Long-term settlement prediction for wastewater biosolids in road embankments

Resources, Conservation and Recycling 77 (2013) 69–77

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Long-term settlement prediction for wastewater biosolids in road embankments M.M. Disfani a,1 , A. Arulrajah a,∗ , V. Suthagaran b,2 , M.W. Bo c,3 a b c

Swinburne University of Technology, Melbourne, Australia Coffey Geotechnics, Melbourne, Australia DST Consulting Engineers Inc., Thunder Bay, Canada

a r t i c l e

i n f o

Article history: Received 1 March 2012 Received in revised form 10 February 2013 Accepted 29 May 2013 Keywords: Biosolids Wastewater Long-term settlement Road embankments

a b s t r a c t An innovative research study was undertaken to characterize the settlement characteristics of aged wastewater biosolids to facilitate its long-term settlement prediction when used as fill material in road embankment applications. Settlement can be sub-divided into compression due to consolidation and deformation attributed to biodegradation. Results of an extensive geotechnical laboratory evaluation including compaction characteristics, shear strength parameters, coefficient of consolidation, compression index, swell index and coefficient of secondary consolidation were used to predict the consolidation settlement of biosolids in road embankments. Other relevant parameters for biodegradation settlement prediction, such as organic content, pH and electrical conductivity of the biosolids were also determined. The biodegradation induced settlement of a road embankment built with aged biosolids was subsequently analyzed by applying an analytical method used previously for municipal solid waste landfills. The adopted model shows that the rate of biodegradation settlement reduces with the reduction in pH values of biosolids. The model also suggests that the time taken for full process of biodegradation decreases dramatically with pH value of the biosolids between 0 and 6 and then increases exponentially with pH value of the biosolids between 8 and 14. A framework has been developed to predict the total settlement of wastewater biosolids in road embankments for end-users. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Waste material is considered any type of material by-product of human and industrial activity that has no lasting value (Disfani et al., 2011; Ali et al., 2011). Extensive amounts of waste are generated daily by various industries and human activities worldwide. Shortage of natural resources, lack of available land space and increasing waste disposal costs, has placed higher urgency and pressure on recycling solid wastes (Aatheesan et al., 2010; Hoyos et al., 2011; Landris, 2007; Disfani et al., 2011; Arulrajah et al.,

∗ Corresponding author at: Faculty of Engineering and Industrial Sciences (H38), Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia. Tel.: +61 3 92145741; fax: +61 3 92148264. E-mail addresses: [email protected] (M.M. Disfani), [email protected] (A. Arulrajah), Sutha [email protected] (V. Suthagaran), [email protected] (M.W. Bo). 1 Faculty of Engineering and Industrial Sciences (H38), Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia. Tel.: +61 3 92144943; fax: +61 3 92148264. 2 Coffey Geotechnics, 126 Trenerry Crescent, Abbotsford, VIC 3067, Australia. Tel.: +6 13 94731300; fax: +61 3 94731350. 3 DST Consulting Engineers Inc., 605 Hewitson Street, Thunder Bay, Ontario, P7B 5V5 Canada. Tel.: +1 807 623 2929; fax: +1 807 623 1792. 0921-3449/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.resconrec.2013.05.009

2012, 2013a). Sustainable usage of waste materials can provide social and economic benefit for governments, industry and consumers and lower carbon footprints (Disfani et al., 2012). Reuse of waste materials in civil and geotechnical engineering applications such as road or pavement applications will reduce the demand for scarce virgin natural resources and simultaneously reduce the quantity of this waste material destined for landfills (Wartman et al., 2004; Poon and Chan, 2006; Tam and Tam, 2007; Arulrajah et al., 2011a; Disfani et al., 2011; Hoyos et al., 2011; Puppala et al., 2011). Sludge is the mixture of solids–water pumped out of wastewater treatment lagoons which possesses the characteristics of a liquid or slurry typically containing 2–15% of oven dried solids (Arulrajah et al., 2011b). Biosolids is dried sludge having the characteristics of an oven dried solid typically containing 50–70% by weight of bulk solids (Arulrajah et al., 2011b). With the increase in the quantity of wastewater biosolids produced annually around the globe and escalating demand for virgin material, recycling and reuse of biosolids has gained significant momentum in a move toward a more sustainable society. In the state of Victoria, Australia, more than 2 million tons of biosolids are currently stored, in lagoons or stockpiles, with a further annual production of 66,700 tons per annum (DNRE, 2002).

