The sediment was dried either in an oven at 90°C or by freeze-drying. An appropriate amount of sediment (0.1-0.5 g) was treated with 20 mL of 0.002 M phosphate solution in a ultrasonic bath for 10 min. The suspension was filtered through 0.2ím filters (Sarto-rius) and analyzed for EDTA as described above.