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UBC Theses and Dissertations

Phosphate recovery from greenhouse wastewater through crystallization Yi, Weigang


An investigation was conducted on the suitablility of phosphate recovery from greenhouse wastewaters by a precipitation/crystallization process. Two groups of jar tests and five months of a pilot-scale reactor operation on site, were carried out. Greenhouse drainage wastewater contains high concentrations of phosphate, calcium, magnesium, potassium, which varies from time to time, depending on the greenhouse operation period. Results from the laboratory jar test show that more than 90% of phosphate could be removed from greenhouse wastewater. Various calcium phosphate salts were also obtained in the process, hydroxyapatite, Ca₅(PO₄)₃OH, was the main product recovered. Phosphate removal was affected by the presence of magnesium in the wastewaters. An increase in magnesium concentration in the wastewaters decreased the phosphate removal rates. The chemical content of precipitates, in terms of calcium, magnesium and phosphate, were affected by the Ca/Mg ratio. The higher calcium content was obtained in wastewaters with the higher Ca/Mg ratios. An addition of magnesium did not affect the potassium content in the precipitates. K-struvite, MgKPO₄.6H₂O, was not the major product in the precipitate, even with addition of a large quantity of magnesium. A second group of lab jar tests was conducted, with magnesium and ammonium addition in three wastewaters of different calcium concentration wastewaters. At any operating pH, the calcium concentration was the major factor determining the phosphate removal efficiency. The addition of ammonium changed the chemical reaction at the lower pH zone (<8.0), moving toward more struvite formation, except when calcium was at a high concentration. However, hydroxyapatite was still the main product of the precipitate that formed. A pilot-scale, fluidized bed reactor was set-up and operated at South Alder Greenhouse, located in Delta, BC. The reactor was tested with two feed strengths during the study period. Results from this operation confirmed that the crystallization process could efficiently remove up to 90% of phosphate under both feed strength conditions. Operating pH and supersaturation ratio of the wastewater were controlling parameters. Crystals obtained grew on the surface of the seeding material, and they were mainly hydroxyapatite.

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