the electrolysis time. Minimizing TOC and COD values is the main objective in SW treatment. However, TOC is the most important role to determine the organic compounds in SW. Figure 1 shows the TOC removal during five hours of electrochemical treatment utilizing the two different DSA-type anodes at the higher current density (j = 20 mA cm–2). As can be seen in Figure 1a), EO process using DSA-O2 without supporting electrolyte reached the maximum removal of total organic carbon at the end of the electrolysis time. Residual TOC concentration achieved 232.66 mg L–1 (79.77 % removal efficiency). Figure 1b) displays the TOC abatement from SW using DSA-Cl2 anode. Alike the DSA-O2 the best EO test was the one where not supporting electrolyte was added, the maximum TOC removal efficiency reached 78.62 % (245.89 mg L–1) unlike the other tests using supporting electrolyte.
The best operating conditions were found at current density of 20 mA cm–2 without supporting electrolyte. TOC, COD and TP removal efficiency were 79.77% and 78.62 %; 89.22% and 79.4%; 96.0% and 64% using DSA-Cl2 (Ti/Ru0,3Ti0,7O2) and DSA-O2 (Ti/IrO2/Ta2O5), respectively. Moreover, a complete discoloration and disinfection were achieved. Electrochemical oxidation test at best operating conditions gave energy consumption and specific energy consumption values for TOC, COD and TP using DSA-Cl2 and DSA-O2 of 24.5 and 26.5 kW h m–3, 27.1 and 28.9 kW h kgTOC–1, 7.14 and 6.88 kW h kgCOD–1 and, 1531.25 and 1104.17 kW h kgTP–1, respectively.
[1] Bustillo-Lecompte C.F., Mehrvar M., 2015. J. Environ. Manag. 161 287–302.
[2] US Environmental Protection Agency. Final Rule; Federal Register. 2004;69(173):54476–54555.
[3] Bustillo-Lecompte, C.F., Mehrvar, M., 2017. J. Clean. Prod. 141, 278–289.
Figure 1