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Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production
Waste Management xxx (2015) xxx–xxx
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production Claudia Kayser ⇑, Tam Larkin, Naresh Singhal Department of Civil and Environmental Engineering, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
a r t i c l e
i n f o
Article history: Received 21 April 2015 Revised 17 July 2015 Accepted 17 August 2015 Available online xxxx Keywords: Biosolids Leachate Metal concentration Shear strength Settlement Overburden pressure
a b s t r a c t Residuals from wastewater treatment operations (biosolids) were mixed with lime, fly ash, lime kiln dust, or two smelter slags to assess their efficacy as potential stabilisation agents by assessing their effects on the shear strength, compressibility, and solids content of mixtures. In addition, the minerals formed and leachate produced during stabilisation were determined. Tests were performed to explore the change of the geoenvironmental properties of the amended biosolids, while under pressure, at different scales using laboratory, pilot and field scale tests. The settlement characteristics of the amended biosolids under a range of applied pressures were determined using a consolidometer. All amended biosolids mixtures showed higher strength than the unamended biosolids, with mixtures containing a combination of 20% fly ash and 20% lime giving the highest (up to eightfold) increase in strength, and that with lime kiln dust and the smelter slags showing the lowest (up to twofold). The biosolids mixtures with only lime gave the second highest increase in strength (up to fourfold), but produced the largest amount of leachate, with higher level of dissolved calcium. The increase in strength correlated with availability of calcium oxide in the mixtures which lead to calcium carbonate formation, accompanied with higher leachate production and settlement during consolidation. Copper, nickel and zinc concentrations increased with alkaline additives and corresponded to higher pH and DOC levels. Nonetheless, concentrations were within the New Zealand regulatory limits for Class A landfills. Ó 2015 Published by Elsevier Ltd.
1. Introduction The enhancement in efficiency of modern wastewater treatment operations has led to production of larger quantities of residuals, or biosolids. Consequently their disposal in a manner that complies with increasingly stringent regulatory requirements can pose a challenge. Problems with disposal arise not only from the material being rich in organic matter and prone to biodegradation, but also from the low solids content (20%) and poor shear strength (3 kPa) (Kayser et al., 2011). Typically, biosolids are disposed following stabilisation (e.g., by digestion, lime addition, lime addition to digested sludge) by spreading on land, use as fill material for land reclamation and rehabilitation, or deposition in monofills or landfills (UN-HABITAT, 2008). Landfill operators are increasingly reluctant to co-dispose large quantities of organically rich materials due to deterioration in strength from long term degradation. Restrictions on land spreading are arising from the lower levels of heavy metals being specified for the material being ⇑ Corresponding author. E-mail addresses: [email protected] (C. Kayser), [email protected]. nz (T. Larkin), [email protected] (N. Singhal).
disposed. Furthermore, the increases in cost, due to larger quantities of lime required to stabilise the residuals, is becoming a consideration for treatment plant operators. Thus, it is not surprising that the use of waste materials in-lieu of, or to supplement, lime addition is gaining philosophical acceptance (Kayser et al., 2011). Our previous study showed that mixing wastewater residuals with locally sourced alkaline waste products (e.g., lime kiln dust, fly ash, smelter slag) increased the shear strength of mixtures by as much as eightfold when 20% lime was added and the pH was maintained above 11 (Kayser et al., 2011). However, few studies have explored the effect of different amendments on the geoenvironmental properties of biosolids, particularly under typical overburden pressures expected at depth in landfills and monofills. A recent New Zealand proposal to dispose biosolids in monofills with depths of up to 35 m provides a useful guide to the pressures that the disposed material are likely to be subjected. While information on the increase in strength of surficial biosolids following their modification with alkaline additives is available (Dirk, 1996; Lee et al., 2002; Lim et al., 2006; Voss, 1996; Yin, 2001), understanding of the behaviour of the mixtures under significant overburden pressures is very limited. In this study we investigate the effect of adding lime and lime mixed with fly ash, lime kiln dust, or two smelter
http://dx.doi.org/10.1016/j.wasman.2015.08.024 0956-053X/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Kayser, C., et al. Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.024
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slags at various concentrations, on the strength, solids content, and settlement under overburden pressures typically encountered in modern landfills. In addition, we characterise the quantity and composition of leachate produced, specifically metals, calcium, pH and organic carbon, under the above conditions. 2. Materials and methodology 2.1. Biosolids and additives Biosolids are the processed sewage sludge using anaerobic digestion and centrifugal dewatering. The biosolids used in this study originated from a local wastewater treatment plant (WWTP) in Auckland, New Zealand, which is one of the largest WWTP in Australasia, treating over 111 billion litres of industrial and domestic wastewater annually. The biosolids are currently disposed after stabilisation with 24% quicklime (CaO) at Pond 2, a 44 ha large biosolids rehabilitation area next to the WWTP. For this study unamended biosolids were collected prior to stabilisation, i.e., prior to mixing with lime. The initial moisture content, solid content and volatile solid content for the unamended biosolids varied from, 76.7–80.3%, 19.7–23.3% and 60–70%, respectively. The waste additives were sourced from local industries and included lime kiln dust (LKD), a low alkaline fly ash (FA) from coal burning, works debris (WD) produced during steel recycling, and smelter slag (KOBM) from steel production. Large particles were separated from WD by sieving through a 0.5 mm mesh; other additives were used as obtained. CaO was added as an alkaline supplement, and used as a reference, since it is commonly encountered as a means of stabilisation of biosolids. Table 1 shows the mixing ratios of additives, which follows our previous experimental work (Kayser et al., 2009, 2011). 2.2. Experimental setup Laboratory experiments were conducted using steel tubes with biosolids under pressure (biorigs) and tanks (pilot scale test), along with in-situ testing in the field. The relative characteristic scales and other characteristics of the testing apparatus are summarised in Table 2. 2.2.1. Biorigs assembly Biorigs were used to test amended biosolids under overburden pressure; the setup allowed for a range of overburden stresses and measurement of the settlement along with collection of leachate (Fig. 1). A biorig frame has a height of 1.89 m and a width of 1.13 m. The steel tubes used in the biorigs are made of very high quality corrosion resistant stainless steel (SAF 2205), with an internal diameter of 75 mm and a length of 500 mm. For a smooth finish, the tubes were honed and sealed using O-rings that were recessed in machined groves in the PVC top and bottom caps. An individual fit was made for each top and bottom cap to a particular tube. This process resulted in a ‘‘no leak system” for pressures up to 316 kPa when applied for up to 6 months. The bottom cap was
Table 1 Mixing ratio of additives. Name of mixture
Percentage of additive used based on the biosolids solid content
FA LKD WD WD + L KOBM Lime
20% 30% 30% 30% 30% 20%
fly ash + 20% lime lime kiln dust works debris works debris + 10% lime smelter slag + 10% lime lime
Table 2 Experimental set-up conditions and scale. Experimental set-up Approximate scale (based on a 44 ha and up to 10 m depth in-situ placement area) Drainage condition Equivalent depth of biosolids layer (m)
Biorig 1:1 108
Tank In-situ 1:1.15 106 1:1
Drained Drained 10 and 20 3
Undrained 0.5–1.5
mounted on a pedestal that was connected to the middle of the cross beam. The top cap was connected to a flat steel plate assembled to a guide rod through the upper part of the frame to ensure alignment. Both caps were designed with an outlet to allow for two way drainage and Tygon tubing connected the bottom and top cap to glass bottles for the collection of gas and leachate (Fig. 1). To separate leachate and solids both caps contained a 0.5 lm stainless steel mesh and a 0.5 bar porous ceramic disk (Soilmoisture Equipment Corp.). For the application of overburden pressure, weights were attached to 2 mm wire ropes, which were guided over a pulley system that provided a twofold mechanical advantage. This allowed the application of either 115 kPa or 225 kPa of overburden pressure following calibration for internal (O-ring and specimen) and external (guiding rod and pulley system) friction (Kayser, 2012). Internal friction was measured by setting up a specimen and replacing the bottom pedestal with an internal pedestal connected to a S-load cell. External friction was measured via a proving ring beneath the top cap. The overburden pressure provided was equivalent to the weight per unit area of either 10 m (115 kPa) or 20 m (225 kPa) overburden of amended biosolids. The five amendments (FA, LKD, WD, WD + L and KOBM (see Table 1)) were mixed with biosolids and employed in the biorigs together with control specimens of unamended biosolids and biosolids with 20% lime. Loading durations of 2 and 12 weeks were used. 2.2.2. Consolidometer To determine the settlement/time characteristics of biosolids, the biorig setup was modified with a pore liquid pressure transducer being installed at the base of the specimen to record the leachate pressure at the undrained face during one way drainage. Additionally, the length of the stainless steel tube used was reduced to 180 mm. To ensure the measurement was of the pore liquid pressure only, a 2 bar high air entry disk (Soilmoisture Equipment Corp.) was used. For the consolidometer test one biosolids specimen with 20% lime was prepared and subjected to four successive vertical pressures of 47 kPa, 109 kPa, 192 kPa and 316 kPa (determined following initial calibrations as described above (Kayser, 2012)), each for a minimum of 40 days. 2.2.3. Tanks To investigate the long term (up to 1.3 years) behaviour of biosolids on a larger scale, a pilot scale study was conducted using four 425 L plastic tanks (1200 mm height and 700 mm diameter, see Fig. 2). These tanks were modified with one leachate collection outlet (bottom) and 12 re-sealable ports (see Fig. 2a) allowing for fortnightly extraction of solids samples and monthly (30 days) measurements of shear strength. Four mixtures were prepared by mixing biosolids by hand with Lime, FA, WD + L and LKD, and placed in a controlled manner to a height of 700 mm and an approximate density of 1.1 t/m3 (representative of field conditions). The outlet of each tank was first fitted with a drainage layer of sand covered by a 125 lm stainless steel filter, designed according to USDA guidelines (USDA, 1994). A biosolids layer was then placed followed by another filter and drainage layer. To create a small overburden pressure (3 kPa), plastic bags filled with sand were used forming the top layer, before the tanks were sealed (Fig. 2b).
