The chemical element phosphorus, which is mainly used as a fertiliser in agriculture, is a key building block for all life forms. Phosphorus cannot be substituted by other elements or produced synthetically. In addition, it is scarce on Earth and the majority of phosphate rock preserves are located in just a handful of countries. A European-wide research project on phosphorus recycling now presents ways of producing the precious raw material from sewage sludge and wastewater.
Phosphorus is a vital element for humans, animals and plants alike. It is required for building strong bones and is a key structural element of DNA and RNA. Phosphorus is also a major constituent of agricultural fertilisers as it is a vital plant nutrient, ensuring high crop yield and quality. It is therefore all the more remarkable that phosphorus is a scarce element, and only found in phosphate rocks in a few regions of the world, including Africa and China. Due to the depletion of mineral phosphorus resources, there are a growing number of new projects aimed at exploring ways for recovering and recycling this precious commodity.
Sewage sludge, which is rich in phosphorus, is becoming increasingly important as a relevant phosphorus reserve. "Sewage sludge is the residual material that is a by-product of wastewater treatment plants and consists mainly of microbial biomass that is generated in the biological wastewater treatment phase," says Dr. Almut Gerhardt, managing director of LimCo International GmbH, which is involved in a European-wide project on the recovery of phosphorus from sewage sludge. The microorganisms clean the wastewater while feeding themselves; they grow and form biomass. Microbial biomass is rich in phosphorus because phosphorus is a key component of DNA and RNA which all organisms need to transmit genetic information. Moreover, cellular energy is stored in molecules called adenosine triphosphate (ATP). "The high phosphate load in wastewater, which stems mainly from the solid and liquid manure used for crop fertilisation, is eliminated by the microorganisms that incorporate it into their biomass," explains Almut Gerhardt.
Sewage sludge has therefore become a popular agricultural fertiliser. However, over the past few years, attention has been drawn to the extreme pollution of soils with heavy metals resulting from using sewage sludge as fertiliser. Sewage sludge also contains a high level of organic micropollutants, which is why it is recommended to refrain from using sewage sludge as a fertiliser. "The European FP7 P-REX project was set up to counteract soil pollution by sewage sludge and make use of its high phosphorus content. The project aims to find ways to recover this valuable element from sewage sludge," says Almut Gerhardt.
P-REX will carry out cost evaluations and market analyses and develop recommendations for efficient phosphorus recovery from sewage sludge and wastewater. Due to world population growth, it has become clear that demand for phosphorus and for fertiliser will continue to increase over the coming years.
"The technical basis for widespread application of phosphorus recycling already exists. However, the method sometimes requires expensive chemicals, so the price of recycled phosphorus is not yet competitive," says Dr. Christian Krabbe from the Berlin Competence Centre for Water (KWB), one of the project partners, and coordinator of P-REX. He recommends partially substituting fossil phosphates with recovered secondary phosphorus to produce agricultural fertilisers. He therefore believes that it will be possible to recycle a substantial proportion of sewage sludge phosphorus into existing value creation chains in a relatively short period of time and without too high an investment.
Institutions and companies from Germany, Austria and Switzerland are involved in the project. One of the partners is a company called LimCo International GmbH from Konstanz, which specialises in testing and monitoring the quality of drinking water, wastewater, process water, rivers and lakes. The company uses toxicological tests to assess the non-hazardous application of recovered phosphorus. "We have a unique real-time-based toximeter that we have used to assess the residual toxic potential of sewage sludge and recovered phosphorus," says Gerhardt.
Together with her colleague Nadja Rastetter, Almut Gerhardt adapted LimCo's toximeter for application in the P-REX project where it was used to determine the toxic potential of sewage sludge and soil. In order to substantiate the harmlessness of sewage sludge-derived phosphorus, LimCo International GmbH used three different indicator species: Gammarus freshwater shrimps, which are important freshwater organisms, duckweed as representative of aquatic plants, and compost worms for soil testing. Almut Gerhardt explains the scope of the toxicological measurements carried out by LimCo: "All tests were carried out according to international standards. Additional sensitive parameters were measured continuously and in real time with our toximeter."
The project will officially end in summer 2015. The results show that the recovery of phosphorus from sewage sludge will become increasingly economically viable over the next few years. However, the project also recommends replacing part of the fossil phosphorus with the recycled product rather than offering secondary phosphorus as an alternative to fossil phosphorus. "Before we invest into a new infrastructure, I think we would be better advised to try and get the most out of the existing one. This is why I think it is important to integrate secondary streams into existing primary streams," concludes Krabbe.