Environ. Sci. Technol. 1998, 32, 383-387
Effects of Freezing Rate, Solids Content, and Curing Time on Freeze/Thaw Conditioning of Water Treatment Residuals PHILIP J. PARKER,* ANTHONY G. COLLINS, AND JOHN P. DEMPSEY Box 5710, Rowley Labs, Clarkson University, Potsdam, New York 13699-5710
The results presented herein are intended to aid in a fundamental understanding of conditioning water treatment residuals by the freeze/thaw process. Residuals were frozen such that the effects of freezing rate, solids content, and curing time were isolated. Residuals were frozen directionally on a copper surface maintained at subfreezing temperatures. In contrast to many of the results in the literature, freezing rate did not seem to affect the final dewaterability. However, the effects of initial solids content and curing time were noticeable. High solids content residuals yielded cake solids contents greater than 30% following freezing, thawing, and drainage under a vacuum. Samples cured at -10 °C for 1 h dewatered much more readily than uncured samples. The results will aid in the design of a mechanical freezing process; the cost optimum conditions for such a process appear to be fast freezing of a high solids content residual, followed by approximately 1 h of curing.
Introduction Dewatering of residuals (sludge) has traditionally been a major concern for the water treatment industry. The industry produces large quantities of hydroxide residuals, the disposal of which often represents a large portion of a water treatment utility’s operating budget. Dewatering is often necessary prior to disposal in order to reduce transportation and disposal costs. However, achieving a high solids content product (>30%) may be difficult or impossible for some types of residuals using mechanical dewatering processes. Residuals are a mixture of water and solids. The solids may be dissolved, colloidal, or bound loosely together as flocs. Flocs are aggregates consisting of impurities removed from the raw water and hydroxide precipitate. These flocs also contain large quantities of “bound” water, which cannot be released by mechanical dewatering. Only water that is not bound, or “free”, can be removed by mechanical dewatering. Following freezing and thawing, flocs are denser and more granular, and some of the bound water has been converted to free water. Freeze/thaw conditioning has been successfully applied to several types of sludges and residuals frozen in a variety of ways (1-12). Despite this near-universal success, there are few full-scale applications of the process. Natural freezing * Author to whom all correspondence should be addressed; phone (315) 268-3844; fax: (315) 268-7636; e-mail: parkerpj@draco. clarkson.edu. S0013-936X(97)00603-2 CCC: $15.00 Published on Web 02/01/1998
1998 American Chemical Society
is often deemed to be unreliable or technically infeasible for all but the coldest climates, while mechanical freezing appears to be too expensive. In an effort to understand the freeze/thaw process and to determine whether it is feasible, researchers have studied several variables affecting the process. For instance, numerous researchers have studied the effects of freezing rate (2, 4, 6). Generally, an increase in freezing rate leads to poorer dewaterability. Also, more flocs are entrapped in the ice at relatively high freezing rates, while at lower rates the growing ice rejects nearly all impurities. Vesilind and Martel (11), Zolotavin et al. (13), and Dempsey et al. (14) investigated the effects of curing time (length of time frozen residuals are stored at subfreezing temperatures) and noted an improvement in dewaterability following curing. Vesilind and Martel (11) studied curing temperature and noted that for similar curing times, better dewaterability was achieved for samples cured at lower temperatures. Vol’khin and Zolotavin (15) and Vesilind et al. (16) thought that improvements in dewaterability might be indirectly due to double-layer compression and increased the electrolyte concentration in the residuals prior to freezing. Vesilind et al. (16) froze samples at -22.5 °C and found no relationship between dewaterability and electrolyte (NaCl) concentration. Vol’khin and Zolotavin (15) used several different electrolytes and noted that the effect of each electrolyte strongly depended on freezing temperature. Finally, the effects of agitation were studied by Vesilind et al. (17) and Baskerville (18). Vesilind et al. suggested that an optimum floc size exists, and agitation prior to freezing may aid in attaining this size. When the flocs were agitated after freezing, Baskerville reported that stirring had only a minimal effect on an alum sludge, while Vesilind et al. reported that dewaterability worsened after agitation of freeze/thaw conditioned biological and alum sludges. Despite the extensive studies of several variables, the optimum conditions for freezing residuals cannot be determined. The purpose of the current study is to determine the individual effects of three variables (freezing rate, curing time, solids content) in order to determine the optimum combination of the selected variables. These optimum conditions can then serve as the basis for the design of a mechanical freezing process.
