The addition of electrostatic precipitator ash to forest industrial wastewater treatments increases the oxygen transfer rate and saves the energy needed for aeration

Abstract

More than 30 m3 of water is needed for each metric tonne of paper. Most of the pulp and paper mills use an aerobic biological wastewater treatment. Aeration is the single most energy demanding process in the wastewater treatment plant; therefore it is desirable to control energy consumption when energy needs to be conserved. The aeration efficiency in pulp and paper industrial effluents is often low.  80 – 90% of added oxygen passes through the water volume and leaves with the off-gases. To increase the aeration efficiency, the process needs the oxygen transfer rate from the air bubble into the water to increase. The bubble size affects the oxygen transfer rate. In small bubbles, with a larger area/volume ratio the oxygen transfer rate is often fast and more efficient. The wastewater properties affect the bubble size. A decreased surface tension ensures smaller bubbles. The aim of this study was to show that a waste material, precipitator ash, could be used to decrease wastewater surface tension and thereby increase the oxygen transfer rate. The results from laboratory aeration trials show that the oxygen transfer rate coefficient (K_La) increases by 25% when small concentrations of electrostatic precipitator ash, from a flue gas treatment, were added to the process effluent.