Review of inventory data for the biological treatment of sewage sludge

Highlights

• Inventory data of biological treatment of sludge address all inputs and outputs.

• Biogas production varies considerably in the 64 datasets on anaerobic digestion.

• Very few data are available on digestate or energy in anaerobic digestion of sludge.

• VS degradation in composting is hard to estimate due to the added bulking materials.

• Data on emissions into the air are currently scarce for sludge composting processes.

Abstract

The biological treatment of municipal sewage sludge, including anaerobic digestion and composting, was reviewed with the purpose of establishing inventory data to address all the inputs and outputs related to sludge treatment. We identified 193 scientific papers, resulting in 64 datasets on anaerobic digestion and 35 datasets on composting. For anaerobic digestion, biogas production varied significantly (up to a factor of four) depending on the sludge. A useful correlation was identified between the amount of methane produced and the degradation of volatile solids. According to statistical tests, no significant differences were found in biogas production for mesophilic and thermophilic digesters. In addition, methane content varied significantly, and very few data were available for digestate composition or for energy consumption and recovery. For composting, accurate estimates relating to the degradation of sewage sludge could not be made, since organic bulking materials were part of the final composted product. Data on emissions to air are currently scarce, which points to the need for more published information. The inventory data evaluated herein are useful in the feasibility assessment of the biological treatment of sewage sludge, for comparing technologies, for example in LCA studies and as a basis for evaluating the performance of a specific biological sludge treatment plant. However, a great deal of the reviewed data originated from laboratory and pilot-scale studies, and so there is a need for more complete datasets on the performance of full-scale technologies, in order to establish full inventories and identify differences in technologies and operational conditions.

Introduction

The biological treatment of municipal sewage sludge is common in rural as well as urban areas. Biological treatment may occur in the absence of oxygen, as in the case of anaerobic digestion, or in the presence of oxygen, as the case of composting. Sometimes, the processes are combined, in that, after digestion, the digested sludge is composted.

The anaerobic digestion of sewage sludge provides stabilisation and allows for the recovery of energy and nutrients. It is estimated that there are more than 132,000 anaerobic digesters in the world, more than 1,300 of which treat sewage sludge (IEA Bioenergy, 2001, World Biogas Association, 2019). Several technologies are available, and the key technological factors are process temperature, dry or wet processing, mixing, retention time, the use of additives and the capture of the generated gas. Lavergne et al., 2018, Gebreeyessus and Jenicek, 2016 provided overviews of common digester technologies for sewage sludge treatment.

The composting of sewage sludge provides stabilisation and allows for the recovery of nutrients and organic matter (Wang et al., 2021). We found no statistics on the number of composting plants dealing with sewage sludge. Several technologies are available, and the key technological factors are the use of bulking materials to improve accessibility to oxygen, and ways to provide air supply – by either aeration or turning (Golbaz et al., 2021, Liu et al., 2020, Ma, 2020, Naserian et al., 2021, Wang et al., 2020). In some cases, composting takes place in the open, while in other cases it takes place in a reactor or a building that controls air ventilation (Han et al., 2018b, Pognani et al., 2011, Shen et al., 2020, Zheng et al., 2018). Benedict et al. (1986) described a number of different sewage sludge composting technologies.

Since information about inventory data describing the performance of biological treatment technologies for sewage sludge is not directly reported in existing studies, we performed a literature review on input and output data describing the performance of anaerobic digestion and the composting of municipal sewage sludge, with the latter applied to both raw and digested sewage sludge. Vermiculture (Guzman et al., 2020, Włóka et al., 2020) and reed beds (Brix, 2017, RozkosnÝ et al., 2020) are also biological treatment options for sewage sludge, but they are not included in this study because of their limited use worldwide. Co-treatment of municipal sewage sludge with other sludge types or with manure is not covered, since general data on sewage sludge can rarely be extracted from co-treatment studies. Inventory data are useful in feasibility assessments relating to the application of biological treatments, for technology comparisons and as a comparative basis for evaluating the performance of a specific biological sewage sludge treatment plant. Inventory data also act as the basis for performing a life cycle assessment (LCA). The purpose of this paper is thus to summarise and analyse inventory data in the scientific literature describing the anaerobic digestion and composting of municipal sewage sludge, thereby enabling us to identify gaps in inventory data on the biological treatment of sewage sludge.