- Email: [email protected]
Use of cotton gin trash (CGT) to form a biological lagoon sealant
Bioresource Technology 69 (1999) 207±213
Use of cotton gin trash (CGT) to form a biological lagoon sealant J.W. Smith, E.W. Tollner* Driftmier Engineering Center, University of Georgia, Athens, GA 30602-4435, USA Received 9 June 1998; revised 27 November 1998; accepted 13 December 1998
Abstract Because of large volumes of cotton gin trash (CGT) and limited options for correct disposal of CGT, a study was launched to ®nd alternative uses for this material. The objective of this report was to explore the possibility of using CGT as a waste lagoon sealant. CGT was applied in 0, 5, and 10 cm layers in 10.15 cm ID acrylic columns with three replicates per treatment. A sandy loam soil was used as a base and as a top soil to hold the CGT in place. The columns were subjected to a constant head of ®ltered tap water. Static compaction was applied at day 7 and kneading compaction at day 12. The columns were destroyed and concentrations of bound extracellular polysaccharides were measured at day 15. A high correlation (R2 0.95) between total EPS concentration and cumulative hydraulic conductivities was observed. The ®nal average hydraulic conductivity of the columns was 2.2 ´ 10ÿ6 cm/s, 2.2 ´ 10ÿ6 cm/s, 1.0 ´ 10ÿ5 cm/s for the 5 cm CGT layer, 10 cm CGT layer, and control columns, respectively. The 5 cm layer had lower hydraulic conductivity and total pollutant loading than the 10 cm layer, suggesting an optimum layer thickness is less than 10 cm. The study suggested that CGT plus compactive eort has excellent potential for sealing waste lagoons. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biological sealing; Microbial gums; Polysaccharides; Cotton gin trash
1. Introduction Treatment lagoons have become an integral waste management tool for complying with the zero discharge requirements of the Clean Water Act of 1972. Biological sealing has been indicated as the primary mechanism in sealing of these lagoons (Chang et al., 1974; Hills, 1976). Extensive research on the pollution of groundwater from lagoons (Ciravolo et al., 1979; Ritter et al., 1984; Westerman et al., 1995; Drommerhausen et al., 1995) has concluded that some soils do not adequately seal, despite a natural biological sealing process. Hills (1976) recommended clay liners in course textured soils (clay <7.5%). Clay liners can cost approximately $12,000/ha. Some swine operations in South Georgia are planning to use plastic liners, at a cost of $50,000/ha (Reed et al., 1995), in order to receive a waste discharge permit. Such measures can increase capital costs ten-fold, making a more economical method of lagoon sealing desirable (Smith et al., 1998). Sealing is ®rst induced by physical blockage from particulates and then biological blockage by microbial * Corresponding author. Tel.: +001-706-542-1653; fax: +001-706-5428806
extra cellular polysaccharides (EPS), microbial gas production, and biomass (Hills, 1976). Materials that contain high C/N ratios and are readily decomposable produce polyuronides, a polysaccharide linked to soil sealing (Avnimelech and Nevo, 1964). Herrman and Elsbury (1987) ranked soil type as the most in¯uential factor in seal development. The next identi®ed factor was the compaction process, including type, weight, and water content. Herrman and Elsbury (1987) noted the most common type of compaction used was static compaction, such as a rubber-tired roller, not because of performance but present practice. In a laboratory study, Mitchell (1965) found that kneading compaction, such as a sheepsfoot roller, generated hydraulic conductivities approximately one order of magnitude lower than static compaction. The study reported here was the second phase of a larger eort to test feasibility of using cotton gin trash (CGT) as a lagoon sealant. Tighter federal clean air laws have prevented the incineration of CGT, renewing interest in alternative utilization strategies (Kennedy, 1994). Tollner et al. (1983) investigated enhancing the biological sealing process by applying CGT into the soil. They observed hydraulic conductivities in the range of 10ÿ5 cm/s, to magnitudes greater than the control, in
0960-8524/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 8 ) 0 0 1 9 7 - 7
208
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
columns applied with a layer of CGT, before any waste was added. However, pollution from the soluble of the CGT caused high organic loading in the ®rst two days of water addition. In the ®rst phase of this study, Smith et al. (1998) measured both bound and free EPS and found no uronides in free EPS, suggesting bound EPS to be critical for biological lagoon sealing. Pre-rinsing CGT had no signi®cant eect on production of EPS and reduced pollutant concentration in the euent; however, it did not reduce total pollutant loading because of higher euent ¯ow rates. The higher euent ¯ow rates could be attributed to the edge eects of the small container used (ID 70 mm), because the pre-rinsed CGT did expand when wetted as did CGT without rinsing. It was also found by Smith et al. (1998) that hydraulic conductivity with CGT decreased with increased concentrations of EPS. Chang et al. (1974) measured the eects of EPS on hydraulic conductivity through soil. Smith et al. (1998) found a smaller change in hydraulic conductivity through CGT, with an equal increase in EPS, than Chang et al. (1974) found in soil. The greater in¯uence of EPS on soil compared to CGT suggests that the sealing advantages due to the production of EPS from the decomposition of CGT could be enhanced from a CGT-soil mixture. The purpose of this study reported here was to further investigate the use of CGT for biological sealing using conditions similar to possible application schemes. The speci®c objectives were to determine: if rinsed CGT could produce similar seal as previously noted from non-rinsed CGT; if compaction plus CGT accelerated sealing; and, if a soil-CGT mixture optimized the eect of increased EPS from CGT and lower hydraulic conductivity of soil. 2. Methods To test these hypotheses, soil columns were constructed with CGT layers and allowed to percolate for 7 days. After this time, a static compaction was applied. At 12 days, a kneading compaction was applied. At 15 days the columns were destroyed and the concentration of EPS at each layer was measured. 2.1. Column preparation The CGT was gathered from a local gin (The Bostwick Gin, Bostwick, GA) in January 1997 and represented the 1996 harvest. The CGT was stored at 2.5°C up to fourteen months at approximately 15% moisture. Before use, the CGT was ground using a ¯ail-type shreader (John Deere shredder attachment for JD 112 mower, Moline, IL). The soil used was taken from an E horizon on the University of Georgia Agronomy farm.
