Towards resource recovery-oriented solutions in agriculture exploiting structural extracellular polymeric substances (sEPS) extracted from aerobic granular sludge (AGS)

In aerobic granular sludge (AGS) for wastewater treatment, microorganisms are embedded in a matrix of extracellular polymeric substances (EPS). A fraction of EPS, the so-called structural EPS (sEPS), can be extracted through physical–chemical methods and form hydrogels. AGS-derived sEPS might be converted into value-added biomaterials, thus contributing to a circular economy-based wastewater sector. This paper aimed to evaluate the feasibility of applying sEPS extracted from AGS and their hydrogels in agriculture. The extraction and hydrogel-forming methods were fine-tuned in terms of chemicals applied and the obtained sEPS and hydrogels were comprehensively characterized for their agronomic properties. It was observed that the chemicals used in the recovery processes influenced the elemental composition of sEPS and derived hydrogels, without significantly modifying the sEPS extraction yield and gelling capacity. Chemical reagents such as K2CO3 (thermo-alkaline extraction) and Ca(NO3)2·4H2O (ionic cross-linking) promoted a K- and N-enrichment of sEPS and hydrogels, with benefits on their agronomic valorization. sEPS hydrogels can store up to 99.5 wt% of water into their 3D structure, exhibiting properties of interest in agriculture. The 50 °C-dehydrated sEPS hydrogels can swell in both water and KNO3 aqueous solution, absorbing up to about 12 g H2O/g TSHydrogel (TS: Total Solids) and behaving similarly to superabsorbent polymer (SAP)-based hydrogels. After swelling in KNO3, they evidenced high K+/NO3– release capacities (133 mg K+ and 187 mg NO3– per g TSHydrogel). Freeze-dried sEPS hydrogels reversibly absorbed and desorbed water under controlled relative humidity conditions. These properties suggested that AGS-derived sEPS might be converted into valuable biomaterials to improve water-holding capacity and nutrient retention of soils, with advantages for both agricultural and wastewater sectors.