Biochar from biosolids microwaved-pyrolysis: Characteristics and potential for use as growing media amendment

Abstract

Biochar, produced from biosolids using microwave pyrolysis technology, is energetically a more efficient alternative to that produced with conventional convective heating. However the potential of microwave generated biochar as a growing media amendment has not been sufficiently explored. Here we produced biochar from biosolids using microwave energy. The pyrolysis expeiments were conducted in two stages, initially using a custom built single mode chamber to explore the energetics and product distribution of the pyrolysis process at different temperatures and secondly in a 1 m3 6 kW multi-mode chamber, to explore potential scale-up of the process. The second phase of the pyrolysis experiments was focused on biochar generation for use in the remainder of this research. Microwave pyrolyzed biochar (MB) was characterised for its chemcal and physical properties. Then, we conducted a greenhouse experiment, where we compared the ability of four growing media mixes that combined pine bark with (i) sphagnum peat and fertilizers; (ii) 20% MB and fertilizers; (iii) 60% MB and fertilizers; and (iv) 60% MB and no fertilizers, to promote plant growth and nutrient uptake and to minimise leaching losses. MB had high mesoporosity (average pore width of 4.46 nm), moderate surface area (75 m2 g−1), elevated nutrient content and low heavy metal concentrations as compared to other biosolids biochars reprted in literatures. Substitution of peat with 60% MB on volume basis reduced leaching loss of nitrate and phosphate from the media but increased leaching loss of ammonium. Addition of MB in conjunction with fertilizer increased plant growth and plant nitrogen and phosphorus use efficiency. Our study has shown microwave pyrolysis as a promising technology for pyrolyzing biosolids and also has demonstrated the synergistic interaction of MB and fertilizer which results in greater plant growth and nutrient uptake and use efficiency.