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The engineering characteristics of the biosolids depend on factors such as the source and type of wastewater the biosolids was generated from (which varies with lifestyle and sewage source), method of treatment used in the treatment plant and biosolids age (US EPA, 2005). These factors are responsible for varying characteristics of biosolids from one treatment plant to the other and from time to time (Arulrajah et al., 2011b, 2013b; US EPA, 2005). The current knowledge of the geotechnical engineering properties of human waste biosolids especially its long-term settlement caused by biodegradation phenomenon is limited. This has consequently led to the unfortunate stockpiling of biosolids in landfills globally. In this research, aged biosolids were sampled from three different stockpiles located at a wastewater treatment plant in Melbourne, Australia. Laboratory tests consisting of index, compaction, consolidation, creep, hydraulic conductivity, shear strength, organic content, pH value and electrical conductivity tests were performed to determine the geotechnical engineering and biodegradation characteristics of aged biosolids. Subsequently, a parametric analysis was undertaken using various pH values for a typical road embankment to predict the long-term settlement caused by biodegradation for the case of using aged biosolids in road embankment fills.

2. Review of past studies Studies to improve the quality of biosolids for reuse in land or agricultural applications have been carried out by various authors (Ratnaparkhe and Sertic, 2005; Thorpe, 2011; Lundie and Peters, 2005). However, limited studies on the geotechnical properties of biosolids derived from human waste for usage in civil and geotechnical engineering applications have been undertaken. There is also no known study on the biodegradation induced settlement of biosolids in road embankments. On the other hand, the geotechnical characteristics of sewage sludge, the primary source of air-dried biosolids, have been studied extensively in recent years in various countries such as Australia, Hong Kong, South Korea, Turkey, Singapore, UK and USA. Vajirkar (2004) studied the strength characteristics of biosolids in blends with municipal solid waste using cone penetration tests which were carried out in Florida, USA. Vajirkar (2004) reported an average internal friction angle of 29◦ for biosolids mixed with municipal solid wastes. Hundal et al. (2005) studied the geotechnical parameters including consolidation characteristics and shear strength parameters of the untreated biosolids generated from municipal wastewater treatment plant in Chicago, USA. Based on laboratory study results; Hundal et al. (2005) concluded that biosolids are suitable fill material for embankment construction. They suggested that the bearing capacity of biosolids can be increased by blending it with topsoil or other residual soils (Hundal et al., 2005). A research study was carried out by O’Kelly (2004, 2005a, 2005b, 2006) on geotechnical characteristics of sludge produced at Tullamore wastewater treatment plant in UK. This research assessed strength, compaction, compressibility and other geotechnical characteristics of sewage sludge. While the source, treatment, storage and age of sewage sludge studied by O’Kelly (2004) is different from the biosolids studied in this paper; the results of O’Kelly (2004, 2005a, 2005b, 2006) confirms the effect of different input levels of domestic and industrial wastewater on the engineering properties of sludge material. O’Kelly (2006) mentioned that the effects of ongoing biodegradation of biosolids should be considered on long-term settlement and factor of safety against slope instability of landfills (O’Kelly, 2006). O’Kelly (2008) in a research on the effect of biodegradation on the consolidation properties of the dewatered sewage sludge, showed that the total volatile solids

percentage in the sludge and specific gravity values of solids are inversely related which can be explained by ongoing biodegradation (O’Kelly, 2008). Biodegradation breaks down the volatile organic solids which results in increasing relative proportion of denser mineral particles, which in turn increases the specific gravity (O’Kelly, 2008). Hyun et al. (2007) analyzed the long-term settlement of municipal solid waste landfills by various settlement estimation methods. Their research was based on the fact that in municipal solid waste landfills, settlement caused by the decomposition of biodegradable solid wastes takes place over a long period, and this settlement considerably contributes to the total settlement (Hyun et al., 2007). Hyun et al. (2007) suggested that the fill age of a municipal solid waste landfill is a critical factor for evaluating the long-term settlement caused by decomposition (Hyun et al., 2007). The findings of their research showed that for the fresh municipal solid waste landfill (fill age less than 5 years) all methods predicted that the long-term settlement potentials are ∼20–60% of the thickness of the landfill. However, the long–term settlement potential rapidly decreased for the intermediately old sites (fill age around 8 years old). This decreased long-term settlement potential is more noticeable in the old sites (Hyun et al., 2007). Therefore, it was shown that the fresh municipal solid waste landfills had more potential of long-term settlements caused by the decomposition of biodegradable organic solids (Hyun et al., 2007). For the sites which the fill age was around 25 years, the possible long-term settlement was estimated to be less than 5% of the fill thickness (Hyun et al., 2007). The geotechnical laboratory experimentation carried out by Suthagaran et al. (2008, 2010) on blends of biosolids and various stabilizers suggested that wastewater biosolids stabilized with certain percentages of cement and lime can be used as engineering fill material. Arulrajah et al. (2011b, 2013b) studied select chemical and engineering properties of wastewater biosolids to assess the possible environmental risks of using biosolids in road embankment fills and provided technical and management suggestions to minimize any possible risks of using wastewater biosolids in road applications. The limited available research findings suggest that biosolids are associated with high compressibility and low bearing capacity while the bearing capacity can be increased by stabilization or blending with other natural aggregates of higher bearing capacity material (Lim et al., 2002; Lo et al., 2002; Suthagaran et al., 2008; Disfani et al., 2009). The available research work also suggests that biosolids, if treated properly and managed in accordance with existing regulations and standards, are safe for the environment and human health (Arulrajah et al., 2011b; US EPA, 2005). Lack of knowledge on long-term settlement of biosolids caused by the biodegradation process and also effect of biodegradation on shear strength properties of biosolids is one the main obstacles in sustainable reuse of biosolids in geotechnical engineering applications.

