Can we feed the world with resource-recovered sewage sludge?: PRESENTATION ONLY

Heitmann, M.1, Fowler. G.1, Perrault, A.2, 1Imperial College London, UK, 2Thames Water Utilities Ltd, UK


Sewage sludge is rich in phosphorus but its recovery is challenging due to matrix complexity. By converting sludge into char via pyrolysis, key elements of interest can be concentrated for easier extraction.

Sewage sludge char (SSC) contains 4.5-6% w/w phosphorus along with other common elements (Fe, Ca etc.) in a partially graphitised carbon matrix. Mineral acids (H2SO4, HCl, HNO3) were found to be effective at converting bound phosphate into phosphoric acid, whilst organic acids (Acetic & Citric) and other extractants (EDTA) were not. Extraction rates in excess of 90% were achieved at stoichiometric phosphate-acid ratios. Due to possible immobilisation within the carbon fraction, heavy metal leaching from SSC was substantially lower (especially copper) than from comparative tests using sewage sludge ash (SSA). Base neutralisation to pH 3, 5 or 7 recovered 42%, 70% and 80% of the phosphate respectively. The availability of Al and Fe were identified as a limiting factor. The precipitate contained approximately 20% w/w P which is a commercially viable concentration for fertiliser production. The extracted char had potential for upgrading to carbon-based adsorbents or as an additive to improve soil structure. This treatment facilitates the complete valorisation of sewage sludge through pyrolysis and phosphorus recovery.

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