Electrooxidation treatment of pulp and paper mill circulating waters and wastewaters

Interest in water treatment by electrochemical methods has grown in recent years. Electrochemical oxidation has been applied particularly successfully to degrade different organic pollutants and disinfect drinking water.This study summarizes the effectiveness of the electrochemical oxidation technique in inactivating different primary biofilm forming paper mill bacteria as well as sulfide and organic material in pulp and paper mill wastewater in laboratory-scale batch experiments. Three different electrodes, boron-doped diamond (BDD), mixed metal oxide (MMO), and PbO₂, were employed as anodes. The impact on inactivation efficiency of parameters such as current density and initial pH or chloride concentration of synthetic paper machine water was studied. The electrochemical behavior of the electrodes was investigated by cyclic voltammetry with MMO, BDD, and PbO₂ electrodes in synthetic paper mill water as also with MMO and stainless steel electrodes with biocides. Some suggestions on the formation of different oxidants and oxidation mechanisms were also presented during the treatment.Aerobic paper mill bacteria species (Deinococcus geothermalis, Pseudoxanthomonas taiwanensis, and Meiothermus silvanus) were inactivated effectively (>2 log) at MMO electrodes by current density of 50mA/cm² and the time taken was 3min. Increasing current density and initial chloride concentration of paper mill water increased the inactivation rate of D. geothermalis. The inactivation order of different bacteria species was M. silvanus>P. taiwanensis>D. geothermalis. It was observed that inactivation was mainly due to the electrochemically generated chlorine/hypochlorite from chloride present in the water and also residual disinfection by chlorine/hypochlorite occurred.In real paper mill effluent treatment, sulfide oxidation was effective with all the different initial concentrations (almost 100% reduction, current density 42.9mA/cm²) and also anaerobic bacteria inactivation was observed (almost 90% reduction by chloride concentration of 164mg/L and current density of 42.9mA/cm² in 5min). Organic material removal was not as effective when comparing with other tested techniques, probably due to the relatively low treatment times.Cyclic voltammograms in synthetic paper mill water with stainless steel electrode showed that H₂O₂ could be degraded to radicals during the cathodic runs. This emphasizes strong potential of combined electrochemical treatment with this biocide in bacteria inactivation in paper mill environments.The results of the experiments showed that electrochemical oxidation could be used in paper mill waters for planktonic bacteria removal. Applying this technique together with some biocides can enhance the oxidation process, keeping the paper mill pipelines clean of slime formed by bacteria. Electrochemical oxidation is also a promising preliminary or tertiary treatment process in pulp and paper mill wastewater treatment, especially against refractory organic compounds which cannot be removed by biological processes.