We generally trust the water we get from our tap without too many questions. But what might we find in it if we did some research? Various chemicals or discarded substances are turning up in our lakes and rivers and may be affecting our health. Often, the quantity of a single chemical is regulated, but combinations rarely are. How much of these contaminants can our municipal water facilities remove, and how much do these substances affect our plants?
Interest in understanding the water we give our plants continues to rise. Fortunately for our plants, the quantities of the types of impurities mentioned above are typically too low to affect plant health, vigor and production. Residuals of many of these substances have not been discovered inside the leaves and fruits of our plants, but many plant species are known to absorb contaminants such as lead, cadmium, chromium, arsenic and various radioactive nuclides or atoms from soils. We need to become and remain aware of what is in our water and keep it as clean as we can.
Along with the normal, undesirable, organic life forms, chemicals or minerals found in tap water, accidents can occur somewhere in the treatment and distribution system, which can add more serious contaminants. These incidents are not common and are much more serious in regards to the safety of human drinking water, but even so, every gardener should stay abreast of any news that might be available from the local water authority.
For the most part, the issues a hydroponic gardener needs to consider regarding effluent water quality include:
- Carbon dioxide
- Alkalinity (pH)
- Suspended solids (iron and algae)
- Odor and color
The issues above are common for hydroponic gardeners to experience and can be more practically ameliorated than soil contaminants.
Water Quality Factors
Municipal water treatment systems routinely do a number of tests looking for unwanted pollutants in the local water.
Hardness represents the combination of calcium and magnesium in the water. These are dissolved solids and are difficult to separate from large quantities of water. Therefore, your water hardness is going to be determined by your municipality’s source water. Keeping your pH levels low is the most practical way to minimize the damage hardness can cause to pipes and drip emitters. Most plants prefer a pH of around 6, which is the same level that minimizes any damage from hard water. The calcium and magnesium salts in your source water are typically not present in a plant-available form, so are definitely undesirable.
Carbon dioxide is present in water in the form of a dissolved gas. Calcium and magnesium combine with carbon dioxide to form carbonates and bicarbonates, and these are what ultimately plug up the works. Plants use CO2 during photosynthesis. This process acts on starches, sugars, oils and proteins—materials that already exist within the plant. The carbon in these materials comes initially from the carbon dioxide in the water. When the oxygen concentration in water containing organic matter is reduced, the carbon dioxide concentration rises.
When adding acid to a bicarbonate-laden nutrient, the result is typically an increase in the CO2 level of the nutrient solution. When that solution is exposed to air, the CO2 is unstable and escapes (bubbles are observed). The CO2 that initially lowered the pH is soon gone and the pH rises again. So, because the pH of the water will only be stable after aeration, it is a good practice to aerate after adding an acid and stabilizing the pH before determining if you have your pH where you want it. Dropping the initial pH to around 5—well below the 6 desired for dealing with hardness—can be beneficial, provided the plants being grown are comfortable with that pH level.
The term used to identify the level of bicarbonate in the water is alkalinity, which represents the ability of the water to neutralize acid. Alkaline water raises the pH, while acid reduces pH. If the tap water you use is highly alkaline, it will raise the pH of your nutrient solution. Checking your nutrient solution frequently for alkalinity is important when high-alkaline tap water is used. It can be confusing when trying to control pH by adding acid if you don’t consider this whole action/reaction between the alkaline water, acid and carbon dioxide. High-alkaline water will resist change from the addition of a pH adjuster, and unless you aerate the water, the pH level will not be stable because the CO2 is slowly escaping.
Iron is a common issue with recirculating systems, as it is found in most nutrient solutions and is required by plants. As iron builds up in the water, it can clog drips and cause brown staining. Iron can generally be removed by aeration followed by two days of settling.
As water contains at least small amounts of nitrogen, algae and other organic life forms will begin to multiply. Once algae has become visible to the eye, it has already been doing damage to your plants for a while. One way to minimize algae growth is to keep the water reservoir dark and free from aeration if nutrients are added to the water, but since aeration is important for stabilizing pH in the reservoir, this can be a conflicting issue. A catch-22, if you will. If you’re not recirculating your water, you can add nutrients to your feed water after it is pumped from the water reservoir. Inline nutrient injectors are available for this purpose. In addition to algae, there are a number of other microscopic life forms present in your reservoir, such as plankton and various minerals.
Turbidity & Salinity
Municipal water systems also test for turbidity and salinity. Turbidity relates directly to the appearance of water, or how clear it is. It is a measurement of the amount of suspended solids in the water. Water treatment plants use settlement basins to remove what will settle and then typically use sand filters to further clean the water they process to remove most of the suspended solids. Because of the steps treatment facilities take, your tap water turbidity reading should be less than 1 NTU (nephelometric turbidity units), which means your water should be clear and clean.
Whether your hydro system is a drip, drain to waste or recirculating system, starting off with the lowest level of salinity is the best way to go, but in recirculating systems, being aware of the salinity and turbidity is even more critical. A tool to consider in managing turbidity is the purchase of a good turbidimeter, which measures total suspended solids, although these are not cheap. Another issue for recirculating systems is the increasing salinity (salt levels or total dissolved solids). Most filters do not remove salinity or reduce TDS, so changing the water frequently is a good way to deal with this issue.
In addition to visible suspended solids, there are chemicals like fluorides, chlorines or monochloramines used by municipal water systems to disinfect our water that can present significant challenges to otherwise healthy plants. For example, high levels of chlorine residuals in the water supplied to plants reduces levels of beneficial bacteria and damages root growth and vigor. The use of available filters for these chemicals can help a lot. Good filtration will remove many contaminants from your watering system.
Reverse Osmosis Filtration
If your tap water is relatively hard and high in salts, an RO filter system can help. A membrane-based filtration system capable of demineralizing nearly all the salts found in our tap water, RO systems are used to purify waste water during the process of making it potable (safe for human consumption). The water is pushed under pressure through a semi-permeable membrane.
To better understand, let’s take a look at osmosis, a process where a weaker saline solution migrates to a stronger saline concentration. When a plant’s roots take up water, this is done by osmosis. This migration requires a semi-permeable membrane that some molecules can pass through but not others (this is the key). Osmosis occurs naturally without requiring energy, but RO requires energy. The membrane used in RO systems is permeable to water molecules but not salts, bacteria and other organics or pyrogens. The pressure for RO is required to overcome the natural force of osmosis.
There are two different water streams in an RO filter: the product stream (pure water provided by the process) and the reject stream (water carrying the pollutants that have been removed). The percent recovery for an RO system is the amount of product water produced versus the amount of water rejected. Industrial systems will have recovery rates from 50-85%, while more economical residential units will run lower than this. The product water from these is just as pure. RO uses cross filtration rather than standard. The reject water is channeled one way, while the product water goes another. This avoids the buildup of contaminants.
The product water from an RO filter is extremely low in salinity and has a slightly lower-than-normal pH as it does not remove much CO2. The calcium and magnesium are removed, so the bicarbonates that plug up the works are not formed. Water processed by an RO system will provide optimal influent water quality. Only distilled water would be superior. In addition, many of the trace pollutants being discovered in our lakes and rivers, and eventually coming from our tap, like arsenic V, can be removed by RO. RO units will typically contain one or more activated carbon filters in addition to their semi-permeable membrane. These carbon filters are among the best methods for removing chlorine, chlorine by-products, pesticides and herbicides.
Starting off with top-quality water is a great first step if you want continued maximum yields from your crops. Keeping your water clean is vital. If your tap water is not quite top-quality, there are things you can do to help. Always keep an eye on water quality to continue getting the results you want.