Envlron. Sci. Technol.
m a , 22, 1201-1207
Settling Speeds of Sewage Sludge in Seaw-atert J. William Lavelle,***Erdogan Ozturgut,§ Edward T. Baker,* Davld A. Tennant,' and Sharon L. Walker$
Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way N.E., Seattle, Washington 981 15-0070, and NOS/Office of Coastal Zone Management, National Oceanic and Atmospheric Administration, 1825 Connecticut Avenue NW, Washington DC 20235
w Laboratory analyses of sludges from four treatment plants were performed to determine solid content, size distributions, and settling spectra of sludge particulates in seawater, The settling experiments were conducted after partitioning samples by sieving into coarse and fine particulate fractions. For the fine fraction, experiments proceeded after controlled mixing of samples with filtered seawater to particulate concentrations low enough ( 10 mg/L) that flocculation was limited during subsequent measurement periods. Results show that 14.6-47.3% of the sludge particulates by weight had diameters greater than 63 pm, with at least 5% exceeding 250 pm. Median to settling velocities of this fraction ranged from 6 X 3 X 10-1 cm/s. The bulk of the particulates were smaller in diameter, and these flocculated in seawater. Resulting aggregates had median settling velocities by volume ranging from 7 X lo4 to 3 X cm/s, with less than 10% of the particles settling more slowly than 10" cm/s. N
Introduction The ability to predict the fate of sewage sludge after ocean discharge requires knowledge of its physical properties, particularly the settling speeds of its constituent particles. Yet this measurement has not been satisfactorily made because of the difficulties of settling measurements in situ and because laboratory studies are beset with their own special set of problems. Specifically, adequate laboratory settling velocity measurements bearing on the fate of ocean discharges of sludge depend on (1)the creation of flocculated particles resembling those found in the ocean, a creation process that is dependent on poorly known ocean mixing conditions, and on (2) being able to prevent the change in the physical properties of the particles after the formation process and during the settling measurements. Few results of settling experiments for sludge particulates are not confounded by continued flocculation during the measurement period. The laboratory work described here primarily addresses the second aspect of this problem, though an attempt was made to create flocculated particles under low shear and during increasing dilution, conditions qualitatively resembling those found in the ocean during the floc formation period. The key to preventing flocculation during settling is to restrict the settling stage to low particulate concentration for which the frequency of particle-to-particle contact is low. Ozturgut and Lavelle (I) found that the settling of clays in seawater was not affected by flocculation when concentrations were 7 ppm or less. On the basis of that result, particulate concentrations near 10 mg/L (-5-10 ppm) were deemed suitable for conducting these measurements. This is also a desirable choice of concentrations because such concentration levels are near those expected in the ocean tens of minutes past the time of discharge. For example, concentrations of 10-40 mg/L were measured Contribution No. 901 from NOAA/Pacific Marine Environmental Laboratory. Pacific Marine Environmental Laboratory, NOS/Office of Coastal Zone Management.
by Jenkins et al. (2)several tens of minutes past the start of a barge dump of sludge in the New York Bight. Past this level of dilution additional flocculation of sludge particulates is likely to be slight, and it is this far-field settling distribution that is of value in predicting the subsequent oceanic dispersion and deposition of sludge. This is not to say that the process of floc formation (e.g., ref 3) up to this point in the dispersion of the sludge is without interest. Indeed, a complete flocculation theory for sludge particulates discharged into the ocean should lead to the same far-field settling velocity distribution. Such a model has yet to be constructed, however; part of the difficulty of doing so will be the specification of oceanic turbulent mixing and shear during the floc formation stage, a problem for the laboratory approach as already noted. The initial work on settling velocities of sewage sludge particulates was done by Brooks ( 4 ) and Myers (5). Faisst (6) extended those results by making measurements at different dilutions in seawater and at different depths in a settling column. Because the settling rates changed with depth and dilution, Faisst (6) acknowledged that flocculations was occurring in the column. Hunt and Pandya (7) studied flocculation of sludge particles under varying shear levels, presented an analysis in terms of second-order kinetics, and reported the settling velocity of the largest aggregates. Wang et al. (8) conducted experiments in a long settling tube after sludge had been mixed in seawater in a fashion to simulate ocean mixing, but initial concentrations remained above 100 mg/L and flocculation during the settling must be suspected. For our experiments, sludge came from four treatment plants: West Point, Seattle; Hyperion, Los Angeles; Middlesex, NJ; Owl's Head, NY. Samples from distant plants were provided to us by Dr. Ronald Gibbs (Middlesex, Owl's Head) and Steven McLean (Hyperion). Samples were acquired in January, 1986, from the West Point plant, in April, 1986, from the other plants. The West Point plant does not actually discharge sludge into the marine environment, but the proximity of the plant to our laboratory made that sludge a useful test sample.
