By Greg Knight, Scott Carr, and Andrew Shaw
Thermal hydrolysis is an innovative wastewater solids conditioning process that boasts many advantages — financial, environmental, and otherwise. Is your plant a good candidate?
The 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).
Incorporating THP isn’t a panacea, and it’s not right in every situation. But where plants are at capacity and need to accommodate future growth, THP enables owners and operators to increase the treatment capacity of existing anaerobic digesters. Utilities with relatively high residuals management costs can benefit from a process that reduces biosolids mass and volume. And generating a better, more valuable end product can increase beneficial reuse and reduce management costs.
A Deeper Dive
Experience with THP in the UK and the U.S. has revealed some important considerations for retrofitting THP to existing facilities. For one, solids need to be screened prior to entering THP facilities. Approximately 5-mm screening is required to prevent problems with buildup of rags and other debris in downstream equipment.
Whereas conventional digestion requires thickening prior to the process, thermal hydrolysis requires upstream dewatering; THP therefore requires two stages of dewatering — one stage before THP and digestion, and another stage afterwards. Cake storage is also required upstream of THP to provide a steady throughput and operational flexibility.
Because thermal hydrolysis requires steam, plants that add THP generally must replace their water boilers with steam boilers. Those with CHP will want to generate steam rather than hot water from the CHP waste heat to power THP operations.
It’s necessary to cool the biosolids material following thermal hydrolysis and prior to digestion. Another consideration when upgrading existing digesters is that the gas piping may not be large enough for the increased biogas production per digester with THP.
Adding THP improves gas production by improving conversion of the energy in the biosolids into biogas. However, the process requires steam, so some of the biogas that is generated typically is used for steam production.
While a lot of existing THP facilities also have CHP, this technology does not always go hand-in-hand with THP. Where electricity costs are high and green energy credits are available — as in Europe and some regions of the U.S. — the generation of additional biogas offers a significant benefit. CHP can be a very good fit for THP because CHP generates a hot exhaust gas that can be used to generate steam. For this reason, it is quite common in Europe to use CHP with THP, where the average price of electricity is higher than in the U.S. More recently, production of RNG is also being carefully considered as an alternative use of the biogas (e.g., for pipeline injection or for vehicle fuel). It is important to examine the economics of biogas utilization options on a case-by-case basis to work out the best and highest use of the gas in a given situation.
Recent THP Innovations
In most applications today, thermal hydrolysis has been used upstream of anaerobic digestion, but there is now a process that allows for use of thermal hydrolysis downstream of anaerobic digestion. The solubilized material leaving the THP is dewatered to 40 percent solids concentration or greater. The sidestream from dewatering, which has a high biodegradable chemical oxygen demand, is sent back to the digesters, which leads to improved gas production and improved conversion of volatile solids. A system like this offers the potential for an easier retrofit of THP to existing processes.
Another new option is intermediate thermal hydrolysis. It entails inserting thermal hydrolysis between two stages of digestion. Owners would process residuals through conventional digestion, then THP, then another stage of digestion to maximize solids conversion and energy recovery.
Both of these emerging approaches could potentially be favorable for facilities with plenty of existing digester capacity. In these situations, owners aren’t driven to get more solids through a limited number of digesters but can reap other benefits. They can benefit from improved digestion performance through better solids conversion and greater gas production, construction and maintenance of a smaller THP facility, and production of a better-quality cake for beneficial use.
About The Authors
Greg Knight serves as the Black & Veatch thermal hydrolysis technical lead in the U.S. and has led process engineering for anaerobic digestion and THP projects on both sides of the Atlantic. He has 14 years of process engineering experience in water, wastewater, and biosolids management.
Scott Carr is Black & Veatch’s Global Practice and Technology Leader for biosolids and residuals management. With 30 years of experience, he has focused his career on all aspects of biosolids and residuals management, including processing and beneficial use of biosolids.
Andrew Shaw is a Global Practice and Technology Leader in wastewater and sustainability for Black & Veatch, as well as an associate vice president. He holds a PhD in environmental engineering and has 20 years of experience of wastewater treatment around the world.