Please cite this article in press as: Kayser, C., et al. Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.024
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Fig. 1. Biorig with leachate and gas collection system.
Fig. 2. (a) Arrangement of re-sealable ports and (b) tank with leachate and gas collection system.
2.2.4. Field testing and sampling Shear vane test were conducted and undisturbed field samples were collected at Pond 2 from depths of 0.5–1.5 m in cells containing 2–8 year old biosolids. 2.3. Specimen preparation and analyses of solids and leachate properties Specimens were prepared by hand mixing additives and biosolids until a visibly homogeneous matrix was achieved. The percentage of additives reported is based on the dry mass of biosolids. Mixtures were compacted in layers in the stainless steel tubes to an initial height of 40 mm in the consolidometer and 390 mm in the biorigs to densities between 1.01 t/m3 and 1.08 t/m3, the
approximate density of biosolids after deposition by the local WWTP. For the measurement of shear strength of specimens from the biorigs and field tests, unconsolidated undrained (UU) triaxial tests were employed according to the New Zealand Standard NZS 4402.6.2.1:1986 (1986). In the case of the biorigs, at the termination of loading, the specimens were extruded from the tubes and trimmed to a length of 150 mm to conduct UU triaxial tests at a cell pressure of either 115 kPa or 225 kPa depending on the biorig consolidation pressure. Hand held shear vane tests were conducted at monthly intervals during the pilot scale study, and during field testing. The tests were carried out following recommendations of the NZ Geotechnical Society Inc. (2001).
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Specimens were measured for solids content according to the New Zealand Standards for soil testing (NZS 4402:1986). Samples were tested in triplicates. The drying temperature is highly critical for biosolids due to its high organic content and temperatures such as e.g., 60 °C (O’Kelly, 2004) and 50 °C (Arulrajah et al., 2011) have been recommended. Analysis by Kayser (2012) showed that there is a linear relationship for solids content between 60 °C and the standard 105 °C (Kayser, 2012). A drying temperature of 105 °C was therefore adopted as per NZS 4402.2.1:1986 (1986) to expedite mixing and to allow a wider comparison with results presented in the literature. Changes in the mineral phase of biosolids due to the addition of additives were ascertained using X-ray diffraction (XRD). The XRD used was a Philips PW 1130 high voltage generator, copper anode X-ray tube, 40 kV, 20 mA, and a scanning speed of 2°/min for 0–60° 2H. Samples were air dried and ring milled. The calcium carbonate (CaCO3) content was quantified following the ASTM Standard for chemical analysis of limestone, quicklime, and hydrated lime (ASTM International, 2006), by using a standard indicator titration method. A 4.6 g dried sample (dried at 60 °C) was used from which the percentage of CaCO3 was determined based on a received wet mass. Environmental scanning electron microscopy (ESEM) analyses were carried out on air dried and platinum coated samples. The ESEM used was a FEI Quanta 200F. Leachate from the biorigs was collected from the top and bottom of the specimen, while tank leachate was collected from the bottom. The volume of biorig leachate is reported as the sum of those two volumes (for unamended biosolids approximately 50:50 and for biosolids mixtures varying from approximately 30:70 to 45:65 for ratio of top and bottom leachate). Leachate was analysed either in duplicates or triplicates for the dissolved organic carbon (DOC) and calcium, copper, nickel and zinc ion concentration (Ca2+, Cu2+, Ni2+ and Zn2+, respectively). For DOC measurement, the leachate was filtered through 0.45 lm PTFE syringe filters and analysed using a Shimadzu SSM-5000A VCSH model. For calcium and metal measurement, samples were prepared by microwave digestion, filtered through 0.45 lm PTFE syringe filters and analysed using a flame atomic absorption spectrometer (AAS, Varian SpectrAA 50). Gas collected from the biorigs was analysed for methane (CH4) and carbon dioxide (CO2) using a GC SRI 8610 gas chromatograph apparatus with a thermal conductivity detector.
3. Results and discussion 3.1. Shear strength and settlement 3.1.1. Biorigs An overview of the shear strength results is given in Fig. 3. The best performing mixtures with respect to strength enhancement were FA mixtures, followed by Lime mixtures. FA and Lime mixtures showed the highest strength after 12 weeks, with the FA mixture specimen doubling in strength under 20 m of overburden pressure. After two weeks loading LKD was the third best performing mixture, but a reduction in shear strength was observed over time for both overburden pressures, due to a reduced availability of CaO. Reduced carbonation took place, since calcium within the LKD was bound in composites such as CaOH2, CaCO3 and CaSO4 prior mixing. This reduced the capability of forming bounds after adding lime to the biosolids. Unamended biosolids performed poorly and showed the lowest overall strength, with only minimal change with time and overburden pressure. While differences between overburden pressure of 10 m and 20 m after 2 weeks were negligible for LKD, WD, WD + L and KOBM mixtures, after
12 weeks these mixtures showed a pronounced increase in strength of 25–60% between 10 m and 20 m of overburden. Comparing drained conditions (biorigs with overburden pressures) and undrained conditions from our previous study (PVC tube with no overburden pressure (Kayser et al., 2011)), it is evident that only Lime and FA mixtures showed a clearly visible increase in shear strength (Fig. 3). All other mixtures showed essentially no difference between (i) drained and undrained conditions and (ii) 10 m and 20 m of overburden pressure. When CaO is mixed with unamended biosolids, it reacts readily with water forming calcium hydroxide (Ca(OH)2) in an exothermic reaction, leading to an immediate increase in solids content. While a correlation between solids content and shear strength has been reported in the literature for both, amended and unamended biosolids (Klein and Sarsby, 2000; Koenig and Bari, 2001; O’Kelly, 2006; Zhang et al., 2008), lower shear strength is not always positively correlated with lower solids content, especially in the case of unamended biosolids (Koenig and Bari, 2001). Results for correlations between shear strength from UU triaxial tests (su tri) and solid content are presented in Fig. 4. Unamended biosolids and LKD mixtures showed no direct relationship between solids content and strength. By contrast, WD, WD + L and KOBM mixtures showed a clear linear relationship, while FA and Lime mixtures showed a strong polynomial relationship. The latter two mixtures exhibited much higher strength, at the same solids content, to that of the LKD, WD, WD + L and KOBM mixtures (Figs. 3 and 4). Following the rapid formation of Ca(OH)2, the formation of CaCO3 and water occurs at a reduced speed under the presence of CO2. The production of CaCO3 increases under high pressures and addition of CO2 (Domingo et al., 2006). In the case of biosolids, the presence of CaCO3 has been suggested to be one of the major reasons for increases in shear strength (Horn, 1990; Kayser, 2012; Lim et al., 2002). XRD analyses of biosolids mixtures in our study identified quartz (SiO2), CaCO3 and Ca(OH)2 as the main crystalline compositions. Both Ca(OH)2 and CaCO3 were found in FA and Lime mixtures after loading for two weeks, but after 12 weeks of loading all Ca(OH)2 was transformed into CaCO3 (Fig. 5). An additional peak at a 2-theat position of around 45° was identified as iron (Fe), which occurred due to the chipping of metal from the crusher in the ring mill prior to testing and was therefore not identified in Fig. 5. No significant difference was observed in the XRD patterns of FA and Lime mixtures loaded under 115 kPa and 225 kPa, with CaCO3 and SiO2 being the only two crystalline composites detectable. To quantify the amount of CaCO3 for FA and Lime mixtures, a standard indicator titration method was carried out. Results showed that larger amounts of CaCO3 were formed under a loading of 225 kPa, 7.9% and 6.9% CaCO3 by wet mass for FA and Lime mixture respectively, compared to that for an overburden stress of 115 kPa, 7.0% and 6.4% CaCO3 by wet mass for the FA and Lime mixture respectively. The increased formation of CaCO3 under higher overburden pressure is consistent with the literature (e.g., Domingo et al. (2006)). For KOBM, LKD, WD and WD + L mixtures no differences in XRD patterns were found due to differences in loading time and pressure, with SiO2 and CaCO3 the only crystalline composites identified. While ettringite was found in FA mixtures in earlier studies (Kayser et al., 2011), no further minerals were detected using XRD in this series of tests, due to a reduced percentage of additives used. The experiments showed that drainage is fundamental to ensure sufficient shear strength when biosolids are amended with alkaline additives. Applying overburden pressure during draining enhances the formation of CaCO3 and further increases the shear strength. Our results are consistent with the literature where large increases in shear strength were found due to the addition of alkaline additives and the application of overburden pressure (Chu et al., 2005). The work of Chu et al. (2005) reports increases in
Please cite this article in press as: Kayser, C., et al. Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.024
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Fig. 3. Shear strength, su
tri,
from PVC tubes after two weeks and from biorigs after 2 and 12 weeks’ of loading (Kayser et al., 2011).
