By Brett W. Maurer and Shobha K. Bhatia
Geotextile tubes have been successfully used by numerous industries to dewater a variety of high-water-content slurries and waste materials. Concurrent with the expanding use of geotextile tubes, the desire to reduce effluent turbidity and improve the speed of dewatering has led to the established use of polymer flocculants, sometimes referred to as dewatering conditioners or accelerants.
Researchers and practitioners alike can attest to the prevalence and success of polymer flocculants, which bind suspended sediments through chemical mechanisms to form “flocs.” While flocculants have become an essential component of geotextile tube dewatering, the fundamentals and mechanisms governing polymer performance are generally overlooked. However, two flocs with identical supernatant turbidities could have different physical characteristics and perform differently in a geotextile tube if formed by contrasting flocculation mechanisms.
The extent to which floc properties influence performance has not previously been discussed in current literature. This article presents a brief overview of flocculation fundamentals and their potential influence on geotextile tube dewatering performance.
By nature, fine particles normally have a net negative surface charge, causing them to repel one another. This inter-particle repulsion opposes aggregation and stabilizes colloids in suspension for long periods of time, resulting in high turbidity.
Because the presence of suspended solids in a geotextile tube input material increases initial soil piping and reduces the overall dewatering rate, it is desirable to aggregate solids. For this to occur, particles must either be bridged via chemical connections or the repulsion forces must be negated so particles are free to collide and combine. While surface chemistry and flocculation are highly complex, the subject may be simplified into two principle mechanisms of flocculation: bridging and charge neutralization.
The bridging mechanism is typically used to describe the destabilization of negatively charged particles by the addition of a non-ionic, anionic, or cationic polymer. In this model, polymer segments are adsorbed on multiple adjacent particles, bridging them together to form an extended network of connections. A conceptual illustration of the bridging mechanism is shown in Figure 1a.
Despite the electrostatic repulsions, anionic polymers can attach to negatively charged surfaces through ion binding if there is a sufficient concentration of metal ions. For example, calcium ions (Ca2+) can promote adsorption by linking carboxyl (–COOH) groups with anionic sites on the particle surface (Elimelech et al., 1995). Because polymer chains must span the distance over which the inter-particle repulsions occur, the most effective bridging polymers generally have high molecular weights (i.e., long chain lengths) (Gregory, 2006).
For anionic bridging polymers, the charge density is also important. The electrostatic repulsion between a highly charged anionic polymer and negatively charged particles generally limits polymer adsorption. A lesser charge repulsion causes the polymer chain to adopt an expanded configuration with many loops and tails (Nasser and James, 2006), which may increase the success of bridging connections.
The charge neutralization mechanism is typically used to explain the destabilization of negatively charged particles by the addition of a cationic polymer. The strong adsorption between the positively charged polymer and oppositely charged particles neutralizes the surface charge, reducing repulsions and allowing collisions and aggregation to occur.
Because the optimum polymer dose is generally that required to neutralize the surface charge, it follows that charge density is the more important cationic polymer property (Elimelech et al., 1995). Because of the strong attractive forces, polymers tend to adopt a flat configuration on the surface of oppositely charged particles, limiting the possibility of the polymer to attach to multiple particles. For this reason, the molecular weight of charge neutralization polymers is less critical than with bridging polymers (Gregory, 2006). A conceptual illustration of the charge neutralization mechanism is shown in Figure 1b.
Floc characteristics and geotextile tube performance
To demonstrate the importance of flocculation mechanisms, a series of laboratory experiments was conducted on a silty dredged sediment with 33% solids content by mass.
Through extensive jar testing, zeta-potential and turbidity vs. dose curves were used to identify the optimum anionic and cationic flocculants from a pool of more than 40 polyacrylamide-derived polymers. Polyacrylamide is nearly always chosen for geotextile tube projects because it has high solubility in water and is easily modified.
For this material, bridging and charge neutralization polymers were both found to be effective and achieved identical supernatant turbidities of 15 nephelometric turbidity units (NTU) at approximately the same dose. As compared to the slurry turbidity prior to flocculation, estimated to be 56,000 NTU, the flocculants reduced turbidity by 99.9%.
The physical characteristics of flocs produced by the anionic and cationic polymers, however, were consistently found to be distinctly different.
The typical cationic floc structure, pictured at left in Figure 2, is composed of discrete spherical particles.The cationic flocs can easily be poured between receptacles in a fluid manner and behave as a collection of individual particles that do not demonstrate inter-particle attraction.