3. Engineering properties of biosolids Samples of biosolids were taken from the top of three existing biosolids stockpiles located at the Western Treatment Plant in the west of Melbourne, Australia. The samples are from aged biosolids stockpiles more than 20 years old and were transferred to the laboratory in large sealed plastic bags. At the Western Treatment Plant, the sewage sludge is pumped from treatment lagoons into sludge drying pans with the drying process in the sludge drying pans taking 6–9 months (DNRE, 2002). In the next step the biosolids are harvested and stored in a biosolids stockpile area. The select engineering properties of this particular wastewater biosolids has been

M.M. Disfani et al. / Resources, Conservation and Recycling 77 (2013) 69–77 Table 1 Select engineering properties of biosolids (Arulrajah et al., 2011b). Test/properties

Result

Moisture content (w) (%) Specific gravity (Gs ) Particle sizes >2.36 mm (%) Particle sizes between 2.36 and 0.075 mm (%) Particle sizes between 0.075 and 0.002 mm (%) Particle sizes between <0.002 mm (%) Coefficient of uniformity (Cu ) Coefficient of curvature (Cc ) Liquid limit (LL) (%) Plastic limit (PL) (%) Plasticity index (PI) (%) Loss on ignition (%) Standard compaction test Maximum dry unit weight ( d,max ) (kN/m3 ) Optimum water content (wopt ) (%) Hydraulic conductivity (10−7 m/s) 1-D consolidation test (oedometer and creep) Initial void ratio (e0 ) Pre-consolidation pressure (kN/m2 ) Coefficient of consolidation (m2 /year) Compression index Recompression index Coefficient of secondary consolidation California bearing ratio (CBR) Swell index (obtained in CBR test) Vane shear test, undrained shear strength (kPa) Direct shear test  (◦ ):  n of 60–240 kPa c (kPa):  n of 60–240 kPa Triaxial shear test (CU)  CU (◦ ):  c of 60–240 kPa c (kPa): ␴c of 60–240 kPa

48–57 1.86–1.88 4–16 40–44 22–33 18–23 100–360 0.3–0.4 100–110 79–83 21–27 35.4–38.5 7.8–8.0 51–53 1.24–1.60 1.49–1.63 190–210 0.04–1.0 0.38–0.45 0.045–0.055 0.003–0.02 0.9–1.0 0.41–0.52 136–152 35 46.1 37.2 6.9

reported previously by Arulrajah et al. (2011b) and the results are briefly discussed in this section and presented in Tables 1 and 2. The moisture content was determined on the basis of the oven dry mass corresponding to a drying temperature of 50 ◦ C instead of the standard drying temperature of 105 ◦ C and was found to vary between 48% and 57%. The specific gravity of biosolids obtained in this research (1.86–1.88) is considerably lower than that of natural soils though comparable to the values reported by other researchers (Arulrajah et al., 2011b; Klein and Sarsby, 2000; Lo et al., 2002; O’Kelly, 2006). Atterberg limit tests were carried out on air-dried biosolids to determine their plasticity characteristics. Liquid limit of 100–110% and plastic limit values of 21–27% were obtained for wastewater biosolids. Based on the Atterberg limit test results and also the behavior of material during plastic limit and liquid limit tests, it was concluded that the fine particles of biosolids are mainly organic silt–sized particles. Although sieve analysis and hydrometer tests results indicate that 44–60% of particles are gravel and sand size Table 2 Chemical contaminants for biosolids (Arulrajah et al., 2011b). Contaminant

Grade C1 limit

Grade C2 limit

Average

Standard deviation

BCC

Arsenic Chromium Copper Mercury Nickel Selenium Zinc DDT and derivatives Organochlorine Pesticides

