As the world rapidly expands its anaerobic digestion and biogas infrastructure in the most part driven to meet renewable energy needs, it's time we turned our attention to an often-overlooked component: ammonia.
While traditionally viewed as a waste product to be eliminated, and of course having a very useful role in digestate fertiliser production, let's not forget it has other properties worthy of exploitation.
Ammonia presents two particularly exciting opportunities that could revolutionize how we think about waste treatment – as an energy carrier and as a building block for protein production. This dual potential could help address two of humanity's most pressing challenges: sustainable energy and food security.
Ammonia as an Energy Carrier
While the hydrogen economy captures most headlines, ammonia offers some surprising advantages as an energy carrier. As Ewan McAdam from Cranfield University has pointed out (at the 8th Conference of the UK Wastewater Network) , although hydrogen has a higher energy content per kilogram (120 MJ/kg versus ammonia's 22.5 MJ/kg), ammonia's ability to be easily liquified gives it an edge. Its energy density per cubic meter (17 MJ/m³) actually exceeds that of hydrogen (11 MJ/m³) at standard temperature and pressure, and it's safer to transport.
In conventional wastewater treatment, we currently spend more than 25% of the total energy (0.63 kWh per m³) converting ammonia to nitrate using bacteria. By capturing this ammonia instead, we could potentially generate 0.19 kWh/m³ – (See Simon Judd's Watermaths eBook) transforming an energy cost into an energy source.
The recovered ammonia can be utilized in several ways:
- Running modified combined heat and power engines
- Mixing with biogas as a fuel
- Producing hydrogen through electrolysis
Our “Take” on Ammonia as an Energy Source
Here's something that might blow your mind: In conventional wastewater treatment, the current generation of biogas plants spend a lot of energy (more than 25% of the total 0.63 kWh per m³) converting ammonia to nitrate using bacteria.
But what if we could capture that ammonia instead?
We could potentially generate 0.63 kWh per m³ of energy! That's quite a flip from using energy to producing it, right?
So how do we actually get this ammonia? The key is catching it at the right moment when its concentration is highest. In anaerobic digestion systems, there are two main sweet spots:
- The offgas from anaerobic membrane bioreactors
- The liquid that comes from dewatering digested sludge (we call this the liquid digestate fraction)
Now, I should mention that this isn't all sunshine and roses – there are some challenges the biogas industry would need to tackle:
- We need to raise the pH to convert ammonium to ammonia
- The process works better at higher temperatures
- The stripping equipment can get fouled up
- We need to concentrate and purify the gases for end use
But here's the silver lining – by skipping the traditional biological nitrification step, we actually save energy. And there's an environmental bonus too: we avoid the potential for releasing nitrous oxide (N₂O), which is a pretty potent greenhouse gas.
The really exciting part is that we can in principle already use this recovered ammonia in several ways:
- Combined heat and power engines can be modified to run on ammonia
- We can mix it with biogas as a fuel
- We can even use it to produce hydrogen through electrolysis
Aspects like corrosion would need tackling but the biggest hurdle right now isn't really technical – it's more about scale. We need to have enough ammonia being produced in one place to justify building the recovery systems. But since ammonia can be transported as a pressurized liquid, this might become less of an issue as the technology develops.
What do you think about this approach? It's pretty interesting how we would be turning what was once considered a waste product into a valuable energy resource, isn't it?
Ammonia as a Protein Source
Beyond energy, ammonia recovery opens another exciting pathway: microbial protein production. This process uses specialized microorganisms to convert ammonia into single-cell protein (SCP), effectively turning a waste stream into a valuable food resource.
The key microorganisms involved include:
- Methylotrophs
- Hydrogen-oxidizing bacteria
- Nitrogen-fixing bacteria
- Selected yeast strains
These microscopic protein factories can produce nutrient-rich biomass suitable for animal feed or potentially human food products, offering a sustainable protein source for our growing world population.
Our “Take” on Ammonia as an Protein Source
Let's talk about something really innovative – how we can turn recovered ammonia into protein using microorganisms. This is a fascinating example of turning what we used to consider a waste product into something valuable for our food systems.