The sandy loam contained 72.9%, 23.0%, and 4.2% of sand, silt, and clay, respectively. Rocks were removed and soil clods broken by screening the soil through a #10 screen. Three percolation columns (Fig. 1) were made from 10.15 cm ID acrylic tubes. The tubes were 152 cm in length with a threaded plug at the bottom. The plug had a 6.4 mm (3/8 in) hose nipple threaded through the center. Five 4.8 mm (1/4 in.) hose nipples were threaded into the side of each column from 0 cm to 20 cm every 5 cm oset by 15°. The bases of all hose nipples were ®lled with glass wool in order to prevent plugging. The attached hoses were used as peizometers. The columns were initially packed with 15 cm of gravel to prevent channeling. A course nylon net was placed over the gravel. Two 540 g layers of soil were packed into the column, forming a 10 cm layer of soil. The CGT was rinsed by placing 127 g of CGT in an 10 l bucket. Three liters of water were added and stirred vigorously for one minute. The water was poured o and the wet CGT was placed in the column, creating a 5 cm layer of rinsed CGT with a bulk density of approximately 0.31 g/cm3 (dry mass). This procedure was repeated in order to produce the 10 cm layer. Another 540 g of soil was placed on top of the CGT. This layer was critical to prevent the CGT from ¯oating to the surface. Three control columns were created by adding a total of 1620 g of soil. Every ®ve cm layer was compacted individually before the next layer was added. Activated carbon ®ltered water was introduced into the columns from the bottom. An activated carbon ®lter was used because there was some evidence in Smith et al. (1998) that the chlorine residual in tap water hindered seal development. A pressure dierential no greater than 10 cm was applied to columns to prevent the compacted material from separating. Once the water level passed the top soil layer, the pressure dierential was increased to the equilibrium point. This process required a 5 day time period for completion. The 6.4 mm (3/8 in) hose connected to the bottom was looped and attached to the outside of the column at the gravel-soil interface in order to prevent unsaturated ¯ow. A ¯oat control valve kept the water at a constant level 127 cm above the soil-gravel interface. Samples were taken from the euent and marked as the midpoint of the sampling period. In order to retrieve enough sample, some sampling periods were as long as 12 h. 2.2. Compaction To study the eect of compaction, two dierent compaction regimes were performed. On day 7 a static compaction was applied. A disk slightly smaller than the inside diameter of the tube was attached to a 2.5 cm steel pipe. The disk was placed ¯at against the top soil surface. A 22.7 kg weight was dropped 15 cm, guided by the
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
209
Fig. 1. Schematic of percolating columns (not to scale).
steel pipe, onto the collar 25 times. On day 12, a kneading compaction was performed. A solid 2.54 cm round stock was attached to the pipe. The pipe had a mass of 8 kg. It was raised 15 cm and dropped 50 times onto the soil surface. 2.3. Hydraulic conductivity For each sample period, the head at each peizometer was recorded. If the levels appeared equal, they were recorded as 0.25 mm for use in hydraulic conductivity calculations. Flow was also determined and Darcy's law was applied to each layer for conductivity data. Measurements were made every day and twice daily after compaction. 2.4. Quanti®cation of EPS At the end of the 15 day trial, the material in the columns was pushed out through the bottom in approximately 5 cm sections. For quanti®cation of EPS, a modi®ed version of the method used by Norberg and Enfors (1982) and outlined in detail by Smith et al. (1998) was used. After hydrolysis of the samples, the
samples were dried, in order to accurately determine the mass of each sample. 2.5. Statistical analyses The statistical analyses consisted of performing several general linear model (GLM) evaluations coupled with evaluating standard error values (computed via spreadsheet) for the logarithm of the cumulative and individual layer hydraulic conductivity values on a daily basis. The NCSS statistical package (Hintz, 1977) was used to make the GLM±ANOVA computations. Main eects were layer thickness and layer position. Time was nested within the main eects. Analyses on cumulative conductivity also were performed which replaced time with compaction as nested eect. A randomized complete block was used for analysing the EPS data. The two CGT layers of the 10 cm column were combined in a composite layer for the EPS analyses. In the event of signi®cant interactions, the GLM or one-way ANOVA procedures were run at each level of one of the interacting variables. An alpha error of 10% was selected as the cuto point for assessing statistical signi®cance. In cases where the time eect was not
210
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
signi®cant the one-way ANOVA model was used. Post hoc tests having a lower alpha are indicated using p 6 xx notation. 3. Results and discussion 3.1. Hydraulic conductivity The hydraulic conductivities of individual layers interacted signi®cantly with both treatment (layer thickness) and time and treatment and layer. The signi®cance of treatment-time interaction and the lack of signi®cance of layer position-time interaction with the log10 transformed individual layer hydraulic conductivity was ascribed to the high variations in conductivity of layers within layer thickness treatments. For example, the ®rst bottom soil layer in the 10 cm CGT treatment was virtually undisturbed while the ®rst bottom soil layer in the 5 cm CGT treatment had some CGT incorporated into it during the kneading compaction. The soil layer below the CGT developed a signi®cantly lower conductivity as time progressed based on analyses by time. Cumulative hydraulic conductivity (Fig. 2) was highly in¯uenced both by treatment and by time (p < 0.01). Over the entire run the hydraulic conductivities of the columns with 5 cm of CGT were signi®cantly lower than the 10 cm columns, which were
signi®cantly lower than the control (p < 0.05). The ®nal average hydraulic conductivities of the columns were 2.2 ´ 10ÿ6 1.9 ´ 10ÿ6 cm/s, 2.2 ´ 10ÿ6 0.6 ´ 10ÿ6 cm/s, 1.0 ´ 10ÿ5 0.02 ´ 10ÿ5 cm/s for the 5 cm of CGT, 10 cm of CGT, and control columns, respectively. The columns with rinsed CGT coupled with compaction reduced conductivity by nearly one order of magnitude compared to the control. This study achieved a similar magnitude of reduction in hydraulic conductivity compared to the control as did the Tollner et al. (1983). The ®rst 15 days of their study involved sand with a layer of CGT. Hydraulic conductivities were relatively constant at 6.4 ´ 10ÿ5 cm/s from 8 to 15 days. The ®nal steady levels of hydraulic conductivity achieved in this study by Tollner et al. (1983) leads one to tentatively conclude that the oxidation-reduction potential of the decomposing layer may limit the sealing process. Some of the dierence in the ®nal hydraulic conductivities between the two studies is a function of soil type. Harr (1962) classi®ed soils by hydraulic conductivity and included ®ne sand, natural clay, and compacted clay with hydraulic conductivities of 5 ´ 10ÿ4 ±10ÿ3 cm/s, 10ÿ6 cm/s and lower, and 10ÿ7 cm/s, respectively. Despite the decrease in hydraulic conductivity by a magnitude form the CGT layer, the ®nal average hydraulic conductivity reached 2.2 ´ 10ÿ6 cm/s and was still above that of compacted clay.
