Recovery of Phosphorus from Sewage Sludge

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In the following, a method to recover phosphorus from sewage sludge as MAP (magnesiumammonium- phosphate, also called struvite) is to be presented. The process uses sulfuric acid to leach phosphorus out of digested sewage sludge. After solid-/liquid-separation, interfering metal ions in the liquid phase are inactivated through complexation. For the subsequent precipitation of the phosphorus, MgCl2 is added. This article describes the results of half-technical experiments (reactor volumes ~ 1m³) and compares them with results of laboratory scale experiments. The quality of the produced MAP is comparable to that of commercial mineral fertilizers in terms of heavy metal concentration. Field experiments to determine the fertilizer value are currently being undertaken.

INTRODUCTION

In countries practicing wastewater treatment, the disposal of sewage sludge is a frequently discussed topic. Some classify sewage sludge as “waste” and insist that it should not be utilized agriculturally in areas with high expectations of product quality. Considering the fact that sewage sludge consists of concentrated pollutants taken out of wastewater, this statement is understandable. However, it is also undeniable that this “waste” contains significant amounts of nutrients, including those among non-renewable resources, such as phosphorus. On the one hand these resources are wasted by disposing sewage sludge without any recovery. On the other hand the production of industrial fertilizer containing phosphorus requires the exploitation of geological sources which leads to a reduction of decomposable rock and therefore contradicts the rule of sustainability. The range of remaining resources is estimated at 100 years for phosphorus reserves and at another 370 years for the reserve bases (ATV-DVWK, 2003).

In Europe, about 79% of the phosphorus consumption is required for agriculture (Johnston and Steén, 2000). Implementing a good fertilization practice the demand of phosphorus for German agriculture would add up to 364.000 t P (an area-specific mean of 50 kg P2O5/(ha·a) is recommended) (UBA, 2001).

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