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Water recycling has become a popular trend in the agricultural industry—one that creates an opportunity for water treatment suppliers to assist growers with the specification, installation and maintenance of water treatment.
From a grower’s perspective, recycling techniques are used to improve the economics of operation. Benefits include higher yields, lower production costs, quicker growing cycles and reduced pest and disease control costs. With these recycling methods, special attention is given to maintaining the nutrient solution (feed water that has been mixed with special additives to optimize plant growth). Proper nutrient solution management is key to the economic return for recycling systems. Without adequate control of this solution, the entire process can prove disastrous.
The challenge of controlling the nutrient solution starts with water. This water management issue typically is not a specialty of the grower, so it creates the need and opportunity for assistance from a water professional.
To better understand growers’ needs, one can investigate the new methods used in water management within greenhouses. There are two different configurations used in most recirculating greenhouse systems. One uses a continuous flow, while the other is a batch system. Control issues are the same regardless of the setup.
• Maintain nutrient levels
• Keep a balanced pH
• Minimize salt (Na and Cl) levels
Continuous Recirculation Systems
The most common of the continuous recirculation systems is the nutrient film technique (NFT). (See Figure 1)
In this configuration, nutrient solution is stored in a tank where the mixture is monitored and adjustments are made. This solution then is recirculated through a distribution system that feeds the nutrients via a thin film to the roots of the plants. It then is returned to the nutrient solution storage tank.
During operation, a number of changes will occur. These changes are summarized in Table 1. It is favorable to continuously monitor these changes and make appropriate adjustments to the nutrient solution based on these changes. In an unregulated system, each of these elements will eventually grow out of spec, thus hampering growth or, in some cases, destroying the crop.
Since the thin film is exposed to the warm environment of the greenhouse, some evaporation occurs, thus reducing the volume of water in the recirculated system. This effect is minimized, however, due to the high humidity levels in greenhouses. Any water loss that does occur will cause a rise in TDS levels.
During growth, plants will extract water and nutrients from the feed solution. This uptake will be relative to the growth cycle of the plant. Some of the major nutrients or macro-nutrients required include calcium, magnesium and nitrogen. In addition to these elements, there are a few nutrients that are required but transported into the plants at low levels. These micro-nutrients include sodium, chloride and iron. Since these constituents are not readily used, their concentration will elevate as water is removed from the system. Eventually, elevated levels will deter plant growth.
Batch Recirculation Systems
With batch systems (Figure 2), a flood and drain operation is very common. This method uses a periodic flow of water to the plants. The nutrient solution is irrigated into a growing medium and the excess water is drained from the system. Eventually, all the water drains back to the nutrient storage tank, where it is maintained.
Unlike the continuous recirculating systems, batch systems do not require constant monitoring. The nutrient solution can be analyzed at the beginning of each cycle and adjustments made appropriately.
The Water Issues
Using some of the fundamental concepts associated with recirculating systems, issues related to the inlet quality of the water can be predicted and addressed. The three main areas of concern with feed water relate to pH, TDS and iron. Although there are many other issues regarding more specific types of crops, these three details must be understood and controlled by the grower.
For most crops, a pH range between 6.5 to 7.5 SU is required. This pH usually is controlled within the recirculation loop since the pH also will fluctuate as a result of cycling the nutrient solution. Common methodology for treatment is the use of chemical feed equipment.
Considering the elaborate distribution systems involved with greenhouse growing systems, high levels of iron will become detrimental to the hardware used. Although most crops require some iron in the water—about 1 mg/l— these levels are low and can be added as a part of the nutrient make-up.
It also is very important to keep the iron in solution form; otherwise, it is worthless to the plant. In many cases, it is better to control iron levels in the feed water by aeration, precipitation and filtration.
TDS or EC
In a recirculated system, optimal growth will be achieved by limiting the variability of the electric conductance (EC) of the nutrient solution. In addition to keeping a consistent EC, it is important to maintain the EC between 1,000 and 2,000 mmhos after it is blended with the required nutrients.
To maintain EC levels, start with the feed water. In environments where changes in feed water quality are common or where total dissolved solids (TDS) levels exceed 1,000 mg/l, reverse osmosis (RO) is the technology of choice.
By using RO as a pretreatment for the nutrient feed water, a consistent quality can be achieved. With this type of performance, the quality of the nutrient water can, in turn, be maintained in the recirculation loop.
In nutrient solutions, these are beneficial elements. Provided the water is being used for irrigation, these hardness ions should not be treated. However, it is possible for the water to be used as pretreatment to an RO or boiler. In these cases, the hardness will need to first be removed and then added back to the nutrient solution through fertilizer additions.
Although high levels of total dissolved solids (TSS) are not detrimental to any crop, the TSS will cause problems with water transport devices including pumps, storage tanks, distribution lines, spray nozzles and other water treatment equipment. The most common treatment here is cartridge filtration.
Free chlorine is detrimental to most crops. Carbon or aeration typically are treatments used in the grower industry for reduction of this oxidant.
There also are many crop-specific treatment issues that face growers, whether they use a recirculating system or not. For any application, growers need a good understanding of the feed water quality as well as the crop’s needs. Most professional growers already know individual crop needs. This means that the best niche for a water treatment professional is to be educated on various water treatment options that will provide a solution that meets crop-specific needs.
About the Author
Jerry has more than 10 years of experience in the water treatment industry. A graduate of Case Western University in Cleveland, he holds a B.A. in Biochemistry and a Masters in Business Administration.