A by-product of municipal wastewater treatment plants is a waste sludge. This sludge holds potential energy that we might be able to use. If we focus on Europe in this post we can look at the graph below to see how much wastewater is treated by different countries.
We can then compare those numbers with the amount of biogas each country also produces.
Germany is dominating here. However, most of this biogas is being produced by co-digesting energy crops, such as corn from viable farmland. Would it be possible to produce this much energy from the waste we produce anyway?
Let’s keep using Germany as an example. In 2013, Germany had a population of 80.6 million people. After treating all of their waste there was 1.8 million tonnes of sewage sludge remaining. If we read up on what the United Nations has to say on excreta and wastewater sludges we see that if we are being optimistic we can expect over 5kWh of energy per kg from this sludge just by burning it. That means by burning all the crap Germany is producing we have created 9 000 000 MWh or enough to power around 300 000 of their homes.
However, Germany’s sludge production is actually not as high as it might be due to many municipal wastewater treatment facilities treating the sludge in biogas plants before incineration, this reduces the amount of sludge by up to 50%. So let’s now look at how much energy they can pull out of the sludge in the form of methane before incinerating it.
An example biogas plant produces around 4 000 MWh per year of energy from wastewater sludge generated by a city with a population of approximately 100 000 people. So we have another 3 224 000 MWh per year of energy from biogas. That is another 100 000 homes!
Ok, so that only gets us to around 1 million tonnes of oil equivalent, which is well under what Germany is currently producing using energy crops. Also, all that energy is usually getting fed back into the plant used to treat the initial influent (part of the energy is used for heating the biogas plant, but a much greater amount is required for powering the initial treatment plants aeration systems). So actually we have no homes being powered by crap…
But new research is investigating how we can get rid of these aeration systems and treat all of the wastewater using robust and energy efficient variations of the biogas plant.
This might be a bit optimistic! But having wastewater treatment plants that are net energy producers rather than energy consumers could become a real possibility in the near future.