Methods Samples from all four treatment plants were acquired at points downstream of the digestors, and in the case of Hyperion at a point after the digested sludge had been mixed with primary and waste-activated effluent typical of the product discharged to the ocean. Samples were taken in l-L plastic bottles to which 1 g of sodium azide was then added. The samples were then refrigerated, either directly or by being placed in an insulated chest together with blue ice during air shipment to the laboratory. The azide was used to retard growth during shipment and storage; its concentration was limited to prevent it causing flocculation. The concentration was found insufficient to prohibit growth under ideal culturing conditions in separate experiments (9),but the azide appears to have been effective under conditions of refrigeration. In the laboratory the samples, in freshwater, were split with a Folsom splitter (e.g., ref lo), one part to be used for bulk analysis and the other for settling. The bulk
Not subject to US. Copyright. Published 1988 by the American Chemical Society
Environ. Sci. Technol., Vol. 22, No. IO, 1988
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analysis fraction was separated by sieving into four size categories: >250,250-126,125-64, and C64 pm. Weight determination of each fraction provided a rough size distribution for the entire sample. The second half of the sample was separated into coarse (>63 pm) and fine (e64 pm) fractions by sieving. The reason for this was that it was not practical for us to count each entire sample, which included large, and frequently fibrous particles, with a multitude of Coulter apertures. Sieving could have caused aggregate breakup, though the sample at that point had not yet been introduced to seawater, the point at which the principal floccule formation might be expected to occur. Sieving also precludes the possibility of the interaction of the two particle sets during the floccule formation and settling stages. Sieving was accomplished by gently washing the sample through a 63-pm sieve with filtered, deionized water to which azide had been added; the wash water increased the volume of the fine sample by approximately a factor of 4 except for the Owl's Head sample, the solid content of which led to a 20-fold volume increase. Only 10-mL samples of the fine fraction were needed, so the fine fraction was split by using both Folsom and Walker (11)splitters. The Walker splitter consists of a rosette of 10 tubes connected to a central chamber. Air bubbling up into the central chamber from the sample chamber continuously mixes the sample while preventing its release to the collecting tubes. When air flow is reversed to slightly pressurize the central chamber, the sample is forced into the rosette of collecting tubes. The resulting 10-mL samples of sludge in freshwater were then refrigerated to await settling analysis. Careful splitting is essential where comparison of different treatments of identical samples are to be made. Coarse Fraction. Settling analyses on the coarse (>63 pm) fraction of particulates were performed in freshwater using a 1-L glass cylinder with a diameter of 6 cm and with large-orifice (2-mm) pipets. The use of freshwater stems from the following considerations: (1)the fibrous content of the coarse fraction dictated gravimetric rather than Coulter Counter volumetric analysis; (2) gravimetric analysis required mass concentrations on the order of 100-200 mg/L in order to get reliable weight estimates from settling experiment subsamples having small volume; (3) particles at concentrations of 100 mg/L have the potential for flocculation in seawater. The objective of this set of experiments was to obtain settling velocity spectra free of the effects of particle interactions. Such interactions are minimized during settling in a low-concentration, freshwater environment. Consequently, -100-150 mg of material was suspended in fresh water and thoroughly mixed with a perforated disk-type stirring rod (12). After an initial sample, samples were taken at an interval of 10 s up to 1min and thereafter a t 2, 3, 5 , 10, and 15 min, at depths of either 11.4 or 23.4 cm depending on the experiment. Between samples up to and including the 1-min sample, the column was remixed and the clock restarted to accommodate sampling at the closely spaced time intervals. Replicate samples for mass analysis, taken at the initial time and at 1rnin after remixing, and visual observations of the particles in the column, suggest that mixing between samples did in fact reestablish a consistent initial state. The resulting sample aliquots were filtered through 0.4-pm polycarbonate filters and dried in a desiccator, and the retained mass was weighed. Fine Fraction. The experiments on the fine fraction were meant to acquire settling velocity spectra of sludge 1202
Envlron. Sci. Technol., Vol. 22, No. 10, 1988
