ABSTRACT

In this work, a variety of polymers were evaluated as precipitation inhibitors for magnesium ammonium phosphate (struvite) in aqueous solution. The polymers tested include 1) homopolymers of acrylic acid and methacrylic acid of varying molecular weight (MW), maleic acid, 2-ethyloxazoline, 2) acrylic acid based co- and terpolymers containing monomers with different functional groups and 3) natural, bio- and hybrid based polymers. The results reveal that polymer performance as a struvite inhibitor is strongly affected by polymer dosage, MW, and ionic charge of the functional group. It has been observed that performance of poly(acrylic acid), PAA, is negatively impacted by increasing the struvite solution supersaturation. Moreover, inhibition of struvite by polymers increases with increasing MW with a range of ~2,000 to 60,000 investigated in this paper. Overall performance reveals the following trend by polymer type: homopolymer > copolymer > bio-, hybrid > terpolymer. SEM and XRD investigations confirm struvite crystals formation in the absence and presence of inhibitors.

INTRODUCTION

Recently, the use of treated waste water as a supplement to feed water in industrial water systems has increased significantly. Waste water contains a variety of contaminants including high levels of phosphorus and nitrogen due to intensive soil fertilization and domestic and/or industrial processes. In some cases, phosphate levels may be sufficiently high to cause the formation of undesirable scale deposits formed through the combination of the orthophosphate with metal ions such as calcium, magnesium, and iron, which may be present in excess in waste water. Precipitation and accumulation of magnesium ammonium phosphate hexahydrate (struvite), on equipment surfaces, pumps, aerators, screens, and pipe walls of anaerobic digestion and post digestion processes is a persistent problem in waste water treatment industry.1,2 Remediation, most of the time, is costly and time consuming. A wide array of waste waters containing nitrogen and phosphorus has been the subject of struvite precipitation and recovery.

Struvite crystallizes in the orthorhombic crystalline structure that may be used as either a fertilizer or raw material for the production phosphorus and it is formed according to the chemical reaction as illustrated in equation (1):

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