Treatment of a Specialty Paper Mill Wastewater Using a Pilot-scale Pulsed Electrocoagulation Unit

Abstract

We used a pilot-scale pulsed electrocoagulation (EC) system capable of treating 0.5~1m^3 h^(-1), with a hydraulic retention time (HRT) of 2.17~6.5 min, and consisting of a homogenizer tank, a pulsed EC reactor, an aerator, a flocculation tank, and a sedimentation tank to treat specialty paper mill wastewater from central Taiwan. The mill has a daily production of 5 t d^(-1), and discharges 1300m^3 d^(-1) of wastewater. It produces mainly filter papers for air conditioners and dehumidifiers, medical industrial paper, home decoration paper, special wrapping and packaging papers, and other specialty papers. A wastewater stream from making black-colored specialty paper was treated in this study. The wastewater was pumped to a 1-m^3 homogenizing tank to even out fluctuations in water quality before entering the treatment unit. The controlled parameters of the pulsed EC unit included the use of either iron or aluminum electrodes, current density, and HRT (flow rate). The wastewater quality variables examined before and after the treatment include electrical conductivity, suspended solids (SS), chemical oxygen demand (COD), and true color. The results indicated that electrical conductivity decreased with increasing current density and HRT; under all current densities, SS removal appeared to have no correlation with the flow rate, and a minimum of 85% SS removal was observed. Removal of COD was dependent on the electrode materials. When iron electrodes were used, due to the low COD of the original wastewater, the post-treatment effluent tended to have large amounts of ferric ions which interfered with the COD measurements. Aluminum electrodes, on the other hand, achieved at least a 40% COD removal at lower current densities. With both electrodes, the higher the current density and the longer the HRT, the greater the true color removal rates became. Under the best operational conditions (a current density of 106.7Am^(-2), and an HRT of 3.25 min) using aluminum electrodes achieved removals of 25.4, 97.14, 76.47 and 70.09% of electrical conductivity, SS, COD, and true color, respectively. The current dissolved air flotation unit costs NT$2.68 (US$0.0812) per ton of wastewater to operate, while the pulsed EC unit costs NT$2.42 (US$0.0733) per ton of wastewater. A proposed 5-stage mechanism of the EC system is also discussed.