Nanofiltration of caustic and acidic cleaning solutions with high COD. 2. Recycling of HNO3
1998
Nanofiltration is suitable not only for the recovery of alkaline cleaning solutions (as described in Part 1 of this work), but also, in principal, for the recycling of acidic cleaning agents. In the present study the nanofiltration of two spent HNO3 solutions, coming from the dairy industry, was investigated. One solution had a rather high COD of 18.5 kg/m3, while the other had 1.8 kg/m3. The highly contaminated acidic solution had been used before an alkaline cleaning and it was shown that the nitric acid could not be recycled for economic reasons. During the cleaning procedure a large portion of mineral compounds was dissolved, which led to a higher osmotic pressure of the spent solution. Besides having a low flux, the permeate quality (COD = 4.45 kg/m3) was also not satisfactory. In the case of the second polluted acid, which was applied (as in the most cases) after an alkaline cleaning step, a high retention (COD) of approx. 93% was achieved. The COD in the permeate totalled 0.45 kg/m3. The average flux was found to be 1.2 10(-5) m3/m2/s. The cross-flow velocity had no influence on the performance of the nanofiltration, whereas the flux climbed linearly with increasing pressure.
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