Phase equil. calcns. gave crystn. fields for sparingly sol. salts esp. struvite, pptg. from the solns. contg. NH4+, Mg2+ and HPO42- ions. At NH4+/PO43-/Mg2+ = 1:1:1 (by moles) starting concns., the struvite ppt. was max. at pH 9.6. The theoretical concns. of NH4+ and P at equil. with struvite are 17 mg N and 37 mg P/dm(3), resp. In N-P-K fertilizer and steel-phopshating waste liquors, pptn. of struvite reduced the NH4+ (420-650) and P (1300-4500 mg/dm(3)) concns. by 97 and > 99% (to 38 mg N and 19 mg P/dm(3)), resp. At an ionic strength of 0.5, struvite was best pptd. at pH 7.4-9.6. Low NH4+ and P concns., e.g., 40 mg N and 10 mg P/dm(3) in a crude sewage, made struvite pptn. impossible.