by Larry Brooke
Taste as we all know, is something personal. Pure water containing no dissolved solids whatsoever, tastes completely flat and offers no nutritional value to either plants or animals. Spring water may be delicious to people, containing dissolved minerals which help build strong bones and teeth. But the same water offered to plants may not offer balanced nutrition and indeed may lock-out essential minerals if, for example calcium or magnesium are in great excess.
One of the advantages of hydroponics is that adjacent growth chambers with varying temperatures, humidity’s, light levels and nutrient solutions can suit very different species. To achieve balanced mineral nutrition we must start with an understanding of our source water content and the effect that it will have on the nutrient requirements of our plants. Water qualities are measured as several properties and contents, all of which effect growth and health of plants, but differently than they affect us. Let’s first list them then examine their effects on plants, then note what to do about them to improve water quality. We will keep in mind that the hydroponic environment is active: the grower controls nutrient composition while the plant removes both water and solutes.
We will look at: pH Hardness Carbonate content in some regions sulphate in others, Salinity, Iron, Sulfides, Organic solutes: humic residues in some regions, pesticides or industrial wastes in others Extraordinary metals etc: selenium for example. Chlorine and products of Chlorination Biological contaminants And of course taste and odor. Finally we need to take a look at your discharge of wastes!
Most municipal water sources are significantly basic; pH 8 to 9, this protects iron pipes from corrosion. On the other extreme, water from decomposing vegetation – forest water and water that has passed through mining areas tend to he significantly acid. The phosphate content of hydroponic nutrients make a good buffer just above pH 7 to ameliorate all but the most extreme waters. However for acid loving plants preferring pH6 or lower, phosphate is a poor buffer: it has little ability to furnish or absorb excess hydrogen at nutrient concentrations. Those of you in limestone regions have an advantage in that carbonate is an effective buffer in this range: it is only necessary to add acid carefully to correct your nutrient solution and it will generally hold for several days before another adjustment is required. It is the rest of the world, those with fine soft rainwater that need help! Fortunately, the better commercial nutrients are well-buffered, but there is a limit to the amount of buffer that can be put into a concentrate or soluble solid. A common agricultural solution is dolomite, a slightly soluble carbonate rock which also carries useful amounts of calcium and magnesium. Usually a bit of silica-mud remains from impurities in the agricultural grade. Another carbonate source could be blackboard chalk, calcium carbonate. These are better for plants than the sodium bicarbonate we might choose fix our digestive difficulties.
While we are on the subject of pH, we like to check our pH meters frequently with not one but two standard buffer values, since the electrodes do respond poorly after heavy use.
Hardness tends to come along with carbonate except in places like California’s Central Valley, where calcium and magnesium sulfates have accumulated, rather than the carbonates elsewhere originally laid down as coral. Since calcium, magnesium and also sulfates are important nutrients, I claim that hardness is a ‘good thing’ as long as you know how much is there and adjust your nutrient appropriately. The calcium or magnesium content of hard water when added to that supplied by a complete nutrient formula may exceed the optimum for your plant’s health. Note that calcium is generally expressed by analytical services as parts-per-million of calcium carbonate, so calcium content is only 40% of the values reported. The same is true for total hardness, calcium plus magnesium.
Since water softeners work by exchanging “good” calcium and magnesium to “bad” sodium, caution is recommended! If you are in a high-sulfate region, know how much is present! While plants need sulfate and can tolerate a considerable excess, nutrients prepared from sulfates when added to high sulfate water and subject to concentration by evaporation and transpiration as can occur in the southwestern US, can become higher in sulfate than you intended.
Salinity is what you can easily follow with your conductivity meter. The problem is that the conductivity meter doesn’t tell you what salts are present, or their true concentrations. The conductivity of nutrient components covers quite a range.
The recommended maximum salinity for drinking water, 500 ppm, is a substantial part of your plants’ salinity tolerance and can be reached or exceeded in some areas by a combination of dissolved salts, of nutritive or negative values. Fortunately most municipal and regional water departments do a rather good job of analyzing their water throughout the seasonal variations and are quite happy to share their detailed reports. County agricultural agents frequently have typical analyses of surface and underground water supplies for a region. Last but not least are the analytical service labs that specialize in water testing: $100 can get you quite a bit of information.
Iron discolors water, but has limited solubility. It tastes better to the plants than to you! Near neutrality ferric iron is quite insoluble. Ferrous iron is more soluble, but readily oxidized by air, making a precipitate that could clog filters if present in large quantities, and in any case becomes rather unavailable to the plants. Thus iron in the water supply can be an inconvenience and does not significantly decrease the amount of iron chelate needed by most species.
Sulfides are readily recognized by the rotten-egg odor when acid is added. One might hope for sulfide to be a sulfur-source if it weren’t for the fact that several other nutrients form insoluble sulfides: zinc, manganese, and copper for example. Sulfides in supply water should be taken seriously.
A standard method for control of sulfides (and iron also) is filtration through a “greensand” carrying a permanganate oxidant (2). Activated carbon has been reported to do a very good job of removing sulfides, but it’s capacity is limited to about 3% of the weight of the carbon used (3).
