A partial flow of the dewatered sewage sludge is at first fully dried in a low-temperature dryer, type BT 20 of HUBER SE. Before incinerated, the partial flow of the sewage sludge dried to 90% dry residue is backmixed with the rest of the dewatered sewage sludge to obtain approx. 45% DR and ensure an autothermal incineration. Backmixing is adjusted to suit the fluidized bed furnace and further depends on the quality of the sewage sludge. The heat supply required by the dryer comes from the incineration exhaust heat.
The location where the mono-incineration plant Halle-Lochau will be erected is not a sewage treatment works. The complete plant must therefore be designed to operate virtually without producing wastewater. Special focus is therefore to be placed on safe disposal/utilisation of the vapours generated with sewage sludge drying. The variant of quasi wastewater-free sewage sludge drying with a supply-exhaust air dryer has turned out to be an efficient and economical sludge drying concept. The exhaust air from the belt dryer is passed on to separate chemical treatment. The wastewater from the acid scrubber is an ammonium sulphate solution (ASL), commonly used as fertilizer in agriculture. The amount of residues generated in the alkaline scrubber stage is so small that the residues can be passed to the incineration process.
At Halle-Lochau, the sewage sludge is thermally utilized in an individually planned stationary fluidized bed furnace of sludge2energy (S2E Fluidizer). The furnace is designed to ensure the supplied sewage sludge can be utilised thermally in a continuous process without adding any auxiliary fuels. A back-pressure steam turbine is used for recovery of the energy from the vapour generated in the boiler plant. Most part of the exhaust steam from the turbine is used to dry the dewatered sewage sludge. Due to the optimized energy concept, the plant’s own electrical energy demand can be fully covered. As the flue gas is cleaned in a dry process, no additional wastewaters are generated which would have to be treated separately. The emission limits required by the 17th BlmSchV are reliably met by this type of flue gas treatment. Flue gas purification is designed to include a pre-separator to ensure that virtually the complete sewage sludge ash is removed from the flue gas already before addition of the sorbent and can thus be used for later phosphorus recovery.
The solution that achieves the highest phosphorus recovery rate and, at the same time, eliminates all contaminants is mono-incineration of sewage sludge with downstream recycling of phosphorus from the generated sewage sludge ashes. Through sludge drying and incineration, the sludge disposal volume is minimized to approximately 10%. The sewage sludge from the fluidized bed furnace at Halle-Lochau (approx. 4.000 t/a), low in pollutants, is an important phosphorus carrier and thus a valuable source material for the future phosphorus fertilizer production.
The current legal framework conditions for sewage sludge utilisation pose great challenges for sewage treatment plant operators. If the presented size of incineration plant is transferred to other locations in Germany, this could lead to the quick expansion of the mono-incineration capacities on those sites and contribute substantially to solving the present case of emergency in the field of sewage sludge disposal. Decentralised mono-incineration plants can therefore be a main component of a future-oriented sewage sludge utilisation concept in Germany.
