Sewage sludge: The perfect phosphorus source?

Sewage sludge contains many nutrients. We can use them as fertilizer in agriculture instead of simply throwing them away. That’s the good side. However, many of the chemicals that we use also end up in sewage sludge. It can contain antibiotic residues and antibiotic-resistant bacteria. Both of these increase the risk of antibiotic resistance spreading in the environment. So is the use of sewage sludge positive or negative?

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The worldwide demand for phosphorus is increasing

The global demand for phosphorus is increasing while the supply of natural phosphorus resources is becoming scarce. Sewage sludge and sewage sludge ash are rich in phosphorus and can therefore be used as a source for phosphorus recovery. Phosphorus is a vital chemical element that is only available to a limited extent on earth. With it as a fertilizer, successful agriculture and an adequate supply of food can be secured. In many places, raw material requirements are covered by imports from other countries. The political and economic dependency can be reduced by recycling processes for the recovery of phosphorus from sewage sludge. There are various methods for this, such as the direct application of sewage sludge on fields, or the recovery of pure phosphorus from the sewage sludge. The latter is the safer option, as we will learn below.

Environmental hazards of sewage sludge application

In addition to nutrients, compost and, above all, sewage sludge also contains proportions of inorganic pollutants: toxic heavy metals such as lead, mercury, cadmium, copper as well as organic pollutants and foreign substances such as dioxins, polychlorinated biphenyls, perfluorinated surfactants, pharmaceutical residues, pathogens, but also nanoparticles, microplastics and so on. Applied to the soil with the fertilizers, these pollutants can accumulate there and enter the food chain via the plants. Groundwater and surface water are at risk if the pollutants enter it through runoff or seepage from arable land. The dangers to the environment and health are difficult to assess since the interactions and transformation processes of the pollutants are often not known and monitoring options, especially for new substances, are often not available. In some countries, its use is already prohibited for these reasons.

Thermal sewage sludge utilization

Thermal sewage sludge recycling has become increasingly important around the world. The reason is the elimination of high transport costs, the demand for resource conservation, as the recovery of phosphorus from sewage sludge ash. Since the phosphorus in the ashes is lost in the case of co-incineration in coal or garbage power plants, mono-combustion is increasingly used. Some major European cities have already started to store this mono-incineration ash in special landfills until suitable processes for phosphorus recovery are available.

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Recovery of phosphorus 

There are different approaches to recovering phosphorus from wastewater and sewage sludge, but they are usually complex and expensive. In some processes, the phosphorus is recovered during wastewater treatment, in others from the ashes after the incineration of the sewage sludge. For a real use of the technologies, some tests in pilot systems and further research are necessary, as the operating costs are still too high. But the development is going in a good direction.

Conclusion

The use of sewage sludge in agriculture can make sense, but no generalizations can be made. For countries with higher antibiotic consumption and poorly equipped sewage treatment plants, where the sewage sludge is not intensively post-treated in digestion towers, it is too risky to use it in agriculture. There continues to be a risk that the sewage sludge disposed of in fields could promote the spread of antibiotic resistance in the environment. On the other hand, the developments in phosphorus recovery look promising and there is a high probability that technologies for this will be widely used in the future.

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