Is my water clean enough or will it harm my plants? The question is simple, the answer is not. Water is essential for every nursery operation; without adequate water, results could be devastating. How do you know that you have clean water? How can you improve your water quality (and potentially your water quantity)?
We will answer these questions, and many more, in a five-part series focused on water treatment in nursery production. This month focuses on sediment filtration, while future articles will focus on agrichemical filtration (fungicide, plant growth regulator, fertilizer etc.), chemical and UV water sanitation, using plants to clean water, and microbial treatment of water.
One of the most common methods of water treatment is physical screening. Typical physical filters include sand, mesh screen, disc, glass, or other substrates that serve as barriers to particulates. During physical screening, the materials in the filter serve as a barrier to stop particles (sediment) from moving through the filter. Filter efficacy is determined by filter size (both the surface area available and the pore size of the filter), sediment particle size and amount, and desired flow rate. These factors impact the cost of installation and upkeep (parts and labor). Saving time and money may be as simple (or complex) as moving your pump intake to the middle of the pond profile, half way between the surface and the bottom, and as far away as you can from where the water enters your pond to minimize particles in the water. This includes ensuring the pump intake does not receive or is near where the filter discharges or backflushes. The further the distance from return water entry to pump intake, the more sediment will drop out (Figure 1). The distance between pump intake and irrigation return water is also beneficial for reducing water-borne plant diseases.
Rapid sand and glass filters consist of tanks that hold sand or glass of a specific particle size, and pumps and pipes that move the water. As water moves through the sand or glass, particulates are trapped and not able to pass through. Sand and glass filters can process large volumes of water quickly, but they are only effective at removing sediment and other larger organic material. Plant diseases and agrichemicals, which can have major negative impacts on your plants, are not removed by these filters.
There are two major benefits of rapid sand filters: they increase the efficacy of water sanitation (chlorine, UV, etc.) and reduce potential for irrigation lines and emitters to become obstructed.
For sand or glass beads, typical particle size ranges from 0.005 to 0.025 inches (125 to 640 µm), but as particle size decreases, more force (larger pumps) are required to move the same volume of water. Regardless of particle size, when pores become clogged with particulates, the system must be back-flushed with water (run in reverse) to clean the filters. Water with more particulates or a system that has smaller particle filters will require more frequent back-flushing. Sand and glass filters do not remove most chemical and biological contaminants; they are mainly used to limit clogging of irrigation lines and emitters and to minimize inactivation of sanitation chemicals (e.g., chlorine).
A number of filters are similar to rapid filters in what they do and how they work. Disc filters (Figure 2) (as well as mesh filters) are also mechanical filters but usually handle smaller volumes of water per unit time than rapid sand filters. Mechanical filters come in a variety of sizes and can remove particles down to 0.006 inches (150 µm). Disc and mesh filters can be used as primary filters when water is relatively free of particulates or if only small volumes need to be treated; they can also serve as secondary filters installed after rapid filters as a second line of defense. Like rapid filters, they must be backwashed periodically to clean particulates out of the screens and do not remove chemical contaminants or most pathogens from the water. Some other types of mechanical filters exist and can be used for specific situations. Paper filters and rotary screens are typically used to remove sediment and large debris from water that is not typically under pressure (for example from a pipe that is returning water to a pond, cistern or other storage area).
Sediment ponds (traps) are not filters per se, but can be used to reduce the volume of sediment entering a containment basin (pond). Ideally, these sediment traps should have a concrete bottom and easy access for equipment to clean them out. A sediment trap is nothing more than a “pre-pond” where water velocity slows down, and sediment can drop out before it reaches the main pond. Using a sediment trap will help you maintain your designed pond volume, by enabling sediment load removal prior to its accumulation in your pond.
Keeping soil in place is in everyone’s best interest. Erosion can be a major problem not only because it can decrease pond capacity and increase filtration problems, but also because of safety concerns (road and ditch structure), poor aesthetics, and potential for agrichemical (fertilizer, fungicides, herbicides, PGRs) movement with sediment that can cause environmental problems. One way to reduce erosion is by applying polyacrylamide. Polyacrylamide, or PAM, is a water-soluble chemical made up of long chains of linked acrylamide (C3H5NO). Cationic forms of PAM have been used safely for about 20 years to reduce sediment movement from agricultural sites after rainfall or irrigation events.
While PAMs are primarily used to reduce erosion, they can also flocculate (cause clumps to form) sediment from water, which also removes any bound pesticide, phosphorus, and microbial residues that are bound to those particles. Pesticide removal depends upon the chemistry of the compound; with removal efficacy ranging from 38 percent to 84 percent depending upon pesticide chemistry (for more information PAM removing specific pesticides, please visit https://goo.gl/ox3Tza). Treatment of water with PAM can also reduce total algal, bacterial, fungal, and microbial biomass in irrigation water. Applying PAM during drip irrigation at 10 ppm reduced erosion into nursery runoff by 93 percent. If you have erosion problem areas, consider trialing PAM to see if it helps control sediment runoff.
Filter socks (Figure 3, page 28) and silt screens are another way to reduce sediment loss. Both filter socks and silt screens are common at construction sites and are used primarily as a sediment trap. They can also retain some chemicals that are bound to sediment (phosphorus, agrichemicals etc.). Filter socks can be filled with a variety of organic media, but are typically filled with composted wood chips. The wood chips and cover (sock) are what slow water movement down and help the sediment to drop out of the water column. The wood chips can be further amended to enhance flocculation.
Costs for filter socks are $3.50 to $15.00 per linear foot with the less expensive price for just wood chips, and the price increasing with additives. Filter socks can provide sediment control when installed and maintained properly, particularly on sites that have elevation changes. Most filter socks have a relatively short lifespan of a few months to a year before they begin to saturate, break down, and lose their effectiveness. There is a new product that is reusable/refillable called Bflexible filter bags, which are designed for catch basins. Filter sock porosity is affected by the particle size of the wood chips and how packed the fill material is. As the wood chips break down, and the pores get filled with sediment, the socks become less porous, which can lead to ponding.
Periodic removal of the sediment that builds in front of the sock helps keep water moving through the sock instead of over it. Proper placement is also very important. If there is poor contact with the ground, the volume of water is too large, or the slope is too steep, the filters will not be as effective. Average removal percent efficiency of compost filter socks varies by contaminant and initial concentration or load. Sediment and phosphorus presence in runoff can be reduced by 59 percent to 65 percent using filter socks. Flow rate capacity and lifespan of filter socks are particularly important considerations in nursery production areas, where uncontaminated water and production runoff events often flood roadways. The capacity of the filter socks to manage sediment, while remaining in place and not backing up water into production areas, is critical for optimal function.
We’ve discussed a few useful tools for reducing sediment and organic matter movement through production areas. Filter socks, silt fences, and PAM can reduce sediment movement “downstream,” while also trapping any contaminants that are attached to the sediment. Any sediment that reaches a pond can be filtered with a sediment trap, and any remaining organic matter or sediment can be removed from the irrigation system via a variety of physical filters. Taking these steps to reduce sediment movement will help reduce chemical inputs into your pond, help maintain a cleaner operation, increase the effectiveness of sanitation, and reduce clogs in your irrigation system. What will you try? If you’ve tried these technologies, let us know what you think. And if you have more questions about these treatment options, feel free to contact us.