Wastewater treatment sans oxygen

Anaerobic digestion is the term used to describe the treatment of wastewater in an oxygen free environment. Normally, municipal wastewater is treated with microorganisms mixed through the liquid with lots of air (so aerobic treatment). The oxygen provides lots of energy for the organisms to grow and quickly consume nutrients in the waste, this prevents the nutrients from entering rivers and streams where they can cause all sorts of problems with the ecological balance of the environment. When consuming the same amount of nutrients, the total mass of microorganisms will be 10 times more when they have been provided with oxygen than not.

It is this huge mass of microorganisms combined with left over solids that is becoming an increasing problem in the developing world. They are treating more and more of their wastewater (which is good) using the same aerobic treatment systems that have been typical in the developed world for the last 100 years. These systems need lots of energy to pump oxygen into the waste and produce a huge amount of highly concentrated sludge that either just gets dumped into a landfill, which is just moving the problem around without really solving it, or incinerated.

Lots of research is now being dedicated to analysing the energy efficiency of treating the sludge using anaerobic digestion. But why are we making these systems more and more complicated? Lets look at the path the wastewater can take from when it first arrives at the treatment plant to when the different parts all exit the plant.

Steps in a typical wastewater treatment process

We are pumping energy into the wastewater (aeration) and then trying to remove it all again straight away in our sludge treatment step. Why do we add all that energy to our wastewater in the first place? Why not just remove the energy that is already contained in it and use it for something else?

Two problems. First, anaerobic digestion is slow. All that energy being pumped into the wastewater is being used to quickly remove the nutrients. If we aren’t pumping energy into our system in the form of oxygen the microorganisms need to just use the energy that is available in the wastewater and they grow and reproduce much more slowly. Second, the microorganisms currently being used in anaerobic digestion are not removing nitrogen from the wastewater. This is bad, because the nitrogen can be used by things like algae in our rivers and streams causing all sorts of problems

However, these two problems are being addressed. The slowness of anaerobic digestion can be improved by clever system design and control. By pumping the wastewater around the system very quickly and increasing mixing with the microorganisms great efficiency improvements can be seen, here the problem comes that more energy is needed to pump the wastewater around quickly and mix everything really well, so we are just putting more and more energy into the system again! Therefore, clever system design is required so that we can reduce both time and energy required for the treatment.

Regarding the second problem, at the end of the 90’s researchers at the Delft University in the Netherlands discovered a bacteria that can convert nitrogen in wastewater (in the form of ammonium and nitrite) into nitrogen gas without needing oxygen. This can mean huge energy savings and reduced greenhouse gas emissions. It could also mean a complete anaerobic wastewater treatment system that either requires much less energy than current wastewater treatment plants or, in the best case, a wastewater treatment plant that is actually producing energy from our waste rather than requiring more energy to treat it.


Energy in waste

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.