Water supply for a coastal hotel facing a salinity cap
A 220-room coastal hotel in Malta uses 32,000 m³ of water a year. Supply comes from a private borehole in a perched aquifer, but chloride has risen from 180 to 420 mg/L over five years and the water utility has warned that abstraction may be capped at 20,000 m³/year. The hotel roof area is 1,200 m²; local mean rainfall is 550 mm and runoff coefficient 0.85. A consultant proposes three options: (a) a small reverse-osmosis plant treating borehole water at 3.5 kWh/m³ with brine discharged 150 m from a Posidonia seagrass meadow; (b) rainwater harvesting into a 60 m³ cistern; (c) greywater treatment and reuse for toilet flushing, estimated at 30% of total demand. The owners want a decision before the next tourist season and have asked for a reasoned, evidence-based recommendation rather than a single technology.
1. Calculate the maximum annual rainwater yield and compare it with the hotel's demand and the proposed abstraction cap. What does this show about relying on rainwater alone?
Read the worked answer
Calculation
Yield = roof area × rainfall × runoff coefficient = 1,200 m² × 0.55 m × 0.85 = 561 m³/year.
Comparison
- 561 m³ is about 1.8% of the 32,000 m³ annual demand.
- It is about 2.8% of the 20,000 m³ abstraction cap.
- If the cap applies, the shortfall is 12,000 m³ before any other measure; rainwater closes less than 5% of that gap.
Judgement
Rainwater cannot be the lead supply option. Even a 60 m³ cistern only buffers a few days of peak summer demand, and rainfall is concentrated in winter while hotel demand peaks in summer. The calculation therefore supports rainwater as a supplementary measure that reduces potable demand in wet months and adds resilience, not as a substitute for the borehole or a treated source. A recommendation based only on 'harvest rain' would fail the volume test and the seasonal-timing test.
2. Recommend a portfolio of measures and state the conditions under which the reverse-osmosis option would be acceptable.
Read the worked answer
Portfolio logic
- Demand reduction first - leak detection, aerators, laundry and pool management. This is cheapest and reduces the size of any treatment plant.
- Greywater reuse - 30% of 32,000 = 9,600 m³/year, the largest single alternative source.
- Rainwater - 561 m³/year, useful but small.
- Borehole/RO for the residual - with the cap, residual demand is about 32,000 − 9,600 − 561 = 21,839 m³; if only 20,000 m³ can be abstracted, the gap is about 1,800 m³ plus whatever the cap implies for water quality. Without the cap, RO would treat a larger share of the 21,839 m³.
Conditions for RO
RO should be acceptable only if:
- brine is discharged through a properly designed diffuser with dilution and salinity monitoring, not a raw pipe 150 m from Posidonia;
- baseline seagrass shoot density and cover are recorded and monitored annually;
- energy use is metered and offset or minimised (3.5 kWh/m³ × volume is significant);
- the plant has a defined chloride target and a trigger for reviewing abstraction;
- implementation is staged, with demand reduction and greywater installed first.
The judgement is conditional: RO is a technical fix for the residual, not a licence to continue unlimited abstraction.
3. Design a monitoring programme to test whether your recommendation is working.
Read the worked answer
A useful programme links each measure to a measurable indicator and a trigger:
| Measure | Indicator | Frequency | Trigger |
|---|---|---|---|
| Borehole | Chloride and abstraction volume | Monthly | Chloride > 500 mg/L or abstraction > cap |
| Greywater | Volume reused, treatment performance (BOD, turbidity) | Weekly/monthly | Reuse < 25% of target |
| Rainwater | Cistern level, overflow, yield | Monthly | Yield < 450 m³/year |
| RO | kWh/m³, recovery rate, brine salinity | Continuous/monthly | Energy > 4 kWh/m³ |
| Seagrass | Shoot density, cover, visual condition | Annual, same season | Decline > 10% against baseline |
| Demand | Total m³ per guest-night | Monthly | No downward trend after fixtures |
Monitoring should include a before/after control at the seagrass site and a reference meadow, because natural variability matters. Results should feed an adaptive review: if chloride keeps rising, the abstraction cap should be enforced regardless of RO capacity.