Three ways an emitter dies
Drip irrigation puts the whole system's reliability through passages well under a millimetre wide, and they block in three ways: physically (sand and silt), chemically (lime scale and iron precipitating inside the lines), and biologically (bacterial slime and algae gluing fine particles into plugs). The standard hazard classification — used internationally and adopted in the South African guideline — reads straight off a water analysis:
| Factor | Minor risk | Moderate | Severe |
|---|---|---|---|
| Suspended solids (mg/L) | under 50 | 50–100 | over 100 |
| pH | under 7.0 | 7.0–8.0 | over 8.0 |
| TDS (mg/L) | under 500 | 500–2 000 | over 2 000 |
| Iron (mg/L) | under 0.1 | 0.1–1.5 | over 1.5 |
| Manganese (mg/L) | under 0.1 | 0.1–1.5 | over 1.5 |
| Bacteria (counts/mL) | under 10 000 | 10 000–50 000 | over 50 000 |
Note what the table implies: perfectly drinkable water can be severe-risk drip water, and water the crop guideline calls fine can kill a drip block in a season.
Iron: the borehole special
Dissolved iron is invisible — the water pumps clear, then oxidises in the lines and drops rust exactly where you cannot reach it. Clogging trouble starts at about 0.2 mg/L, and practical guidance treats water above 0.5 mg/L as unusable in drip systems untreated — while the crop itself would happily take 5 mg/L. Worse than the rust is what farms it: iron bacteria, which feed on dissolved iron and build the red-brown jelly (ochre) that sheets through laterals and plugs emitters wholesale. Manganese does the same, more slowly and in black.
The reliable cure happens before the water enters the system: oxidise, settle, filter. Aeration works, but its speed depends on pH — at pH 8 dissolved iron converts in about half a minute; at pH 6 it takes days — so acidic borehole water gets chlorine dosed as the oxidant instead (budget about 0.64 mg/L of chlorine per mg/L of iron, injected upstream of the filters so the rust is caught, not delivered). Heavily ironed water (above ~4 mg/L) is best aerated into a settling dam first. A catalytic media filter does the oxidation and capture in one vessel and is the standard packaged answer at farm scale.
Filter to a tenth
The sizing rule is simple: filtration should remove particles larger than about one-tenth of the smallest emitter passage — smaller particles matter too, because they bridge across an opening like stones jamming a funnel. In practice: standard drippers want 120–150 mesh (around 130–100 micron); clog-sensitive and pressure-compensated drippers 200 mesh (75 micron); micro-sprinklers get away with 150–200 micron.
Which filter type depends on what the water carries:
- Screen filters — clean borehole water with a little sand and silt. Cheap, simple; hopeless against algae and slime, which smear across the mesh.
- Disc filters — the general-purpose choice for mixed loads; avoid where pumped sand is the main problem (grit lodges between the discs).
- Sand/media filters — the first choice for dam and river water carrying algae and organics, and for oxidised iron; depth filtration that holds real dirt loads, backwashed in pairs so one vessel cleans while the other works.
- Hydrocyclones — a spin-separator ahead of everything else where the borehole pumps sand.
Backwash on pressure difference, not on the calendar — screens and discs at roughly 20–35 kPa across the filter, media beds at about 70 kPa.
Keep the lines alive
Filtration alone does not stop slime and scale forming inside the laterals, so maintenance is chemical. Chlorine for the biology: either a continuous low dose holding 1–2 mg/L of free chlorine at the far end of the furthest lateral (check with an ordinary DPD test kit), or periodic shock doses of 10–20 mg/L held in the lines for an hour or two. Acid for the lime: where the Langelier index says the water scales, continuous acid dosing to just below pH 7 prevents it, and a remedial acid slug (held in the lines for around 12 hours, then flushed) dissolves what has formed. Two safety rules are absolute: never let acid and chlorine products meet in a tank or injection point — the reaction releases chlorine gas — and flush laterals at 0.3 m/s or better, ends open, until the water runs clear.
Rule of thumb
Screen the analysis against the clogging table, not the crop table. Treat iron before the manifold — oxidise, settle, filter — and never send it down the lines to precipitate there. Filter to a tenth of the emitter passage, choose media filters for surface water and screens only for clean boreholes, and hold a chlorine residual at the end of the furthest line often enough to prove the system is still yours and not the bacteria's.