20 400 100 1 60 3 200 0.5

60 3000 2000 5 270 50 2500 1

19.0 732.7 847.8 4.4 115.7 5.8 1780.0 <3.0

1.789 42.866 29.158 0.443 5.465 0.408 81.731 0.000

20.5 767.8 871.7 4.8 120.1 6.2 1847.0 <3.0

0.05

0.5

Note: values are in mg/kg of dry weight

<0.75

0.000

<0.75

71

particles (larger than 0.075 mm in size) it is believed that in reality these particles are mainly smaller silt size particles lumped together during the long term stockpiling of biosolids (Arulrajah et al., 2011b). Particle size distribution along with Atterberg limit results suggest that biosolids sourced form WTP can be classified as organic silt of medium to high plasticity according to both USCS and Australian Soil Classification System (ASTM, 2010; Standards Australia, 1993). Results of standard proctor compaction tests (on two samples from each stockpile) on air-dried biosolids material suggest that all samples have typical convex shaped compaction curves similar to that of natural clay soils. Values of 7.8–8.0 kN/m3 and 51–53% were obtained for maximum dry unit weight and optimum water content of biosolids respectively which suggest that higher optimum water content and lower maximum dry unit weight are the main differences between compaction characteristics of biosolids and those of natural clays soils. Biosolids samples mixed with optimum water content were left for 24 h for curing and then compacted inside the permeability cell to reach a minimum of 95% density ratio. In the next step; biosolids specimens were subjected to vacuum pressure to remove air, saturated with de-aired water and then went through the permeability phase using falling head method. An average hydraulic conductivity value of 1.24–1.60 × 10−6 (m/s) was obtained which is considered low to very low according to permeability classification introduced by Terzaghi et al. (1996). The obtained permeability values are an indication of poor drainage characteristics of biosolids (Head, 1994). Oedometer 1-D consolidation tests were performed on three biosolids specimens compacted with standard compaction energy inside rings with the diameter of 63.5 mm. Each loading stage lasted for 24 h (for consolidation) and 7 days (for creep) tests and was planned at stress levels of 31, 62, 124, 248, 496 and 992 kPa. The consolidation properties of the biosolids indicate the biosolids has medium to high consolidation potential. California bearing ratio (CBR) test specimens were prepared using standard compaction energy and subsequently were soaked in water for 4 days under a 4.5 kg surcharge. The CBR test results of untreated compacted biosolids (0.9–1.0%) clearly indicate the high compressibility potential of untreated biosolids which accentuates the need for stabilization before using the material in engineering fill applications. Shear strength tests undertaken on the biosolids include laboratory vane, direct shear and triaxial shear tests. Laboratory vane shear test conducted on untreated biosolids specimens obtained undrained shear strength of 136–152 kPa, indicating the compacted samples are stiff. Direct shear tests were conducted on biosolids specimens compacted to reach at least 95% of maximum dry density and then consolidated for 24 h under the saturated condition. Results of a strain controlled shear phase (shear speed < 0.005 mm/min under normal stress levels of 60–120–240 kPa) suggest a drained internal friction angle (ϕ ) of 35◦ and a drained cohesion (c ) of 46.1 kPa. It is believed that the high friction angle obtained for compacted biosolids is due to the high content of coarse particles (about 50%) in aged biosolids (Arulrajah et al., 2011b). Consolidated undrained (CU) triaxial shear tests were performed on the untreated biosolids specimens with a density ratio equal or higher than 95% of the density obtained in standard compaction tests. Samples were compacted in 5 layers using a dynamic tamping approach. All test specimens reached minimum B value (Skempton’s pore pressure coefficient) of 0.95 before starting a 24 h volume controlled consolidation phase which was then followed by a strain-controlled shear phase (with the rate of 0.01 mm/min). Using the effective stress Mohr circles and with confining pressures ranging from 60 to 240 kPa a drained internal friction angle (␸ cu )

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of 37.2◦ and a drained cohesion (c ) of 6.9 kPa was obtained for biosolids. Considering that the conditions in a triaxial test replicate most field situations (Wartman et al., 2004), it is recommended to use the triaxial test results in the design of biosolids in embankment fill applications (Arulrajah et al., 2011b). The chemical properties of the biosolids based on 6 samples (2 from each stockpile) are presented in Table 2. Biosolids contaminants are classified based on BCC as either Grade C1 or Grade C2 . Biosolids are deemed safe if BCC is below C1 limit and can be used without any restrictions. Biosolids are deemed unsafe if BCC is above C2 limit (Arulrajah et al., 2011b). Biosolids within C1 and C2 limits can be used subject to specified guidelines (EPA Victoria, 2004). Table 2 indicates that all the samples tested had contaminants below unsafe level (C2 ). It is to be noted that DDT and organochlorine pesticides could not be measured accurately due to apparatus limitation as the apparatus available could only accurately measure DDT and organochlorine Pesticides, that are 3.0 mg/kg and 0.75 mg/kg respectively (Arulrajah et al., 2011b).

parameters. Chakma and Mathur’s (2007) modified model is presented in Eq. (3): d(Ms ) = [−kj × (T, pH, ) × Msj ], dt

