Iron, Biofilm and a Venturi on the Suction Line

A screen filter is a surface filter with small dirt-holding capacity. It can catch sand grains, but dissolved ferrous iron passes straight through, oxidises on contact with air inside laterals and filters, and forms a floc that blinds the screen and cements onto emitter walls. Making the screen bigger only lengthens the interval between cleanings; it does not remove the iron. The 0.9 bar rise in three days is a symptom of floc loading, and the brown slime plus 35% discharge loss confirms biofilm/iron deposition, not sand.

Specify:

1. Media (sand) filter as the primary filter, sized for the system flow, with a screen or disc filter downstream as a safety filter.
2. Oxidation ahead of the media filter (chlorination or aeration with a contact/retention volume) so dissolved iron becomes filterable floc.
3. Injector moved to the discharge side, downstream of the filters, with a check valve, vacuum relief and backflow prevention, and at least 15–30 m of mainline before the first lateral so the fertiliser mixes before it reaches emitters.
4. A flush valve at the end of each sub-main.

Injecting on the suction side risks corrosion and erratic dosing and must be corrected first.

Sequence matters - treat, then flush, then measure.

1. Mechanical first: open lateral and sub-main ends and flush at high velocity (target 0.3–0.5 m/s at the lateral end) until the water runs clear; clean the filter and check the sand bed for channelling.
2. Chlorination to oxidise iron and kill biofilm. Use a shock dose per label/extension guidance (commonly cited in the 50–100 mg/L range held for a few hours) with the system full, then flush every lateral until the chlorine smell is gone. Continuous low-dose (1–2 mg/L) during irrigation is the maintenance option where injections are reliable.
3. Acidification to dissolve carbonate and iron deposits, targeting roughly pH 4 in the treated water for 30–60 minutes, then flush. Acid and chlorine must never be mixed or injected together.
4. Block 3 separately: if laterals remain dead after treatment, they are probably beyond recovery - cut open one emitter to confirm and replace only those laterals.

Verification: measure emitter discharge at head, middle and tail of the same lateral before and after (a graduated cylinder and a stopwatch are enough); record the clean-filter ΔP; re-check after two weeks. If discharge returns to within about 10% of nominal and ΔP stays under 0.5 bar, the programme worked.

Safety rules:

• Store acid and chlorine separately, in labelled, closed containers, never in unmarked bottles near the pump.
• Full PPE: chemical goggles/face shield, acid-resistant gloves, apron, boots; eyewash water at the site.
• Always add acid to water, never water to acid.
• Never mix acid and chlorine - it releases chlorine gas. Flush the injection line between products.
• Inject chemicals downstream of the filters with a check valve and vacuum breaker, and post warning signs at the head.
• Nobody irrigates or works on the lines while chemicals are in the system; the field is flushed before workers re-enter.

Practice change: the farmer currently stops the pump right after injecting fertiliser, leaving concentrate sitting in the main and laterals. He should finish injection, then run clean water for 20–30 minutes (enough to clear the longest lateral), and only then stop. Fertigation should also be scheduled early in the set, not at the very end, so the nutrient is pushed into the root zone rather than left in the pipe.