From Drains to Gains - Wastewater Empowers Green Hydrogen

Direct electrification is difficult, if not impossible, in some end-use sectors, including steel-making, chemical production, long-haul aviation and maritime shipping. But there is a viable solution for decarbonising these end uses – “green” hydrogen and other fuels produced from renewable electricity.

No matter from IRENA's 1.5℃ scenario, or Princeton's Net-Zero America report, H ₂  is an important element in hard-to-abate sectors - where direct electrification is hard to achieve.

H ₂  comes in many colors - grey H ₂  from fossil fuel sources, blue H ₂  which is grey H ₂  + CCS (carbon capture and storage), and green H ₂  which is produced from water electrolysis powered by renewable electricity. Unfortunately, most of the world's current H ₂  supply is grey H ₂ , with green H ₂  expected to take over in the coming decades.

Grey H ₂  bears large carbon intensity. With CCS (blue H ₂ ), this carbon intensity can be partly offsetted. Green H ₂  remains the cleanest method of production with one underlying requirement - the electricity source has to be renewable.

 Read more : green H ₂  also has a higher tolerance of leakage than blue H ₂  to not invoke increase of atmospheric methane - a potent greenhouse gas.

Although the water consumed for hydrogen production will not have a significant impact globally, the importance of considering local water contexts when planning hydrogen development cannot be overstated, especially chronic water risks such as water stress.

Water electrolysis - a process that uses electrical energy to break down water molucules to H ₂  and O ₂  - is a thirsty process as the name suggests. Stoichiometrically, 9 kg of H ₂ O is needed to produce 1 kg of H ₂ . However, electrolyzers take clean water only - the water purification process itself will have water loss. Then, there is cooling water - water needed to cool down the heat-emitting process. The total demand can ammounts to well over 30 kg of H ₂ O per kg of H ₂ .

Total quantity-wise, water availability is not a big issue - as the total water demand is significantly smaller than other water-heavy sectors such as agriculture. However, green H ₂  can easily become one of the largest water users in a region - pose water stress to local environments and compete with essential water needs.

Unfortunately, one key requirement of green H ₂  - renewable energy - often does not come in water-abundant regions.

Across the world, safely reused wastewater is grossly undervalued as a potentially affordable and sustainable source of water, energy, nutrients and other recoverable materials.

For regions that are promising for developing green H ₂  but are faced with high water stress, water resource recovery plants, or in short, WRRFs, provide a easily-accessible and viable water options.

Besides their convenient location, another prime reason why WRRFs might be a good opportunity to enable green H ₂  is - they typically have a much larger water flow than what a H ₂  plant would require. This means WRRFs can just provide a slipstream which would be enough to enable green H ₂  production without disturbing the original water flow.

But of course, you need to consider a lot of things for water reuse projects: build a water pipe between WRRF and H ₂  production site, pumping the water, treating the water...... That's why we built an optimization framework to try to determine whether this is economically feasible, and applied it on the current H ₂  producers and WRRFs in the United States. I know - nobody likes to read Methods section, so we saved the boring part in the paper published.

Reusing treated wastewater effluents for green H ₂  production could save 66% (62% - 99% from the worst case to the best case) of freshwater for the clean hydrogen economy given the current distribution of H ₂  producers and WRRFs. States that currently has the highest H ₂  activity (such as Texas, Louisiana, California) will benefit the most.

The average flow diversion for green H ₂  production is < 1.5% on state-level, meaning reusing wastewater effluents for green H ₂  production likely will not induce a significant disturbance to the original water cycle.

The calculated minimal water selling price - which is based on the cost of service for providing the reclaimed water - is comparable to, or even lower than, the typical industrial water cost. This means WRRFs can price the reclaimed water provided to the H ₂  producers at a profitable yet competitive price.

The interaction between WRRFs and green H ₂  might be beyond water - the possibilities of tunneling the produced O ₂  for aeration, or utilizing the waste heat from electrolyzer are all exciting and worth exploring.

U.S. is mainly operated under two water rights systems - the Riparian Doctrine and the Prior Appriopriation Doctrine. What that essentially means is that WRRFs not necessarily have the right to dispose/use their effluent any way they want. However, many states do have green lights on regarding beneficial reuse. It would be interesting to see how state-level regulations unfold. We are optimistic that such reuse makes economic and environmental sense, and perhaps equally importantly - unlike potable reuse which people always have a hard time accepting, most people don't care if the H ₂  they directly or indirectly use comes from freshwater or impaired water at all.

This storymap is based on the  study  published in Environmental Science & Technology titled "Water Resource Recovery Facilities Empower the Electrolytic Hydrogen Economy". Please cite us if you find this page or the paper useful!