Coagulant precipitated and biologically stabilized sewage sludge impacts nutrient availability, and risk of nutrient and micropollutant leaching in sludge-amended soils

Coagulants used in wastewater treatment affect subsequent processes including sludge management, recycling, and disposal methods. However, their direct impact on the nutrient availability and leaching, as well as micropollutant leaching on the soil and drainage water from sludge-treated soils have not been fully investigated. This study compared the effects of using biologically stabilized (i.e., anaerobic digestion and composting) sludges (biosolids) produced from chemical precipitation with typically used inorganic coagulant to recently available organic coagulants as soil amendment agents following the growth of Poa pratensis var. Goa. A microplot system equipped with perforated layer and leachate collection port was utilized. A sprinkler system built based on Finnish Meteorological Institute data of rain duration, intensity, and frequency was used to simulate two rain events over the experimental period providing water required for plant growth. The coagulants tested included an inorganic coagulant (IC), polyaluminium chloride (PAC), and organic coagulants, (OC) polyamine (pAmine) and chitosan (Chit). Overall, different coagulant-derived biosolids showed a measurable effect on the availability and leaching of nutrients in addition to the growth of Poa pratensis. Highest leaching of total nitrogen (tot-N) across all coagulants and rain events was observed for composted pAmine. Comparatively, total phosphorous (tot-P) leaching was highest for digested pAmine after the first rain event and digested Chit after the second rain event. Furthermore, Al concentration was found to be highest for digested PAC sludge while Fe concentration was found highest in the control. Evidently, nutrient concentrations in soil samples showed lowest tot-N in composted pAmine while tot-P concentrations remained lower than the tot-N concentrations in this study at 0.71-0.99 mg l^-1 for all coagulants and treatments. PAC-sludge fertilized plants showed the lowest total chlorophyll (tot-Chl) concentrations in the leaf for both composted and digested treatments, even lower than the control while pAmine and Chit-sludge fertilized plants showed higher tot-Chl in both treatments. Statistical analysis shows no significant difference in tot-Chl between different treatments (p<0.05, CI=90%). In general, tot-N and tot-P concentrations in grasses were comparable at 20-26 g/kg DM and 3-4 g/kg DM, respectively which signifies the optimum utilization of nutrients by the plant. Micropollutants (e.g., Bisoprolol, carbamazepine, cetirizine, caffeine, diclofenac, paracetamol, tetracycline, etc.) previously measured in initial biosolids were found to be lower than the detection limit in drainage water (<0.050 µg/L) as well as soil samples (<1.0 µg/kg). The fertilizer potential of different coagulants applied was investigated successfully in this study. The selection of the most suitable coagulant will depend on the objective of the wastewater treatment facility towards reutilization of the biosolids. Higher nutrient leaching may have been exhibited by pAmine but it showed the lowest Al and Fe leaching across all coagulants and treatments indicating safer disposal options considering metals concentration. Comparably, pAmine and Chit biosolids induces higher tot-Chl in Poa pratensis indicating better plant health. Lastly, micropollutant leaching and pollution were observed to be low to negligible when using these coagulant biosolids as soil amendment agents.