Page 715 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

670 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

in water systems that have such a problem. As stated previ-
ously, MnO 4 is a strong oxidant that in itself may oxidize TABLE 21.5

free Mn 2þ (in solution) to MnO 2 . Reductions in Metal Concentrations in Industrial
Wastes a
21.3 PRACTICE Parameter Concentrations in mg=L
Metal Cadmium Copper Chromium Nickel Zinc
Removal of hardness and heavy metals involves first adding
anions that are both cheap and have low solubility product Raw feed 4805 4136 26,000 18,617 259.865
when combined with the target ion. After the reaction has Lime only b 103 673 58 1,561 6,629
occurred, the task is to cause particle growth to form a Lime-sulfide c 8 70 86 751 422
settleable floc with polishing=removal by filtration.
Source: Robinson, A. K. and Sum, J. C., Sulfide precipitation of heavy
metals, Report EPA-600=2-80-139, Industrial Environmental
21.3.1 LIME SOFTENING Research Laboratory, Office of Research and Development, U.S.
Environmental Research Laboratory, Cincinnati, OH, 1980.
Factors involved in the lime softening precipitation process a Treatment train includes rapid-mix, settling, filtration.
include temperature, return of precipitated sludge, mixing b Lime only refers to the addition of calcium hydroxide after pH adjustment,
turbulence, and time of mixing. Most often the process is i.e., 8.0 pH 11.0.
carried out in a reactor clarifier, which involves rapid mix in c Lime-sulfide refers to adding soluble sulfide with the lime in the rapid mix.
a center feed well, removal of solids by means of a sludge
pocket at the bottom center, upward passage of the precipitate
suspension through a sludge blanket, overflow of the clarified and filtration (Banerjee, 2002, p. 181). Sulfide is an effective
water from peripheral weirs (or weirs placed in a radial alternative to hydroxide for removals of Cd, Cu, Zn, As, Se,
configuration). Filtration is recommended as a final process etc. Removals to 0.1 mg=L may be expected. Sources of
in the treatment train. sulfide include sodium sulfide, hydrosulfide, or the slightly
Alternatively, for large installations involving hard soluble ferrous sulfide introduced as slurry. On the negative
groundwater, the same treatment train as alum coagulation side, hydrogen sulfide gas, and sulfide toxicity and odor are
may be used for precipitation softening, but dedicated to potential problems (Banerjee, 2002, p. 182).
softening, that is, rapid-mix with lime added, flocculation, Hydroxide treatment with lime after pH adjustment to 8.0
settling, and filtration. For hard surface water, softening may pH 11.0 is a standard treatment to remove heavy metals
be carried out simultaneously with alum coagulation, with (Robinson and Sum, 1980). The process includes rapid-mix,
lime being added in the rapid-mix with alum. settling, and filtration. In experiments with five metals, that is,
cadmium, copper, chromium, nickel, and zinc, the concentra-
tions were reduced as indicated in Table 21.5. As seen, the
21.3.2 PRECIPITATION OF HEAVY METALS
sulfide polishing generally gives lower effluent concentrations,
Mining sites are a source of heavy metals, for example, As, Cd, which is explained by the solubility products being lower than
Hg, Pb, Se, and anions such as F ,SO 4 , also Fe, Ni, Zn, Cu, the hydroxides. Raw water concentrations are quite high in the

2
are not uncommon. In most cases, the drainage from mines is case of chromium, nickel, and zinc.
acidic and is likely to come from a reducing environment, that Figure 21.3 shows the tanks for treatment of a mining
is, low redox potential. For example, iron is likely to be in the waste, which is classed as acid mine drainage with pH < 4;
ferrous state, that is, Fe . If and when such water reaches the
2þ
surface, oxidation to Fe 3þ may occur. As a next step, Fe(OH) 3
will form, depending also on pH (see, e.g., the Pourbaix dia-
gram for iron such as shown in Snoeyink and Jenkins, 1980,
p. 362), causing deposits on the stream bed, with esthetic and
ecological consequences (e.g., red deposits on rocks and clog-
ging of insect niches in the gravel stream bottom, respectively).
Similar reactions occur with Pb, Zn, Cd, and Hg, except that the
elements are ‘‘heavy metals,’’ and are toxic.

21.3.2.1 Common Chemical Reactions
The standard treatment method for removal of heavy metals
from wastewater is chemical precipitation with hydroxide,
sulfide, or carbonate anions. The most common anion used
is hydroxide, with lime being the cheapest source and easiest
to use in operation (Banerjee, 2002, p. 181). Treated effluent
concentrations of metals such as copper, lead, and zinc are FIGURE 21.3 Photograph of settling. (Courtesy of John H. Smith
typically 0.5 mg=L by hydroxide precipitation after settling III, Sepco, Inc., Fort Collins, CO.)