Phosphorus is an important raw material, especially as an agricultural fertilizer. However, in water bodies, it degrades water quality. Therefore, since the 1980s, phosphate precipitation has been one of the main processes in municipal wastewater treatment plants. Phosphorus is bound with salts in sewage sludge. However, this raw material is also increasingly scarce and must be recovered there. This can be achieved, for example, when it is present in bound form as vivianite. Researchers at the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) investigated the factors that promote the formation of vivianite and increase the amount of recoverable phosphorus.
There are many good reasons for phosphorus recycling. Phosphate rock is becoming increasingly polluted and dependent on several countries for supply. As such, it has been on the European Union’s list of ‘important raw materials’ since 2014. The German government also passed the Sewage Sludge Ordinance in 2017. According to it, by 2032, large plant operators will have to ensure that phosphorus is included. Sewage sludge is collected.
Precipitation in sewage sludge produces vivianite, an iron-phosphorus compound from which phosphorus can be recycled relatively easily. “However, until now it was not clear what conditions in the sewage treatment plant favored the formation of vivianite. We also used the precipitation of phosphorus from water to reduce the nutrient load and improve water quality.” We are also interested in rehabilitating lakes that are depleted,” explains IGB researcher Michael Hupfer, who led the study. The team analyzed the properties and composition of sludge samples from 16 wastewater treatment plants and the process parameters of the plants in order to identify the factors that influence the formation of vivianite.
High iron content promotes the formation of vivianite, high sulfur content inhibits vivianite formation
High iron content proved to be the most important factor promoting the formation of vivianite. A high sulfur content reduces the formation of vivianite. “There are precipitants containing sulfur and precipitants without sulfur. A comparison shows that using a precipitant containing sulfur increases the sulfur content of the sludge, which can hinder the formation of vivianite.” Therefore, the choice of precipitant can have an important impact on phosphorus recycling,” said IGB PhD student Lena Heinrich, lead author of the study. .
Adjusting the conditions makes a difference. In 16 wastewater treatment plants, the percentage of phosphorus bound to vivianite varied from about 10% up to 50%. This range shows great potential for increasing vivianite yields.
“For us as aquatic ecologists, this finding is very important because iron-bearing sediments may also help restore eutrophic and nutrient-contaminated lakes. The stable vivianite inside will probably one day also be available for phosphorus recovery,” Hupfer said.
This research Comprehensive Environmental Science.
Sewage sludge can be an excellent sustainable fertilizer
Lena Heinrich et al., Formation of vivianite in digested sludge and its control factors in municipal wastewater treatment, Comprehensive Environmental Science (2022). DOI: 10.1016/j.scitotenv.2022.158663
Courtesy of Forschungsverbund Berlin eV (FVB)
Quote: Improved Phosphorus Recycling from Sewage Sludge (14 October 2022)
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