Residuals Management - Public Water Systems - page 753

Note that for the case where r 1, Equation 23-8 reduces to Equation 23-7.
Use of the 100 percent rejection assumption for reverse osmosis systems will be
conservative in overestimating the concentration of the concentrate, and will result in
a worst-case scenario. This approach may also be used for EDR systems. The errors
should be in the range of 15 percent and under. For NF membrane systems, assumption
of complete rejection of all species leads to more significant errors. Rejections of 70
and 90 percent should be used, respectively, for monovalent and multivalent ions. 1

Characteristics of Sludges
Characteristics of water treatment plant solids affect handling, thickening, dewatering,
conveying, and disposal processes that reduce the volume of the sludge to produce a
material suitable for disposal or recovery.

Coagulant Sludges

Alum Sludges. In the absence of significant organic pollution of the raw-water, co-
agulant sludges are essentially biologically inert and have a near-neutral pH. The
sludge is generally thixotropic (the plastic nature of the sludge changes with agitation)
and gelatinous. However, coagulant sludges from plants treating river water with a
fairly high silt content are not as gelatinous as sludge from plants obtaining raw water
from clear mountain streams, lakes, or quiescent reservoirs.
Coagulant sludges such as alum sludge may be characterized at varying solids
contents, as shown in Table 23-9.
Various raw-water quality parameters and sludge characteristics are shown in Table
23-10 for five water treatment plants in the United States. The Moline, Illinois, water
also required a lime dose of 141 mg / L. The Washington, D.C., water was low in color,
but had a high suspended solids level, and turbidities ranging as high as 160 NTU. In
considering recovery of alum, the aluminum content and dissolved inorganic and or-
ganic solids are of importance. 20
The effects of the type of coagulation mechanism on alum sludge properties are
shown in Table 23-11. 21 The data show that the solids content after gravity settling
and vacuum dewatering depends on the raw-water turbidity, coagulant dose, and co-
agulation mechanisms. Because the pH at which coagulation occurs will determine the
coagulation mechanism, pH will also affect the residuals solids content.
Another factor influencing the dewaterability of alum sludges is floc density. Settled
and dewatered cake solids increase as the suspended solids level in the floc increases. 1
In a study about the effects of organic matter on floc density, when flocs had more
organic matter (as indicated by the amount of TOC removed), their density declined,
dewatering rates decreased,and dewatered cake solids decreased (see Fig. 23-2).

TABLE 23-9. Alum Sludge Characteristics
Solids Content, Percent Sludge Character

0-5 Liquid

8-12 Spongy, semisolid

18-25 Soft clay

40-50 Stiff clay