One beautiful summer's evening, Felix flies his model aeroplane and performs a few stunts.
My plane's battery is empty once more.
It's getting dark already. Let's recharge the batteries overnight. Even your own batteries will be fully charged again tomorrow, and you'll be fit and ready to take off.
I don't even have any batteries myself.
Just like every other living thing, your body cells need energy.
Even birds?
Yes.
And deer, whales, trees and so on?
Yes, of course — all of them.
What about the bacteria in sewage treatment plants?
Absolutely.
Where do they get their energy from?
From the wastewater and the nutrients it contains. This is food for the bacteria. They obtain the energy they need to survive from it, among other things. You have to eat something every day, too.
And when I eat, the bacteria benefit from it, too!
What do you mean?
Well, if I eat and drink a lot, I need to go to the toilet quite often. Ultimately, what I consume ends up with the bacteria in the sewage treatment plant.
Wastewater actually has a significant energy content. The proportion of organic matter in wastewater is expressed as the chemical oxygen demand (COD) and results in a theoretical energy potential per inhabitant per year that could cover the electrical energy needs of an average family for 2–3 weeks. Additionally, we supply an average of 0.66 kg of phosphorus and 4 kg of nitrogen.
Then I act as a battery for the little creatures.
You, I and everyone else feeds them.
Is there actually a lot of energy in the water?
It should be noted that only a small amount of this energy can be used. The sewage treatment plant itself also needs energy – between 20 and over 50 kWh per inhabitant per year. Most of this is needed to aerate the biological treatment step. There, bacteria convert nitrogen-containing compounds into nitrogen gas. A considerable amount of chemical energy is lost in this process, as carbon is simultaneously converted into carbon dioxide. Some of the remaining carbon is converted into methane gas in the digester. Many sewage treatment plants now use this methane to produce heat and electricity. However, the maximum amount of electricity generated in this way corresponds to only 10% of the theoretical energy content of the wastewater.
That's not very much, is it?
At least some sewage treatment plants cover their own electricity needs with this. An increasing number of plants are considering how they can save energy and extract as much as possible from the process. However, this requires a fundamental rethink: rather than consuming as little energy as possible, the aim of the activated sludge process — the heart of every modern sewage treatment plant — is to mineralise organic contaminants to a high degree with the help of bacteria.
Can't the bacteria be taught to produce other substances?
Not necessarily, but the conditions and processes can be altered to encourage other biochemical reactions that produce substances which can be used for energy.
That sounds complicated!
It's easier than it sounds. The aim is to produce as much methane as possible in the digestion tower under anaerobic conditions — meaning without oxygen. To achieve this, the microorganisms require carbon. That's why ideas are being developed to work out how to transfer as much carbon as possible from the pre-treatment stage at the beginning of the process to the digestion tower.
The bacteria in the digester will be delighted!
The operators of the sewage treatment plant will be pleased, too. If this process control is successful, then in the aerobic stage...
That's where it bubbles nicely, right?
Yes, that's exactly right. Less oxygen is needed, so less energy is required for aeration.
Oh dear! Then it won't bubble so much anymore.
Don't worry — the bacteria still need enough air to break down the nitrogen compounds. You have to make sure the aerobic bacteria aren't neglected either.
Sharing is caring!
When the carbon is optimally distributed, nitrogen degradation continues and the energy balance improves at the same time. We can contribute to this, too: coagulants can help sewage treatment plants generate more energy.
Why is that?
According to calculations by the Swedish IVL Institute (Johansson, Nilsson & Rahmberg, 2025), the targeted use of coagulant in primary treatment can reduce the CO₂ footprint of the wastewater treatment plant while improving its energy balance. Pre-precipitation relieves the aeration process by diverting the organic load to the digestion tower. Consequently, less aeration energy is required and more methane gas is produced in the digester. These differences are particularly noticeable when compared to pure biological phosphorus removal (Bio-P). In the IVL study, pre-precipitation produced the best results, followed by simultaneous precipitation and Bio-P. However, this does depend on local conditions: the plant must be equipped with sludge digestion and primary treatment, and have sufficient organic load (i.e. carbon) that can be partially separated in primary treatment without starving the aeration process.