4.1. Methodology Prediction of waste compression due to biodegradation has historically been carried out by applying the rheological model for the long-term compression in peat (Edil et al., 1990). Yen and Scanlon (1975) and Park and Lee (1997, 2002) have previously proposed methods of biodegradation prediction for various soil conditions. Prediction of biosolids (and other waste) decomposition rates, in relationship with initial chemistry of waste has been reported by Rowe et al. (2001), Melillo et al. (1982); Gower and Son (1992) and Stump and Binkley (1993). Settlement due to biodegradation occurs over a very long period. Magnitude of biodegradable settlement is furthermore dependent upon biodegradable fraction (Shah, 2000). The biodegradable fraction could be estimated applying Eq. (1) presented by Shah (2000): BF = 0.83 − (0.028 × LC)

(1)

where BF is biodegradable fraction, LC is lignin content as a percentage of volatile solids (VS) and 0.83 and 0.28 are empirical constants. According to Eq. (1) depending upon the lignin content, biodegradable fraction could vary between 10 and 83%. Depending upon the biodegradable fraction, the rate of biodegradation can be classified between slowly degradable to rapidly degradable biosolids (Shah, 2000). A generalized equation relating the biodegradation decay of waste to its settlement was given by Park and Lee (1997) and is presented in Eq. (2). S(t) = H0 × ED × {1 − exp(−k × t)}

(2)

where S(t) is settlement at time t in meters, H0 is the initial height of waste in meters, ED is total expected strain in fraction, k is the first order kinetic constant in day−1 and t is time since the start of decay in days. However, Park and Lee’s (1997, 2002) model does not take into consideration several parameters such as moisture content, bulk density, pH and temperature for predicting settlement due to biodegradation. Chakma and Mathur (2007) however proposed a modified model, which takes into consideration the abovementioned

(3)

where, Ms1 , Ms2 , Ms3 and Ms4 are the masses of the non degradable, slowly degradable, moderately degradable and rapidly degradable solid waste with their respective rate constants k1 , k2 , k3 and k4 (Chakma and Mathur, 2007). The decay rate of biodegradable wastes (k) are categorized as 0.00, 0.00001, 0.0001 and 0.001 day−1 for non-biodegradable, slowly biodegradable, moderately biodegradable and rapidly biodegradable materials respectively (Findikakis and Leckie, 1979).  is a function of temperature (T), pH and moisture content () in fraction as defined by Chakma and Mathur (2007) and is presented in Eq. (4):



(T, pH, ) =

T ×  × exp −0.288(pH − 7)2



1 + exp(0.25T − 18)

(4)

The volume of waste at time t (Vs ) can be calculated using Eq. (5) developed by Chakma and Mathur (2007):

4. Long-term settlement predictions of biosolids The long–term settlement prediction of biosolids is essential to evaluate its performance in various geotechnical applications, especially when it acts as a part of load bearing structure such as fill material under road pavements.

(j = 1, 2, 3, 4)

Vs (t) =

j=4  fj × Msj × exp{−kj × (T, pH, ) × t}

pj

j=1

(5)

where j is the density of waste material, fi is the percentage of biodegradable waste under different categories and Msj is the mass of each category of waste material. The strain due to biodegradation is estimated using Eq. (6) proposed by Chakma and Mathur (2007): εb (t) =

 V − V (t)  s i Vi

(6)

where Vi , is the initial volume the waste material layer. Finally, the settlement due to biodegradation at any time (t) can be computed applying Eq. (7): Sb (t) = Hi × εb (t)

(7)

where Hi is the initial thickness of waste layer. 4.2. pH and electrical conductivity tests The pH and electrical conductivity are two vital parameters for the corrosivity. For the investigation of biodegradation settlement of biosolids and the corrosion potential of biosolids, pH and electrical conductivity tests were performed on untreated biosolids. Electrical conductivity is a measure of the dissolved material in an aqueous solution, which relates to the ability of the material to conduct electrical current through it (Serway, 1998). Electrical conductivity is measured in the units of Seimens per unit length (S/m). The higher the dissolved material in water or soil sample, the higher the electrical conductivity of that material (Serway, 1998). The electrical resistivity of a soil is an indicator showing how strongly the soil opposes the flow of electric current and is measured in unit of -m (Serway, 1998). The unit conversion of -m and S/m is presented in Eq. (8): ˝-m =

1 S/m

(8)

In general, the corrosivity toward a buried object in the highway embankment fill is dependent on a number of parameters, “including fill material’s resistivity, water content, dissolved salts, pH, presence of bacteria and the amount of oxygen available at the buried metal surface” (Coburn, 1987). It is generally understood that no one parameter equation can be used to accurately forecast the corrosivity of a particular embankment fill material or soil. Nevertheless, electrical resistivity is commonly utilized as an

