Phosphorus is an elementary building block of life for which there is no substitute. Phosphate enters plants through the soil and this is how it reaches humans and animals. Farmland is usually treated with fertilisers but these are produced from fossil phosphorus deposits found in precious few places in the world. The mining process is also associated with great environmental pollution and high energy consumption. Until now, the EU has been dependent on imports of this critical raw material.
Phosphorus is not only used in agriculture, but is also used as a food additive, detergent or flame retardant. In order to reduce the dependency on imports and to avoid losing the vital element inert in landfill sites or building materials (like cement), the plan is to recover phosphorus from streams, such as sewage sludge, fermentation residues or liquid manure. Up to 50 per cent of the phosphorus required in this country could be recovered from sewage sludge.
In the long term, phosphorus (P) recovery from wastewater and other residue flows rich in P such as sewage sludge will be indispensable to securing the global food supply. What’s more, Germany’s options for disposing of sewage sludge are going to change markedly in the near future because of:
a) the provisions of the amended German sewage sludge ordinance (AbfKlärV of Sep. 27,2017)
(b) the phasing out of coal-fired power (loss of co-combustion capacities), and
(c) the problem of nitrate, organic and/or pathogenic pollutants entering the environment when sewage is spread on farmland.
The question of how to dispose of sewage sludge is sure to become an ever more pressing issue in Germany and abroad.
The question of how to dispose of sewage sludge is sure to become an ever more pressing issue in Germany and abroad. The trend these days is to solve the problem via thermal disposal in central, mono-incineration plants. Building a plant that incinerates nothing but sludge next to a waste-to-energy plant would make sense. The operator could take advantage of synergies in logistics, CHP generation, flue gas cleaning, and plant staffing, which would certainly reduce overhead.
It remains to be seen how plant operators are going to meet the German sewage sludge ordinance’s phosphorus recovery requirements. In most cases, this issue remains unresolved. In theory, P could be recovered from the ashes of mono-incinerated sewage sludge. Many processes are in development or at the pilot stage, but as yet none has proven effective. All involve some sort of tradeoff between feasibility, effectiveness, economic efficiency and environmental impact.
Joining forces with partners in industry and research, ZSW is developing a new method of incinerating sewage sludge in a fluidized bed. The aim is to optimize the properties of the ash for subsequent use, particularly to provide ash rich in P for crops and thereby close the phosphorus loop.
To this end, sewage sludge was incinerated under real-world conditions alongside additives containing calcium to produce ash of stable quality with a high phosphorus content of up to 10% of mass. As part of the ReCaPhos project, an EU-funded individual fellowship, researchers are conducting theoretical and experimental investigations to identify favorable conditions for combustion and beneficial properties in sewage sludge, and thoroughly analyze the process. They will apply their findings to the fluidized bed process in a lab equipped with a 10-kW fluidized reactor to separate ash at high temperatures, and array of measuring instruments.