12 Characterization of Particles in Digested
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Sewage Sludge W I L L I A M K. F A I S S T
1
Keck Laboratories, California Institute of Technology, Pasadena, C A 91125 Some physical properties of several digested sludges were measured to better characterize the sludge particle system. The particle size distributions of two sludges were meas ured with a Coulter Counter. Number counts were in excess of 10 /L and the majority of the particles had diameters less than 5 μm. Particle surface areas were at least 20 m /L. Particle sizing by filtration was attempted but proved unsuc cessful because of membrane pore clogging. Sedimentation experiments for sludge in artificial seawater showed decreas ing sedimentation velocities with increasing dilution (and hence lower particle number concentration). A comparison of sedimentation velocities for four sludges from different sources showed that sedimentation velocities increased with increasing solids content. Data presented suggest that flocculation of the sludges in seawater increased the sedi mentation velocities. 12
2
O e w a g e sludge is the liquid-solids suspension resulting from the sedi^ mentation phase of wastewater treatment. Quantities of sludge produced in the United States are increasing as the population grows and as wastewater treatment facilities are upgraded i n an attempt to improve receiving water quality. Treatment and disposal are difficult and costly for sludge because of its contrary physicochemical nature and because it contains most of the trace metals and persistent synthetic organics from the influent sewage. Sludge treatment and disposal often represent one-quarter to one-half the total cost of wastewater treatment. Current address: Brown and Caldwell, Consulting Engineers, 1501 North Broad way, Walnut Creek, C A 94596. 1
0-8412-0499-3/80/33-189-259$06.00/0 © 1980 American Chemical Society
Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
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260
PARTICULATES
IN WATER
Sludge suspensions are typically less than 5 % solids by weight, but the solids are usually the most important part of the system. They may contain more than 99% of trace metals such as cadmium, zinc, and lead ( I ) , 95% of organics such as dichlorodiphenyltrichloroethane ( D D T ) and polychlorinated biphenyls ( P C B ) (2), and substantial amounts of other organics. Pathogenic agents such as the eggs of the parasite Ascaris lumbrociodes are also found in sludge (3). The particle size distribution has been found by Karr and Keinath (4) to influence sludge dewaterability strongly. Sludge particulate matter has also been impli cated in the alteration of the ocean bottom and overlying water column near the Los Angeles County Sanitation District's ( L A C S D ) White Point effluent outfalls ( 5 , 6 , 7 ) . The size distribution of sludge particles w i l l directly affect its physical behavior on ocean discharge and its impact on many kinds of marine life. In the presentation that follows, experiments to characterize the sludge particle system, including particle size and particle density, are described, and particle size distributions are reported. Quiescent sedi mentation experiments for several sludges in seawater are also described. These experiments were carried out to better understand the general nature of the sludge particle system and to use as inputs for specific modeling studies for sludge discharges to the ocean. Since sludge behavior in the complicated ocean system is affected by both physical and chemical interactions not easily described mathematically nor easily simulated in the laboratory, this work attempted to bring together several types of information on the sludge particles to develop simple yet reason able assumptions for modeling efforts (see Ref. 1 for further details). The detailed particle size information should also be useful when considering the particularly vexing problems of thickening or dewatering digested sludges. Sludge Physical Properties The most basic measure of the solids in sludge is total residue on evaporation ( T R O E ) , which includes both the suspended and dissolved matter remaining after evaporation at 105°C. The dissolved fraction— nonfilterable residue on evaporation ( N R O E ) — i s typically less than 2,000 mg/L, vs. 20,000-50,000 mg/L for digested sludges T R O E . T R O E and percent volatile residue for the nine sludge samples used in this work are given in Table I. The percent volatile is fairly constant for sludges from the three sources, reflecting good stabilization as a result of anaerobic digestion. Particle Size and Particle Density. A n introduction to the impor tance of particle size distribution for sludge has already been presented.
Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
12.
FAISST
261
Particles in Digested Sewage Sludge Table I.
Digested Sludge Solids Analyses Solids TROE
Percent Volatile
Hyperion Digester 5C
.02374
57.1
LACSD Primary
.02429
51.9
0
.02528
52.3
d
.02880
54.8
.02402
58.2
Hyperion Thermophilic*
.01831
56.6
Hyperion Mesophilic*
.02452
55.6
LACSD Primary'
.02315
54.0
OCSD
.02839
45.9
Source
a
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6
LACSD Primary
0
b
LACSD Primary
b
Hyperion Digester 3 C
d
b
a b c d c T
e r
TROE is the total residue on evaporation, expressed as g solids/g wet sludge. LACSD stands for County Sanitation Districts of Los Angeles County. Used for shallow columns sedimentation experiments. Used for particle sizing by Coulter Counter. Used for tall column sedimentation experiments. OCSD stands for Orange County Sanitation District.
Particle size together with particle density also affect the terminal settling velocity of particles released in the water column. F o r particles settling in environments where the Reynolds number: R
pdv
B
(1)
is less than 0.5, Stokes L a w holds:
18
d
2
(2)
where v is the particle settling velocity; g the acceleration of gravity; p the mass density of particle; p the mass density of fluid; p. the absolute viscosity; and d the particle diameter. B
s
Kavanaugh and Leckie; Particulates in Water Advances in Chemistry; American Chemical Society: Washington, DC, 1980.
262
PARTICULATES
IN WATER
Both p and p. vary with temperature. For natural materials, p varies from close to 1.0 g/cm for some biological growths to 2.65 g/cm for typical siliceous minerals to about 4.0 g/cm for garnet sands. For Stokes L a w v is proportional to d , where particle diameter is the more sensitive parameter. The importance of the density term (p — p) on sludge sedimentation is difficult to evaluate but is most critical where p approaches p, for example, with biological floes. The solids in sewage have been classified classically by size accord ing to Rudolfs and Balmat (8) as: s
3
3
3
2
s
s
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s
1. 2. 3. 4.
Settleable solids Supracolloidal solids Colloidal solids Soluble solids
>100 /xm 1 /xm-100 /xm 1 m/xm-1 /xm
