Choosing a Sand Media Filter
Over most of my career I have been asked what type of Sand Media Filter a grower should purchase. This question has many parts: is the filter manufactured from stainless steel or carbon steel, does it have a single chamber or double chamber and is the collector welded stainless-steel, removable stainless-steel or plastic. All great options, but what is best for a grower? What characteristics should be evaluated when deciding on the type and manufacturer? The rest of this article is my opinion. I won’t say that any sand media filter will not work, I just believe there are types of sand media tanks that work better than others for unique water conditions.
How does a Sand Media Tank work?
Sand Media tanks are pressure vessels that have an inlet on the top and an outlet on the bottom.
The area between the inlet and outlet uses sand as a filtering surface for debris.
This sand has various degrees of filtration but most common media for agriculture is #16 or #20 silica sand. As with all media types, the more the debris collects on the surface of the sand the greater the degree of filtration.
Once the pressure between the inlet and the outlet reaches the differential pressure of 7.5 psi, an automatic valve is actuated and one tank is closed off from the inlet, forcing clean water from the other tank or tanks to push upward from the outlet and forcing the debris through the backwash line. This forces a great amount of water through the tank -up to 200 gpm. Plan on a large collection area for the water that will discharge from the backwash line.
When should you use a sand media filter?
When should you use a sand media filter instead of an auto screen filter or manual filter? Sand media filters can be used in with most water sources. They are great for removing organic material from irrigation water. They also work well to remove silts and other small inorganic particles. The only time sand media filters should not be used is when the water source carries a sand particle with a higher density than the media in the tank.
Can the sand media filter work with any type of water source?
The simple answer is yes, but it depends on the construction of the sand media filter body, manifolds and valves. Water sources in most agricultural areas have trace amounts of different minerals, organic material and inorganic material. The sand media filter can handle all these materials. The sand media tank is also exposed to the environment and might be exposed to salt water, high humidity and high heat. With the correct material, it can withstand any of these environmental conditions.
Sand Media Filter Construction
There are several types of sand media filters made from a variety of metals and plastics produced by many manufacturers. Sometimes there is confusion over what is and isn’t important in their construction.
Let us examine these sand media filter characteristics. Starting with material, there are two main options: stainless steel and carbon steel. There are also sand media filters constructed of fiberglass and molded plastic, but these are less common in agriculture.
Now consider the method used to collect the filtered water and disperse the backwash water.
There are two basic methods for building collectors. The first type is single chamber sand media tanks with collectors/ diffusers attached to outlet. The collectors are constructed of a variety of materials like injection molded Polyethylene, PVC replaceable stainless-steel wedge wire screen and weld in place stainless-steel wedge wire screen.
The second method is a double chamber tank with small plastic diffusers mounted into a steel plate. The plate provides a great method of distribution of the flow of the backwash water.
Now, to decipher these options and arrive at the which is the best sand media filter for your irrigation application. To help us make a good decision, first evaluate the materials of the tanks. This article will focus on the two most popular materials used in agriculture, Stainless Steel and Carbon Steel.
Stainless Steel sand media tanks are lighter in weight, the manifold is lighter in weight, and both are resistant to rust inducing environmental conditions. The tanks are typically rated at 80 PSI and only one manufacturer is 100 PSI. If the collector is made of stainless steel and welded into the outlet it eliminates the need for a safety screen.
Carbon Steel sand media tanks are heavier, coated on the outside with baked-on epoxy. In the past, carbon steel tanks required painting every couple of years. There has been great improvement on the epoxy coatings to a point today that carbon steel tanks are resistant to rust inducing environmental conditions for their entire service life. In addition, carbon steel tanks with stainless steel collectors that can be welded into the outlet can eliminate the need for a safety screen.
Since both materials will work in many of the same applications, what else could be evaluated to determine which media filter manufacturer to choose? The collector comes in next as the most important item to evaluate when choosing a media filter manufacturer.
