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Description
This study aimed to valorize nutrient-rich wastewater from the corn starch industry by using it as a source of plant-relevant nutrients, primarily nitrogen and phosphorus, to enrich anaerobically digested sewage sludge (ADSS). ADSS obtained from the Public Utility Company “Vodovod i kanalizacija” Subotica, Serbia, was enriched with corn starch industry wastewater in a continuous packed-bed column containing approximately 100 g of digestate. The enrichment was carried out over 24 h in a cylindrical plexiglass column with an inner diameter of 40 mm and a height of 180 mm, while 0.7 L of wastewater was continuously mixed and circulated through the column using a peristaltic pump at a flow rate of 4 mL/min.
The enriched digestate was then subjected to pyrolysis at 400, 550, and 700 °C under an argon atmosphere to produce biochars with varying degrees of thermal transformation. The resulting materials were evaluated using a leaching test to determine how pyrolysis temperature affected nutrient release and the mobility of potentially toxic elements. Particular attention was given to balancing the retention of plant-relevant elements and the stabilization of hazardous elements within the carbonaceous and mineral matrix.
The results showed that pyrolysis temperature strongly influenced elemental leaching behavior. Biochars produced at higher pyrolysis temperatures, particularly 550 and 700 °C, generally showed reduced leaching of most potentially toxic elements, indicating improved stabilization within the solid matrix and enhanced environmental safety. This effect may be related to the incorporation of metals into stable carbonaceous and mineral phases. Meanwhile, the biochars retained key plant-relevant elements, including Ca, K, Mg, P, and, to a lesser extent, residual N.
Overall, the combination of nutrient enrichment and controlled pyrolysis produced a stabilized, mineral-enriched biochar with potential as a soil conditioner. These findings suggest that this approach may support the sustainable valorization of both anaerobically digested sewage sludge and nutrient-rich industrial wastewater from the corn starch industry.
Keywords: anaerobically digested sewage sludge, corn starch industry wastewater, pyrolysis, biochar, leaching test, soil conditioner