Fig. 4. Solids content versus shear strength, su
tri.
Fig. 5. XRD analyses for mixtures under 20 m overburden pressure using (a) Lime mixtures and (b) FA mixtures after 2 and 12 weeks (Kayser, 2012).
strength from 100% to 175% for a mixture of biosolids, cement and copper slag and the application of overburden pressure of 80 kPa under drained conditions, stating that a combination of both chemical and mechanical treatment is needed for significant strength increase. 3.1.2. Consolidometer There is a paucity of studies on the consolidation and settlement properties of biosolids and those that exist generally focus on unamended biosolids (Aydilek et al., 2000; Klein and Sarsby, 2000; Koenig et al., 1996; Lo et al., 2002; O’Kelly, 2005, 2008). In the work reported here, four load increments were applied to a specimen of lime stabilised biosolids over a period of 160 days, with each load increment lasting 40 days. Table 3 gives a summary of the settlement characteristics for each load increment. An axial strain of 34% occurred during the first pressure increment and
Table 3 Settlement characteristics. Number of load increment Overburden pressure, kPa Settlement per load increment, mm Total axial strain at end of each load increment, % Maximum pore leachate pressure per load increment, kPa Time to maximum leachate pressure, days Maximum pore leachate pressure increment/ vertical stress increment, %
1 47 13.82 34.53
2 109 4.62 46.11
3 192 3.76 55.84
4 319 3.00 63.00
47
25
21
17
0.6 100
2.0 43
2.6 24
5.4 13
increased to a total of 63% at the end of the fourth increment. Leachate pressure, measured at the undrained face, reached the magnitude of the first vertical stress increment (47 kPa, i.e., Skempton’s
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near the drained face could have occurred simultaneously resulting in reduced pressure readings. Concurrently, the solid skeleton of the mixture was maturing and stiffening from the reaction of the biosolids to the lime. The hardening of the biosolids will result in a reduction of a transfer of stress from the solid skeleton to the pore liquid. After about 30 days, all excess pore pressure had dissipated independently of the overburden pressure applied, indicating the end of the apparent primary consolidation phase. The coefficient of consolidation (cv), which describes the rate of settlement, was determined using Taylor’s and Casagrande’s method (Head, 1980) and was found to be approximately 0.01 m2/year by both methods (Kayser, 2012). 3.2. Tank and field Fig. 6. Hand held shear vane measurements, su (Kayser, 2012).
vane,
taken once every 30 days
B = 1) after 15 h. Subsequent load increments did not result in a maximum leachate pressure equal to the vertical stress increment, but were 43%, 24% and 13% of the vertical stress increment (Table 3). While the initial leachate pressure built up slowly at the undrained face of the specimen, dissipation of excess pressure
Following the biorigs experiment a pilot scale study was carried out, increasing the scale from that of the biorigs, 1:1 108, to that of a tank (0.7 m diameter by 1.2 m height), 1:1.15 106 (Table 2). Results of the shear strength measurements from hand held shear vane are presented in Fig. 6. The shear strength for all mixtures increased for the first 150–220 days, with insignificant change for the remaining days. A few kPa variations in the results (up to 15%) are not unusual and relates to inhomogeneity and point of measurement in the tanks (Fig. 2a). The FA mixture
Fig. 7. Loading time versus normalised leachate production for (a) biorigs, 12 weeks, 20 m, (b) biorigs, 12 weeks, 10 m, (c) pilot scale experiment and (d) loading time versus axial strain (biorigs, 12 weeks, 20 m).
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resulted in the highest strength readings overall, with a maximum reading of 27.9 kPa (at 5 months). The second best performing mixture was Lime, which was only marginal better than that of WD + L and LKD, and thus differed to the biorigs tests, where both FA and Lime mixtures performed similarly well. The difference in results between tank and biorig is related to the large quantity of biosolids to be mixed for the tanks, where hand mixing resulted in less homogenous mixture than for the biorigs. While less homogenous, the tanks represent a relatively accurate simulation of the in-situ mixing procedure at the WWTP. Hence, fly ash mixed with lime can result in significant strength increase even in a reduced homogenous mixture, if drainage is allowed. Shear strength measurements (UU triaxial and hand held shear vane) on five field specimens with lime percentage assumed to range from 20–30% varied between 5.3–12.2 kPa (UU triaxial) and 8.4–22.0 kPa (hand held shear vane). Measurements of the pH of solids were above 12 for biosolids aged two and three years, while for ages in excess of 3 years the pH was approximately 8.5. One sample was measured with a pH of 11.6 (5 years) showing the variability within the results related to mixing procedure and weathering in the field. No trend in regard to time (2–8 years) or depth (0.5–1.5 m) of placement was found. Taking into account weather conditions, e.g., rain, wind, temperature, it can be said that pH and shear strength results are similar to those observed during the pilot scale study (Fig. 6). The small quantity of field samples and experimental tests was dictated by (a) the lack of accurate determination of the actual time of deposition, (b) existing subsurface obstruction such as geogrids, geotextiles and oxidation pond sludge, and (c) lime dosage of mixture at time of deposition.
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3.3. Leachate volume and composition During both biorig and pilot scale experiments large amounts of leachate were collected from biosolids mixtures. An overview of the volume of leachate collected with time is given in Fig. 7a–c. For comparison purposes, the normalised leachate production in percentage (ratio of volume of leachate (L) collected to initial mass of liquid within the biosolids mixture (kg)) is presented. The corresponding settlement, change in axial strain (defined as the change in height over the initial height) with time, for the biorig specimens is shown in Fig. 7d. When considering leachate production as a function of overburden it is shown that with 10 m of overburden the amount of leachate production is approximately one half that for the case of 20 m of overburden at the end of testing (84 days). In the case of the pilot scale study leachate was collected over a period of 480 days and volumes were below that of the biorigs. It can further be seen that for all biosolids mixtures higher volumes of leachate resulted in higher axial strain/higher settlement (Fig. 7a and d). While the settlement and volume of leachate varied depending on loading duration and mixture, the ratio of normalised total leachate production to final axial strain showed a constant ratio of 1.14 L/kg with a standard deviation of 0.12 L/kg for all amended biosolids (12 weeks and 20 m). Unamended biosolids exhibited a different behaviour with an average ratio of 2.48 L/ kg (values varying between 1.6 L/kg and 3.7 L/kg). The constant ratios (1.14 L/kg and 2.48 L/kg) are due to the initial average bulk density of the mixtures (mean = 1.09 t/m3, standard deviation = 0.02 t/m3) and the unamended biosolids (mean = 0.99 t/m3, standard deviation = 0.03 t/m3) being close to that of water/leachate (Kayser, 2012). Biodegradation within the specimens of the
Fig. 8. ESEM images of (a) unamended biosolids, (b) Lime mixture after 12 weeks and 20 m overburden pressure, (c) FA mixture prior to testing and (d) as in (c) after 12 weeks and 20 m overburden pressure.
