Phosphorus Recovery – Reliable Analysis of Sewage Sludge with ICP-OES

Phosphorus Recovery – Reliable Analysis of Sewage Sludge with ICP-OES

Phosphorus and phosphorus compounds are essential for all life on earth. They therefore are an essential base for fertilizers and animal feed. With the world population growing, the global food demand is continuously increasing, which is why natural phosphorus reserves are in danger of depleting. Furthermore, phosphate deposits are limited to a few regions worldwide. A productive alternative to phosphate mining is the recovery of phosphorus from sewage sludge. The European Environment Agency estimated in 2021, that phosphorus recovery from 50 percent of currently unused sewage sludge could replace up to 10 percent of phosphorus fertilizer currently used on agricultural land ⁽¹⁾. Sewage sludges however also contain pollutants, such as heavy metals. Precise analytical methods are crucial to determine the recovery potential of sewage sludge and help to take a further step to a sustainable circular economy.

The practice of applying untreated sewage sludge as a fertilizer directly to agricultural fields has been becoming less common during the last decades and will be prohibited in some countries within a few years. Pollutants and heavy metals contained in the sewage sludge, such as cadmium, lead, and mercury or even arsenic, are limiting direct use. To make use of the valuable nutrients in sewage sludge, their recovery has been decided in an EU-wide strategy. Currently, only 20 percent of sewage sludge is recycled in municipal wastewater treatment plants. The reasons are usually the high proportion of pollutant content, which is why sludges and ashes are often landfilled or used as secondary fuel.

The suitable analysis method

A key challenge to analyzing sewage sludge is that the samples are remarkably high in matrix. This means that they contain a wide range of elements in quite different concentrations, as well as a high amount of salt compounds. Ideally, the different element contents – basic substances for fertilizers, as well as critical elements – are determined in one measurement cycle per sample without time-consuming sample preparation.

A well-suited method for the reliable, routine determination of major and minor trace elements in sewage sludge is inductively coupled plasma optical emission spectrometry (ICP-OES). ICP-OES is especially attractive because of its capability for multi-elemental analysis, its broad linear dynamic range, and its measurement sensitivity. Sample preparation and measurement procedures are described in standards, such as EPA Method 3050B⁽⁴⁾ and 6010C⁽⁵⁾, DIN EN 16174:2012-11⁽⁶⁾ and ISO standard 11885:2007⁽⁷⁾.

With ICP-OES, samples are atomized over argon plasma at a temperature from 5,000 to 10,000 K, and then ionized. The emitted electromagnetic radiation is measured with a spectrometer. The measured values reveal details about the element contents.

Benefits

Multi-element analysis of sewage sludge provides wastewater treatment plant operators with reliable data for deciding on how to proceed with the sewage sludge in accordance with the law and standards. Depending on the phosphorus contents and pollutant load, they can choose suitable methods, that also meet economic aspects. 

Two options are mentioned here as examples: Pyrolysis is suitable if the phosphorus content is high and heavy metal load is low. The sewage sludge is processed in smaller pyrolysis reactors at 600 °C, which can be operated directly at the sewage treatment plant. Phosphorus is recovered from the incineration ash. In case of higher heavy metal contents, a thermochemical method can be used: Magnesium chloride or hydrochloric acid is added to the sewage sludge at 900 °C to precipitate the phosphorus. 

With the recycling of phosphorus and other basic elements, valuable raw materials for fertilizers can be recovered locally or decentralized. This is not only eco-friendly, but also helps to become more independent from the few naturally occurring phosphorus deposits. Precise knowledge of the pollutant load prevents heavy metals and other pollutants from entering the environment. Phosphorus recovery from sewage sludge is therefore a major step for approaching a more sustainable circular economy. 

Sources

⁽¹⁾https://www.deutsche-phosphor-plattform.de/eu-umweltagentur-naehrstoff-und-kohlenstoffrecycling-aus-klaerschlamm/

⁽²⁾https://eur-lex.europa.eu/legal-content/DE/TXT/PDF/?uri=CELEX:52020DC0474

⁽³⁾Verordnung über die Verwertung von Klärschlamm, Klärschlammgemisch und Klärschlammkompost, https://www.gesetze-im-internet.de/abfkl_rv_2017/

⁽⁴⁾EPA Method 3050B: Acid Digestion of Sediments, Sludges, and Soils, https://www.epa.gov/esam/epa-method-3050b-acid-digestion-sediments-sludges-and-soils

⁽⁵⁾EPA Method 6010C: Inductively Coupled Plasma - Atomic Emission Spectrometry, https://19january2017snapshot.epa.gov/homeland-security-research/epa-method-6010c-sw-846-inductively-coupled-plasma-atomic-emission_.html

⁽⁶⁾Schlamm, behandelter Bioabfall und Boden - Aufschluss von mit Königswasser löslichen Anteilen von Elementen, https://www.beuth.de/de/norm/din-en-16174/148093733

⁽⁷⁾Water quality — Determination of selected elements by inductively coupled plasma optical emission spectrometry (ICP-OES), https://www.iso.org/standard/36250.html