After 1.5 years of continuous operation in Pau-Lescar, France, results are now available that confirm the controversially discussed benefits of Hydrothermal Carbonization (HTC) of sewage sludge.
In the project implemented by the French Suez group, in a first step sewage sludge is treated by an anaerobic digestion process to produce biomethane. The digested sludge is then dewatered and upgraded by the “Ultra” HTC-process into almost-dry HTC coal, which is incinerated locally for climate-friendly energy generation.
Nitrogen is removed from the side stream of HTC process water and added to the digestion process to enhance biomethane production.
While the advantages of the HTC process due to its low energy consumption compared to conventional sewage sludge drying methods have long been recognized in the industry, the effects of integration at a wastewater treatment plant with sludge digestion have now been examined. The focus was particularly on the long-term effects of treating the HTC process water on sludge digestion and the purification performance of the wastewater treatment plant.
With the integration of the HTC plant, approximately 20% more biomethane was generated by sludge digestion. This significant increase was achieved partly due to the additional easily degradable carbon supplied by the HTC process water, and partly because the effectiveness of sludge digestion was substantially improved. According to Suez, the “Ultra” process can even be operated without any heating costs.
The purification performance of the wastewater treatment plant in Pau-Lescar, designed for 190,000 inhabitants, was only moderately affected for the parameter COD (Chemical Oxygen Demand) by a 3.15 mg/l increase. In Europe, the COD limits for treated wastewater typically range between 50 and 125 mg/l, depending on the country and the size of the treatment plant.
The “Ultra” process was tested in 2014 in cooperation between Suez and TerraNova Energy in Maribor, Slovenia, demonstrating energy-efficient mechanical dewatering of carbonized sewage sludge to 65% dry matter. The first commercial installation followed in 2016 under the name TerraNova®️ultra in Jining, China.