Transforming Sewage Sludge and Biomass Ash into High-Phosphorus Biochar for Agriculture

A study demonstrates that co-pyrolyzing sewage sludge with potassium and calcium-rich biomass ash significantly enhances phosphorus bioavailability in biochar. This cost-effective process transforms waste into a slow-release fertilizer, promoting sustainable agriculture and reducing dependence on finite phosphate rock resources.

December 12, 2024

1–2 minutes

Guo, et al (2025) Enhanced phosphorus bioavailability of biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More derived from sewage sludge co-pyrolyzed with K, Ca-rich biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion. Ash Ash is the residue that remains after the complete More. *Water Research.* https://doi.org/10.1016/j.watres.2024.122901

A new study explores an innovative method to enhance phosphorus (P) bioavailability in biochar derived from sewage sludge by co-pyrolyzing it with potassium (K) and calcium (Ca)-rich biomass ash, such as sunflower straw ash (SSA). Phosphorus, a critical nutrient for plant growth, often exists in forms within sewage sludge that are not easily accessible to plants, limiting its agricultural use.

Researchers found that adding 50% SSA during pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More at 600°C significantly increased the proportion of bioavailable phosphorus (Bio-P) in the resulting biochar, reaching an impressive 92.1% compared to just 9.5% for sludge-only biochar. This improvement stems from the transformation of Fe-phosphate in sludge into more bioavailable K and Ca-phosphates, such as K2CaP2O7 and Ca5(PO4)3OH, facilitated by the K and Ca in SSA.

Further analysis using density functional theory (DFT) calculations revealed that these new phosphate forms exhibit stronger interactions with organic acids, making them more effective as a nutrient source. The co-pyrolyzed biochar also met the standards for slow-release P and K fertilizers, offering potential for sustainable agricultural applications.

By utilizing biomass ash, a byproduct often discarded in landfills, the method provides a cost-effective and environmentally friendly solution to improve sludge recycling, enhance soil fertility, and reduce reliance on finite phosphate rock resources. This study underscores the promise of integrating waste management and resource recovery for sustainable agriculture.