Advantages and Disadvantages of Reverse Osmosis Systems By Tom Blessington, Regional Specialist, Central Maryland Research and Education Center, University of Maryland Cooperative Extension Service Often times water quality problems exist in greenhouses which include: high water alkalinity, excessive solids in water, excessive mineral content in water requiring some form of purification system. It is important to get water tested prior to growing plants. Most common water problem is total dissolved solids (TDS) TDS is mainly composed of soluble salts. High TDS decreases plant growth and causes salt burn on leaves. Hard watr high in calcium, magnesium, iron or manganese can cause unsightly salt deposits on leaves. Primary uses of purified water: -To irrigate salt sensitive plants -During mist propagations. - Holding solution for cut flowers REVERSE OSMOSIS (RO) Reverse osmosis serves as a water purification system where the water is forced under high pressure through a very fine membrane that filters out the dissolved solutes Most commonly used water purification system Based on osmosis: Osmosis: if two solutions with different concentrations of solute or salt and are divided by a membrane, then water from the lower concentration solution will move to the solution with the higher concentration Pressure induced reverse osmosis: applying pressure to the solution with the higher concentration can force water to travel from the solution with higher concentration to the solution with lower concentration In irrigation systems pressure is applied to water on one side of a membrane, forcing it through by reverse osmosis The solutes are left behind and relatively pure water collects on the other side of the membrane The membrane must have pores small enough to filter out the solutes Less expensive membranes are available that purify water about 50-70%, they require less pressure (about 100-300 psi), reducing energy costs Factors influencing rate of water purification and percent of TDS removed from water: - Pressure of the system (150-400 psi) - Type of membrane - Temperature - TDS of water The efficiency of the RO system depends on the cleanliness and condition of the membrane Two main types of membranes: cellulose acetate (thin film composite) and polyamide (hollow fiber) Commercial RO systems can remove 95 to 99% of the salt present in the water Advantages of RO systems: - Effective at removing most TDS - Different salts are removed at varying efficiencies - Calcium and magnesium are generally removed more efficiently then sodium, potassium, lithium, nitrate, chloride and borate - RO is economical (~ 2 cents per gallon) Disadvantages of RO systems: - Produces brine water waste - Check on local government regulations for dumping brine water waste - Requires pressures of 150-400 psi | - High energy costs - Membranes are relatively expensive - Costly maintenance and replacement of membranes contribute significantly to total production cost - Some compounds can make the membrane deteriorate or clog- ex. chlorine and sediment in the water - Therefore it is important to pretreat the water by suspending and removing solid material, antiscaling treatments, adjusting the pH, dechlorinating and softening