The chemical constituents of wastewater are typically classified as inorganic and organic.
Inorganic chemical constituents of concern include nutrients, nonmetallic constituents, metals, and gases. Inorganic nonmetallic constituents considered in this section include pH, nitrogen, phosphorus, alkalinity, chlorides, sulfur, other inorganic constituents, gases, and odors. Metallic constituents are considered in Section 2–5.
Sources of Inorganic Nonmetallic Constituents
The sources of inorganic nonmetallic constituents in wastewater derive from the back- ground levels in the water supply and from the additions resulting from domestic use, from the addition of highly mineralized water from private wells and groundwater, and from industrial use. Domestic and industrial water softeners also contribute significantly to the increase in mineral content and, in some areas, may represent the major source. Occasion- ally, water added from private wells and groundwater infiltration will (because of its high quality) serve to dilute the mineral concentration in the wastewater. Because concentra- tions of various inorganic constituents can greatly affect the beneficial uses made of the waters, the constituents in each wastewater must be considered separately.
pH
Because the concentration of the species of most chemical constituents is dependent on the hydrogen-ion concentration in solution, the hydrogen-ion concentration is an important quality parameter of both natural waters and wastewaters. The usual means of expressing the hydrogen ion concentration is as pH, which is defined as the negative logarithm of the hydrogen ion concentration:
pH5 2log10[H1] (2–27) The concentration range suitable for the existence of most biological life is quite narrow and critical (typically 6 to 9). Wastewater with an extreme concentration of the hydrogen ion is difficult to treat by biological means, and if the concentration is not altered before
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discharge, the wastewater effluent may alter the concentration in the natural receiving waters.
For treated effluents discharged to the environment, the allowable pH range usually varies from 6.5 to 8.5.
The hydrogen ion concentration in water is connected closely with the extent to which water molecules dissociate. Water will dissociate into hydrogen and hydroxyl ions as follows:
H2O Sd H11 OH (2–28) Applying the law of mass action [Eq. (2–7)] to Eq. (2–28) yields
[H1][OH2]
[H2O] 5K (2–29) where the brackets indicate concentration of the constituents in moles per liter. Because the concentration of water in a dilute aqueous system is essentially constant, this concen- tration can be incorporated into the equilibrium constant K to give
[H1][OH2]5Kw (2–30) Kw is known as the ionization constant or ion product of water and is approximately equal to 1 3 10214 at a temperature of 25°C. Equation (2–30) can be used to calculate the hydroxyl ion concentration when the hydrogen ion concentration is known and vice versa.
With pOH, which is defined as the negative logarithm of the hydroxyl ion concentra- tion, it can be seen from Eq. (2–30) that, for water at 25°C,
pH 1pOH514 (2–31) The pH of aqueous systems typically is measured with a pH meter (see Fig. 2–14). Various pH papers and indicator solutions that change color at definite pH values are also used. The pH is determined by comparing the color of the paper or solution to a series of color standards.
Chlorides
Chloride is a constituent of concern in wastewater as it can affect the final reuse applications of treated wastewater. Chlorides in natural water result from the leaching of chloride con- taining rocks and soils with which the water comes in contact, and in coastal areas from saltwater intrusion. In addition, agricultural, industrial, and domestic wastewaters discharged to surface waters are a source of chlorides.
Figure 2–14
Typical meter used for the measurement of pH and specific ion concentrations.
Human excreta, for example, contain about 6 g of chlorides per person per day. In areas where the hardness of water is high, home regeneration–type water softeners will also add large quantities of chlorides. Because conventional methods of waste treatment do not remove chloride to any significant extent, higher than usual chloride concentrations can be taken as an indication that a body of water is being used for waste disposal. Infiltration of groundwater into sewers adjacent to saltwater is also a potential source of high chlorides as well as sulfates.
Alkalinity
Alkalinity in wastewater results from the presence of the hydroxides [OH2], carbonates [CO322], and bicarbonates [HCO32] of elements such as calcium, magnesium, sodium, potassium, and ammonia. Of these, calcium and magnesium bicarbonates are most com- mon. Borates, silicates, phosphates, and similar compounds can also contribute to the alkalinity. The alkalinity in wastewater helps to resist changes in pH caused by the addition of acids. Wastewater is normally alkaline, receiving its alkalinity from the water supply, the groundwater, and the materials added during domestic use. The concentration of alka- linity in wastewater is important where chemical and biological treatment is to be used (see Chaps. 6 and 7, respectively), in biological nutrient removal (see Chap. 8), and where ammonia is to be removed by air stripping (see Chap. 11 and 15).
