Enhancement of geotechnical properties of municipal sewage sludge for sustainable utilization as engineering construction material

Highlights

• Sewage sludge (SS) exhibited expansive behaviour due to high content of Montmorillonite.

• Lime (CaO) treatment improved SS strength, swelling and plasticity characteristics.

• CaO induced cementation via gyrolite and hydrates of Ca and Ca–Al–OH silicates.

• Unconfined compressive strength increased to 1102 kPa after 28 d of lime treatment.

• SS treated with 6% CaO and cured 28 d found suitable for subbase pavement construction.

Abstract

The present study demonstrates potential utilization of lime-stabilized sewage sludge (SS) for construction purposes. SS samples were constituted using 0, 2, 4, 6 and 8% of lime by weight of the SS and cured for 7, 14 and 28 d. Stability of the lime-modified SS was evaluated through determination of geotechnical, microstructural and mineralogical properties. A large improvement in unconfined compressive strength (UCS) from 207 kPa to 1102 kPa was achieved after 28 d of curing SS with 6% lime. The improvement in UCS is attributed to formation of cementing compounds as a result of pozzolanic reactions. Marked reductions in swell pressure of 47, 51, 54 and 58% were obtained with the 2, 4, 6 and 8% lime treatments. Also, the plasticity index decreased with increasing lime content for different curing periods, indicating the SS becomes much more workable and less expansive. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) after lime treatment confirmed formation of the new compounds responsible for the improved engineering properties of SS. The study concludes that lime treatment can be an alternative to stabilize SS for potential use as a construction material for subbase of flexible pavement as per IRC (Indian Road Congress) 37–2012 guidelines.

Introduction

Developing countries like India lack lined drain systems, leading to high sediment content of wastewater at sewage treatment plants. The high sediment content of the sewage sludge (SS) allows alternative ways of disposal. The SS has traditionally been used for agriculture and brick manufacturing as it is free and inexpensive. SS contains essential nutrients, but often contains toxic metals which can affect yield, soil quality, and human health (Lee et al., 1998) and clean up requires much energy (Atiyeh et al., 2000; Elvira et al., 1998). SS can be used in construction material such as bricks (Tay, 1987; Weng et al., 2003; Liew et al., 2004; Ramadan et al., 2008; Chiang et al., 2009; Mageed et al., 2011; Babu and Ramana, 2013; Ukwatta Pitiye and Mohajerani, 2016; Ukwatta et al., 2016) as a replacement of clay and can be an alternative for SS management. Rabie et al. (2019) found that up to 15% SS by weight could be used in concrete mix. Yang et al. (2017) reported that dewatered SS cake containing FeCl₃ + quick lime and Fenton’s reagent + red mud could be used as landfill covers. Effort has also been made to recycle SS using epigenic earthworms i.e., vermicomposting (Benitez et al., 1999; Gupta and Garg, 2008; Suthar, 2009), but metals in SS resulted in higher mortality of the earthworms (Gogoi et al., 2015; Taki et al., 2019).

Researchers recently started using SS for geotechnical applications. Ayininuola and Ayodeji (2016) utilized sludge ash to enhance the shear strength of soil. The increase in strength after 7% (optimum) addition of sludge ash was attributed to formation of cementitious product binding the particles together. Ayodele et al. (2016) reported an increase in soil strength as SS increased from 2 to 16% and De Figueirêdo et al. (2013) suggested use of lime-stabilized mixtures of soil and SS as road base and back filling material. It was found that the admixtures could significantly enhance the California Bearing Ratio, Unconfined Compressive Strength, Indirect-Tensile Strength, and Resilient Modulus. Iqbal et al. (2019) demonstrated use of drinking water sludge (DWS) blended with crushed concrete and incineration ash as a geotechnical material. The blend of crushed concrete enhanced compaction, compressibility and bearing capacity of DWS for use as road subgrade. Lin et al. (2007) used SS ash and hydrated lime in 0, 2, 4, 8 and 16% by weight of the soil, to stabilize the pavement subgrade soil. A remarkable increase in the shear strength parameter from 30 to 70 kPa was observed. Lim et al. (2002) similarly evaluated improvement in the engineering properties of sludge modified by lime, fly ash and loess amendments. The unconfined strength of the lime and fly ash exceeded 100 kPa which satisfied construction material criteria. A proper lime mixing ratio maintained a pH of 12 which sterilized the mixture (Lim et al., 2002). The toxicity characteristic leaching procedure (TCLP) analysis indicated heavy metal leachability below regulation criteria.

SS production and disposal has reached a point where the development of technology is imperative for efficient and sustainable management. As stated by Peccia and Westerhoff (2015), “Sludge management practice must shift from treatment of a liability toward recovery of the embedded energy and chemical assets, while continuing to protect the environment and human health. The shift will require new research, treatment technologies, and infrastructure which must be guided by application of green engineering principles to ensure economic, social, and environmental sustainability”. The quality of SS produced in India is highly complex and unpredictable, as unlike developed countries, the waste management system in India has no proper sorting system and control. Dumping of SS into landfills is not practiced in India as high cost and space requirements make it an unsustainable option. Also, there is high potential for leaching of heavy metals to the groundwater and reaching drinking water sources (Kumar et al., 2010, 2013). For a developing country like India, this is not good as much of the country suffers from acute water crises (Kumar et al., 2011). Hence, there is a need for technology that is neither energy dependent nor detrimental to the environment.

The present study aims at utilizing SS as source material for geotechnical purposes. The study reports that SS studied is expansive in nature, meaning it exhibits swelling shrinkage response, which to our knowledge has not been reported in the literature. There is also a dearth of literature on the geotechnical properties of SS for civil engineering purposes in India. Most studies conducted on SS (e.g., Lim et al., 2002; Lin et al., 2007) have not considered critical geotechnical properties such as swell pressure and differential free swell index before utilizing it as civil engineering construction material. In addition, other studies only illustrate changes in geotechnical properties for SS in combination with other materials and do not discuss specific civil engineering applications. The present study will help in managing SS and understanding its application in civil engineering such as foundation soil, pavement and embankment construction.

The objectives of the present study were to: (i) evaluate improvements in geotechnical properties (swelling, plasticity and shear strength) of lime-modified SS; (ii) investigate its application as an engineering material for construction purposes; and (iii) evaluate the metal leaching potential of lime-treated SS using the TCLP test. Morphological and mineralogical changes in the modified skeleton of lime-treated SS are explored as a function of cementation resulting from pozzolanic reaction through scanning electron microscopy (SEM), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR).