Phosphorus recovery from sewage sludge ashes

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Phosphorus is a limited resource and its availability will decrease in the next decades. Therefore, it is important to study viable ways to recover phosphorus before it gets dispersed in the environment in too low concentrations.
This study aims at evaluating the potential recovery of phosphorus from sludge ashes which contain 80% of the phosphorus discharged in urban wastewater.

The regulations in force at European, national and regional level are limited to the reuse of sludge in agriculture, where only 30% of the phosphorus is bioavailable to crops.
The Italian production of sewage sludge is about three million tons per year with an average content of dry matter of 25%, of which about 400 000 tons are produced in Lombardy.
The average concentration of phosphorus in sewage sludge is 2.10% (on dry basis). The average fraction of sludge ash is about 30%, which corresponds to a content of 7% of phosphorus (P) in the ashes of sludge or 16% as (P2O5), a value that is comparable to the content in phosphate rocks (10.9% Florida, 27.1% Morocco). Of this, around 70% can be made available to crops.

Today the recovery of phosphorus from the sludge produced by Italian wastewater treatment plants could be more difficult than in other European countries, for two main reasons: the first reason is the strong dilution of wastewater in most cases, which determines a low concentration of phosphorus in the influent (4-5 mg/l against 8-9 mg/l as typical values in Northern European countries). As a consequence, with a limit to the effluent of 1 mg/l, the percentage of phosphorus removed is lower. Secondly, the use of iron and aluminum salts for the chemical precipitation of phosphorus makes it more difficult to extract phosphorus from the ashes.
The possibility of recovering sewage sludge material, recently defined by the acronym STRUBIAS (struvite, biochar, ashes) depends, above all, on the legislation that should encourage the circular economy.

Phosphorus is a limited resource and its availability will decrease in the next decades. Therefore, it is important to study viable ways to recover phosphorus before it gets dispersed in the environment in too low concentrations.
This study aims at evaluating the potential recovery of phosphorus from sludge ashes which contain 80% of the phosphorus discharged in urban wastewater.

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The regulations in force at European, national and regional level are limited to the reuse of sludge in agriculture, where only 30% of the phosphorus is bioavailable to crops.
The Italian production of sewage sludge is about three million tons per year with an average content of dry matter of 25%, of which about 400 000 tons are produced in Lombardy.
The average concentration of phosphorus in sewage sludge is 2.10% (on dry basis). The average fraction of sludge ash is about 30%, which corresponds to a content of 7% of phosphorus (P) in the ashes of sludge or 16% as (P2O5), a value that is comparable to the content in phosphate rocks (10.9% Florida, 27.1% Morocco). Of this, around 70% can be made available to crops.

Today the recovery of phosphorus from the sludge produced by Italian wastewater treatment plants could be more difficult than in other European countries, for two main reasons: the first reason is the strong dilution of wastewater in most cases, which determines a low concentration of phosphorus in the influent (4-5 mg/l against 8-9 mg/l as typical values in Northern European countries). As a consequence, with a limit to the effluent of 1 mg/l, the percentage of phosphorus removed is lower. Secondly, the use of iron and aluminum salts for the chemical precipitation of phosphorus makes it more difficult to extract phosphorus from the ashes.
The possibility of recovering sewage sludge material, recently defined by the acronym STRUBIAS (struvite, biochar, ashes) depends, above all, on the legislation that should encourage the circular economy.


Planned activities within PerFORM WATER 2030

The experimental activities regarding Phosphorus recovery from sewage sludge ashes are part of the Recovery of energy and materials research line included in PerFORM WATER 2030. They include the study and characterization of sludge produced in Lombardy, and of sludge ash produced in the laboratory and by sludge mono-incineration processes.

Objectives and expected results

  • Evaluation of the potential recovery of phosphorus from sludge ash.

  • Development of laboratory tests for the extraction of phosphorus from ashes.

Description of the activities

  • Evaluation of the potential recovery of phosphorus from sewage sludge, with particular reference to the Lombardy Region.

  • Tests for the extraction of phosphorus from ashes produced by sludge combustion in a laboratory furnace and ashes deriving from sludge thermal processes.

Pilot site

The experimental activities will be located at San Giuliano Milanese West WWTP (Wastewater treatment plant) operated by CAP Group.
For further information, please refer to the specific web-page dedicated to demonstration sites of PerFORM WATER 2030.

Partners involved

VOMM takes care of the construction of a demonstration sludge combustion plant producing the sewage sludge ashes (SSA), while DICA-AMB Department of Politecnico di Milano will supervise sampling and analyses of sludge, off-gases (in cooperation with LEAP, Laboratory of Energy and Environment - Piacenza) and ashes and will perform the lab-tests on extraction of phosphorus from ashes. POLIMI will also evaluate the potential recovery of phosphorus from ashes deriving form sludge produced in Lombardy.

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