What can be done when there is no more carbon available?
Sewage treatment plants with digestion towers are increasingly using suitable organic waste materials, such as kitchen waste and grease trap residues. These materials usually contain compounds that can be quickly converted into methane.
This lets the methane bacteria make lots of biogas!
That's right — this also increases electricity production. However, using these co-fermentates can cause various problems that sewage treatment plant operators must address using appropriate measures. For example, the residues may contain high levels of sulphur, which enters the biogas in the form of hydrogen sulphide.
Eww, the biogas will really smell!
At these concentrations, hydrogen sulphide is odourless and highly dangerous. When used in biogas utilisation, however, it would damage the gas engine, which would be very costly. Fortunately, we can help remove this harmful gas with products such as Donau Bellamethan and activated carbon from Donau Carbon. This means that nothing stands in the way of energy production at the sewage treatment plant.
An increasing number of sewage treatment plants are using the thermal energy contained in wastewater. Wastewater has a relatively constant temperature. This can be harnessed using heat pumps to either heat the sewage treatment plant itself or provide district heating for neighbouring settlements. As this heat is considered a form of renewable energy, it would lead to lower CO₂ emissions. In Styria, for instance, the potential has been calculated at over 500 GWh per year (Mach und Partner ZT-GmbH, 2022) if sewage treatment plants with a capacity of over 5,000 PE, located near settlements in Styria, were to be fitted with heat recovery systems. This calculation assumes a 5°C reduction in wastewater temperature. The potential greenhouse gas savings are estimated at 47,000 t CO₂-eq/a. The electricity required for the heat pump has also been taken into account. This is because energy is required for every energy generation process. If this were not the case, we would have a perpetual motion machine. However, the laws of physics do not permit this, so such machines only exist in phantasy.
You get out what you put in.
That's right. According to the laws of thermodynamics, I cannot create or destroy energy; I can only convert it into other forms. For example, I could use electrical energy to produce hydrogen. This hydrogen then contains chemical energy, which can be converted back into heat and used to produce electricity again.
Isn't hydrogen also produced when metal is added to acid?
That's right, this has been common knowledge for centuries: In 1530, Paracelsus described a gas being produced when iron was dissolved in sulphuric acid. In 1650, Sir Turquet de Mayerne discovered that the gas was combustible and called it 'combustible air'. Around 1700, Nicolas Lemery recognised that the gas could react explosively with air.
Boom! Just like the balloon in chemistry class!
This is known in chemistry as detonation gas reaction for good reason. By the way, Paracelsus's experiment produced iron sulphate; had he used hydrochloric acid instead, iron chloride would have been produced. He thus created a coagulant, although this application was not relevant in the 16
th century.
The explosion range for a hydrogen-air mixture is very wide at 4–76%, and the required ignition energy is very low at 0.017–0.02 mJ (millijoules), which is much lower than that of methane at 0.28 mJ. A small spark or even static electricity is enough...
BANG!
That's why we ensure that in our ferric chloride plant the acid doesn't come into contact with metal — we don't want it to explode!
I'd rather not run my model aeroplane on hydrogen, then. Isn't there a wastewater battery?
Something similar, theoretically – a microbial fuel cell, for example.
Great! Are they already available for model aeroplanes?
The technology is still a long way from being put into practice. From today's perspective, the achievable current densities are too low.
It's a shame, as it would have been very practical, and I wouldn't have had to wait so long for the battery to recharge. I would only have to go to the toilet quickly.
Sources
Johansson, K., Nilsson, L., & Rahmberg, M. (2025).
Carbon footprint assessment of chemical and biological phosphorus removal. IVL Swedish Environmental Research Institute ltd. INCOPA.
Mach und Partner ZT-GmbH. (2022).
ABWASSERWÄRMEPOTENZIAL STEIERMARK Endbericht. Graz: Land Steiermark, Abteilung 14, Wasserwirtschaft, Ressourcen und Nachhaltigkeit.
Header image created with the support of ChatGPT (AI-generated).