Application of extracellular polymeric substances recovered from aerobic granular sludge as bio-based hydrogels for plant cultivation systems

Wastewater treatment plants generate large amounts of excess sludge, whose management represents an environmental and economic challenge, motivating resource recovery strategies in line with European Union Directive 2024/3019. Among these, the extraction of extracellular polymeric substances (EPS) enables conversion of sludge-derived biopolymers into sustainable hydrogel materials. However, the effects of structural EPS (sEPS) on nutrient availability and plant performance remain poorly understood. This study investigated aerobic granular sludge derived sEPS as hydrogel-based substrates for in vitro seedling cultivation. Garden cress (Lepidium sativum L.) was grown in substrates containing 0, 1, 2, and 4 g TSsEPS L-1 sEPS, applied as stand-alone hydrogels (sEPS-CM) or combined with Murashige and Skoog medium, with and without sucrose (MS + sEPS-CM; MS + sEPS + S-CM). Nutrient availability, Ca/N stoichiometry, and plant responses were evaluated. In MS-based systems, increasing sEPS concentrations reduced bioavailable Ca2+ and inorganic nitrogen, inducing a shift in Ca/N stoichiometry. Germination was not affected. In stand-alone hydrogels, the best responses were observed at 1 g TSsEPS L-1, with maximum shoot and root growth and vitality, whereas 4 g TSsEPS L-1 showed inhibitory effects. Nutrient-rich systems, with increasing sEPS concentrations, reduced dry biomass, pigments, soluble sugars, and photosynthetic performance, with the strongest negative effects at 4 g TSsEPS L-1. The MS + sEPS + S system showed the highest performance, highlighting the role of carbon availability. Overall, sEPS act as structural matrices and regulators of nutrient availability. Moderate concentrations (∼1 g TSsEPS L-1) are compatible with plant development, whereas high levels induce nutrient limitation, highlighting potential as bio-based substrates for sustainable cultivation systems.