Phosphorus recovery from anaerobic digester supernatants using a pilot-scale struvite crystallization process

The performance of a pilot-scale, struvite crystallization process, for recovering phosphorus from various supernatants, was investigated. Using synthetic supernatant, as well as anaerobic digester supernatants from two full-scale wastewater treatment plants located in Vancouver, B.C., Canada, it was found that over 90% phosphate removal efficiency was achieved through controlling the supersaturation ratio, operational pH and magnesium dosage in the supernatant. With a high supersaturation ratio, or high operational pH, or high magnesium dosage, excellent phosphate removal was achieved. However, an operational pH greater than 8 was not necessary;; it was possible to control the process using a high magnesium dosage in place of a higher operating pH, but without a compromise in phosphate removal. Chemical analysis of the harvested crystals indicated very pure struvite (in excess of 91%), with only small amounts of residual calcium, carbonate, and trace metals. Most crystals exceeded 2 mm in mean diameter.