7.2 billion people live on this planet. All these people need to eat. Eating has its consequences, namely excrement in the form of feces and urine. The problem is, that people leave their excrement behind as waste products. This amounts to about 1 billion kilograms of feces and 14 billion liters of urine each day! The process of food consumption and subsequent excretion by a total urban population is what I call ´Urban Metabolism´. Our present-day urban metabolism causes a variety of negative environmental effects, such as eutrophication, oxygen depletion, fish kills, and harmful algal blooms. These effects are both caused by human population numbers and because of the fact we consider excrement to be waste. Furthermore, urban areas are generally found in the vicinity of water, which is thus where much of our excrement ends up. Since population numbers are not so easily changed, let´s discuss the way we treat our excrement. The importance of discussing this is clear when we consider the reported environmental effects. More importantly, we can do something about it! Let´s consider how we can change a threat to our environment and start appreciating our excrement as a valuable resource.
To understand the threat, we need to understand that excrement is a fertilizer for plants. It fertilizes because it contains the elements nitrogen and phosphorus, which are essential nutrients for plant growth. Therefore, by dumping our excrement as waste, we continuously add these two elements to the earth system and never use them again. If the concentrations of nitrogen and phosphorus are high, this can cause ´nutrient pollution´. Excess amounts of nutrients in surface waters have proven to be detrimental to the environment. That´s not all. In addition to food, another important urban source of phosphorus is detergents. Your everyday dishwasher detergent or hand soap likely contains a lot of phosphorus! A schematic representation of all urban sources of phosphorus or nitrogen is shown in figure 1. In the late 20th century, detergents were a major source of urban phosphorus. Luckily, several countries started to ban the use of phosphorus-based detergents, which caused a rapid decline in urban phosphorus pollution in Europe since the 1980s.
![Figure 1: Sources and sinks of nitrogen and phosphorus in urban areas [Morée, 2013]. The fraction of people without sewers or without waste water treatment is still very large, causing nitrogen and phosphorus to end up in surface waters.](http://scisnack.com/wp-content/uploads/2014/07/1-300x248.jpg)
Figure 1: Sources and sinks of nitrogen and phosphorus in urban areas [Morée, 2013]. The fraction of people without sewers or without waste water treatment is still very large, causing nitrogen and phosphorus to end up in surface waters.
A positive effect of this concentration in sewers is the possibility to apply waste water treatment. At present, modern waste water treatment facilities can remove ~80% of nitrogen and ~90% of phosphorus. However, only a very small fraction of the global population is connected to such advanced facilities and most sewers are not connected to any form of waste water treatment at all. On a global scale, urban areas are localized and concentrated point sources of nitrogen and phosphorus. When high concentrations of nitrogen and phosphorus reach surface waters, they are likely to cause negative effects such as harmful algal blooms.
![Figure 2: Total surface water loadings of nitrogen and phosphorus for 1900, 1950 and 2000 [Morée, 2013]. The right hand column 'Agriculture' shows how much nitrogen and phosphorus was used in agriculture for these years. While industry was the most important source of nitrogen and phosphorus in the early 20th century (pink) to surface water, it is now clear that human population numbers dominate the system (blue). Animals (green) are mostly important for agricultural nitrogen and phosphorus.](http://scisnack.com/wp-content/uploads/2014/07/2-300x266.jpg)
Figure 2: Total surface water loadings of nitrogen and phosphorus for 1900, 1950 and 2000 [Morée, 2013]. The right hand column ‘Agriculture’ shows how much nitrogen and phosphorus was used in agriculture for these years. While industry was the most important source of nitrogen and phosphorus in the early 20th century (pink) to surface water, it is now clear that human population numbers dominate the system (blue). Animals (green) are mostly important for agricultural nitrogen and phosphorus.
We have to eat. We have to face the consequences of eating. But both the lack of waste water treatment facilities in many areas of the world and the fact that global population numbers continue to increase create a worrisome future. But there is hope, as shown by the decline in surface water nutrient pollution in Europe, where waste water treatment is applied and phosphorus containing detergents are getting banned. The world may even be able to use its ´waste´ as fertilizers again, which will ensure a healthy urban metabolism and a closed nutrient cycle.
Bon appetit!
References:
Morée, A. L., A. H. W. Beusen, A. F. Bouwman, and W. J. Willems (2013), Exploring global nitrogen and phosphorus flows in urban wastes during the twentieth century, Global Biogeochem. Cycles,27, 836–846, doi:10.1002/gbc.20072.