Efficient phosphorus recycling

According to a recent study, plants can effectively utilize the phosphorus in sludge from wastewater treatment plants. Plants can utilize the phosphorus in wastewater sludge treated with the KemiCond and ferric-coagulation methods even more efficiently than they can make use of soluble mineral fertilizers. This information is based on initial research results from MTT Agrifood Research Finland.

A study supervised by Principal Research Scientist Helena Kahiluoto (right) at MTT examined how effectively plants use and store phosphorus from various organic sources, such as manure and wastewater sludge. Phosphorus utilization was tested on Italian ryegrass and flax. The researchers sowed the plants and determined their phosphorus content.

The comparative study used wastewater sludge treated in various ways as well as digested, composted and untreated manure. These were compared with mineral fertilizers and unfertilized soil. The study included pot experiments in a growth chamber as well as a two-year field experiment, which is still in progress.
Kemira supplied the project with ferric-coagulated wastewater sludge samples that had been treated by means of the KemiCond method, digestion, composting, lime stabilization and ammonia hygienization. The samples were mixed in the sandy soil used as the substrate.
The field experiment is halfway through, but the final results of the pot experiment will soon be available.
According to Kahiluoto, treated sludge and other organic sources of phosphorus gave good results: the plants were able to utilize the phosphorus even more efficiently than they can use mineral fertilizers. “In addition to composted wastewater sludge, the promising group of sources included sludge treated with KemiCond.”
The amount of iron seems to be key when using ferric-coagulated wastewater sludge. The effects of iron were studied by comparing sludge from wastewater treatment plants that use different dosages of coagulants. Plants are able to utilize phosphorus efficiently even if the ferric coagulant dosage is at the level typically used in the Nothern Baltic area, which is higher than that in Central Europe.
Helena Kahiluoto has studied organic nutrient sources and nutrient recycling for a long time. “I was not surprised by the results, even though the general belief still seems to be that plants are not able to utilize organic sources of phosphorus as efficiently as mineral fertilizers.”
Nutrients in organic soil conditioners have been thought to be less available and more slowly available for plants than those in mineral fertilizers.
“The results of our pot experiment do not support this belief,” says Kahiluoto. However, she points out that the pot experiment is only part of the results. The field experiment will provide more practical information.
“The results of the two-year field experiment will be interesting. The soil ecosystem serves as a buffer and contains interactions that are difficult to predict. The pot experiment indicates the mechanisms involved more clearly.”
Senior Research Scientist Outi Grönfors (left), was responsible for project management at Kemira. She finds the initial results to be very interesting and is pleased with the way the study is producing profound information on how plants utilize phosphorus.
“It’s interesting to learn that wastewater sludge treated with the KemiCond method as well as ferric-coagulated wastewater sludge are highly available for plants”, says Grönfors. “The initial results are encouraging in terms of using wastewater sludge as a phosphorus source.”
“Wastewater sludge should not be seen as waste, because it serves as a source of nutrient and organic matter essential for plant growth.”
When phosphorus becomes even more scarce, interest in recycling will increase. Phosphorus can be regarded as a strategically important raw material, as it is absolutely necessary in food production. Of the phosphorus used in Europe, 95 percent is imported. Phosphorus is extracted from phosphate rock, a non-renewable resource.
For all living organisms, phosphorus is an important building block. Phosphorus cycles in the food chain, and human and animal feces have high phosphorus content. Phosphorus emissions to water with wastewater and fertilizers cause eutrophication to a high degree.
According to Grönfors, the facts are in favor of phosphorus recycling. “It’s fantastic that the phosphorus in wastewater sludge can be recycled and reused virtually indefinitely.”
However, wastewater sludge must be treated appropriately to remove contaminants. “The sludge used as a fertilizer on fields must be safe and clean. The KemiCond treatment is used to disinfect the sludge and remove pathogens.”

Did you know that:

  • Phosphorus is among the most important nutrients for plants. In addition, it is essential as animal feed in food production.
  • The global demand for phosphorus is increasing with population growth, the need for food and the use of fertilizers in agriculture.
  • Phosphorus is extracted from phosphate rock, a non-renewable resource. Major phosphorus deposits are located in China, Morocco and the United States, among other countries.
  • Human and animal feces have high phosphorus content.

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