(70f) Particle Size Distribution of Water Treatment Plant Residuals

Water Treatment Plant (WTP) Residuals are produced during processes such as coagulation, softening, sedimentation and filter back-washing processes. Though not toxic and hazardous, handling and disposal of theses materials have become difficult due to the increasingly stringent legislation. These wastes contain large quantities of water that cannot be removed by conventional drying methods. Water can be removed if WTP residuals can be subjected freeze thaw, water gets removed. Also due to this freeze, the residuals, the particle size distribution of these materials changes significantly, from a fine-grained plastic condition (clays) to a coarse grained nonplastic (sands) condition.

In a study funded by the American Water Works Association to determine the criteria for monofilling theses residuals, authors performed particle size distribution tests such as Sieve analysis; Hydrometer analysis and tests using Particle Size Analyzer (PSA) were performed. In their fresh states, combined sieve and sedimentation analyses were carried out on residual samples such as JCD. Hydrometer and PSA tests were employed to determine grain size distribution of fresh residual HWD. However in the case of some fresh residual samples, hydrometer test was not successful since the sample formed a gel. The grain size distribution of that residual was determined by PSA. For sample JCD in the fresh state, grain size distribution by PSA was not possible, since the sampling chamber choked up, probably due to the high calcium compounds present in that sample. For dry and freeze and thaw samples, the grain size distribution was determined by combined sieve and hydrometer and PSA methods.

Grain distribution of the six residuals considered for this study under fresh, dried and freeze-thawed conditions respectively. Some of the parameters required for classification such as the uniformity coefficient, coefficient of curvature, sand, silt and clay fractions have been shown determined. The specific surfaces for all the residuals corresponding to the average size (D50) were calculated, assuming the solid particles were to be perfectly spherical. The residuals were classified according to the Unified Classification System. Detailed data will be presented in the final paper. A sample data is shown in Table 1.

An examination of the results presented in the Table 1 and the grain size distribution data clearly indicates the increases in sizes of particles of residuals, due to drying and freezing and thawing. Initially in fresh state, all these residuals were predominantly fine-grained. Upon drying, and freezing and thawing, increase in the sizes of particles took place, making the residuals coarse-grained materials. The values specific surface, defined as the ratio of surface area to weight, dropped significantly for all the residuals. Increase in particle size is more pronounced in the residual JCD, where the value of specific surface decreased from 23800 cm2/g in the fresh state to 476 cm2/g for weathered sample due to freezing and thawing. Discussion for the possible causes will be presented later.

The grain size distribution of residual HWD in its fresh state as obtained by hydrometer gives smaller sizes of the particles than that of PSA. This may due to the fact that no dispersing agent solution was used in PSA and some particles might have flocculated. Also, it has to be pointed out here that both PSA and hydrometer techniques do not make absolute determinations of particle size. In PSA, particle size is measured from the diffraction patterns created by the laser beam on the sample. The hydrometer technique is based on the measurement of settling velocities.

The mechanisms responsible for growth in particle size are (1). The Dehydration of residuals upon drying and (2) Change in plasticity due to change in the nature of organic matter. Van Schulenborgh, (1954), also noticed similar phenomenon which caused the increase in grain size of organic topsoils due to air-drying.

The dehydration of soil by drying: may result in a vigorous cementing action of the colloids on the soil aggregate. The dispersion of such aggregates requires a rehydration of the colloidal particles and this rehydration may be very slow. WTP residuals contain humus, which possess gel-like properties. During drying, the alteration of gel structure causes permanent reduction in volume. Moreover, air has entered the gel structure and this air can hardly be removed on re-wetting.

