When the pores become clogged with particulates, the pressure required to force the water through the substrate increases, therefore treated water volume is reduced unless pressure is increased. These systems must be back-flushed to remove collected particulates and maintain the effectiveness of the filter . Smaller particle size filter media clog more easily and require more frequent back-flushing. As the amount of particulates in the intake water increases, so does the frequency of back washing, which can waste water if it is not recaptured by the system. These systems do not remove most chemical and biological contaminants, but are mainly used to limit clogging of irrigation lines and emitters and to minimize inactivation of sanitation chemicals via sorption to non-target particulates .Disc filters are mechanical filters that typically handle smaller volumes of water per unit time than rapid sand filters . Disc filters can remove particles up to 150 μM and are used as primary filters when water is relatively free of particulates or if only small volumes need to be treated; when larger particulates may cause clogging , they can also serve as secondary filters behind rapid sand filters . Like rapid sand filters,arandanos cultivo they must be back washed periodically to clean particulates out of the discs.
Disc filters do not remove chemical contaminants or most pathogens from the water. Other types of mechanical filters are 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. These systems are generally used in greenhouses to filter recaptured water from the operation.Activated carbon is not a stand-alone treatment and should be paired with another filtration system to increase treatment efficacy . Activated carbon has a large, porous internal surface area with filter pore sizes ranging from 10 to 500 μm and can be manufactured to desired particle size with a low acid/base reactivity. Activated carbon is positively charged and can adsorb organic, moderately polar compounds and negatively charged contaminants depending upon source material and pyrolysis, oxidation, and purification methods . The internal structure of activated carbon influences its capacity to adsorb contaminants . Activated carbon is used extensively in micropropagation applications to mitigate effects of inhibitory compounds on plantlet growth . As the volume of water per unit time increases, carbon filters become less effective because contact time with the activated carbon decreases. Water pH, ions present , and concentration of other contaminants also influence the efficacy of carbon filters . Activated carbon systems require periodic maintenance including replacement or regeneration of carbon once it has been saturated, which depends on water volume and contaminant loads.
Carbon filters remove some pesticides . Economic losses associated with stunted or deformed non-target crops can be attributed to presence of residual ancymidol or paclobutrazol at concentrations as low as 3 or 5 μg L−1 , respectively . Detection and remediation of plant growth regulators with activated carbon are currently being evaluated. The cost of the technology, along with its potential to remove beneficial compounds such as residual metals applied as fertilizer, may make its application less useful in some circumstances. Additional research on efficacy and economics of carbon filters would benefit growers, particularly in the area of plant growth regulator removal, which is a concern particularly in greenhouses.Membrane filters work by exerting pressure on water on one side of a membrane to sieve particles from the water stream . The permeate, or filtered water, is pushed through the filter while the retentate, or concentrated waste stream, must be disposed of or treated . Membrane filters facilitate removal of contaminants with particle sizes ranging from 0.1 to <0.0005 μm . Within this size range, filter classifications are defined by pore size and membrane pressures as identified in Table 2. The pore sizes of material from which membranes are derived differ and thus influence their applications and the types of contaminants that can be controlled. Membranes can become clogged over time and may require periodic remediation to manage fouling. Remediation may consist of back washing for micro-filtration and ultra-filtration or use of acid or alkaline detergents to mitigate inorganic or organic fouling, respectively . Membranes can be produced which avoid fouling by using pre-filters or membrane surface modifications, whether to alter hydrophobic/hydrophilic ratios for nano-filtration and reverse osmosis or to manage electrostatic attraction sources so membranes actively repel fouling agents .
Contaminant remediation using membrane filters is considered prohibitively expensive for use in most container production systems except in asexual plant propagation of high value crops—where high-quality water is critical, due to installation and maintenance costs, pumping costs, downstream processing costs, and rapid clogging of filters .Anionic, water-soluble polyacrylamide are long chains of linked acrylamide . They have been used since 1995 as an additive to reduce irrigation-induced sediment loss, promote infiltration, and induce flocculation and aggregation of suspended solids from irrigated production runoff . The PAMs used in agriculture contain less than 0.05% of acrylamide monomer, which is considered toxic to humans . Anionic PAMs are considered safe in the environment, as they have a low aquatic toxicity in comparison with cationic and non-ionic forms .Use of PAMs has primarily focused on mitigation of erosion, but when PAMs are used to flocculate suspended solids from water, they also remove any bound pesticide, phosphorus, and microbial residues that are adsorbed to those particles. Pesticide removal depends upon the chemistry of the compound; efficiencies depend upon the compound evaluated with removal averaging 78.7% for bifenthrin, 38% for bupirimate, 49% for atrazine, 49% for chlorothalonil, 54% for endosulfan, 84.2% of cis-permethrin, and 71.2% of transpermethrin , though it is difficult to differentiate between flocculation of sediment-bound pesticides and pesticide removal by PAM alone. Dissolved reactive phosphorus was not removed, but particulate P was . Sojka and Entry reported that PAM treatment reduced total algal, bacterial, fungal, and microbial biomass in irrigation water. Applying PAM during irrigation with rates as low as 1–2 kg/ha halted 94% of erosion from irrigated furrows and 92.9% sediment reduction when PAM was injected at 10 mg/L into nursery production runoff . Estimated cost per acre in 2008 was $10 to $30 per acre at these application rates .Filter socks are used primarily as a sediment trap or to retain some chemicals from construction site runoff, but in recent years filter socks have also been evaluated for mitigating sediments and agrichemicals from surface water runoff in agricultural fields . Filter socks can be filled with a variety of organic media, primarily composted wood chips,maceteros grandes reciclados that can be further amended with inorganic adsorbents/precipitants or synthetic additives such as PAM or other polymers to enhance flocculation, depending on their purpose . Filter socks cost between $3.50 and $15.00 per linear foot for continuous non-amended or amended filter socks, with amended socks having higher costs. Filter socks can provide significant sediment control when installed and maintained properly . 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; however, there are reusable/refillable Bflexible filter hand bags^ for catch basins which have been recently introduced . Filter sock flow-through rate, and subsequent ponding prior to the sock, is affected by substrate packing density and particle size . If sediment levels build up to the point where water crests the filter sock instead of flowing through it, the filters are less effective.Also, if the socks have poor contact with the ground, the volume of water is too large, or the slope is too steep, the filter sock can be bypassed, reducing treatment effectiveness. Hydraulic flow-through rate may better predict sediment and phosphorus removal than particle size distribution alone .
