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Modern pulp mills can produce biofuels and value-added bio-products, such as lignin products. However, these bio-products are often present in solutions that are too dilute to be separated in a cost-efficient manner, and concentration by filtration is difficult due to their high fouling properties. In this research, challenging liquor, tannin-containing spruce bark extract, was concentrated by forward osmosis using soda ash from the soda recovery boiler as a draw solution. Soda ash, which consists of about 90% sodium sulphate, could be considered as a cost-free draw solution, since part of it needs to be discharged daily in order to prevent enrichment of potassium and chloride in the chemical circulation of pulp mills. Draw properties of soda ash solution were compared with the properties of pure salts, namely sodium sulphate and sodium chloride. Soda ash generated the same osmotic pressure and similar flux than sodium sulphate, but was a weaker draw solution compared with sodium chloride. When the draw solution concentration was 20% at 40°C, the flux was 16 and 13 LMH with soda ash and sodium sulphate, respectively. Using FO, water recovery over 80% could be reached, which is double compared with water recovery obtained by nanofiltration. However, the salt concentration of the feed solution was increased due to the reverse salt flux of the draw solution, which may hinder the further processing of tannin-containing spruce bark extract.

Internal concentration polarization (ICP) severely limits water flux performance in forward osmosis (FO). We investigated the use of ultrasound to mitigate ICP. Various parameters affecting the performance of the novel ultrasonically-assisted FO were studied, such as ultrasonic frequency and constant versus pulsed operation. With either deionized water or polyphenolic tannin solution as the feed and sodium sulphate as the draw solution, the water flux was nearly doubled for a thin film composite FO membrane upon the application of a 20 kHz ultrasound, with stronger enhancement achieved when the ultrasound was applied to the support layer of the membrane. High frequencies of 573 and 1136 kHz had much weaker effects. Pulsed application of ultrasound can significantly reduce the energy consumption of sonication. For the first time, the current study provides compelling evidence that ultrasonic vibrations applied to porous support structure of an FO membrane is highly effective in mitigating ICP.

This study focused on membrane filtration of neutralized pond water, which may be necessary when good quality water is required for hydrometallurgical processes. Neutralized mine water can still have fairly high metal and sulphate levels, which can hinder discharge and reuse possibilities. Both nanofiltration and reverse osmosis are effective in removing metals and sulphate, but scaling can be a severe problem. Microfiltration as a pre-treatment method, although meant for particle removal, seemed to decrease the amount of scalants, thus delayed scaling on the membrane surface and increasing water recovery for both nanofiltration and reverse osmosis. It is possible that the presence of particles in the feed water promoted crystal growth in the turbulent flow and caused the removal of dissolved constituents. Alternatively, supersaturation could have occurred, allowing microfiltration to remove the scalants as particles. The Liqum sensor indicated that redox values started to increase again just before scaling began due to precipitation in the supersaturated membrane concentrate solution. Thus, the sensor seemed to provide real time, in-situ, early-stage scaling warning.

Results from an experimental study of electric and/or ultrasonic field assisted filtration are presented. Both electric and ultrasonic fields can reduce fouling of the filtration medium and have significant influence on filtration capacity. The extent of filtration improvement is affected mostly by particle size, surface charge, acoustic frequency, and field strengths. Theoretical examinations of the use of electric field and/or ultrasonic field to enhance filtration efficiency were laid out. Some aspects regarding orthokinetic interaction in acoustic agglomeration were considered. Energy consumptions of the filtrations of different suspensions used in experiments were also determined. For pyrite suspension the best results with ultrasound were obtained when filtration was done as follows: 15‐s negative pressure filtration, 15‐s negative pressure filtration with 22‐kHz ultrasonic burst (0.5 s) and 20‐s drying with negative pressure. In this case the moisture content of cake was 7% while in the reference c...