particulates as they might be found after discharge and mixing in the ocean. As already mentioned, the poorly known conditions of mixing and shear to which sludge particulates are subjected after discharge in the ocean preclude a demonstrably authentic re-creation of that mixing environment in the laboratory. The conditions of low shear and increasing dilution during the period of floccule formation in the ocean can, however, be qualitatively reproduced. Thus, the fine fraction was introduced to the bottom of a dry 3.5-L settling tube with a diameter of 10.6 cm to which a bottom port had been inserted. Filtered seawater was then introduced at a controlled rate while the contents of the tube were stirred by a magnetic bar rotating at a slow speed when the volume was small and by perforated disk thereafter. The dilution rate, controlled by a flow meter, was adjusted over a 30-min dilution period so that a concentration of 500 mg/L was reached a t 2 min, 100 mg/L was reached at 10 min, and 10 mg/L was reached at 30 min. Quantification of these low shear conditions during this mixing stage was not attempted. Theoretical dilution values for both diffuser and barge discharges are given by Koh (13). After sample mixing, the settling column was lowered into a 100-L water bath that was used to insulate the column from changes in room temperature. Because temperature equilibration of the settling column and the tank took hours to achieve, the column was left overnight in the bath with a magnetic stirrer moving just above stall speed to keep the column mixed. Maintaining a constant-temperature bath was found to be critically important. Backlighting the settling tube during preliminary experiments revealed convective current cells in the settling column when the temperature of the bath varied as little as f0.3 "C about a mean temperature. Heat exchange through the walls of the settling tube apparently resulted in temperature gradients between the core and the near-wall fluid within the tube, and the result was convective mixing symmetrical about the tube axis. If the bath temperature fell below the mean, the cellular current moved upward in the center of the tube and downward at the walls, and when the temperature of the bath was above the mean, the cell currents were in the opposite direction. Accordingly, a temperature controller with a ic0.02 OC range was used in the bath, and the water in the bath was well circulated with three aquarium pumps. A high-precision thermistor positioned near and, in some test cases, in the settling tube, showed that temperature was stabilized to within the f0.02 "C specifications of the heater. The bath temperature was kept near 24 OC, slightly higher than maximum room temperature. Thereafter, no further convective currents attributable to fluctuations of bath temperature were observed. Heat flux directly into the upper, open end of the settling tube that appeared to cause some convection was eliminated by closing the tube with a cap insulated with Styrofoam, through which a pipet sampling guide had been drilled. The foam extended very near the column fluid surface, which was maintained at the same level as the bath fluid so that no heat flux though the sides of the tube exposed to air would occur. Convective currents were all but eliminated by these means; however, some slight vertical motion occurred around the pipet position immediately after sampling despite the slow removal of the pipet from the settling column. This effect may have resulted from the disequilibrium of the temperatures of the pipet and fluid. Further details of the dilution procedures of the sludge and the configuration of the apparatus are given in Tennant et al. (11).
The settling experiment for the fine fraction, given this apparatus, proceeded in the conventional way (12). Samples were taken with standard volumetric pipets (0.80.9-mm orifice) at the initial time and at a schedule that resembled a near doubling time scale up to 8 h at a column depth of 15 cm. Multiple samples at some of these sampling times and a t shallower depths were taken in lieu of overnight samples a t the 15-cm level; a 24-h sample was also taken. No effort was made in the sampling or analysis stage to prevent the breakage of flocs, for reasons to be discussed below. The resulting aliquots had a volume of 25 mL and a concentration of 1-10 mg/L. This small volume and low concentration dictated that the particle content be measured by electronic particle counter rather than by gravimetry (1, 14), though optical measurements might also have been used for this purpose (7, 15). Aliquots were measured by Coulter Counter with 70- and 200-pm-aperture tubes. To insure good overlap of the two size distributions, samples were counted on both apertures at times as close together as practical (-4 h). The number of samples taken during each experiment (-20) prevented all samples being counted on a single day. Some samples were stored while being refrigerated and gently shaken. Agitation was used to prevent the particles from settling to the bottom of the sample container and there agglomerate, a result preliminary tests had suggested would happen. The principal concern was not the size distribution of the sample, however, but the stability of the size distribution over the time period between the analysis of the samples using Coulter apertures of different sizes. The combined 70- and 200-pm-aperture data gave total volume concentrations for the size range 2.1-80.0-pm diameters. Measurements of total volumes of flocculated particles are not made without some