Waste not

At this point, Minazzoli consulted with Dr. Jatal Mannapperuma of the California Institute of Food and Agricultural Research (CIFAR). Besides consulting with California growers, Mannapperuma operates a mobile trailer housing several membranes for purposes of experimentation.

First, they attempted using tubular ceramic membrane ultrafiltration (UF) as the prefiltration prior to the spiral RO. The filtrate from the ceramic UF unit provided an acceptable feed for the spiral RO, but the dust flowing through the ceramic membrane eroded the membrane surface, reducing its life. 

Mannapperuma next recommended evaluating polymeric tubular RO membranes, and Peter Allan, sales engineer for PCI Membrane Systems was consulted.  The 1/2-in. tubular channels in PCI RO membranes do not require prefiltration, and the polymer membrane surface is more resistant to abrasion than inert materials such as ceramics. Hence, National Raisin could accomplish its goal of sugar concentration in one step rather than two.

The scale-up

Once an initial trial in the CIFAR trailer proved that the tubular RO concentrated the sugar to the levels prescibed by the distillery, additional scale-up trials were arranged directly with Allan to determine the size of the final system.  The larger-scale trials were also successful, and a full-scale system was subsequently installed.  

Once the concentrated sugar water (called "retentate" in membrane-filtration parlance) has been removed, the remaining water (called the "permeate") is actually lower in dissolved solids than the well water feeding the plant. As a result, it can be re-used in the raisin-washing process or sent to irrigate nearby vineyards without concern about odor or soil contamination.

The membrane filtration plant installed at National Raisin incorporates 80 Model B1 filtration modules and is designed so that it can be easily expanded to 120 modules to meet increased demand. Membrane life is guaranteed for a year, and the first set was replaced after a year of use.

National Raisin is continuing its program of optimizing RO use for maximum return on its investment.  Demand for grape sugar water tends to fluctuate , even dropping to zero occasionally, but the savings on Champion Raisin's sewer bill alone amount to about $300,000 per year -- enough to keep the system's return-on-investment within the originally scheduled three years. Any additional income that comes from selling the concentrated sugar water to distilleries will simply accelerate the return.