Work

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

Year: 2025

Type: article

Abstract: • Total solids were reduced and toxic metals removed from stabilised sludge. • EDTA and process solution were recycled by alkalinisation and reused. • ZVI addition reduced leaching of toxic residual metals from stabilised sludge. • Concentration and plant-availability of P increased in stabilised sludge. • Stable performance and cost-efficiency of new process were demonstrated. 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.

Source: Water Research

DOI: 10.1016/j.watres.2025.124653

Authors: Barbara Fojkar, Domen Lestan

Institution: University of Ljubljana

Language: English

Topic: Phosphorus and nutrient management

Subfield: Waste Management and Disposal

Field: Environmental Science

Domain: Physical Sciences

SDGs: Responsible consumption and production, Clean water and sanitation

Open Access status: hybrid

Estimated APC paid: $4,660