Enhancing waste management and nutrient recovery: Preparation of N slow-release fertilizer using sewage sludge and its release behavior, effects on ryegrass (Lolium perenne L.) growth

Abstract

Efficient recovery and utilization of the nitrogen(N) contained in sewage sludge (SS) is a key consideration in the treatment of SS. In this study, a novel sludge hydrogel sphere slow-release N fertilizer was prepared based on the N-rich property of SS, using SS as a substrate and Fe³⁺(extracted from drinking water sludge (DWTS)), CMC, and ATP synergistically constructed as an encapsulated skeletal material. Based on the comprehensive consideration of hydrogel sphere formation performance, mechanical strength and encapsulation effect, the optimum mass ratio for the preparation of sludge hydrogel sphere was: M_SS: M_CMC: M_HATP: M_DWTS = 300: 4: 6:0.07; the concentration of HCl-modified ATP was 2.25 mol/L, and the number of coating layers was 1. The cumulative N release rates increased gradually with increasing soil moisture content, temperature and pH. The Korsmeyer-Peppas model can well describe the N release kinetic from sludge hydrogel spheres (R² = 0.9509). Potting experiments showed that the sludge hydrogel spheres not only promoted ryegrass growth, but also improved plant quality. The N retention mechanism of sludge hydrogel spheres mainly includes the physical barrier of coating and the adsorption of NH₄⁺ by electronegative electrostatic gravitational force of hydrogel spheres. And the degradation and rupture of the coating in the later stage guaranteed the N release. This study provides new ideas for the preparation of slow-release fertilizers from solid waste resource utilization.