Pretreatment of sewage sludge with hydrothermal carbonization (HTC) significantly increases the energy production of sewage sludge incineration plants and can also be flexibly adjusted seasonally. A recent study by the engineering firm Umwelttechnik & Ingenieure GmbH shows that pretreatment of just a quarter of the sewage sludge volume increases the energy surplus of renewable electrical energy by 50 to 100%. The thermal energy delivered to a connected local heating network can be adjusted according to demand. Many currently planned and publicly funded sewage sludge incineration projects are controversial.
As a result of the amendment to the Sewage Sludge Ordinance (AbfKlärV), around 44 sewage sludge incineration plants are currently being planned or built in Germany. The primary purpose of incineration is waste disposal and preparation for subsequent recovery of phosphorus from the remaining ash. Most of the energy released during incineration is used for the plant’s own energy supply and for pre-drying the sewage sludge, which is supplied by the municipal wastewater treatment plants with a water content of about 75%.
However, many of these construction projects financed by wastewater fees are controversial: Citizen protests complain of increasing truck traffic, rising environmental pollution, lack of ecological benefit, unclear phosphorus recovery strategy and high costs. A recent study now shows a way to significantly increase the environmental benefits of these projects:
Pretreatment in the TerraNova®ultra process produces a regenerative coal from the sewage sludge by hydrothermal carbonization, which is almost free of water. This saves transport costs and truck journeys and significantly increases the energy surplus of sewage sludge incineration. The study by the engineering firm Umwelttechnik & Ingenieure GmbH calculates an increase in the production of green electricity by 50% in the event that a quarter of the total volume of sewage sludge is pre-treated using the TerraNova®ultra process. In summer, when the heat demand at the connected heating network is low, the electricity production is even more than doubled. In a sewage sludge incineration plant of typical size with 38,000 t of sewage sludge (dry matter), up to approx. 3.5 GWh of additional green electricity are thus generated – this corresponds roughly to the needs of 1,000 families.