The typical anionic floc structure, pictured at right in Figure 2, is composed of a more continuous or interconnected mass of large flocs. The anionic flocs have large global voids but appear dense on a more confined scale. Individual anionic flocs are difficult to distinguish and cannot easily be poured or transferred because the floc structure tends to behave as one cohesive “clump.”
This behavior is indicative of the bridging mechanism. The high molecular weight and extended chain configurations favor the formation of extensive bridging networks. Thus, two flocs formed from the same material may produce identical turbidities but have different physical characteristics and, consequently, form different filter cake structures.
To investigate the significance of floc structure on geotextile tube performance, the pressure filtration test (PFT), a geotextile filtration performance test, was conducted using a high-strength woven polypropylene geotextile. The PFT uses a 600ml slurry sample and applies 34kPa pressure, representative of the pressure generated in a geotextile tube. The test results, presented as filtrate volume passing through the geotextile vs. time, are shown in Figure 3.
A distinct difference in the filtration performance can be seen for the anionic and cationic flocculants. On average, the dewatering rate when using the anionic polymer was 2.6 times greater than with the cationic polymer. In terms of piping, the anionic flocculant also had a slight advantage due to its larger, more interconnected, floc structure that limited passing of flocs through the geotextile. However, following dewatering, the filter cake height was on average 10% greater when using the anionic polymer, meaning its cationic counterpart produced a more compact filter cake.
In addition to the PFT, measurements of the floc’s resistance to shear and compression were undertaken using the critical mixing speed and compressive yield stress tests, both developed in the field of colloid science. Under forces representative of those found in a full-scale geotextile tube, the cationic flocs were found to be less resistant to both shear and compression. Together, the filter cake height and compressibility provide an indication of how much material can be treated per geotextile tube, which could be significant on high-volume geotextile tube projects.
To put these findings to the test, the pressure filtration test was performed using multiple fillings, representative of field practice. By comparing the dewatering rate of the initial fillings with those of subsequent fillings, the initial dewatering rate was better maintained when using anionic polymer. In other words, the dewatering rate had less degradation. These findings agree with the results of the compression measurements, which found that anionic polymer produced flocs that were more resistant to compression. If less compression occurs under loading, such as the overburden pressure created by multiple filling phases, then it could be expected that permeability, and hence dewatering rate, would be better maintained.
A case study of the geotextile filtration performance of flocculated dredged sediment has been presented; other sediments having different composition and surface chemistry may or may not have similar trends. For this particular silty sediment with net negative charge, both anionic and cationic polymers achieved identical turbidities but had different performance in geotextile filtration tests.
The cationic flocs were found to be smaller, more discrete, less shear resistant, more compressible, and slower to dewater than anionic flocs. The ultimate flocculant selection may depend on the specific needs of the geotextile tube project.
If the strength of flocs and dewatering rate are most important, an anionic polymer is preferred for this material. In contrast, if compact sediment is preferred so the mass of sediment treated per geotextile tube is maximized, a cationic flocculant is preferred for this material.
The selection of polymer flocculants should extend beyond a visual or pseudo-experimental analysis. Many factors in addition to turbidity should be taken into consideration when selecting a polymer because it has been shown that flocculation mechanisms may significantly affect geotextile tube dewatering performance. A more thorough and scientific evaluation of floc characteristics could dictate capital savings and project success.
Brett Maurer is a teaching assistant in the Syracuse University Department of Civil and Environmental Engineering.
Shobha Bhatia is the Laura J. and L. Douglas Meredith Professor for Teaching Excellence in the Syracuse University Department of Civil and Environmental Engineering. She is Technical Program co-chair for Geosynthetics 2013 (April 1-4 in Long Beach, Calif.).
The authors would like to thank G. Lebster of WaterSolve LLC and D. Hunter of BASF Corp. for sharing their industry experience and providing polymer flocculants; and P. Kaye and V. Ginter of TenCate Geosynthetics for providing geotextiles used in this study.
Elimelech, M., Gregory, J., Jia, X., Williams , R.A. (1995). Particle Deposition and Aggregation: Measurement, Modelling and Simulation, Butterworth-Heinemann Publishing, 448 pp.
Gregory, J. (2006). Particles in Water: Properties and Processes, IWA Publishing, 188 pp.
Nasser, M.S., James, A.E. (2006). “The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behavior of kaolinite suspensions,” Separation and Purification Technology 52 (2), 241-252.