While all plants’ water requirements are unique, so too are their water sources, and rarely do the two’s qualities align with one another. This is why the first two questions we ask when designing a water system are:
1. What are the requirements of the crop?
2. Is there an analysis on the available water source?
Discrepancies between these two questions along with the requirements of equipment are why filtration is required in every greenhouse. The importance of filtration and the pros/cons of three common filter types are described below.
Filters are a must for well and surface sourced waters, and recommended for municipal water. The three most common filter types are: disc filters, media filters, and reverse osmosis. When looking for which filter is appropriate the following characteristics must be considered: method of irrigation, maintenance, budget, volumetric flow of water, and what impurities need to be filtered.
Disc filters work by forcing water between discs that have small ridges. As the water flows through the gaps between the ridges of the discs particulates too big to fit are filtered out. These filters are great at removing particulates, easy to maintain, low cost, and are customizable for most volumetric and filter size applications. Where they lack is that they can clog up quickly if not sized correctly, and are poor at removing organic contaminants.
Media filters have been around since earliest man and their concept is based on natural principals. This type of filter works by forcing water to percolate through media (sand, gravel, activated carbon), and the gaps between the media capture particulates. Media filters are typically used for large flow rates and are effective at removing organic and inorganic contaminants. Media filters are considered more reliable than disc filters, but are also more expensive than disc filters, take more floor space, and require more water for back flushing.
Reverse osmosis (RO) is a relatively new technology to be used in the agricultural industry, but that doesn’t mean it should be ignored. RO works by forcing water through semipermeable membranes that filter out everything from bacteria to atomic ions. Its ability to remove even the smallest of impurities puts RO in a league of its own when it comes to filtration. However, that doesn’t mean every irrigation system should use it, as RO can become expensive quickly in both up front and maintenance cost. For this reason RO is often only recommended as a worse case when sodium or other ionic concentrations in the source water are too high.
Strategies to make RO more economical include placing filters and water softeners upstream of the RO system to reduce maintenance cost. Another approach is to only send a fraction of the water through the RO membranes. For example if the influent water has a sodium concentration of 100ppm and a concentration of 50ppm is desired only 50% of the influent needs to be sent through the RO system.
We like to think of filters as an insurance policy we’ll use every month. Filters should be used in every irrigation system, whether it be drip, sprinklers, ebb and flood, etc. - it’s just too cost efficient not to. But like insurance policies, they aren’t all the same and the only way to know which one is best for your irrigation application is to get a water analysis and determine what needs extracted.
Jacob Carson is a Water Systems Engineer within the Commercial Greenhouse Division at RBI. He believes that every growing operation is unique and should be treated as such. His aim is to engineer based on efficiency and sustainability. Jacob is currently in his third year at the University of Cincinnati studying Environmental Engineering.