Geosynthetics International, 2003, 10, No. 4 Long-term filtration performance of nonwoven geotextile-sludge systems A. H. Aydilek 1 and T. B. Edil 2 1 Assistant Professor, Department of Civil and Environmental Engineering, University of Maryland, 1163 Glenn Martin Hall, College Park, Maryland 20742, USA, Telephone: +1 301 314 2692, Telefax: +1 301 405 2585, E-mail: aydilek@eng.umd.edu 2 Professor & Chair Geological Engineering Program, and Professor, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA, Telephone: +1 608 262 3225, Telefax: +1 608 263 2453, E-mail: edil@engr.wisc.edu Received 13 January 2003, revised 15 May 2003, accepted 21 June 2003 ABSTRACT: A study of the filtration behaviour of contaminated wastewater treatment sludges was conducted. The laboratory portion of the study included a series of filtration tests with different nonwoven geotextiles. Filtration performance of the sludge–nonwoven-geotextile systems was also observed in field test cells by exhuming geotextile samples from the cells after exposure followed by analysis. The results indicated that the sludges could be filtered with nonwoven geotextiles selected on the basis of geotextile permittivity. The standard gradient ratio test did not always reflect the filtration performance, and therefore other clogging ratios should be considered. Two commonly used geotextile constriction sizes, O 95 and O 50 , could not be related to either clogging or piping. Additionally, ratios of geotextile constriction size to soil particle size in the existing filter criteria did not always predict the observed filtration performance. KEYWORDS: Geosynthetics, Capping, Filtration, Gradient ratio test, Image analysis, Nonwoven geotextile, Pore constriction size, Sludge REFERENCE: Aydilek, A. H. & Edil, T. B. (2003). Long-term filtration performance of nonwoven geotextile-sludge systems. Geosynthetics International, 10, No. 4, 110–123 1. INTRODUCTION The retirement of large industrial waste storage facilities in accordance with environmental regulations has become a critical cost issue for industry and a challenge to the geotechnical community. Many facilities were constructed prior to the emergence of modern environ- mental regulations and contain a variety of contami- nated high water content materials. Some examples are PCB (polychlorinated biphenyls) containing wastewater treatment sludges, contaminated harbour dredgings, waste pickle liquor sludges, asbestos-containing sedi- ments, and contaminated river bottom sediments. These materials are typically contained in surface impound- ments such as lagoons, ponds or old quarries. One of the least costly remediation alternatives is capping (Grefe 1989; Zeman 1994). Among various types of capping options, composite caps are usually preferred, in which a geotextile component serves as a reinforcement, separa- tion, and filtration layer. The emphasis in this paper is on the filtration performance of nonwoven geotextiles in such a com- posite cap. This study was undertaken as part of the development of a closure system for the PCB- contaminated wastewater treatment sludges in Madison, Wisconsin. For the evaluation of the filtration perform- ance, both clogging and retention behaviour of these filters had to be investigated. The low water solubility of PCBs in contaminated sludges and their partition on the solid phase requires that the retention performance of these filters be analysed considering more stringent piping rate limits than the existing criteria. On the other hand, high organic content in sludge promotes clogging, which requires an assessment of the applic- ability of existing anti-clogging criteria to sludge. To respond to this need, laboratory soil filtration tests (gradient ratio and filter press tests) and geotextile permittivity tests were conducted with various sludge– geotextile systems. The same tests were also performed on a reference silty sand having the same particle size distribution as the sludge. The long-term filtration performance of sludge–geotextile systems was also observed in four field test cells capped using a light- weight fill. Permittivity tests and image analyses were performed to quantify the degree of clogging observed in the laboratory and in the field. Finally, comparisons were made between the laboratory and field results. This paper presents the results of analyses performed using nonwoven geotextiles; woven geotextiles were previously presented by Aydilek and Edil (2002). 110 1072-6349 # 2003 Thomas Telford Ltd Geosynthetics International 2003.10:110-123.