Fig. 2. Average, high, and low hydraulic conductivities with respect to time. Error bars around the average points represent one standard error, N 3.
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
211
The kneading compaction created a CGT-soil mixture. A more intense kneading of the 10 cm, which incorporated more of the bottom CGT, may have resulted in reduced hydraulic conductivity. The layers below the penetration depth of the kneading compaction had the lowest hydraulic conductivity (Fig. 2). The lower hydraulic conductivities could possibly be a function of the position in the column. The lowest layers can collect a larger amount of particulates deposited from the above layers. The same phenomenon was also true in the control column where the second bottom soil layers had ®nal hydraulic conductivities a magnitude greater than all of the prior layers. In the columns with CGT, the ®rst bottom layer had signi®cantly lower ®nal hydraulic conductivities. Since the ®rst bottom layer in the column with 10 cm of CGT was undisturbed by the kneading compaction and had the lowest hydraulic conductivity, the reduction in hydraulic conductivity cannot be attributed to a complementary eect of soil-CGT mixture. Smith et al. (1998) found no uronide accumulation in free EPS, so it is unlikely free EPS produced in the CGT layer traveled down into the subsequent soil layer and aected its hydraulic conductivity. A possible scenario explaining the lower hydraulic conductivity of the ®rst bottom layer is that particulates and microbial biomass from the CGT plugged pores in the ®rst bottom layer and did not travel to second bottom layer. A similar proposed mechanism for sealing was discussed by Hills (1976) and observed by Toebati et al. (1986). Average pore size in the Toebati et al. (1986) study was reduced after introducing media to a percolating column. They concluded the reduction in pore size indicated partial plugging of larger pores from microbial biomass accumulation due to preferential ¯ow through the largest pores. The presence of microbial biomass in subsequent layers below the CGT also supports the presence of bound EPS in the layer below the CGT. It is
also possible that the linking of uronides to sealing could actually be a relationship of biomass and sealing, since uronide accumulation was only found in bound EPS. Measurements of high uronide concentration could actually represent high biomass concentrations which are responsible for the seal formation.
Fig. 3. Five cm CGT layer column EPS and hydraulic conductivity (K) as a function of layer. Error bars are one standard error, N 3.
Fig. 4. Control column EPS and hydraulic conductivity (K) as a function of layer. Error bars are one standard error, N 3.
3.2. Production of EPS A signi®cant two way-interaction existed between layer and treatment for the production of EPS. When a cumulative concentration of EPS was analyzed, treatment was signi®cant (p < 0.01). If the production of EPS is viewed on unit CGT basis, the values are normalized to an average of 2.3 mg EPS/g CGT (s 0.4), which corresponds to the production of EPS noted by Smith et al. (1998) of 2.59 mg EPS/g CGT in an anaerobic environment at 14 days. There was a signi®cant (p < 0.05) interaction between layer and treatment. The dierence in the trends with EPS and layer are evident in Figs. 3±5. The results of compaction were dependent on the thickness of the CGT layer. The column with the 5 cm layer of CGT had the top soil layer integrated into the CGT layer and a portion of the ®rst bottom soil layer. The column with 10 cm layer or CGT had the top soil layer completely mixed with the top CGT layer, but the bottom CGT layer had minimal disturbance. When analyzed within each treatment, concentrations of EPS were signi®cant with layer (p < 0.05). In the control layer, only the top layer had a signi®cantly dierent amount of EPS (p < 0.05). In the columns with a 5 cm CGT layer, only the CGT layer had a signi®cantly dierent amount of EPS (p < 0.05). In the columns with a 10 cm layer of CGT, all layers were signi®cantly dierent (p < 0.05). An example of the insensitive response of hydraulic conductivity solely to EPS is shown in Fig. 3. The CGT layer has the highest concentration of EPS, and the
212
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
Fig. 5. Control column EPS and hydraulic conductivity (K) as a function of layer. Error bars are one standard error, N 3.
greatest hydraulic conductivity. Referring to Fig. 4, one may note nearly double the concentration of EPS in the bottom CGT layer compared to the top, but no signi®cant dierence in hydraulic conductivity (p < 0.05). The control layer (Fig. 5) had signi®cantly lower (p < 0.01) concentrations of EPS, with the top layer having the highest average concentration of 0.22 mg/g of soil. These results lead to the conclusion that uronide production must be accompanied with appropriate interaction with a solid matrix to accomplish sealing. When the total mass of EPS is normalized by the total column mass, it has a high correlation (R2 0.95) to hydraulic conductivity of the column (Fig. 6). Only two 5 cm columns were used in the correlation, because the third column had an uncharacteristically low hydraulic conductivity of 5.6 ´ 10ÿ8 cm/s which is nearly two magnitudes lower than the others, suggesting some other mechanism was responsible for the sealing.