M.M. Disfani et al. / Resources, Conservation and Recycling 77 (2013) 69–77

30m

X = 3H

3 1

1.5

Nonstructural Fill

X = 3H

Engineered Fill

1

73

Biosolids

In-situ soil or rock

Nonstructural Fill

5m 0.5m

Impermeable geotextile separator or 0.5 m impermeable clay layer

Fig. 1. Geometry of road embankment for settlement analysis.

indicator of embankment fill material or soil corrosivity. Observation of soil drainage and/or measurement of pH and supplement resistivity measurements were conducted by various researchers including Coburn (1987) and Davie et al. (1996). The general relationship that exists between soil resistivity, pH and corrosion of ferrous metals is presented in Table 3. However, because of other factors, the relationship is not always valid and considerable variation in the ranges tabulated can occur (Coburn, 1987; Davie et al., 1996). The effect of solution pH upon the corrosion rate of iron has been studied previously by Scully (1990). The pH and electrical conductivity tests were performed in accordance with the Australian standard on “Methods of testing soils for engineering purposes, method 4.3.1: soil chemical tests – determination of the pH value of a soil – electrometric method, AS 1289.4.3.1-1997” (Standards Australia, 1997) on 4 samples of untreated biosolids. The average pH value was determined to be 4.67 and electricity conductivity was found to be 1975 ␮S/cm. Electrical conductivity and pH values of untreated biosolids specimens are presented in Table 4. The loss on ignition (LOI) values of 35.4–38.5% were obtained for the biosolids by igniting dry powdered biosolids material in a muffle furnace at a temperature of 440 ◦ C for a sufficient period of time (Arulrajah et al., 2011b). Ignition loss is an indirect measure of the organic content of the dry specimen mass, which can be used to assess the state of biodegradation of biosolids. Results of organic content tests presented in Table 4 were used to cap the limit of the biodegradation settlement in the analytical analysis. 5. Analysis of biodegradation settlement and corrosivity Analysis of the biodegradation settlement of a typical road embankment using untreated biosolids is important to assess the influence of important input parameters and any uncertainty involved in their determination. Biodegradation settlement of the biosolids in highway embankments depends on factors such as density, moisture content, temperature and pH. It is common practice for engineers and researchers to use the available empirical relationships between soil properties to estimate the parameters required for an engineering analysis. In the biodegradation settlement analysis of aged biosolids studied in this research, several assumptions were made. Firstly the aged biosolids in this study is considered slowly degradable waste as it is believed it has passed the active decomposition stage after 20–25 years of stockpiling. Secondly it was assumed that after completion of the embankment construction, biodegradation will occur concurrently with consolidation. Thirdly; the settlement prediction due to biodegradation was based on the remaining

residual thickness of biosolids layer after the completion of primary and secondary consolidation settlement of the biosolids layer. The last assumption is that the percentage of degradable material (fi ) in the biosolids layer was assumed to be equal to the average organic content value obtained in experimental study. The typical geometry for a 5 m high road embankment using biosolids is shown in Fig. 1. This geometry is based on an acceptable design for the usage of biosolids in road embankments based on guidelines specified by the local road authority for the use of biosolids in road embankments. Requirements are imposed on the usage of biosolids in road embankments which include the maximum allowable thickness of only 0.5–1 m of biosolids, placement above design flood levels and requirement for biosolids not be placed within 1 m of the subgrade level (VicRoads, 2007). To minimize the effect of long term decomposition of biosolids that can lead to large settlements, the thickness of the biosolids layer was restricted to a conservative 0.5 m. An impermeable geomembrane separator was used to encapsulate the biosolids and to prevent any seepage or leaching of biosolids into the fill material. Results of oedometer and creep tests were used to predict the settlement of the biosolids layer due to primary and secondary consolidation. 0.057 m of primary and secondary consolidation settlement was computed to have occurred due to primary and secondary consolidation in a period of 6 months and the remaining 0.443 m was subjected to biodegradation settlement prediction. A parametric study consisting of the sensitivity of the biodegradation settlement with varying pH values for the untreated biosolids layer depicted in Fig. 1 was carried out. Fig. 2 presents the prediction of biodegradation settlement with varying pH values over a long span of time calculated using Eq. (7). The effect of pH value in the biodegradation settlement was analyzed by reducing the pH value to 3 and 4 and also increasing the pH value to 5. The biodegradation settlement of the embankment depicted in Fig. 1, 22 years after the construction was found to be 138 mm for the actual pH value of 4.67. The biodegradation settlement was reduced to 80 mm and 14 mm with reduced pH values of 4 and 3 respectively while it increased to 153 mm with the increased pH value of 5. Fig. 2 also suggests that for pH values of 4, 4.67 and 5 it takes between 50 and 200 years to reach the maximum biodegradation settlement while for pH value of 3, this takes more than 1000 years. The time taken for fully biodegradation process with varying pH values of biosolids is presented in Fig. 3. Fig. 3 implies that the time taken for fully biodegradation process decreases dramatically with pH value of the biosolids mixture between 3 and 6 (acidic range) and then increases exponentially with pH value of the biosolids between 8 and 11 (alkaline range). Fig. 3 suggests that the time