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the end of testing, independent of loading time and pressure. FA mixtures showed high leachate production when under high overburden pressure, but reduced with reduced loading, although an increase in leachate production was found with time due to its alkaline content. The volume of the leachate production was thought to be mainly due to: (i) an increase in permeability leading to a reduction in bound water, and (ii) the production of water during the formation of CaCO3. The increase in permeability due to increase in void spaces was shown using ESEM analyses. A distinct difference was found between unamended biosolids (Fig. 8a) and Lime and FA mixtures (Fig. 8b–d), with the latter showing numerous micro pores. Comparing FA mixtures before and after 12 weeks of curing (Fig. 8b and c), a reduction in void spaces was observed, which is most likely related to the formation of calcium carbonate. A reduction in void space with addition of alkaline additives was also observed by Lim et al. (2006) who attributed it to the formation of cement hydration products. Fig. 9. Total dissolved calcium in leachate for (a) biorig experiment after two and 12 weeks of loading and (b) for the pilot scale study.
biorigs was determined based on the measurement of CH4 and CO2, and only unamended biosolids resulted in gas production, for which the average gas concentration of CH4 and CO2 after 12 weeks loading was 45% and 19% respectively. The production of CH4 and CO2 in unamended biosolids corresponds to measurement of low solids pH (7.8) and the initial increase in volume (Fig. 7d). For both biorig and pilot scale experiments, it was found that Lime mixtures resulted in the largest production of leachate at
3.3.1. Calcium Calcium concentrations in leachate were analysed to assess the transformation of additives between the solid and aqueous phases. For the biorigs, concentrations ranged from 795 mg/L (WD mixture) up to 2748 mg/L (FA mixtures), with minor Ca2+ being detected for unamended biosolids (<250 mg/L). While Ca2+ concentrations were relatively similar after 12 weeks’ loading, a generally higher Ca2+ concentration was observed in leachate collected from specimens with higher lime content. For the pilot scale study, Ca2+ concentration varied between 1670 mg/L (LKD mixture) and 3973 mg/L (Lime mixture). While the calcium concentration stayed
Fig. 10. Cu2+, Ni2+ and Zn2+ concentration measured in leachate collected from (a) the biorigs experiment and (b) the pilot scale study.
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approximately constant, the total dissolved calcium (as presented in Fig. 9) increased with time and overburden pressure, which relates to the increase in the volume of leachate (Fig. 7a–c). Specimens containing higher amounts of lime resulted in higher readings indicating a higher leachability due to excess calcium within the biosolids matrix. Lime mixtures had the highest readings, followed by FA mixtures with WD having the lowest amount of Ca2 + after 12 weeks and 225 kPa overburden pressure. Higher leachability of Ca2+ for Lime mixtures compared to FA mixtures correlates with the lower shear strength and lower CaCO3 formation within the Lime mixture. The leaching of Ca2+ is also one of the main reason for pH increase of the leachate (see Section 3.3.3), as discussed by Asavapisit et al. (2005).
3.3.2. Copper, nickel and zinc Three typical metals quantified in biosolids are copper, nickel and zinc (Wang and Viraraghavan, 1998), and results for these three metals in this study are summarised in Fig. 10. In New Zealand, the maximum allowable ion concentrations for Class A landfill leachate for Cu2+, Ni2+ and Zn2+ are 5, 10 and 15 mg/L, respectively (Ministry of the Environment, 2004). Zn2+, Cu2+ and Ni2+ in general exhibit lowest mobility at pH levels around neutral to slightly acid and alkaline conditions (pH 6–8) due to a reduced formation of hydroxide ion complexes (Kumpiene et al., 2008; van der Sloot et al., 1996, 1997). With the addition of alkaline additives, an increase in metal solubility occurs which relates to an increase in pH and DOC (Kumpiene et al., 2008; Merrington et al., 2003; van der Sloot et al., 1996, 2001). A summary of various studies by Merrington et al. (2003) showed that a large percentage of Cu2+,
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Ni2+ and Zn2+ are either organically or carbonate bound within sewage sludge. Thus, explaining the increase in metal concentration with increased leachability of DOC. A relationship between increased metal leachability and increase in pH and DOC was found for all three metals and resulted from the addition of alkaline additives (Fig. 11a–c). While an increase in concentration of up to 4.0, 11.5 and 9.7 times that of unamended biosolids occurred for Cu2+, Ni2+ and Zn2+ respectively, the levels were all below the maximum levels for Class A landfills. The greatest metal leachability was measured within the first two to three months of testing after which the concentrations stayed either constant (Cu2+ and Ni2+), or slowly decreased with time (Zn2+) (Fig. 10b). Merrington et al. (2003) showed that the largest metal leachability in soil occurred immediately after the addition of sludge, which indicates that the highest metal concentration occurs almost immediately and will reduce in the long term.
3.3.3. Dissolved organic carbon and pH The level of DOC in leachate of landfills for example is important due to its potential for enhancing the growth of hazardous microorganisms when infiltrating the ground (Kuchar et al., 2006; van der Sloot et al., 2001). Bednarik et al. (2004) stated that, especially in the case of hydraulic binders, the leachability of organic components occurs very easily and thus could cause harm to the environment. Results from the biorigs and tanks showed that mixtures containing additives had high levels of DOC, especially those containing additional lime, compared to unamended biosolids. It was found that while the DOC content increased with time, no change due to different overburden pressures was found
Fig. 11. Biorig results: relation between pH and (a) copper concentration, (b) nickel concentration, (c) zinc concentration, and (d) DOC.
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(Kayser, 2012). The increase in DOC over time is due an increase in soluble low molecular mass organic components, which form during the hydration process (Glasser, 1997). Fig. 11d depicts the relationship between pH and DOC level for leachate collected from the biorigs after 12 weeks of loading. Mixtures with no lime (WD mixture) produced the lowest pH readings (9.9–10.2) and thus the lowest DOC readings for amended mixtures (DOC 6.4–13.3 mgC/ mL), with unamended biosolids being the lowest (pH 7.8, DOC 1.5 mgC/mL). Lime mixtures had the highest values for pH (11.4–12.4) and with that, the highest DOC readings (13.5– 23.0 mgC/mL). DOC concentration within the leachate from the pilot scale study increased with time for all mixtures and ranged from 18.8 gC/L (pH 11.0) for WD + L mixtures to 27.7 gC/L (pH 12.0) for Lime mixtures (Kayser, 2012). 4. Conclusions This study reports on the shear strength, settlement, and leachate volume and composition of biosolids amended with fly ash (FA), KOBM, lime kiln dust (LKD) and smelter slag (WD) with and without the addition of lime. Experimental equipment involved specially developed biorigs simulating different overburden pressures, followed by a pilot scale study and field tests to investigate time and scaling effects. Results show that alkaline additives can enhance the shear strength and stiffness of biosolids but result in a more compressible material (larger settlements resulting in larger disposable volume of material). At the same time the leachability of organic matters (DOC) and metals (Cu2+, Zn2+ and Ni2) increase, but metal concentration stay within the requirements for Class A landfills in New Zealand. Strength measurements showed that independent of the testing method (unconsolidated undrained triaxial or hand shear vane), FA and Lime mixtures (20% lime) showed the highest strength increase, followed by WD + L and KOBM mixtures (10% lime), with WD and LKD mixtures (0% lime) exhibiting the lowest increase. For FA and Lime mixtures, strength increase was mainly related to the formation of CaCO3 which was favoured under higher overburden pressure and drained conditions. Mixtures with a clear increase in strength with time, after the addition of additive, lead to a positive correlation between solid content and strength. While solids content may be positively correlated with strength it cannot be used solely for strength characterisation of all biosolids, either amended or unamended, since the change in strength is highly dependent on the resulting chemical reactions involving a particular additive. Field testing results were limited and while they showed no trend in regard to the duration of field placement, results were comparable to those found in the pilot scale study. Leachate volume, concentrations of DOC, calcium, copper, nickel and zinc were determined for the biorig and pilot scale experiments. Overall, Lime mixtures showed the largest production of leachate, followed by FA mixtures, with WD + L and LKD mixtures having the lowest rate of production. Leachate analyses showed that a lower mobility of Ca2+ can be related to higher strength readings, depending on the total amount of CaO available. Leachability of Ca2+ was also the main source of increase in leachate pH and DOC, which further related to increased solubility of Cu2+, Zn2+and Ni2+. Acknowledgements The study was funded by Watercare Services Ltd. Claudia Kayser received a Technology for Industry Fellowship from the Foundation for Research, Science and Technology, New Zealand.