Organic Solutes & Extraordinary Metals
In a few areas of the country extraordinary water contaminants exist. Selenium, for example, has been a recent concern in California. Boron is in excess in some other locations and other species may be found by chemical analysis of local water. These require individual attention, first to the question are they good or bad for your plants, or for food products. We might note that some water plants happen to be excellent for removal of some such materials. Water hyacinths, bulrush (4) and others have been used to improve waste-grade waters before further treatment.
Chlorine & Byproducts
Most municipal water supplies are chlorinated. While I have heard that tiny amounts of oxidant chlorine can have a beneficial effect on some plants, the frequent excessive chlorination leads me to suggest aeration: an air stream bubbled through source water for a few minutes will purge most of the chlorine and “trihalomethane” type products of chlorination.
Biological contaminants – the bugs that the chlorine was added to kill – are likely to flourish in the hydroponic environment. This is an argument for (1) cleanliness, and (2) against “topping-up,” rather than changing, nutrient solution for long periods.
An active disinfectant alternative to chlorine is ozone. Ozone is preferred to chlorine in many municipal water treatment systems but costs more. Recently a simple electrochemical synthesis of ozone has been found, which could be applied on a local scale. This method was described in a Ph.D. thesis by Michael Gnann(5) and I have further tested it at the NASA Ames Research Center. What Gnann discovered is that a lead oxide electrode, as used in storage batteries, with a phosphate buffer in place of the sulfuric acid of the storage battery, will generate about 25% ozone with the oxygen when current is applied in the sense to “overcharge” the cell. This is about 10 times the ozone generated by the usual electric discharge in air, at much lower electrical power cost. Since ozone is a faster bactericide and virucide than chlorine, water sterility is reached much more rapidly. Ozone does not remain active as long as chlorine – it is not a permanent protection. Excess ozone can be removed from water and must be removed from air, by active carbon. Ozone oxidizes carbon compounds in water, and would certainly attack any chelates in nutrient solutions.
Taste and Odour
Tastes and odors of water are most likely to come from humus residues in country water if sulfides are not present. One can easily be concerned about possible effects on vegetable products. These organic residues are removed in municipal treatment facilities by flocculation: a process that must be carefully controlled and is not convenient for individual installations. Alternatively activated carbon is excellent for cleanup of organic contamination in water, but it has a limited capacity. Regeneration is by steam at quite high temperatures, so activated carbon is generally replaced rather than regenerated, except in large facilities.
Let’s look at some of the water purification methods that are available, and examine them for hydroponic water improvement. The first that comes to mind is the traditional distillation. Except for the most advanced multistage distillation technology of large installations, it is quite energy intensive; secondly the product water is unnecessarily pure! Indeed, for most municipal use, distillation plants allow a partial bypass so that the water is not devoid of salts. Solar distillation deserves consideration, especially in tropical and regions where high salt content can make water unsuitable for plant consumption without purification, and where plentiful solar energy can be utilized. A unique solar distillation system has been investigated at General Hydroponics Co. and is a collaborative effort including Dr. Hillel Soffer of Ein Gedi, Israel, Dr. Michael Reid of U.C. Davis, and the authors of this paper.
Laboratory deionizers also produce unnecessarily pure water for hydroponic purposes, are difficult to regenerate, and are fairly expensive.
Reverse osmosis is worth some discussion. At the University of California Water Technology Center we had a series of research studies on reverse osmosis, though the significant early development was at UCLA and other labs. The advantages and disadvantages can be listed:
Moderate energy cost (pumping power is not a lot more than the work actually required to separate water from salts). Tradeoff is possible between salt rejection and water flux. (Membranes prepared for partial salt rejection have high water output: “tight” membranes have lowered water output in exchange for high rejection, not needed in most hydroponic applications). Not difficult to operate as a small size installation. Since RO has attained a mass market for industrial and even for home use, the prices are competitive. But there are disadvantages:
The membranes are easily blocked by precipitates, either from hardness in the water exceeding its solubility in the brine concentrate, or from other materials not fully removed by prefilters.
The waste stream is large. In common practice in single-stage, low pressure, RO systems, the waste stream is more than half of the input water! As a result it is not a concentrated brine and can be used for other purposes. Membranes are susceptible to degradation and failure from bacterial attack and/or from chlorine oxidation.
A superficial review of home-quality RO units has even appeared in Consumer Reports(6), along with other water treatment devices of interest. CR tested water containing 600 ppm of salt, at a low pressure (45 psi), and found that 10 to 25% of the feed water reached the product stream. “The fundamental problem is that as the salt concentration increases, in the water NOT passing through the membrane, the pressure required to overcome the osmotic pressure of this water increases. To achieve greater product water recovery, higher water pressure is needed, probably more membrane length, but the risk of membrane clogging from precipitated carbonates or calcium sulfate is greater. Large RO installations have pretreatment steps which may be more involved than the RO part, just to protect the membranes. Sodium hexameraphosphate maybe used to inhibit precipitation, but the amount used is critical.
RO is fine for home-hydroponic use where the waste-water can go on to the garden outdoors, but is rarely needed for water of average quality.
