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Methods

Distillation
- Multi-stage flash distillation (MSF)
- Multiple-effect distillation (MED)
- Vapor-compression (VC)
Ion exchange
Membrane processes
- Electrodialysis reversal (EDR)
- Reverse osmosis (RO)
- Nanofiltration (NF)
- Membrane distillation (MD)
- Forward osmosis (FO)
Freezing desalination
Geothermal desalination
Solar desalination
- Solar humidification–dehumidification (HDH)
- Multiple-effect humidification (MEH)
- Seawater greenhouse
Methane hydrate crystallization
High grade water recycling
Wave-powered desalination

The solar humidification–dehumidification method (HDH) is a thermal water desalination method. It is based on evaporation of sea water or brackish water and subsequent condensation of the generated humid air, mostly at ambient pressure. This process mimics the natural water cycle, but over a much shorter time frame.

Overview

The simplest configuration is implemented in the solar still, evaporating the sea water inside a glass covered box and condensing the water vapor on the lower side of the glass cover but not below the unevaporated seawater. More sophisticated designs separate the solar heat gain section from the evaporation-condensation chamber. An optimized design comprises separated evaporation and condensation sections. A significant part of the heat consumed for evaporation can be regained during condensation. An example for such an optimized thermal desalination cycle is the multiple-effect humidification (MEH) method of desalination.^{[citation needed]}

Solar humidification takes place in every greenhouse. Water evaporates from the surfaces of soil, water and plants because of thermal input. In this way the humidification process is naturally integrated within the architecture of the greenhouse. Several companies like Seawater greenhouse utilize this inherent feature of a greenhouse in order to conduct desalination inside the atmosphere of the facility.^{[citation needed]}

Design

The method can be optimized by using various effects in the categories of thermal energy collection and storage for continued nocturnal operation, choice of site location, various evaporation effects, as well as condensator design and provision of cooling energy to harvest distillate from the moist air. A Desalination Greenhouse using all of the effects in all categories, with an emphasis on the optimized combination of the effects including synergies, is the IBTS Greenhouse. The Global water cycle also includes all sub-effects of HDH, like increased evaporation over the oceans surface and surface increase by wind, making the generation of freshwater on the planet so efficient.^{[citation needed]}

Tests

There are successful small-scale agricultural experimentation done in arid regions such as Israel, West Africa, and Peru. The major difficulty lies in effectively concentrating the energy of sun on a small area to speed up evaporation.^[1]