Page 244 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Coagulation 199

600 2. Regarding Al hydrolysis products, the organic
Fulvic acid ligands compete with OH .

500 Humic acid 9.3.2.4 Organics in Wastewaters
3. NOM adsorbs on amorphous Al(OH) 3 precipitate.
X (mg organic/g Al(OH) 3 ) 300 Wastewaters include municipal and industrial sources, which
400

include a wide variety of organic substances. Municipal was-
tewaters include fats, proteins, carbohydrates, urea, etc. Indus-
200
including synthetic organics, depending on the industry.
100 trial wastewaters may include variety of contaminants,
9.3.2.4.1 Composition of Municipal Wastewater
0 Table 9.3 shows a distribution of organic matter sizes in a
0 10 20 30 40 50
typical raw municipal wastewater and compared with the
Concentration of organic (mg/L)
same fractions for effluent after secondary settling (provided
FIGURE 9.5 Isotherm showing equilibrium concentrations of here for reference). As seen, organic matter is given as volatile
organics in solution and adsorbed to Al(OH) 3 . (Adapted from solids (VS), TOC, and chemical oxygen demand (COD),
Mazet, M. et al., Water Res., 24(12), 1509, December 1990.) which are different laboratory measures of organic matter in
water.
for HA and FA) in equilibrium with preformed aluminum
9.3.2.4.2 Chemical Treatment
hydroxide precipitate. Maximum capacities of the aluminum
Effluent from primary settled municipal wastewater can be
hydroxide precipitate for HA and FA were about 540 mg
rendered clear to the eye by alum coagulation, which can be
HA=g Al(OH) 3 and 560 mg FA=g Al(OH) 3 . About 0.8
demonstrated readily by a laboratory jar test. As noted
fraction of the uptake occurred within 30 min with stirring
(Section 9.3.1), lime and alum have long been recognized as
at 50 rpm.
agents that precipitate nonsettleable solids in water. In tertiary
9.3.2.3.4 Summary for NOM Removal treatment of municipal wastewaters, that is, effluent from
secondary clarifiers, lime is used frequently as a coagulant
Chemical reactions that relate to NOM removal (Edzwald,
(e.g., City of Colorado Springs, Rawhide Power Plant,
1993, p. 29) include
Wellington, Colorado).
1. Under neutral or acidic pH conditions, humic and FA
9.3.2.5 Coagulation of Synthetic Organics
organic ligands may complex with Al, creating a
demand for Al that must be satisfied before precipi- Regulated organics included some 615 contaminants
tation of Al(OH) 3 can occur. (Champlin and Hendricks, 1993). In a spiking experiment

TABLE 9.3
Distribution of Organic Matter in Wastewater and Secondary Effluent
VS TOC COD
Substance Fraction a (mg=L) (%) (mg=L) (%) (mg=L) (%)
Wastewater Soluble 116 48 46 42 168 40
Colloidal 23 9 12 11 43 10
Supra-colloidal 43 18 22 20 87 21
Settleable 59 25 29 27 120 29
Total 241 109 418
b
Secondary effluent Soluble 62 67 16.5 69 46 74
Colloidal 6 7 1.5 6 3 5
Supra-colloidal 24 26 6 25 13 21
Settleable 0 0 0 0 0 0
Total 92 24 62
Source: Adapted from Rickert, D.A. and Hunter, J.V., J. Water Pollut. Control Fed., 44(1), 135, January 1972.
a 3
Definitions: Settleable, >100 mm; supra-colloidal, 1–100 mm; colloidal, 10 –1 mm; soluble, <1 mm.
b
Treatment was conventional activated sludge.