How to Size Filtration for a Nursery or Greenhouse Irrigation System

Filtration is the part of a nursery irrigation system that gets sized last and blamed first. The filter that plugs every afternoon, the spray stakes drifting off pattern, the dripline losing a lateral a week: most of it traces back to a filter picked from a catalog page rather than from the water and the emitters.

This guide covers the method we use when a grower calls about a new house or a block: what the water source tells you, the three numbers that fix size and rating, how screen, disc and media differ, where the filter belongs relative to the injector and zone valves, and the undersizing mistakes we see most. By the end you should be able to write down a flow rate, a micron target and a filter type and bring them to a quote.

Start from the water source, not the filter catalog

The water decides the filter type before flow rate decides the size. Each source carries a characteristic load, and each load defeats a different kind of element.

Well water is usually low in organics but carries sand and grit, and fine sand wears a filter element and an emitter alike. A centrifugal separator ahead of the filter spins that grit out first. Amiad's Sand Separator Series and the Rivulis F1300 Hydrocyclone Filter both exist for this job, so the filter behind them is not doing work it was never meant to do. Behind a separator, a screen element handles what remains.

Pond, canal and other surface water is the opposite problem: algae, weed fragments and fine organic matter that a screen strains on its surface and blinds over. This is the water disc and media filtration were built for, and on a heavy algae load a media tank is often the only thing that runs a full set without constant flushing. Surface water with visible weed and debris also wants a coarse pre-filter, such as the Amiad ABF Series, ahead of the fine stage.

Municipal water is the easiest case. It still needs a filter, because pipe scale and construction debris still arrive, but a screen at the head end and one at each zone usually covers it.

Recycled and reuse water combines organics with a biological load. Treat the source with chlorination to hold biofilm and algae down, then filter with disc or media, because a screen alone will not keep up.

The three numbers: flow rate, finest emitter, available pressure

Once the water is characterized, three numbers fix the filter. Write them down first.

Flow rate. Size the filter to the flow that actually passes through it, not to the pipe diameter it sits in: the largest zone, or the largest combination of zones running at once, in gallons per minute. A body is rated by gpm, not pipe size. The Amiad TAF Series and the Manual Plastic Amiad Series both run to 220 gpm, the Spin Klin Compact Angle 2" is rated to 66 gpm, and the Hunter HY Series tops out at 20 gpm. Published maximums assume clean water; on dirty water a partly loaded element passes less before it needs a flush, so leave headroom.

Finest emitter. Every dripline, spray stake and mister carries a filtration requirement from its manufacturer, and that number, not the filter catalog, sets the rating. The filter has to catch particles well below the emitter's smallest passage, because particles bridge. Mesh and micron run in opposite directions: a higher mesh count is finer, a lower micron figure is finer. Hunter's HY Series zone filters, which belong in front of every drip and micro zone, carry a 150-mesh stainless screen; Amiad's Scanaway Series takes drip systems down to 50 micron, and the Manual Plastic Arkal Series disc line reaches 20 micron.

Available pressure. A filter costs pressure to run and more to flush. If a spray stake or drip block lacks the head to drive a conventional filter, Netafim's 2" Compact LP Disc-Kleen was built for that case.

Screen, disc or media: match the element to the load

The three element types differ in where they hold dirt, which decides what survives your water. This is how to read the filtration range.

A screen strains on its surface. It is simple, quick to clean and right for inorganic particles: sand behind a separator, pipe scale, municipal supply. Its weakness is organic matter, which lays over the mesh and blinds it. Manual screens run from small plastic housings to Amiad's steel-bodied Manual Steel Series for mainline duty where plastic will not take the pressure.

A disc filter stacks grooved discs so dirt is trapped through the depth of the stack, not on a face. That gives far more capacity for organic matter, and it filters finer in the same body size: the Manual Plastic Arkal Series disc line reaches 20 micron against 50 micron for the Manual Plastic Amiad Series screen. Disc is our default for pond and reuse water on a single house. Toro's F100 Series body takes either a stainless screen or a disc element, so the filter can be re-specified if the water changes.

