Removal of Phosphate from Sewage as Amorphous Calcium Phosphate

Abstract

This paper discusses the progress of laboratory scale and a larger pilot plant campaign directed at evaluating a new process for the removal and recovery of phosphate from municipal sewage (after secondary treatment) and certain industrial waste streams. The laboratory scale experiments (conducted at flow rates up to 75 l hr−1) on real sewage (total P around 10 mg l−1) and on a variety of simulated industrial and sewage streams (total P up to 160 mg l−1) were very successful in removing phosphorus to >98%, and produced a phosphate containing ‘product’ containing up to 18% elemental P (i.e. around 80% Ca3(PO4)2), and some of these laboratory scale tests are described in detail. However, when tested in the field with real sewage (flow rate 2 000 1 hr−1) the overall results were disappointing. Although phosphorus removal was excellent and achieved expectations (>95% removal of P from the sewage stream), the phosphate levels in the ‘product’ were far below those achieved in the laboratory work. This had dramatic effects on reagent utilisation and negative impact on the economic evaluation of the project. Also, despite general similarities between the process design and equipment used in both the laboratory scale and pilot plant equipment, during the pilot plant tests certain operational problems were encountered which were not anticipated from the laboratory scale work. The reasons for the discrepancies between the results obtained from laboratory and field experience are discussed with reference to a number of factors including: (1) functional differences in certain equipment (i.e. filter units), and differences in physical scaling factors between laboratory and pilot plant operations, (2) the effect on field operations of unforeseen components in real effluents specifically high bicarbonate levels and the presence of propionic acid, and, (3) inadequate flexibility in the construction of the pilot plant which precluded easy changes to plant configuration and introduction of new equipment. The pilot plant used in this work was built in a very robust manner, which is unsuitable for an experimental facility and better suited for construction of plant to continuously demonstrate an already proven process.