Thermal hydrolysis is a pre-treatment to the conventional anaerobic digestion of organic waste. In general terms, it is a two-stage process:
This combined action fractures the cell structure and makes the waste more biodegradable, improving the anaerobic digestion performance and yielding more biogas. In addition, thermal hydrolysis sterilizes the sludge. The destruction of pathogens, or pasteurization, results in high-quality sludge that is suitable for land application as fertilizer or compost even under the most stringent regulations. Finally, this pre-treatment adjusts the rheology of the material to such an extent that loading rates to the anaerobic digester can be doubled. The dewaterability of the sludge is also significantly improved to up to 40% total dry solids, resulting in less biosolids. While research and development of this technology started back in the 90s, the technology has come long ways since evolving from the initial batch processes into the cutting-edge TH4+ process.
TH4+, or Thermal Hydrolysis for +, is teCH4+ innovative process that comprises the following steps:
The organic waste is fed into dosification vessels and heated up with vapors from the heat recovery section (Step 3) for optimum heat integration and energy efficiency. Two parallel dosification vessels sequentially feed the organic material to pressurization tanks, where the waste is pressurized using compressed air or steam. This allows the organic material to flow through the process without the need for pumps or any other mechanical means.
The pressurized material enters a mixer, where live steam is injected to achieve the temperature setpoint in an extremely rapid manner. In conventional processes, the material to be hydrolyzed is kept at high temperatures for approximately 30 minutes, long enough for the material to undergo secondary reactions that reduce its methanogenic potential. This limits standard hydrolysis temperatures to 180ºC. The TH4+ process overcomes this limitation by means of an exceptionally quick heating time of below 5 seconds. This greatly minimizes the impact of the secondary reactions even at temperatures as high as 220ºC. The hot material is flashed to a regulation tank that provides a stable pressure throughout the system.
The content of the regulation tank is fed to the flash vessel through a series of flash valves. This second sudden decompression further fractures the cells structure, making the material more soluble