You know how traditionally in wastewater treatment, we already mentioned how the water treatment industry spends a lot of energy converting ammonia to nitrate just to get rid of it?
Well, what if instead of doing that, we could capture that ammonia and use it to grow protein? As the text mentions, the conventional approach of converting ammonia to nitrate eats up more than a quarter of the energy used in wastewater treatment (about 0.63 kWh per cubic meter) (See Simon Judd's Watermaths eBook).
That's a lot of energy we could potentially save!
The key, just like with energy recovery, is catching the ammonia at the right moment. There are two prime spots where we can grab it:
- From the gases coming off anaerobic membrane bioreactors
- From the liquid that comes from dewatering digested sludge
Now, once we've captured the ammonia, here's where it gets really interesting. We can use it to feed special microorganisms that turn it into protein. These tiny protein factories are pretty amazing – they can create what we call “single-cell protein” or SCP. Think of it as a sort of microscopic protein farm!
The process does face some similar challenges to energy recovery:
- We need to raise the pH to convert ammonium to ammonia
- Higher temperatures make the process work better
- We have to deal with equipment getting gunked up
- The gases need to be concentrated and purified
But here's where it gets really practical – the key microorganisms for microbial protein production that we already mentioned above can do it for us.
The end product is a protein-rich biomass that could potentially be used for animal feed or even human food products.
Though, not everyone will be thrilled at the prospect of eating a slab of pork or vegetarian meat that was prepared using sewage treatment plant rubbish!
The biggest challenge, similar to energy recovery, is having enough ammonia in one place to make the system economically viable. But here's a silver lining – since ammonia can be transported as a pressurized liquid, we might be able to collect it from multiple locations.
This whole concept really fits into the essential drive towards the circular economy.
It's about turning what we once considered waste into valuable resources. Sure, there are challenges, but as the only limitation is commitment.
What do you think about this approach? It's pretty amazing how we can potentially turn wastewater treatment byproducts into protein, isn't it?
Common Recovery Points and Challenges
Both applications – energy and protein production – rely on capturing ammonia at its highest concentration points in the treatment process:
- From anaerobic membrane bioreactor offgas
- From centrate produced during sludge dewatering
The process faces several technical challenges:
- pH adjustment for ammonium-to-ammonia conversion
- Temperature optimization
- Surface fouling management
- Gas concentration and purification needs
However, the benefits extend beyond the direct products. By avoiding traditional biological nitrification, we save energy and prevent the release of nitrous oxide, a potent greenhouse gas.
Scale and Implementation
The primary hurdle for both applications isn't technical but rather one of scale. Sufficient ammonia volumes are needed at a single location to justify the capital investment in recovery systems.
However, ammonia's transportability as a pressurized liquid offers potential solutions through centralized processing facilities.
Perhaps the biggest consideration against ammonia would be the health and safety aspects of ammonia leakage, and yet the alternatives are not exactly safe, if leaked, either!
Conclusion
As we stand at the crossroads of climate change and food security challenges, it's time for some blue-sky thinking about ammonia in anaerobic digestion (AD) processes.
The use of ammonia for natural fertiliser production isn't the only use for this by-product of the anaerobic digestion process. The additional potential of ammonia for both energy and protein production invites us to reimagine our waste treatment AD facilities not as mere energy production and cleaning stations, but as biorefinery platforms producing renewable energy and sustainable protein.
Imagine future anaerobic digestion facilities where every molecule of ammonia is carefully managed and directed either toward energy production or protein synthesis, depending on local needs and market conditions. Such facilities could help power our communities while simultaneously providing sustainable protein sources for our growing population.
The technology exists, the potential is proven, and the need is clear. What's required now is the vision and commitment to implement these solutions at scale. As we design the next generation of anaerobic digestion facilities, let's ensure ammonia recovery takes a more prominent role – not as a waste product to be eliminated, but as a valuable resource to be harnessed for a more sustainable future.
In the end, our only true limitation is our imagination and commitment to innovation. The path to both renewable energy and sustainable protein production might just run through the ammonia in our waste streams. Isn't it time we started walking that path?
