Influence of the Addition of Different Metals to an Activated Sludge System on the Enhanced Biological Phosphorus Removal

Abstract

Abstract Activated sludge systems can be operated to select bacteria which accumulate phosphorus as polyphosphate. By these means, phosphate can be removed without the addition of chemical precipitants. This contribution presents results of experiments with a bench‐scale purification plant for domestic sewage. The goal was to find the concentrations of relevant metal cations at which the biological P removal may be affected by competing chemical (precipitation) or physical (adsorption) processes. For this goal, increasing amounts of iron and calcium, respectively, were added into the pilot plant. During the addition of iron, the proportion of chemically (iron‐) bound phosphorus increased from 10 to more than 50%. The P‐release rate substantially decreased with increasing amount of added iron. An addition of small amounts of iron enhanced the long‐term stability of the P elimination as a whole. During the experimental period with addition of calcium, the proportion of Ca‐bound phosphorus increased from 1 to 2% to almost 15%. In batch experiments a measurable Ca‐phosphate precipitation took place at a pH value of at least 8.0 and a Ca‐concentration of at least 100 mg/l. The increase in hardness of the influent waste water didn't produce any positive effect on the stability of the enhanced biological phosphorus removal. The metal ions Ca 2+ , Mg 2+ and K + serve as counter‐ions in the polyhosphate chains. They were identified and quantified by X‐ray spectrometry in combination with scanning transmission electron microscopy. A release of Mg 2+ and K + occured simultaneously with the degration of polyphosphates (PP). The PP bound to Ca was not redox sensitive.