ED (ElectroDialysis) is a technology for producing fresh water from salt or brackish water. By creating a voltage difference - also in a stack - the ions from the salt water are forced to move through the membranes, thus achieving desalination.
In the ED stack, the polarity of the voltage difference is reversed at certain time intervals, and at the same time the discharge flows of concentrate and product are swapped. This regularly removes any contamination at the membranes and electrodes. Because of this exchange, this process is also called ED-Reversal (ED-R).
ED consumes less energy per volume, and can handle a wider variety of process water than conventional technologies. ED also contributes to a higher degree of water reuse.
The availability of sufficient clean water in an increasing global problem. As a result, there is increasing interest in technologies to desalinate water. In the desalination process, requirements are increasing with regard to the reduction of waste streams (Zero Liquid Discharge / Minimal Liquid Discharge) and with regard to energy saving in connection with cost reduction and reduction of CO2 emissions.
Worldwide, approx. 104 million cubic meters of seawater and brackish water are desalinated every day. This volume is increasing by about 5% annually. At this point in time, ED as a desalination technology forms a relatively small share of this at about 3%. This share is also annually growing. The lifetime of the stacks and membranes within these plants is about 5 years, so a replacement market exists. The total available market for ED has a value of approx. € 400 mln.
REDstack's objective is to grow to a good market share in this replacement market. Partly by replacing stacks within existing plants, but also replacing stacks in newly realized plants with REDstack stacks.
REDstack focuses on some specific applications of ED in the desalination market.
ED for brackish water desalination
Brackish water desalination is a way to obtain low-cost drinking water. ED has proven its feasibility and high performance in brackish water desalination and desalination of amino acids and other organic solutions. ED is particularly suitable for lower salinity to remove a high amount of salt. ED consumes less energy than other technologies for desalination of brackish water.
ED for desalination of brine water (to achieve Zero Liquid Discharge)
There are several emerging markets related to the recovery of water and other valuable constituents from (waste) water streams, in which ED technology has the potential to play an important role. REDstack is orienting itself towards these markets.
Worldwide, many efforts aim on the field of Zero Liquid Discharge (ZLD). This is an application where no wastewater is produced anymore. The intention is to recover all water from the wastewater stream and make it reusable within production, thus relieving the environment and greatly reducing the water demand of industry. Here, ED technology can also play an important role by strongly concentrating the wastewater flow.
ED technology can also be used to recover nutrients from wastewater streams and wash water from air scrubbers. The extraction of ammonia in particular, which can then be used as a useful fertilizer, is an interesting application, certainly within the context of the nitrogen problem as it currently exists in the Netherlands.
ED for desalination of cooling tower water
Water abstraction for the processing industry and for the production of electricity (mainly cooling water) accounts for 42% of total water abstraction in the EU. The EU energy sector currently requires about 74 billion m3 of fresh water to operate. Most of the water is returned to the environment, but this consumes 3.8 billion m3 of freshwater. The total water requirements of the energy sector exceed those of agriculture and public water supply.
A typical 500 MW thermoelectric power plant consumes 26.5 m3 / min. Cooling towers are an important part of cooling systems. Hot water returns to the top of the cooling tower and drips down over the fill material in the tower. As it drips down, it comes into contact with the ambient air that washes up through the tower by forced draft. That contact causes a small amount of water to be lost and some of the water to evaporate.
During evaporation, the salts present remain, increasing their concentration in the circulating cooling stream. If this process is continued (concentration cycle), less soluble minerals such as calcium, magnesium and silica may finally precipitate insolubly and cause scaling. To prevent unwanted increases in salt concentration, some of the water is drained for disposal. Fresh water replenishment is delivered to the tower basin to compensate for the loss of evaporated water, blown-off water and sluicing water.
Treatment of cooling tower water presents several challenges, such as high TSS, TDS, hardness and alkalinity, possible presence of algae and bacteria (e.g. Legionella) or pathogens. Apart from the above, various chemical additives can be introduced to prevent scaling, lower pH, inhibit corrosion and eliminate biological contaminants (biocides).
ED technology is used to treat make-up water before the dissolved mineral concentration becomes corrosive to the system. By desalinating the make-up water, the number of cycles of concentration could be increased and overall water consumption reduced. And thus reducing the amount of brine that must be disposed of.
ED for seawater desalination
ED can be used as a stand-alone technology for seawater desalination, but also as a complementary technology to existing Reverse Osmosis (RO) plants to increase productivity.