Wash membrane filters by soaking in distilled water before use because they may contribute significant amounts of phosphorus to samples containing low concentrations of phosphate Use one of two washing techniques: (a) soak 50 filter in 2 L distilled water for 24 h (b) soak 50 filters in 2 L distilled water for 1 h, change distilled water, and soak filters an additional 3 h.