If you're serious about minimizing the waste your hydroponics garden produces, you need to get serious about your water quality. No matter which hydroponic system you choose, the first step to nutrient solution management is obtaining clean water.  Plants use a tremendous amount of water to support life.  In fact, for every carbon molecule assimilated through photosynthesis, about 500 water molecules are lost through transpiration!  So topping off the reservoir from a clean water source is very important in order to prevent trace elements in the water from accumulating to toxic levels over time.  The goal of zero runoff is to replace all of the water lost through transpiration, while providing only enough mineral salts to replace the specific elements removed by the plant.  Nothing should go down the drain.

In the Netherlands rain water is collected from the greenhouse roof.  Rain sensors on the greenhouse roof detect rainfall, and the computer starts the collection process about 15 minutes after the rain begins.  The short delay allows enough time for most of the dust and debris to be washed from the roof before the rainwater is collected.  If there is not enough rainfall to support all of the needs of the plants, supplemental water sources must be tapped, especially during the dry season.  In the Netherlands, where zero runoff is the national standard, any well water removed from the ground must be carefully metered, and it must be replaced by law during the rainy season.

Different water sources have different levels of minerals, measured in parts per million (ppm).  If you are considering commercial hydroponic production, a water analysis should be performed before the first crop is planted.  A water analysis should test for all essential elements, pH, EC, sodium and bicarbonate levels.  If all mineral elements are within tolerance, a custom nutrient formula can be easily created for a specific crop and climatic conditions.  But if even one element is at toxic levels, the water should be filtered and treated first.

City water can also be a good source of raw water for hydroponics since it is already pretreated, but not all city water is alike.  Some sources have innate problems that may be detrimental to plant growth.  Chlorine levels are a well-known example.  Chlorine in the form of chloride is an essential element for plant growth in trace amounts, but high levels of chlorine can be toxic to plants, especially if high levels of sodium are also present.  Occasionally, water treatment plants may have to shock the water supply with excessively high levels of chlorine to treat biological contaminates.  So it's a good idea to remove excess chlorine from your source water before irrigating your plants. To remove excess chlorine, simply let the water sit over night or place an air stone in the reservoir.  The chlorine gas will dissipate quickly and become harmless to plants.  Chloramine, on the other hand, is a more recalcitrant form of chlorine and it stays in the water longer.  The US Environmental Protection Agency claims that the normal level of chloramines in drinking water is safe for plants, but it can be detrimental to fish or to compost teas.  If you are concerned, passing water through an activated charcoal filter will adequately remove chloramines from city water.

Well water may also be used in hydroponic systems as long as no mineral is at toxic levels.  Some well water is classified as "hard water", meaning that it has high levels of bicarbonates, usually as calcium carbonate.  When phosphoric acid is used to lower the pH of well water, the bicarbonates are burned off as carbon dioxide and water, but some of the acid reacts with the calcium to form calcium phosphate.  Calcium phosphate is what your bones are made of.  It is 95% water insoluble and it precipitates out of solution as lime scale.  Once precipitated, the calcium and the phosphate become unavailable to the plant, and it can start to clog up drip irrigation lines and submersible pumps.  Also, because of the "buffering capacity" associated with high levels of bicarbonates, it could easily take ten times as much acid to lower the pH of the nutrient solution to target levels compared to city water, with a danger of phosphorus toxicity. 

So if you have hard water, make sure to use a nutrient solution specifically formulated for hard water.  A hard water nutrient is formulated to compensate for the higher levels of calcium, magnesium and sulfate usually found in hard water, and it is more acidic than a normal nutrient formula.  Dilute nitric acid is the preferred acid for treating hard water since nitric acid doesn't form lime scale, but it isn't recommended for hobbyists.  CAUTION!  Nitric acid is an extremely aggressive acid that can cause severe burns and other safety risks if handled improperly!  So it is recommended that you use a good hard water nutrient, instead.  A premixed hard water nutrient should include some of its nitrogen as nitric acid, but at levels safe for handling.  It will also have proportionately higher levels of ammonium-N.  When ammonium ions are taken up by the plant, hydrogen ions are exuded from the roots, helping to naturally lower the pH of hard water.

Don't use water from a water softener.  Water softeners remove calcium and magnesium ions from the water, but replace them with sodium ions.  Sodium is not an essential element for plant growth.  It can accumulate in the plant tissue to toxic levels.  If you must use water from a water softener, use potassium chloride salt instead of sodium chloride in your water softener.  Potassium ions will be substituted for the calcium and magnesium ions instead of sodium.  Since potassium is required by plants in relatively high quantities, it will be much less detrimental for plant growth.  Also, try to avoid using water from a hot water heater.  Hot water heaters may contain toxic levels of iron and other heavy metals. 

The best way to remove excess minerals from your water is to use a reverse osmosis (RO) filter.  An RO filter has a membrane that only allows pure water to flow through it, removing all of the mineral ions from the raw water.  Therefore, when using RO water all of the minerals in your nutrient solution will come from your fertilizers, not from the water supply.  In this way, a nearly perfect nutrient balance can be achieved.

The only down side to RO water, besides the cost of producing it, is that all of the bicarbonates are removed.  With no bicarbonates, there is no buffering capacity, so pH can swing wildly during periods of rapid plant growth.  During the vegetative growth stage, plants take up proportionately more nitrate-N, a negatively charged ion.  To maintain internal balance, the plant must exude hydroxyl ions (OH-) from the roots, causing the pH of the nutrient solution to spike upwards.  Conversely, during heavy fruiting and flowering, plants take up proportionally more potassium in the form of positively charged ions.  As a result, the plant roots release hydrogen ions (H+) back into the water, sometimes causing the pH to crash.  Therefore, if you want to use RO water, it's a good idea to add at least 10% tap water back into the mix to help buffer changes in pH.  Or better yet, condition the RO water with a potassium bicarbonate-based buffering solution at about 60 ppm before adding your fertilizer.

To maximize quality and yield, it's always best to take periodic water samples and discuss the results with a qualified hydroponics consultant.  If you start with clean water and top off with clean water, you are well on your way to growing bumper crops of nutritious fruits and vegetables in the most environmentally-responsible way possible!

(To learn more about hydroponics and nutrient solution management, please refer toThe Science of Hydroponicse-book series written by Sue and Harley Smith.  Send e-mail inquiries to prohydotech@gmail.com)