Background Using the above studies as a guide, three variables were studied: freezing rate, initial total solids content, and curing time. The rationale for selection of these variables is explained as follows. Freezing Rate. As water is frozen from residuals, the solids may be “entrapped” within the forming ice. If flocs are rejected instead of entrapped, they will “migrate” ahead of the growing ice, effectively thickening the unfrozen residuals. Several investigators have noted that more flocs are entrapped for rapid freezing than for slow freezing (4, 7, 19). Parker et al. (20, 21) used microscopic studies to help explain this freezing rate effect. They noted that at high freezing rates the interface was dendritic, and flocs were entrapped as the dendrites moved through the residuals. At lower rates, a smooth interface was evident, and flocs “piled up” in the unfrozen residuals at the interface. If rejection and entrapment can be termed a primary effect of freezing rate, there is also an important secondary effect, namely, dewaterability. Clements et al. (2) and Dempsey et al. (14) note that the benefits of freezing are not completely realized unless the residuals are completely frozen. Residuals VOL. 32, NO. 3, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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that are thawed following incomplete freezing are similar in appearance to centrifuged residuals, and do not have the desired granular consistency. In order for complete freezing to occur, all flocs must be entrapped in the ice matrix. Despite the connection between freezing speed and dewaterability, only limited results are available in the literature to correlate the two variables. Some researchers have noted that faster freezing leads to poorer dewatering (7, 12), but they have not actually measured dewaterability as a function of freezing rate. Only Hung et al. (4) and the present study have done so. Hung et al. observed that settleability worsened but filterability improved as freezing rates increased. Solids Content. The amount of entrapment depends on the solids content of the residuals to be frozen as well as the freezing rate. This is illustrated by the phenomenon of rhythmic banding (7, 19). Rhythmic banding has been observed in directionally frozen ice and appears to be the result of a buildup of flocs in the unfrozen residuals at the interface. As the solids content increases at the interface due to migration, a thickened layer of residuals is formed that resists the flow of water to the interface. When this layer becomes sufficiently thick, water cannot flow to the interface rapidly enough for pure ice to grow, and the flocs in the gel layer are entrapped. After this layer is entrapped, the process repeats itself as another layer builds up and is entrapped. This cycling between rejection and entrapment can occur at a constant freezing rate, suggesting that the increase in solids content at the interface influences entrapment. The relationship between entrapment and solids content has not been investigated before. Possibly this is because of complexities that arise when measuring entrapment as a function of freezing speed and solids content. For instance, measuring entrapment as a function of some known solids content is difficult. The initial solids content in the unfrozen residuals is easily measured. However, due to migration, the solids content in the melt varies as freezing progresses. Moreover, the variation in solids content at the interface is not solely a function of the initial solids content but also of freezing rate. Presently, there is no model describing the variation of solids concentration in the melt (specifically at the interface) as a function of freezing rate. The authors are currently studying this problem in a parallel project. Curing Time. Curing time refers to the length of time the frozen residuals are stored after being completely frozen. A preliminary experiment illustrated the importance of curing. A 10 cm diameter cylinder of residuals was frozen downward to a depth of 16 cm at an average rate of 3 mm/h. Entrapment occurred to varying degrees throughout the depth of the frozen residuals. After completion of freezing, the frozen residuals were melted directionally (opposite to the direction of growth) by running water over the interface end. Flocs entrapped near the interface were “clayey” and soft and did not resemble coffee grounds as often reported in the freeze/ thaw literature. However, these flocs had only been stored (or cured) in the ice for a short time. Conversely, flocs entrapped near the top of the cylinder, which had been cured for up to 53 h (16 cm at 3 mm/h), were granular. The soft, clayey flocs apparently retained more bound water than the granular flocs. Assuming that all flocs were entrapped under similar conditions, this experiment proved that flocs must be cured to maximize dewaterability and that the effects of curing are manifested after very short curing times. Although the beneficial effects of curing have been noted (11, 14), its effects have not been isolated from freezing rate effects. For example, in Hung et al. (4), 15 cm long cylinders were frozen at rates ranging from 1.41 to 72.6 µm/s (0.5126.1 mm/h). Thus, total freezing times ranged from 0.57 to 30 h. Assuming that samples were thawed immediately after 384
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freezing was complete, the initially entrapped flocs were cured nearly 30 h for the slowly frozen samples and only 0.57 h for the most rapidly frozen samples. Therefore, improvements in dewaterability noted for the slowly frozen samples may have been due to the lower freezing rates, as suggested by the authors, or to increased curing time. However, there is no way of differentiating between curing and rate effects for the experimental apparatus employed. There are a few instances in the literature where curing time has been investigated. Vesilind and Martel (11) froze samples at a single temperature (-6 °C) and then cured the samples for 12, 24, and 48 h. Dempsey et al. (14) compared the results of curing for 2 h to 2 weeks. However, neither of these studies considered the time that flocs were entrapped before completion of freezing as curing time. Nor did they investigate the importance of short curing times (less than 1 h). From an experimental point of view, curing time is not trivial to isolate. In practice, it is impossible to freeze residuals and determine the effects of “zero curing”, i.e., freezing a sample without any curing whatsoever. Such an experiment can only be accomplished if an infinitely thin sheet of ice is formed and immediately thawed. Hence, in order to isolate curing time in a practical manner, residuals should be frozen in the thinnest layers possible.