uncertainty, however. As pointed out by Treweek and Morgan (16), an electronic particle counter theoretically measures only the volume of the solid material making up the aggregate but not the volume of the enclosed electrolyte. This fact is advantageous for our purposes because when one is concerned only with total solid particle volume concentration and not individual particle sizes, preserving the integrity of the individual particles after pipeting and during subsequent handling and counting stages theoretically should not matter. The measurement of the solid particle volume of a single aggregate or the sum of solid particle volumes of its pieces should be the same. That the volume concentration could be independent of the particle size distribution was suggested in a preliminary experiment that we ran to determine the effects of storage on a sludge sample. That experiment showed the particle size distribution could shift to larger particle sizes without a significant change in the total volume concentration estimate. Practical considerations for an electronic particle counter can make the actual and measured values for volume somewhat different, however. Treweek and Morgan (16) mention two concerns. First, if a large aggregate passes through a small aperture, the particle is elongated and the resulting electrical impulse is broader but not as high as a single particle of equivalent solid particle volume. The counter’s estimate of the volume, based on the height of the impulse, is somewhat less than the actual equivalent solid volume. If a large aggregate moves through an aperture sufficiently large that the particle is not elongated, the aperture may be too large to be entirely efficient in sensing the constituent small particles. An underestimation of the equivalent solid particle volume may also result. Using empirical results for the effective electrical resistance
Table I. Size and Weight Percentages size fractions, % solids by&, >250 250-126 125-64
-0
-0
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loo
101
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Figure 1. Changes in mass concentration versus time for the coarse fraction (>63 pm) of sludges from four treatment plants. The sampling depth, h , is indicated. Coarse Fraction
10-2
10-1
100
101
SETLING VELOCITY (cm/s)
Figure 2. Cumulative settling curves by weight for the coarse fraction. Hyperion A and B represent experiments that differ only in initial concentration (800 and 120 mg/L, respectively).
material, the effect called differential settling. Some of the settling material was clearly fibrous in nature. Because the experiment was conducted in freshwater, the effect cannot be attributed to ionically induced flocculation. When initial concentrations were reduced to 100-150 mg/L and the sampling depth was reduced to 10 cm, the effect was not observable. The results for the Hyperion A sample and for the West Point sample, which had an initial con-
-
1204
20
0.0 Ibl Ib TIME (tec)
Environ. Sci. Technol., Vol. 22, No. 10, 1988
40
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Figure 4. Volume size distribution for the fine fraction of Owl’s Head sludge at the start of the settling experiments. (A) Sample mixed in seawater; (B) sample mixed in freshwater. The distribution is much broader and the peak shifts from 7 to 4 pm in freshwater. Solid and dashed lines correspond to 70- and 200-gm Coulter aperture sizes.
centration of 1500 mg/L, are overestimates of the settling velocities of these materials at lower concentrations. The median settling velocity by weight of the coarse fraction varied from 6 X to 3 X 10-1cm/s. Except for Hyperion, less than 10% of the coarse fraction settled with velocities less than cm/s. This velocity could be expected of particles greater than 63 pm in diameter, assuming particle sphericity and Stokes settling, only if the particles had densities of 1.04 g/cm3 or less. No visually discernable amount of material accumulated at the surface of the tube in any of these experiments. Less than 10% of each sample contained particles settling at more than 1 cm/s, except for the more heavily concentrated West Point sample affected by differential settling (Figure 2). Settling of particles of velocities much greater than 1cm/s could go unsampled with the experimental arrangement that was employed, though visual observations of the cessation of turbulence and the settling in the tube lead us to believe that their fractional contribution was not significant, Faisst (6) too observed no settling at speeds greater than 1cm/s in experiments on Hyperion sludge. The relative absence of higher velocity materials may reflect the screening and grit removal at the treatment plants. Fine Fraction. Experiments on the fine fraction of particulates were conducted after both moderate and rapid mixing in seawater and after freshwater dilution. Several of the measured size distributions are shown in Figures 3-5.
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Flgure 5. Volume size distribution for the fine fraction of the Middlesex sludge at (A) the start of a settling experiment in seawater and (B) after 25 h had elapsed. Solid and dashed lines correspond to 70- and 200-pm Coulter aperture sizes.
