Monitoring and optimizing the co-composting of dewatered sludge: A mixture experimental design approach

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Abstract

The management of dewatered wastewater sludge is a major issue worldwide. Sludge disposal to landfills is not sustainable and thus alternative treatment techniques are being sought. The objective of this work was to determine optimal mixing ratios of dewatered sludge with other organic amendments in order to maximize the degradability of the mixtures during composting. This objective was achieved using mixture experimental design principles. An additional objective was to study the impact of the initial C/N ratio and moisture contents on the co-composting process of dewatered sludge. The composting process was monitored through measurements of O2 uptake rates, CO2 evolution, temperature profile and solids reduction. Eight (8) runs were performed in 100 L insulated air-tight bioreactors under a dynamic air flow regime. The initial mixtures were prepared using dewatered wastewater sludge, mixed paper wastes, food wastes, tree branches and sawdust at various initial C/N ratios and moisture contents. According to empirical modeling, mixtures of sludge and food waste mixtures at 1:1 ratio (ww, wet weight) maximize degradability. Structural amendments should be maintained below 30% to reach thermophilic temperatures. The initial C/N ratio and initial moisture content of the mixture were not found to influence the decomposition process. The bio C/bio N ratio started from around 10, for all runs, decreased during the middle of the process and increased to up to 20 at the end of the process. The solid carbon reduction of the mixtures without the branches ranged from 28% to 62%, whilst solid N reductions ranged from 30% to 63%. Respiratory quotients had a decreasing trend throughout the composting process.

Highlights

► We monitored the co-composting of sludge with amendments and organic solid wastes. ► A mixture experimental design approach was used and response surfaces were drawn. ► Sludge and food waste mixtures at a 1:1 ratio (ww) maximized degradability. ► The initial moisture and initial C/N ratio did not affect the composting process. ► The bio C/bio N ratio and the respiratory quotients were illustrated versus time.

Introduction

Wastewater sludge disposal is a major problem worldwide due to the continuously increasing amounts produced. Since the trend is to reduce landfilling of biodegradable waste in Europe, sludge pretreatment becomes necessary (EUC, 1999). According to Spinosa (2011), a precondition for biosolids landfilling is to produce a biologically stable and adequately dewatered material. Therefore, alternative techniques to treat sludge have been developed. Co-composting of municipal wastewater sludge with other organic substrates has been widely used over the past decades. Materials used as amendments in the process are the organic fraction of municipal solid waste, sawdust, wood chips and tree trimmings/branches. The addition of energy amendments, such as the organic fraction of municipal solid wastes, aims to enhance the overall degradation of a mixture. The addition of structural amendments, such as sawdust and wood chips, aims to reduce the moisture content of the mixture to near optimal levels and to increase the free air space (FAS) in order to facilitate oxygen transfer through the solid material (Haug, 1993).

Mixing of sludge with other organic materials is usually performed based on practical guidelines, sometimes using the initial C/N ratio and moisture content of the mixture as design parameters. The effect of initial physicochemical parameters (e.g. temperature, air flow rate, bulking agent content, bulking agent particle size, initial moisture content, C/N ratio) on the aerobic degradation of wastewater sludge mixtures has been studied (Gea et al., 2003, Gea et al., 2007, Liang et al., 2003, Tremier et al., 2005, Banegas et al., 2007, Tremier et al., 2009, Mohajer et al., 2009, Mohajer et al., 2010, Pasda et al., 2005). The initial C/N ratio, in particular, has been found to affect the composting process for several organic substrates (Nakasaki et al., 1992, Huang et al., 2004, Ponsá et al., 2009). On the other hand, de Guardia et al. (2010a) reported recently that the initial C/N ratio is irrelevant to the aerobic biodegradability of 5 organic wastes. The bio C/N and bio C/bio N ratios are new alternative indices that have been recently proposed to follow the composting process (Sánchez, 2007, de Guardia et al., 2010a, de Guardia et al., 2010b).

Regression analyses and advanced experimental design techniques have been used to study the impact of initial physicochemical parameters on the composting of wastewater sludge (Gea et al., 2003, Mohajer et al., 2009). However, the impact of the contents of individual components contained in a mixture on the degradability of the overall mixture has not been well studied.

The objective of this work was to optimize the co-composting of dewatered sludge with other organic amendments by studying the degradability of the resulting mixtures. The organic amendments were food wastes, mixed paper, sawdust and tree branches that were added in variable percentages to the dewatered sludge. The goal was to locate optimal mixing ratios of the above components to maximize the decomposition process of the mixture during aerobic composting. This was achieved via the application of regression analysis and the adoption of principles of mixture experimental design (MED). A secondary objective was to investigate the effect of initial C/N ratio and moisture content (MC) on the degradation process. The degradability of the mixtures was monitored via several parameters, such as the O2 uptake rate, CO2 evolution, organic matter, solid carbon and nitrogen reductions and temperature.

Section snippets

Substrates

Six (6) components (i.e. substrates) were used to prepare all mixtures in this study. These components were: dewatered sludge (DWSL), office paper (OFP), newsprint (NP), food waste (FW), sawdust (SW) and tree branches (BRC). OFP and NP were mixed at 1:1 ratio to simulate mixed paper waste (MXP). FW was considered as energy (fuel) amendment and MXP, SW, BRC as drying and structural (bulking) amendments (Haug, 1993). Batches of 50 kg of DWSL were obtained as needed from the nearby publicly owned

Characterization of mixtures

Table 2 includes the initial properties of all individual materials used in this work.

Table 3 includes the initial and final properties of the mixtures of the 8 runs. The term “overall mixture” includes both the branches and the RMF. Initial moisture contents of the mixtures ranged from 56% to 65% (ww). Initial C/N ratios of the mixtures ranged from 19 to 30. The above range of initial properties in the mixtures was the basis of the experimental design that aimed in maintaining initial C/N

Conclusions

  • 1.

    According to empirical equations (2), (3), sludge and food waste mixtures at a 1:1 ratio (ww) lead to maximum degradability. Structural amendments should be kept to less than 30% to reach thermophilic temperatures. These results should be taken into account in practical applications.

  • 2.

    Initial moisture content and initial C/N ratio did not correlate significantly with the degradability of the mixtures as opposed to the bio C/N ratio that correlated significantly with the degradability of the

Acknowledgments

This research work was financially supported by the General Secretariat of Research and Technology, Ministry of Development (Greece) over the period 2006–2008. This paper would not have been possible without the active participation of the following undergraduate students: Anastassios Apostolou, Georgios Giannakis, Dimitrios Kalpakis, Georgios Lazaridis and Margarita Margariti.

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