Scope of Study This study sought to quantify the effects of freezing rate, solids content, and curing time on dewaterability. Results from freezing one type of water treatment residuals are reported here (alum residuals from the Potsdam, NY, water treatment facility). Dewaterability following freeze/thaw conditioning can be predicted for a given freezing rate, solids content, and curing time. However, the results can be applied only to a single residual and a single freezing configuration (thin, horizontal layers frozen on a cold surface). The effects of additional components in the residuals, such as powdered activated carbon and polymer, are currently being studied. Eventually the dewaterability of many types of water treatment residuals following freeze/thaw conditioning can be predicted.
Materials and Methods The alum residuals used in this study were taken from the sedimentation basins at the Potsdam (NY) water treatment facility. The residuals averaged 1% solids content. Residuals were frozen in thin layers, and the solids content prior to freezing was varied by centrifugation. Following freezing, the frozen residuals were either immediately thawed or cured. Four freezing rates, four solids contents, and four curing times were employed. Directional freezing experiments took place in the test cell shown in Figure 1. The test cell consists of a 10 cm diameter polyethylene cylinder (7.5 cm tall) with a copper disk (thickness 0.55 mm) sealed to the bottom. Approximately 200 mL of residuals (corresponding to a depth of 2.5 cm) was placed in the test cell, which was then inserted into
FIGURE 1. Test cell.
FIGURE 2. CST apparatus. a piece of rigid insulation and capped with the same insulation. The insulated test cell was then floated on a chilled antifreeze bath. Directional freezing occurred in an upward direction until the residuals froze to a thickness of 0.4 cm, at which time freezing was terminated. The unfrozen residuals on top of the ice were completely removed, and the resulting ice disk was either immediately melted or cured at -10 °C. A run consisted of freezing four samples of residuals at a single solids content and freezing rate until the ice had grown to a thickness of approximately 0.4 cm. From each run, one of the frozen disks was immediately thawed, and the remaining three were cured at -10 °C for 1, 6, and 24 h. Three more runs were performed at the same freezing rate and a different solids content. Solids content was varied by centrifugation (Beckman J2-21 centrifuge). Three rotational speeds were employed: 1000, 5000, and 10 000 rpm for 30 min. A total of four solids contents were tested, corresponding to one uncentrifuged and three centrifuged samples: 1%, 3%, 5%, and 10%. The four runs were then repeated for three additional freezing rates. The temperature of the antifreeze bath was set by controlling the ambient air of the cold room in which the experiments took place. Four temperatures were used (-3.5, -7.5, -15, and -30 °C) corresponding to four freezing rates (0.5, 2.7, 12.1, and 24.0 mm/h). Average freezing speed was determined by dividing the total time of freezing by the thickness of the ice disk (0.4 cm). Freezing speeds and solids content were varied to give a range of behaviors expected in any freezing process, from complete rejection to nearly complete entrapment. Rejection was quantified by measuring the solids content in the melted ice disk. Complete rejection occurred for the 1% solids content sample frozen at -3.5 and -7.5 °C. Complete entrapment did not occur, even for the thickest sample frozen at the highest rate; however, entrapment was greater than 90% for these samples. One method used to determine dewaterability was capillary suction time (CST). CST is measured with the apparatus shown in Figure 2. Residuals are poured into the stainless steel collar, which rests on a sheet of filter paper. The rate at which water flows through the filter paper is measured by the timer rings set in the plexiglas blocks. Thus easily dewatered residuals will exhibit a low CST reading. Vesilind (22) has suggested presenting CST results in terms of filterability (χ), calculated by
χ)φ
[ µCt ]
where φ is the dimensionless instrument constant; µ is the viscosity (N s m-2); C is the solids concentration (mg L-1); and t is the capillary suction time (s). Therefore, large values
FIGURE 3. Filterability results. of χ indicate residuals that are readily dewatered. Unlike CST, χ is not unique to solids content or the specific instrument being used and thus can be used for comparison purposes. Dewaterability was also quantified by the solids content of a cake remaining after vacuum filtration. Ten grams of residuals was placed on a filter apparatus holding a Whatman No. 42 filter paper (pore size
