Unexpected benefits arise from proper environmental cleaning water disposal.


National Raisin of Fowler, Calif., was faced with an interesting predicament. Sales and production of its products were rapidly increasing, but so were its wastewater costs. 

To reap the benefits of the increase in sales and production, National Raisin needed to improve its method of cleaning the raisins. Fortunately, thanks to a reverse osmosis membrane system, the company has been able to cut its wastewater costs and open a potentially lucrative source of additional income.


RO membrane
Close-up arrangement of RO membranes


The Company

Fowler, Calif., is an agricultural community with lush grape vineyards and expansive farmland. With a population of less than 6,000, it is where the majority of raisins are produced.

The National Raisin Company is one of the largest independent raisin processors in the industry. Its 60-acre, state-of-the-art facility in the heart of the fertile San Joaquin Valley processes more than 100 million pounds of raisins annually.

Brothers Ernest, Krikor and Kenneth Bedrosian founded National Raisin Company in 1969 with a mission to achieve the highest possible quality standards of any processor within California’s raisin industry. Its raisins are packaged with the Champion Raisin brand.


The Cleaning Process

Before shipment, the raisins undergo a 35-cycle cleaning process (the Laser Kleen process) and a complete, thorough, documented inspection by the company’s quality assurance team. 

As an added quality guarantee, National Raisin has contracted with the U.S. Department of Agriculture (USDA) for full-time quality inspection services to work jointly to further verify the company’s standard as the “World’s Cleanest Raisin.”


The Wastewater Disposal Problem

Cleaning means wastewater. National Raisin realized that its wastewater costs resulting from cleaning the raisins were rising rapidly. In an area of the country in which water is not taken for granted, the company decided it needed to find ways to improve this important part of its business. 

In processing about 200 tons of raisins daily with its Laser Kleen systems, the company generated between 60,000 and 80,000 gallons per day of wastewater, primarily from the raisin washing process. 

Since the product is sun-dried outdoors, the raisins have a fine coating of dust blown onto them from the sandy, San Joaquin Valley soil. This must be washed off before packaging.

Regulations concerning water run-off and the need to protect much-needed local aquifers are strict. 

The Bedrosian family—long-time residents of the area and solid, corporate citizens—wanted to do everything possible to provide for a sustainable business. This meant that standard solutions for disposing of the wastewater would not suffice.

If dust were the only problem, simple settling tanks or filters could eliminate it, and the raisins’ wash water could be re-used for irrigation and other purposes, or disposed of at the local wastewater plant at a minimal cost. However, 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. 

Wash water that contains dissolved sugars creates a high biological oxygen demand (BOD) that increases the cost of disposal. Extensive paperwork for permits, ongoing regulatory review and expensive steps are required to process this type of wastewater.“This is a small town, and we know just about everybody,” said 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.”

For all these reasons, the Bedrosian family wanted to find an alternative to land application of its wastewater and side products. Trucking the wastewater to the local municipal treatment plant was not an attractive answer. The municipal plant charges a premium to process water with high BOD. In National Raisin’s situation, the cost would have been about $50,000 per month, or more.


stacked circulation
RO membranes in a stacked circulation arrangement


Reverse Osmosis, Tubular Membranes

If National Raisin could remove the sugar from the wastewater, it could return the water to the land. This plan would reduce municipal wastewater charges and eliminate the environmental concerns associated with land application of BOD effluent.

The decision to remove sugar from the wash water before disposal was easy. In fact, it would be an added benefit. If the grape-sugar concentration in the wash water were high enough, then it could be sold to local distilleries as an ingredient for grape alcohol. This alcohol is used to make fortified wines such as sherry, port and brandy. One local distillery said it would be interested in purchasing the wash water if it was a minimum of 8 percent sugar. This meant that the sugar content had to be doubled or quadrupled from the 2 to 4 percent normally released in the raisin wash water. 

The more difficult decision was choosing the best process to concentrate the raisin wash water. Several were available. The two most logical choices were using evaporation or using reverse osmosis (RO). Unfortunately, even state-of-the-art, high-efficiency evaporators operating under vacuum require large amounts 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 to force water through a filter membrane. This offers many benefits either in place of conventional evaporation technology or in conjunction with it, including:

  • Low sugar content can be concentrated with reverses osmosis to a higher level that may be acceptable for other uses without the need for an evaporator. 
  • For higher sugar concentrations, reverse osmosis can provide an energy efficient pre-concentration to reduce the size and cost of the final evaporation system. 

National Raisin’s plant engineer, John Minazzoli, first considered spiral RO elements that are relatively inexpensive and require the least amount of floor space. However, dust and other grape solids (bits of stems and skins) were blocking the small channels in these spiral elements. 

Conventional pre-filters used upstream of the spiral elements also became blocked. 

Ultimately, through contacts with the California Institute of Food and Agricultural Research (CIFAR) in Davis, Calif., and others, National Raisin was introduced to a representative of an Ohio-based membrane distributor who suggested tubular RO membranes. 

The important advantage tubular RO membranes provided were ½-inch tubular channels that do not require pre-filtration, and the polymer membrane surface is more resistant to abrasion than inert materials in pre-filters, such as ceramic membranes. With the tubular membranes, National Raisin could accomplish its goal of sugar concentration in one step instead of two. Pilot trials then proved that 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, it was decided to install 80 modules from the company fitted with tubular RO membranes in a continuous system, with expansion possible up to 120 modules in the future as National Raisin increases its capacity. 


Close-up arrangement of RO membranes


Re-Use and Increased Profit

Once the full system was running and the concentrated sugar water (the retentate) had been removed, the remaining water (the permeate) was actually lower in dissolved solids than the well water that feeds National Raisin’s plant. Therefore, this water was available for re-use or could be sent to irrigate nearby vineyards without concerns about odor or soil contamination. National Raisin is continuing its program of optimizing RO use for maximum return on its investment. 

The original cost of the installation was recouped in less than 2 years. Demand for grape sugar water tends to fluctuate (even dropping to zero occasionally), but the savings on National Raisin’s wastewater disposal bill alone amounted to about $300,000 per year. The disposal savings by themselves were enough to keep the system’s return on investment within the original plan of 3 years. Any additional income from selling the concentrated sugar water to distilleries is an added bonus. 

National Raisin Company found a smarter way to solve its wastewater problem. By doing the right thing for the community and protecting the environment, National Raisin also enjoyed unexpected business benefits.