Guide

Choosing a drip irrigation filter: screen, disc or media?

Every drip system needs a filter: the water passages in a dripper are tiny, and sand, silt, algae and some minerals block them. The right filter depends on what your water carries and how fine a filter your dripper's maker asks for. This guide shows how to find out what is in your water, how screen, disc and media filters differ, and how to size and clean them.

Written for farmers, greenhouse and nursery growers, homeowners and estate and facility managers.

What a filter must stop

FAO names solid particles carried in the water as the chief cause of clogging, and also the easiest to solve: sand, silt, clay, algae and bits of plant. A filter cannot stop everything. USDA notes that filters cannot remove dissolved minerals, algae cells or bacteria, and UC ANR that a 200 mesh filter lets fine sand, silt, clay and fine algae through, to settle in the lines. Flushing the lines and treating the water deal with what a filter misses.

Fineness is given in microns, the size of the openings in thousandths of a millimetre, or in mesh, a number that rises as the openings get smaller. FAO's handbook equates 200 mesh with 75 microns, and one maker's sheets equate 120 mesh with 130 microns and 80 mesh with 200 microns. USDA gives fine sand as 100 to 250 microns, silt as 2 to 50 and clay as smaller than 2.

Seven causes of drip irrigation failure

Sources: FAO (1985), Irrigation and Drainage Paper 29 Rev. 1: Water Quality for Agriculture, section 5.7; USDA-NRCS (1997), National Engineering Handbook Part 652: Irrigation Guide, chapter 6; UC ANR (undated), Maintenance of Microirrigation Systems: flushing; FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 3, valves and filters; Netafim (2024), DripNet PC AS TWD and MWD product sheet.

Know your water first

FAO recommends a complete analysis before a drip system is designed, and a series of tests for surface water, whose quality can change even within one watering. Penn State notes that groundwater is fairly clean and may need only a screen or disc filter, while streams and ponds carry bacteria, algae and other life that make a media (sand) filter a necessity.

The seasons change the water too. In the north the dry, dusty harmattan can blow from October to May, and FAO notes that a severe one leaves thick deposits of dust, so in those months check open tanks and ponds for it and expect to clean the filter more often. USDA notes that algae live in almost all still surface water and that sunlight and nutrients make them grow, so it helps to keep a storage tank covered.

Water source	What it often carries	Filters to discuss
Town supply	It varies. Ask the supplier for a recent test; if there is none, have one done	A screen or disc filter
Borehole or well	Some sand, and minerals that can settle out	A screen or disc filter, with a sand separator first if it pumps sand
River, dam, pond or open tank	Silt, clay, algae, plant matter and small creatures, changing with the season	A media filter first, then a screen or disc filter
Water with iron	Iron that turns into a reddish-brown deposit when it meets air	Treatment to settle the iron out, then filtering
Hard water	Lime that builds up in drippers, more at high pH and in heat	Not a filter problem: control the pH, or have the lines cleaned with acid on expert advice; FAO warns that the acid is dangerous to handle

Sources: FAO (1985), Irrigation and Drainage Paper 29 Rev. 1: Water Quality for Agriculture, section 5.7; Penn State Extension (2016), Drip Irrigation for Vegetable Production; USDA-NRCS (1997), National Engineering Handbook Part 652: Irrigation Guide, chapter 6; FAO (1994), CIFA Occasional Paper No. 21: Aquatic Plants and Wetland Wildlife Resources of Nigeria, section 1.1; Netafim (2024), DripNet PC AS TWD and MWD product sheet; FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 3, valves and filters.

Reading a water test

Ask the laboratory for at least the items below. FAO's table rates each for the risk that it clogs drippers, and calls it a first approximation rather than firm rules: a slight-to-moderate or severe result is a reason for more tests and a design, not a verdict.

For iron, FAO treats 5 mg per litre as the maximum for drip and notes that in practice a value above 2.0 mg per litre may be near that maximum, because filtering becomes too costly; iron must first be turned solid with air or chlorine before a filter can catch it.

Test	No restriction	Slight to moderate	Severe
Suspended solids, mg per litre	Below 50	50 to 100	Above 100
pH	Below 7.0	7.0 to 8.0	Above 8.0
Dissolved solids, mg per litre	Below 500	500 to 2,000	Above 2,000
Iron, mg per litre	Below 0.1	0.1 to 1.5	Above 1.5
Manganese, mg per litre	Below 0.1	0.1 to 1.5	Above 1.5
Hydrogen sulphide, mg per litre	Below 0.5	0.5 to 2.0	Above 2.0
Bacteria, number per ml	Below 10,000	10,000 to 50,000	Above 50,000

Source: FAO (1985), Irrigation and Drainage Paper 29 Rev. 1: Water Quality for Agriculture, section 5.7.

Screen, disc or media

USDA puts the practical limit of screen filters and sand separators at about 74 microns, and of media (sand bed), disc and cartridge filters at about 25 microns. FAO's handbook gives screens from 60 to 200 mesh and discs from 40 to 600 mesh (400 to 25 microns). The two sources differ on discs: FAO finds them effective on all kinds of dirt, algae included, while Penn State advises against them where the water carries a lot of plant matter.

