Environmental assessment and agronomic performance of biosolids-derived biochar in controlled environment studies

Environmental assessment and agronomic performance of biosolids-derived biochar in controlled environment studies

Increasing regulatory restrictions on traditional land application of biosolids (treated sewage sludge) in Australia has driven the need to explore alternative treatment technologies that render biosolids (BS) suitable for beneficial use while mitigating environmental risks. Thermal treatment of biosolids is currently being investigated in Australia as an alternative management practice; however, the characteristics of the biochar product is poorly understood. This thesis evaluates the safe land application of biosolids-derived biochar (BDB) produced from the gasification of BS at an Australian wastewater treatment plant using two contrasting Australian soil types (Ferrosol and Chromosol) at three application rates (2.5%, 5% and 10% w/w). Leaching columns were used to assess the environmental risk of heavy metal concentration and mobility in soils treated with BDB compared with BS. The study applied the novel use of p-XRF as a rapid and low-cost technique for investigating leaching behaviour. While BDB-amended soils contained higher total metal concentrations, the leachates showed significantly lower concentrations of soluble copper and chromium compared to those treated with BS. The study underscores a potential mitigation of environmental risks associated when applying BDB as a soil amendment. The combined assessment of agronomic and heavy metal dynamics of BDB was performed using glasshouse studies and compared to BS. BDB significantly increased the dry matter yield of ryegrass, compared to control. At 2.5% w/w application rate BDB increased yield by 180% in Chromosol and 192% in Ferrosol compared to control. Higher application rates of BDB did not affect agronomic efficiency. Importantly, BDB significantly reduced heavy metal uptake (chromium, copper, and zinc) by ryegrass compared to BS. Soil incubation studies supported these findings, showing that 2.5% BDB increased urease activity by 50% in Chromosol and 23% in Ferrosol, relative to control soil, which is beneficial for nitrogen cycling. While BDB at the same rate decreased acid phosphatase activity to 12% in Chromosol and 20% in Ferrosol, which shows a potential impact on phosphorous availability. The study advocates for a 2.5% application rate of BDB to optimize agricultural outputs while safeguarding soil and plant health. The findings also highlight the importance of tailored soil management practices that consider specific soil characteristics, and application rates to optimize benefits and minimize environmental risks. Further research is recommended to explore land application of BDB using field trials to understand long-term impacts and connect the results of the current research observed in controlled environment studies.

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