Common arguments for why cities should reduce and recycle food waste are to lower greenhouse gas emissions, increase levels of service to households, reduce the need for landfills and/or incinerators, and to close the loop. But there is one more very compelling reason to address food waste: to solve the World’s phosphorus crisis. And cities, as aggregators of phosphorus, are at the centre of the solution.

Phosphorus – The Challenge

Phosphorus is an essential nutrient for life. Without phosphorus, food production is not possible. Thus, just like water, it is a key element of life. At the current rate of consumption, many countries will face a crisis as their phosphorus reserves are depleted.

The use of phosphorus started as mineral fertiliser in agriculture. Now it is also used in a wide variety of products like pharmaceuticals, flame retardants and chemicals¹. Over the last 50 years the use of phosphorus has increased from 4 till 24 Mton per year².

Historical sources of phosphorus fertilizers (1800-2010).

Image sourced from Cordell, 2014

Demand vs supply

The demand for phosphorus is increasing due to this wider application, a rising global population and global trend towards more meat and dairy-based diets, which are significantly more phosphorus intensived³. It is expected the demand for phosphorus will increase by 50-100% by 2050⁴.

On the supply side, there are four issues making phosphorus scarce:

1. Reserves of phosphorus are limited. Depending on which scientists you ask, the world’s phosphate rock reserves will last for another 35² to 400⁵ years¹. Limited supply can result in price shocks which already happened in 2008 when prices increased with 800%.
2. It becomes harder to mine good quality. The concentration of phosphorus in mined rock is decreasing and the concentration of unwanted clay particles and heavy metals like cadmium are increasing.
3. There are no substitutes. Where oil can be substituted with other sources of energy, phosphorus has no substitute for food production³.
4. Reserves are unevenly distributed. It is estimated that the world resources of phosphate rock are in total 300 billion tons. 85 to 90% of world’s remaining reserves are controlled by only five countries which are Morocco, China, Algeria, Syria and Jordan. China, the U.S. and South Africa keep their mined phosphorus for their own use whereas Morocco and Jordan are the largest exporters⁶. Morocco controls up to 85 percent of the remaining phosphate rock reserves. However, many of Morocco’s mines are located in Western Sahara, which Morocco has occupied against international law⁵. Importing countries are thus vulnerable to price fluctuations and supply disruptions in producing countries³.

Countries like India, Europe and Australia need to prioritize securing a supply of phosphorus to sustain their food production. The EU is addressing the issue on the look for alternative solutions⁶.

Algae polluted water in brackish water

More than 384 million urbanites (46 percent of all people living in the 100 largest cities) get their drinking water from watersheds with high nutrient pollution. As watersheds are exploited for agricultural purposes, and as agriculture intensifies, the use of fertilizers increases and more fertilizers end up in the water. The task of raw water quality maintenance seems harder for the developing world than for the developed. Cities that are likely to have the biggest increase in nutrient loading from agriculture are located in Brazil, Argentina, and parts of sub-Saharan Africa⁷.

The solution

The current linear model of mining, using and wasting should become more circular: a model in which phosphorus is used again and again and again. This would enhance the self-sufficiency of a country through a more sustainable use of phosphorus. Cities have a crucial role in this value chain: almost all nutrients enter cities in the form of food that people buy and consume.

Prevention

Cities can partially prevent unnecessary consumption of food, and hence unnecessary demand for phosphorus. The average diet today results in the use of around 3.2 kilograms phosphate per year. This is 50 times greater than the 1.2 grams per person per day recommended daily intake³. The best way to limit phosphorus in our diet is to limit intake of fast food, convenience food, processed food and beverages⁸. A public awareness campaign for healthy food and/or food waste reduction thus also helps to reduce the use of phosphorus. According to the waste hierarchy, prevention is prefered to recycling. As an example, read about San Fransisco where information and education is one of the five success factors.

Urban Mine

Next to prevention, cities should become an urban mine. Cities could support the harvest of phosphorus from recycling of manure, waste water, animal by-products (e.g. bones), and food and other green waste (via composting or via ashes)⁶. One important source is wastewater. Examples of harvesting nutrients are available: In Amsterdam, the local water company Waternet, recovers about 900 ton/year struvite from wastewater⁹.

Another form of harvesting is food waste recycling. Food waste is the solid waste fraction containing the most phosphorus per kilogram, it can easily be recovered and is a substitution possibility of nutrients to chemicals. In a case study in Sweden, three forms of recycling food waste were compared: recovery from residual waste (incineration), recovery from source separated organic waste (compost), and recovery from source separated food waste (anaerobic digester). The highest yield is to recover phosphorus from the digestate of digested food waste¹⁰. Recovery from municipal solid waste incineration is less favourable due to the trace metal content in the residu. This reduces potential application to agricultural land¹¹.

Conclusion

Cities could become a hub of recovery within the phosphorus chain. They should become an urban mine and make the chain of phosphorus circular. This will limit dependency on uneven distributed sources and decrease excessive nutrient polluted areas within their territory. This can be achieved through implementing legislation, policies and programs aimed at recycling food waste to recover phosphorus, or better yet, reducing the demand for phosphorous by food waste prevention programs.