Fig. 6. Eect of EPS concentration of entire column with hydraulic conductivity (K) (not including one point with uncharacteristically low hydraulic conductivity of 5.6 ´ 10ÿ8 cm/s).
3.3. Euent concentrations The 10 cm column had signi®cantly (p < 0.01) higher ammonia and lower nitrate (Fig. 7). The Ammonia roughly doubled, which would indicate washout of preexisting soluble ammonia on mass basis by the percolating water. The reverse of nitrate compared to ammonia suggests some metabolic eects during the course of the percolation. Perhaps a more anaerobic environment existed in the 10 cm CGT layer, causing denitri®ers to more thoroughly remove preexisting nitrate in the 10 cm layer compared to the 5 cm layer. The large variation in the orthophosphate prevented it from being signi®cantly greater, but all measurements from the 10 cm CGT column were greater than the 5 cm CGT column. Variation in the CGT is an issue requiring better understanding. Further studies are planned on nutrient
Fig. 7. Five day cumulative nutrient loading from columns with 5 and 10 cm CGT layers. Error bars represent one standard error (plus only for ortho P and nitrate), N 3.
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
release form CGT for pond sealing purposes. No bene®ts from a larger amount of CGT for the production of EPS were found; the larger amounts CGT had higher cumulative loading of pollutants and higher hydraulic conductivities. The potential pollutant loading with increased CGT layer thickness also justi®es the need for optimizing the layer thickness. 4. Conclusions Rinsed CGT and the described soil compaction regime resulted in an average hydraulic conductivity of 2.2 ´ 10ÿ6 cm/s, approximately an order of magnitude lower than an identically treated control without CGT. The kneading compaction resulted in an order of magnitude reduction in all treatments compared to the static compaction, as noted by Mitchell (1965). The lowest hydraulic conductivity did not occur in mixing the soil with CGT rich in EPS. A high correlation was observed between cumulative EPS normalized by total mass of the column and total column hydraulic conductivity, although evidence suggests that EPS must be plugging a soil matrix to be ecacious. Additional soil treatment compaction regimes are needed to understand the ecacy of CGT as a sealant. Adding additional CGT beyond a 10 cm layer is detrimental to seal formation and causes increased pollution potential. The optimum CGT layer thickness need not exceed 10 cm of CGT. CGT layer thickness may be as low as 5 cm. References Avnimelech, Y., Nevo, Z., 1964. Biological clogging of sands. Soil Science 98, 222±226. Chang, A.C., Olmstead, W.R., Johnson, J.B., Yamashita, G., 1974. The sealing mechanism of waste ponds. Journal of the Water Pollution Control Federation 46 (7), 1715±1721.
213
Ciravolo, T.G., Martens, D.C., Hallock, D.L., Collins, E.R., Konregay, E.T., Thomas, H.R., 1979. Pollutant movement to shallow groundwater tables from anaerobic swine waste lagoons. Journal of Environmental Quality 8 (1), 126±130. Drommerhausen, D.J., Radclie, D.E., Brune, D.E., Gunter, H.D., 1995. Electromagnetic conductivity surveys for dairiee for groundwater nitrates. Journal of Environmental Quality 24(6), 1083±1091. Harr, M.E., 1962. Groundwater and Seepage. McGraw-Hill, New York. Hills, D.J., 1976. In®ltration characteristics from anaerobic lagoons. Journal of Water Pollution Control Federation 48 (4), 695±709. Hintz, J., 1977. NCSS97, Number Cruncher Statistical Systems, Kaysville, UT. Herrman, J.G., Elsbury, B.R., 1987. In¯uential factors in soil liner construction for waste disposal facilities. In: Geotechnical Practice for Waste Disposal '87: Proceedings of the Specialty Conference. University of Michigan, Ann Arbor, Michigan, 15± 17 June, pp. 522±536. Kennedy, B., 1994. Gin wate utilization: How our gins handle waste. In: Proccedings of the 1994 Beltwide Cotton Conference. National Cotton Council, p. 608. Norberg, A.B., Enfors, S., 1982. Production of extracellular polysaccharide by zoogloea. Environmental Microbiology 44, 1231±1237. Mitchell, J.K., 1965. Fundamentals of Soil Behavior. Wiley, New York. Reed, S.C., Crites, R.W., Middlebrooks, E.J., 1995. Natural Systems for Waste Management and Treatment. McGraw-Hill, New York. Ritter, W.F., Walppole, E.W., Eastburn, R.P., 1984. Eect of an anaerobic swine lagoon on groundwater in sussex country. Delaware, Agricultural Wastes 10 (4), 267±284. Smith, J.W., Tollner, E.W., Eiteman, M., 1998. Microbial Gum Formation From the Decomposition of Cotton Gin Trash. Bioresource Technology, October, 1998, accepted. Toebati, H.M., Raiders, R.A., Donaldson, E.C., McInerney, M.J., Jenneman, G.E., Knapp, R.M., 1986. Eect of microbial growth on pore entrance size and distribution in sandstone cores. Journal of Industrial Microbiology 1, 227±234. Tollner, E.W., Hill, D.T., Busch, C.D., 1983. Physical and biochemical transformation in residues decomposing under small hydraulic gradients. Agricultural Wastes 7, 127±146. Westerman, P.W., Human, R.L., Feng, J.S., 1995. Swinelagoon seepage in sandy soil. Transactions of ASAE 38 (6), 1749±1760.
