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Phosphate Fertilizers from Sewage Sludge Ash – Design of an Industrial Manufacturing Plant

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Wastewater treatment plants (WWTP) in EU-27 countries produce more than 10.000.000 tons of sewage sludge (dry mass) every year. Municipalities and wastewater treatment boards operate altogether 110 sludge-to-energy plants, combust 2.500.000 tons of sewage sludge, produce energy and more than 1.000.000 tons of ash. This ash contains about 200.000 tons of phosphate (as P2O5), equivalent to 2/3 of the current total European phosphate rock production, which at present is irreversibly lost to landfills and construction materials (Hermann, 2009).

Phosphorus is a limited resource that is vital for all living organisms and essential for crop production. It cannot be replaced by any other element. Current static rock phosphate reserves are estimated at 16 billion tons. With an annual global consumption of 160 million tons, natural resources are going gradually into depletion and even the United States, second largest phosphate rock producer in the world, is not self-sufficient any more (Jasinski, 2010).

Sludge from municipal WWTP with effective phosphorus removal contains up to 10% of P2O5and is the largest single secondary resource. As a consequence, development and implementation of technologies for phosphorus recovery from wastewater and biosolids are a widely acknowledged strategic objective.

Whereas most research institutes and companies have focused recovery efforts on phosphorus rich side streams from sludge dewatering processes, ASH DEC, in cooperation with the German Federal Institute of Material Research and –Testing in Berlin, has developed a technology to produce phosphate fertilizers from ash that remains after thermal valorization of sewage sludge.In comparison to all upstream wastewater treatment residues, sludge ash presents the highest concentration of phosphates (as P2O5) of up to 25% (Adam, 2008)

ASH DEC's phosphorus recovery technology is a thermo-chemical process that removes heavy metals and transforms stable tri-calcium phosphates into plant available magnesium- and calcium phosphate compounds. It is also known as SUSAN technology after a joint, EU-co-funded R&D project.

To produce heavy metal free phosphate fertilizers, ash is mixed with an additive of natural magnesium and calcium salts and fed into a gas-fired rotary kiln. At a temperature of 1.000°C salts decompose and release gaseous HCI that reacts with the targeted metals cadmium, lead, copper and zinc to compose gaseous heavy metal chlorides. Simultaneously, the solid fraction of the salts reacts with phosphorus to compose plant available phosphate compounds.

Combustion and process gases leave the kiln to the air pollution control system. Downstream, the process gas is cooled to condense heavy metal chlorides on dust particles. A sequence of dry baghouse filters removes the heavy metal dust and chlorines, sulfates and other pollutants from the gas. Whereas the heavy metal dust is disposed of, chlorines and sulfates are recycled to the process and the product respectively. Only 5% of the ash-salt mixture remains as a waste. Contrary to all other phosphate recovery technologies on the market, only the SUSAN process can recover virtually all phosphates that are removed from the wastewater (Mattenberger, 2008). After implementation and two years successful operations of a process controlled pilot plant in Leoben, Austria, company has started to design and engineer an industrial phosphate fertilizer manufacturing plant.

Many lessons have been learned from the 2-years pilot plant operations, leading to significant modifications in the plant design. However, the basic concept of the manufacturing plant proved to be transferable to the industrial scale.

Experience has taught that you cannot transfer a technology directly from the lab-scale to the industrial scale without the intermediate step of a test- or a semi-industrial pilot plant. If ASH DEC would not have invested 2 million Euro to the pilot plant, large parts of the industrial plant, after the early stages of operations, would have to be turned-off and retrofitted to accomplish a reasonable industrial performance.
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Keywords: Phosphorus recovery; biosolids; phosphate fertilizer; sewage sludge ash; sludge incineration

Document Type: Research Article

Publication date: 01 January 2011

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