Pilot-scale struvite crystallisation tests using anaerobic effluent from potato processing industries were performed at three different plants. Two plants (P1 & P2) showed high phosphate removal efficiencies, 89 + 3% and 75 + 8%, resulting in final effluent levels of 12 + 3 mg PO4/3- -P/L and 11 + 3 mg PO4/3- -P/L, respectively. In contrast, poor phosphate removal (19 + 8%) was obtained at the third location (P3). A noticeable difference in the influent Ca2+/PO4/3- -P molar ratio was observed between the test sites, ranging from 0.27 + 0.08 (P1), 0.62 + 0.18 (P2) and 0.41 + 0.04 (P3). A negative effect on struvite formation occurred when a Ca2+/PO4/3- -P molar ratio of 1.25 + 0.11 was obtained after initial pH increase in the stripper at P3. A full-scale struvite plant treating 90–110 m3/h of anaerobic effluent from a diary industry also showed Ca2+ interference. Initially in this plant, influent phosphate levels ranging from 40 to 45 mg PO4/3- -P/L were decreased to below 10 mg PO4/3- -P/L, but no struvite was produced. A shift in Ca2+/PO4/3- -P molar ratio from 2.69 to 1.36 by an increased phosphate concentration resulted in average total phosphorus removal of 78 + 7%, corresponding with effluent levels of 14 + 4 mg Ptotal/L(9 + 3 mg PO4/3- -P/L). Under these conditions pure spherical struvite pellets of 2–6 mm were produced