Use of cotton gin trash (CGT) to form a biological lagoon sealant J.W. Smith, E.W. Tollner* Driftmier Engineering Center, University of Georgia, Athens, GA 30602-4435, USA Received 9 June 1998; revised 27 November 1998; accepted 13 December 1998
Abstract Because of large volumes of cotton gin trash (CGT) and limited options for correct disposal of CGT, a study was launched to ®nd alternative uses for this material. The objective of this report was to explore the possibility of using CGT as a waste lagoon sealant. CGT was applied in 0, 5, and 10 cm layers in 10.15 cm ID acrylic columns with three replicates per treatment. A sandy loam soil was used as a base and as a top soil to hold the CGT in place. The columns were subjected to a constant head of ®ltered tap water. Static compaction was applied at day 7 and kneading compaction at day 12. The columns were destroyed and concentrations of bound extracellular polysaccharides were measured at day 15. A high correlation (R2 0.95) between total EPS concentration and cumulative hydraulic conductivities was observed. The ®nal average hydraulic conductivity of the columns was 2.2 ´ 10ÿ6 cm/s, 2.2 ´ 10ÿ6 cm/s, 1.0 ´ 10ÿ5 cm/s for the 5 cm CGT layer, 10 cm CGT layer, and control columns, respectively. The 5 cm layer had lower hydraulic conductivity and total pollutant loading than the 10 cm layer, suggesting an optimum layer thickness is less than 10 cm. The study suggested that CGT plus compactive eort has excellent potential for sealing waste lagoons. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biological sealing; Microbial gums; Polysaccharides; Cotton gin trash
1. Introduction Treatment lagoons have become an integral waste management tool for complying with the zero discharge requirements of the Clean Water Act of 1972. Biological sealing has been indicated as the primary mechanism in sealing of these lagoons (Chang et al., 1974; Hills, 1976). Extensive research on the pollution of groundwater from lagoons (Ciravolo et al., 1979; Ritter et al., 1984; Westerman et al., 1995; Drommerhausen et al., 1995) has concluded that some soils do not adequately seal, despite a natural biological sealing process. Hills (1976) recommended clay liners in course textured soils (clay <7.5%). Clay liners can cost approximately $12,000/ha. Some swine operations in South Georgia are planning to use plastic liners, at a cost of $50,000/ha (Reed et al., 1995), in order to receive a waste discharge permit. Such measures can increase capital costs ten-fold, making a more economical method of lagoon sealing desirable (Smith et al., 1998). Sealing is ®rst induced by physical blockage from particulates and then biological blockage by microbial * Corresponding author. Tel.: +001-706-542-1653; fax: +001-706-5428806
extra cellular polysaccharides (EPS), microbial gas production, and biomass (Hills, 1976). Materials that contain high C/N ratios and are readily decomposable produce polyuronides, a polysaccharide linked to soil sealing (Avnimelech and Nevo, 1964). Herrman and Elsbury (1987) ranked soil type as the most in¯uential factor in seal development. The next identi®ed factor was the compaction process, including type, weight, and water content. Herrman and Elsbury (1987) noted the most common type of compaction used was static compaction, such as a rubber-tired roller, not because of performance but present practice. In a laboratory study, Mitchell (1965) found that kneading compaction, such as a sheepsfoot roller, generated hydraulic conductivities approximately one order of magnitude lower than static compaction. The study reported here was the second phase of a larger eort to test feasibility of using cotton gin trash (CGT) as a lagoon sealant. Tighter federal clean air laws have prevented the incineration of CGT, renewing interest in alternative utilization strategies (Kennedy, 1994). Tollner et al. (1983) investigated enhancing the biological sealing process by applying CGT into the soil. They observed hydraulic conductivities in the range of 10ÿ5 cm/s, to magnitudes greater than the control, in
0960-8524/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 8 ) 0 0 1 9 7 - 7
208
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
columns applied with a layer of CGT, before any waste was added. However, pollution from the soluble of the CGT caused high organic loading in the ®rst two days of water addition. In the ®rst phase of this study, Smith et al. (1998) measured both bound and free EPS and found no uronides in free EPS, suggesting bound EPS to be critical for biological lagoon sealing. Pre-rinsing CGT had no signi®cant eect on production of EPS and reduced pollutant concentration in the euent; however, it did not reduce total pollutant loading because of higher euent ¯ow rates. The higher euent ¯ow rates could be attributed to the edge eects of the small container used (ID 70 mm), because the pre-rinsed CGT did expand when wetted as did CGT without rinsing. It was also found by Smith et al. (1998) that hydraulic conductivity with CGT decreased with increased concentrations of EPS. Chang et al. (1974) measured the eects of EPS on hydraulic conductivity through soil. Smith et al. (1998) found a smaller change in hydraulic conductivity through CGT, with an equal increase in EPS, than Chang et al. (1974) found in soil. The greater in¯uence of EPS on soil compared to CGT suggests that the sealing advantages due to the production of EPS from the decomposition of CGT could be enhanced from a CGT-soil mixture. The purpose of this study reported here was to further investigate the use of CGT for biological sealing using conditions similar to possible application schemes. The speci®c objectives were to determine: if rinsed CGT could produce similar seal as previously noted from non-rinsed CGT; if compaction plus CGT accelerated sealing; and, if a soil-CGT mixture optimized the eect of increased EPS from CGT and lower hydraulic conductivity of soil. 2. Methods To test these hypotheses, soil columns were constructed with CGT layers and allowed to percolate for 7 days. After this time, a static compaction was applied. At 12 days, a kneading compaction was applied. At 15 days the columns were destroyed and the concentration of EPS at each layer was measured. 2.1. Column preparation The CGT was gathered from a local gin (The Bostwick Gin, Bostwick, GA) in January 1997 and represented the 1996 harvest. The CGT was stored at 2.5°C up to fourteen months at approximately 15% moisture. Before use, the CGT was ground using a ¯ail-type shreader (John Deere shredder attachment for JD 112 mower, Moline, IL). The soil used was taken from an E horizon on the University of Georgia Agronomy farm.