The Double Chamber tank has a plate with molded collectors mounted on the plate. Pro: The double chamber is great for backwash uniformity. This provides a good stirring of the sand helping to remove the debris and keeping the sand clean. This is very important to increase the longevity of the sand. Because of the plate and formed collectors, it is very important to keep the pressure differential below 10 PSI. Con: If the pressure differential goes up above 10 PSI there is a risk to plate and welds holding the plate in place. This filter requires additional steel for the plate, so the price is typically higher.
Single Chamber tanks have the collectors attached to the outlet. Pro: The single chamber tank is easier to produce and often are less expensive than double chamber tanks. Con: The collector requires gravel or a central manifold and a lot of arms to create a uniform backflush.
Reviewing the available collector styles will reveal additional choices and a way to narrow down the manufacturers.
Molded collectors are injection molded plastic pieces assembled on a central shaft with fine slots, which are smaller than a #20 silica sand. The collectors are mounted on a central PVC Manifold that is attached to the outlet. Pro: The collector can be installed after epoxy coating the inside of the tank. No gravel is needed - only media sand. Con: These molded pieces can deform under high pressure. When close to the side wall in high heat the units can soften and deform. Held in place by a stainless-steel bolt and nut. The collectors extend straight out from the Manifold. They require a safety screen.
PVC Collectors are molded and assembled on a central shaft with fine slots. The individual units are attached to a central manifold and the manifold is attached to the outlet. Pro: PVC is strong and does not deform under pressure and heat. The collector is assembled in the tank after epoxy coating. Only sand is used for the media (no gravel). Con: The PVC can be damaged if shovels or other hard objects are inserted to check the sand. Held in place by a stainless-steel bolt and nut. The collectors extend straight out from the Manifold. They require a safety screen.
Removable Stainless-steel wedge wire screen with fine slots are assembled to a central PVC hub. The slots are welded to a threaded stainless shaft threaded into the PVC. The manifold is attached to the outlet. Pro: The collector is constructed of strong material. The unit uses gravel to help disperse the back-flush water. The collector is assembled after the epoxy coating. Con: The metal is threaded into PVC. Held in place by a stainless-steel bolt and nut. The collectors extend straight out from the Manifold. They require a safety screen.
Stainless-steel wedge wire screen with fine slots welded onto a central hub. The collector is welded onto the outlet of the tank. Pro: The collector is constructed of strong material. The unit uses gravel to help disperse the back-flush water. The ends of the collectors bend upward allowing the collector to sit lower in the tank and allowing a greater depth of sand for filtration. No safety screen is required. The welded screen can withstand great pressures. Con: The collector is welded onto the outlet, so a special coating is used to protect the inside of the tank from corrosion.
Sand Media Filter Summary
In summary, the options available are unique. Stainless-steel has good material characteristics and is considered a good option for most water sources. The only issue with stainless-steel is microbes can attack and destroy the stainless-steel in a brief time like 2 years. If your water source is known to have these microbes, consider using the option of sacrificial anodes. Carbon steel works great and with the improvement in manufacturing process they epoxy will hold up in harsh environmental conditions. Carbon steel tanks have operated in the field for up to 25 years.
Double Chamber tanks are good media tanks, but the added cost is often a breaking point. Single chamber tanks have improved their backflush stirring capability with balanced collectors and dispersing gravel making them an ideal choice.
The manufacturer differences have worked down to the collector. I think the best option is weld in place as this option removes the cost a safety screen.
Reviewing all the supplied information has led Grow Irrigation to select Everfilt as a supplier for providing sand media filters. They are the only company to offer as a standard 100 PSI stainless-steel tank. They offer welded-in-place collectors that can safely handle a 40 PSI pressure differential, preventing media sand from contaminating the irrigation system. Safety is a big factor in selecting filters and Grow Irrigation will only provide a product that will preform under very harsh conditions in most water sources and not rupture.
I did not have room in this article to discuss other parts of the system such as the backflush valve and the controller. There are differences in these pieces of equipment, but they can be discussed in another article.
For comments and information please contact me.