Science - April 15, 2010

Spending an environmentally friendly penny

The toilet of the future can be found in a housing estate in Sneek. Introducing separation 'at the bowl' means a new lease of life for a large proportion of the raw materials. Good for the environment and your wallet.

The principle works
Marthe de Graaff has made significant progress towards the toilet bowl of the future. 'My PhD dissertation shows you can easily treat concentrated toilet wastewater; you can extract energy from it and remove fertilizers such as phosphate and nitrogen.' But she is also cautious. 'There are still quite a few questions. For example, about residues of medicines: this process is able to remove some of those, but not all. I don't yet know what the situation is with pathogens. And then there is the production of the powerful greenhouse gas N 2 O.'
But the principle works, says the student, who will get her Environmental Technology PhD on Friday. That has become clear from the practical application in Sneek and the water institute Wetsus' lab in Leeuwarden. Since 2006, 32 households in a newly built housing estate in Sneek have had a vacuum toilet in their homes. These toilets, the kind used on airplanes, use only one litre of water when flushed. According to De Graaff, that in itself reduces water consumption by 25 per cent in the average home. That means savings of twelve euros per person on an annual basis. But the real innovation is in the separation and processing of 'black' toilet wastewater.
The black water (faeces and urine) in Sneek is collected in a garage where an anaerobic biogas plant converts it into methane. A small-scale combined heat and power plant (CHP) converts that into electricity and heat. De Graaff improved this system and added a few steps so that most of the nitrogen is removed as well and the phosphate reclaimed.
De Graaff uses two processes to eliminate the nitrogen. First, oxygen is used to partially convert ammonium (NH 4 + ) into nitrite (NO 2 - ). Then a special kind of bacterium converts both compounds anaerobically into nitrogen gas.
This method removes 86 per cent of the nitrogen. Magnesium is added to precipitate the remaining phosphates out of the solution in the form of struvite. De Graaff claims that ten per cent of the global production of phosphate fertilizers would no longer be needed if all human faeces and urine were treated in this manner.
But there is also a hidden problem. The conversion of ammonia into nitrite also produces a small amount of nitrous oxide (laughing gas). 'Less than two per cent of the nitrogen is converted into N 2 O. But this is a greenhouse gas that does happen to be three hundred times worse than CO 2 , so this is still a problem. Incidentally, conventional water purification plants also produce nitrous oxide.
Weighing against this is the fact that the whole process is self-supporting in terms of energy. De Graaff's machine extracts 1.8 cubic metres of methane from one cubic metre of black water. According to her, the electricity and heat generated from this are more than enough to keep the purification and recycling going.
At present, the location in Sneek is only producing biogas. The purification and recycling will be added in a follow-up experiment. However, De Graaff does not think each household will purify its own toilet wastewater in future. 'That would be too small-scale and complicated. People will want to go to the toilet without having to think about all that. But it's perfectly possible at the housing estate level.'