Understanding Filtration in Hydroponic Systems
A clean hydroponic system with high-quality water and nutrient inputs is one of the most effective ways of avoiding pathogen problems and producing a great crop. These days there is a general move away from the use of chemical water/nutrient treatments, towards more sustainable and natural approaches that help maintain a healthy microbial balance as well as controlling any nasties.
Filtration as a water purification process has been around for a very long time, initially as a way of cleaning up drinking water and preventing human disease. However, this technology has been adopted by greenhouse and hydroponic growers and is one of the most reliable options for water and nutrient solution treatment.
Why Filter?
Water supplies can contain many contaminants and crops vary in their sensitivity to certain water quality issues. Hydroponic water supplies that originate from rainwater, dams, rivers, reservoirs, shallow bore/wells, and streams are likely to have come into contact with plant and/or food safety pathogens. As well as microbial contamination, soil and other organic particles can be found in many water supplies. Filtration can be used to remove contaminates over a wide range of particles sizes resulting in a clean and high-quality water supply. Filtration used within a hydroponic system is mainly for the purpose of cleaning up recirculating solutions which have passed through root systems before returning to the nutrient reservoir.
Recirculating systems such as NFT, may have filtration systems to remove particles of substrate, pieces of root system and other debris.
Types of Filtration for Hydroponics
Mechanical Filtration
Several filtration methods may be incorporated into hydroponic systems for different purposes. Coarse or mechanical filters on recirculating systems are used to catch debris in the nutrient solution as it returns to the reservoir; these may include particles of growing substrate, pieces of root systems, and undissolved nutrient scale and precipitates. Synthetic mechanical filters (which are used to remove contaminants such as sand or soil grains, algae, roots, and other small particles) typically have a relatively large pore size in the range 10,000-100,000nm. In comparison, slow sand filters have pore sizes in the range 1000-1000nm. Rapid sand filters may be used in hydroponic systems to remove large particles from the drain water before it is reused within the irrigation system or before unwanted nutrient leachate is disposed of. The returning nutrient solution may then go through an additional synthetic filter to remove any undissolved fertilizers and avid clogging the irrigation system. Similar filters are often used to pretreat water and nutrient solutions before undergoing other disinfection methods such as UV or ozone.
These types of coarse filters may also be included ‘inline’ within the irrigation system to catch any debris before it reaches drippers or emitters and causes blockages. Mechanical filters for hydroponics are relatively inexpensive and some innovative growers construct their own and incorporate these successfully into a wide range of different systems.
Filters can be constructed by the grower such as this shade cloth material used as a coarse mechanical filter.
Membrane Filtration
Other types of filtration aim to remove pathogens from recirculating nutrient solutions, but may also be used to treat water supplies before being added to a hydroponic system. These are: membrane filtration, which includes micro filtration (pore size 100-1000nm), ultra filtration (10-100 nm), nano filtration (1-10nm) reverse osmosis (< 1nm), and slow sand filtration or bio filtration. There are several membrane-type filtration systems that can be used to remove microbes from water; the most efficient use a combination of different sized filters to progressively remove smaller and smaller particles as the water or solution flows past. Micro filters (100-1000 nm pore size) generally remove suspended particles and most bacteria; ultra filtration (10-100 nm pore size) can remove proteins, fungi and viruses, while reserve osmosis (<1 pore size) is used to remove unwanted ions from water supplies as well as pathogens.
The smaller the pore size of the filtration unit being used, the more the flow of water is inhibited and slowed, and higher pressures and frequent cleaning of the filters are required to prevent clogging. Many systems have automated back washing of the filters to prevent frequent clogging and improve efficiency. If used correctly, membrane filtration has been shown to be effective for several different pathogens, however the system requires regular maintenance to ensure it is operating correctly. Over recent years, the filtration technology developed for use in the food industry and for water purification has seen some good advancement in filtration systems which is more reliable and cheaper than it has been in the past.
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Activated Carbon Filters
Activated carbon or charcoal filters are a relatively inexpensive tool for water treatment and are widely available in a range of different sizes. These are more often used on water sources from municipal or city water supplies where water treatment chemicals may become an issue for sensitive hydroponic crops. Carbon filters consist of canisters filled with activated carbon that removes certain chemical and compounds including chlorine and other water treatment chemicals and organic compounds. Activated carbon is a material specifically produced to achieve a large internal surface area (500-1500 m2/g) for the adsorption of certain substrates. While these can be highly effective for some water supplies, the activated carbon requires frequent replacement for the filter to remain effective.
Different grades of sand and gravel are the raw material used to construct highly effective slow sand filters.
Biological Filtration: Slow Sand Filter
Bio filtration is a highly effective method of filtration which has long been used to clarify drinking water supplies. The advantage of bio filtration systems is they carry out complete filtration from removal of large particles and sediments, down to controlling a range of pathogens. The most widely used form of bio filtration is in the form of slow sand filters — while these are still mostly constructed of sand of different particles sizes, other materials such as granulated rockwool have proven equally as successful. Slow sand filters are not only highly effective, they can be constructed from inexpensive materials and are reliant on populations of beneficial microbes which develop inside the large surface area of the filter material to control and suppress plant pathogens. Slow sand filters are relatively easy to construct and require a column of gravel/sand or suitable alternate material which is graded by particle size.
The top of the filter must be open to the air as oxygen is a vital component of biological filtration with the base of the filter filled with coarser grade sand or gravel (8-16mm) with finer grades of sand towards the top of the filter (1-2 mm). To operate an effective slow sand filter, the nutrient solution or water to be treated must be dripped or sprayed onto the surface of the filter so as not to dislodge or disrupt the filter surface. The water or nutrient solution then slowly flows down through the filter material and out the base to be incorporated back into the hydroponic system. Studies have shown that the ideal flow range through these types of filtration systems is around 100 l/m2/hour for optimal pathogen elimination. Under these conditions with a sufficiently slow flow rate, studies have found that root rot pathogens such as Phytophthora and Pythium can be eliminated completely while Fusarium, viruses, and nematodes are removed at a rate of 90-99.9 percent. Nutrient solution flowing through the filter undergoes biological filtration, however, this process will not change the physical or chemical nature of the solution, so EC, pH, and levels of individual nutrient ions will not alter during filtration. Another advantage of this system is that treated nutrient solution leaving the base of the filter is enriched with populations of beneficial microorganisms which may further assist with pathogen control and reduction when returned to the hydroponic system.
Summary
While mechanical, membrane, activated carbon, and bio filtration are all well proven methods for hydroponic water and/or nutrient solution treatment, some filters commonly used for water supply treatment are not ideal for hydroponic use. These include ‘water softeners’ which use ion exchange resins to remove minerals from hard water sources (typically calcium and magnesium) and replace these with sodium. Sodium can rapidly build up in recirculating hydroponic systems and become toxic, thus water softeners are best restricted to use for evaporative cooling walls where the removal of these minerals prevents scaling and clogging and not used for water supplies destined for hydroponic nutrient solutions.
Correctly used, filtration is a vital component of hydroponic systems. From prevention of irrigation component clogging, removal of unwanted sediments and organic material, and control of pathogens in both raw water sources and recirculating nutrient solutions, filtration is an efficient and practical tool for the hydroponic grower.