The sandy loam contained 72.9%, 23.0%, and 4.2% of sand, silt, and clay, respectively. Rocks were removed and soil clods broken by screening the soil through a #10 screen. Three percolation columns (Fig. 1) were made from 10.15 cm ID acrylic tubes. The tubes were 152 cm in length with a threaded plug at the bottom. The plug had a 6.4 mm (3/8 in) hose nipple threaded through the center. Five 4.8 mm (1/4 in.) hose nipples were threaded into the side of each column from 0 cm to 20 cm every 5 cm oset by 15°. The bases of all hose nipples were ®lled with glass wool in order to prevent plugging. The attached hoses were used as peizometers. The columns were initially packed with 15 cm of gravel to prevent channeling. A course nylon net was placed over the gravel. Two 540 g layers of soil were packed into the column, forming a 10 cm layer of soil. The CGT was rinsed by placing 127 g of CGT in an 10 l bucket. Three liters of water were added and stirred vigorously for one minute. The water was poured o and the wet CGT was placed in the column, creating a 5 cm layer of rinsed CGT with a bulk density of approximately 0.31 g/cm3 (dry mass). This procedure was repeated in order to produce the 10 cm layer. Another 540 g of soil was placed on top of the CGT. This layer was critical to prevent the CGT from ¯oating to the surface. Three control columns were created by adding a total of 1620 g of soil. Every ®ve cm layer was compacted individually before the next layer was added. Activated carbon ®ltered water was introduced into the columns from the bottom. An activated carbon ®lter was used because there was some evidence in Smith et al. (1998) that the chlorine residual in tap water hindered seal development. A pressure dierential no greater than 10 cm was applied to columns to prevent the compacted material from separating. Once the water level passed the top soil layer, the pressure dierential was increased to the equilibrium point. This process required a 5 day time period for completion. The 6.4 mm (3/8 in) hose connected to the bottom was looped and attached to the outside of the column at the gravel-soil interface in order to prevent unsaturated ¯ow. A ¯oat control valve kept the water at a constant level 127 cm above the soil-gravel interface. Samples were taken from the euent and marked as the midpoint of the sampling period. In order to retrieve enough sample, some sampling periods were as long as 12 h. 2.2. Compaction To study the eect of compaction, two dierent compaction regimes were performed. On day 7 a static compaction was applied. A disk slightly smaller than the inside diameter of the tube was attached to a 2.5 cm steel pipe. The disk was placed ¯at against the top soil surface. A 22.7 kg weight was dropped 15 cm, guided by the
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
209
Fig. 1. Schematic of percolating columns (not to scale).
steel pipe, onto the collar 25 times. On day 12, a kneading compaction was performed. A solid 2.54 cm round stock was attached to the pipe. The pipe had a mass of 8 kg. It was raised 15 cm and dropped 50 times onto the soil surface. 2.3. Hydraulic conductivity For each sample period, the head at each peizometer was recorded. If the levels appeared equal, they were recorded as 0.25 mm for use in hydraulic conductivity calculations. Flow was also determined and Darcy's law was applied to each layer for conductivity data. Measurements were made every day and twice daily after compaction. 2.4. Quanti®cation of EPS At the end of the 15 day trial, the material in the columns was pushed out through the bottom in approximately 5 cm sections. For quanti®cation of EPS, a modi®ed version of the method used by Norberg and Enfors (1982) and outlined in detail by Smith et al. (1998) was used. After hydrolysis of the samples, the
samples were dried, in order to accurately determine the mass of each sample. 2.5. Statistical analyses The statistical analyses consisted of performing several general linear model (GLM) evaluations coupled with evaluating standard error values (computed via spreadsheet) for the logarithm of the cumulative and individual layer hydraulic conductivity values on a daily basis. The NCSS statistical package (Hintz, 1977) was used to make the GLM±ANOVA computations. Main eects were layer thickness and layer position. Time was nested within the main eects. Analyses on cumulative conductivity also were performed which replaced time with compaction as nested eect. A randomized complete block was used for analysing the EPS data. The two CGT layers of the 10 cm column were combined in a composite layer for the EPS analyses. In the event of signi®cant interactions, the GLM or one-way ANOVA procedures were run at each level of one of the interacting variables. An alpha error of 10% was selected as the cuto point for assessing statistical signi®cance. In cases where the time eect was not
210
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
signi®cant the one-way ANOVA model was used. Post hoc tests having a lower alpha are indicated using p 6 xx notation. 3. Results and discussion 3.1. Hydraulic conductivity The hydraulic conductivities of individual layers interacted signi®cantly with both treatment (layer thickness) and time and treatment and layer. The signi®cance of treatment-time interaction and the lack of signi®cance of layer position-time interaction with the log10 transformed individual layer hydraulic conductivity was ascribed to the high variations in conductivity of layers within layer thickness treatments. For example, the ®rst bottom soil layer in the 10 cm CGT treatment was virtually undisturbed while the ®rst bottom soil layer in the 5 cm CGT treatment had some CGT incorporated into it during the kneading compaction. The soil layer below the CGT developed a signi®cantly lower conductivity as time progressed based on analyses by time. Cumulative hydraulic conductivity (Fig. 2) was highly in¯uenced both by treatment and by time (p < 0.01). Over the entire run the hydraulic conductivities of the columns with 5 cm of CGT were signi®cantly lower than the 10 cm columns, which were
signi®cantly lower than the control (p < 0.05). The ®nal average hydraulic conductivities of the columns were 2.2 ´ 10ÿ6 1.9 ´ 10ÿ6 cm/s, 2.2 ´ 10ÿ6 0.6 ´ 10ÿ6 cm/s, 1.0 ´ 10ÿ5 0.02 ´ 10ÿ5 cm/s for the 5 cm of CGT, 10 cm of CGT, and control columns, respectively. The columns with rinsed CGT coupled with compaction reduced conductivity by nearly one order of magnitude compared to the control. This study achieved a similar magnitude of reduction in hydraulic conductivity compared to the control as did the Tollner et al. (1983). The ®rst 15 days of their study involved sand with a layer of CGT. Hydraulic conductivities were relatively constant at 6.4 ´ 10ÿ5 cm/s from 8 to 15 days. The ®nal steady levels of hydraulic conductivity achieved in this study by Tollner et al. (1983) leads one to tentatively conclude that the oxidation-reduction potential of the decomposing layer may limit the sealing process. Some of the dierence in the ®nal hydraulic conductivities between the two studies is a function of soil type. Harr (1962) classi®ed soils by hydraulic conductivity and included ®ne sand, natural clay, and compacted clay with hydraulic conductivities of 5 ´ 10ÿ4 ±10ÿ3 cm/s, 10ÿ6 cm/s and lower, and 10ÿ7 cm/s, respectively. Despite the decrease in hydraulic conductivity by a magnitude form the CGT layer, the ®nal average hydraulic conductivity reached 2.2 ´ 10ÿ6 cm/s and was still above that of compacted clay.
