Sewage sludge is a byproduct of wastewater treatment plants. While it can serve as a valuable energy source and organic waste, it also contains pollutants and must be treated responsibly. A new treatment facility in Karlovac, Croatia, will prepare sewage sludge for safe disposal and energy recovery at the nearby cement plants. To ensure the sludge is dewatered enough to be used in cement kilns, an innovative drying unit has been built, combining solar energy and Pipelife industrial underfloor heating.
The amount of sewage sludge produced in the EU and worldwide is rising; therefore, sustainable solutions for its treatment and disposal are continuously sought. In 2021, the Karlovac water utility company Waterworks and Sewage (Vodovod i kanalizacija d.o.o. – ViK) started constructing a solar drying facility for sewage sludge. The plant is situated in the city of Karlovac, on the Kupa bank, and the project has been co-financed by the Cohesion Fund of the EU.
In the plant, the sludge is evenly distributed over a foundation in a 10 to 50-centimeter-thick layer and aerated by mechanical mixers. The greenhouse-like building has a transparent roof and wall panels, letting in as much sunlight as possible. As a result, the temperature inside the drying unit increases, facilitating evaporation.
Depending on the composition and the quantity of the sludge being processed, as well as on the local weather conditions, it is possible to obtain a product that consists of 60 - 90% of dry solids while the volume of the sludge is reduced by roughly two-thirds.
Dried sewage sludge can be safely disposed of via incineration, which is becoming the fastest-growing disposal practice in the EU. Incineration destroys the harmful pathogens in the sludge and generates ash — a stable and reusable product.
Furthermore, incineration can be combined with energy recovery systems, offering a more sustainable alternative to energy generation from fossil fuels. According to research, incineration of sewage sludge emits 58% fewer emissions than natural gas and 80% less than hard coal and fuel oil for the same amount of energy.
In Karlovac, sewage sludge can be incinerated at the nearby cement plants if the dry solids content is at least 90%. Depending on the weather, additional thermal treatment may be necessary for the drying process to ensure the sludge meets the requirements for incineration.
Pipelife Croatia supplied the Karlovac facility with an underfloor heating system normally used in industrial environments. Its specific purpose, in this case, is to heat the dewatered sludge when solar drying is insufficient or when the air temperature inside the greenhouse drops below 15˚C.
The additional heat source allows for a smaller surface area of the drying unit and helps reduce its dependence on weather conditions and the change of seasons in an energy-efficient way. In total, more than 11,000 meters of polyethylene (PE-RT) 20x2 mm pipes were installed for the underfloor heating system.
The construction works of the Karlovac plant have been successfully completed, and the trial run was started last month. The current annual sludge output in the area is around 2,000 tons, equaling 28,000 PE.
However, the new plant’s design capacity is almost 100,000 PE, allowing for the safe treatment of even greater amounts of sewage sludge in the future.
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