The effect of acid pretreatment on the anaerobic digestion and dewatering of waste activated sludge D.C. Devlin ⇑ , S.R.R. Esteves, R.M. Dinsdale, A.J. Guwy University of Glamorgan, Sustainable Environment Research Centre, Upper Glyntaff, Pontypridd CF37 4AT, Wales, UK article info Article history: Received 7 September 2010 Received in revised form 7 December 2010 Accepted 8 December 2010 Available online 21 December 2010 Keywords: Biosolids Methane yield Oxitop Pretreatment Waste activated sludge abstract Waste activated sludge (WAS) is difficult to degrade in anaerobic digestion systems and pretreatments have been shown to speed up the hydrolysis stage. Here the effects of acid pretreatment (pH 6–1) using HCl on subsequent digestion and dewatering of WAS have been investigated. Optimisation of acid dosing was performed considering digestibility benefits and level of acid required. Pretreatment to pH 2 was concluded to be the most effective. In batch digestion this yielded the same biogas after 13 days as com- pared to untreated WAS at 21 days digestion. In semi-continuous digestion experiments (12 day hydrau- lic retention time at 35 °C) it resulted in a 14.3% increase in methane yield compared to untreated WAS, also Salmonella was eradicated in the digestate. Dewatering investigations suggested that the acid pre- treated WAS required 40% less cationic polymer addition to achieve the same cake solid content. A cost analysis was also carried out. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Sewage sludge should be adequately stabilised prior to its use via land application and this can be performed via different treat- ment options. Typically it has been achieved by composting, aero- bic treatment, lime treatment, incineration, chemical stabilisation, landfilling and anaerobic digestion (AD). Treatment of sewage sludge via AD has been carried out widely and technology has re- cently received increased interest in many countries due to the renewable energy it can produce. In the UK alone 1.3 million tonnes (dry solids) of sewage sludge are produced every year (Water UK, 2006) and water companies treat over 60% of this sewage sludge using AD. Rudd et al. (2003) concluded that the most viable method of treatment was AD, which results in a product that can be used in agriculture. These authors based their decision on the ‘best practicable environmental option’ for treating sewage sludge, for which they considered four categories: technical reliability, cost, environmental sustainability and environmental nuisance. Digested sludge, often known as biosolids, contains many important nutrients and trace elements vital to plant growth and it is often part of a farm management plan. The safe sludge matrix is a voluntary code of practice be- tween the UK water industry, the Environmental Agency and the British Retail Consortium, which was introduced to reassure the public about the use of biosolids in agriculture and lays down two categories of sludge treatment defined on the log reduction of an indicator organism. The two categories are conventionally treated sludges and enhanced treated sludges, which should have a 2 log and a 6 log reduction in Escherichia coli, respectively. Along with E. coli reduction enhanced treated sludge should be free of Salmonella. Treatment via AD results in a reduction of the amount sludge solids for disposal and also a decrease in odour problems associ- ated with residual putrescible matter and at the same time green energy is produced in the form of biogas (Appels et al., 2008). This biogas can be either upgraded to allow it to be directly used as transport fuel or injected into the gas grid or it can be desulphur- ised and dried then utilised in combined heat and power (CHP) sys- tems to provide the Waste Water Treatment Plant (WWTP) with electricity and heat or export in the case of excess. Since this en- ergy is from a renewable source it also has the added benefit, in a number of countries, of attracting incentives for the energy pro- ducer. Any improvement in AD efficiency will therefore lead to a further reduction of sludge for transport and disposal. In addition to this it is likely that biogas yield will increase and hence a greater amount of renewable energy produced resulting in better plant environmental performance and economies. It is widely accepted that primary sludge is readily digestible whilst secondary sludge or WAS is difficult to digest (Lafitte-Trou- que and Forster, 2002). The physical state of microbial cells present in WAS makes them an unfavourable substrate for microbial deg- radation as most of the organics are encased within microbial cell membranes. The cells are protected from osmotic lysis because of the semi rigid structure of the cell envelope (Muller et al., 1998). This meant that often high HRTs are necessary (20–30 days) to 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.12.043 ⇑ Corresponding author. Tel.: +44 0 1443 482227. E-mail address: dcdevlin@glam.ac.uk (D.C. Devlin). Bioresource Technology 102 (2011) 4076–4082 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech