Life Cycle Assessment of the Mesophilic, Thermophilic, and Temperature-Phased Anaerobic Digestion of Sewage Sludge

Iryna Lanko,Laura Flores,Ivet Ferrer,Vladimir Todt,John A Posada,Marianna Garfí,Pavel Jenicek

doi:10.3390/w12113140

Abstract

In this study the environmental impact of the anaerobic digestion (AD) of sewage sludge within an activated sludge wastewater treatment plant (WWTP) was investigated. Three alternative AD systems (mesophilic, thermophilic, and temperature-phased anaerobic digestion (TPAD)) were compared to determine which system may have the best environmental performance. Two life cycle assessments (LCA) were performed considering: (i) the whole WWTP (for a functional unit (FU) of 1 m3 of treated wastewater), and (ii) the sludge line (SL) alone (for FU of 1 m3 of produced methane). The data for the LCA were obtained from previous laboratory experimental work in combination with full-scale WWTP and literature. According to the results, the WWTP with TPAD outperforms those with mesophilic and thermophilic AD in most analyzed impact categories (i.e., Human toxicity, Ionizing radiation, Metal and Fossil depletion, Agricultural land occupation, Terrestrial acidification, Freshwater eutrophication, and Ozone depletion), except for Climate change where the WWTP with mesophilic AD performed better than with TPAD by 7%. In the case of the SL alone, the production of heat and electricity (here accounted for as avoided environmental impacts) led to credits in most of the analyzed impact categories except for Human toxicity where credits did not balance out the impacts caused by the wastewater treatment system. The best AD alternative was thermophilic concerning all environmental impact categories, besides Climate change and Human toxicity. Differences between both LCA results may be attributed to the FU.

Highlights

In conventional activated sludge wastewater treatment plants (WWTP), excess sludge is continuously formed at the biological reactor of the wastewater treatment line
The results of the life cycle assessments (LCA) for the whole WWTP (WWTP-LCA) are shown in the Figure 2. This figure includes all environmental impact categories considered in this study, and within each impact category there are three scenarios: mesophilic, thermophilic, and Temperature phased anaerobic digestion (TPAD)
Results are shown for the whole WWTP, and separately for the wastewater treatment line and the sludge treatment line; this disaggregation of results was done to better identify the contributions of each process stage to the overall impacts

Summary

Introduction

In conventional activated sludge wastewater treatment plants (WWTP), excess sludge is continuously formed at the biological reactor of the wastewater treatment line. WWTP operational expenses to handle the produced excess biological sludge, namely waste activated sludge (WAS), together with primary sludge may go up to 50% [1,2,3]. It has been a long time since AD was adopted as one of the most effective solutions of sewage sludge treatment in terms of sludge reduction, stabilization, Water 2020, 12, 3140; doi:10.3390/w12113140 www.mdpi.com/journal/water. Water 2020, 12, 3140 and resource recovery [4,5,6]. Population growth and natural resources exhaustion made it crucial to find a way out of this situation. Sludge is considered as a source of substances that can be recovered and reused [9,10,11]

Objectives

Methods

Results

Conclusion