Parameter
Henry’s constant,
atm
Henry’s constant, unitless
Temperature coefficients
A B
Air 66,400 49.68 557.60 6.724
Ammonia 0.75 5.61 3 1024 1887.12 6.315
Carbon dioxide 1420 1.06 1012.40 6.606
Carbon monoxide 53,600 40.11 554.52 6.621
Chlorine 579 0.43 875.69 5.75
Chlorine dioxide 1500 1.12 1041.77 6.73
Hydrogen 68,300 51.10 187.04 5.473
Hyrogen sulfide 483 0.36 884.94 5.703
Methane 37,600 28.13 675.74 6.880
Nitrogen 80,400 60.16 537.62 6.7392
Oxygen 41,100 30.75 595.27 6.644
Ozone 5300 3.97 1268.24 8.05
Sulfur dioxide 36 2.69 3 1022 1207.85 5.68
a Adapted in part from Crittenden et al. (2012), Cornwell (1990), and Hand et al. (1998).
2–4 Inorganic Nonmetallic Constituents 101
The unitless form is obtained by noting that at 1.0 atm pressure and 0°C, the volume occupied by 1.0 mole of air is 22.414 L. At other temperatures, 1.0 mole of air is equal to 0.082 T L of air, where T is temperature in Kelvin, K (273.15 1 °C). Using these conver- sions, the unitless form of Henry’s law is
Hu5 cHatm (mole gas/mole air)
(mole gas/mole water) d a mole air
0.082 T Lb a L
55.6 mole waterb
Hu5 a H
4.559 Tb
(2–50) For example, at 20°C, Hu equals
Hu5 c H
4.559(273.15120)d 5H3(7.4931024) at 208C
If atmospheric conditions prevail and Henry’s constant is expressed in terms of atm?m3/ mole (another form of Henry’s law used commonly in the literature), the unitless form of Henry’s law is obtained as follows:
Hu5 H
RT (2–51)
where Hu5 Henry’s law constant, unitless as used in Eq. (2–49)
H 5 Henry’s law constant values expressed in atm?m3/mole
R 5 universal gas law constant, 0.00008205 atm?m3/mole?K
T 5 temperature, K 5 273.15 1 °C
EXAMPLE 2–7 Saturation Concentration of Oxygen in Water What is the saturation of
oxygen in water in contact with dry air at 1 atm and 20°C?
Solution–Method 1 Using Eq. (2–46) 1. Dry air contains about 21 percent oxygen by volume (see Appendix B). Therefore,
pg5 0.21 mole O2/mole air 2. Determine xg.
a. From Table 2–7, at 20°C, Henry’s constant is
H54.113104atm (mole gas/mole air)
(mole gas/mole water) b. Using Eq (2–46), the value of xg is
xg5PT H pg
5 1.0 atm
4.113104atm (mole gas/mole air)
(mole gas/mole water)
(0.21 mole gas/mole air)
55.1131026 mole gas/mole water
3. One liter of water contains 1000 g/(18 g/mole) 5 55.6 mole, thus
ng
ng1nw
55.1131026
ng
ng155.6 55.1131026
Because the number of moles of dissolved gas in a liter of water is much less than the number of moles of water,
ng155.6<55.6 and ng<(55.6)5.1131026
ng<2.8431024 mole O2/L 4. Determine the saturation concentration of oxygen.
Cs<
a2.8431024 mole O2
L b a 32 g
mole O2
b a103 mg 1 g b (1 g/103 mg)
<9.09 mg/L
Solution–Method 2 Using Eq. (2–49) 1. The density of air at 20°C from Appendix B is 1204 kg/m3.
2. The percent of oxygen in air from Appendix B is about 23.18 percent oxygen by weight.
3. Determine the saturation concentration of oxygen.
a. From Table 2–7, at 20°C, the unitless form of Henry’s constant is
Hu5 30.75 b. Using Eq (2–49), the value of Cs is
Cs5 Cg
Hu
Cs5 (1.204 kg/m3)(103 g/kg)(0.2318)
30.75 59.08 g/m359.08 mg/L
Comment The computed values (9.09 and 9.08 mg/L) are essentially the same as the value given in
Appendix E (9.09 mg/L). It should be noted that the values for the Henry’s law constant given in Table 2–7 will vary depending on the source and the method used to derive them.
Also, the relationship at different temperatures is not linear.
