The MPP Process (a one-stage CCA treatment and accelerated fixation process) generates ~ 250 litres of kickback per cubic metre of timber treated. Composition of kickback is mainly wood sugars and traces of copper, chromium and arsenic. In pilot plant scale trials, MPP process kickback was filtered through a Reverse Osmosis (RO) membrane of a type found from earlier small-scale trials to be the most effective for this operation. To reduce the total organic carbon (TOC) from 2000 ppm to around 100 ppm in the shortest time, preliminary pilot plant trials showed an optimum operating temperature of 20°C and pressure of 25 bar . Using this schedule, a final trial was undertaken in which 300 litres of kickback was separated into 275 litres of permeate and 25 litres of retentate. TOC was reduced from around 2000 ppm down to 98-108 ppm and CCA to less than 1 ppm in the permeate. To determine whether the permeate could be used as make-up water in a treatment plant operation, CCA solutions (1.6% oxide) were prepared using 0, 50 and 100% permeate containing 0, 65.6 and 131.2 ppm TOC respectively. After heating to 75°C and cooling to room temperature daily for 21 days, there were no differences in the amount of CCA sludge produced in each solution, indicating that permeate could be safely used as make-up water at the treatment plant. Impregnating retentate into MPP-treated material was investigated as an option for its disposal.

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