Salinity, reverse osmosis and the water bill at Mgarr
A 2.4 ha vegetable holding in the Mgarr valley grows 0.6 ha of cherry tomatoes under unheated plastic tunnels and 1.0 ha of open-field courgettes; the rest is rotation/fallow. The metered borehole's electrical conductivity has risen from 1.9 to 3.1 dS/m over four seasons, and tunnel tomato yields fall sharply above 2.5 dS/m. The water authority caps abstraction at 4,000 m³/year; the farm already uses 3,700 m³ and pays €0.55/m³. A reverse-osmosis (RO) unit would cost €18,000 installed, treat up to 1,000 m³/year of borehole water at 65% recovery, use 1.1 kWh per m³ of permeate at €0.22/kWh, and require brine disposal by licensed tanker at €9/m³. A 500 m³ lined reservoir collecting tunnel-roof runoff would cost €22,000; local average rainfall is 550 mm and the collectable roof area is about 1,000 m² with a runoff coefficient of 0.8. The bank offers 7-year loans at 4.5%.
1. Calculate the annual volume of RO permeate and brine, the full annual cost per m³ of permeate (include capital recovery over 7 years at 4.5% and operating costs), and the average harvestable roof runoff. Use these figures to compare the two options.
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Step 1 – RO mass balance. Feed 1,000 m³ at 65% recovery gives 650 m³ permeate and 350 m³ brine. Energy = 650 × 1.1 × €0.22 = €157.30. Brine disposal = 350 × €9 = €3,150. Operating cost = €3,307.30/year.
Step 2 – capital recovery. Annuity factor for 7 years at 4.5%: 0.045/(1−1.045^−7) = 0.1697. Annual capital charge = €18,000 × 0.1697 = €3,054.60.
Step 3 – unit cost. (€3,307.30 + €3,054.60)/650 = €9.79/m³.
Step 4 – rainwater. 1,000 m² × 0.55 m × 0.8 = 440 m³/year. Using the same 7-year charge, €22,000 × 0.1697/440 = €8.49/m³, before pumping or treatment.
Interpretation. The reservoir water is slightly cheaper and low in salt, but 440 m³ is only about 12% of current use. RO gives 650 m³ of low-salinity water but at a high cost and with a brine-disposal obligation. Neither alone solves the salinity and volume problem; the decision should focus on blending, demand reduction and protecting the tunnel crop, not simply the lowest €/m³.
2. The tunnels use 1,800 m³/year. At EC 3.1, the excess over 2.5 dS/m is 0.6 dS/m, and each 0.1 dS/m above 2.5 is associated with a 2.5% yield loss. Tunnel yield at EC 2.5 is 48 t/ha. Estimate the yield and revenue loss at EC 3.1, and assess whether using 300 m³ of RO permeate to dilute the borehole water is financially justified.
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Excess EC = 3.1 − 2.5 = 0.6 dS/m. Number of 0.1 dS/m steps = 6. Yield loss = 6 × 2.5% = 15%.
Lost yield = 48 t/ha × 0.15 = 7.2 t/ha. Over 0.6 ha = 4.32 t. At €1.10/kg, revenue loss = 4,320 kg × €1.10 = €4,752/year.
Cost of 300 m³ RO permeate = 300 × €9.79 = €2,937/year. If 300 m³ of permeate is blended with 1,500 m³ of borehole water, the arithmetic blend EC is approximately (1,500 × 3.1 + 300 × 0.4)/1,800 ≈ 2.65 dS/m - better, but still above 2.5. Blending 400–450 m³ would be needed to get close to 2.5, costing €3,916–€4,405, which is close to the value of the lost yield.
The financial case is therefore marginal. It becomes stronger only if the dilution is targeted at the most sensitive young plants, if the packhouse rejects fruit grown above 2.5 dS/m, or if the yield loss at 3.1 dS/m is greater than the assumed 15%. Monitor EC, sodium and chloride in the soil and water, and record yield by block before committing.
3. Recommend one investment for this year, with conditions. Include two monitoring indicators and explain how the choice addresses both salinity and the abstraction cap.
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Recommendation: install the lined reservoir first, and budget for a small RO unit only for the most sensitive tomato block if the packhouse imposes a hard 2.5 dS/m limit. The reservoir reduces borehole abstraction, captures winter rain and provides low-salinity water for blending; it also has a lower running cost and no brine stream. However, 440 m³/year will not solve the salinity problem alone.
Conditions: (1) confirm the grant or loan before committing, and keep the abstraction below 4,000 m³/year; (2) secure a written brine-disposal contract and a licensed receiving site before running RO; (3) use drip irrigation, mulch and a leaching fraction to manage salt build-up; (4) switch part of the tunnel area to a more salt-tolerant crop if EC cannot be held below 2.5.
Monitoring indicators: weekly borehole and blend EC (target below 2.5 dS/m in the tunnel), and monthly metered volumes against the 4,000 m³ cap. Also record yield per m² and fruit grade so the investment can be judged after one season.