| Summary: | In this study, the pilot-scale struvite crystallization process developed at UBC was
operated continuously for recovering phosphate, in the form of struvite, from anaerobic digester
supernatants from the Annacis Island Wastewater Treatment Plant and the Lulu Island
Wastewater Treatment Plant. In addition, the process performance for a synthetic supernatant
with high phosphate concentration (100-190 mg/L) was verified.
Study results showed that the process was capable of removing more than 90 % of
ortho-phosphate from both the synthetic supernatant and the digester supernatants.
Approximately 90 % of removed phosphate was recovered as harvestable struvite crystals. The
desired phosphate removal efficiency was achieved through controlling the inlet supersaturation
(SS) ratio, operational pH and magnesium dosage in the supernatant. It was possible to reduce
the effluent ortho-phosphate concentration to less than 5 mg/L through choosing optimal
operational conditions.
Chemical analysis of the recovered crystals showed very pure struvite (91.2 % and 94.1
% by weight for crystals produced from the Annacis supernatant and the Lulu supernatant,
respectively.) with small amounts of calcium and carbonate, and traces of iron and aluminum.
Most of recovered crystals were round, hard and larger than 2 mm in mean diameter over the
course of the study. The crystal retention time in the reactor and the magnesium dosage in the
supernatant were identified as two major factors affecting the size, density, hardness and
morphology of recovered struvite crystals.
Determination of the struvite solubility product (Ksp) showed that there were significant
differences among Ksp values for the synthetic, Annacis and Lulu supernatants, due to
combined effects from impurity ions and suspended solids, as well as other unknown factors. In
this scenario, the conditional solubility product (Ps) was considered to be more useful than Ksp
in operating the struvite crystallization process and predicting process performance.
A cost analysis showed that the application of air stripping or a high magnesium dosage
in the supernatant could not reduce the total chemical costs, since both Annacis and Lulu
supernatants used in this study had a high pH value around 8.0. However, a further study on
other anaerobic digester supernatants with low pH was recommended.
Two struvite models developed by Britton and Potts, respectively, were used to predict
the process performance and determine the operational parameters. Both were validated through
comparing the predicted results with the actual operational data. The comparison showed that
the former predicted the actual results with relatively high accuracy; however, the latter
demonstrated a large deviation from the real results, probably caused by the Ksp value used in
the model. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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