Theoretical and practical experience of calcium phosphate inhibition in RO waters

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The increasing trend over the last ten years to use waste water as a feed source for reverse osmosis membrane plant has resulted in an increase in calcium phosphate scaling on membrane surfaces. This is due to much higher levels of phosphate in waste water typically 10-30 mg/l as compared with a groundwater source which may have only 1-5 mg/l of phosphate. These higher phosphate levels resulting from effluent breakdown in primary sewage treatment causes calcium phosphate solubility to be exceeded in reverse osmosis membrane systems resulting in scale formation.

Conventional antiscalant chemistry based on threshold inhibition and crystal distortion has had poor results in high
phosphate high pH waters. Dosing large quantities of acid to lower the scaling tendency is no longer an acceptable approach. This paper investigates the complexities of multiple species of calcium phosphate with varying solubility’s all contributing to an amorphous deposit. Research conducted by the medical community into calcium phosphate based cement for bone and teeth repair has been applied to theories of calcium phosphate precipitation and crystal growth. Highly active calcium phosphate threshold inhibitors were tested in the laboratory and a development product Genesys PHO (Antiscalant B) formulated which was then used in three waste water pilot plants and compared with conventional
antiscalants.

The results of this beta testing showed the new formulation out performed the conventional antiscalants and that calcium phosphate scaling could be controlled without the need for pH correction through acid dosing in once through RO plants. The third RO pilot plant was a recycled system where scale formation was not prevented. The results from another two high pH, high phosphate agricultural leachate waters are also presented.

Understanding the mechanisms of calcium phosphate deposition and the lack of crystal growth hasenabled a highly active threshold inhibitor antiscalant to be formulated which is effective at preventing calcium phosphate scale formation in high phosphate; high pH feed waters to RO membranes without the need for acid dosing.

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