National Raisin Co. of Fowler, Calif., had good news and bad news. The good news was that sales and production of its Champion Raisin products were rapidly increasing. The bad news was that their wastewater costs were increasing just as fast.
Fortunately, a new membrane filtration system manufactured by PCI Membrane Systems, Inc. allowed National Raisin to not only cut their wastewater costs, but they have also opened up a potentially lucrative source of income.
Processing about 50,000 tons of raisins per year, National Raisin is the second largest processor and distributor of raisins in the U.S. The company generates between 60,000-80,000 gal/day of wastewater, primarily from the raisin-washing process. Raisins have a fine coating of dust blown onto them from the sandy soil of the Central Valley of California, and this dust needs to be washed off prior to packaging.
If this dust was the only problem, simple settling tanks or filters could eliminate it and the wash water cold be re-used for irrigation and other purposes, or disposed of at the local wastewater plant at a very minimal cost.
The real problem with the wash water is that, when it washes away the dust on the raisins, some of the sugar in the raisins also dissolves into the water. The wash water now has sugar in it, and that creates a high biological oxygen demand (BOD).
Land application, or irrigation, of water with BOD requires a special permit that can be time-consuming and expensive to obtain. In addition, more paperwork and ongoing regulatory review are necessary to maintain permits, and regulations for land applications in California are getting tighter all the time. This regulation is considered necessary to maintain general groundwater quality. Offensive odors can also be produced when sugar-laden wash water is disposed of via land application.
Alternative approach
For all the reason listed previously, the Bedrosian family, owners of the National Raisin Co., wanted to find an alternative to land applications. The family has extensive roots in the Fowler area, near the raisin processing plant, and are involved in local civic activities.
"This is a small town," said Ernie Bedrosian, president of National Raisin Co. "There are only four to five thousand people and we know just about everybody. There are cheaper ways to dispose of the raisin wash water, but we wanted to do the right thing for the community."
As time-consuming and environmentally unfriendly as land applications can be, sending the water to the local municipal wastewater plant, was not an attractive alternative. It costs more to process water with high BOD, so municipal wastewater plants charge more for BOD-related services--about $50,000 per month in National Raisin's case.
Not surprisingly, the packer decided it would be more economical to remove the sugar from the wash water. This would reduce municipal wastewater charges and eliminate the environmental concerns that came with land applications.
So, the decision to remove sugar from the wash water before disposal was easy. Making it even more so was the fact that, if the grape sugar concentration in the wash water was high enough, it could be sold to local distilleries to make grape alcohol.
This alcohol, in turn, is used to make fortified wines like sherry and port, and to make brandy. One local distillery said it would be interested in purchasing the water if it was a minimum of 8% sugar. This meant that the sugar content had to be doubled or quadrupled from the 2-4% which was normal in the raisin wash water.
The more difficult decision then would be to decide how best to concentrate the raisin wash water, because there were several options.
Prefiltration unnecessary
The most logical choices were evaporation or reverse osmosis (RO). Even state-of-the-art, high-efficiency evaporators, operating under vacuum, require
a lot of energy to boil away enough wash water to concentrate the sugar to the desired level. RO, on the other hand, only requires enough energy to generate pressure that forces water through a membrane that retains and concentrates the sugar. Thus, RO seemed the most likely approach.
Plant engineer John Minazzoli said they considered spiral RO elements, which are relatively inexpensive and require the least floor space. Bust dust and other grape solids--bits of stem and skin--were found to block the small channels in these spiral elements. Conventional pre-filters used upstream from the spiral elements also became blocked.
At this point, Minazzoli raised the question with Dr. Jatal Mannapperuma from the California Institute of Food & Agricultural Research (CIFAR). Dr. Mannapperuma consults with growers all over California and operates a mobile trailer that houses several membrane options for experimentation.
First, they tried 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 unfortunately, the dust flowing through the ceramic membrane eroded the membrane surface, reducing its life.
At this point, Dr. Mannapperuma recommended evaluating polymeric tubular RO membranes, and Peter Allan, a sales engineer for PCI Membrane Systems was brought in to consult.
The tubular channels in PCI RO membranes do not require prefiltration and the polymer membrane surface is more resistant to abrasion then inert materials such as ceramics. Essentially, National Raisin could accomplish their goal of sugar concentration in one step instead of two.
The initial trial in the CIFAR trailer proved that the tubular RO concentrated the sugar up to the 8-10% levels required by the distillery and additional scale up trials were then arranged directly with PCI's Allan to determine the size of the final system. The larger scale trials were also successful, and a full-scale system was installed.
Once the concentrated sugar water had been removed, the remaining water was actually lower in dissolved solids than the well water that feeds the plant. Therefore, it can be re-used in the raisin washing process or sent to irrigate nearby vineyards without any concerns about odor or soil contamination.
The membrane filtration plant installed at the National Raisin Co. incorporates 80 Model B1 filtration modules and is designed so that it can easily be expanded 50% to 120 modules to meet increased demand in the future. Membrane life is guaranteed for a year, and the first set was replaced after a year of use.
"PCI Membranes was very good to work with," said Minazzoli. "Peter Allan and the people at their main plant were very knowledgeable and helpful. Even after the system was installed, they were there to assist us whenever we needed them."
National Raisin is continuing their program of optimizing RO use for maximum return on their investment. Demand for grape sugar water tends to fluctuate--even dropping to zero occasionally, but the savings on National Raisin's sewer bill alone amounts to $300,000 per year.
