Drip system with a clogged sand filter, low emitters and an undersized pump
A farmer has 1.5 ha under drip irrigation with 3.0 m between rows and 0.5 m between emitters, giving 10,000 emitters rated at 2.0 L/h. The 5 hp submersible pump delivers 18 m3/h at 1.0 kg/cm2 at the filter outlet. The system has a sand filter followed by a screen filter. The pressure difference across the sand filter is 1.1 bar; when clean it is 0.2 bar. Measured emitter discharge ranges from 0.8 to 1.9 L/h. Water tests show EC 1.8 dS/m, iron 2.8 mg/L, hardness 320 mg/L as CaCO3, and visible algae in the open storage tank. The farmer runs the pump 9 hours daily and applies about 30 mm every third day. He complains that the crop is wilting and that some rows receive less water than others.
1. Calculate the system's design application rate and the runtime needed for 24 mm net at 85% efficiency. Does the 5 hp pump match the 10,000 emitters?
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At the rated 2.0 L/h, the block requires 10,000 × 2.0 = 20,000 L/h = 20 m3/h. The pump supplies 18 m3/h at the stated pressure, so the pump is slightly undersized relative to the emitters' rated flow; in practice the emitters run below 2.0 L/h, which matches the measured range.
With 18 m3/h over 15,000 m2, the application rate is 18/15,000 = 0.0012 m/h = 1.2 mm/h. For 24 mm net at 85% efficiency, gross depth = 24/0.85 = 28.2 mm. Volume = 28.2 mm × 15,000 m2 = 423 m3. Runtime = 423/18 = 23.5 hours per irrigation, or about 7.8 hours per day if applied every third day. The farmer's 9 hours daily is therefore close to the theoretical requirement, but only if the filter is clean and all emitters are performing. With the current clogging and 1.1 bar pressure loss, the applied depth is much lower and uneven, so the wilting is explained by distribution failure rather than by the schedule alone. The pump curve should also be checked because filter cleaning and friction losses add head.
2. Diagnose the emitter clogging and the filter pressure differential. Distinguish physical, chemical and biological causes, and explain the risk of mixing treatments.
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A pressure differential of 1.1 bar across a sand filter that is 0.2 bar when clean means the bed is heavily loaded or backwashing is ineffective. The low and variable emitter flows-0.8 to 1.9 L/h-show that clogging is already in the laterals, not only in the filter.
The causes are likely combined. Physical: fine sand, silt and tank algae reaching the laterals. Chemical: hardness 320 mg/L as CaCO3 can precipitate calcium carbonate, and iron 2.8 mg/L can oxidise to iron oxide, especially after aeration in the tank. Biological: algae and bacterial slime from the open, unshaded tank. The screen filter alone will not remove dissolved iron or scale.
Never mix acid and chlorine: acid can release toxic chlorine gas, and strong acid with hypochlorite is dangerous. Treat sequentially-backwash and flush, then acidify to dissolve scale, flush thoroughly, then chlorinate to control biological growth, and flush again. Check compatibility with the crop and use label directions.
3. Give a two-week rehabilitation and scheduling plan, including monitoring indicators.
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Days 1–2: Open the laterals and flush the mainline and sub-main at high velocity. Backwash the sand filter until the sight glass runs clear; if the bed is cemented, use air scour or replace the sand. Clean the screen filter and remove algae from the tank; cover the tank to exclude light.
Days 3–5: Apply an acid treatment through the system to dissolve carbonate and iron deposits, following label concentration and contact time, then flush. Do not mix with chlorine. Days 6–8: Apply a chlorine treatment for biological slime, flush, and check emitter flows at several points. Replace badly clogged or damaged emitters rather than increasing pressure.
Days 9–14: Restore the schedule for 24 mm net every third day, about 7.8 hours per day at 18 m3/h, split into two runs if possible. Monitor pressure difference across the sand filter (backwash at 0.5–0.7 bar above clean), emitter discharge in at least three zones, soil moisture, and crop wilting. Keep a log of water use and filter cleaning. If iron remains high, consider aeration and settling before the filter, and install a fertigation/acid injection point with a non-return valve.