Experiments on thermal treatment of sewage sludge / green cuttings at Outotec

In 2011, Outotec has acquired and further developed the AshDec® process to produce a phosphate fertilizer from sewage sludge ash by thermochemical treatment. The purpose is to remove heavy metals from the feed material and to convert the insoluble phosphate species created in the incineration step to highly plant available calcium-sodium/potassium phosphates. In the process, sewage sludge ash is mixed with sodium/ potassium additives and dried sewage sludge as reducing agent and heated up to about 900 °C in a rotary kiln. Heavy metals are vaporized under reducing conditions to the gas phase and are condensed in the filter system to be disposed with the filter dust.

The goal of the investigation is to adapt the AshDec® process using a rotary kiln and sewage sludge ash to the fluidized bed technology and to produce a phosphorous fertilizer from sewage sludge in one process step. Scoping tests were performed at the Frankfurt Research Center of Outotec in a 50 mm fluidized bed reactor to determine optimal process conditions. The raw material was a mixture of sewage sludge and green cuttings provided by our project partner Soil Concept. Soil Concept built a demonstration plant for producing fertilizers from renewable resources by thermal treatment, also with the idea to utilize a modified AshDec® process.

It turned out that the additive dosing to the fluidized bed resulted in the formation of agglomerates which made the operation difficult. As a result of the bed sintering, the produced fertilizer samples had lower phosphorus plant availability than expected (max. 50 %, AshDec®: 90 %) because the reactants were not sufficiently mixed. Furthermore, the sand starting bed, which was introduced at the beginning of each test, could not be separated from the sample sufficiently, which resulted in a dilution of the sample. However, a significant removal of heavy metals was achieved, with the best results observed with reducing atmosphere and additive dosage.

The outcome of the scoping tests was used to give advice to Soil Concept on how to operate the production plant. The dosing of the tested additives to the fluidized bed reactor cannot be recommended. While heavy metal reduction can be achieved in the existing plant, a further process step would be needed to increase the P plant availability of the fertilizer.

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