NANOFILTRATION OF CAUSTIC AND ACIDIC CLEANING SOLUTIONS WITH HIGH CODPART 2 - RECYCLING OF HNO3

Nanofiltration is suitable not only for the recovery of alkaline clean ing solutions (as described in Part 1 of this work), but also, in prin cipal for the recycling of acidic cleaning agents. In the present stud y the nanofiltration of two spent HNO3 solutions, coming from the dair y industry, was investigated. One solution had a rather high COD of 18 .5 kg/m(3), while the other had 1.8 kg/m(3). The highly contaminated a cidic solution had been used before an alkaline cleaning and it was sh own that the nitric acid could not be recycled for economic reasons. D uring the cleaning procedure a large portion of mineral compounds was dissolved, which led to a higher osmotic pressure of the spent solutio n. Besides having a low flux, the permeate quality (COD = 4.45 kg/m(3) ) was also not satisfactory. In the case of the second polluted acid, which was applied (as in the most cases) after an alkaline cleaning st ep, a high retention (COD) of approx. 93% was achieved. The COD in the permeate totalled 0.45 kg/m(3). The average flux was found to be 1.2 10(-5) m(3)/m(2)/s. The cross-flow velocity had no influence on the pe rformance of the nanofiltration, whereas the flux climbed linearly wit h increasing pressure. (C) 1998 Elsevier Science Limited. All rights r eserved.