Performance of Geosynthetics in the Filtration of High Water Content Waste Material

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1 INDIAN GEOTECHNICAL SOCIETY CHENNAI CHAPTER Performance of Geosynthetics in the Filtration of High Water Content Waste Material T. Arun 1 and K. Ilamparuthi 2 ABSTRACT: Filtration mould was fabricated in the laboratory for dewatering studies using flyash on woven geotextiles. Filtration tests were conducted on flyash slurry using two woven geotextile G1 and G2 with AOS of 75microns and 150 microns respectively by varying parameters like initial water content and head of sludge. The relation between sludge-water systems for the geotextiles used in this study was established and the relationship obtained was identical to that of the response of soil water system as reported by Koerner and Ko (1982). From the response curves filtration efficiency and dewatering efficiency were determined. For the sludge and geotextile combination critical water content was determined at which both the efficiencies were close to 100%. The sludge cake formed during the process of dewatering was analyzed for characterizing its quality with reference to density, residual water content, void ratio and cake height. The effect of these parameters due to change in initial water content, head of sludge and opening size were established. KEYWORDS: Geosynthetics, Flyash, Filtration, Sludge, Water content Introduction A high proportion of waste generated annually takes the form of slurries or sludges with high water contents. These waste materials are usually characterized by a high percentage of fine particle sizes. They are generated from a variety of sources such as mining operations, tannery sludge, paper mills, and agriculture and industrial sites. For example, millions of cubic meters of flyash are disposed of annually from the thermal power plant and tannery sludge from tannery industry. The high water content and toxic nature of these materials are preventing them to reuse or safe disposal. Dewatering must first be accomplished to reduce the volume of water. Then the dewatered materials can be transported to a landfill or beneficially used as construction materials in dike enforcement, wet land restoration or creation and other uses if they are free from objectionable materials. Among the alternatives for dewatering high water content sludges geotextile tubes comprise an emerging technology, as one of the numerous applications of geosynthetics in civil engineering. In the recent years, a number of tests in the laboratory have been conducted to characterise geosynthetic materials for the purpose of dewatering and dewatering high water content sludges (Aydilek and Edil, 2003; Fourie and Addis, 1999; Liu and Chu, 2006; Muthukumar and Ilamparuthi, 2006; Haung and Luo, 2007). Still more needs to be known on various parameters pertaining to sludge and geotextile in selecting suitable geotextile material for dewatering sludges. With these review tests were conducted on two waste materials in this study, but the results of tests on flyash only presented here. Experimental Investigation Materials and Properties Flyash used for the study has collected from Thermal Power Station, Ennore. It was collected in the dry form and preliminary tests were conducted in the laboratory. The properties of the flyash are presented in Table 1. Figure 1 illustrates the particle size distribution of Flyash. Two types of woven geotextiles were taken and their properties are presented in Table 2. The values presented are as reported by the manufacturers. Table 1 Properties of Flyash Property Value Specific gravity, Gs 2.14 Liquid limit, % 36 Fines content (<0.075mm), % 94 Fine sand, % 6 Silt, % 88 Clay, % 6 Fig. 1 Particle Size Gradation of Fly Ash 1 P.G Student, College of Engineering, Guindy, Anna University, Chennai , E -mail thangaveluarun@yahoo.co.in 2 Professor, College of Engineering, Guindy, Anna University, Chennai , kanniilam@gmail.com

