Evaluation of the methods used for the determination of orthophosphate and total phosphate in activated sludge extracts

The standard orthophosphate method for water could be used, but attempts to increase its range are likely to introduce errors due to turbidity formation, especially with samples at high ionic strength. The standard persulphate digestion procedure for total phosphate (TP) was found to give slightly inferior recoveries from activated sludge samples relative to a procedure in which the acid and persulphate concentrations were raised, and in which samples of high TP were prediluted. Irrespective of the latter, a post-digestion dilution is necessary to avoid interferences in the subsequent orthophosphate determination due to the higher ionic strength and acidity of the digest. Acid-hydrolysable phosphates at concentrations ca. 1 to 75 mg P/ℓ (as TP) interfere positively in the standard molybdate-ascorbic acid orthophosphate determination, and to approximately the same extent in related methods which were designed to limit such interference. Additional study results are discussed.