Process and Operations

Waste not

A new membrane filtration system helps a raisin company reduce its wastewater costs and increase its revenues

For Fowler, Calif.-based National Raisin Company, the good news was that sales of its Champion Raisin products were rising quickly. And the bad? The company's wastewater costs were rising just as quickly.


A new membrane filtration system from PCI Membrane Systems, Inc., helped to rectify the situation, reducing wastewater costs and providing National Raisin with additional source of income.


Processing some 50,000 tons of product per year, National Raisin is the second largest processor of its kind in the U.S. The company also generates between 60,000 and 80,000 gallons per day of wastewater, primarily from washing dust off the raisins prior to packaging.


If the dust -- which originates from the region's sandy soil -- had been the only problem, simple settling tanks or filters could have been used to eliminate it, with resulting wash water re-used for irrigation or dispatched at minimal cost to a local wastewater plant.  However, the problem was that sugar from the raisins would have dissolved into the wastewater, creating a high biological demand (BOD).  Land application (irrigation) of water with BOD requires a special permit that can be time-consuming and expensive to obtain. Further, regulations for land application in California are getting tighter all the time.  The regulations are considered necessary to maintain general groundwater quality in the area.


For these reasons, the Bedrosian family, which own National Raisin and are commited to protecting the local environment, sought an alternative to land applications. As time-consuming and environmentally unfriendly as land application would have been, sending the water to the local municipal wastewater plant presented problems of its own.  Due to the higher cost of processing water with high BOD, National Raisin would have been required to pay a $50,000- per-month premium.


Hence, the company decided it was more economical to remove the sugar from the wash water before disposal, thereby reducing related charges and eliminating environmental concerns associated with land application.


Additional income stream


The Bedrosians also discovered that grape sugar concentration from the wash water could be sold to local distilleries to make grape alcohol, providing the concentrations were high enough. One local distillery expressed interest in purchasing the water from National Raisin if it was a minimum of 8 percent sugar, which meant the sugar content in the wash water needed to be doubled or quadrupled.


The question was how best to concentrate the raisin wash water.


The most logical choices were evaporation or reverse osmosis (RO). Even high-efficiency evaporators operating under vacuum would have required large quantities of energy to boil away sufficient amounts of wash water to achieve the desired sugar concentrate level. RO, on the other hand, only required enough energy to generate the pressure necessary to force water through a membrane that retained and concentrated the sugar. Thus, RO seemed a more reasonable approach.


Plant Engineer John Minazzoli says that spiral RO elements were considered, since they are relatively inexpensive and require minimal floor space.  However, it was discovered that dust and other grape solids (bits of stems and skins) blocked the small channels in these spiral elements.  Conventional pre-filters located upstream from the spiral elements likewise became blocked. 


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.



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