Pyrolysis of urban waste streams: Their potential use as horticultural media

•Physical properties of the biochar are desirable for use as plant growing media substrate.•The biochar has high surface area.•The biochar is rich in major plant nutrients which come from the biosolids component of feedstock.•The biochar contains below permissible levels of heavy metals and PAHs are below detection level.

Biochar produced from mixture of biodegradable urban resources such as biosolids and greenwaste was slowly pyrolysed at 650 °C and characterised for its chemical and physical properties. The biochar was then compared to published values for growing media. It was found that this biochar has high surface area, inherent nutrient content, high porosity, high stability, desirable water holding capacity, air filled pore space and bulk density. Comparing the results of characterisation of this biochar with ideal potting substrate showed the highly porous and nutrient rich biochar were desirable however the high salt content was problematic. Fourier transform infrared analysis shows that carboxyl/carbonyl bands were very weak in biochar spectra due to loss of carboxyl groups in the decomposition of carbohydrates in high temperature pyrolysis which also contributed to low cation exchange capacity. The pyrolysis process has multiple advantages such as waste management, energy generation in the form of syngas and long term sequestration of carbon in biochar.