Mineral fertiliser from sewage sludge
Beside other elements, phosphorus is essentially required for cell building and is mainly mined outside Europe. Our phosphorus resources are limited. Their price will therefore rise significantly in the future. Phosphorus is a main component of fertilisers and ends up in our food chain either directly via plants or indirectly via animal products. In the human organism phosphorus, as adenosine triphosphate (ATP), has the role of the energy supplier for our cells. About 2 g phosphorus per capita are flushed down the toilet every day. The phosphorus compounds contained within wastewater are removed from the water by microorganisms (“biological phosphorus elimination”) and finally end up in the sewage sludge in the course of the continued treatment process.
Depending on pH and phosphate concentration the phosphorus bound in the microorganisms (sewage sludge) can however be set free again. Frequently, undesired MAP crystallisation processes (MAP = magnesium ammonium phosphate) take place in the digester or later in the mechanical sludge dewatering system. MAP crystals settle in these systems and, in the course of time, form tenacious layers which later lead to severe operating troubles and uncalculable costs.
In Berlin, they have made a virtue out of a necessity. They apply a patented method to control MAP crystallisation in a separate process and selectively remove the crystals from the sewage sludge [source: www.bwb.de]. The desired size of the crystals can be achieved by changing the residence time. A suspension of digested sludge and crystals is discontinuously removed from the process and passed to a HUBER Grit Washer RoSF4/t with a fluidized bed system which separates the suspension simply but effectively into sludge and MAP crystals.
The homogenous fluidized bed system with pressure measurement has clear advantages compared to fixed bed systems as MAP density is always the same, irrespective of grain size. As also the majority of the organic particles are washed out, the end product does actually not have much to do with a sewage treatment plant or with sewage sludge anymore. The HUBER Grit Washer RoSF4/t has operated to the full satisfaction of the customer in Berlin for more than a year meanwhile and produces about 3 tons of clean MAP a day. The produced MAP material, which contains also the nutrients nitrogen and magnesium in addition to phosphorus, is of excellent quality. The MAP undergoes routine analysis before it is sold to be used for agricultural purposes. Even if the sales revenues cannot cover the ongoing costs, the investment pays off nevertheless as a lot of operational problems in digesters, pipelines and centrifuges are avoided. As a side effect, this solution saves phosphorus – after all, a resource we need more urgently than mineral oil.