Removing Contaminants From Recycled Water
A convergence of factors – regulatory trends, public perception, crop quality, and cost efficiency – has sparked considerable interest among growers in the technology of water recycling. Indeed, the greenhouse industry at the local, state and federal levels has been promoting the conservation and recycling programs already implemented by growers as a “proactive” response to the increased concern among citizens and within the government over the quality of groundwater and runoff water.
It may be premature to proclaim that this effort by growers has staved off the threat of stricter federal and state mandates. Needless to say, the American public is unlikely to lose interest in industrial or agricultural practices that may degrade the quality of water and, by extension, the environment. Likewise, the Environmental Protection Agency, along with other governmental and private-sector “watchdogs,” are unlikely to reverse course when all indicators point toward a continuing and increasingly sharper scrutiny of practices deemed a threat to water quality.
The spectre of stricter water quality regulations provides growers with a strong incentive to reduce discharge and waste volumes. But a number of these techniques also have been shown to increase yields, lower production costs, speed up growing cycles, and reduce pest and disease control costs.
The key to any recycling system used in commercial growing is its impact on nutrient solution management. A grower who successfully manages the nutrient solution while utilizing recycled water will realize both a decrease in the cost of fertilizer and an increase in crop quality. Conversely, the consequences can be disastrous to a grower who cannot control the nutrient solution.
The challenge of controlling nutrient solutions starts with the water, the quality of which varies widely across the nation. Some growers are luckier than others by simple virtue of having access to good water. (If you haven’t had your water thoroughly analyzed, now is the time to do it!)
The contaminants of interest to growers are numerous. In some cases, the level of a particular contaminant may be within acceptable boundaries for the production of floral crops. In other cases, the presence of one or several contaminants will mandate that the water be treated.
As basic guidelines, the following parameters should be analyzed for each water source (typical sources include streams, lakes, wells and municipal water treatment centers).
pH Preferred Range Variability in U.S. water sources
6.5 — 7.5 SU 5 — 8.5 SU
The pH of the water will give a measurement of reactivity. Most water in the U.S. is nearly neutral, measured as a pH of 7. A pH lower than 7 is considered acidic, while a pH higher than 7 is considered alkaline. The pH of water can be measured with an electronic analyzer (typically accurate to 0.1 of a pH point), or with a liquid test kit (which uses a colorimetric indicator to signify pH).
Adding the appropriate chemicals is the preferred way to maintain an adequate pH in the nutrient solution. Acid is injected into the system if the pH is too high, while a caustic material is used if the pH is too low. These adjustments can be done manually by sampling the solution, calculating the amount of chemical required, and then adding the chemical.
Alkalinity Preferred Range Variability in U.S. water sources
N/A 10 — 500 mg/l
In association with pH, alkalinity plays a role in determining the “neutralization” capacity of the water. Most growers understand that adding acid to water with a high pH initially will lower alkalinity without significantly changing the pH. Once the alkalinity is reduced, the acid will begin to reduce the pH. The alkalinity of the water must be known to properly predict the volume of chemical required to adjust the pH. Colorimetric test kits are available for the measurement of alkalinity.
Treatment of excessively alkaline nutrient solution alkalinity is done in conjunction with the adjustment of pH.
EC (electric conductivity) Variability in U.S. water sources
150 — 2,300 umhos 75 — 15,000+ umhos
TDS (Total Dissolved Solids) 100 — 1,500 mg/l 50 — 10,000+ mg/l
EC (electric conductivity) is a measurement of all ions in the water – sodium, magnesium, calcium, iron, chloride, sulfate, nitrate, and many others. This measurement also relates to the TDS (total dissolved solids) in the water. As with pH, the EC of water varies greatly across the U.S. For instance, water supplies in some parts of the Northeast have an EC of less than 100, while the EC level of water in some areas of the Southwest exceeds 10,000.
EC measurement is performed using an electronic device that directly measures electrical conductivity. This tool is refered to as conductivity analyzers or TDS analyzers.
Among the several treatment methods for reducing EC, reverse osmosis (RO) is the most common in the industry, including the greenhouse industry. This technology uses a membrane as a barrier to dissolved salts, inorganic molecules and organic molecules with a molecular weight greater than approximately 100.
Water molecules pass freely or “permeate” through the membrane, creating a purified product stream. Most commercial membranes are designed to prevent or “reject” more than 95 percent of the contaminants. This means that 5 percent or less of salts will permeate through the membrane.
