Environ. Sci. Technol. 1984, 18, 119-121
Sewage Sludge Coagulatlon and Settling in Seawater James R. Hunt* and Jltendra D. Pandya
Division of Sanitary, Environmental, Coastal, and Hydraulic Engineering, Department of Civil Engineering, University of California, Berkeley, California 94720 ~~
Sewage sludge removal from seawater is shown to be a continuous process of aggregate production by coagulation followed by settling of the largest aggregates. Anaerobically digested sewage sludge coagulation experiments in artificial seawater were conducted at fluid shear rates of 0, 0.25, 0.5, 1, 2, 4, and 8 s-l with a rotating cylinder apparatus. The sludge removal rate was second order in sludge concentration and dependent on the fluid shear rate. The settling velocity of aggregates produced at a given shear rate was found to be constant.
Introduction Many human activities generate waste products that are frequently discharged into oceanic and coastal waters. Typical waste products include sewage effluent, sewage sludge, dredged sediments, industrial wastes, drilling muds from offshore oil exploration, and resuspended bottom sediments during mining operations. As a result, particles and pollutants associated with particles are introduced into oceanic waters and the fate of these particles must be known to adequately evaluate disposal alternatives. Hunt (1) has demonstrated the importance of particle coagulation in controlling particle removal from seawater. The purpose of this note is to present particle coagulation kinetic information for an anaeobically digested sewage sludge in artificial seawater and to show that the settling velocity of particle aggregates can also be determined. Such data are required as input to models predicting the fate of particles in oceanic waters. The recent modeling efforts of Jackson (2) and Koh (3) do not account for the dominance of particle coagulation in the marine environment. Morel and Schiff (4) and Hunt (5) have shown in rather different ways that overall particle removal by coagulation and settling has a second-order dependence on particle concentration at a fixed verticle distance from the surface, 2:
where C is the total suspended particle mass per fluid volume and b is the rate parameter dependent on fluid and particle properties. Equation 1can be integrated at a fiied depth, z , from time tl to tz: i
This second-order result is obtained once the aggregate size distribution has reached a quasi-dynamic steady state. As particles coagulate through the distribution, the shape of the size distribution is maintained but the magnitude decreases. Particle coagulation by Brownian motion, fluid shear, and possibly diferential sedimentation transports particles through the size distribution up to a size where aggregate settling becomes dominant, and particles are removed from the fluid volume (6). When coagulation is balanced by settling, the mass balance expression is
ac -- -us--ac _ at
0013-936X/84/0918-0119$01.50/0
az
(3)
where usis the aggregate settling velocity. For a fixed time tl, eq 3 can be integrated by using eq 1 from a depth z1 to depth z 2 giving
(4) If simultaneous measurements of concentration are obtained at two depths then the aggregate settling velocity can be obtained once b is known. A simpler procedure is possible if simultaneous measurements of particle concentration are obtained over time. If eq 2 is evaluated at z = zz and the expression for C(zz,tl)is substituted into eq 4, we obtain
For an appropriate choice of t z and tl so that C(zl,tl) = C(zz,t2),eq 5 becomes
The time lag, t2 - tl, required for the concentration at depth zz to reach the concentration at depth z1provides a procedure for determining the aggregate settling velocity.
Experimental Methods Experiments were conducted with sewage sludge in seawater to test the validity of eq 2 and 6, to measure the rate parameter at different fluid shear rates and to determine the settling velocity of the aggregates. Anaerobically digested primary and waste-activated sewage sludge was obtained from the East Bay Municipal Utility District, Oakland, CA. The sludge was screened through a 60-mesh sieve followed by a 200-mesh sieve to remove all particles greater than 74 pm. All experiments used this sample of sludge that was stored at 4 "C. The artificial seawater was prepared from the recipe of Lyman and Fleming (7) and was filtered through 0.22-pm Millipore filters to remwe suspended particles. The fluid temperatures were in the range 22-25 O C . The coagulation experiments were conducted in the annular gap between two concentric cylinders where the fluid shear rate was controlled by the outer cylinder rotation rate. The inner cylinder's diameter was 8.0 cm with an annular gap of 0.9 cm and a height of 15 cm. The cylinders were aligned vertically to allow aggregate settling following production. The experiments were started by Teleasing a few milliliters of sieved sludge into artificial seawater stirred a few seconds by a magnetic mixing bar. The mixed sludgeseawater suspension was then poured into the gap between the rotating cylinders with little flotation of sludge particles. During each experiment 5-mL samples were withdrawn at various times at fixed depth(s) below the liquid level. Shaken samples were measured for light transmittance at 500 nm in a 1-cm quartz cell. Optical absorbance was related to sludge concentration by means of a Calibration curve developed from sludge-in-seawatersolutions of known mass concentration. During one experiment,
0 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 2, 1984 119
O.Oz5
r I
Table I. Sludge Removal Rate Parameter as a Function of Fluid Shear Rate
(a 1.5 cm
G, s - ' 0 0.25 0.5 1
0.005
I
0
I
I
I
1
I
1
IO
20
30
40
50
60
TIME (min)
I
I
I
IO
20
30
I 40
1
J
50
60
TIME (mtn) Figure 1. Sewage sludge coagulation at G = 2 s-'. (a) Sampling depth Of 1.5 Cm (0) gives C-' = 1.13 X IO-* (3.43 X 10-')t, and at a (3.49 X 10-6)t. (b) depth of 5.0 cm ( O ) , C-' = 0.99 X Sampling depth of 1.5 cm ( 0 )gives C-' = 1.19 X -I-(3.25 X 10-')t, and at a depth of 7.5 cm (H), C-I = 0.99 X f (3.27 X 10-')t. The units of time, t , are seconds and for C-',mg-' L.
