Ask a group of growers about using filters in irrigation systems and nearly all will agree that it’s important to have one. Ask them about what’s the best mesh size for the filter cartridge and about half will answer correctly. Lastly, ask this group to relate mesh size of the cartridge to micron size and the percent with the correct answer will dwindle to single digits.
It is important to understand the terminology and principles of water filtration and to know how to choose between filter options. Filtration underlies all other water-treatment technologies because it provides a key pretreatment before chemical disinfestants can be used successfully. For example, some treatments, including ultraviolet light, require clear water for rays to penetrate water. Oxidizers such as chlorine are used up, reacting with peat or other particles if they are not filtered out of water before treatment.
Filters differ in their coarseness and role in an irrigation system. Coarse filters remove duckweed, sand and peat particles to prevent clogging and abrasion of plumbing. Membrane filtration (i.e., reverse-osmosis systems) are so fine that they can actually remove disease spores from water.
Filtration options for greenhouses and nurseries: [Click here for chart]
Water source differences
Greenhouse operators choose from several water sources (well, surface or municipal) when building greenhouses. Less common sources include manmade containment ponds, which capture water from rain and irrigation.
Municipal water is usually the cleanest irrigation water source (free of sediment, organic load, chemical contamination, algae and pathogens) because municipalities filter, clean, treat and deliver high-quality water to customers. As a result, municipal water can also be expensive. There’s a correlation here that many growers fail to acknowledge. The processes water is put through by municipalities to improve its quality cost money. Growers who use non-municipal water should perform some of the same filtration and treatment processes, and none of these steps are free.
What are the dirtiest water sources? All water types can have their challenges.
Well water is generally free of pathogens and algae, but it can contain silt, which can clog irrigation nozzles. Surface water from lakes and streams can contain silt, especially after heavy rain, and slow-moving streams or shallow lakes may also contain pathogens and algae.
The dirtiest water source
Ponds, both natural and manmade, are among the potentially dirtiest water sources for greenhouses. Small, stagnant ponds often have high levels of turbidity and microbial contamination. The presence of algae in these ponds is the rule because of fertilizer contamination (both nitrogen and phosphorus).
Duckweed, a common, simple plant that grows on the surface of stagnant ponds, is often confused with algae. A misconception is that duckweed presence is bad and an indicator of poor water. To the contrary, duckweed covering a pond is actually cleaning the water through bioremediation. Duckweed is actually an ally in the battle against microbial contaminants and poor water quality.
Mesh and microns
There is a relationship between filter mesh and micron size. Mesh sizes of 200, 300 and 400 are capable of filtering to approximately 75, 50 and 35 micron, respectively. To add perspective, an oxalis seed is 1,000 microns in size, some algae are 25 micron, a Pythium zoospore is 15 micron and a Ralstonia bacterium is only 2.5 micron.
Filtration goals
You should obtain advice from an irrigation expert before installing a greenhousewide filtration system because one size does not fit all. A greenhouse operation using municipal water and one-directional irrigation may need only 200-mesh (75-micron) filtration to operate efficiently.
A second location, using pond water that receives runoff from the operation and also uses flood floors, may need several stages of filtration to achieve better than 600-mesh (25-micron) filtration. Even at this level of filtration, Pythium zoospores will not be caught.
A possible multistaged filtration system may look something like this:
* The first stage would be a coarse screen to filter out duckweed and other floating debris as the water leaves the pond.
* A second stage would consist of a sand filter located in a pumphouse.
* A third and final stage could be either a paper or belt filter that removes particles to less than 10 microns. This filter is located in the production area to treat water as it leaves the flood area and returns to a holding tank.
Although designing a filtration system is not simple, one conclusion is very clear. With few exceptions, every greenhouse stands to benefit from additional filtration. Irrigation water can become gradually contaminated by algae and other microbial organisms over time, from the water source itself, greenhouse surfaces and biofilm present in the plumbing system.
A logical filtration goal for growers is to determine the micron level of filtration currently being achieved and simply take the next step up an imaginary filtration ladder. For instance, a grower using 200-mesh (75-micron) filters could consider changing to 300 mesh (50 micron). If the quality of the water source is good, this may be as easy as replacing 200-mesh cartridges with 300-mesh cartridges. If the water source is relatively dirty, stepping up filtration to 300 mesh may require a second filter downstream from the first.
Similarly, a greenhouse operation equipped with flood floors and a containment pond water supply definitely needs a filtration system that is a well-designed, staged system to properly filter water leaving the pond and leaving the flood floor as well. Anything short of incorporating such a system will result in most chemical or other disinfestation treatments to be ineffective.
Filtration costs
Filtration is relatively inexpensive when compared with the additional maintenance, plumbing equipment replacement and crop loss costs from disease that can result from inadequate water pretreatment. An effective filtration system will actually reduce the operating cost and increase the efficacy of chemical water treatments.
By Peter Konjoian, Ratus Fischer, Paul Fisher and Bill Argo
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Paul Fisher, University of Florida, Environmental Horticulture Department, (352) 392-1831, Ext. 375; pfisher@ufl.edu; Peter Konjoian, Konjoian’s Floriculture Education Services; Ratus Fischer, Fischer EcoWorks; and William Argo, Blackmore Co.
May 2008