Reverse Osmosis Technology

Transcription

1 Reverse Osmosis Technology RO Membrane Pressure Gauge Sediment Pre-Filter Carbon Block Filter The basic Reverse Osmosis (RO) system consists of a sediment pre-filter, a carbon block pre-filter and an RO membrane. An RO membrane works by straining the water through extremely small pores which allow pure water to pass through while blocking any solutes. On one side of the membrane you have the concentrate (feed water with dissolved solutes). On the other side you have pure water. Due to osmotic pressure, the pure water will tend to migrate through the semipermeable membrane to the concentrate side to achieve equilibrium. By applying pressure to the concentrate side, the flow is reversed so that the concentrate migrates to the pure water side. Hence the term reverse osmosis. By their nature, RO membranes are highly susceptible to clogging. Without some type of pre-filtering, the life of the RO membrane would be too short to be useful. The first stage of an RO system is the sediment filter which traps particulates such as dirt and rust which will clog the carbon block filter and the RO membrane. The second stage is the carbon block filter which removes chlorine, volatile organic compounds, taste and odor. The final stage is the RO membrane. A pressure gauge is often added between the pre-filters and the membrane. This lets you know when the prefilters are too clogged to allow enough pressure for the RO membrane to function properly. For ultrapure water, a de-ionization cartridge may be added downstream of the RO membrane.

2 Why Use Reverse Osmosis In my previous writings I have detailed why you probably will not get the rated flow from your Reverse Osmosis system and why waste water is a necessary factor for all RO systems. Then why put up with the added expense and maintenance of an RO system? Can t you just grow with tap water and forget all of the added hassles of maintaining an RO system? The answer is an unequivocal Yes. We all know that you can throw some seeds in dirt and water them out of the hose bib and they will grow. Then why do we go through all of the trouble of setting up a hydroponics system with expensive lighting, hydrations systems and nutrient solutions? The advantages seem obvious. We do this in order to maximize production and quality, and to shorten the time necessary for our plants to produce. You wouldn t dream of setting up a hydroponics system and not using grow lights, then why would you leave out a crucial item like a reverse osmosis system? After all, isn t the basis of a hydroponics system...water? WHY SHOULD YOU CONSIDER AN RO SYSTEM TO BE A CRUCIAL ELEMENT OF A HYDROPONICS SYSTEM? If you currently use a hydroponics system then you are aware of the multitude of nutrients that are available. These costly additives are formulated for a variety of situations. They are all based on using water that is close to 0 ppm of Total Dissolved Solids (TDS). If the feed chart says to bring the nutrient solution to 1200 ppm and you are starting with water that is at 500 ppm, then it will be extremely hard, if not impossible, to dial in the appropriate nutrient concentration. Much of your hardness has already been wasted on contaminants instead of nutrients. WHY IS THE HARDNESS OF THE NUTRIENT SOLUTION SUCH AN IMPORTANT FACTOR? In a previous paper regarding how Reverse Osmosis systems work, I discussed osmotic pressure. Simply put, a solution of low concentration will migrate through a membrane to a solution of higher concentration in order to achieve equilibrium of the solvent which, in this case, is water. RO systems use water pressure to reverse this flow thereby producing purified water. Plants use osmotic pressure to absorb nutrients through their roots. If the concentration of the nutrient solution is higher than the concentration in the roots, then the flow will be reversed and the roots will actually release water and nutrients into the growing medium. DO I STILL NEED TO USE REVERSE OSMOSIS IF THE WATER IN MY AREA HAS A LOW HARDNESS FACTOR? Even if your local water supply has a low TDS factor, you are still wasting prime nutrient uptake space on contaminants. Local water supplies vary from location to location and even by time of year in the same location. By using purified water, you keep the baseline of your water source consistent, allowing you to dial in the appropriate nutrient solutions regardless of fluctuations in the quality of your water supply. If you are using organic nutrients, then it is imperative that all chlorine and chloramines in the water be removed or the live organisms in your nutrient solution will be adversely affected. If your water source has a low TDS factor, then the life of your membrane and filters in your RO system will be greatly extended thereby reducing necessary maintenance. WHY SHOULD YOU USE AN RO SYSTEM FROM SPECTRAPURE? Unlike most RO manufacturers that have sprung up since the recent proliferation of hydroponics stores, SpectraPure has been designing and manufacturing RO systems for commercial and laboratory applications since Due to the demanding nature of these applications, we utilize extensive in-house quality control and support. Our support staff are extremely knowledgeable and participate in the actual designs of our RO systems. All SpectraPure RO systems are backed by a 3-year warranty.

