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Thermal hydrolysis is an innovative wastewater solids conditioning process that boasts many advantages — financial, environmental, and otherwise. Is your plant a good candidate? By Greg Knight, Scott Carr, and Andrew Shaw T he thermal hydrolysis process (THP) has been compared to a pressure cooker. It conditions wastewater solids at a high temperature and pressure to improve digestibility. Injected steam heats the solids and maintains them at a temperature of approximately 165°C and a gauge pressure of 600 kilopascals (kPA), or 87 psi, for 20 to 30 minutes, after which the pressure is released. The combination of high temperature and rapid depressurization makes the material more biodegradable for the anaerobic digestion that follows. An additional benefit is that the resulting biosolids are pathogen-free, achieving "Class A" status. A number of configurations are available, including batch and continuous processes. The Cambi Group AS developed THP technology approximately 20 years ago, but other European and U.S. suppliers also now offer versions of this technology. The Benefits Of THP Increased biodegradability of wastewater residuals yields increased digester-loading rates, production of cake with higher solids content, a biosolids product that meets the top standards for land application, and increased biogas production. Because it improves digestibility and the solids are easier to mix and pump at higher solids concentrations, THP can be used to increase digester loading rates. This makes it appealing to facilities that need to process more solids in existing systems or need to minimize the size and number of new digesters. Improved conversion of volatile solids in the digestion process leads to other benefits, including better dewaterability and a drier cake product. Treating solids at a high temperature also yields a Class A biosolids product for fertilizer use according to U.S. EPA regulations for land application. The cake product from THP facilities also has fewer odors than that from conventional digestion facilities, which makes it more appealing for beneficial reuse. Farmers spend a lot of money on fertilizer that is rich in nitrogen and phosphorus. Biosolids are also rich in nitrogen and phosphorus, so reusing very stable biosolids as a fertilizer reduces fertilization costs for farmers, reduces management costs for utilities, and provides a very real environmental benefit through sustainable reuse. We also know that, globally, our phosphorus resources are limited, so reusing phosphorus through land application of biosolids is an environmentally sustainable practice. It is important to understand that THP doesn't necessarily increase energy recovery from a given quantity of solids because of the need to provide process steam. However, adding THP allows facilities with existing digesters to more than double their throughput capacity, which results in a significant increase in net biogas production. This can result in an equivalent increase in energy production for facilities with combined heat and power (CHP) or those producing renewable natural gas (RNG). 32 wateronline.com n Water Innovations Warming Up To Thermal Hydrolysis United Utilities doubled digester throughput and electricity production at the Davyhulme Wastewater Treatment Works in Manchester, U.K., by adding thermal hydrolysis. Utilities with relatively high residuals management costs can benefit from a process that reduces biosolids mass and volume.