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Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production Claudia Kayser ⇑, Tam Larkin, Naresh Singhal Department of Civil and Environmental Engineering, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
a r t i c l e
i n f o
Article history: Received 21 April 2015 Revised 17 July 2015 Accepted 17 August 2015 Available online xxxx Keywords: Biosolids Leachate Metal concentration Shear strength Settlement Overburden pressure
a b s t r a c t Residuals from wastewater treatment operations (biosolids) were mixed with lime, fly ash, lime kiln dust, or two smelter slags to assess their efficacy as potential stabilisation agents by assessing their effects on the shear strength, compressibility, and solids content of mixtures. In addition, the minerals formed and leachate produced during stabilisation were determined. Tests were performed to explore the change of the geoenvironmental properties of the amended biosolids, while under pressure, at different scales using laboratory, pilot and field scale tests. The settlement characteristics of the amended biosolids under a range of applied pressures were determined using a consolidometer. All amended biosolids mixtures showed higher strength than the unamended biosolids, with mixtures containing a combination of 20% fly ash and 20% lime giving the highest (up to eightfold) increase in strength, and that with lime kiln dust and the smelter slags showing the lowest (up to twofold). The biosolids mixtures with only lime gave the second highest increase in strength (up to fourfold), but produced the largest amount of leachate, with higher level of dissolved calcium. The increase in strength correlated with availability of calcium oxide in the mixtures which lead to calcium carbonate formation, accompanied with higher leachate production and settlement during consolidation. Copper, nickel and zinc concentrations increased with alkaline additives and corresponded to higher pH and DOC levels. Nonetheless, concentrations were within the New Zealand regulatory limits for Class A landfills. Ó 2015 Published by Elsevier Ltd.
1. Introduction The enhancement in efficiency of modern wastewater treatment operations has led to production of larger quantities of residuals, or biosolids. Consequently their disposal in a manner that complies with increasingly stringent regulatory requirements can pose a challenge. Problems with disposal arise not only from the material being rich in organic matter and prone to biodegradation, but also from the low solids content (20%) and poor shear strength (3 kPa) (Kayser et al., 2011). Typically, biosolids are disposed following stabilisation (e.g., by digestion, lime addition, lime addition to digested sludge) by spreading on land, use as fill material for land reclamation and rehabilitation, or deposition in monofills or landfills (UN-HABITAT, 2008). Landfill operators are increasingly reluctant to co-dispose large quantities of organically rich materials due to deterioration in strength from long term degradation. Restrictions on land spreading are arising from the lower levels of heavy metals being specified for the material being ⇑ Corresponding author. E-mail addresses: [email protected] (C. Kayser), [email protected]. nz (T. Larkin), [email protected] (N. Singhal).
disposed. Furthermore, the increases in cost, due to larger quantities of lime required to stabilise the residuals, is becoming a consideration for treatment plant operators. Thus, it is not surprising that the use of waste materials in-lieu of, or to supplement, lime addition is gaining philosophical acceptance (Kayser et al., 2011). Our previous study showed that mixing wastewater residuals with locally sourced alkaline waste products (e.g., lime kiln dust, fly ash, smelter slag) increased the shear strength of mixtures by as much as eightfold when 20% lime was added and the pH was maintained above 11 (Kayser et al., 2011). However, few studies have explored the effect of different amendments on the geoenvironmental properties of biosolids, particularly under typical overburden pressures expected at depth in landfills and monofills. A recent New Zealand proposal to dispose biosolids in monofills with depths of up to 35 m provides a useful guide to the pressures that the disposed material are likely to be subjected. While information on the increase in strength of surficial biosolids following their modification with alkaline additives is available (Dirk, 1996; Lee et al., 2002; Lim et al., 2006; Voss, 1996; Yin, 2001), understanding of the behaviour of the mixtures under significant overburden pressures is very limited. In this study we investigate the effect of adding lime and lime mixed with fly ash, lime kiln dust, or two smelter
http://dx.doi.org/10.1016/j.wasman.2015.08.024 0956-053X/Ó 2015 Published by Elsevier Ltd.
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slags at various concentrations, on the strength, solids content, and settlement under overburden pressures typically encountered in modern landfills. In addition, we characterise the quantity and composition of leachate produced, specifically metals, calcium, pH and organic carbon, under the above conditions. 2. Materials and methodology 2.1. Biosolids and additives Biosolids are the processed sewage sludge using anaerobic digestion and centrifugal dewatering. The biosolids used in this study originated from a local wastewater treatment plant (WWTP) in Auckland, New Zealand, which is one of the largest WWTP in Australasia, treating over 111 billion litres of industrial and domestic wastewater annually. The biosolids are currently disposed after stabilisation with 24% quicklime (CaO) at Pond 2, a 44 ha large biosolids rehabilitation area next to the WWTP. For this study unamended biosolids were collected prior to stabilisation, i.e., prior to mixing with lime. The initial moisture content, solid content and volatile solid content for the unamended biosolids varied from, 76.7–80.3%, 19.7–23.3% and 60–70%, respectively. The waste additives were sourced from local industries and included lime kiln dust (LKD), a low alkaline fly ash (FA) from coal burning, works debris (WD) produced during steel recycling, and smelter slag (KOBM) from steel production. Large particles were separated from WD by sieving through a 0.5 mm mesh; other additives were used as obtained. CaO was added as an alkaline supplement, and used as a reference, since it is commonly encountered as a means of stabilisation of biosolids. Table 1 shows the mixing ratios of additives, which follows our previous experimental work (Kayser et al., 2009, 2011). 2.2. Experimental setup Laboratory experiments were conducted using steel tubes with biosolids under pressure (biorigs) and tanks (pilot scale test), along with in-situ testing in the field. The relative characteristic scales and other characteristics of the testing apparatus are summarised in Table 2. 2.2.1. Biorigs assembly Biorigs were used to test amended biosolids under overburden pressure; the setup allowed for a range of overburden stresses and measurement of the settlement along with collection of leachate (Fig. 1). A biorig frame has a height of 1.89 m and a width of 1.13 m. The steel tubes used in the biorigs are made of very high quality corrosion resistant stainless steel (SAF 2205), with an internal diameter of 75 mm and a length of 500 mm. For a smooth finish, the tubes were honed and sealed using O-rings that were recessed in machined groves in the PVC top and bottom caps. An individual fit was made for each top and bottom cap to a particular tube. This process resulted in a ‘‘no leak system” for pressures up to 316 kPa when applied for up to 6 months. The bottom cap was
Table 1 Mixing ratio of additives. Name of mixture
Percentage of additive used based on the biosolids solid content
FA LKD WD WD + L KOBM Lime
20% 30% 30% 30% 30% 20%
fly ash + 20% lime lime kiln dust works debris works debris + 10% lime smelter slag + 10% lime lime
Table 2 Experimental set-up conditions and scale. Experimental set-up Approximate scale (based on a 44 ha and up to 10 m depth in-situ placement area) Drainage condition Equivalent depth of biosolids layer (m)
Biorig 1:1 108
Tank In-situ 1:1.15 106 1:1
Drained Drained 10 and 20 3
Undrained 0.5–1.5
mounted on a pedestal that was connected to the middle of the cross beam. The top cap was connected to a flat steel plate assembled to a guide rod through the upper part of the frame to ensure alignment. Both caps were designed with an outlet to allow for two way drainage and Tygon tubing connected the bottom and top cap to glass bottles for the collection of gas and leachate (Fig. 1). To separate leachate and solids both caps contained a 0.5 lm stainless steel mesh and a 0.5 bar porous ceramic disk (Soilmoisture Equipment Corp.). For the application of overburden pressure, weights were attached to 2 mm wire ropes, which were guided over a pulley system that provided a twofold mechanical advantage. This allowed the application of either 115 kPa or 225 kPa of overburden pressure following calibration for internal (O-ring and specimen) and external (guiding rod and pulley system) friction (Kayser, 2012). Internal friction was measured by setting up a specimen and replacing the bottom pedestal with an internal pedestal connected to a S-load cell. External friction was measured via a proving ring beneath the top cap. The overburden pressure provided was equivalent to the weight per unit area of either 10 m (115 kPa) or 20 m (225 kPa) overburden of amended biosolids. The five amendments (FA, LKD, WD, WD + L and KOBM (see Table 1)) were mixed with biosolids and employed in the biorigs together with control specimens of unamended biosolids and biosolids with 20% lime. Loading durations of 2 and 12 weeks were used. 2.2.2. Consolidometer To determine the settlement/time characteristics of biosolids, the biorig setup was modified with a pore liquid pressure transducer being installed at the base of the specimen to record the leachate pressure at the undrained face during one way drainage. Additionally, the length of the stainless steel tube used was reduced to 180 mm. To ensure the measurement was of the pore liquid pressure only, a 2 bar high air entry disk (Soilmoisture Equipment Corp.) was used. For the consolidometer test one biosolids specimen with 20% lime was prepared and subjected to four successive vertical pressures of 47 kPa, 109 kPa, 192 kPa and 316 kPa (determined following initial calibrations as described above (Kayser, 2012)), each for a minimum of 40 days. 2.2.3. Tanks To investigate the long term (up to 1.3 years) behaviour of biosolids on a larger scale, a pilot scale study was conducted using four 425 L plastic tanks (1200 mm height and 700 mm diameter, see Fig. 2). These tanks were modified with one leachate collection outlet (bottom) and 12 re-sealable ports (see Fig. 2a) allowing for fortnightly extraction of solids samples and monthly (30 days) measurements of shear strength. Four mixtures were prepared by mixing biosolids by hand with Lime, FA, WD + L and LKD, and placed in a controlled manner to a height of 700 mm and an approximate density of 1.1 t/m3 (representative of field conditions). The outlet of each tank was first fitted with a drainage layer of sand covered by a 125 lm stainless steel filter, designed according to USDA guidelines (USDA, 1994). A biosolids layer was then placed followed by another filter and drainage layer. To create a small overburden pressure (3 kPa), plastic bags filled with sand were used forming the top layer, before the tanks were sealed (Fig. 2b).
