Water Treatment for Hydrogen Production through Electrolysis

Required Water Quantities 

Depending on the electrolysis technology used, an electrolyzer consumes about 10 to 11 kg of water per kg of hydrogen. This is partly because, in addition to the portion converted into hydrogen and oxygen, some of the water used is "lost" through evaporation, and some becomes wastewater. 

Since electrolyzers must be fed with ultrapure water, the raw water used must be treated accordingly. Depending on the achievable yield of the water treatment, consumption increases accordingly. 

Furthermore, electrolyzers need to be cooled. If cooling is done exclusively with water, the cooling water requirement is approximately five to six times the water requirement for electrolysis. 

According to Eurowater, a 1-MW electrolyzer requires around 200 l/h of ultrapure water and about 400 l/h of cooling water when using evaporative coolers.

Quality Requirements and Treatment 

The ultrapure water used for electrolysis, known as Ultra Pure Water (UPW), must meet certain requirements: 

• Electrical conductivity: < 0.5 µS/cm 

• TOC (Total Organic Carbon): < 0.1 mg/L 

• Silicates: < 0.1 mg/L 

• Total hardness: < 0.1 °dH 

The quality requirements for PEM electrolyzers are even higher than those for alkaline electrolyzers. 

Not only must the UPW used meet certain quality characteristics, but also the cooling water. The materials used should be protected from corrosion, and the formation of deposits and biofilms should be minimized. The guideline values for cooling water can be found in VDI Guideline 3803. 

Three steps are always necessary for UPW production: 

1. Pretreatment 

1. Desalination (also called "make-up") 

1. Post-treatment (also called "polishing") 

Each of these three steps can consist of multiple processes. Techniques such as reverse osmosis, ion exchange, and distillation are used for treatment. 

The first stage of water treatment includes mechanical and chemical processes. These include filtration to remove suspended solids, activated carbon filtration to remove organic compounds, pH adjustment, and water softening. This pretreatment ensures that the water is low in germs and particle-free. This may require chlorine disinfection and precipitation or flocculation. The crucial step in water treatment is desalination. The effort required for desalination varies depending on the water's salt content. Post-treatment is always similar, regardless of the raw water resource. During polishing, further desalination takes place, and dissolved gases must be removed from the water. 

Many of these water treatment methods are energy-intensive, which can affect the climate balance of hydrogen production. Additionally, the construction and operation of modern water treatment systems are very expensive.

Water Scarcity and Use of Seawater 

Electrolysis is particularly suitable in sunny regions, as the potential for solar energy is very high and thus electricity costs are very low. However, these regions are often affected by water scarcity. 

It should not be overlooked that when exporting hydrogen, a portion of water is always exported as well. This applies regardless of whether hydrogen or other energy carriers such as ammonia or methanol are transported. 

Using drinking water for hydrogen production in these water-scarce regions is therefore ethically questionable. One possible solution would be the use of seawater for electrolysis. However, technologies for using salt water present technical challenges. 

When seawater is used as raw water, the ultrapure water yield is lower than when using freshwater. Seawater desalination plants have a yield of about 50 percent or less. In comparison: The production of ultrapure water from ground or surface water can be achieved with a yield of 70-80 percent. In absolute numbers, according to Eurowater, this means that for 1 m³ of UPW, approximately 1.4 m³ of groundwater, 1.5 m³ of surface water, or about 3.3 m³ of seawater is needed.

Critical Points 

Using seawater requires more complex pretreatment, and the energy requirement for reverse osmosis is significantly higher. Additionally, desalination plants used for water treatment produce highly concentrated wastewater. These concentrates have increased salt concentration and sometimes contain chemicals as well. 

The production of pure and ultrapure water is already widely practiced today, as many industrial sectors and research facilities require ultrapure water. Currently, wastewater streams are discharged into the seas worldwide. 

However, if significantly more ultrapure water is produced for hydrogen production in the future, the amount of wastewater concentrates will increase accordingly and could damage ecosystems. 

Hydrogen Ramp-up Could Improve Water Treatment 

The hydrogen ramp-up also offers opportunities regarding water treatment, as it has a different economic significance for electrolysis. Electrolysis is the expensive process, while water treatment is comparatively less significant in terms of cost. This makes concentrate post-treatment processes more economically viable.