Determining the feasibility of phosphorus recovery as struvite from filter press centrate in a secondary wastewater treatment plant

Abstract

In this study, the workability of a pilot-scale, fluidized reactor was examined to determine effectiveness in removing, and recovering, phosphorus as struvite, from centrate at Lulu Island Wastewater Treatment Plant (LIWWTP), Richmond, British Columbia. The crystallization process was run continuously over a period of 5 months in two runs (Run 1 for 4 months and Run 2 for a month). In addition to efficient recovery of phosphorus as struvite, the study also investigated factors that affect the growth of struvite. Chemical analyses were conducted on the harvested struvite to determine its purity. Results showed that the reactor was capable of removing over 90% of phosphate and 4% of ammonia-nitrogen, with greater than 85% of the phosphate removed being recovered as harvestable struvite crystals. It was possible to achieve over 90% P-removal at a pH of 7.5; this is contrary to the information found in literature, which recommends that a higher pH (8.2–9.0) is required. Factors that affected phosphate removal were the operating pH, the reactor supersaturation ratio (SSR), the N:P and Mg:P molar ratios. Analysis of the harvested product showed that the crystals were composed of nearly pure struvite (96% by weight), with small amounts of calcium and traces of other metals. High resolution SEM pictures were taken of the inside of the crystals to determine the influence of Mg:P molar ratio on the compactness of the crystals.

Keywords:

• Centrate
• crystallization
• nutrient removal
• phosphorus recovery
• sludge treatment
• struvite
• wastewater

Acknowledgment

The authors would like to thank The University of British Columbia, Natural Science and Engineering Research Council of Canada (NSERC), Greater Vancouver Regional District (GVRD) and Stantec Consulting Ltd., Vancouver, B.C., Canada for their financial support towards this research. The authors are also very grateful to the staff at Lulu Island Wastewater Treatment Plant for their support and help during the study.