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Table 3 Soil corrosivity classification (Coburn, 1987; Davie et al., 1996). Parameters

Classification

Resistivity ( -cm)

Little corrosive

Mildly corrosive

moderately corrosive

Corrosive

Very corrosive

>10,000

2000–10,000 5000–10,000

1000–2000 2000–5000

500–1000 700–2000

<500 <700

5.0–6.5

<5.0

>5.0 and <10.0

pH

Table 4 pH, organic content and electrical conductivity of biosolids. Untreated biosolids sample

Temperature (◦ C)

pH

Electrical conductivity (␮S/cm)

Organic content (%)

Sample 1 Sample 2 Sample 3 Sample 4 Average

21.4 21.5 21.6 21.8 21.6

4.65 4.67 4.68 4.69 4.67

1 871 2 001 2 093 1 935 1 975

36.5 35.8 35.4 38.5 36.6

Time (years) 0

100

200

300

400

500

600

700

800

900

1000

0.00 Untreated : Actual pH=4.67 Untreated : pH=3 0.03 Untreated : pH=4 Untreated : pH=5 Settlement (m)

0.06

0.09

0.12

0.15

0.18

Fig. 2. Biodegradation settlement of 5 m high embankment using untreated biosolids.

taken for full biodegradation of biosolids with the pH value of 3 (1000 years) reduces to 50 years with a pH value of 5. The pH value of untreated biosolids is in acidic range in the pH scale and is shown in Fig. 3. Even though the untreated biosolids is in the acidic range, biodegradation settlement analysis shows that, untreated biosolids with the more acidity (i.e. pH value of less than 3) takes a longer time time than the untreated biosolids with less acidity for fully biodegradation process. The time for completing fully the biodegradation process for the untreated biosolids in 5 m

embankment with a pH value below 3 is more than double that with a pH value above 3. Effect of changing moisture content on biodegradation settlement of biosolids layer is shown in Fig. 4 for actual moisture content (optimum moisture content that the biosolids layer will be compacted at) and for lower and higher moisture contents. Fig. 4 suggests that while moisture content level of biosolids does not have an effect on the amount of biodegradation settlement reached after 100 years, biosolids layers with higher moisture contents reach the maximum settlement at a faster rate. Fig. 5 depicts the effect that changing temperature will have on biodegradation settlement of biosolids layer for a range of possible occurring temperatures. Fig. 5 suggests that an increasing temperature will accelerate the biodegradation process significantly though the final achieved biodegradation settlement after 160 years is the same for all possible scenarios. The average conductivity and resistivity values of untreated biosolids are 1975 ␮S/cm and 4534 -cm respectively. Based on soil corrosivity classification presented in Table 4 and conductivity measurements of untreated biosolids, it can be concluded that untreated biosolids are moderately corrosive. The pH values of untreated biosolids were compared with corrosivity classification of materials based on their pH value presented in Table 4. The corrosivity of the untreated aged biosolids is classified as moderately corrosive to corrosive. This biosolids corrosivity classifications based on pH values is also in agreement

Time (years) 0

100

200

300

0.00 Untreated : Actual moisture content = 51% Untreated : moisture content = 30% 0.03

Untreated : moisture content = 40% Untreated : moisture content = 60%

Settlement (m)

0.06

Untreated : moisture content = 70%

0.09

0.12

0.15

0.18

Fig. 3. Time taken for biodegradation settlement versus pH value.

Fig. 4. Biodegradation settlement with changing moisture content.

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M.M. Disfani et al. / Resources, Conservation and Recycling 77 (2013) 69–77 Time (years) 0

100

200

300

75

information for calculating the consolidation settlement following conventional consolidation equation as described in Das (2006).

0.00 Untreated : T = 21.6 degree Celsius

6.3. Secondary compression (creep) settlement

Untreated : T = 10 degree Celsius

0.03

One dimensional consolidation test with extended load duration (7 days rather than standard 1 day) will provide the change of void ratio ( e) over a full logarithmic period on the time scale (e.g. between 1000 min and 10,000 min). Coefficient of secondary compression (C˛ ) can be calculated following Eq. (11) (Holtz et al., 2011).

Untreated : T = 30 degree Celsius

Settlement (m)

0.06

Untreated : T = 40 degree Celsius

0.09

e

log t

0.12

C˛ =

0.15

Secondary (creep) consolidation can be determined following Eq. (12) suggested by Holtz et al. (2011):

0.18

Ss = Fig. 5. Biodegradation settlement with changing temperature.

with the effect of pH value upon the corrosion rate of iron by Scully (1990).