3 Benefits of Using RO Water In a Hydroponics Nutrient Solution Target Nutrient Concentration ppm 1200 PPM 700 ppm 1190 ppm 600 PPM Available Nutrient Space Starting Hardness 0 PPM WITHOUT RO FILTRATION 500 ppm TDS WITH RO FILTRATION 10 ppm TDS In this example, we show the effects of using filtered and non-filtered water when adding nutrients to a hydroponic solution. We are starting with tap water having a TDS of 500 ppm with a desired goal of 1200 ppm after adding the nutrients to the solution. Without RO filtration, you will have 700 ppm of beneficial nutrients and 500 ppm of contaminants. Using RO filtration, the starting hardness is 10 ppm (500 ppm tap water put through an RO membrane with 98% rejection). This means you will have 1190 ppm of beneficial nutrients and 10 ppm of contaminants. Without RO filtration, your final concentration of beneficial nutrients to contaminants is 1.4:1; with RO filtration that ratio would be 119:1. Using RO water results in much more space for beneficial nutrients, less contaminants which can be destructive to the organic nutrients and a more stable ph.

4 Waste Water Facts One of the most important (and misunderstood) parameters in an RO system is the waste-to-product ratio (the ratio ). The ratio can be calculated by measuring the waste water rate and comparing that to the product water rate. These are expressed as ratios (e.g. 2:1 or 3:1). A 3:1 waste water ratio means that for every 4 gallons of water used, 3 gallons of water would go down the drain and 1 gallon of pure water would be produced. Reverse Osmosis Membranes are made from an inert, synthetic film material with billions of micro-pores that are the size of pure water molecules. As tap water passes by the surface of this material, pressure is utilized to force pure water molecules through to the product water output port. Water molecules with pollutants attached to them are rejected and sent down the drain. All Reverse Osmosis membranes (regardless of size) need to have some minimum amount of waste water to keep the membrane surface flushed clear of built-up hardness and other contaminants. If a membrane is allowed to become fouled, it will lose its ability to produce purified water at a reasonable rate. The micropores plug up, leaving fewer and fewer locations for the pure water to make it through the membrane. Production goes down, while the waste water rate stays the same, resulting in a greater and greater waste water ratio. It is important to understand that waste water ratios as stated by RO system manufacturers are an arbitrary figure. All RO systems use a Flow Restrictor to regulate, or restrict the amount of waste water that escapes to the drain. The amount of restriction in the flow restrictor used dictates the waste water ratio. The end user may set that ratio at their discretion. When deciding on the optimum waste water ratio, the factors to consider are the hardness of the water source, the desired life of the RO membrane compared to the cost of the water that is lost down the drain and the cost to prefilter the extra waste water. If you use a low waste water ratio then the concentration of the minerals in the membrane housing is higher. This effect may be mollified by frequent flushing of the membrane. In any case, flushing the membrane housing at the end of the cycle is of extreme importance. If the highly concentrated water is left in the housing, the minerals in the water will form deposits on the membrane surface, drastically reducing the life and efficiency of the membrane. Frequent flushing of the membrane and adjusting the waste water flow rate to the optimum ratio are the most important factors in extending the life of the membrane. In locations with high hardness a water softener ahead of the RO system would be recommended. This would allow lower waste water ratios while increasing the life of the membrane and prefilters. As you can see, waste water ratios are much more arbitrary than RO system manufacturers would have you believe. The end user is faced with determining how to balance the life of the membrane and filters with the amount of waste water generated. We highly recommend using an automated flush system when employing low waste water ratios.

5 Flushing Your Reverse Osmosis Membrane The most important part of your Reverse Osmosis system is the membrane. This is where the pure water is produced. Basically, the other parts of your RO system exist to extend membrane life. The pre-filters work to take out particulates that would rapidly clog the RO membrane. As the membrane is used, the microscopic pores become clogged with particulates, biological matter and mineral scale. As fouling occurs, the pressure necessary to force water molecules through the membrane increases resulting in less product water and more waste water. More than 85% of American homes have hard water (water with a high mineral content). Usually, the principal cause of hard water is calcium carbonate. This is what causes lime scale on your water fixtures. These minerals have very low solubilities. When their concentration exceeds their saturation points, they precipitate out of the water and form a scale. As the scale forms on the RO membrane it provides a nucleation site that increases the rate of formation of additional scale. Because of the low solubilities, scaling is difficult, if not impossible, to remove once it has formed on the membrane. As the RO unit is used, pure water is extracted from the feed water resulting in higher concentrations of minerals and contaminants in the water surrounding the membrane. Higher waste water ratios are helpful in flushing the surface of the membrane and in reducing the concentration of minerals and contaminants in the membrane housing, but flushing is necessary to maintain the efficiency and life of an RO membrane. It is always recommended to flush the membrane before and after every use. If highly-concentrated solution is left in the housing when the RO unit is turned off, then the amount of scaling on the membrane is greatly increased (think of making rock candy when you were a child). Also, if the RO unit is in use for an extended period of time, we recommend periodic flushing during the cycle in order to reduce the concentration of the water in the membrane housing. With the flush valve in the closed position (the standard position when the RO unit is in use), the water is ported through the flow restrictor which increases the back pressure resulting in water molecules being forced through the RO membrane. Some water makes it through the flow restrictor resulting in waste water. The length of the flow restrictor determines how much waste water makes it through the flow restrictor and how much water is forced through the membrane (waste water ratio). Without waste water, all contaminants would be left in the membrane housing resulting in a severely shortened lifespan for the RO membrane. With the flush valve in the open position, the water flow is diverted past the flow restrictor, resulting in an unimpeded flow to the drain. This flushes the membrane surface and reduces the concentration of minerals and pollutants in the water surrounding the membrane in the housing.