Oxygen (O2). Dissolved oxygen (DO) is required for the respiration of aerobic
microorganisms as well as all other aerobic life forms. However, O2 is only slightly soluble in water. The actual quantity of O2 (and other gases too) that can be present in a solution is governed by (1) the solubility of the gas, (2) the partial pressure of the gas in the atmosphere, (3) the temperature, and (4) the concentration of the impurities in the water
2–4 Inorganic Nonmetallic Constituents 103
(e.g., salinity, suspended solids). The interrelationship of these variables is delineated in Chap. 6 and is illustrated in Appendix E, where the effect of temperature and salinity on DO concentration is presented.
Because the rate of biochemical reactions that use O2 increases with increasing temperature, dissolved oxygen levels tend to be more critical in the summer months. The problem is compounded in summer months because stream flows are usually lower, and thus the total quantity of O2 available is also lower. The presence of DO in wastewater is desirable because it prevents the formation of noxious odors. The role of O2 in wastewater treatment is discussed in Chaps. 5, 7, 8, and 9.
Hydrogen Sulfide (H2S). Hydrogen sulfide is formed, as mentioned previously,
from the anaerobic decomposition of organic matter containing sulfur or from the reduc- tion of mineral sulfites and sulfates. It is not formed in the presence of an abundant supply of oxygen. This gas is a colorless, inflammable compound having the characteristic odor of rotten eggs. Hydrogen sulfide is also toxic, and great care must be taken in its presence.
High concentrations of H2S can overwhelm olfactory glands, resulting in a loss of smell.
This loss of smell can lead to a false sense of security that is very dangerous. The black- ening of wastewater and sludge usually results from the formation of H2S that has com- bined with the iron present to form ferrous sulfide (FeS). Various other metallic sulfides are also formed. Although H2S is the most important gas formed from the standpoint of odors, other volatile compounds such as indol, skatol, and mercaptans, which may also be formed during anaerobic decomposition, may cause odors far more offensive than that of H2S.
Methane (CH4). The principal by-product from the anaerobic decomposition of the
organic matter in wastewater is methane gas (see Chaps. 10 and 13). Methane is a color- less, odorless, combustible hydrocarbon of high fuel value. Normally, large quantities of CH4 are not encountered in untreated wastewater because even small amounts of oxygen tend to be toxic to the organisms responsible for the production of CH4. Occasionally, however, as a result of anaerobic decay in accumulated bottom deposits, CH4 has been produced. Because methane is highly combustible and the explosion hazard is high, access ports (manholes) and sewer junctions or junction chambers where there is an opportunity for gas to collect should be ventilated with a portable blower during and before the time required for operating personnel to work in them for inspection, renewals, or repairs. In treatment plants, CH4 is produced from the anaerobic treatment process used to stabilize wastewater sludges (see Chap 13). In treatment plants where CH4 is produced, notices should be posted about the plant warning of explosion hazards, and plant employees should be instructed in safety measures to be maintained while working in and about the structures where CH4 may be present. Methane is also a serious greenhouse gas with an impact of over 25 times that of CO2 (U.S. EPA, 2008).
Odors
Odors in domestic wastewater are usually caused by gases produced by the decomposition of organic matter or by substances added to the wastewater. Fresh wastewater has a distinc- tive, somewhat disagreeable odor, which is less objectionable than the odor of wastewater which has undergone anaerobic (devoid of oxygen) decomposition. The most characteris- tic odor of stale or septic wastewater is that of hydrogen sulfide, which, as discussed previ- ously, is produced by anaerobic microorganisms that reduce sulfate to sulfide. Industrial wastewater may contain either odorous compounds or compounds that produce odors
during the process of wastewater treatment. The management of odors from wastewater treatment plants is considered in Chap. 16.
Public Concern. Odors have been rated as the foremost concern of the public relative to the implementation of wastewater treatment facilities. Within the past few years, the control of odors has become a major consideration in the design and operation of waste- water collection, treatment, and disposal facilities, especially with respect to the public acceptance of these facilities. In many areas, projects have been rejected because of the concern over the potential for odors. In view of the importance of odors in the field of wastewater management, it is appropriate to consider the effects they produce, how they are detected, and their characterization and measurement.
Effects of Odors. The importance of odors at low concentrations in human terms is related primarily to the psychological stress they produce rather than to the harm they do to the body. Offensive odors can cause poor appetite for food, lowered water consumption,
Table 2–8