Simultaneous aerobic stabilisation of sewage sludge and chelator-assisted removal of toxic metals in a closed-loop process

Fojkar, Barbara (author), Leštan, Domen (author)

Sewage sludge is a valuable source of phosphorus but contains toxic metals that hinder its use as fertilizer. We present a novel process that simultaneously stabilises sludge through post-aerobic digestion (12 days, pH 5.8–7.6, solid-to-liquid ratio 1:25 (dry w/v)) and removes toxic metals using ethylenediaminetetraacetate (EDTA, 10 mM). On average, 74.4% of EDTA was successfully recycled by alkalinization of process solution with quicklime (CaO) to pH 12.5 and reused in subsequent batches. Chelator losses were compensated by the addition of Na-EDTA. Toxic metals, along with excess Ca and Na, were effectively removed from the recycled process solution, enabling closed-loop operation without wastewater generation. During five consecutive batches, total solids in the sludge were reduced by an average of 23.2%, and 74, 61, 26, and 43% of Zn, Cu, Cr, and Pb, respectively, were removed. The leaching of residual toxic metals from the stabilised and decontaminated sludge was further reduced by 55–88% through the addition of zero-valent iron (3% dry w/w). The process also increased the phosphorus concentration in the sludge by 15% and improved its phytoavailability up to 1.8-fold (as determined by ammonium-lactate extraction). This process is sustainable: it preserves valuable nutrients, can be integrated into existing wastewater treatment plant operations, generates only 17% solid waste (relative to the dry weight of the sludge), and produces no wastewater. Due to the recycling of costly EDTA, reagent costs were halved, reaching €73 t$^{-1}$ of dry sludge.

• Language: English
• Keywords: sewage sludge, phosphorous, toxic metals, EDTA, aerobic stabilization, closed process
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: BF - Biotechnical Faculty
• Publication status: Published
• Publication version: Version of Record
• Year: 2026
• Number of pages: 15 str.
• Numbering: Vol. 288, pt. A, art. 124653
• PID: 20.500.12556/RUL-174623
• UDC: 628.3:502.175
• ISSN on article: 1879-2448
• DOI: 10.1016/j.watres.2025.124653
• COBISS.SI-ID: 251096579
• Publication date in RUL: 07.10.2025

Record is a part of a journal

• Title: Water Research
• Publisher: Elsevier
• ISSN: 1879-2448
• COBISS.SI-ID: 23055365

Licences

• License: CC BY-NC 4.0, Creative Commons Attribution-NonCommercial 4.0 International
• Link: http://creativecommons.org/licenses/by-nc/4.0/
• Description: A creative commons license that bans commercial use, but the users don’t have to license their derivative works on the same terms.

Secondary language

• Language: Slovenian
• Keywords: blato komunalnih čistilnih naprav, fosfor, EDTA, aerobna stabilizacija, zaprti proces

Projects

• Funder: ARRS - Slovenian Research Agency
• Project number: J7-50152
• Name: Dekontaminirano biooglje iz blat čistilnih naprav kot učinkovito gnojilo in sredstvo za izboljšanje tal

• Funder: ARRS - Slovenian Research Agency
• Project number: P4-0085
• Name: Agroekosistemi