Influence of interelectrode distances in electrocoagulation: is there any possibility and advantages to operate at micro-distances with low-conductivity effluents?

Abstract

It has been proposed for the first time to investigate the possibility to implement micro-inter-electrode distances in electrocoagulation (EC) in order to improve both the treatment and energy efficiencies compared to conventional EC cells with centimetric distances. The study has been performed in a microfluidic monopolar flow-by filter-press cell for the treatment of simulated and real low-conductivity (0.5-1 mS cm^-1) laundry wastewaters. The influences of interelectrode distance (d_elec) (100-10,000 μm), applied current density (j_app) (10-200 mA cm^-2), and types of anode materials (iron, aluminium and stainless steel) have been studied. The removal of representative organic pollutant (i.e., paracetamol at 15 mg L^-1) as well as of total organic carbon (TOC) content (312 mg-C L^-1) from actual wastewater was noticed, including at micro-distances. Optimal treatment capacities were obtained with d_elec of 0.5 mm (57% TOC removed), 3 mm (58% TOC removed) and 10 mm (41% TOC removed) and with j_app of 70 mA cm^-2, 40 mA cm^-2 and 20 mA cm^-2 respectively, using stainless steel anode. It led to reduced energy requirement at micro-distances (16 kWh g-TOC^-1 at 500 μm) compared to millimetric gap (19 kWh g-TOC^-1 at 3 mm, 40 kWh g-TOC^-1 at 10 mm). Contrastingly, more sludge was generated with micrometric distance (172 g-sludge g-TOC^-1 at 500 μm) compared to larger gaps (95 g-sludge g-TOC^-1 at 3 mm, 87 g-sludge g-TOC^-1 at 10 mm) due to higher optimal j_app at low distances. The efficiency was maximal with an aluminium electrode, but this anode remained inapplicable with micro-distances using the current reactor design, given the high sludge production between the cathode and anode.