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Thermal hydrolysis is an advanced wastewater solids conditioning process that boasts both financial and environmental advantages. With the process in wide usage across the UK, it is now starting to gain traction in the U.S., as wastewater utilities analyze whether their facilities would make a good candidate.

The thermal hydrolysis process (THP) increases the biodegradability of wastewater residuals, thereby boosting digester loading rates, increasing biogas production, and producing a biosolids product that can be used for land fertilization.

“The reasons plants are now examining whether to install THP are varied,” said Greg Knight, Black & Veatch Technology Lead for THP. “Some facilities need to process more solids through existing digesters, and some have a requirement to reduce digester volume for new systems.”

Knight said the process kills pathogens in biosolids, thereby meeting the highest U.S. Environmental Protection Agency criteria for land application as a fertilizer, known as Class A.

With THP, solids are easier to mix and pump at higher solids concentrations, which leads to increased digester loading rates. This can be appealing to facilities that need to process more solids in existing systems or need to minimize the size and number of new digesters. (To learn more about the benefits of THP, watch this Black & Veatch webinar.)

Some facilities have realized savings in biosolids end-use and disposal costs, resulting from a reduction of volatile solids and better dewaterability, which leads to decreased wet cake volume, according to Martin Jolly, Black & Veatch UK Technical Director.

“In Europe and some regions of the U.S., revenue from energy production can provide a significant additional consideration, due to high electricity costs and credits associated with green energy production,” Jolly said. He noted that at Davyhulme, Manchester, UK, Black & Veatch completed the construction of a $160 million plant in 2014 that is capable of generating 12 megawatts of electricity from sewage sludge.

Knight said that many THP facilities include combined heat and power (CHP) systems to produce electricity. Some facilities are also considering conversion of biogas to renewable natural gas as an alternative to CHP.

How Thermal Hydrolysis Works

THP has been compared to a pressure cooker, according to Scott Carr, Black & Veatch’s Global Practice and Technology Leader for biosolids and residuals management. 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. Then the pressure is released. The combination of high temperature and rapid depressurization makes the material more biodegradable for the anaerobic digestion process that follows.

Specifically, the high temperature and the rapid decrease in pressure causes cells to rupture in the waste activated sludge and break down extracellular polymer, making more material available for anaerobic digestion. This step also kills pathogens and makes the hydrolyzed sludge more digestible for microbes, significantly improving digester loading and performance. A number of configurations can be designed for plants, including batch and continuous processes.

The Benefits of Thermal Hydrolysis

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 nitrogen and phosphorus fertilizers,” Carr said. “Biosolids are also rich in nitrogen and phosphorus, so reusing biosolids as a fertilizer reduces fertilization costs for farmers, reduces management costs for utilities, and provides a real environmental benefit through sustainable reuse.”

He noted that global phosphorus resources are limited, so reusing phosphorus through land application is an environmentally sustainable practice.

Andrew Shaw, Global Practice and Technology Leader in wastewater and sustainability for Black & Veatch, said it is important to understand that THP doesn’t necessarily increase energy recovery at a facility because of the need to provide process steam.

“Adding THP improves gas production by improving conversion of the energy in the biosolids into biogas,” Shaw said. “However, the process requires steam, so some of the biogas that is generated typically is used for steam production.”

As the number of facilities that use thermal hydrolysis grows, those considering the technology will have more opportunities to see it in operation first-hand and understand both the challenges and successes.

“Incorporating THP isn’t a panacea, and it’s not right in every situation,” Shaw said. “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. Adding THP allows facilities with existing digesters to more than double their throughput capacity.”

He added that utilities with relatively high residuals management costs can benefit from a process that reduces biosolids mass and volume.

“Generating a better, more valuable end-product can increase beneficial reuse and reduce management costs,” Shaw stated.

Published originally on Black & Veatch Solutions.