However, substrate particle size influences hydraulic flow-through rate; thus, both flow rate capacity and particle distribution are pertinent factors when designing filter socks for sediment control. Average removal percent efficiency of compost filter socks varies by contaminant and initial concentration or load , with concentration reductions reported for sediment , total and/or soluble phosphorus , or pesticides , and 18% for alachlor. Flow rate capacity and lifespan of filter socks are especially pertinent 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 a critical design factor and requires further investigation. One option to investigate is alternate layouts that capture sediment without impeding flow, similar to stream restorations .The first step in irrigation water treatment is typically physical removal of macro-particles via filtration, followed by the addition of chemical disinfectants to reduce the spread of water-borne diseases. Chemical applications are more effective when carbon-based particulates are removed prior to chemical treatment, because organic compounds create a demand on active ingredient of chemical treatments . Chemical treatment efficacy declines if high levels of organic matter are present in water, necessitating higher concentrations of chemical to treat the same volume of water . Whereas filtration removes physical impurities from irrigation water that are larger than single-celled microbes, chemical treatment is targeted towards the removal of biological contaminants . Removal of pathogens and other biological organisms improves crop health and system longevity. This section covers some of the more commonly adopted disinfection technologies including chlorine, copper, peroxides, silver, ozone, and ultraviolet light. A review of a wide range of treatment technologies not discussed herein can be found in Raudales et al. .The most commonly used chemical treatment is chlorination either as a solid , liquid , or gas or generated through an electrolysis process. The presence of 0.5 to 2 ppm of free chlorine at the sprinkler head is recommended to ensure adequate sanitation . Chlorine levels should be routinely checked during crop production, as changes in water quality and the amount of organic matter in treated water impact the chlorine residual that will exit the sprinkler head. Hypochlorous acid , the most important sanitizing form of dissolved chlorine, is favored over the weaker hypochlorite form at pH below 7.5. Therefore, acidification of irrigation water is often desirable prior to chlorine injection to increase chlorine efficacy. Many operations use chlorine for disinfection because of its cost effectiveness and relative ease of use. The major concern with chlorine is the additional safety precautions that are required for its use, which vary by type of chlorine. Extensive discussions related to efficacy, dose, costs and benefits, and timing of chlorine injection are available in Newman , Raudales et al. , and Stewart-Wade .Ozone is a strong oxidant that disinfects by producing a reduction-oxidation reaction in pathogens and other organic constituents . An ozone production system uses electricity to split oxygen molecules to form ozone . Ozone breaks down into peroxides and other oxygen radicals, providing additional disinfection. No additional inputs are required, and no persistent by-products are produced. Ozone and by-products degrade quickly in water, so direct testing is difficult, but in-line monitors that also control injection concentration are typically employed . Ozone activity is reduced in the presence of organic matter, high pH, and/or high concentrations of nitrite, manganese, iron, or bicarbonate . For a high level of disinfestation, Runia reported that a dose of 10 g O3/m3 water with a 1-h contact time at a pH 4 resulted in kill of 99.9% of bacteria and fungi. However, this process requires injecting ozone into a storage cistern rather than in-line injection, as can be used for other chemical treatment technologies. Other studies with ozone have varied contact times , with effective control achieved with ozone doses ranging from 0.01 ppm O3 for algae control to 1.6 ppm O3 to control Phytophthora cinnamomi chlamydospores and 1.75 ppm O3 to kill Fusarium oxysporum conidia . Ozone is one of the most expensive water treatment options in terms of installation cost, with electricity being the main operating cost . Potential human health effects from ozone exposure require fail-safes and adequate venting, thus reducing the popularity of ozone for treating pathogens in irrigation water .Slow sand filters have been used since 1804 to cleanse contaminants from water for both drinking water and industrial uses and have been adopted by the European horticultural community to remove phytopathogens from reused irrigation water since the early 1990s .