Abstract

A seed-induced crystallization process was developed to remove and recover phosphorus from waste water, i.e. attention has been especially drawn to gain a phosphorus-rich product, which can be recycled by the phosphorus industry and/or may directly be used as a fertilizer. The necessity of phosphor-us recovery from phosphorus sinks is due to the fact that the world reserves of phosphorus rock will run out within the following 80-100 years. On the other hand, phosphor-us is an essential nutrient for all forms of life, which cannot be replaced by any other element.

For the development of this phosphorus recovery process, continuous-flow fixed-bed column and expanded-bed experiments were carried out in the laboratory scale using calcite (cc) and tobermorite-rich crushed gas concrete (cgc) as seeds. The experiments revealed that cgc was much more efficient in phosphorus removal from organic-enriched waste water compared to calcite seeds. Under suitable conditions, a phosphorus removal efficiency of 80-100% could be achieved for a time period of up to 4 months yielding a product containing about 10% P per weight.

Furthermore, the physical and chemical properties of the product obtained proved the ability for phosphorus recovery. The results of FTIR and XRD investigations showed the transformation of cgc into calcite, the formation of hydroxyl apatite-like compounds on the seed material during the reaction with waste water, and finally the residual quartz originating from the cgc. A comparison of the results of laboratory- and half-scale fixed-bed column experiments revealed a shortened life-cycle of the columns with higher phosphor-us concentrations of the inflow waste water, i.e. the velocity of Ca depletion within the fixed-bed system is governed by the P concentration.

Active filtration using cgc seems a promising development for phosphorus removal and recovery from waste water. Thus, it should be possible in the near future to replace state-of-the-art methods of waste water treatment plants (WWTPs), such as metal precipitation or enhanced biological phosphorus removal (EBPR), which have the only aim of phosphorus removal, by a rather simple technology with the paramount goal of phosphorus recovery.