From Waste to Resource: Biosolids from Sludge Treatment Wetlands as Biofertilizers and Biostimulants

This research assessed biosolids from sludge treatment wetlands (STW) as possible biofertilizers, comparing them with digestate, compost, and microalgae biomass in greenhouse trials involving three crops (i.e., lettuce, radish, and ryegrass). Additionally, the potential biostimulant effects were evaluated through bioassays conducted under controlled conditions using water extracts of the biosolids. The biosolids showed a notable increase in radish growth compared with the unfertilized control, with increases of 93% and 95% in fresh and dry weight, respectively. The protein content of all crops grown under biosolids treatment was similar to that of microalgae, and it exceeded the control treatments by 6%, 9%, and 17% for lettuce, ryegrass, and radish, respectively. Furthermore, using biosolids reduced nutrient loss through leaching (i.e., N mineral forms), which was observed in urea treatments. Phytohormone-like bioassays revealed that biosolids extracts showed gibberellin-like activity, aiding seed germination and growth. Biosolids extracts also delayed senescence by slowing chlorophyll breakdown in wheat leaves and stimulated secondary root formation in bean seedlings. Overall, the results demonstrated the dual role of STWs’ biosolids as biofertilizers and biostimulants, with performance comparable to urea and most of the biofertilizers used for comparison. Future studies should include field trials to confirm agronomic performance under real conditions.