Evaluation of the nutrients cycle, humification process, and agronomic efficiency of organic wastes composting enriched with phosphate sludge

Composting is now at the core of the environmental policy and the circular economy as one of the best options for recycling organic waste leading to a high-value final product. However, composting organic waste suffers from several drawbacks in the resulting product, such as insufficient maturity, unstable organic matter (OM), accumulation of phytotoxic substances, and low amounts of nutrients. This study investigated the effectiveness of adding phosphate sludge (PS) to improve the green waste (GW) composting process, as well as its effectiveness when combined or not with sugar beet waste (SBW), GW, and oil mill waste (OMW) in the composting process of food waste (FW). The impact on the fertilizer quality of final products was also assessed. Composting treatments and controls were conducted in large-scale trials for 150 days. The composting process parameters, nutrient dynamics, phytotoxic effect, and agronomic capacity of the compost product were evaluated. The physicochemical analyses demonstrated that the addition of PS increased the pile temperature and prolonged thermophilic period by two weeks compared to controls. The PS addition brought pH, electrical conductivity (EC), and carbon to nitrogen ratio (C/N) values of FW and GW composts to maturity acceptable level. The results also indicate that adding PS alone or in combination stimulated the nitrification process in FW composting with a nitrate concentration of 69.91 mg/L recorded in the treatment received the combined addition of PS and GW. The results of Fourier-transform infrared (FT-IR) spectroscopy analysis showed a significant increase in humification by adding PS in mixtures of FW and GW. The resulting treated composts showed high total content of nutrients, no phytotoxic effect on seed germination, and growth enhancement of tomato plant compared to control compost, NPK fertilizer, and no amended soil.