Although activated carbon and membrane filtration are typically used more in greenhouse production, they definitely are technologies that would be appropriate in some nursery production scenarios. Activated carbon or membrane filters are tools worth considering if you recycle water or have problems with water quality.
Activated carbon (charcoal) filters
Activated carbon is a material that is produced when a carbon-based material (wood, coconut fiber, or coal, for example) is heated under specific conditions, increasing both surface area and number of reactive binding sites. The internal structure of activated carbon is affected by production method, which also influences its capacity to adsorb contaminants. Because of carbon source variability and different production methods, once you settle on an activated carbon supplier, request their routine test results regarding the consistency of their product. Activated carbon is positively charged and can adsorb organic, moderately polar compounds and negatively charged contaminants (such as paclobutrazol, chloramines, and some herbicides). Activated carbon is a highly porous material, which allows for both physical adsorption and chemical binding of impurities. A teaspoon of activated carbon has as much surface area as a soccer field.
Activated carbon is not a stand-alone treatment and should be paired with another filtration system to increase treatment efficacy. Without a pre-filter, activated carbon can clog quickly and lose its effectiveness.
Activated carbon is typically used in closed greenhouse systems to ensure plant growth regulators and other compounds are not being reapplied to non-target crops. Activated carbon is used extensively in micropropagation (tissue culture) applications. As water recycling becomes more common, activated carbon will likely be included in nursery production as well. As flow rate increases, carbon filters become less effective as contact time with the activated carbon decreases. Proper planning of filter design is critical, with regard to particle size and total volume of activated carbon, to ensure adequate treatment at the highest water flow rates. One should also know the quality of the water passing through the filter, as pH, ions present (e.g., KI, KCl, and NaCl), and concentration of other contaminants (plant growth regulators, PGRs) also impact the efficacy and life-span of carbon filters.
Maintenance of activated carbon includes replacing or regenerating carbon once it has been saturated. The frequency of replacement/regeneration varies based on the volume of water processed and contaminant loads, but a good maintenance checkpoint is about every 18 months.
Potential drawbacks with the use of carbon filters include: (1) It is difficult to know when the activated carbon is becoming less effective (binding sites saturated). You do not want to find out you have a problem by noticing stunted growth in your plants. For this reason, (2) the activated carbon within the filters should be replaced at regular intervals based on design criterion (water treatment volume and typical contaminants present).
In terms of cost, installation requirements for a carbon filter are similar to those for installing a rapid sand filter. Granular activated carbon can be regenerated several times before it needs to be replaced, which decreases operating costs (though frankly your supplier likely has facilities better suited to regenerating the material – there’s likely a way to develop a service agreement for activated carbon reuse). It is also important to be aware that activated carbon removes beneficial minerals such as micronutrients along with residual PGRs. This is a concern particularly if fertigation solution is injected prior to water flowing through an activated carbon filter.
If you recycle water, routinely spray PGRs, and non-target crops at your operation are stunted or deformed, residual chemicals in your recycled water may be reducing plant quality – and thus hurting your bottom line. Low concentrations of PGRs in recycled water can also slow growth of less sensitive plant species, this slowing of growth can be difficult to detect. Levels as low as 3-5 ppb (parts per billion) can impair plant growth.
Activated carbon filtration is a relatively new technology. Additional research on removal efficacies and the economics of treatment for both greenhouse and nursery production are necessary. This technology is currently being used and tested mainly in greenhouse production, but there are numerous applications were it makes sense for nursery production as well.
Pressure-driven membrane filters
Membrane filters work by exerting pressure on the incoming water to push it through a membrane. The size of the pores in the membrane determine which substances are filtered out and which substances are allowed through. Table 1 lists different filter pore sizes, and the particles they are able to remove. The larger the pore size, the higher the flow rate that can be treated with the same amount of membrane surface area. The concentrated retentate (residual water) consisting of all the materials that are filtered out are then disposed of or treated. For reverse osmosis, the membrane pores are so small that only water molecules can fit through them.
A number of different types of materials can be used to make membranes including ceramic, mineral, organo-mineral, or polymeric compounds. Each material has benefits and drawbacks that need to be considered including service life, cost, ease of use, and reusability.
A membrane system needs to be sized to meet the flow requirements at your operation and will require maintenance. Membranes can become clogged over time and may require periodic remediation (weekly to yearly) to manage fouling. Remediation may consist of backwashing for micro-filtration and ultra-filtration or the use of acid or alkaline detergents to mitigate inorganic or organic fouling, respectively. Fouling can be reduced by using a pre-filter (think rapid sand filter, just like one would use before a carbon filter). Membranes can also be manufactured to minimize fouling (they have altered hydrophobic/hydrophilic ratios) for nano-filtration and reverse osmosis or to manage electrostatic attraction sources so membranes actively repel fouling agents (nano-filtration, ultra-filtration).
Contaminant remediation using membrane filters is expensive for use in most container production systems except areas where high value crops are produced (e.g., plant propagation) — where high-quality water is critical. If it is difficult to get consistent, high-quality water, membrane filtration may be worth considering. Membrane filters can be designed to yield high-quality water that can then be used to irrigate sensitive plants or blended with lower-quality water, for operation-wide irrigation.