Sewage sludge (biosolids) is a byproduct of waste water treatment that accumulates in large quantities. In Austria the total annual sludge production in 2005 at municipal and industrial waste water treatment plants (WWTP) amounted to 264,000 tons total solids (TS), which corresponds to about 880,000 tons a year based on 30% solids content. With regard to resource management different options of sludge recycling are available. Energy recovery by anaerobic sludge digestion is usually a common way to stabilize sewage sludge at WWTP >50,000 population equivalents (PE). Sludge mass reduction results in biogas production which is converted into electric energy and used for the operation of the waste water treatment plant. Different waste management strategies are followed after anaerobic digestion. Mono-incineration of digested sludge is not very efficient due to the high water and low energy content. Co-incineration with organic waste or in cement factories is a further practicable option. Since phosphorus is a limited resource, recovery of phosphates from the ash is an issue of increasing interest and.
Sewage sludge is an important source of nutrients for plants and very useful as soil amendment. However, in Europe its use in agriculture is a matter of controversial discussion. Landfilling is one of the alternative options for sludge disposal. According to the new Austrian landfill regulation, which became effective in 2004, disposal of solid waste with a total organic carbon content (TOC) higher than 5% is not allowed in new landfills. Since both the TOC content and the calorific value of digested sewage sludge do not meet legal requirements, disposal of sewage sludge will be possible in the future only after incineration.
Lab-scale experiments were performed to estimate to what extent the organic matter content, and consequently the calorific value and the biological activity, of digested sludge can be reduced by an additional aerobic sludge stabilization step. Adequately stabilized sewage sludge consists of slowly degradable particulate organic matter, mainly microbial cell membranes. These particulate organic compounds need to be hydrolyzed before biological degradation can take place. Hydrolysis is therefore expected to play a key role in the performance of an additional sludge stabilization step. A switch from an anaerobic to aerobic environment in the post-stabilization phase provided more favorable conditions for the degradation of residual sludge organic solids.
|
