690 Research Article Received: 19 April 2013 Revised: 5 July 2013 Accepted article published: 13 July 2013 Published online in Wiley Online Library: 9 August 2013 (wileyonlinelibrary.com) DOI 10.1002/jctb.4171 Anaerobic digestion of aerobic granular biomass: effects of thermal pre-treatment and addition of primary sludge ´ Angeles Val Del R´ ıo, ∗ Tania Palmeiro-Sanchez, M ´ onica Figueroa, Anuska Mosquera-Corral, Jos ´ e L Campos and Ram ´ on M ´ endez Abstract BACKGROUND: Management of the sludge generated in the wastewater treatment plants accounts for more than 50% of their operational costs. To minimise these costs, technologies capable of reducing the production of sludge in the plant need to be developed, such as aerobic granular systems. The aggregation state of aerobic granular sludge (AGS) could be a limiting factor for its further anaerobic digestion. Therefore in this work the feasibility of anaerobic digestion of AGS has been studied under three conditions: (1) raw AGS, (2) thermal pre-treated AGS and (3) a mixture of thermal pre-treated AGS with primary sludge. RESULTS: The values obtained for anaerobic biodegradability and reduction of solids in the case of raw AGS were 44% and 32%, respectively. Thermal pre-treatment of AGS at 133 ◦ C enhanced the anaerobic digester performance, in terms of solids reduction, by approximately 47%. The mixture of thermal pre-treated AGS with primary sludge provided better results for solids removal than in the case with only thermal pre-treated AGS. CONCLUSION: Anaerobic digestion of AGS has a similar performance as that reported for waste activated sludge, which indicates that the aggregation of the biomass into granules does not seem to limit the anaerobic process. c  2013 Society of Chemical Industry Keywords: aerobic granules; anaerobic digestion; primary sludge; thermal pre-treatment INTRODUCTION Nowadays, in developed countries, the management of large amounts of produced waste is of concern. Treatment of the wastewater produced, from urban or industrial origin, is an example. In this sense the European Union, through Directive 91/271/EEC for Urban Waste Water Treatment, 1 has a prime objective to protect the environment from the adverse effects of urban wastewater discharges and discharges from certain industrial sectors. To fulfil these requirements wastewater treatment systems are applied, with the consequent production of sludge, which, as a waste, needs to be treated. In wastewater treatment plants (WWTPs) sludge is generated mainly from the primary treatment (primary sludge, PS) and the secondary treatment that involves biological processes (waste activated sludge, WAS). The mixture of these solids is treated subsequently in a stabilisation step to reduce the pathogens content, followed by a dehydration step to decrease the volume. One of the most used technologies to stabilise the sludge is anaerobic digestion, due to its ability to (1) reduce the amount of solids for final disposal, (2) destroy most of the pathogens present in the sludge, (3) limit possible odour problems associated with residual putrid matter, and (4) transform organic matter into biogas which can be used to produce energy in the same installation, for example to heat the anaerobic digester or to dehydrate the final sludge. 2 The anaerobic biodegradability of WAS is limited by the hydrolysis step; 3 furthermore, its low carbon/nitrogen (C/N) ratio (between 6 and 16) is also reported to be a problem for anaerobic digestion. 4 The suggested optimum C/N ratio is in the range of 20–30 to ensure the presence of sufficient nitrogen supply for the cell production and the biological degradation of the organic matter. 5 The co-digestion of WAS with PS can be a solution to overcome the difficulties of treating WAS alone and to adjust its unbalanced quantity of nutrients. Bouallagui et al. 6 observed that the addition of PS to WAS during anaerobic digestion is beneficial in that it increases the solids removal and the specific biogas production. Anaerobic biodegradability of PS is normally higher than that of WAS. 7 – 9 Therefore the application of pre-treatment to anaerobic digestion is often used to enhance the biodegradability of the WAS in preference to PS pre-treatment. 10,11 The energy required to pre-treat the PS is too high compared to the subsequent slight increase in the amount of obtained biogas. 9 The pre-treatment methods to improve the anaerobic biodegradability of the sludge ∗ Correspondence to: ´ Angeles Val Del R´ ıo, Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain. E-mail: mangeles.val@usc.es Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, E-15782, Santiago de Compostela, Spain J Chem Technol Biotechnol 2014; 89: 690–697 www.soci.org c  2013 Society of Chemical Industry