Since the product is sun-dried outdoors, the raisins have a fine coating of dust blown onto them from the sandy soil found in the Central Valley. This needs to be washed off before packaging.

If dust were the only problem, simple settling tanks or filters would eliminate it, and the raisins’ wash water could be reused for irrigation or sent to the local wastewater plant at very minimal cost. However, the real problem with the wash water is as it washes away the dust on the raisins, it picks up some of the sugar in the raisins. The dissolved sugars create a high biological oxygen demand (BOD) that increases the cost of disposal, estimated to be about $50,000 per month at the local municipal water treatment plant. Plus, the disposal requires extensive paperwork for permits, ongoing regulatory review and a variety of expensive steps to process the wastewater.

“This is a small town, and we know just about everybody,” says President Ernie Bedrosian, the eldest of the three brothers. “There are cheaper ways to dispose of the raisin wash water, but we wanted to do the right thing for the community.”

The decision to remove sugar from the wash water before disposal was easy. And there was an added benefit: If the grape sugar concentration in the wash water were high enough, it could be sold to local distilleries as an ingredient for grape alcohol.

One local distillery wanted a minimum of 8 percent sugar. Therefore, the sugar content had to be doubled or quadrupled from the 2 to 4 percent normally contained in the raisin wash water. Two methods to concentrate the sugar level, evaporation and reverse osmosis (RO) were considered. RO was chosen because of its much lower energy requirements.

National Raisin’s plant engineer, John Minazzoli, first considered spiral RO elements, which are relatively inexpensive and require the least amount of floor space. “However, dust and grape solids [bits of stems and skins] were blocking the small channels in these spiral elements. Conventional pre-filters used upstream from the spiral elements also became blocked,” says Minazzoli.

Ultimately, National Raisin was introduced to Peter Allan, now with Ohio-based Membrane Specialists LLC, the exclusive distributor of PCI Membrane products in the Americas. PCI has several filtration solutions including RO.

PCI tubular RO membranes have 1/2-in. tubular channels, which don’t require pre-filtration, and a polymer membrane surface that is more resistant to abrasion than inert materials in pre-filters. With these membranes, National Raisin could accomplish its goal of sugar concentration in one step instead of two.

Pilot trials proved the tubular RO membranes would concentrate the sugar up to the required 8 to 10 percent levels for processing by a distillery. After initial trials were completed, National Raisin decided to install 80 B1 PCI Membrane modules from Xylem, fitted with AFC99 tubular RO membranes in a continuous system, with the expansion capability up to 120 modules for future capacity.

Once the full system was running, and the concentrated sugar water (called “retentate”) had been removed, the remaining water (called the “permeate”) was actually lower in dissolved solids than the well water that feeds National Raisin’s plant. Therefore, this water is available for reuse, or it can be used for irrigation without any concerns about odor or soil contamination.

The original cost of the installation was recouped in less than two years. Demand for grape sugar water tends to fluctuate, but the savings on National Raisin’s wastewater disposal bill alone amount to around $300,000 per year. The disposal savings by itself is enough to keep the system’s ROI within the original plan of three years. Any additional income from selling the concentrated sugar water to distilleries is an added bonus.  

For more information: Darren Reed, darren.reed@xyleminc.com; Peter Allan, peter.allan@membranespecialists.com