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Low temperature distillation enables cost-effective seawater desalination

Swiss company Watersolutions AG, has introduced the Watersolutions LTD system, a patented, cost-effective thermal process for seawater desalination based on the principle of low temperature distillation.

“Low Temperature Distillation (LTD) represents an exciting advancement in thermal desalination technology," said Espen Mansfeldt, CEO of Watersolutions.

"The Watersolutions LTD system is simple to install, robust and highly efficient with low running and maintenance costs.

"We believe it has enormous potential to provide the world’s ever growing population with really clean water in a very cost-effective, energy efficient and environmentally friendly way,” 

The Watersolutions LTD system condenses water at low temperature and pressure, using waste heat (50-110° C) from thermal processes including renewable energy sources such as solar energy or geothermal energy.

The system requires significant amounts of low grade waste heat (6 – 30 MW), which can be derived from any source including thermal power plants, district cooling systems, general industry, mining and waste incineration.

The LTD technology can either work alongside other technologies or as a standalone plant. The configuration is determined by the customer’s needs and the availability of waste heat. “It is an ideal complement to existing technologies,” said Mansfeldt.

A pilot plant (pictured above) in El Gouna, Egypt, with a design capacity of 500 cubic meters per day (m3/d), has proven the principle, with very pure water being produced reliably and efficiently.

While investment costs (CAPEX) associated with LTD are very competitive, the major savings are in operating costs (OPEX) excluding depreciation, which are projected at only 1/3 -1/2 of existing processes.

Unlike conventional desalination technologies, where the main cost is related to energy usage, the LTD process uses free, low grade waste heat that cannot be used otherwise.

The electricity requirement for LTD is very low. For example, the Watersolutions LTD system with one cascade can produce pure water at less than 1.0 kilowatt hour per cubic meter (kWh/m3). In contrast, SWRO typically uses 3.5 – 4.5 kWh/m3 of water production.

Another benefit is the high conversion ratio of LTD, with only 1.5 m3 of seawater needed to produce 1.0 m3 of clean water (< 10 ppm of dissolved solids).

Also included in OPEX are the maintenance and replacement costs of parts such as membranes, which become worn over time. With LTD, which has no membranes and no interior pipe bundles, the requirement for maintenance is very low.

Watersolutions’ patented LTD process condenses water at low temperature/pressure, using waste energy or solar power.

A schematic of Watersolutions’ patented LTD process which condenses water at low temperature/pressure, using waste energy or solar power.

In general, the LTD process has advantages where the salt content is high, the price of electricity high, part load flexibility is necessary, and/or where a minimum of maintenance is required.

In addition, the LTD technology is particularly suited to treat problematic industrial waste water from various sources such as produced water, mining and industrial waste.

“Our LTD system can deal with a wide range of salinity, whereas many desalination plants are designed to treat a specific type of feedwater," said Mark Lehmann, Watersolutions’ Chief Technology Officer.

"The LTD process can even produce clean water from produced water that is highly saline and contaminated with oil.

"It can also accommodate variations in the plant load, running efficiently from 10 – 110% of plant design capacity.

"The process is self-adjusting, and the amount of water produced is proportional to the amount of waste heat provided,” added Lehmann.

LTD claims to work efficiently over a broad range of salinity. Because the process is very tolerant to the salinity of the feedwater, it can even handle brine concentrate from RO.

As a result, retrofitting an existing RO plant with an LTD system would be an efficient way to increase the plant’s capacity.

“With little or no investment in the peripherals, the capacity of a plant could be more than doubled using the brine as feedwater,” Mansfeldt noted.

The brine from an LTD plant can be concentrated close to the saturation level of salt, thus making drying of salt and minerals easier and Zero Liquid Discharge (ZLD) an opportunity.

The Watersolutions LTD system is modular, scalable and easy to install. The units are available in two sizes – a large module that produces 1000-2000 m3/d (pending the amount of waste heat available and number of cascades) and a medium module with capacity of 500-1000 m3/d.

These units can be combined to scale up production as needed. Watersolutions is currently targeting plants of 500-20,000 m3/d capacity.

The expected lifetime of an LTD plant is 25 to 30 years, due to the simplicity of the design, the combination of high quality materials used, and its ability to function at relatively low pressure.

Due to the robustness of an LTD plant and its high conversion ratio, the plant operates using a relatively small amount of chemicals for pretreatment.

“Seawater desalination provides the only sustainable new source of fresh water, and our LTD system not only provides a robust, cost-effective and efficient solution – it is also ideally positioned to capture growth both in the desalination market and in the market for treating industrial wastewater including produced water from oil and gas exploration,” said Mansfeldt.

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