In my third blog post, I introduced the problem of water pollution and the need to reduce urban water pollution. In this blog post, I will be diving deeper into one major water pollutant from cities –

Phosphorus.

Algae bloom caused by excess phosphorus (and nitrogen) in water (Source)

Metson et al. (2022) urge cities to move towards the goal of net-zero phosphorus, a clear reference to cities’ increasing commitment to the net-zero carbon agenda. Excess phosphorus (and nitrogen) in urban waters add excess nutrients to the water which encourages excess algae growth, a process also known as eutrophication. Not all algae blooms are toxic, but they all cause water to become contaminated and unsuitable for human usage. Eutrophication (Figure 1) costs the United States (US) around 2.2 billion dollars per year and can result in loss of access to drinking water due to cyanobacteria (which produces cyanotoxins) contamination from algae blooms, which is what happened in the city of Toledo, US, in 2014 (Metson et al., 2022).

Figure 1: Process of eutrophication which leads to contaminated water (Hessein et al., 2019)

How does phosphorus end up in our water sources?

The biggest demand for phosphorus comes from food. Of all the phosphorus sold worldwide, 85% is for making fertilizers for crops, 10% for animal feed supplements and 5% is used in the chemical industry (Metson et al., 2022). With increasing economic development, the consumption of meat products has greatly increased over the years.

Figure 2: Phosphorus footprint of different food groups (Metson et al., 2016)

This has tremendous implications for phosphorus consumption as much more phosphorus is needed to produce meat, dairy and eggs as compared to fruits or vegetables (Figure 2). In more wealthy countries where meat consumption is significantly greater (Argentina and USA as compared to India and Ghana), the difference in phosphorus footprint is even more pronounced (Figure 2).

Today, around 14 million tons of phosphorus enter global aquatic ecosystems per year (Metson et al., 2022). In cities, phosphorus is lost through runoff where urban wastes such as excreta from humans and animals and fertilizers for lawns and urban farms are washed into waterways. Phosphorous in stormwater runoff in urban areas could range from 0.19 mg/L in open areas to about 0.32 mg/L in residential areas (Minnesota Pollution Control Agency, 2023) which is almost more than 10 times the recommended concentration of 0.035 mg/L of phosphorus in healthy rivers (Environmental Protection Agency (EPA), 2023). Given that cities are where the most economic activity, meat consumption and runoff due to built-up areas occur, there is indeed a great need for cities to move towards net-zero phosphorus – not zero phosphorus as phosphorus is still a necessary nutrient needed to support the ecosystem (Metson et al., 2022).

The simplest solution to excess phosphorous in water is having urban food and waste systems become more circular (Metson et al., 2022). Where possible, wastes should be recycled and, if not, properly managed, such as returning organic wastes to nature. Proper drainage systems and infrastructures are essential as well to minimize runoff towards urban waterways. From the demand perspective, there should be an encouragement toward a less meat-heavy diet.

However, where water is already contaminated, phosphorus can be treated in 3 different ways before or after they enter waterways – physical, chemical and biological. The physical method involves using filters and membrane technology which contaminated water passes through and phosphorous is removed. This method is often costly to implement on a large scale (Riber et al., 2021). The Phosphate Elimination and Recovery Lightweight (PEARL) membrane, developed by Riber et al. (2021), aims to reduce the cost of such membranes by using nanotechnology. The PEARL membrane is a nanocomposite consisting of an iron oxide layer (which attracts phosphorous) attached to a cellulose sponge that has been developed to be effective in controlling phosphorous levels and is scalable and cost-effective (Riber et al., 2021). Using the chemical method, calcium (in the form of lime) or aluminium and iron can be added to phosphorous-rich water to cause phosphate to precipitate out of the water (Lenntech, 2023). The biological way of removing phosphorous from water includes introducing phosphorous-accumulating organisms (PAOs) that consume the phosphorous and are subsequently removed as sludge (Lenntech, 2023). Find out more here!

More and more sources of water are getting polluted with excess phosphorous and algae blooms over time, especially with warmer weather associated with climate change encouraging the growth of algae (EPA, 2022). The higher the concentration of phosphorus in the water, the higher the cost of treating phosphorus-contaminated water. At a city level, it is important to have relevant legislation in place to prevent careless handling of phosphorus-rich waste and to reduce runoff where possible for surface sources of phosphorus not to be washed straight into rivers and waterways. At an individual level, understanding the impacts of our food choices and taking baby steps to reduce our demand for meat makes all the difference.

References