2 44 STUDENT S PAPER COMPETITION 2009 Methodology Table 2 Properties of Geotextiles Properties Material G1 G2 Mass per unit area, (g/cm 2 ) Tensile strength in warp/weft, ( kn/m ) 55/40 55/52 Elongation at peak load, IS 1969, (%) Apparent opening size, ASTM D4751, (µ) Water flow rate, ASTM D4491, (l/m 2 /s) 9 32 Fiber type PP PP Puncture strength, ASTM D4833, ( N ) Filtration tests were conducted for two types of geotextiles. Flyash sludge was used for the study. The efficiency of geotextile to dewater sludge is influenced by different factors and in this study the following factors are given due importance (i) water content (ii) head of sludge and (iii) AOS of geotextile. The effect of water content and the head were studied by changing the initial water content and the head of sludge for flyash. Increments were fixed based on the pilot test conducted. Filtration Efficiency and dewatering efficiency were computed from the test results. Filter cake formation at the end of every test were characterized for their properties. Experimental Procedure The dry flyash was mixed with water to prepare model sludge. The sludge prepared at specific water content and head were filled in the filtration chamber keeping the valve in the collection chamber closed to hold the flow until filling is complete. Filtrate were collected from the outlet continuously and measured simultaneously to observe the flow rate variation during the test for a test period of 24hours. Sludge volume will be considerably reduced by conducting the process. The filtrate was later tested for total suspended solids. After completing test, sludge cake had formed in the filtration tube on the upstream of the geotextile. The density of the top and bottom layer of sludge is found out by using thin wall samplers of 17mm height and 26mm diameter. Samples were taken in a diagonal direction both from the top and bottom part of filter cake. Similarly samples were also taken at top and bottom layer for determination of water content and analysis of particle size distribution. The cake is then removed from the upstream of geotextile and the geotextile is tested to get permittivity value as per ASTM D Results and Discussions Flow Response of Test on Flyash Figure 3 illustrates the flow response for the test Filtration Apparatus Filtration apparatus comprises of two components namely filtration tube and collecting chamber as shown in Figure 2. All these are made up of acrylic material. Filtration mould of 60cm high and 15cm inner diameter dewatering column were used to carry laboratory dewatering tests on flyash and tannery sludge. The collecting chamber is a funnel like arrangement with inner diameter same as the inner diameter of filtration tube. A filter plate specially made of stainless steel is used in between filtration tube and collecting chamber in order to avoid sagging of geotextile. Fig. 3 Comparison of Flow Response Curves for Different Water Contents Using G1 5mm Steel mesh support Geotextile 130mm 600mm 150mm Top cover Slurry Acrylic tube Fig. 2 Filtration Test Setup Rubber gasket Filtration tube Filtrate Collecting chamber carried out on the combination of initial water content and head of the sludge. The flow rate is characterized by a factor called coefficient of initial flow response (C if). Generally for a given water content and opening size of geotextile, the flow rate is high initially and decreases with time. The rate of decrease of flow is linear up to certain time and thereafter changes its slope. The change in rate of flow in the later part is very gentle. The flow rate increases with increase in head as well as initial water content. The flow rate follows the same pattern for the G2 as well, for the water contents of 100% and 500%. The main difference between the results is high discharge in G2 than G1 at any given filtration time. The C if value increases with increase in head as well as initial water content (Figure 4 in the next page). This is very much true for G2 whereas for G1 the increase in C if for the initial water content higher than 300% is marginal. Based on AOS of geotextile, C if value is high for higher opening size geotextile and vice versa.

3 GEOSYNTHETICS & FILTRATION OF HIGH WATER CONTENT WASTE 45 Efficiency of Geotextiles Figure 6 shows the plot between efficiency and water content for 100mm initial head. At lower water content, the dewatering efficiency is low and with increasing water content, it increases. Filtration efficiency is high at lower water content and with increasing water Fig. 4 Coefficient of Initial Flow Response with Variation of Head for Different Water Content Response of Discharge Curves for Flyash Figure 5 shows the comparison of discharge curves for different head using G1 geotextile. Generally the discharge increases with increase in time at decreasing rate and in some situation the rates of Fig. 5 Discharge Curves for Different Head using G1 Fig. 6 Efficiency of Geotextile and Initial Water Content Relation for Flyash Using G1 content it decreases. The decrease in efficiency is minimal for both the geotextiles. These two efficiencies when plotted on the same graph against water content of sludge they intersect at a point, which is known as critical water content at which both has the same value. The geotextile functions to the fullest capability at this water content. Almost identical response as obtained for G1 was obtained for G2 also between the efficiency and water contents. The critical water content value increases with increase in head of sludge for both the geotextiles (Fig 7). The dewatering efficiency decreases as the head of the sludge increases and filtration efficiency slightly increases as the head increases due to the lesser opening size of geotextiles chosen in this study. discharge is constant or almost nil indicating constant cumulative discharge. It is because of no more water will be expelled from the filtration tube. The rate of increase in cumulative discharge is very high at the initial time and then it decreases with respect to time. The slope of the slanted line is high for 500mm head curve when compared to other two curves. The cumulative discharge value increases with increase in water content keeping head constant. With the increase in head the cumulative discharge value for specific initial water content is high. The cumulative discharge value for higher opening size geotextile is higher than the lesser opening size. Flow Response with respect to AOS For the water content of 100%, the difference in flow rate curve between G1 and G2 is marginal. However with increasing head, the difference between the flow rate value for G1 and G2 increases for a constant initial water content. The second stage of the flow rate curves between G1 and G2 doesn t show any marked difference for 100% water content with varying head. Beyond transition time the curves are no more distinct. The discharge curve shows higher value for G2 than G1 because of larger opening size. The slope of the slanted portion of the discharge curve goes on increases with varying head for 100% water-content. Fig. 7 Critical Water Content with Head for Flyash The transition time increases with increase in head corresponding to any water content. Transition time doesn t show much variation with the increase of initial water content for a particular head in case of G2. Smaller is the opening size of geotextile higher is the transition time irrespective of the head of the sludge. Characteristics of Filter Cake Grain Size Analyses for Flyash The top and bottom part of filter cake formed is used for the grain size analyses. For the 100% water content combinations with different head, the variation in

