Heavy metals speciation study revealing merits of anaerobic co-disposal of municipal solid waste with discrete paper mill sludges: An experimental investigation in simulated landfill bioreactors

Highlights

• Co-landfilling of PMS with MSW showed reduction in bioavailability of heavy metals.

• Movable forms of Cu, Cr, Pb and Zn increased in MSW disposed landfill bioreactor.

• ETS co-disposal decreased toxicity of all analyzed heavy metals after landfilling.

• Bioavailabilities of Cr, Pb and Ni were unaltered in case of DPS co-disposal.

• Transformation of heavy metals were greatly influenced by pH, VS and MC.

Abstract

Speciation of heavy metals (Cu, Cr, Pb, Ni, Cd and Zn) during anaerobic co-disposal of two distinct paper mill sludges viz. effluent treatment plant sludge and de-inking plant sludge with municipal solid waste in anaerobic landfill bioreactor was investigated. Effects of moisture content, volatile solids and pH on fractionation of heavy metals over the course of landfilling were also analyzed. Study revealed enhancement in bioavailability of Cu, Cr, Ni and Cd on sole disposal municipal solid waste in bioreactor landfill over 300 days. However, co-disposal of effluent treatment plant sludge with municipal solid waste showed reduction in bioavailability of every analyzed heavy metals with maximum increment in stable residual forms of Cu, Cr, Pb, Ni and Zn by 3.70 %, 0.73 %, 3.08 %, 1.17 % and 3.45 % (dry weight), respectively. De-inking sludge co-disposal was also found effective for reducing bioavailability of Cu, Cd and Zn while bioavailability of Cr, Ni and Pb was unaltered. Strong correlations between increase in residual forms of heavy metals and volatile solid reduction obtained for both co-disposed bioreactors.

Introduction

Rapid population growth associated with industrialization in developing countries has contributed to unparalleled increase in the generation of solid waste in past several decades, which is associated significantly with public health and environmental hazards [[1], [2], [3]]. In most of the developing countries, landfilling is the foremost method of treatment and disposal of municipal solid waste (MSW), including dumpsites and some properly engineered sanitary landfills [[4], [5], [6]]. Uncontrolled dumping of MSW undergoes long term anaerobic digestion in landfills, ultimately releasing heavy metals and affecting groundwater quality, native vegetation and soil characteristics [[7], [8], [9], [10]]. However, in view of rapid degradation, to counter tremendous waste generation rate, sustainable technologies such as operating landfill as bioreactor has attained significant interest in the past several years. Bioreactor type landfilling not only enhances the degradation of organic matter but also releases the contaminants in a relatively shorter span of time [11].

Nowadays, heavy metals pollution in landfilled waste is gathering attention worldwide because of its severe effects on the environment. Over the course of anaerobic digestion in landfills, heavy metals change their states according to substrate nature, prevailing pH condition, degradation phase and moisture in the degrading medium [12]. Hence, the bioavailability of heavy metals gets altered by above mentioned parameters. Several other mechanisms responsible for transformation, transportation and accumulation of heavy metals in a landfill system are presented in Fig. 1. The primary investigation of heavy metal behavior during landfilling is the assessment of total metal concentration, but it can only address the effects of heavy metal toxicity and stimulation in anaerobic digestion, though, it may not be sufficient to state regarding the bioavailability of heavy metals [13,14]. The bioavailability of heavy metals relies on their chemical speciation, which in turn is dependent on the prevailing conditions of landfill [15]. Bioavailability of heavy metals by assessing chemical speciation could also provide insights about the eco-toxicity of digestate [16].

In general, one-third of the total heavy metals (Cu, Cr, Pb, Ni, Cd, Zn) from landfilled MSW were found to exist in mobile and reactive forms, which take a very long time to immobilize [17]. Therefore, many other wastes such as incineration bottom ash, sewage sludge and lime sludge from paper industries were experimented as co-disposal substrates for immobilization of heavy metals under landfilling, anaerobic digestion and composting conditions [[18], [19], [20], [21]]. However, non-hazardous wastes from some of the industries, such as paper mill sludge (PMS) could also be beneficial for co-disposal with MSW for immobilization the heavy metals. Lignocellulose content present in such sludges could be advantageous for not only reducing the mobility of heavy metals but additional organic loading may increase biogas generation as well [22,23]. Co-disposal of lignocellulose waste could also provide additional adsorption sites to capture mobile heavy metals. In addition, humic substances formed during anaerobic co-digestion have strong binding tendency with heavy metals, which reduce its eco-toxicity to plants [24]. Effluent treatment plant sludge (ETS) and De-inking plant sludge (DPS) are major lignocellulosic wastes generated from a pulp and paper mill [25,26]. Previously, it was well proven that cardboard and paper waste (also the component of MSW) could be used as low-cost adsorbent material for the exclusion of heavy metals from mine drainage [27]. Likewise, Devi and Saroha [28] have shown reduction in bioavailability and mobility of heavy metals even during the pyrolysis of ETS from pulp and paper mill. DPS as adsorbent for effective removal of pollutants also reinforces its use as co-substrate material for heavy metal transformation and immobilization during anaerobic landfilling process [29].

Although, most of the studies have been conducted to evaluate the transformation in chemical speciation in raw, composted or anaerobically digested MSW and various types of sludges including sewage sludge and paper mill sludges [[30], [31], [32], [33], [34], [35]]. However, only a few studies were dedicated to the evaluation of bioavailability and speciation of the heavy metals during the landfilling process [36,37]. To the best of our information, the study examining the bioavailability of the heavy metals during co-disposal of industrial organic sludge with MSW under anaerobic landfill bioreactor is not reported. Therefore, the scarcity in information limits the knowledge of such industrial organic sludge regarding its co-disposability with MSW under accelerated moisture anaerobic landfilling conditions. Therefore, an effort was made to assess the variation in partitioning of heavy metals during individual co-disposal of two different paper mill sludges (ETS and DPS) with MSW in anaerobic landfill bioreactor with leachate recirculation.

In this study, three simulated landfill bioreactors with leachate recirculation were operated under anaerobic conditions for 300 days to inspect the chemical speciation of heavy metals over the course of anaerobic disposal of MSW and PMS co-disposed MSW. Two distinct PMSs viz. ETS and DPS were used for co-disposal with MSW. The solid samples collected from each bioreactor was analyzed for pH, VS, moisture content, total concentrations and speciation of Cu, Cr, Pb, Ni, Cd and Zn. The results attained could provide insights to comprehend the patterns in the transformation of fractions of different heavy metals during the landfilling process of MSW and PMS co-disposed MSW.