Combined energy and removal efficiency of electrochemical wastewater treatment for leather industry

Abstract

Wastewater treatment and low energy consumption are two important environmental issues. In this work, a challenging treatment problem of leather industry wastewater is chosen as a case study to combine the issue of energy and treatment efficiency using a common denominator of oxygen demand. Tannery wastewater is known for its highly contaminated structure. Due to the acidic nature of the effluent, conventional methods do not perform well for tannery wastes, so an electrochemical process is a reasonable approach for the problem. Since electrochemical reactors use electricity, the energy efficiency of such plants should be simultaneously considered for overall sustainability. The electricity production partially results in carbon footprints and consumption of oxygen. In this work, the electrochemical treatment results were assessed through total chromium removal and chemical oxygen demand (COD) reduction. As expected, the electrochemical process parameters (reactor types, electrode variations, electrical current density, etc.) as well as the process duration affect the treatment performance. Meanwhile, these parameters also affect energy consumption. Although the energy consumption has been previously researched, (i) “the oxygen equivalent of the required energy” and (ii) “oxygen demand reduction of the wastewater” were not considered simultaneously before. In this work, the used energy is represented in terms of consumed oxygen at the current electric generation profile (in Turkey and in USA). It is argued that, as the consumed energy increases, the consumed oxygen also increases, causing a separate pollution (in terms of COD and other indirect effects). The combined production/consumption profiles as well as energy efficient parameter settings are investigated by experiments. It is observed that the combined oxygen demand (i.e. reduced on the treatment side and increased at the power generation side) starts exhibiting an overall increase at early treatment stages. For example, at an electrical current setting of 20 mA/cm2 and pH of 7, using electro-coagulation with aluminum electrodes, treatment reaches to 82% COD removal (from 1024 mg/L to 180.71 mg/L) at 8.33 kWh/m3, corresponding to consuming 3.23 g/L oxygen at average energy production settings in Turkey. The oxygen consumption using electro-Fenton exhibits similar results. The necessity of renewable energy utilization or an earlier treatment termination is concluded.