Filter	How it works	Suits	Cleaning
Screen	A fine mesh sleeve holds dirt on its surface	Clean groundwater, town supply, or after a media filter	Open the flush valve, or wash the sleeve
Disc	Grooved rings, stacked tight, catch dirt in their grooves	Most waters; ask about heavy plant matter	Loosen and spray the rings; some clean themselves
Media	A tank of sand or crushed rock traps dirt through its depth	Rivers, dams, ponds and open tanks	Backflush to waste
Sand separator	Spins the water so sand drops into a chamber	Water that carries sand, before the main filter	Empty the chamber

Water from the source, carrying sand, silt and bits of plant
Screen filter: a fine mesh holds back anything larger than its holes
Disc filter: a stack of grooved rings catches dirt in its grooves
Media filter: a deep bed of sand or gravel traps dirt through its depth
Clean water on its way to the drip lines
Backflush: water sent back through the bed carries the dirt out to waste

A screen, a stack of discs and a bed of sand each catch dirt in their own way, and the sand is cleaned by backflushing it to waste.

Filtration and fertigation

Sources: USDA-NRCS (1997), National Engineering Handbook Part 652: Irrigation Guide, chapter 6; FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 3, valves and filters; Penn State Extension (2016), Drip Irrigation for Vegetable Production.

Match the filter to your dripper and your flow

Pressure is how hard the water pushes inside the pipe, measured in bar; given as a height of water, one bar is about 10 metres of head. Flow is how much water passes in a set time, in litres per hour (L/h) for a dripper, or cubic metres per hour (m³/h) for a filter, a pump or a pipe. One cubic metre is 1,000 litres.

Take the fineness from your dripper's or drip line's sheet: one maker asks for 130 microns for its smaller drippers and 200 for larger ones, and Penn State calls for 150 to 200 mesh on most drip tapes and 100 mesh on clog-resistant lines. Finer than needed has a cost: USDA notes that the finer the mesh, the faster it plugs, and that two filters, or a larger filter area, give longer between cleanings.

Then check the flow: a filter is rated for a flow at a stated fineness, and must pass the flow of the largest zone it serves. Penn State adds that a sand separator works properly only when sized for its flow.

Drip tape or dripline: which suits your crop?

Sources: Netafim (2024), DripNet PC AS TWD and MWD product sheet; Penn State Extension (2016), Drip Irrigation for Vegetable Production; USDA-NRCS (1997), National Engineering Handbook Part 652: Irrigation Guide, chapter 6; AZUD (undated), AZUD Modular 100: technical sheet; FAO (2007), Handbook on Pressurized Irrigation Techniques: annex, units conversion table.

Cleaning and backflushing

Fit a pressure gauge before and after the main filter: the gap between the two readings grows as the filter fills, and USDA uses it to judge when the filter needs cleaning. Penn State backflushes media filters when the gap is about 0.35 to 0.55 bar higher than when they were clean, and cleans screens at about the same point. FAO's handbook cleans the filters thoroughly before every irrigation, and sets automatic filters to clean themselves at a set gap, for example 0.5 bar, or on a timer.

Backflushing sends water backwards through a media filter's sand to lift it and carry the dirt out to waste. Penn State notes that it needs the right flow, which is why media filters usually come in pairs, one cleaning the other, and you need somewhere to drain the waste water (USDA). To clean a screen or disc filter by hand:

Read both gauges and note the gap between them.
Stop the water to the filter and open its drain.
Take out the screen or the disc stack.
Wash the screen clean; for discs, loosen the stack and spray the rings apart.
Check the screen for tears and the seals for wear: Penn State warns that a neglected screen can tear and let dirt through.
Refit it, let the water back in slowly, and read the gauges again: the gap should be back near its clean value.

Sources: USDA-NRCS (1997), National Engineering Handbook Part 652: Irrigation Guide, chapter 6; Penn State Extension (2016), Drip Irrigation for Vegetable Production; FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 3, valves and filters; FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 20, operation and maintenance.

Pressure lost across the filter

Every filter takes some pressure from the water, a little when it is clean and more as it fills. FAO's handbook gives filter friction losses of about 0.3 to 0.5 bar. Penn State gives about 0.14 to 0.21 bar across a clean media tank, and a normal drop of about 0.07 to 0.21 bar across a screen.

In FAO's sample design for a hectare of watermelons, the control station at the start of the system (a gravel filter, a disc filter, a fertiliser injector and valves) takes 0.90 bar of the 2.65 bar the pump must supply. Read the loss at your flow from the maker's chart, and allow for more as the filter fills.

Choosing a pump for a borehole, well, river or tank

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 3, valves and filters; Penn State Extension (2016), Drip Irrigation for Vegetable Production; FAO (2007), Handbook on Pressurized Irrigation Techniques: chapter 14, sample design for watermelons.

What to measure

A laboratory water test with the items in the table above; for a river, dam or open tank, several through the season
A clear bottle of water from the source, left to stand: sand settles in seconds, while silt and clay can stay cloudy for hours or longer
The fineness your dripper or drip line's sheet asks for, in microns or mesh
The flow of the largest zone the filter will serve
The pressure at the filter's inlet, with the pump running
Where the backflush water can drain away

Sources: FAO (1985), Irrigation and Drainage Paper 29 Rev. 1: Water Quality for Agriculture, section 5.7; Penn State Extension (2016), Drip Irrigation for Vegetable Production.

What to compare on the spec sheet

On the filter's sheet	Why it matters
Type: screen, disc or media	It must suit what your water carries
Fineness, in microns or mesh	It must meet what your dripper's maker asks for
Rated flow at that fineness	It must cover the flow of your largest zone
Pressure loss at your flow, from the maker's chart	It adds to the pressure your pump must supply
Highest and lowest working pressure	Your system's pressure must sit between them
Filtering area	A larger area runs longer between cleanings
Cleaning: by hand, flush valve or automatic	It sets the labour, and whether it cleans itself
Limits on the water: pH, salt and temperature	Your water test must fall within them