The world is facing an acute shortage of clean and safe freshwater. What are the alternative technologies which can solve this crisis? Briefly discuss any three such technologies citing their key merits and demerits.

Introduction: With over two billion people lacking safely managed drinking water, several engineered technologies now supplement rivers and aquifers. Three stand out, each carrying clear trade-offs between cost, energy and reliability.

1. Desalination (reverse osmosis): seawater is forced through semi-permeable membranes that strip out salts to yield potable water. Merits — a near-limitless, drought-proof source ideal for coastal cities, with mature technology; the Minjur and Nemmeli plants supply Chennai. Demerits — very energy-intensive and capital-heavy, and the concentrated brine discharged back to sea raises salinity and harms marine life.

2. Wastewater treatment and reuse: municipal sewage is purified through multi-stage treatment for irrigation, industry or even drinking. Merits — a continuously generated, drought-proof supply that also reduces pollution of rivers, and it is far cheaper than desalination. Demerits — it needs costly advanced infrastructure, and the "toilet-to-tap" stigma slows public acceptance despite rigorous purification.

3. Atmospheric water generation and fog harvesting: moisture is condensed from air or trapped on mesh nets. Merits — decentralised and useful in remote or emergency settings; fog nets are low-cost and almost energy-free. Demerits — output depends heavily on humidity, so yields stay small and unreliable in the arid regions that need water most.

Conclusion: No single fix works everywhere — coastal cities suit desalination, urban areas suit reuse, and humid uplands suit fog harvesting. A locally matched portfolio, combined with strong demand-side conservation, offers the durable answer to freshwater stress.