Media filters pass water through a bed of sand, glass or other granular media and hold organic load through the whole bed. They are the answer when a screen or disc blinds too fast. Toro's Aqua-Clear Sand Media Filters run 50 to 400 gpm across 18" to 36" fiberglass tanks; Amiad's Media Series is rated to 330 gpm per unit. Media needs a backwash cycle and the flow to drive it: a station, not a fitting.

Manual, semi-automatic or self-cleaning

The right cleaning method is decided by how often the element will load, multiplied by how far it sits from the person meant to clean it.

A manual filter is opened, the element pulled and rinsed. On clean well or municipal water at a greenhouse head that may be a monthly job, and a manual housing is the right buy. On surface water it may be twice a day, and a manual filter that needs cleaning twice a day is a filter that does not get cleaned; the block then runs dirty until a plugged emitter or a pressure drop makes someone look.

Semi-automatic filters clean without opening the housing. Amiad's Brushaway Series sweeps the screen with internal brushes when you turn the handle, without stopping irrigation, and the Scanaway Series does the same with a suction scanner down to 50 micron. The Rivulis F6400 Semi-Automatic Screen Filter works on the same principle.

Automatic self-cleaning filters flush themselves through a controller such as Amiad's ADI-P Controller or Netafim's Backflush Controllers. The smallest fit a greenhouse head: the Amiad TAF Series at 2" to 3", and the Spin Klin Compact Angle 2" for a single house. Where supply cannot drop during a flush, look at designs that keep filtering through the cycle: Spin Klin Batteries, where one unit backflushes while the rest keep filtering, Netafim's SandStorm Double Chamber, or the Rivulis F3440 Continuous Circulation Screen Filter. At a remote pump with no electrical supply, the Rivulis F3240 / F3240-IQ Automatic Screen Filter is hydraulically driven.

Where the filter sits: separator, primary, injector, zone

Filtration on a nursery is a chain, not one unit, and the order matters.

1. Sand separator first, on well water, directly after the pump so grit never reaches an element.
2. Coarse pre-filter next, on surface water with weed and debris, ahead of the fine stage.
3. Primary filter after that, sized to the full system flow and rated to the finest emitter on the whole system, with a pressure gauge on each side. The difference between the two gauges is the only honest measure of how loaded the element is, and it is what an automatic controller flushes on.
4. Fertilizer injector downstream of the primary filter, so the primary is not loaded with undissolved fertilizer or with precipitate from what was injected. The emitters still need protecting from that, which is the job of a secondary filter placed after the injector.
5. Zone filters at each valve: small inline units such as Hunter's HY Series Filters, or the HFR Filter Regulators, which combine a 150-mesh screen, a 25 or 40 psi regulator and a flush cap in one fitting. The primary protects the mainline; a zone filter protects the emitters from anything that enters the sub-main after it.

Chlorination sits at the source, upstream of the whole chain, so what it kills is caught by the filters rather than delivered to the emitters. Regulate pressure after the filter, not before, so the filter sees full line pressure to flush against.

Flushing, maintenance and the undersizing mistakes we see

A filter is sized correctly when it holds the rating the emitters need, at the flow the system draws, for long enough between cleanings that the cleaning actually happens. Most failures miss one of those three.

• Sizing to pipe diameter. A 2" line does not need a 2" filter; it needs one rated for the gpm through it, with headroom for dirty water.
• Sizing to the catalog maximum. Those figures assume clean water. A filter run at its maximum on pond water spends the day flushing.
• Taking the rating from the filter, not the emitter. The finest emitter on the block sets the micron target.
• A screen on surface water. It blinds, and the usual response is to pull the screen out, after which there is no filtration.
• No separator on a sandy well. Grit wears elements and drippers alike; a hydrocyclone ahead of the filter stops it.
• No secondary filter after the injector, so the emitters get whatever the fertilizer brought in.
• Too little pressure to flush. A media tank or automatic screen needs flow and head to clean itself; check that first.
• A manual filter no one can reach. At a remote pump it wants a semi-automatic or hydraulic automatic unit.

Flush on the gauges, not the calendar. Log the differential and the date; if the interval shortens through a season, the water has changed and the filter may need to change with it. With flow, emitter list and water source in hand, request a quote and ask us about sizing.