C˛ t H0 log 1 + e0 tp

(11)

(12)

where e0 is the initial void ratio, H0 is the thickness of biosolids layer, and t and tp represent the time interval the calculation is aimed for. 6.4. Biodegradation settlement

6. Framework for wastewater biosolids settlement prediction In order to provide a simplified approach for potential end-users to predict the total settlement of biosolids in embankment fill applications a framework has been developed. The total settlement of wastewater biosolids has been separated into 4 different components while for each component the required laboratory testing and governing equations have been introduced. Eq. (9) can be used in calculating the total settlement of wastewater biosolids.

In order to obtain biodegradation induced settlement of biosolids layer, water content of biosolids material placed in the embankment fill, pH value of biosolids, biodegradable fraction (can be capped at organic content obtained in the laboratory), temperature of layer and the rate of biodegradation (slow, moderate or rapid which relied on age of biosolids) are required. Section 4 described how each of these parameters can be determined and Eq. (7) shall be used for calculating Sb .

St = Si + Sc + Ss + Sb

6.5. Future work

(9)

where St is the total settlement of biosolids layer at time t, Si is the immediate settlement, Sc is the consolidation settlement, Ss is the secondary compression (creep) settlement and Sb is the biodegradation induced settlement. 6.1. Immediate settlement Assuming that the elastic theory can appropriately represent immediate settlement of compacted biosolids, Eq. (10) can be used to calculate the settlement under a uniformly distributed load (Holtz et al., 2011). Si =

q0 B (1 − 2 )Is Eu

(10)

where Si is the immediate settlement of biosolids layer under applied stress of q0 , B is the characteristics dimension of the loaded area as outlined by Holtz et al. (2011), Eu is the undrained Yung’s modulus,  is the Poison’s ratio and Is is the shape factor as defined by Holtz et al. (2011). Eu and  can be obtained from a triaxial test with volume change measurements.

To fully investigate the deformation (settlement) of wastewater biosolids, particularly the biodegradation induced settlement, a future physical laboratory model needs to be developed. This future physical model should incorporate the effects of immediate, consolidation, creep and biodegradation (decomposition) settlement for biosolids in embankment fills. Temperature, water content and organic content are the parameters that their effect on the settlement should be studied. Using the results from this future physical model, the current constitutive models for deformation of clays can be extended to accommodate biodegradation settlement. Validation of the constitutive model with this physical model specifically for biosolids will ensure there is confidence from end-users, consultants and alike in the future usage of biosolids in embankment fills. This proposed future research work will be able to determine and model the “missing link” of the biodegradation induced settlement behavior of biosolids for implementation in the design and usage of biosolids in embankment fills. 7. Conclusion

6.2. Consolidation settlement One dimensional consolidation testing using oedometer consolidation cell under the design load range would provide all required information including T50 , T90 , coefficient of consolidation (Cv ), compression index (Cc ), coefficient of volume compressibility (mv ) and swell (recompression) index (Cr ) if required. These parameters along with the characterization test results (specific gravity, water content and compaction test results) provide all required

The biosolids samples have high moisture content, liquid limit and plasticity indices that are comparable to common organic soils. The particle density of biosolids was found to be substantially less than that of natural inorganic soils. The compaction tests results indicated that maximum dry density varied only slightly with the moisture content changes. Biosolids has significantly high optimum moisture content which indicates poor workability as a construction material without treatment. The consolidation characteristics

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of biosolids indicate that biosolids have a similar behavior to organic soils, but with a higher compression potential. A sensitivity analyses method was adopted to predict the biodegradation settlement of biosolids using parameters such as pH value, moisture content, temperature, density and biosolids biodegradation stage. The trend of the time taken for the fully biodegradation process of a 0.5 m biosolids layer in a 5 m embankment fill clearly indicates sensitivity of pH value in the biodegradation process. The maximum rate of biodegradation process is expected at a pH value of 7 (neutral). Increasing values of moisture content and temperature accelerate the biodegradation process while not affecting the attained biodegradation settlement after 100 and 160 years respectively. Electrical resistivity is an important parameter used in evaluating the corrosivity of a material. High resistivity is associated with low corrosion potential. The electrical conductivity and pH tests indicate that the untreated biosolids exhibit acidic nature and is classified as moderately corrosive to corrosive. Due to the complex nature of corrosion mechanism, a single parameter may not be sufficient to evaluate corrosivity of a material. A framework has been proposed to provide a guideline for future end-users to calculate the total settlement of biosolids layers in embankment fill taking into account immediate, consolidation, creep and biodegradation settlement.

Acknowledgements The authors would like to acknowledge the Smart Water Fund for funding this research project (Project No: 42M-2059). The Smart Water Fund is an initiative of the Victorian Government and the Victorian water industry in Australia aimed at encouraging innovative solutions to water conservation, water management and biosolids management.

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