Fosforo Apha

Standard Methods for the Examination of Water and Wastewater

4500-P

PHOSPHORUS*#(1)

4500-P A.

Introduction

1. Occurrence Phosphorus occurs in natural waters and in wastewaters almost solely as phosphates. These are classified as orthophosphates, condensed phosphates (pyro-, meta-, and other polyphosphates), and organically bound phosphates. They occur in solution, in particles ordetritus, or in the bodies of aquatic organisms. These forms of phosphate arise from a variety of sources. Small amounts of orthophosphate or certain condensed phosphates are added to some water supplies during treatment. Larger quantities of the same compounds may be added when the water is used for laundering or other cleaning, because these materials are major constituents of many commercialcleaning preparations. Phosphates are used extensively in the treatment of boiler waters. Orthophosphates applied to agricultural or residential cultivated land as fertilizers are carried into surface waters with storm runoff and to a lesser extent with melting snow. Organic phosphates are formed primarily by biological processes. They are contributed to sewage by body wastes and food residues, andalso may be formed from orthophosphates in biological treatment processes or by receiving water biota. Phosphorus is essential to the growth of organisms and can be the nutrient that limits the primary productivity of a body of water. In instances where phosphate is a growth-limiting nutrient, the discharge of raw or treated wastewater, agricultural drainage, or certain industrial wastes to thatwater may stimulate the growth of photosynthetic aquatic micro- and macroorganisms in nuisance quantities. Phosphates also occur in bottom sediments and in biological sludges, both as precipitated inorganic forms and incorporated into organic compounds. 2. Definition of Terms Phosphorus analyses embody two general procedural steps: (a+) conversion of the phosphorus form of interest to dissolvedorthophosphate, and (b) colorimetric determination of dissolved orthophosphate. The separation of phosphorus into its various forms is defined analytically but the analytical differentiations have been selected so that they may be used for interpretive purposes. Filtration through a 0.45-µm-pore-diam membrane filter separates dissolved from suspended forms of phosphorus. No claim is made thatfiltration through 0.45-µm filters is a true separation of suspended and dissolved forms of phosphorus; it is merely a convenient and replicable analytical technique designed to make a gross separation.
© Copyright 1999 by American Public Health Association, American Water Works Association, Water Environment Federation

Standard Methods for the Examination of Water and Wastewater
Membrane filtrationis selected over depth filtration because of the greater likelihood of obtaining a consistent separation of particle sizes. Prefiltration through a glass fiber filter may be used to increase the filtration rate. Phosphates that respond to colorimetric tests without preliminary hydrolysis or oxidative digestion of the sample are termed ‘‘reactive phosphorus.’’ While reactive phosphorus is largelya measure of orthophosphate, a small fraction of any condensed phosphate present usually is hydrolyzed unavoidably in the procedure. Reactive phosphorus occurs in both dissolved and suspended forms. Acid hydrolysis at boiling-water temperature converts dissolved and particulate condensed phosphates to dissolved orthophosphate. The hydrolysis unavoidably releases some phosphate from organiccompounds, but this may be reduced to a minimum by judicious selection of acid strength and hydrolysis time and temperature. The term ‘‘acid-hydrolyzable phosphorus’’ is preferred over ‘‘condensed phosphate’’ for this fraction. The phosphate fractions that are converted to orthophosphate only by oxidation destruction of the organic matter present are considered ‘‘organic’’ or ‘‘organically bound’’...