A venturi that will not suck and a pond-water drip system slimed with algae

The physics A venturi creates suction from the pressure drop across its throat. Rated suction (250 L/h here) is quoted at a substantial differential, typically 0.6–1.0 bar, and suction falls steeply as the differential shrinks.

• Measured differential = 2.55 − 2.40 = 0.15 bar.
• That is roughly one-fifth of the minimum needed to approach rated flow, so drawing only 40 L/h (about 16% of rating) is exactly what the hydraulics predict. The injector is not broken; it is starved of differential.

Checks on the suction side (cheap, do these first)

1. Air leak at the suction hose connection or a perished hose - the commonest cause of poor suction.
2. Clogged injector throat or suction strainer; the pond is algae-rich, so this is likely.
3. Drum height and suction lift; the drum must be at or above the injector if possible.
4. A partly closed valve downstream or a bypass that is passing water around the venturi and killing the differential.

Changes

• Create the differential deliberately: throttle the injector outlet valve so that the drop across the venturi is about 0.8–1.0 bar while still leaving enough head for the block, or fit a booster pump, or replace the venturi with a unit rated at the available differential, or move to a diaphragm/water-powered proportional dosing pump.
• Fix the pump intake: a floating intake or screened bucket, moved away from the algal mat, so that flow and pressure stop fluctuating.
• Fit pressure gauges before and after the injector and record them during every fertigation.

Injection rate

• 19:19:19 gives roughly 1.4 dS/m per g/L, so 80 g/L stock ≈ 112 dS/m.
• Required ratio r = (target EC − source EC) ÷ (stock EC − source EC) with source ≈ 0.6 dS/m: (2.0 − 0.6) ÷ (112 − 0.6) ≈ 1.25%.
• At 10 m³/h that is 125 L/h of stock, not 40 L/h.
• Cross-check the present situation: 40 L/h ÷ 10,000 L/h = 0.4%, which adds about 0.45 dS/m to a 0.6 dS/m source, giving ≈ 1.05 dS/m - close to the measured 0.9 dS/m. The arithmetic and the field reading agree, so the problem is injection rate, not fertilizer quality.
• Always confirm with the EC meter in the field and adjust; do not trust the venturi scale.

Filtration

• Pond water needs sand or media filtration followed by a screen, not a screen alone. The screen passes algal cells and fine organic matter that then grow and form slime inside the laterals.
• Use a 120–150 mesh screen (about 100–125 µm) downstream of the media filter for drip.
• Check backwash capability: a media filter typically needs 15–20 m³/h for proper backwash, more than this 10 m³/h pump can give. Either install a filter sized for a lower backwash rate, provide a backwash storage tank and pump, or accept frequent manual cleaning and budget for it.

Water treatment

• Continuous chlorination to keep about 1–2 mg/L free chlorine at the end of the lateral; shock dose 10–20 mg/L for 30–60 minutes when slime is established, then flush. Chlorine is far more effective at pH below 7.5 and when the lines are clean.
• Acid injection periodically to control carbonate scale and lower pH; check with a pH meter and never mix acid and chlorine in the same drum.
• Flush lateral ends regularly at 0.5 bar above operating pressure; flush until the water runs clear.
• Control algae in the pond itself by shading, reducing nutrient inflow and cleaning the intake screen.

Missing controls

1. Backflow protection: no non-return valve on the injector line and a drinking-water dug well only 10 m away. Fertilizer and acid can siphon into the aquifer and the well. Fit a non-return valve, a vacuum relief/air inlet valve, and inject downstream of the filter and check valve. Maintain an air gap between the suction hose and the stock solution surface where possible.
2. Chemical storage: stock and acid must be in labelled, sealed, bunded containers away from the well and the pump, with a spill kit and clean-water eyewash nearby.
3. Operator protection: gloves, goggles and apron; no eating or smoking near chemicals; know the first-aid instructions for the products used.
4. Records: log date, product, quantity, injection duration, EC and pH, so a problem can be traced.

Correct sequence

1. Start the pump, run clean water until the system is at full pressure and the far-end pressure is correct.
2. Check the venturi differential and confirm suction with clean water in the drum.
3. Inject the stock solution at the calculated rate, monitoring EC at the emitter (target 2.0 dS/m) and pH.
4. Stop injection; continue pumping clean water for 20–30 minutes to push the fertilizer out of the mainline and laterals - this prevents slime and salt build-up at the emitters.
5. Stop the pump, then flush the laterals; open and clean the screen; record everything.
6. Never leave fertilizer in the lines overnight; never inject into a system that is not running; never inject before the filter.