Enhancing food waste compost with sewage sludge biochar: impacts on stability, nutrient dynamics, and agronomic performance in maize

The sustainable management of organic waste, including food waste (FW) and sewage sludge (SS), is crucial for mitigating environmental impacts and promoting nutrient recycling. This study investigated the effect of sewage sludge biochar (SSB) on FW composting, specifically regarding the quality, stability, and agronomic performance of the resulting co-compost. SSB, produced at 300 °C (BC300) and 500 °C (BC500), was mixed with FW at various proportions (0 %, 5 %, 10 %, 15 %, 20 % w/w). Adding SSB enhanced the physical and chemical properties of the co-composts, notably increasing phosphorus (P), calcium (Ca), sulfur (S), and zinc (Zn) levels, and improving nutrient retention. Moreover, higher biochar levels, especially BC500, lowered volatile matter and increased ash content, indicating greater compost stability. Importantly, heavy metal levels remained within regulatory limits, ensuring the environmental safety of the co-composts. Co-composts with higher biochar additions (15 %–20 %) notably increased chlorophyll content in maize plants, with a 49 %–60 % rise in the SPAD index compared to the unfertilized control. However, maize dry matter production was not affected by biochar addition, probably because the nutrient solution supplied to the plants limited the effect of the co-compost. Conversely, combining co-compost with mineral fertilization significantly boosted plant dry matter yield, exceeding both fertilized and unfertilized controls. These findings suggest that incorporating biochar into co-composts can improve soil nutrient availability and promote plant health, supporting more sustainable farming methods. Future research should focus on assessing the long-term impacts of this co-compost, particularly under field conditions, to better understand its role in gradual nutrient release and crop growth.