|May 6: Reed plants reduce sewer sludge by 90 percent
On the German island of Norderney, sewer sludge is pumped into environmentally isolated reed fields. The growing plants over six years time reduce the slurrie's volume by 90 percent.
Fridolin Mai, a marathon runner and 'Abwassermeister' (sewage clearing master) at Norderney first takes his time to lecture us on the methods and challenges of biological sewage treatment. All the sewage of the island ends up in this one installation with three larges basins. One of the challenges is the varying population of the island. It's number of inhabitants is only 6.500. But that number is easily multiplied by what Mai calls the 'white industry', or tourism. In the winter there are about 13.000 souls on the island, and in the summer even three times more (50.000). So the sewage clearing plant has to be able to cope with largely varying 'inputs'.
The main installation runs very much like any similar plant elsewhere. The sewage is first filtered from solid debris (condoms, toilet paper and the like), then bacteria are added are the mixture, which is led into a basin where air is added. The sewage sludge is continually stirred to stimulate the aerobe biological degrading process.
In the much more quiet second stage, after the bacteria have done their bit, sediments sink to the bottom of the basin from which it is slowly and steadily removed. The clear water on top flows over the rim and can be recycled.
The problem is usually with the sediment, the sewage sludge, which in the case or Norderney has a volume of 16 thousand cubic meters per year. Without further treatment this has to shipped back to the main land for disposal.
However, since 1991 the Norderney sewage plant has special reed basins, isolated by plastic folie from the surroundings, into which the sewage sludge is fed. Over the years, reed grows and the volume in the basin slowly accumulates. After a year of rest, the 'soil' under teh reed dries and solidifies. When it is then cleared, it only has a fraction of the original volume. In numbers: over the period from 1991 to 2005 246 thousand cubic meters of sewage sludge were fed into the (now nine) reed basins. In twelve clearings, only 17 thousand cubic meters came out. “A volume reduction of 93 percent”, Mai proudly states.
Volume reduction is important, because all waste has to be shipped back from the island to the German mainland for further disposal (Entsorgung). In principle the compacted sludge could be used for as manure for agriculture. But as there is no agriculture on the island, this is no option. Besides, there are some doubts on the suitability as fertiliser because of the possible contents of heavy metals.
The sewage sludge reduction process looks simple enough, it is practised since 1991 by the German firm Ekoplant. Still, it doesn't seem to be well-known outside of Germany. The Dutch mayor of Ameland, Albert de Hoop for example, has visited the plant twice. He would like to adopt a similar system at Ameland instead of the current transports to incendiaries on the mainland.