Phosphorus (P) is an elementary building block of life for which – like water – there is no substitute. All living organisms need phosphorus in the form of phosphate. Plants, animals and humans obtain it from soil. Depleted farmland is usually replenished with fertilizers. The problem with these fertilizers is that they are produced using mineral phosphorus resources that are finite, non-renewable and increasingly contaminated with cadmium, uranium and the like. The largest deposits are found in just five countries, most being in Africa. Mining pollutes the environment and consumes a great deal of energy.
Dependent on offshore sources, the EU has to import this critical resource, which is so vital to the economy. This indispensable element also figures prominently in other products such as food additives, detergents and flame retardants. To reduce this dependence on imports and ensure a precious resource is not lost to landfills or left to lie inert in building materials such as cement, phosphorus should be recovered from waste flows such as sewage sludge, fermentation residues and liquid manure. Up to 50 percent of Germany’s demand could be recovered by recycling 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.
The risk of ash softening during combustion increases with the fuel’s ash content. For example, if the fluidized bed process is to remain stable, ash may not under any circumstances soften. Otherwise, the bed material will not agglomerate or it will cake in the reactor. This is why ZSW determines the critical temperature at which ash softens in advance for ash mixtures via rheometric characterization. Then the fluidized bed can be run safely below this temperature point.
This underlying principle for this measuring technique is that the torque of a stirrer increases as the viscosity of a fine particle bed increases because the particles stick together as the ash softens.
The test cell is an electrically heated furnace. Inside it is a clay crucible containing the particle bed that is to be tested. This bed is stirred at a specific speed. The temperature of the furnace rises continuously to a peak of 1,300°C. The test ends when the torque exceeds the threshold value.
Particle size distribution in solids is a fundamental characteristic that influences the material’s behavior, for example, its reactivity. Particle size distribution also places demands on the process in terms of fluidization, gas/solid separation and the like. With our sieve tower and laser diffraction device, we can conduct quick comparative analyses of samples ranging from 2 µm to approx. 2 mm. A divider splits the sample into ten comparable individual samples to be measured by three optics with different size ranges. This hardware comes with software tools that enable us to immediately view and assess these readings. The picture on the right shows the experimental setup for particle size determination via laser diffraction.