A second membrane process is electro dialysis: this was developed in the 1960s as a method to remove salts from water, but RO, which removed the water from the salts, proved more economical for large installations.
Electro dialysis membranes are more durable than RO membranes, but cost more and the usual mounting in clamped stacks tends to leak. The method has found application in some regions of moderate water needs. It works by passing an electric current through ion-exchanging membranes of alternating charge: the salts in the source stream separate into positive ions going one way, negatives the other. The membranes in the alternating stack are arranged so that ions can pass through the first membrane but not the second of opposite charge: this creates a brine stream from the accumulating salts. If the feed water has a moderately low conductivity, the resulting electrical resistance between the membranes causes a substantial power loss.
Ion exchange is the process used in water softeners and in laboratory deionizers, with the disadvantages noted above. There are many ways to use ion exchangers, however, and we can look for a more suitable method.
First, to recall the usual arrangements: in water softening a cation exchanger is charged with sodium, and exchanges the sodium ions for calcium or magnesium ions. In deionizers, two exchangers are mixed: one for cations and one for anions. The cation exchanger is initially in the acid form: the cation is hydrogen. The anion exchanger is in the base form: the anion is OH, hydroxyl salts in the water exchange their cations for hydrogen ion, and their anions for hydroxyl ion. Since cations and anions are present in equivalent quantities, the hydrogen and hydroxyl neutralize, forming water. Regenerating the exhausted resin involves separating the mixture of many grains of the two resins, by a density difference, and exchanging the resins with strong acid and strong base. The resin grains are them remixed and put back into use. A process investigated by the University of California – Water Technology Center is the use of weak-acid ion-exchange resins, which are easier to regenerate to the acid form, than the strong-acid resins usually employed. Strong acid resins contain the functional group of sulfuric acid, a strong acid. Weak acid resins contain the functional group of carbonic acid or acetic acid.
If we exchange calcium carbonate-containing hard water using only a weak-acid exchanger, the water will acquire hydrogen ions from the resin, displaced by the calcium ions. To some extent these hydrogen ions will be neutralized by the carbonate ions in the water, so that if we do not need to remove too much dissolved salts, and our water is initially basic as is usual, we don’t need to remove the anions and can use a single exchanger.
Regeneration of ion-exchanger is by acid: the usual sulfuric -acid risks precipitation of calcium sulfate, so hydrochloric acid would be considered next. However please consider what you will do with the waste. If you are going to pour it down the drain, hydrochloric acid is quite corrosive. Consider, however, that the calcium and magnesium are of fertilizer value, even if chloride isn’t. If the exchanger is regenerated with nitric acid at somewhat higher cost, the resulting calcium and magnesium nitrate solution has practical fertilizer value. One immediate caution: nitric acid is a strong oxidizer and can be dangerous. If you do not have safe mixing facilities to dilute concentrated nitric acid to 10% or less, purchase diluted nitric acid. There is another source of very good water: your greenhouse. Plants transpire quite a lot: that is their method of imbibing nutrients. Almost all of the water taken up by the roots is released to the atmosphere as humidity.
Dehumidification within the greenhouse offers several advantages. By lowering humidity within the greenhouse, the plants are encouraged to transpire more and thus take in more nutrient. In regions of water scarcity, recycling transpired water can significantly reduce overall water requirements. Additionally, water recovered from dehumidification contains no dissolved solids and is thus practically pure.
Dehumidifiers operate quite efficiently since the heat removed by the cooling coils from the inlet air stream is rejected to the exit air stream, with some increase in temperature. The fan power can be used as part of your greenhouse ventilation scheme. The theoretical energy cost to cool and condense humidity is about 2.6 kilowatt-hours per gallon of water, at 75% efficiency that becomes 3.5 kwh per gallon. To get this water at lower cost requires a source of “cold.” Temperatures cool enough to help condense humidity might be found underground, or in nighttime air, or from evaporation of very poor, even saline, water, if there is no spray carried to the plants. Ocean water is cool even in tropical areas if you go down a moderate distance. If your greenhouse is saturated with water at 70°F, or has a dew point of 70°F, the water content has a vapor pressure of 17.5 mm of mercury and half of it can be condensed a t 49?F, given a sufficient “cold” source to remove the heat content of the condensing water.
Any complete discussion of water requires inclusion of wastewater. Hydroponics discharges include used nutrients and when water purification systems are used, RO or other brines, and resin regenerant wastes. Discharges might be to the homeowner’s garden or to municipal or septic-tank sewage systems. A first consideration is corrosion of piping by acids. This is industrially controlled by passing acid wastes through limestone gravel as a convenient neutralizing agent. Most sewage systems have very low tolerances for pesticides and bactericides. If this is a problem, one solution might be evaporation of the wastewater leaving a solids residue that can be bagged for disposal. The evaporation pond became favored in California’s Central Valley for large quantities of agricultural drainage wastes, when recycling was abandoned. Discharges to the land require your consideration of what is in the water and who is going to use it next. Some RO brines are really quite good, in that they are not highly concentrated. One might not want to drink this water, but it can be used for washing or irrigation if the salinity or hardness is not out of tolerance.