THE PROS AND CONS OF THERMAL DRYING OF SEWAGE SLUDGE

Experience with thermal drying in Europe has been mixed. The last 20 years has seen the process growing in popularity but now there appears to be a decline in the drying plant market. Initially there was considerable debate about the relative benefits of drying untreated sludge v anaerobically digested sludge. Not having to provide digesters for, say a green field site, meant a potential reduction in capital expenditure and therefore was a very attractive option. However, concerns emerged about the ability to control the release of odours both during the process and after the product has been applied to land.It became apparent that the nature and composition of untreated sludge produced a dried product having different characteristics to those derived from treated sludge. In the first instance this was noticed in the granulation process and the final density of the product. Research into the fibre content of the sludge produced some interesting comparisons between the products and their relationship to the type of sludge processed. A worldwide survey of plants was undertaken to provide a limited database on which to base some conclusions. It became apparent that the fibre content of the sludge could be used to determine if a pelletiser should be included in the flow sheet.A number of safety issues have arisen and these are described. The role of the UK Government Health and Safety Executive in establishing a guidance document for the design and operation of sludge driers is described and some of their recommendation given.While the above refers to the production of a product with a dry solids content over 90%, systems are on the market that are more suitable to drying sludge to much lower ds content. These vary from the use of drying technology to reach the sludge autothermic limit prior to incineration to the latest developments, which involve the drying of the sludge to around 60%ds followed by lime treatment to create a product suitable for the recycling market.