Schalsa and others (1965) investigated the influence of air-drying on volcanic soils from Frutillar and Santa Barbara regions of Chile. These soils were rich in organic matter. They have observed that the effect of air-drying on the sand and clay fractions was more pronounced than silt fraction. Maximum increase in sand fraction occurred in the B-horizon of the Frutillar soil and amounted to 93%. The clay fraction of the same sample decreased by 57%. On the other hand, the sand content of the A-horizon of the Santa Barbara was not significantly affected by air drying, whereas there was a 68% increase in sand fraction with the B-horizon. Air-drying decreased the silt fraction in the A-horizon of the Frutillar soil by 13%. In the Santa Barbara soil, air-drying increased the amount of silt by 10% in the A-horizon, but caused a 26% decrease in the B-horizon.

It is interesting to note that WTP residuals contain the necessary Al. Fe and Si elements for the mechanism stated above to happen. Several references can be cited in which the bridging action of organic materials to form stable aggregates is indicated (Kohnke 1968, USDA Soil Survey Manual 1951). According to Berger (1965), soils with a good organic content and also those high in iron oxides have a good granular texture. Soils with fine texture acquire their granular structure because of the binding of the particles together by iron oxides and by gums and resins formed by the decomposition of organic matter. In this study, of the above two mechanisms cited, the binding due to iron oxide is possible, since all the three residual samples tested contained iron oxide.

Weil (1998) offers the following explanation for cementation: During flocculation, individual colloidal particles coagulate together into tiny clumps or floccules. The floccules can then be bound together by cementing agents such as the microbial polysaccharides or iron oxides to form the stable aggregations in the topsoil.

The mechanism of cementation due to organic matter can be utilized to explain the increase in particle sizes of the residual HWD. The sample JCD has a significant amount of lime content. This compound causes cementation, similar to that of lime stabilization. Therefore the significant increase in grain size of JCD samples can be attributed to the presence of organic contents and calcium oxide. Residual HWD contains also metallic oxides in small quantities and hence the dominant cause for increase in grain sizes for these samples can be attributed to cementation by organic contents. More detailed explanation is presented in the doctoral thesis of one of the authors (Basim, 1999)

Table 1-Grain Size Data for HWD, and JCD at Different Conditions.

Residual HWD JCD Fresh Dry Freeze/Thaw Fresh Dry Freeze/Thaw % fines 86 48 35 100 30 24 % sand 14 52 65 --- 70 76 D10(mm) 0.004 0.006 0.006 0.0008 0.014 0.014 D30(mm) 0.012 0.024 0.06 0.0034 0.08 0.1 D50(mm) 0.04 0.4 1.4 0.006 0.28 0.3 D60(mm) 0.024 0.24 0.8 0.008 0.5 0.38 Cu 6 40 133.3 10 35.7 27.14 Cc 1.5 0.4 0.75 1.8 0.9 1.88 Specific Surface (cm2/g)* 2820 282 81 23800 510 476

*The specific surfaces are calculated by assuming the particles are as perfect spheres and the average size D50 is used in these calculations.

References

Basim, Swamy C., ? Physical and Geotechnical Characterization of water Treatment Plant Residuals?, Dissertation presented towards partial fulfillment of the degree of Doctor Of Philosophy, New Jersey Institute of Technology, New Jersey, May 1999.

Berger, and Kermit C., Sun, Soil and Survival ? An Introduction to Soils, University of Oklahoma Press, 1965.

Kohnke, H. 1968. Soil Physics, McGraw Hill, Inc., New York,

Schalsa, E.B., Gonzalez, C., Vergara, I., Galindo, G., and Schatz, A., 1965. ?Effect of Drying on Volcanic Ash Soils in Chile?, Proceedings of the Soil Science Society of America, 29, 481.

USDA, 1951. Soil Survey Manual, Handbook 18,

Van Schuylenborgh, J., 1954. The Effect of Air-drying Of Soil Samples upon Some Physical Soil Properties, Netherlands Journal of Agricultural Society, 2,50,

Weil, Ray R., 1998. Laboratory Manual for Introductory Soils, Sixth Edition, Kendall Hunt Publishing Company.