Aspects of digestibility of surplus activated sludge and its potential for biogas production
Winter, P., Pearce, P., Lee, K., and Ganidi, N., Thames Water Research and Development, UK
(free)
Surplus activated sludge (SAS) is produced as part of the biological treatment process and requires further processing within the sludge stream. Within Thames Water the activated sludge (AS) process is the most widespread process in terms of volume of wastewater treated and it is estimated that approximately 180,000tds of SAS were produced in 2007. Frequently, the quantities of SAS produced exceed design criteria e.g. as a result of increased plant loading or chemical precipitation of phosphorous. High quantities of SAS result in problems in the sludge stream, in particular compromising the performance of the anaerobic digestion process and subsequent dewaterability.
The paper will discuss aspects of digestibility of SAS and how it may be influenced by the way our activated sludge plants are operated. This will be illustrated through tests showing the potential of various SAS to produce volatile fatty acids.
It will also be discussed if a benchmark could be established to understand maximum biogas yield. An example is data from laboratory scale digestion of SAS homogenised at 800 bar that show an average 26% increase in specific gas production compared to a control. A further aspect involves the anaerobic digestion process and if separate digestion of SAS and primary sludge could be beneficial for the treatment process. Given their very different composition it may be expected that optimum conditions for anaerobic digestion are also different. The paper argues for a better understanding of the optimum conditions for SAS and for primary sludge. The paper suggests that it is necessary to carefully address the ‘local’ problem to find techniques and concepts that improve digestibility and biogas production from SAS.
