Greenhouse Water Treatment and Filtration
Water Filtration for Suspended Solids
The suspended solids (SS) in irrigation water are the inorganic particles such as clay, silt, and other soil constituents, as well as organic matter such as plant material, algae, and bacteria. Suspended solids need to be removed from irrigation water to prevent clogging piping, valves, nozzles, and emitters. Suspended solids are more of a problem with surface water. Ground water, although usually clean, may contain fine particles of sand or suspended iron. There is a wide variety of filter systems used to remove contaminants from either source water or water recovered for reuse in recirculating systems. Often, filters are used as a pre-treatment step prior to other physical disinfection systems, such as UV, that require relatively clear water in order to perform optimally or chemical disinfection where organic materials can tie up the active disinfection component. Ideally, total suspended solids should be less than 20 mg/L. Coarse materials can rapidly clog filtration systems designed to remove finer particles (e.g., microfiltration) and should be removed first to increase the efficiency and longevity of a more expensive downstream filtration system. The capital and operating costs of filtration systems generally increase with decreasing pore size, from very low for in-line screens to very high for nano- and ultrafiltration.
Pre-Treatment or Pre-Filtration
Pre-treatment or pre-filtration of irrigation water involves the use of various filter types to remove organic and inorganic particulate matter (debris, sediment, soil particles, algae, etc.) from the water prior to treatment for pathogens. Pre-filtration is important for two reasons. Firstly, larger particulate matter has the potential to clog the irrigation system (e.g., emitters). Secondly, the effectiveness of many pathogen treatments (e.g., all chemical treatments using oxidizers, UV) is decreased significantly in the presence of particulate organic matter.
Centrifugal Sand Separators
Centrifugal sand separators, in theory, are not actually filters but are used as pre-filtration devices for other types of filters (Figure 19.3). A centrifugal sand separator removes larger particles of sand, silt, or other abrasive grit particles that can lead to the premature degradation of irrigation system components. These contaminants can reduce the efficiency of the irrigation system equipment by plugging and clogging valves and emitters. A sand separator is effective in removing up to nearly 98 percent of particle sizes too large to fit through an equivalent 200 mesh (0.074mm) filter. The device does not remove organic material.
System Flushing
These filters need little maintenance, but they require regular flushing. The amount of sediment in the incoming water, the volume of water used, and the capacity of the collection chamber at the bottom of the filter will determine how often and how long the flushing valve needs to operate.
Rotating Drum Filter
Rotating drum filters (RDFs) are a type of mechanical filter that use a rotating drum to remove solids from water (Figure 19.4). Rotating drum filters can handle a broad range of solids content, from fine suspensions to larger materials, and they are available in a range of sizes and configurations to suit specific process requirements. The rotary drum filter is a mechanical filter because it uses physical means (filter panel) to remove solid particles and debris from the water by trapping it and discharging it out of the filter without any chemical or biological means.
Fiber Media Filter
With a fabric (sometimes called paper) media filter (Figure 19.5), water passes into a collection basin and drains through a filter fabric. As particles clog the pores in the fabric, the water level rises in the collection basin. A sensor turns on a motor, which unrolls fresh fabric. Fabrics vary widely in their thickness and pore size. This system unit will work effectively for the basic water needs of a smaller greenhouse operation and the high-volume needs of a flood floor system. Since the contamination is discharged via the cloth, the cloth filter does not require any rinse water for cleaning.
Media Filters
Media filters (Figure 19.6) have been used extensively in filtering water in greenhouse operations. Media filters are often used to remove organic materials (e.g., bacterial slimes and algae), fine silt, or other fine organic or inorganic materials from ponds and surface water. These filters trap contaminants in irrigation water in a deep column of sand, recycled glass, packed mineral or glass fibers, and/or other dense substrates. The most notable of these is conventional sand of selected sizes placed in pressurized tanks that filter contaminants as the water flows through the filtering media. The main body of the tank contains sand, which is the active filtering ingredient. The sand is placed on top of a thin layer of gravel, which separates it from an outlet screen.
Sizing of Sand Media Filters
The manufacturer’s recommendations for sand particle sizing and uniformity coefficient must be followed to provide required filtration and ensure system hardware integrity. Proper sizing of the sand media filter is critical to system performance.
System Flushing
As the media fills with particulate matter, the pressure drops across the media tank increases, forcing water through smaller and fewer channels. This will eventually disable a media filter, requiring that clean water from one tank be routed backwards through the dirty tank to clean the media. Typically, media filters should be backflushed when the pressure drop across the filter reaches about 10 lb/in2 (psi) or as recommended by the manufacturer.
Screen Filters
Screen filter products are popular because they are inexpensive, easy to install, and take up less space than a sand media filter. They come in various shapes and sizes, although most are horizontal cylinder types. Screen filters (Figure 19.7) are most frequently used for removing inorganic contaminants. Screen filters are recommended for the removal of very fine sand or larger inorganic debris. It is normally not effective to use screen filters for the removal of heavy loads of algae or other organic material since filters clog rapidly, requiring too frequent cleaning to be practical.
Sizing Screen Filters
Sizing of screen filters is based on the maximum particle size allowable by the emitter, the quality of the irrigation water, the flow volume between required cleanings, and the allowable pressure drop across the filter. Information about the maximum allowable particle size should be available from the emitter manufacturer.
System Flushing
As the media fills with particulate matter, the pressure drops across the media tank increases, forcing water through smaller and fewer channels. This will eventually disable a media filter, requiring that clean water from one tank be routed backward through the dirty tank to clean the media. Typically, media filters should be backflushed when the pressure drop across the filter reaches about 10 lb/in2 (PSI) or as recommended by the manufacturer. During the backwash process, the direction of the water reverses through the sand bed. As this takes place, the dirty sand bed lifts, allowing individual particles to separate from each other.
Disc Filters
Disc filters (Figure 19.8) are a cross between a screen filter and a media filter, with many of the advantages of both. Disc filters are good at removing both particulates, like small amounts of sand, and organic matter. Disc filters are better than screen filters for retaining algae. The screening element of a disc filter consists of stacks of thin, doughnut-shaped, grooved discs, forming a three-dimensional filter cartridge. The stack is enclosed in corrosion and pressure-resistant housing. Each individual disc contains grooves molded into its surface. These molded grooves provide for the mesh (or micron) rating of the filter. Typical options include 80 mesh (0.18mm), 100 mesh (0.15mm), 120 mesh (0.13mm), and 200 mesh (0.074mm) ratings.
System Flushing
Most disc filters must be cleaned manually by separating the disks and reversing the water flow through them, but automatic backflush types are available. A disk filter requires less water volume for backflushing than does a sand media filter.
Membrane Filters
Traditional membrane filters, also referred to as cartridge or surface filters, are the perpendicular flow membranes used for the sterile filtration of wines just prior to bottling. They are called membrane filters since they collect particles at the surface. Membranes allow for a more precise pore size differentiation, so the different levels of product flow can provide the grower with choices, depending on the need for clarification.
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