Fig. 2. Average, high, and low hydraulic conductivities with respect to time. Error bars around the average points represent one standard error, N 3.
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
211
The kneading compaction created a CGT-soil mixture. A more intense kneading of the 10 cm, which incorporated more of the bottom CGT, may have resulted in reduced hydraulic conductivity. The layers below the penetration depth of the kneading compaction had the lowest hydraulic conductivity (Fig. 2). The lower hydraulic conductivities could possibly be a function of the position in the column. The lowest layers can collect a larger amount of particulates deposited from the above layers. The same phenomenon was also true in the control column where the second bottom soil layers had ®nal hydraulic conductivities a magnitude greater than all of the prior layers. In the columns with CGT, the ®rst bottom layer had signi®cantly lower ®nal hydraulic conductivities. Since the ®rst bottom layer in the column with 10 cm of CGT was undisturbed by the kneading compaction and had the lowest hydraulic conductivity, the reduction in hydraulic conductivity cannot be attributed to a complementary eect of soil-CGT mixture. Smith et al. (1998) found no uronide accumulation in free EPS, so it is unlikely free EPS produced in the CGT layer traveled down into the subsequent soil layer and aected its hydraulic conductivity. A possible scenario explaining the lower hydraulic conductivity of the ®rst bottom layer is that particulates and microbial biomass from the CGT plugged pores in the ®rst bottom layer and did not travel to second bottom layer. A similar proposed mechanism for sealing was discussed by Hills (1976) and observed by Toebati et al. (1986). Average pore size in the Toebati et al. (1986) study was reduced after introducing media to a percolating column. They concluded the reduction in pore size indicated partial plugging of larger pores from microbial biomass accumulation due to preferential ¯ow through the largest pores. The presence of microbial biomass in subsequent layers below the CGT also supports the presence of bound EPS in the layer below the CGT. It is
also possible that the linking of uronides to sealing could actually be a relationship of biomass and sealing, since uronide accumulation was only found in bound EPS. Measurements of high uronide concentration could actually represent high biomass concentrations which are responsible for the seal formation.
Fig. 3. Five cm CGT layer column EPS and hydraulic conductivity (K) as a function of layer. Error bars are one standard error, N 3.
Fig. 4. Control column EPS and hydraulic conductivity (K) as a function of layer. Error bars are one standard error, N 3.
3.2. Production of EPS A signi®cant two way-interaction existed between layer and treatment for the production of EPS. When a cumulative concentration of EPS was analyzed, treatment was signi®cant (p < 0.01). If the production of EPS is viewed on unit CGT basis, the values are normalized to an average of 2.3 mg EPS/g CGT (s 0.4), which corresponds to the production of EPS noted by Smith et al. (1998) of 2.59 mg EPS/g CGT in an anaerobic environment at 14 days. There was a signi®cant (p < 0.05) interaction between layer and treatment. The dierence in the trends with EPS and layer are evident in Figs. 3±5. The results of compaction were dependent on the thickness of the CGT layer. The column with the 5 cm layer of CGT had the top soil layer integrated into the CGT layer and a portion of the ®rst bottom soil layer. The column with 10 cm layer or CGT had the top soil layer completely mixed with the top CGT layer, but the bottom CGT layer had minimal disturbance. When analyzed within each treatment, concentrations of EPS were signi®cant with layer (p < 0.05). In the control layer, only the top layer had a signi®cantly dierent amount of EPS (p < 0.05). In the columns with a 5 cm CGT layer, only the CGT layer had a signi®cantly dierent amount of EPS (p < 0.05). In the columns with a 10 cm layer of CGT, all layers were signi®cantly dierent (p < 0.05). An example of the insensitive response of hydraulic conductivity solely to EPS is shown in Fig. 3. The CGT layer has the highest concentration of EPS, and the
212
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
Fig. 5. Control column EPS and hydraulic conductivity (K) as a function of layer. Error bars are one standard error, N 3.
greatest hydraulic conductivity. Referring to Fig. 4, one may note nearly double the concentration of EPS in the bottom CGT layer compared to the top, but no signi®cant dierence in hydraulic conductivity (p < 0.05). The control layer (Fig. 5) had signi®cantly lower (p < 0.01) concentrations of EPS, with the top layer having the highest average concentration of 0.22 mg/g of soil. These results lead to the conclusion that uronide production must be accompanied with appropriate interaction with a solid matrix to accomplish sealing. When the total mass of EPS is normalized by the total column mass, it has a high correlation (R2 0.95) to hydraulic conductivity of the column (Fig. 6). Only two 5 cm columns were used in the correlation, because the third column had an uncharacteristically low hydraulic conductivity of 5.6 ´ 10ÿ8 cm/s which is nearly two magnitudes lower than the others, suggesting some other mechanism was responsible for the sealing.
