Leaching of Phosphorus From Ashes of Co-Combustion of Sewage Sludge and Wood

Municipal sewage sludge has a high concentration of phosphorus, which should be recovered because phosphorus is a limited natural resource. In this work, sewage sludge was co-fired with wood in a FBC boiler. The aim of the investigating was to study the solubility of phosphorus in the ashes, by leaching as an alternative to the phosphorous recovery method of using the ashes directly on farmlands. The fly ashes from the boiler’s secondary cyclone and bag filter were leached at various pH-values and the release of phosphorus was measured. Only acidic leaching was applied. The ashes precipitated with Al₂(SO₄)₃ released nearly all phosphorus at a pH-value of 1, whereas the ashes precipitated with Fe₂(SO₄)₃ did not release all phosphorus even at the very low pH of 0.5. The concentrations of phosphorous in the leachate must be compared with liquid phosphorus sources such as human urine or liquid animal manure used as fertilisers. This may result in that the leachate has to be processed further. A continuation of the work to investigate to what extent the leachate is contaminated with toxic trace elements is necessary.