Regardless of the type of RO system, a constant drain (reject flow) is required. This reject flow contains all of the non-permeated contaminants. The efficiency or “recovery” of the RO system is determined by the ratio of reject flow to permeate flow. A typical commercial RO design will provide efficiency of about 50 percent for every one gallon processed; a half gallon is permeated into good water while the other half gallon is processed to drain.
The use of RO as a pretreatment for the nutrient solution assures consistent quality, which in turn is maintained in the recirculation loop.
Total Suspended Solids Variability in U.S. water sources
<5 mg/l 1 — 50+ mg/l
The TSS (total suspended solids) refers to the amount of filterable material in the water. Although high TSS levels have minimal impact on crops, the TSS causes problems with water transport devices, including pumps, storage tanks, distribution lines, spay nozzles, and other water treatment equipment.
The measurement of TSS is typically done in a laboratory. One way to measure this parameter is to filter 100 ml of water through a filter. The filter is then dried and weighed. The residue filter cake, which represents the TSS in the water, is measured by its weight. With this device, the size (in microns) of the suspended solids can be classified.
Filters are utilized to remove TSS. The most commonly used filters are disposable cartridge filters, but bag or backwash filters are employed in situations that entail significant levels of TSS.
Also important is the filter rating. Most cartridge filters will have a rating of one, five, 20 or 50 microns, which refers to the size of the solids that are removed. A 20-micron filter will protect most water treatment devices.
Free Cl2 preferred range Variability in U.S. water sources
<0.05 mg/l 0 — 5 mg/l
Free chlorine is detrimental to nearly every crop. This strong oxidant is added to municipal water supplies as a residual disinfectant. The goal of most municipalities is to provide a residue of 1 mg/l of Cl2 at the furthest point in the distribution network. This means that locations close to the processing plant will have significantly higher values.
A colorimetric test kit can be used to detect free Cl2. This type of kit provides a rough estimate of the residue Cl2 levels to an accuracy of 0.1 mg/l.
Activated carbon is the typical means by which Cl2 is removed. The carbon can be incorporated into a filter cartridge; however, this type of filter requires constant monitoring and replacement.
Most commercial applications use the carbon in a contact media tank. These types of systems can generally operate nine to 12 months before requiring replacement of the media.
Another means of removing Cl2 is to route water to a retention pond. Because Cl2 levels diminish with time and exposure to the atmosphere, facilities that employ retention ponds rarely need additional treatment to bring Cl2 levels to acceptable levels.
Hardness preferred range Variability in U.S. water sources
<20 mg/l 20 — 1,500+ mg/l
Many water supplies contain dissolved solids such as calcium and magnesium. Dissolved calcium and magnesium ions are typically referred to as “hardness” in the water. Although these elements are not directly detrimental to crops, they can cause excessive wear on equipment such as water heaters, boilers, steamers, pumps and humidifiers. Removing hardness from the water significantly extends the life of equipment in contact with the water.
Water is most commonly softened through an ion exchange process (a highly efficient means of removing hardness thanks to the development of synthetic resins). In this process, the resins are used to remove calcium and magnesium from the water by exchanging their ions, in a sense, with the “soft” ions of sodium or potassium.
Resins eventually become exhausted, at which time a regeneration sequence is used to remove concentrated deposits of hardness from resin beds, while replenishing beds with soft ions. Sodium chloride (table salt) is typically used for the regeneration, although potassium chloride also is suitable.
Iron Preferred Range Variability in U.S. water sources
<0.1 mg/l 0 —- 10+ mg/l
The detrimental affect to water processing equipment of iron in the water is similar to that calcium and magnesium. A field test kit can be used to detect iron, although it announces its presence fairly obviously by staining faucets.
A water softener can remove iron in addition to removing calcium, magnesium and other agents of hardness; care must be taken, however, to minimize the oxidation potential. This means ensuring to the fullest extent possible that the water does not come in contact with air. Iron will precipitate if oxidizers are introduced into the stream, which creates unfavorable conditions for the desired ion exchange. A filter system with at least a five-micron rating is the recommended treatment for a situation in which oxidation is significant.
While there are many other possible water contaminants, an understanding of the previously listed contaminants will go a long way toward helping to manage the water system of any grower’s operation.