Ultrafiltration has been the state-of-the-art technology for several years now in the field of polishing juices. The cloudy juice is separated into clear permeate and cloudy retentate. The retentate becomes increasingly enriched with solids, and thus determines the operating time and economic efficiency of the UF installation.

The retentate can be processed in two ways by means of separators and decanters:

1. Secondary-stream separation of the retentate circle by means of self-cleaning separator

The circulation capacity of the retentate cycle is, depending on the particular installation, five to 25 times greater than the flux rate (permeate flow). The retentate becomes increasingly enriched with solids and, above a certain concentration factor, requires subsequent batch processing for recovering the valuable juice.

The cleaning cycles are determined by layer building on the membranes and also the concentration. The cleaning intervals and permeate performance can be improved significantly if a self-cleaning separator is used in the secondary stream to the retentate cycle. For instance, if a secondary stream cycle of between 10 and 20% of the circulation cycle of the retentate is used, solids are continuously separated and excessive concentration is prevented.

A similar effect can be achieved if the separator is installed in the feed of the balance tank in the UF installation and if the entire quantity of juice is preclarified. In this case, the separator throughput has to be adapted to the permeate throughput. A smaller centrifuge can be used for secondary-stream separation in the retentate cycle.

2. Batch concentration of retentate by means of decanters

Batch processing of the retentate, depending on the UF installation type, achieves concentration of centrifugally separable solids of between 50 and 60% (by volume). The retentate is diluted to approx. 20% (by volume) of centrifugally separable solids; it is then heated and decanted when hot.