Urban Metabolism

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.

Despite these efforts, the amount of nutrients continues to grow as our urban metabolism ever increases. The global urban-metabolism-related impacts of urban areas worsened during the twentieth century due to rapid population growth, urbanization, and sewer construction. Before the general presence of sewer pipelines (~1950), urban excrement was directly dumped into rivers or used as fertilizers. Yes, fertilizers! As we just discussed, excrement contains the plant nutrients nitrogen and phosphorus. Interestingly, the fertilizer value of feces was much appreciated in many parts of Europe and Asia only about 100 years ago. This is in contrast to our modern society, in which we provide nitrogen and phosphorus to plants using artificial fertilizers. We extract nitrogen from the atmosphere using the Haber-Bosch process and we mine phosphorus from phosphate rocks. This touches upon another problem of our urban metabolism, since these phosphate rocks may deplete within the next 150 to 200 years. On the other hand however, nitrogen is an ‘infinite’ resource since we are able to take it from the atmosphere which consists of 78% nitrogen. The application of urban excrement as fertilizers has now been banned in most countries in the world due to hygiene considerations. This may seem to be a good thing, but the downside is that if excrement is collected in sewers instead of being reused, two things happen: 1. The excrement becomes more concentrated and 2. it becomes waste.

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.

Because so many people lack sewers and waste water treatment facilities, about 7.7 Teragrams of urban nitrogen (the weight of more than one million elephants) and 1.0 Teragram of urban phosphorus ended in surface waters in the year 2000. In 1900, these values were about 3.5 times lower for nitrogen and about 4.5 times lower for phosphorus. With the relative contribution of industries and animal nutrient excretion in decline, humans are now the main source of urban nitrogen and phosphorus (figure 2). Though urban nutrient pollution is much smaller than the agricultural nutrient pollution of surface water, the concentrated characteristics of the urban pollution causes major environmental problems in coastal ecosystems and rivers along densely populated areas. We have learnt from the decline in surface water nutrient pollution in industrialized areas that the combined effects of waste water treatment and banning of phosphorus-based detergents can strongly reduce this pollution. However, it would be even better if we could convert the nitrogen and phosphorus back from our sewage, thereby also solving the problem of finite phosphorus resources.

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.