How to turn sewage into a phosphorus mine

Phosphorus, as the writer Isaac Asimov stated in 1974, is an element that we can consider the “bottleneck of life”. Its role in agriculture as a fertilizer has made it an essential and irreplaceable component of life. Its exhaustion could put the future of our society at risk.

Historically, farmers already used the phosphorus contained in animal manure as fertilizer. In the 19th century, in addition to organic matter, it was discovered that the use of bones and rocks rich in phosphorus also improved crops. For this reason, the fertilizer industry became interested in rock phosphate as a rich source of phosphorus. Today, phosphate fertilizer production accounts for most of the world’s demand for phosphate rock.

The world’s reserves of phosphate rock are under the control of a limited number of countries, such as China and Morocco, whose trade policies could pose a risk to their future supply. This has led the European Union, dependent on the import of this non-renewable and irreplaceable resource, to include phosphate rock on its list of critical raw materials.

Phosphorus as a solution and a problem

The complexity of the chemistry of phosphorus explains that, despite being an essential element for life, it also has the power to kill it. We can say that phosphorus has gone from being a nutrient to being a global pollutant.

The continuous discharge of fertilizers, detergents and wastewater with high concentrations of phosphorus is causing a reduction in the quality of water near densely populated areas.

Excess phosphorus in water can cause a process called eutrophication – the uncontrolled growth of algae. This phenomenon negatively affects water quality since excess algae consumes oxygen and, in addition, generates large amounts of organic matter.

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The challenge of treatment plants

European regulations on urban wastewater treatment (Water Framework Directive 2000/60 / CE together with Council Directive 91/271 / CEE) establish maximum values ​​for phosphorus concentration in treated water effluents, which are makes it a challenge for wastewater treatment plants (WWTP).

The conversion of soluble phosphorus to an insoluble compound that can be easily separated from aqueous solution is the foundation of existing methods for the removal of this element. Chemical precipitation of struvite, a mineral containing phosphorus, is the best established procedure in a large number of plants internationally. This method is applied to the sludge that comes from anaerobic digestion.

However, the design of numerous water treatment plants that are currently in operation in Spain did not include anaerobic sludge digestion in their design. The development of increasingly stringent regulations regarding the concentration of phosphorus puts these plants in a difficult situation. Therefore, it is necessary to explore new methods with which phosphorus can be eliminated.

Recovery of phosphorus from waste

It goes without saying that the reduction, reuse and recycling of phosphorus from waste is key to solving in a practical and economical way the problems caused by the scarcity, the supply risk and the environmental impact of this element.

Urban wastewater treatment plants can represent a liquid mine for obtaining phosphorous due to the constant and high incoming concentration of this element. This content comes from:

  • Human and animal excretions.

  • Cleaning products and detergents.

  • Fertilizers that come from the soil.

From the laboratory to the treatment plant

At present, our research group (Group of Engineering and Environmental Management of the Department of Engineering of the University of Malaga) is carrying out a project that seeks to provide solutions to the problems related to the high concentration of phosphorus in the effluents of the WWTP. This project is funded by the Junta de Andalucía and the European Union through aid for knowledge transfer activities between the agents of the Andalusian knowledge system and the productive fabric.

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The research focuses on the proposal for improvements in a real plant that presents the previously described problem: high concentrations of phosphorus in the treated wastewater. To do this, we scale the process from the laboratory to its real application through a pilot plant.

In an initial stage, from the laboratory we identified the point of the plant where a greater amount of phosphorus was concentrated. Based on these results, we carried out tests that served as the basis for designing a pilot plant with which to work with higher volumes of wastewater. The investigation culminated in field trials that served to confirm the success of the investigation.

In the different stages of the project we use various reagents to precipitate phosphorus compounds. In this way, we are able to establish the bases to revalue the phosphorus contained in wastewater as a fertilizer.

With this project, the circular economy is promoted by taking advantage of the phosphorus contained in wastewater. In short, a critical raw material for the European Union is recovered while reducing the environmental impact of phosphorus pollution in water.

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