Thermal Hydrolysis Process at Växjö WWTP, Sweden

Thermal Hydrolysis Process has been implemented in existing projects designed to reduce disposal quantities and the cost of building and operating digesters, as well as increasing the biogas production and achieve hygienisation.  

Components installed in the solution

The THP-unit consists of pre-dewatering equipment, pulper, reactors, flash-tank, heat exchanger, and steam boiler. The steam boiler is fueled with wood pellets. The resulting biosolids are used as a REVAQ-certified fertilizer in agriculture.

The pre-treatment consists of steam explosion (6 bar, 165 °C for about 20 minutes), which leads to cell rupture and therefore improved substrate availability. Thus, enhanced degradation rate and increased methane production in the digestion process can be achieved. The treatment also provides hygienisation and improves the material dewatering properties.

Solution provider

Operational mode

The Municipality of Växjö has recently started operating a pre-treatment plant for source-separated household food waste. The aim is to increase the capacity of existing biogas plant produce biogas also from food waste either co-digestion. In 2030 the new plant will treat 8 600 metrics tons of dry matter annually according to existing plans, of which 67% is sludge and 33% is food waste.

The thermal hydrolysis process was installed at Sundet biogas plant in Växjö in September 2014 to handle the increasing amounts of sludge and food waste. It resulted in efficient sludge degradation and increased methane production. The production of biogas is 20 % higher than conventional technology. All the biogas will be upgraded to vehicle gas and used in the transport sector.

Sundet WWTP has a capacity of 95 000 PE with an average daily flow of 20 000 m3/d.

The results showed an increased biogas production corresponding to 500 kWh/t TS substrate after the installation of thermal hydrolysis, 26% higher gas production compared to the period before the installation. The energy required for the thermal hydrolysis was of the same magnitude as the increased biogas production but it can be justified as biogas as a higher economic value. The pre-treatment also provided hygienisation. The plant’s technical complexity increased after the thermal hydrolysis process was installed, leading to higher competence requirement for the personnel operating the plant.

The production and quality of the resultant bio solids save transport and energy costs as well; the end product can be applied directly to agricultural processes or composted or dried for use as fertilizer or bio-fuel. In addition to optimizing energy-efficiency and lowering operating costs, THP also eliminates odour problems associated with the treatment of organic materials. This solution can be combined with cogeneration plants, which produce green electricity and provide hot steam for the Thermal Hydrolysis Process. Resultant biogas can also be cleaned for use as vehicle fuel or as a replacement for natural gas.

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