Biogas production from low-organic-content sludge using a high-solids anaerobic digester with improved agitation Xiaocong Liao a,b , Huan Li a,⇑ a Key Laboratory of Microorganism Application and Risk Control of Shenzhen, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China b Joint Research Center of Urban Resource Recycling Technology of Graduate School at Shenzhen, Tsinghua University and Shenzhen Green Eco-Manufacturer High-Tech Co. Ltd., ShenZhen 518055, China highlights  High-solids digestion recovered bioenergy efficiently from low-organic-content sludge.  A new stirrer was designed for high-solids digestion of low-organic-content sludge.  High-solids digestion exhibited similar performance with conventional low-solids ones.  High volumetric biogas yield was achieved without any accumulated inhibitors. article info Article history: Received 16 January 2015 Received in revised form 15 March 2015 Accepted 17 March 2015 Keywords: Sludge Anaerobic digestion Biogas Ammonia Volatile fatty acids abstract Sludge with a low organic content has poor biochemical methane potential. High-solids digestion is a possible method to recover bioenergy economically from this kind of sludge, but the blocked mass trans- fer is a major obstacle. A pilot scale high-solids anaerobic digester equipped with an enhanced stirring system was designed and operated continuously for 9.5 months to evaluate the feasibility of bioenergy recovery from low-organic-content sludge. The results showed that high-solids anaerobic digestion can evolve successfully from low-solids status. Although the system once suffered slight inhibition derived from ammonia, it then stabilized with volatile fatty acids concentration of 200–400 mg/L and free ammo- nia concentration less than 250 mg/L, exhibiting similar removal rates of organic solids and biogas yields as achieved using low-solids digestion. Statistical analyses proved that the organic removal rate was almost proportional to the organic content of feed sludge. The organic removal rate was nearly 35% when the organic content of feed sludge was 50%, while the system tended to failure when the organic content of feed sludge was less than 38%. High-solids anaerobic digestion, when combined with improved agita- tion, is an effective method for bioenergy recovery from sludge with organic content of 40–50%. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction During conventional biological processes of municipal wastewater treatment, a large quantity of waste activated sludge and primary sludge are produced. Sludge treatment cost accounts for up to 50% of the total operation cost of wastewater treatment plants [1]. Hence, sludge reduction is the first step of sludge treat- ment and disposal. Anaerobic digestion is widely used for sludge reduction because it can also transform organic matter into biogas [2,3], making sludge a potential bioenergy source instead of just a residue requiring disposal [4]. The volatile solids (VS) content in sludge total solids (TS) commonly are in the range of 60–80% (by weight), and 50–65% (by weight) of VS can be converted into bio- gas during conventional anaerobic digestion when feed TS is 2–6% and solids retention time (SRT) is 15–25 d. Under these conditions, the collected biogas can simultaneously produce heat energy to maintain the digester at a moderate temperature and generate electricity for output [5]. Therefore, conventional anaerobic digesters are routinely used at wastewater treatment plants in developed countries. However, differences in wastewater characteristics and the development of wastewater treatment plants bring a new chal- lenge to the bioenergy production from sludge, especially in devel- oping countries. In some wastewater treatment plants, the organic content in sludge solids is often lower than 50% due to special treatment processes, e.g. biological nutrient removal [6]. During the subsequent anaerobic digestion, the feed sludge with low http://dx.doi.org/10.1016/j.apenergy.2015.03.082 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: li.huan@sz.tsinghua.edu.cn (H. Li). Applied Energy 148 (2015) 252–259 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy