Every day cities in Europe discard a useful nutrient-rich resource that could be used to grow crops. Ironically, we treat and process human wastes while we mine non-renewable phosphate and potassium and we consume fossil fuel to make nitrogen fertiliser. Since wastewater after all is rich in nitrogen, phosphate and potassium, we could switch such waste to useful fertiliser. This precisely what the SWITCH research project, funded by the European Union, attempted to do. Project scientist Ralf Otterpohl professor of civil and environmental engineering at Hamburg University of Technology, Germany, talks to youris.com about recent endeavours to utilise human waste as a valuable agricultural resource, based on the revolutionary sanitation concept known as Ecological Sanitation, or ecosan.

What is this ecosan approach?
The ecosan approach links wastewater systems to agriculture in a way that provides safe nutrients to agriculture. We are looking to recycle those substances that come to the cities with food, that are consumed and that go into the toilet. We want to bring them back to the land, as otherwise they go out to the water cycle and often cause problems there.

What has human waste got that agriculture could want?
There is a very large amount of nutrients in excreta. And, in fact, we treat a lot of nitrogen, phosphate, and potassium. But also trace elements and this is something that should be recovered. Otherwise, we pollute our water resources and at the same time deplete the land of these substances.  We are thinking about really large-scale systems, so that you recover nutrients for agricultural production outside cities. 

What about safety: is it really okay to put such waste back on agricultural land?
We have to be very careful. This is a clear finding from the SWITCH project. There are quite significant concentrations of pharmaceutical residues and hormones from the pill in urine. We do not want these in our agriculture for food production. They could be taken up by the plants. We are working on technologies to recover nutrients and to destroy such pharmaceutical residues. We are developing toilet systems that can recycle nutrients, possibly through membrane technology, and hold back most of the harmful substances. In this way we can produce a very clean fertiliser. With some technologies, you can even end up with a fine white powder.

Can Europe benefit from this approach?
There are low tech and also high tech approaches. You can use black water for toilet flushing, so that you save on freshwater, and you can process the water so that you produce just a small volume of waste with concentrated levels of nutrients. You can then eliminate harmful substances from this waste. This is technically complex, but can be done. 

We have just finished a pilot project in downtown Hamburg in a real-life situation. It is a public toilet near the railway station. But we are also planning a new hotel project which will be larger again. Visitors will be told that a special system is in place to conserve water and recover resources. Hotels like these systems and they market them to customers. The big advantage is that the toilet looks exactly the same. It is especially attractive to hotels in isolated areas where there is no wastewater infrastructure and in regions with sensitive habitats.

What else do we need to do different in our future cities?
We must find ways to deal with different wastewaters in different ways. We need different types of system for processing wastewater from toilets and wastewater from the rest of the house, so the so-called “grey water” from showers and washing machines. There should also be a focus on reducing toxic chemicals in household products that are harmful to waste water recovery and harmful to people too. And there is also a need to focus efforts on reducing pharmaceutical residues in wastewater.

You also see benefits for agriculture in this approach?
We can collect excreta through these processes in cities and transport them to rural areas, without any smells involved. And they can be composted ideally alongside charcoal to make humus – the living part of soil. This will improve the soil properties and help address soil degradation, which is a major problem around the world and a threat to our future food supplies.

Will all this research affect the way cities are planned for waste water in future?
It should be happening today. There are huge new developments being built every day and they are using old and outdated systems that are not appropriate where water is scarce. And they do not address the need to recover nutrients.  The water profession is very slow to change. 

What do farmers think of this approach to fertilising their crops?
They have concerns about accepting such fertiliser but use of night soil (i.e. human excreta collected at night) is still very common around the world. With proper inputs, we can produce a substance that is as good if not better than commercial fertilisers, some of which have problems due to the presence of heavy metals such as uranium and cadmium. When we show farmers the white powder, produced after further processing, they are more convinced. The eye does not deceive here. When we test this stuff for micro-pollutants we find none.