Alkalinity is determined by titrating against a standard acid; the results are expressed in terms of calcium carbonate, mg/L as CaCO3. For most practical purposes alkalinity can be defined in terms of molar quantities, as
Alk, eq/m35meq/L5[HCO32]12[CO322]1[OH2] 2[H1] (2–32) The corresponding expression in terms of equivalents is
Alk, eq/m35(HCO32)1(CO322)1(OH2)2(H1) (2–33) In practice, alkalinity is expressed in terms of calcium carbonate. To convert from meq/L to mg/L as CaCO3 it is helpful to remember that
Milliequivalent mass of CaCO3 5 (100 mg/mmole)
(2 meq/mmole) (2–34)
550 mg/meq
Thus 3 meq/L of alkalinity would be expressed as 150 mg/L as CaCO3. Alkalinity, Alk as CaCO3 5 3.0 meq
L 3 50 mg CaCO3
meq CaCO3
5150 mg/L as CaCO3
Nitrogen
The elements nitrogen and phosphorus, essential to the growth of microorganisms, plants, and animals, are known as nutrients or biostimulants. Trace quantities of other elements, such as iron, are also needed for biological growth, but nitrogen and phosphorus are, in most cases, the major nutrients of importance. Because nitrogen is an essential building block in the synthesis of protein, nitrogen data will be required to evaluate the treatability of waste- water by biological processes. Insufficient nitrogen can necessitate the addition of nitrogen to make the waste treatable. Nutrient requirements for biological waste treatment are dis- cussed in Chaps. 7 and 8. Where control of algal growths in the receiving water is necessary, removal or reduction of nitrogen in wastewater prior to discharge may be desirable.
Sources of Nitrogen. The principal sources of nitrogen compounds are (1) the nitrogenous compounds of plant and animal origin, (2) sodium nitrate, and (3) atmospheric nitrogen. Ammonia derived from the distillation of bituminous coal is an example of
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nitrogen obtained from decayed plant material. Sodium nitrate (NaNO3) is found principally in mineral deposits in Chile and in the manure found in sea birds’ rookeries.
The production of nitrogen from the atmosphere is termed nitrogen fixation. Because fixation is a biologically mediated process and because NaNO3 deposits are relatively scarce, most sources of nitrogen in soil/groundwater are of biological origin.
Forms of Nitrogen. The chemistry of nitrogen is complex because of the several oxidation states that nitrogen can assume and the fact that changes in the oxidation state can be brought about by living organisms. To complicate matters further, the oxidation state changes brought about by bacteria can be either positive or negative depending upon whether aerobic or anaerobic conditions prevail. The oxidation states of nitrogen are summarized (Sawyer et al., 2003):
2III 0 I II III IV V
NH3 — N2 — N2O — NO — N2O3 — NO2 — N2O5 (2–37) The most common and important forms of nitrogen in wastewater and their corresponding oxidation state in the water/soil environment are ammonia (NH3, 2III), ammonium (NH41, 2III), nitrogen gas (N2, 0), nitrite ion (NO22, 1III), and nitrate ion (NO32, 1V).
The oxidation state of nitrogen in most organic compounds is 2III.
Total nitrogen, as reported in Table 2–6, is composed of organic nitrogen, ammonia, nitrite, and nitrate. The organic fraction consists of a complex mixture of compounds including amino acids, amino sugars, and proteins (polymers of amino acids). The com- pounds that comprise the organic fraction can be soluble or particulate. The nitrogen in these compounds is readily converted to ammonium through the action of microorganisms in the aquatic or soil environment. Urea, readily converted to ammonium carbonate, is seldom found in untreated municipal wastewaters.
Organic nitrogen is determined analytically using the Kjeldahl method. The aqueous sample is first boiled to drive off the ammonia, and then it is digested. During digestion the organic nitrogen is converted to ammonium through the action of heat and acid. Total Kjeldahl nitrogen (TKN) is determined in the same manner as organic nitrogen, except that the ammonia is not driven off before the digestion step. Total Kjeldahl nitrogen is, there- fore, the total of the organic and ammonia nitrogen. An alternative method is the perfulfate digestion procedure in which organic nitrogen is oxidized to nitrate nitrogen at high Table 2–6