Every fertilizer contains phosphorus. But this raw material has to be imported, mostly from politically unstable regions. At the same time, phosphorus is contained in the sewage sludge of every sewage treatment plant. The solution is recycling and has now become reality in Hamburg for the first time.

Challenge: Even ash can be recycled

At the end of the biological process in a sewage treatment plant there are two main substances: water and sewage sludge. The purified water can easily be returned to the natural cycle. Sewage sludge is already being used for biogas production in Hamburg and subsequently thermally exploited. What’s left is ashes. But even that does not have to be waste.

From ashes, for example, phosphoric acid can be obtained for the production of mineral fertilizer. But also plaster for the construction industry and various metal salts for wastewater treatment. This results in several material cycles, but phosphorus is the most interesting of them.

Solution: Sewage plant becomes a supplier of raw materials

Hamburg Wasser operates a sewage treatment plant in the Port of Hamburg, which produces 1.5 million cubic meters of sewage sludge every year. It is thermally recovered directly on site and transformed into heat and electricity. In addition, during the digestion process digester gas is produced, which is purified and supplied to the public gas supply. This leaves behind around 20,000 tonnes of sewage sludge ash per year, which previously had to be disposed of.

By 2029 at the latest, large sewage treatment plants are obliged to recover the phosphorus contained in sewage sludge and recycle it. A requirement which Hamburg Wasser took into consideration years ago and in 2015 installed a pilot plant for phosphorus recovery. After the successful test operation, the plant was dismantled again in 2017 and the planning of a large-scale plant started. By 2020, the world’s first large-scale plant for the recovery of phosphorus from sewage sludge ash will go into operation.

Hamburg Wasser is thus the pioneer of a new recycling technology, which meets with great interest worldwide. The plant is operated by Hamburg based Phosphorrecyclinggesellschaft mbH, a joint venture between Hamburg Wasser and Remondis Aqua Industrie GmbH Co. KG. The plant is to be built in 2019 and put into operation in 2020. The entire project is funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety with funds from the Environmental Innovation Program.

For the recovery of phosphorus from sewage sludge ash, the TetraPhos process developed by Remondis is used. Hamburg sewage sludge ash is expected to produce around 7,000 tonnes of ultra pure phosphoric acid per year. This raw material is in high demand by the fertilizer industry, but is also used in corrosion protection and chip production.

One process for four recyclables

Additional by-products of phosphorus recycling are raw materials in the form of metal salts and gypsum. The salts can be used for wastewater treatment. The gypsum is used, for example, in the form of gypsum panels in the building materials industry. Even the remaining ash can be recycled as an additive in the building materials industry.

In addition to gas, heat and electricity, Hamburg Wasser is thus opening up four additional raw material cycles for the benefit of the environment with the new recycling plant.

August 2019

Phosphorus Recycling
in Numbers

1.4 million
tons of sewage sludge per year
tons of sewage sludge ash after incineration
tons of high purity phosphoric acid


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