Historically, the focus sewage sludge digestion was to stabilise the sludge and reduce the odour emitted. Pathogen reduction, solids destruction and biogas production then became important. In recent years a number of advanced digestion processes have become available which can be added to the conventional digestion process. Initially advanced treatment methods for treating sludge were more associated with bacterial quality and not economics, however recently the focus has shifted. This paper deals with the economics of advanced digestion.
The advanced treatment processes affect the digestion process by improving the conversion of organic material particularly cell matter to biogas. This is carried out using different methods such as high pressure steam injection, increased heating, physical and electrical rupturing of cells. Each process has associated consequences that change the economics of the treatment of sludge.
This paper looks at the following advanced digestion techniques. Some of these processes are well proven with numerous sites around the world whilst others are only just tested at full scale:
- Thermal hydrolysis
- Enhanced enzyme hydrolysis
- Thermophilic digestion
- Cell Rupture.
An assessment is made of the economics of increased biogas production and improved dewaterability against of the potential increased cost of treating the liquors. The costs associated with liquor treatment is affected by the change in concentration of recycled material particularly ammonia. The disposal route of the sludge cake is potentially the most influential factor in the choice of advanced treatment process.