Efficacy of vermitechnology integration with Upflow Anaerobic Sludge Blanket (UASB) and activated sludge for metal stabilization: A compliance study on fractionation and biosorption

Highlights

• Prominent efficacy of vermitechnology integration with wastewater treatment plant.

• Significant compliance between K_d estimated through biosorption and fractionations.

• Vermitechnology found to be more efficient with activated sludge (AS) than UASB.

• 1:1(Sample:Cowdung) mixing ratio showed higher metal stabilization than 2:1(S:CD).

Abstract

Efficacy of vermi-transformation for metal partitioning and transformation from Upflow Anaerobic Sludge Blanket (UASB) and Activated Sludge (AS) was investigated. Sludge samples were mixed with cow dung (CD) in two combinations (1:1 (UASB/AS:CD)) & (2:1(UASB/AS: CD)). Fractionation study revealed that Zn, Cd & Pb were associated with reducible fractions, and Cr, Cu with oxidizable fractions. Higher removal efficiency for 1:1 (UASB/AS: CD) combination over 2:1 (UASB/AS: CD) implies the non-significant contribution of cow dung during the metal stabilization process. After vermi-remediation, maximum metal removal was achieved at 1:1 ratio than 2:1 in AS. In UASB, 1:1 ratio worked better for Cr, Zn & Cd, whereas for Cu & Zn 2:1 ratio resulted in efficient removal. Overall for both AS and UASB, efficiency was found to be higher in 1:1 treatment ratio. The value of K_d (Bio sorption) was highest in Cu followed by Cr, which indicates the closer association with the metal bound organic matter (R² ≥ 0.99). Based on the compliance study between two estimated sorption coefficients K_d (Biosorption & Fractions), vermi-remediation was found to be effective for AS than UASB. Therefore, the obtained results clearly validate the feasibility of integration of vermi-remediation as a potential promising ecological techniques for removing metal contaminant from the wastewater. Further research is required to study the decontamination of emerging contaminants with such integrated technology, which have physico-chemical properties different than metal ions.

Introduction

Rapid growth in human population, economic framework, and infrastructure has resulted in significant upsurge in sewage sludge production (Adam et al., 2007; Nowak et al., 2010; Azizi et al., 2013; Naoum et al., 2001). In past, activated sludge (AS) process has been widely used for the treatment of domestic and industrial wastewater. AS process has high efficiency, operational flexibility and possibility of nutrient removal (Von Sperling et al., 2001). However, there are several disadvantages associated with AS: high mechanization, construction and operational costs, sophisticated operation and generation of large amount of sludge (Von Sperling et al., 2001; Von Sperling, 1997). Generated sludge contains different types of contaminants, the most prominent is the presence of heavy metals, PPCPs, bacteria, viruses which makes its disposal very challenging (Carballaa et al., 2007). In recent time, use of Upflow Anaerobic Sludge Blanket (UASB) has overcome the disadvantages of mechanized aerobic systems especially because of the absence of energy consumption and lower excess sludge generation (Chernicharo, 1997). Still it is observed that the treated effluent and sludge, most of the time is unable to comply with the existing discharge standards (Chernicharo, 1997).

In India, wastewater treatment is served by 234 sewage treatment plants (STP) which yield approximately 38,354 MLD (million litres daily) sludge that is projected to reach 1,32,000 MLD marked by 2051. AS process is the mostly favored technology in India accounting for 59.5% of the total installed capacity followed by UASB technology, covering 26% of the total installed capacity (Kaur et al., 2012; Mukherjee et al., 2015). The cost of sludge treatment is very high which restricts its feasibility in several countries of the world. Moreover, re-use avenues like biogas production, filling material, brick production, etc. from sewage sludge are also cost intensive and require special infrastructural facilities. The indigenous process vermitechnology has emerged as an effective waste-recycling approach which is cost effective, easily executable, and rapid in organic waste conversion (Gogoi et al., 2015; Sahariah et al., 2015). Earthworms can accumulate toxic metals in their body with the help of metal binding proteins and also modify the substrate through humification which restricts the mobility of metal species into environment to a greater extent (Goswami et al., 2014; Gogoi et al., 2015; Sahariah et al., 2015; Das et al., 2015, 2016).

The present work tends to fill the gaps in the research works previously carried by vermi-remediation on sewage sludge samples. Despite a number of literature is available on treatment of AS sludge by vermi-technology, there is a dearth of literature pertaining to the effectiveness of toxic removal from UASB sludge by vermi-technology. Though numerous studies are available on AS sludge, still studies related to understand the speciation of metal during vermi-remediation is seldom reported. UASB being a rarely used technique in India, the scope of vermi-remediation has never been explored and also metal speciation study is very much unlikely reported. The scope of using sorption coefficient (K_d) to check the efficacy of vermi-remediation for sludge, as an additional tool for realizing its potential along with fractionation study and metal removal efficiency has never been explored.

In light of that, the current study aims at understanding the role of vermitechnology in cleaning the toxic sewage sludge collected from two different facilities, AS and UASB sludge. The present study aims to understand the efficacy of vermiremediation with the aid of sorption coefficients, two sorption coefficients were used in the present study i.e. K_d (Biosorption) and K_d (Fractions). Both the Kd values were computed by the ratio of total metal concentration (C_T) to the leachable metal or metal present in liquid (C_L). In case of K_d biosorption, the sum of different fractions such as exchangeable (F1), carbonate based (F2) & Fe/Mn based fractions (F3) were taken as C_L and for K_d fractions the sum of F1& F2 fractions was considered as C_L. Also, the compliance between two estimated sorption coefficients for investigating vermi-remediation efficacy was explored, this objective clearly demarcates the uniqueness of the present study with other existing studies on vermi-remediation, where this aspect has never been considered to the best of our knowledge.

The uniqueness of the present study is inclusion of sequential extraction of metals at different phase along with vermicomposting which will further help to understand the metal removal efficacy at different stages and also to understand the temporal variation of physico-chemical parameters of the substrate (AS and UASB sludge). Tessier scheme ( Tessier et al., 1979) has been widely employed for metal fractionation in soils and sediments, but it has rarely been applied to sewage sludge (Pérez-Cid et al., 1999). Sequential extraction provides information about both mobile and stable fractions of metals (Borgese et al., 2013; Kumar et al., 2013a, 2013b; Liang et al., 2014; Özkaraova Güngör and Orkun, 2014; Skipperud and Salbu, 2015). However, under certain condition non-mobile fraction of metals can become mobile fractions and are likely to cause heavy contamination in groundwater through leaching (Kumar et al., 2013a). Therefore, precise assessment of mobility of heavy metals is necessary to evaluate potential mobility (Kumar et al., 2015).

Based on the above discussion, the following five folds objectives are defined for this study: (i) Comparative evaluation of percentage metal removal from the sludge collected from AS and UASB through vermi-conversion technique. (ii) Predicting the change in metal speciation during the process of vermicomposting. (iii) Articulating the best mixing ratio for metal removal through vermicomposting. (iv) Compliance study between sorption estimated through metal fractionation and biosorption (through earthworm). (v) Efficacy of integration of vermiconversion technique in AS and UASB process.