Solid and dotted curves correspond to results from 70- and 200-pm Coulter Counter apertures. The overlapping distributions allow a calculation of total volume between 2.1 and 80 pm. The distributions at the start of experiments for the Hyperion, Owl's Head, and Middlesex sludges (Figures 3, 4A, and 5A) all show size maxima well away from the smallest particle size measured (2.1 pm). The broad maxima for the Hyperion sample between 10 and 20 pm is somewhat different from the narrower distributions centered at 7 and 10 pm for the Owl's Head and Middlesex samples. There is no evidence in these distributions for a substantial contribution to particle volume from particles less that 2.1 pm, so the use of Coulter apertures of smaller size was judged not necessary. Few particles greater than 50 pm appear in these distributions. The flocculating effect of seawater on particle sizes is seen in the comparison of parts A and B of Figure 4, those being Owl's Head samples prepared in seawater and in freshwater. The maximum in size occurs at 4 rather than 7 pm under freshwater conditions, and the distribution is much broader. The size distributions also change during the measurement period because of particle settling. Figure 5 shows the initial and 25-h distributions for Middlesex sludge settling in seawater. After 25 h has elapsed, the overall concentration is much reduced and the contribution of the larger particles to total volume is slight. In this case, particles at unmeasured diameters less than 2.1 pm can contribute significantly to total volume, but measurements with the apertures necessary to investigate this range were not made. Total volume estimates at these longer times must therefore be regarded as less accurate. These size distributions must be viewed with caution. They may not represent the sizes of particles as they actually settled in the settling tube. Treweek and Morgan (16) point out that, for an aggregate, the electronic particle counter measures the solid particle volume but not the volume of the enclosed electrolyte; consequently, the aggregate actually has a larger diameter than that estimated by the counter. Moreover, we made no attempt to prevent (or cause) breakage of flocs in the handling, storage, or analysis after subsamples had been removed from the settling tube, save for trying to maintain the constancy of the size distributions between Coulter counting with apertures of different sizes. Indeed, it seems likely that some breakage during these procedures would occur (18, 19). Maintenance of the integrity of the flocs as they had settled in the tube after subsampling was, in our opinion,
not critical to the outcome of this experiment because, as stated earlier, the settling velocity estimates depend only on the relative changes of total volume concentration in time. Measuring the actual size distribution of the settling material was not a goal of this experiment. Total volume concentrations at each sampling time were calculated from distributions like those shown in Figures 3-5. Total volume concentration estimates (solid points, Figure 6) were used to infer the change of the volume concentrations in time. For each experiment, theoretical volume concentration change curves (Figure 6) were constructed from the initial size distribution (e.g., Figure 3) by using a range of effective densities, the magnitude of which is designated on each curve (in gm/cm3). The computation assumes a single density and Stokes settling for all particles in the distribution and takes the initial size distributions (e.g., Figures 3-5) BS representative of the size of particles settling in the tube. While each of these assumptions can be questioned, the settling velocity information can be summarized with a single effective density in this way. The volume concentration change data of Figure 6 allow comparison of results from different treatment plants and of different laboratory sample preparation procedures. The sample pair from Hyperion (Figure 6A,B) represent sludges mixed with seawater under moderate and rapid stirring. Under moderate stirring, the particles in seawater had an effective density of 1.5 g/cm3, while under more rapid stirring the effective density was 1.3 g/cm3. Reduction of effective density under more vigorous mixing was observed in all pairs of similar experiments. More shear during the production of flocs in these cases apparently resulted in smaller flocs or flocs of lower density. Two results with Middlesex particulates (Figure 6C,D) permit the comparison of samples prepared and settled in seawater and in freshwater. Settling times are seen to lengthen considerably if particles have not undergone flocculation in seawater. In freshwater, concentrations remained unchanged at the sampling depth (15 cm) for -5 h before clearing began. The Owl's Head and West Point samples (Figure 6E,F) show differences in the speed of settling consistent with their initial particle size distributions. One clear aspect of the data of Figure 6 is the variance with respect to the smooth decreasing curves that have been constructed to represent the changes of concentration in time. Possible sources of subsample contamination were examined in numerous preliminaryexperiments using glass beads; some sources were so identified and eliminated. Another cause for the scatter of data about each curve at sample times greater than 4 h stems from the slight mixing of the column while sampling at multiple depths. Though every precaution was taken not to disturb the column, particles were observed, by backlighting the column, to undergo slight motion after each sample was withdrawn by pipet. Samples taken at nearly the same time (>4h) but at different depths often had nearly the same concentrations. Small particles of low density are easily subject to slight currents. Cumulative settling curves by volume for the fine fraction of sludge (Figure 7) were constructed from the volume concentration loss relative to the initial concentration (solid curves, Figure 6). The settling velocities of the fine fraction range from less than to -4 X cm/s, with the to 2.6 X cm/s. These median ranging from 0.7 X distributions show sludge as having a wide range of settling velocities. The distributions of Faisst (6), cumulative settling curves by weight rather than volume, show many Environ. Sci. Technol., Vol. 22, No. 10, 1988
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A
Hyperion, Los Angeler (LA-2)
-
B
Hyperion, Lor Angeles (LA-))
15
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Fine Fraction I
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Flgure 8. Changes In volume concentration versus time for the fine fraction (
