Clear plants are a clean business. At least when you consider what comes in impure at the front and comes out clear at the back. And what remains? Well … In the bamberg district, this amounts to around 70,000 metric tons of clear sludge per year. And they have their work cut out for them: in addition to valuable nutritional elements such as phosphorus, there are also organic compounds – such as perfluorinated surfactants (PFT) and pollutants from plasticizers, flame retardants and detergent ingredients. In addition, there are drugs such as antibiotics or hormones and their degradation products.
This list is far from complete – and yet already gives enough cause for concern. Especially when you consider that in 2015, for example, more than a third of the 71,000 tons of clear sludge produced in the district of bamberg was used for agricultural purposes. That means a good 24,000 tons ended up on local fields. At least the spreading of clear sludge on grassland, fruit and vegetable plantations and in forests is prohibited.
With a rate of 34 percent, the district was also well above the bavarian average. There it was only 14.8 percent in the same year.
To change this – and because the requirements of the clear sludge ordinance have become stricter in recent years and will probably continue to do so – seven municipalities in the southern district (frensdorf, altendorf, buttenheim, hirschaid, strullendorf, memmelsdorf and gundelsheim) have joined forces to develop and implement a new concept.
The goal: in the future, clear sludge should primarily be thermally recycled. So far, this method of disposal – incineration in refuse-derived fuel power plants or in the cement industry – has accounted for a smaller share than agricultural recycling and recycling through reclamation/landscaping (39 percent), which mainly includes the export of sludge to fill opencast lignite mining pits in eastern germany.
The problem with thermal recycling is that the sludge has to be dried beforehand, which requires special equipment and consumes additional energy. The wet sludge produced in the clarification plant consists of around 95 percent water and has a dry residue content of around five percent. In order for it to burn, this must be increased to about 25 to 30 percent with presses or centrifuges, according to the state office for the environment (lfu). For incineration processes, such as co-incineration in cement plants, further drying may also be necessary.
Scientists from the institute for energy technology (ife) at the east bavarian technical university in amberg-weiden were commissioned by the district to investigate where such a drying plant could be located. Industrial waste heat sources such as the gunreben company in strullendorf, the animal carcass disposal plant in walsdorf and the mull cogeneration plant in bamberg could therefore be considered as energy suppliers.
Favorite location is strullendorf. There is also the local clearing plant very close to the gunreben company, additional areas for the drying plant were available.
But drying alone is not enough. The task was to develop an "economically viable concept", says ife head prof. Markus brautsch. However, according to the federal statistical office, bavaria’s mull power plants are already operating at the limits of their capacity. And in the cement industry, demand for combustible clear slurry declined.
In any case, the recovery of the valuable phosphorus would be optimal. But this is only possible with mono-combustion. However, the first plant in bavaria in which only clear sludge is incinerated is currently being planned in straubing, lower bavaria, following a positive decision by the city council. For the seven communities in the southern district of bamberg alone, something like this would probably be too big a job.