Fig. 6. Eect of EPS concentration of entire column with hydraulic conductivity (K) (not including one point with uncharacteristically low hydraulic conductivity of 5.6 ´ 10ÿ8 cm/s).
3.3. Euent concentrations The 10 cm column had signi®cantly (p < 0.01) higher ammonia and lower nitrate (Fig. 7). The Ammonia roughly doubled, which would indicate washout of preexisting soluble ammonia on mass basis by the percolating water. The reverse of nitrate compared to ammonia suggests some metabolic eects during the course of the percolation. Perhaps a more anaerobic environment existed in the 10 cm CGT layer, causing denitri®ers to more thoroughly remove preexisting nitrate in the 10 cm layer compared to the 5 cm layer. The large variation in the orthophosphate prevented it from being signi®cantly greater, but all measurements from the 10 cm CGT column were greater than the 5 cm CGT column. Variation in the CGT is an issue requiring better understanding. Further studies are planned on nutrient
Fig. 7. Five day cumulative nutrient loading from columns with 5 and 10 cm CGT layers. Error bars represent one standard error (plus only for ortho P and nitrate), N 3.
J.W. Smith, E.W. Tollner / Bioresource Technology 69 (1999) 207±213
release form CGT for pond sealing purposes. No bene®ts from a larger amount of CGT for the production of EPS were found; the larger amounts CGT had higher cumulative loading of pollutants and higher hydraulic conductivities. The potential pollutant loading with increased CGT layer thickness also justi®es the need for optimizing the layer thickness. 4. Conclusions Rinsed CGT and the described soil compaction regime resulted in an average hydraulic conductivity of 2.2 ´ 10ÿ6 cm/s, approximately an order of magnitude lower than an identically treated control without CGT. The kneading compaction resulted in an order of magnitude reduction in all treatments compared to the static compaction, as noted by Mitchell (1965). The lowest hydraulic conductivity did not occur in mixing the soil with CGT rich in EPS. A high correlation was observed between cumulative EPS normalized by total mass of the column and total column hydraulic conductivity, although evidence suggests that EPS must be plugging a soil matrix to be ecacious. Additional soil treatment compaction regimes are needed to understand the ecacy of CGT as a sealant. Adding additional CGT beyond a 10 cm layer is detrimental to seal formation and causes increased pollution potential. The optimum CGT layer thickness need not exceed 10 cm of CGT. CGT layer thickness may be as low as 5 cm. References Avnimelech, Y., Nevo, Z., 1964. Biological clogging of sands. Soil Science 98, 222±226. Chang, A.C., Olmstead, W.R., Johnson, J.B., Yamashita, G., 1974. The sealing mechanism of waste ponds. Journal of the Water Pollution Control Federation 46 (7), 1715±1721.
213
Ciravolo, T.G., Martens, D.C., Hallock, D.L., Collins, E.R., Konregay, E.T., Thomas, H.R., 1979. Pollutant movement to shallow groundwater tables from anaerobic swine waste lagoons. Journal of Environmental Quality 8 (1), 126±130. Drommerhausen, D.J., Radclie, D.E., Brune, D.E., Gunter, H.D., 1995. Electromagnetic conductivity surveys for dairiee for groundwater nitrates. Journal of Environmental Quality 24(6), 1083±1091. Harr, M.E., 1962. Groundwater and Seepage. McGraw-Hill, New York. Hills, D.J., 1976. In®ltration characteristics from anaerobic lagoons. Journal of Water Pollution Control Federation 48 (4), 695±709. Hintz, J., 1977. NCSS97, Number Cruncher Statistical Systems, Kaysville, UT. Herrman, J.G., Elsbury, B.R., 1987. In¯uential factors in soil liner construction for waste disposal facilities. In: Geotechnical Practice for Waste Disposal '87: Proceedings of the Specialty Conference. University of Michigan, Ann Arbor, Michigan, 15± 17 June, pp. 522±536. Kennedy, B., 1994. Gin wate utilization: How our gins handle waste. In: Proccedings of the 1994 Beltwide Cotton Conference. National Cotton Council, p. 608. Norberg, A.B., Enfors, S., 1982. Production of extracellular polysaccharide by zoogloea. Environmental Microbiology 44, 1231±1237. Mitchell, J.K., 1965. Fundamentals of Soil Behavior. Wiley, New York. Reed, S.C., Crites, R.W., Middlebrooks, E.J., 1995. Natural Systems for Waste Management and Treatment. McGraw-Hill, New York. Ritter, W.F., Walppole, E.W., Eastburn, R.P., 1984. Eect of an anaerobic swine lagoon on groundwater in sussex country. Delaware, Agricultural Wastes 10 (4), 267±284. Smith, J.W., Tollner, E.W., Eiteman, M., 1998. Microbial Gum Formation From the Decomposition of Cotton Gin Trash. Bioresource Technology, October, 1998, accepted. Toebati, H.M., Raiders, R.A., Donaldson, E.C., McInerney, M.J., Jenneman, G.E., Knapp, R.M., 1986. Eect of microbial growth on pore entrance size and distribution in sandstone cores. Journal of Industrial Microbiology 1, 227±234. Tollner, E.W., Hill, D.T., Busch, C.D., 1983. Physical and biochemical transformation in residues decomposing under small hydraulic gradients. Agricultural Wastes 7, 127±146. Westerman, P.W., Human, R.L., Feng, J.S., 1995. Swinelagoon seepage in sandy soil. Transactions of ASAE 38 (6), 1749±1760.