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Fig. 1. Biorig with leachate and gas collection system.
Fig. 2. (a) Arrangement of re-sealable ports and (b) tank with leachate and gas collection system.
2.2.4. Field testing and sampling Shear vane test were conducted and undisturbed field samples were collected at Pond 2 from depths of 0.5–1.5 m in cells containing 2–8 year old biosolids. 2.3. Specimen preparation and analyses of solids and leachate properties Specimens were prepared by hand mixing additives and biosolids until a visibly homogeneous matrix was achieved. The percentage of additives reported is based on the dry mass of biosolids. Mixtures were compacted in layers in the stainless steel tubes to an initial height of 40 mm in the consolidometer and 390 mm in the biorigs to densities between 1.01 t/m3 and 1.08 t/m3, the
approximate density of biosolids after deposition by the local WWTP. For the measurement of shear strength of specimens from the biorigs and field tests, unconsolidated undrained (UU) triaxial tests were employed according to the New Zealand Standard NZS 4402.6.2.1:1986 (1986). In the case of the biorigs, at the termination of loading, the specimens were extruded from the tubes and trimmed to a length of 150 mm to conduct UU triaxial tests at a cell pressure of either 115 kPa or 225 kPa depending on the biorig consolidation pressure. Hand held shear vane tests were conducted at monthly intervals during the pilot scale study, and during field testing. The tests were carried out following recommendations of the NZ Geotechnical Society Inc. (2001).
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Specimens were measured for solids content according to the New Zealand Standards for soil testing (NZS 4402:1986). Samples were tested in triplicates. The drying temperature is highly critical for biosolids due to its high organic content and temperatures such as e.g., 60 °C (O’Kelly, 2004) and 50 °C (Arulrajah et al., 2011) have been recommended. Analysis by Kayser (2012) showed that there is a linear relationship for solids content between 60 °C and the standard 105 °C (Kayser, 2012). A drying temperature of 105 °C was therefore adopted as per NZS 4402.2.1:1986 (1986) to expedite mixing and to allow a wider comparison with results presented in the literature. Changes in the mineral phase of biosolids due to the addition of additives were ascertained using X-ray diffraction (XRD). The XRD used was a Philips PW 1130 high voltage generator, copper anode X-ray tube, 40 kV, 20 mA, and a scanning speed of 2°/min for 0–60° 2H. Samples were air dried and ring milled. The calcium carbonate (CaCO3) content was quantified following the ASTM Standard for chemical analysis of limestone, quicklime, and hydrated lime (ASTM International, 2006), by using a standard indicator titration method. A 4.6 g dried sample (dried at 60 °C) was used from which the percentage of CaCO3 was determined based on a received wet mass. Environmental scanning electron microscopy (ESEM) analyses were carried out on air dried and platinum coated samples. The ESEM used was a FEI Quanta 200F. Leachate from the biorigs was collected from the top and bottom of the specimen, while tank leachate was collected from the bottom. The volume of biorig leachate is reported as the sum of those two volumes (for unamended biosolids approximately 50:50 and for biosolids mixtures varying from approximately 30:70 to 45:65 for ratio of top and bottom leachate). Leachate was analysed either in duplicates or triplicates for the dissolved organic carbon (DOC) and calcium, copper, nickel and zinc ion concentration (Ca2+, Cu2+, Ni2+ and Zn2+, respectively). For DOC measurement, the leachate was filtered through 0.45 lm PTFE syringe filters and analysed using a Shimadzu SSM-5000A VCSH model. For calcium and metal measurement, samples were prepared by microwave digestion, filtered through 0.45 lm PTFE syringe filters and analysed using a flame atomic absorption spectrometer (AAS, Varian SpectrAA 50). Gas collected from the biorigs was analysed for methane (CH4) and carbon dioxide (CO2) using a GC SRI 8610 gas chromatograph apparatus with a thermal conductivity detector.
3. Results and discussion 3.1. Shear strength and settlement 3.1.1. Biorigs An overview of the shear strength results is given in Fig. 3. The best performing mixtures with respect to strength enhancement were FA mixtures, followed by Lime mixtures. FA and Lime mixtures showed the highest strength after 12 weeks, with the FA mixture specimen doubling in strength under 20 m of overburden pressure. After two weeks loading LKD was the third best performing mixture, but a reduction in shear strength was observed over time for both overburden pressures, due to a reduced availability of CaO. Reduced carbonation took place, since calcium within the LKD was bound in composites such as CaOH2, CaCO3 and CaSO4 prior mixing. This reduced the capability of forming bounds after adding lime to the biosolids. Unamended biosolids performed poorly and showed the lowest overall strength, with only minimal change with time and overburden pressure. While differences between overburden pressure of 10 m and 20 m after 2 weeks were negligible for LKD, WD, WD + L and KOBM mixtures, after
12 weeks these mixtures showed a pronounced increase in strength of 25–60% between 10 m and 20 m of overburden. Comparing drained conditions (biorigs with overburden pressures) and undrained conditions from our previous study (PVC tube with no overburden pressure (Kayser et al., 2011)), it is evident that only Lime and FA mixtures showed a clearly visible increase in shear strength (Fig. 3). All other mixtures showed essentially no difference between (i) drained and undrained conditions and (ii) 10 m and 20 m of overburden pressure. When CaO is mixed with unamended biosolids, it reacts readily with water forming calcium hydroxide (Ca(OH)2) in an exothermic reaction, leading to an immediate increase in solids content. While a correlation between solids content and shear strength has been reported in the literature for both, amended and unamended biosolids (Klein and Sarsby, 2000; Koenig and Bari, 2001; O’Kelly, 2006; Zhang et al., 2008), lower shear strength is not always positively correlated with lower solids content, especially in the case of unamended biosolids (Koenig and Bari, 2001). Results for correlations between shear strength from UU triaxial tests (su tri) and solid content are presented in Fig. 4. Unamended biosolids and LKD mixtures showed no direct relationship between solids content and strength. By contrast, WD, WD + L and KOBM mixtures showed a clear linear relationship, while FA and Lime mixtures showed a strong polynomial relationship. The latter two mixtures exhibited much higher strength, at the same solids content, to that of the LKD, WD, WD + L and KOBM mixtures (Figs. 3 and 4). Following the rapid formation of Ca(OH)2, the formation of CaCO3 and water occurs at a reduced speed under the presence of CO2. The production of CaCO3 increases under high pressures and addition of CO2 (Domingo et al., 2006). In the case of biosolids, the presence of CaCO3 has been suggested to be one of the major reasons for increases in shear strength (Horn, 1990; Kayser, 2012; Lim et al., 2002). XRD analyses of biosolids mixtures in our study identified quartz (SiO2), CaCO3 and Ca(OH)2 as the main crystalline compositions. Both Ca(OH)2 and CaCO3 were found in FA and Lime mixtures after loading for two weeks, but after 12 weeks of loading all Ca(OH)2 was transformed into CaCO3 (Fig. 5). An additional peak at a 2-theat position of around 45° was identified as iron (Fe), which occurred due to the chipping of metal from the crusher in the ring mill prior to testing and was therefore not identified in Fig. 5. No significant difference was observed in the XRD patterns of FA and Lime mixtures loaded under 115 kPa and 225 kPa, with CaCO3 and SiO2 being the only two crystalline composites detectable. To quantify the amount of CaCO3 for FA and Lime mixtures, a standard indicator titration method was carried out. Results showed that larger amounts of CaCO3 were formed under a loading of 225 kPa, 7.9% and 6.9% CaCO3 by wet mass for FA and Lime mixture respectively, compared to that for an overburden stress of 115 kPa, 7.0% and 6.4% CaCO3 by wet mass for the FA and Lime mixture respectively. The increased formation of CaCO3 under higher overburden pressure is consistent with the literature (e.g., Domingo et al. (2006)). For KOBM, LKD, WD and WD + L mixtures no differences in XRD patterns were found due to differences in loading time and pressure, with SiO2 and CaCO3 the only crystalline composites identified. While ettringite was found in FA mixtures in earlier studies (Kayser et al., 2011), no further minerals were detected using XRD in this series of tests, due to a reduced percentage of additives used. The experiments showed that drainage is fundamental to ensure sufficient shear strength when biosolids are amended with alkaline additives. Applying overburden pressure during draining enhances the formation of CaCO3 and further increases the shear strength. Our results are consistent with the literature where large increases in shear strength were found due to the addition of alkaline additives and the application of overburden pressure (Chu et al., 2005). The work of Chu et al. (2005) reports increases in
Please cite this article in press as: Kayser, C., et al. Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.024
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Fig. 3. Shear strength, su
tri,
from PVC tubes after two weeks and from biorigs after 2 and 12 weeks’ of loading (Kayser et al., 2011).