+
+
absorbance measurements and suspended mass determination were made simultaneously to demonstrate the validity of the calibration curve as suggested by Bradley and Krone (8). Even though sludge is a heterogeneous particulate suspension, there was no evidence that sludge had different fractions that would coagulate at different rates and change the calibration.
Results and Discussion Sludge coagulation experiments were conducted at fluid shear rates of 0, 0.25, 0.5, 1, 2, 4, and 8 s-l within the rotating cylinder apparatus at an initial concentration of approximately 100 mg L-l. Figure 1presents examples of the data obtained during experiments at G = 2 s-l in this case. The data are plotted as inverse concentration vs. time following eq 2, and the removal rate parameter, b, was determined by finding the slope of the least-squares line fitting the data. The data fall on a straight line, indicating that the sewage sludge concentration in seawater decays following second-order kinetics. The same removal rate parameter was found for initial sludge concentrations of 50 and 200 mg L-l at G = 2 s-l. In eight separate determinations at G = 2 s-l, the removal rate parameters were in the range 3.0 X lo4 to 3.5 X lo4 mg-l L s-l, indicating a 10% variation about the mean. For all the sludge coagulation experiments an initial time lag of at most 10 min was observed before suspended solids decreased at a given depth. The time required to first produce settleable aggregates cannot be quantitatively assessed from these experiments because of its partial dependence on the initial mixing procedure. Table I summarizes the measured removal rate parameters for sewage sludge at shear rates from 0 to 8 s-l. The removal rate parameter increases with increasing fluid shear as expected for a system dominated by shear coagulation. With very low or zero fluid shear, particle coagulation by Brownian motion and probably differential sedimentation continuously produce aggregates capable 120
Environ. Sci. Technol., Vol. 18, No. 2, 1984
1.0 x 1.4 x 1.5 X 2.5 X
G , s-'
b, mg-' L s-'
2
3.3 x
4
4.2 X
8
9.1 x
Table 11. Range of Sludge Aggregate Settling Velocities as a Function of Fluid Shear Rate G, s-'
settling velocity, ern s-'
0 0.5 2
2.8 x 10-3 to 3.3 x 3.5 x 10-3 t o 4.1 x 10-3 8.8 X t o 10.0 x 10.2 x 10-3 t o 11.3 x 10-3
8
0005l 0
b , mg-' L s-'
of settling. Morel and Schiff (4) calculated removal rate parameters within 0.5 order of magnitude of 2 X lo-' mg-l L s-l from Faisst's (9) data for quiescent conditions, G = 0 s-l. These values were less than observed in our experiments at G = 0 s-l, indicating an expected dependence on sludge characteristics and solution temperature. The data in Table I indicate that particle coagulation controls particle removal, but the data do not say anything about the settling velocity of the aggregates that are produced. The settling velocity of agggregates formed at a given shear rate can be determined by obtaining simultaneous samples at two depths in the rotating cylinder apparatus. Figure l a contains sludge concentration data at depths of 1.5 cm and 5.0 cm below the liquid surface in the rotating cylinder apparatus. The lines fitting the data points are parallel as expected, but the lines are offset with a lower concentration at 1.5 cm than at 5.0 cm for all times. The data indicate that aggregates produced by coagulation require time to settle out of the apparatus, and the time required for settling between two depths is the horizontal distance between the two lines. In Figure l a the horizontal distance between regression lines is 6.6 min at C = 90 mg L-' and 5.8 min at C = 50 mg L-l, and these are the times requried for aggregates produced at G = 2 s-l to settle a vertical distance of 3.5 cm. Thus, the data indicate a settling velocity in the range 8.8 X to 10 X loe3cm s-l. To check that the settling velocity is constant at a given shear rate, another experiment was conducted with samples withdrawn at 1.5 and 7.5 cm below the fluid surface in the rotating cylinder apparatus. For a separation in sampling depths of 6.0 cm, Figure l b shows a horizontal time of 10-10.3 min separating the lines and gives a settling velocity in the range cm s-l. The agreement in calculated 9.7 X to 10 X settling velocity for experiments at different vertical separations indicates that aggregates settle at a constant rate. Table I1 summarizes the range in settling velocities observed at shear rates of 0,0.5,2, and 8 s-'. Because the calculation of settling velocity requires determination of horizontal distance between lines, the result has a range of values because regression lines were not exactly parallel. The aggregate settling velocity increases with incresing fluid shear rate as expected for a suspension dominated by coagulation. At higher fluid shear rates there should be a greater range in aggregates dominated by shear coagulation which would produce larger aggregates with greater settling velocity. Morel and Schiff s ( 4 ) analysis of Faisst's (9) data indicated no concentration gradient with depth under quiescent conditions, implying rapid settling of sludge aggregates once formed.
