By: David Pearson
National Raisin Company, Fowler, Calif., recently received
news that had dual implications. The good news was that sales and production of
its Champion Raisin products were rapidly increasing. The bad news was that
their wastewater costs from cleaning the raisins were increasing just as fast.
Fortunately, thanks to a new membrane filtration system, National Raisin not
only has been able to cut its wastewater costs, but it also has opened up a
potentially lucrative source of additional income.
While this particular solution currently is used only in
raisin processing, producers of any dried fruit (prunes, dried apricots, etc.),
as well as many other sugary waste streams, likely would find similar benefits.
Processing about 50,000 tons of raisins per year (200 tons
per day), National Raisin is the second largest processor and distributor of
raisins in the United States. However, the company generates between 60,000 and
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 in the
Central Valley of California, and this needs to be washed off before packaging.
If this dust was the only problem, simple settling tanks or
filters could eliminate it and the wash water could be re-used for irrigation
and other purposes, or disposed of at the local wastewater plant at very
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. The wash water now has sugar in it, creating a high
biological oxygen demand (BOD). To complicate matters, land application
(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
application in California are getting tighter all the time. This regulation is
considered necessary to maintain general groundwater quality. Offensive odors
also can be produced when sugar-laden wash water is disposed of via land
application.
For all of these reasons, the Bedrosian family, owners of
National Raisin Company, wanted to find an alternative to land applications of
their sludge. They were raised in the Fowler area near the raisin processing
plant and are involved in local civic activities. They take pride in their
community and the company always has been committed to protecting the local
environment.
"This is a small town," said president Ernie
Bedrosian, the eldest of three brothers who own the company. "There are
only four or 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
application of wastes can be, sending the water to the local municipal
wastewater plant is not an attractive alternative. Since it costs more to
process water with high BOD, municipal wastewater plants charge their customers
more?about $50,000 per month more in National Raisin's case.
Not surprisingly, the packer decided it would be more
economical to remove the sugar (the source of the BOD) from the wash water.
This plan would reduce municipal wastewater charges and eliminate the
environmental concerns that came with land application.
So, the decision to remove sugar from the wash water before
disposal was easy. In fact, as an added benefit, if the grape sugar
concentration in the wash water was high enough, it could be sold to local
distilleries to make grape alcohol. This type of alcohol is used to make
fortified wines such as sherry and port as well as brandy. One local distillery
said it would be interested in purchasing the wash water if it was a minimum of
eight percent sugar. This meant that the sugar content had to be doubled or
quadrupled from the two to four percent normally released in the raisin wash
water.
The more difficult decision would be to pick the best
process to concentrate the raisin wash water, since there were several options.
Wastewater Options
The most logical choices were using 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. On the other hand, reverse osmosis
only requires energy enough 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 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
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 of treatment
with Dr. Jatal Mannapperuma from the California Institute of Food and Agricultural
Research (CIFAR). 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, Mannapperuma recommended evaluating polymeric
tubular RO membranes and Peter Allan, sales engineer for PCI Membrane Systems,
Inc., was brought in. The 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. In other words, 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 membranes concentrated the sugar up to the 8 to 10 percent levels
required by the distillery, and additional scale up trials were then arranged
to determine the size of the final system. The larger scale trials also were
successful, and a full-scale system was installed.
Re-Use
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 that feeds the plant. Therefore, it can be
reused 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 National Raisin
Company incorporates 80 Model B1 filtration modules and is designed so that it
can be easily expanded 50 percent (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.
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 these savings on
Champion Raisin's sewer bill alone amount to around $300,000 per year. These
savings are enough to keep the system return-on-investment within the original
plan of 3 years. Any additional income that comes from selling the concentrated
sugar water to distilleries will just speed things up.
About The Author: David Pearson is the general manager of PCI Membranes, Inc., Cincinnati, OH.