Fig. 4. Solids content versus shear strength, su
tri.
Fig. 5. XRD analyses for mixtures under 20 m overburden pressure using (a) Lime mixtures and (b) FA mixtures after 2 and 12 weeks (Kayser, 2012).
strength from 100% to 175% for a mixture of biosolids, cement and copper slag and the application of overburden pressure of 80 kPa under drained conditions, stating that a combination of both chemical and mechanical treatment is needed for significant strength increase. 3.1.2. Consolidometer There is a paucity of studies on the consolidation and settlement properties of biosolids and those that exist generally focus on unamended biosolids (Aydilek et al., 2000; Klein and Sarsby, 2000; Koenig et al., 1996; Lo et al., 2002; O’Kelly, 2005, 2008). In the work reported here, four load increments were applied to a specimen of lime stabilised biosolids over a period of 160 days, with each load increment lasting 40 days. Table 3 gives a summary of the settlement characteristics for each load increment. An axial strain of 34% occurred during the first pressure increment and
Table 3 Settlement characteristics. Number of load increment Overburden pressure, kPa Settlement per load increment, mm Total axial strain at end of each load increment, % Maximum pore leachate pressure per load increment, kPa Time to maximum leachate pressure, days Maximum pore leachate pressure increment/ vertical stress increment, %
1 47 13.82 34.53
2 109 4.62 46.11
3 192 3.76 55.84
4 319 3.00 63.00
47
25
21
17
0.6 100
2.0 43
2.6 24
5.4 13
increased to a total of 63% at the end of the fourth increment. Leachate pressure, measured at the undrained face, reached the magnitude of the first vertical stress increment (47 kPa, i.e., Skempton’s
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near the drained face could have occurred simultaneously resulting in reduced pressure readings. Concurrently, the solid skeleton of the mixture was maturing and stiffening from the reaction of the biosolids to the lime. The hardening of the biosolids will result in a reduction of a transfer of stress from the solid skeleton to the pore liquid. After about 30 days, all excess pore pressure had dissipated independently of the overburden pressure applied, indicating the end of the apparent primary consolidation phase. The coefficient of consolidation (cv), which describes the rate of settlement, was determined using Taylor’s and Casagrande’s method (Head, 1980) and was found to be approximately 0.01 m2/year by both methods (Kayser, 2012). 3.2. Tank and field Fig. 6. Hand held shear vane measurements, su (Kayser, 2012).
vane,
taken once every 30 days
B = 1) after 15 h. Subsequent load increments did not result in a maximum leachate pressure equal to the vertical stress increment, but were 43%, 24% and 13% of the vertical stress increment (Table 3). While the initial leachate pressure built up slowly at the undrained face of the specimen, dissipation of excess pressure
Following the biorigs experiment a pilot scale study was carried out, increasing the scale from that of the biorigs, 1:1 108, to that of a tank (0.7 m diameter by 1.2 m height), 1:1.15 106 (Table 2). Results of the shear strength measurements from hand held shear vane are presented in Fig. 6. The shear strength for all mixtures increased for the first 150–220 days, with insignificant change for the remaining days. A few kPa variations in the results (up to 15%) are not unusual and relates to inhomogeneity and point of measurement in the tanks (Fig. 2a). The FA mixture
Fig. 7. Loading time versus normalised leachate production for (a) biorigs, 12 weeks, 20 m, (b) biorigs, 12 weeks, 10 m, (c) pilot scale experiment and (d) loading time versus axial strain (biorigs, 12 weeks, 20 m).
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resulted in the highest strength readings overall, with a maximum reading of 27.9 kPa (at 5 months). The second best performing mixture was Lime, which was only marginal better than that of WD + L and LKD, and thus differed to the biorigs tests, where both FA and Lime mixtures performed similarly well. The difference in results between tank and biorig is related to the large quantity of biosolids to be mixed for the tanks, where hand mixing resulted in less homogenous mixture than for the biorigs. While less homogenous, the tanks represent a relatively accurate simulation of the in-situ mixing procedure at the WWTP. Hence, fly ash mixed with lime can result in significant strength increase even in a reduced homogenous mixture, if drainage is allowed. Shear strength measurements (UU triaxial and hand held shear vane) on five field specimens with lime percentage assumed to range from 20–30% varied between 5.3–12.2 kPa (UU triaxial) and 8.4–22.0 kPa (hand held shear vane). Measurements of the pH of solids were above 12 for biosolids aged two and three years, while for ages in excess of 3 years the pH was approximately 8.5. One sample was measured with a pH of 11.6 (5 years) showing the variability within the results related to mixing procedure and weathering in the field. No trend in regard to time (2–8 years) or depth (0.5–1.5 m) of placement was found. Taking into account weather conditions, e.g., rain, wind, temperature, it can be said that pH and shear strength results are similar to those observed during the pilot scale study (Fig. 6). The small quantity of field samples and experimental tests was dictated by (a) the lack of accurate determination of the actual time of deposition, (b) existing subsurface obstruction such as geogrids, geotextiles and oxidation pond sludge, and (c) lime dosage of mixture at time of deposition.
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3.3. Leachate volume and composition During both biorig and pilot scale experiments large amounts of leachate were collected from biosolids mixtures. An overview of the volume of leachate collected with time is given in Fig. 7a–c. For comparison purposes, the normalised leachate production in percentage (ratio of volume of leachate (L) collected to initial mass of liquid within the biosolids mixture (kg)) is presented. The corresponding settlement, change in axial strain (defined as the change in height over the initial height) with time, for the biorig specimens is shown in Fig. 7d. When considering leachate production as a function of overburden it is shown that with 10 m of overburden the amount of leachate production is approximately one half that for the case of 20 m of overburden at the end of testing (84 days). In the case of the pilot scale study leachate was collected over a period of 480 days and volumes were below that of the biorigs. It can further be seen that for all biosolids mixtures higher volumes of leachate resulted in higher axial strain/higher settlement (Fig. 7a and d). While the settlement and volume of leachate varied depending on loading duration and mixture, the ratio of normalised total leachate production to final axial strain showed a constant ratio of 1.14 L/kg with a standard deviation of 0.12 L/kg for all amended biosolids (12 weeks and 20 m). Unamended biosolids exhibited a different behaviour with an average ratio of 2.48 L/ kg (values varying between 1.6 L/kg and 3.7 L/kg). The constant ratios (1.14 L/kg and 2.48 L/kg) are due to the initial average bulk density of the mixtures (mean = 1.09 t/m3, standard deviation = 0.02 t/m3) and the unamended biosolids (mean = 0.99 t/m3, standard deviation = 0.03 t/m3) being close to that of water/leachate (Kayser, 2012). Biodegradation within the specimens of the
Fig. 8. ESEM images of (a) unamended biosolids, (b) Lime mixture after 12 weeks and 20 m overburden pressure, (c) FA mixture prior to testing and (d) as in (c) after 12 weeks and 20 m overburden pressure.
