A study on biological clogging of nonwoven geotextiles under leachate flow

Abstract

This paper presents results of long-term permittivity tests using leachate to evaluate biological clogging of nonwoven geotextiles. Three types of geotextiles with varying masses per unit area were used in the tests. The identification and quantification of microorganisms in the geotextile were carried out as well as microscopic investigations. The accuracies of semi-empirical models to evaluate the kinetics of bacteria growth and to correlate hydraulic properties and microbiological parameters were examined. Permittivity tests under increasing water heads were also performed on the geotextile samples already subjected to long-term leachate flow in order to evaluate the values of water heads required to wash the biofilms out of the geotextile pores. The results of the tests showed the marked reduction of geotextile permeability due to biological clogging and that the results of the predictions by semi-empirical methods were consistent with the biological mechanisms observed.

Introduction

Geotextiles have been extensively used in drainage and filtration of geotechnical and geoenvironmental works for the last four decades. These materials can be very cost-effective in comparison with traditional granular drainage layers, particularly in regions were sand and gravel are scarce or their exploitations are restricted by environmental regulations. Geotextiles are easy to transport and install and by being manufactured products their characteristics and properties can be assured.

Many studies examining the behaviour of geotextiles in drainage and filtration can be found in the literature (Giroud, 1982; Gourc and Faure, 1990; Bhatia et al., 1991; Fannin et al., 1994; Giroud, 1996; Lafleur, 1999; Palmeira and Gardoni, 2000, for instance). Most of these studies have investigated the retention capability, hydraulic properties and clogging potential of geotextiles, particularly for applications as drains and filters in geotechnical engineering works. However, the marked growth of geotextile applications in environmental projects has also demanded research on these products performance in leachate collection systems of landfill and filters of mining tailings facilities, for instance.

Drainage and filter layers in landfills have to cope with more severe clogging mechanisms than in other types of engineering works. Leachate is a complex and heterogeneous material that may cause physical, chemical and biological clogging of filters and studies from different researchers have reported granular and geotextile clogging under laboratory and field conditions (Cancelli and Cazzuffi, 1987; Koerner and Koerner, 1990; Brune et al., 1994; Cazzuffi and Cossu, 1993; Fourie et al., 1994; Mlynarek and Rollin, 1995; Kossendey et al., 1996; Rollin, 1996; Fleming et al., 1999; Silva et al., 2002; Fleming and Rowe, 2004; Rowe, 2005; Rowe and McIsaac, 2005; McIsaac and Rowe, 2005, McIsaac and Rowe, 2006, McIsaac and Rowe, 2007). In the laboratory, most of the studies have examined the performance of geotextile filters in filtration devices where leachate (either real or synthetic) was used.

Cancelli and Cazzuffi (1987) conducted permittivity tests on nonwoven geotextiles using leachate under a range of normal stresses from 0 to 200 kPa. The geotextile specimens were subjected to a continuous flow of leachate in a hydraulically closed system. The authors observed a significant decrease of geotextile permeability caused mainly by sedimentation of suspended solids in the leachate on the geotextile. Silva et al. (2002) obtained similar results in large scale filtration tests on geotextiles using leachate from a dump in Brasilia, Brazil. Cazzuffi and Cossu (1993) performed similar tests and investigated the effect of immersion of the geotextile specimen in leachate prior to the permittivity test.

Fourie et al. (1994) carried out a study similar to the ones described above but with a sand layer on top of the geotextile specimen in order to avoid the influence of the suspended solids in the leachate and under aerobic conditions. These authors comment that aerobic conditions are likely to accelerate the formation of bioclogging in the geotextile and suggest the addition of an antimicrobial agent to the geotextile during manufacturing to inhibit the attachment of bacteria to surfaces. However, the lifetime of this agent has to be compatible with the lifetime expected for the drainage system.

Rollin and Dennis (1987) pointed out the importance of a satisfactory performance of the drainage system in landfill applications and discussed the selection of geotextiles with particular reference to its function in the work (drainage, filtration or separation). These authors examined clogging risks and suggested maximum and minimum values of geotextile filtration opening sizes and permeability levels depending on the geotextile role in the landfill. Rollin and Denis emphasised the importance of quality assurance programmes in this type of work.

The effects of aerobic and anaerobic conditions, different geotextiles, with and without soil (Ottawa sand) were evaluated in tests with leachates from different landfills by Koerner and Koerner (1990). Different levels of clogging were observed varying from no clogging up to complete geotextile clogging. The authors recommend the use of cleanout procedures for the filter, such as leachate, water or nitrogen gas backflush or vacuum extraction to extend its lifetime. It is likely that the mound of leachate on the clogged filter will build up in the waste until it becomes large enough to cause breakthrough of the clogged filter, causing the leachate to flow rapidly through the drainage system until further filter clogging occurs and the process repeats itself.

Corcoran and Bhatia (1996) presented a study on the conditions of exhumed samples of nonwoven heat bonded geotextile as filter in the collection trench at Fresh Kills landfill, NY, USA. The installation of the geotextile was more than 4 years old and hydraulic and mechanical properties of the geotextile were investigated. The results of permittivity tests showed a reduction of geotextile permittivity from 0.8 to 0.0615 s⁻¹, caused by partial clogging of the geotextile by soil particles intrusion. The authors believe that most of the particles penetration into the geotextile filter occurred in the early period of installation before the geotextile had formed a natural graded filter with the adjacent soil, reaching a stable and acceptable situation. No biological clogging was observed and Corcoran and Bhatia attribute that to the low values of BOD₅, which are characteristic for the Fresh Kills landfill leachate. Based on this study the authors recommended the use of nonwoven geotextiles in future collection systems at that landfill. Junqueira et al. (2006) also investigated the performance of geotextile filters in experimental waste cells and concluded that for the duration (6 years) of the experiments both granular and geotextile filter performed satisfactorily.

It is clear from the works performed so far that biological clogging is a complex mechanism that can cause failure of a drainage system to fulfil its role in a landfill. This paper investigates the performance of geotextile filters under conditions prone to yield to biological clogging and the development of the mechanisms involved in such process.