Barratt, P. and Spooner, J., Eco-Solids International(free)
Cellruptor is a particle and cell disruption technology which has a number of applications for pre-treatment of biomass feedstocks to different processes. The application here is for the pre-treatment of Secondary Activated Sludge (SAS) for subsequent mesophilic anaerobic digestion.
Since the completion of full-scale prototype tests (5 tonnes/hour), a Cellruptor process unit has now been designed to treat up to 20 tonnes/hour of thickened SAS (6% dry solids) at a large sewage treatment works in the UK. It is the first installation of its kind in the world, in that it will treat only SAS. The aim is to increase biogas production from a new anaerobic digestion installation, and so increase Combined Heat and Power (CHP) output from gas engines on site. On an annual basis, up to 1200 MWh additional generated power is expected. At the same time the process will target decreased digestate solids, and lower dose rates of chemical additives during sludge handling.
This paper presents the background to the project, the design of the process, and the targets for improvement of the existing anaerobic digestion. The new process plant, to be commissioned at the end of 2010, has a predicted biogas yield enhancement of up to 40% based on SAS, which equates to an increase of approximately 12% across the whole site.
Keywords: Anaerobic digestion, Biogas, Carbon Dioxide, Cellruptor, Pre-treatment, Secondary Activated Sludge (SAS).
As anaerobic digestion becomes recognised globally as an important technology in harnessing renewable, carbon-based energy, there is a focus on feedstock optimisation to bring about the maximum biogas yields at any given installation. In the treatment of municipal wastewaters by conventional aerobic wastewater treatment processes eg activated sludge, there are two principal biosolids (sludge) components which lend themselves to anaerobic digestion. These are the primary biosolids separated before the wastewater receives aerobic treatment, and the secondary biosolids, comprising the excess activated sludge from the subsequent aerobic treatment. The latter are often referred to as Secondary or Surplus Activated Sludge (SAS).
SAS is an interesting proposition as a source of renewable carbon for energy production, as it is generated in large quantities (often in similar volumetric flow rates to that of primary sludges), and contains significant quantities of organic carbon. However, the recovery of this renewable energy through anaerobic digestion processes shows relatively poor conversion of the Volatile Solids (VS) in SAS to biogas in conventional mesophilic digesters, which typically operate at Hydraulic Residence Times (HRT) in the municipal wastewater industry of 13 to 18 days.
Normally SAS, during mesophilic anaerobic digestion, may show between 25 and 35% Volatile Solids Destruction (VSD) depending on feedstock and digester conditions, thus leaving much of the VS unconverted, and therefore of no renewable energy value, and an additional burden as the solid fraction of the digestate.