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the end of testing, independent of loading time and pressure. FA mixtures showed high leachate production when under high overburden pressure, but reduced with reduced loading, although an increase in leachate production was found with time due to its alkaline content. The volume of the leachate production was thought to be mainly due to: (i) an increase in permeability leading to a reduction in bound water, and (ii) the production of water during the formation of CaCO3. The increase in permeability due to increase in void spaces was shown using ESEM analyses. A distinct difference was found between unamended biosolids (Fig. 8a) and Lime and FA mixtures (Fig. 8b–d), with the latter showing numerous micro pores. Comparing FA mixtures before and after 12 weeks of curing (Fig. 8b and c), a reduction in void spaces was observed, which is most likely related to the formation of calcium carbonate. A reduction in void space with addition of alkaline additives was also observed by Lim et al. (2006) who attributed it to the formation of cement hydration products. Fig. 9. Total dissolved calcium in leachate for (a) biorig experiment after two and 12 weeks of loading and (b) for the pilot scale study.
biorigs was determined based on the measurement of CH4 and CO2, and only unamended biosolids resulted in gas production, for which the average gas concentration of CH4 and CO2 after 12 weeks loading was 45% and 19% respectively. The production of CH4 and CO2 in unamended biosolids corresponds to measurement of low solids pH (7.8) and the initial increase in volume (Fig. 7d). For both biorig and pilot scale experiments, it was found that Lime mixtures resulted in the largest production of leachate at
3.3.1. Calcium Calcium concentrations in leachate were analysed to assess the transformation of additives between the solid and aqueous phases. For the biorigs, concentrations ranged from 795 mg/L (WD mixture) up to 2748 mg/L (FA mixtures), with minor Ca2+ being detected for unamended biosolids (<250 mg/L). While Ca2+ concentrations were relatively similar after 12 weeks’ loading, a generally higher Ca2+ concentration was observed in leachate collected from specimens with higher lime content. For the pilot scale study, Ca2+ concentration varied between 1670 mg/L (LKD mixture) and 3973 mg/L (Lime mixture). While the calcium concentration stayed
Fig. 10. Cu2+, Ni2+ and Zn2+ concentration measured in leachate collected from (a) the biorigs experiment and (b) the pilot scale study.
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approximately constant, the total dissolved calcium (as presented in Fig. 9) increased with time and overburden pressure, which relates to the increase in the volume of leachate (Fig. 7a–c). Specimens containing higher amounts of lime resulted in higher readings indicating a higher leachability due to excess calcium within the biosolids matrix. Lime mixtures had the highest readings, followed by FA mixtures with WD having the lowest amount of Ca2 + after 12 weeks and 225 kPa overburden pressure. Higher leachability of Ca2+ for Lime mixtures compared to FA mixtures correlates with the lower shear strength and lower CaCO3 formation within the Lime mixture. The leaching of Ca2+ is also one of the main reason for pH increase of the leachate (see Section 3.3.3), as discussed by Asavapisit et al. (2005).
3.3.2. Copper, nickel and zinc Three typical metals quantified in biosolids are copper, nickel and zinc (Wang and Viraraghavan, 1998), and results for these three metals in this study are summarised in Fig. 10. In New Zealand, the maximum allowable ion concentrations for Class A landfill leachate for Cu2+, Ni2+ and Zn2+ are 5, 10 and 15 mg/L, respectively (Ministry of the Environment, 2004). Zn2+, Cu2+ and Ni2+ in general exhibit lowest mobility at pH levels around neutral to slightly acid and alkaline conditions (pH 6–8) due to a reduced formation of hydroxide ion complexes (Kumpiene et al., 2008; van der Sloot et al., 1996, 1997). With the addition of alkaline additives, an increase in metal solubility occurs which relates to an increase in pH and DOC (Kumpiene et al., 2008; Merrington et al., 2003; van der Sloot et al., 1996, 2001). A summary of various studies by Merrington et al. (2003) showed that a large percentage of Cu2+,
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Ni2+ and Zn2+ are either organically or carbonate bound within sewage sludge. Thus, explaining the increase in metal concentration with increased leachability of DOC. A relationship between increased metal leachability and increase in pH and DOC was found for all three metals and resulted from the addition of alkaline additives (Fig. 11a–c). While an increase in concentration of up to 4.0, 11.5 and 9.7 times that of unamended biosolids occurred for Cu2+, Ni2+ and Zn2+ respectively, the levels were all below the maximum levels for Class A landfills. The greatest metal leachability was measured within the first two to three months of testing after which the concentrations stayed either constant (Cu2+ and Ni2+), or slowly decreased with time (Zn2+) (Fig. 10b). Merrington et al. (2003) showed that the largest metal leachability in soil occurred immediately after the addition of sludge, which indicates that the highest metal concentration occurs almost immediately and will reduce in the long term.
3.3.3. Dissolved organic carbon and pH The level of DOC in leachate of landfills for example is important due to its potential for enhancing the growth of hazardous microorganisms when infiltrating the ground (Kuchar et al., 2006; van der Sloot et al., 2001). Bednarik et al. (2004) stated that, especially in the case of hydraulic binders, the leachability of organic components occurs very easily and thus could cause harm to the environment. Results from the biorigs and tanks showed that mixtures containing additives had high levels of DOC, especially those containing additional lime, compared to unamended biosolids. It was found that while the DOC content increased with time, no change due to different overburden pressures was found
Fig. 11. Biorig results: relation between pH and (a) copper concentration, (b) nickel concentration, (c) zinc concentration, and (d) DOC.
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(Kayser, 2012). The increase in DOC over time is due an increase in soluble low molecular mass organic components, which form during the hydration process (Glasser, 1997). Fig. 11d depicts the relationship between pH and DOC level for leachate collected from the biorigs after 12 weeks of loading. Mixtures with no lime (WD mixture) produced the lowest pH readings (9.9–10.2) and thus the lowest DOC readings for amended mixtures (DOC 6.4–13.3 mgC/ mL), with unamended biosolids being the lowest (pH 7.8, DOC 1.5 mgC/mL). Lime mixtures had the highest values for pH (11.4–12.4) and with that, the highest DOC readings (13.5– 23.0 mgC/mL). DOC concentration within the leachate from the pilot scale study increased with time for all mixtures and ranged from 18.8 gC/L (pH 11.0) for WD + L mixtures to 27.7 gC/L (pH 12.0) for Lime mixtures (Kayser, 2012). 4. Conclusions This study reports on the shear strength, settlement, and leachate volume and composition of biosolids amended with fly ash (FA), KOBM, lime kiln dust (LKD) and smelter slag (WD) with and without the addition of lime. Experimental equipment involved specially developed biorigs simulating different overburden pressures, followed by a pilot scale study and field tests to investigate time and scaling effects. Results show that alkaline additives can enhance the shear strength and stiffness of biosolids but result in a more compressible material (larger settlements resulting in larger disposable volume of material). At the same time the leachability of organic matters (DOC) and metals (Cu2+, Zn2+ and Ni2) increase, but metal concentration stay within the requirements for Class A landfills in New Zealand. Strength measurements showed that independent of the testing method (unconsolidated undrained triaxial or hand shear vane), FA and Lime mixtures (20% lime) showed the highest strength increase, followed by WD + L and KOBM mixtures (10% lime), with WD and LKD mixtures (0% lime) exhibiting the lowest increase. For FA and Lime mixtures, strength increase was mainly related to the formation of CaCO3 which was favoured under higher overburden pressure and drained conditions. Mixtures with a clear increase in strength with time, after the addition of additive, lead to a positive correlation between solid content and strength. While solids content may be positively correlated with strength it cannot be used solely for strength characterisation of all biosolids, either amended or unamended, since the change in strength is highly dependent on the resulting chemical reactions involving a particular additive. Field testing results were limited and while they showed no trend in regard to the duration of field placement, results were comparable to those found in the pilot scale study. Leachate volume, concentrations of DOC, calcium, copper, nickel and zinc were determined for the biorig and pilot scale experiments. Overall, Lime mixtures showed the largest production of leachate, followed by FA mixtures, with WD + L and LKD mixtures having the lowest rate of production. Leachate analyses showed that a lower mobility of Ca2+ can be related to higher strength readings, depending on the total amount of CaO available. Leachability of Ca2+ was also the main source of increase in leachate pH and DOC, which further related to increased solubility of Cu2+, Zn2+and Ni2+. Acknowledgements The study was funded by Watercare Services Ltd. Claudia Kayser received a Technology for Industry Fellowship from the Foundation for Research, Science and Technology, New Zealand.
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Please cite this article in press as: Kayser, C., et al. Amendment of biosolids with waste materials and lime: Effect on geoenvironmental properties and leachate production. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.08.024