Printversion
Phosphorus Recycling from Waste Water Sludges
Mineral phosphorus is a limited resource but an essential element for all animals, plants and humans. Hence phosphorus is necessary as nutrient in agriculture. Due to the finiteness of the resource and in order to achieve natural cycles of matter the phosphorus recovery from waste water sludge is a recent research target. Problems are harmful substances in the sludge like heavy metals and harmful organic substances. Several processes have been developed already to gain purified and agricultural applicable phosphorus from waste water sludge. Examples are the Phostrip process or phosphorus recovery from waste water sludge ash. The target of the PHOXNAN project is an optimised process for phosphorus recovery with the combination of low pressure wet oxidation (LOPROX) and membrane filtration. The LOPROX process has been developed by the project partner Bayer Technology Services GmbH for industrial waste water treatment. During the process the main part of the biological matter is oxidised by applying pure oxygen under acidic conditions and temperatures of 120°-220°C. Due to the reusage of the oxidation heat the process works completely autothermic. After applied the LOPROX process the phosphorus is existent as dissolved agent. The second process stage which will be developed at the IVT consists of firstly an ultrafiltration to remove the remaining particles and subsequently a nanofiltration to achieve the separation of phosphorus from other unwanted dissolved substances like heavy metals and harmful organics. Nanofiltration membranes are tight charged membranes possessing ionic selectivity due to electrostatic interactions. Under the acid conditions achieved in the LOPROX stage the membrane is positive charged and therefore cations are retained to a higher amount than anions. Furthermore phosphorus is existent mainly as H2PO4- in acidic conditions having a better permeability than double charged ions. The aim is the accumulation of phoshorus in the permeate and the separation from the strongly remained heavy metals and harmful organic substances. The final step of the process to develop is the precepitation of phosphorus by using MgO or CaO to gain a plant available form of phosphorus. 
|
