Grower 101: Reverse Osmosis –The Pros and Cons
Osmosis is the passing of a liquid through a membrane from alesser concentration to a greater concentration. Eventually, both liquids wouldbe of equal concentrations. Figure 1, page 48, demonstrates how this reactiontakes place. A good example of osmosis is how plants uptake water.
Reverse osmosis (RO) puts pressure on the greaterconcentrated liquid and forces it through the membrane to thelesser-concentrated liquid, hence the term, reverse osmosis. The membrane trapsparticles and impurities down to 0.0009 micron, and the effluent or permeatewater is very clean and free from impurities. Figure 1 demonstrates how thisworks. So at its most basic level, reverse osmosis filters impurities from aliquid, namely water.
By starting with water that is free from impurities andminerals, RO water can help make growing more calculable, since the waterquality is constant. Nutrients can be better controlled without having to worryabout what is in the water source at the beginning of the irrigation process.Contaminants in the water source, such as iron, manganese, calcium, magnesiumand chlorine, can react with the nutrients and cause problems with fertilizermix.
Growing Without Contaminants
Growing with good water and adding nutrients at the grower’sdiscretion make growing more calculable. Not having iron, manganese, calcium ormagnesium makes setting up the EC of a nutrient solution easier since thefertilizers are not reacting with the impurities in the water. The onlynutrient minerals that are added to the irrigation water are ones that are anadvantage to the plant. Total dissolved solids (TDS) reveals the conductivityof 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 costconsiderations are the most objectionable. Plus, wastewater is another downsideof this technology. There are two types of membranes that are used in ROdesigns, and each has their uses and limits. The cellulose triacetate membranedoes not offer the rejection rate of thin film composite (TFC) membranes.However, chlorine can break down TFC membranes faster and cause prematurefailure of the system, so a carbon pretreatment is recommended to removechlorine.
Pretreatment of the influent water must be considered or themembranes will foul out. Failure to address the pretreatment issue can causeextensive maintenance and labor costs. Without pretreatment, membranes becomeclogged and need to be replaced or cleaned sooner than normal. When used withsoftened water, the membranes last longer, since other minerals are removedbefore reaching the system. The use of a softener creates more cost and laborwhen utilizing RO but must be considered to offset the higher cost of membranereplacement, labor and downtime of the water treatment system.
Another factor to consider when using RO is the wastewater thatis created. In general, depending on pressure and size of the unit, 4 gal. ofwater are required to produce 1 gal. of RO water. This would not be a goodtreatment option in areas where water use is restricted. There are newersystems on the market that are more efficient and produce less waste, but forthe amount of water a grower needs, these systems have a long way to go beforebeing adopted within the industry.
RO water is very aggressive, since it does not contain anyminerals, and can be very corrosive, especially to metal piping. RO watershould never be run through galvanized or copper pipes because they will bedestroyed by the water’s aggressive nature. Pipes, tubing, drippers, mistersand foggers must be able to stand up to RO water.
System Sizes and Costs
The size of an RO water system varies greatly frommanufacturer to manufacturer. There are a few factors to consider when lookingat cost, namely how much water is needed per day. Most systems are rated on thenumber of gallons per day (GPD) that the system can produce. Residentialsystems can produce anywhere from 15-50 GPD without a pressure pump, the costranges from $200-600 and the rejection rates are acceptable. A system with apressure pump can produce 75-150 GPD for $800-1,000, but their rejection ratesare even greater because of the extra pressure.
Commercial and industrial RO systems escalate in price, butso does the volume of water being produced and the need for high-pressure pumpsand large storage tanks. These systems can produce volumes of water rangingfrom a few thousand gallons per day to more than several hundred thousand perday. The costs vary from application to application and manufacturer tomanufacturer. Keep in mind the storage areas for large amounts of processed waterneed to be addressed, and areas built for tanks increase the cost of theinvestment.
Most growers, especially new or inexperienced ones, wouldbenefit from most any type of water treatment that removes impurities. Removalof contaminants that promote microbial growth, such as iron and bacteria, canmake a huge difference in growing, and the impact would be seen from reducedroot rot problems, such as Pythium or fungal diseases that are introduced bywater-borne bacteria. Propagation areas seem to be where most growers targettheir water treatment strategies, but many are incorporating treatment in otherareas as well.
RO would be a great asset to any grower, as nutrientsolutions could be controlled and calculated for a variety of crops andduplicated for consistency. RO is usually a hefty initial investment, but ifincorporated correctly, it can have a fast return on investment.