Grower 101: Reverse Osmosis --The Pros and Cons
Find out if this water-filtering process could benefit your business.
Osmosis is the passing of a liquid through a membrane from a
lesser concentration to a greater concentration. Eventually, both liquids would
be of equal concentrations. Figure 1, page 48, demonstrates how this reaction
takes place. A good example of osmosis is how plants uptake water.
Reverse osmosis (RO) puts pressure on the greater
concentrated liquid and forces it through the membrane to the
lesser-concentrated liquid, hence the term, reverse osmosis. The membrane traps
particles and impurities down to 0.0009 micron, and the effluent or permeate
water is very clean and free from impurities. Figure 1 demonstrates how this
works. So at its most basic level, reverse osmosis filters impurities from a
liquid, namely water.
By starting with water that is free from impurities and
minerals, RO water can help make growing more calculable, since the water
quality is constant. Nutrients can be better controlled without having to worry
about what is in the water source at the beginning of the irrigation process.
Contaminants in the water source, such as iron, manganese, calcium, magnesium
and chlorine, can react with the nutrients and cause problems with fertilizer
Growing Without Contaminants
Growing with good water and adding nutrients at the grower's
discretion make growing more calculable. Not having iron, manganese, calcium or
magnesium makes setting up the EC of a nutrient solution easier since the
fertilizers are not reacting with the impurities in the water. The only
nutrient minerals that are added to the irrigation water are ones that are an
advantage to the plant. Total dissolved solids (TDS) reveals the conductivity
of water, but EC measures the actual electrical production of these minerals.
Water that is pure will not conduct electricity or have an EC and TDS reading.
The implementation of RO can be quite ambiguous, and cost
considerations are the most objectionable. Plus, wastewater is another downside
of this technology. There are two types of membranes that are used in RO
designs, and each has their uses and limits. The cellulose triacetate membrane
does not offer the rejection rate of thin film composite (TFC) membranes.
However, chlorine can break down TFC membranes faster and cause premature
failure of the system, so a carbon pretreatment is recommended to remove
Pretreatment of the influent water must be considered or the
membranes will foul out. Failure to address the pretreatment issue can cause
extensive maintenance and labor costs. Without pretreatment, membranes become
clogged and need to be replaced or cleaned sooner than normal. When used with
softened water, the membranes last longer, since other minerals are removed
before reaching the system. The use of a softener creates more cost and labor
when utilizing RO but must be considered to offset the higher cost of membrane
replacement, labor and downtime of the water treatment system.
Another factor to consider when using RO is the wastewater that
is created. In general, depending on pressure and size of the unit, 4 gal. of
water are required to produce 1 gal. of RO water. This would not be a good
treatment option in areas where water use is restricted. There are newer
systems on the market that are more efficient and produce less waste, but for
the amount of water a grower needs, these systems have a long way to go before
being adopted within the industry.
RO water is very aggressive, since it does not contain any
minerals, and can be very corrosive, especially to metal piping. RO water
should never be run through galvanized or copper pipes because they will be
destroyed by the water's aggressive nature. Pipes, tubing, drippers, misters
and foggers must be able to stand up to RO water.
System Sizes and Costs
The size of an RO water system varies greatly from
manufacturer to manufacturer. There are a few factors to consider when looking
at cost, namely how much water is needed per day. Most systems are rated on the
number of gallons per day (GPD) that the system can produce. Residential
systems can produce anywhere from 15-50 GPD without a pressure pump, the cost
ranges from $200-600 and the rejection rates are acceptable. A system with a
pressure pump can produce 75-150 GPD for $800-1,000, but their rejection rates
are even greater because of the extra pressure.
Commercial and industrial RO systems escalate in price, but
so does the volume of water being produced and the need for high-pressure pumps
and large storage tanks. These systems can produce volumes of water ranging
from a few thousand gallons per day to more than several hundred thousand per
day. The costs vary from application to application and manufacturer to
manufacturer. Keep in mind the storage areas for large amounts of processed water
need to be addressed, and areas built for tanks increase the cost of the
Most growers, especially new or inexperienced ones, would
benefit from most any type of water treatment that removes impurities. Removal
of contaminants that promote microbial growth, such as iron and bacteria, can
make a huge difference in growing, and the impact would be seen from reduced
root rot problems, such as Pythium or fungal diseases that are introduced by
water-borne bacteria. Propagation areas seem to be where most growers target
their water treatment strategies, but many are incorporating treatment in other
areas as well.
RO would be a great asset to any grower, as nutrient
solutions could be controlled and calculated for a variety of crops and
duplicated for consistency. RO is usually a hefty initial investment, but if
incorporated correctly, it can have a fast return on investment.