4 46 STUDENT S PAPER COMPETITION 2009 grain size curve between top and bottom sample is very less or it is similar in nature (Figure 8). On increasing the lower opening size. With increase in head for particular water content, the percentage finer will also increase for top sample. In the bottom sample, with increase in head the percentage fines decreases for particular water content. Density of Filter Cake ( r f ) Fig. 8 Grain Size Distribution of Top and Bottom Sample water content or head, the top sample shows higher percentage of fines than the bottom sample (Figure 9). The fines content goes on increases as the head increases keeping constant water content for top sample. The difference between top and bottom is more pronounced for the higher opening size of geotextile than The density increases with head in general and between the top and bottom samples, bottom sample density is always higher irrespective of water content and density for the soil cakes obtained using both the geotextiles (Table 3). However the density of top sample reduced with head for the lower water content. With the increase in water content, the density decreases for top and bottom samples for a given head. Density of bottom sample increases for the geotextile with larger opening size. Density of top sample doesn t show much variation between the two geotextiles however higher opening size has provided higher density to the filter cake formed. Residual Water Content Residual water content (w r) is more in the top part of cake than bottom part of the cake in all the tests conducted as shown in Table 3. Residual water content is higher in cake formed from the sludge with higher initial water content and residual water content increases marginally with initial water content. Similar response is seen for the geotextile G2 which has higher opening size than G1. Further between the two geotextiles, the residual water content in the bottom sample is almost equal and the maximum difference is about 3% particularly for the lower water content sludge drained at lower head. Conclusions Fig. 9 Grain Size Distribution of Top and Bottom Sample of 500% Water Content The individual influence of the factors like water content of the sludge, head of the sludge and AOS of geotextiles on the flow response of sludge-water system was analysed. Table 3 Results of Density of Filter Cake and Residual Water Content Material G1 G2 r f w r r f w r Initial Water content Head Top Bot Top Bot Top Bot

5 GEOSYNTHETICS & FILTRATION OF HIGH WATER CONTENT WASTE 47 > Flow rate increases with increasing water content and opening size of geotextile for a given particle size distribution of sludge. For given water content, flow rate is higher for bigger opening size geotextile. The coefficient of initial flow response increases with opening size, head of sludge and water content. > The time for formation of filter cake is more for smaller opening size geotextile and reduces with increasing opening size. The time at which the flow ceases is more for smaller opening size geotextile. > Filtration efficiency is less affected by water content, head of sludge and AOS whereas dewatering efficiency increases with water content and decreases with increase in head for lesser opening size geotextile. Critical water content is different for the geotextiles tested. The critical water content for a particular geotextile increases with increase in head of sludge. Between the geotextiles used, the critical water content is less for the higher opening size geotextile. > Particle size curves for top and bottom sample doesn t show much variation with respect to varying head. With the increase in water content, fines in the top part of filter cake is more than at the bottom part. With high discharge head similar trend is exhibited by the filter cake for both the geotextiles G1 and G2. References Aydilek and Edil, (2003), Long-term filtration performance of nonwoven geotextile-sludge systems, Geosynthetics International, 10, No. 4, pp Fourie and Addis, (1999), Changes in filtration opening size of woven geotextiles subjected to tensile loads, Geotextiles and Geomembranes, 17, pp Huang and Luo, (2007), Dewatering of reservoir sediment slurry is using woven geotextiles. Part I: Experimental results, Geosynthetics International, 14, No. 5, pp Liu and Chu, (2006), Modelling the slurry filtration performance of non woven geotextile, Geotextiles and Geomembranes, 24, pp Muthukumaran and Ilamparuthi, (2006), Laboratory studies on geotextile filters as used in geotextile tube dewatering, Geotextiles and Geomembranes, 24, pp