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containing the PO4 ions. These three ions collectively are called orthophosphates. As in the case of the nitrogen forms ammonia, nitrite and nitrate, the orthophosphates can also cause eutrophication in receiving streams. Thus, concentrations of orthophosphates should be controlled through removal before discharging the wastewater into receiving bodies of water. The orthophosphates of concern in wastewater engineering are sodium phosphate (Na3PO4), sodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), and ammonium hydrogen phosphate [(NH4)2HPO4]. They cause the problems associated with algal blooms.

When phosphoric acid is heated, it decomposes losing molecules of water forming the P-O-P bonds. The process of losing water is called condensation, thus the term condensed phosphates and, because they have more than one phosphate group in the molecule, they are also called polyphosphates. Among the acids formed from the condensation of phosphoric acid are dipolyphosphoric acid or pyrophosphoric acid (H4P2O7), tripolyphosphoric acid (H5P3O10), and metaphosphoric acid (HPO3)n. Condensed phosphates undergo hydrolysis in aqueous solutions and transform into the orthophosphates. Thus, they must also be controlled. Condensed phosphates of concern in wastewater engineering are sodium hexametaphosphate [Na(PO3)6], sodium dipolyphosphate (Na4P2O7), and sodium tripolyphosphate (Na5P3O10).

When organic compounds containing phosphorus are attacked by microorganisms, they undergo hydrolysis into the orthophosphate forms. Thus, as with all the other phosphorus species, they have to be controlled before the wastewaters are discharged.

Orthophosphate can be determined in the laboratory by adding a substance that can form a colored complex with the phosphate. An example of such a substance is ammonium molybdate. Upon formation of the color, colorimetric tests may then be applied. The condensed and organic phosphates all hydrolyze to the ortho form, so they can also be analyzed using ammonium molybdate. The hydrolysis are normally done in the laboratory at boiling-water temperatures.

2.1.23 Acidity and Alkalinity

Acidity and alkalinity are two important parameters that must be controlled in the operation of a wastewater treatment plant. Digesters, for example, will not operate if the environment inside the tank is acidic, since microorganisms will simply die in acid environments. The contents of the tank must be buffered at the proper acidity as well as proper alkalinity.

Acidity is the ability of a substance to neutralize a base. For example, given the base OH- and a species HCO-, the reaction of the two species in water solution is

Thus, in the previous reaction, because HCO_ has neutralized OH_, it has acidity and it is an acid.

Alkalinity, on the other hand, is the ability of a substance to neutralize an acid. For example, given the acid HCl and the species HCO_, they react in solution as follows:

In the previous reaction, because HCO3_ has neutralized the acid HCl, it has alkalinity. Alkaline substances are also called bases. From the above two reactions of HCO_, it can be concluded that this species can act both as an acid and as a base. A substance that can act both as an acid and as a base is called an amphoteric substance.

Alkalinity in wastewaters results from the presence of the hydroxides, carbonates, and bicarbonates of such elements as calcium, magnesium, sodium, and potassium, and radicals like the ammonium ion. Of the elements, the bicarbonates of calcium and magnesium are the most common. The other alkalinity species that may be found, although not to a major extent as the bicarbonate, are HSiO-, H2BO-, HPO-2, and H2PO-. Alkalinity helps to resist the change in pH when acids are produced during the course of a biological treatment of a wastewater. Wastewaters are normally alkaline, receiving this alkalinity from the water supply and the materials added during domestic use. Alkalinity is determined in the laboratory by titration using a standard concentration of acid. The reverse is true for the determination of acidity in the laboratory.

2.1.24 Fats, Oils, Waxes, and Grease

Organic compounds with the general formula RC-OH are called organic acids,

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