6 Factors Affecting Production Of Reverse Osmosis Water There are many factors to consider when determining which RO system will fill your needs. RO systems are normally defined by the flow rating of the membrane expressed in gallons per day (GPD). In order to determine this rating, companies use a standard of 60 psi line pressure (80 psi for 1000 GPD membranes), 77 F water temperature and 500 parts per million (ppm) of Total Dissolved Solids (TDS). Any changes from those conditions will have an effect on the amount of water your RO system produces. Also, the stated flow rate of an RO membrane has a variance of ±20%. As you can see, just because your system is rated at 200 GPD, your actual flow rate may be quite different from the one stated on the box. The factor that has the most effect on water production is your household line pressure. This has a direct correlation to output. Let s take, for example, a household water pressure of 30 psi which is not uncommon. That is ½ of the standard pressure of 60 psi. That means that your RO system which is rated at 200 GPD will only produce 100 GPD and that is discounting the rated variance and any other factors which affect production! Another big factor is water temperature. As the temperature of the water decreases, the viscosity increases. This requires more pressure to get the same amount of water through the membrane. The effect of water temperature may be seen in the Temperature Correction Factor Table () shown below. As you can see from the table, a water temperature of 60 F will drop your production by 25% / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / The third factor we are concerned with is osmotic pressure. The flow of water through a membrane in response to differing concentrations of solutes on either side generates a pressure across the membrane called osmotic pressure. In other words, as the hardness of the tap water increases, greater tap water pressure is required to force the water through the membrane. Although this is a factor in RO production, it has much less effect on the output than line pressure and water temperature.

7 How much water will my Reverse Osmosis System actually produce? We are often asked why an RO system does not produce the rated amount of water (e.g. 100 gallons per day). Production rates depend on many factors such as the flow rate of the RO membrane, water temperature, water pressure and the TDS (Total Dissolved Solids) of the water supply. The rated gallons per day (GPD) of the RO membrane are based on 60 psi of line pressure, a 77 F operating temperature and 500 ppm of total dissolved solids (TDS). These are industry standards. In actuality, these factors will vary in practical applications. In order to calculate the actual production rate of the membrane, we use a Pressure Correction Factor (PCF) and a Temperature Correction Factor (). Therefore, the expected GPD for the membrane is calculated by multiplying the rated GPD times the PCF and the (GPD = Rated GPD x PCF x ). PRESSURE CORRECTION FACTOR The PCF is easily calculated using the following formula: PCF = Line Pressure (in psi) 60, with 60 being the line pressure at which the membrane rating was calculated. If the line pressure at your RO installation site is 30 psi, then your PCF would be.5 (30 60). Right away, you can see that this factor alone would cut your production rate in half. You may use a booster pump to increase the line pressure but we do not recommend anything over 80 psi. TEMPERATURE CORRECTION FACTOR The is harder to calculate. For that, we use the following table: 41.0 / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / MEMBRANE OUTPUT CALCULATION EXAMPLE GPD (60 psi) What is the expected GPD from a 100 GPD System at 50 psi pressure and 60 F water temperature? PCF = = = (from Table above) Expected GPD = = 62.8 GPD 62.8 GPD ( ± 20%* ) would be the Actual Production Rate *Manufacturers state their GPD ratings based on a variance of ±20% MEMBRANE OUTPUT CALCULATION EXAMPLE GPD (80 psi) What is the expected GPD from a 1000 GPD System at 50 psi pressure and 60 F water temperature? PCF = = = (from Table above) Expected GPD = = 471 GPD 471 GPD ( ± 20%* ) would be the Actual Production Rate *Manufacturers state their GPD ratings based on a variance of ±20%