Environ. Sci. Technol. 1904, 18, 121-123
These results for sewage sludge coagulation and settling can be compared with earlier results for clean clay particles
in artificial seawater (1). At G = 2 s-l the removal rate parameters for clays were 7-27 times greater than the value for sewage sludge, The clay rate parameters were reported in (parts per million by volume)-l s-l which is similar to mg-l L s-l units for sludge because the sludge density is ~ , one multiple depth sampling expernear 1g ~ m - Only iment was conducted with a clay mineral. At G = 4 s-l the removal rate parameter for illite was 5 x 10-~ ppm-l s-l, and the settling velocity was 4 X cm s-l (5). while the clay coagulation rate parameter was about 10 times greater than the sludge’s rate parameter at 4 s-l, the clay settling velocity was less than half the expected sludge settling velocity from Table 11. Removal rate parameters and settling velocities must be viewed as separate factors probably controlled by the aggregate porosity. A porous aggregate has a large collision cross section per unit volume of particles but also has a low settling velocity because of a small density difference between aggregate and fluid (see ref 1).
S u m m a r y and Conclusions The removal of sewage sludge particles from seawater is controlled by particle coagulation and the settling of aggregates as they are formed. Laboratory experiments in a rotating cylinder apparatus measured removal rate and aggregate settling velocity as a function of fluid shear rate. Limited experience to date suggests that particle coagulation rate and aggregate settling velocity are very dependent on fluid and particle properties. For this reason, specific data must be obtained for each waste under consideration. The two-step process of a continuous production of large aggregates followed by the settling of these aggregates must be incorporated into the modeling efforts discussed by Jackson (2) and Koh (3)if predictive capability is expected for particulate pollutants in ocean waters.
Two important topics have not been addressed in this note. First, the method of introducing concentrated waste materials into oceanic waters probably has a major impact on particle coagulation because of high particle concentrations and intense fluid shear. Second, particle removal and aggregate settling velocity are strongly dependent on the fluid shear rate which is poorly known in oceanic waters. Both of these areas require further study.
Acknowledgments We thank an anonymous reviewer for suggesting the mathematical analysis of settling.
Literature Cited (1) Hunt, J. R. Enuiron. Sei. Technol. 1982, 16, 303. (2) Jackson, G. A. Environ. Sei. Technol. 1982, 16, 746. (3) Koh, R. C. Y. Environ. Sei. Technol. 1982, 16, 757. (4) Morel, F. M. M.;Schiff, S. L. In “Ocean Disposal of Municipal Wastewater: The Impact on Estuary and Coastal Waters”; Myers, E., Ed.; MIT Sea Grant College Program: Cambridge, MA;in press (also: Ralph M . Parsons Laboratory for Water Resources and Hydrodynamics, Massachusetts Institute of Technology, Cambridge, 1980,Report 259). (5) Hunt, J. R. Ph.D. Thesis, California Institute of Technology, Pasadena, CA, 1980. (6) Hunt, J. R. J . Fluid Mech. 1982, 122, 169. (7) Lyman, J.; Fleming, R. H. H. Mar. Res. 1940, 3, 134. (8) Bradley, R. A,;Krone, R. B.J . Sanit. Eng. Div., Am. SOC. Civ. Eng. 1971, 97 (SAl),59. (9) Faisst, W. K. Environmental Quality Laboratory, California Institute of Technology, Pasadena, CA,1976,EQL Report No.13.
Received for review February 28,1983. Accepted August 5,1983. This work was partially supported by US.Department of Commerce, National Oceanographic and Atmospheric Administration, Office of Marine Pollution Assessment (NA81RAD00035).
Affinity of Hydrophobic Pollutants for Natural Estuarine Colloids in Aquatic Environments Ranmali D. Wljayaratne and Jay C. Means” Chesapeake Biological Laboratory and Department of Chemistry, University of Maryland, Solomons, Maryland 20688-0038
The interaction of natural estuarine colloids with a model herbicide [14C]atrazine was studied by using gel chromatography techniques. Results indicate that dissolved organic carbon binds atrazine and holds the resulting aggregate of molecules in stable colloidal suspension. Hydrophobic compounds held in such a manner show behavior characteristic of the natural organic matter rather than the behavior they would normally exhibit in solution. Oxidation of the compound-colloid complex by hydrogen peroxide or hypochlorite resulted in the partial destruction of the complex and release of bound herbicide. The average molecular weight of the estuarine polymer was estimated to be 1OOOO. This molecular weight is decreased with oxidation.
Introduction Natural waters contain significant quantities of dissolved or colloidal (
