Continuing advances in wastewater-to-energy technology
September 24, 2020
How thermal hydrolysis accelerates digestion to efficiently process wastewater while recovering valuable resources
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The consequences of climate change include deadly droughts, forest fires, high intensity storms, floods, and more frequent extreme weather events. But one of the most significant ramifications of climate change isn’t always in the news: The global water crisis.
Not only do millions of people lack access to clean water, but most of the water being used is returned to clean water sources as is. In fact, the UN estimates that approximately 1.8 billion people use a source of drinking water that has been contaminated with under-treated (or untreated) wastewater.
One of the ways we are combatting this challenge is through One Water—a concept that integrates all forms of water as a shared resource. A key aspect of One Water is, of course, wastewater. How can we safely process wastewater? How can we get the most value out of it? Over the years, engineers have made good progress on recovering resources, but there is opportunity for more.
As improvements and (perhaps more importantly) attitudes change, industry professionals are progressing towards a near paradigm shift—treating wastewater as an opportunity and less like a requirement. Indeed, in addition to valuable water in wastewater, there are significant organic resources we can reuse in many ways. But how do we extract those resources? How can we use them? This blog will explore how thermal hydrolysis is able to effectively treat those recovered organic resources.
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One of many steps in the wastewater treatment process leading towards thermal hydrolysis.
What is thermal hydrolysis processing?
The way in which we manage our wastewater has gone through a remarkable transformation since the ancient Babylonians first developed the cesspit around 4,000 BCE. That was quite commonplace until about 100 BCE when the Romans invented latrines to safeguard sanitation. Now, in more developed communities, wastewater is transferred from residential, commercial, and industrial enterprises to the wastewater treatment plant (WWTP). Once there, through a series of biological, chemical, and physical processes, the suspended and soluble matter is separated from the liquid wastewater.
Next, the organic solids are transferred to an anaerobic digestion system and converted to biogas. The remaining residuals, known as biosolids, are used as a soil amendment. Anaerobic digestion is where thermal hydrolysis processing (THP) can make a big impact.
THP uses heat and pressure to convert the anaerobic digestion feedstocks more efficiently into biogas. Although there are several process variations on the market, the process typically injects steam into a reactor with highly concentrated organics to hydrolyze and solubilize the long-chained carbon molecules. THP offers:
- A faster digestion process than conventional treatment methods
- An intensified anaerobic digestion process resulting in much less volume
- Faster conversion of organics to biogas
- A finer residual product with no pathogens and significantly less pathogen indicators
- A high quality biosolids product that dewaters to 28% dry solids with 90% less odor
- A reduced quantity of reusable product enhancing the concentration of valued nutrients and lower operating costs.
A look at a thermal hydrolysis, heat exchanger, and biogas storage system at the Crawley Sewage Treatment Plant.
How does THP fit into the overall treatment process?
To better understand THP, lets break the wastewater treatment process down into five key reactions:
- Raw Sludge Screening, Thickening, and Pre-Dewatering – Wastewater arrives at the WWTP, but there’s A LOT of it. The solids are separated from the liquid using a thickening process to reduce the overall volume of sludge, preparing it for dewatering. Thickened sludge is screened by a six-milimetre strain press prior to dewatering. The THP process requires a feed concentration of 15-18% dry solids
- Thermal Hydrolysis – Before digestion, steam is injected into the sludge and processed for approximately 30 minutes in a batch reactor. Heat recovery from the combined heat and power system can be more complex because for THP it must be in the form of steam. After the batch heating process, disinfected sludge is flashed to a final reactor which suddenly releases pressure and further hydrolyzes the material. Once through the thermal hydrolysis process, sludge is transferred to the anaerobic digestion system
- Advanced Digestion – Next, a sequence of biological processes breaks down biodegradable material even further. Biogas is created as a result and is used to power co-generation systems or produce vehicle fuel. The THP process achieves Class A biosolids. It also accelerates the bioconversion process so the anaerobic digesters do not have a shorter solids retention time as compared to conventional digestion systems. The feed concentration and lower solids retention time significantly reduce the anaerobic digester volume required. The rate of digestion compels owners who want to maximize biogas capture to consider series digestion or mechanical systems to force the biogas out of the sludge prior to dewatering
- Final Dewatering – Thermal hydrolysis further enhances the process of separating the digested solids from the liquid, reducing the overall quantity of biosolids products. THP sludge dewaters very well with belt filter presses, which require less energy as compared to other liquids/solids separation equipment, further minimizing parasitic power requirements
- Biosolids production – THP produces Class A EQ organic material that we can use in many ways improving soil health, reducing landfill disposal of valuable organics, and reducing greenhouse gas emissions.
Provided by Science Direct, the following chart visualizes the entire the treatment process and highlights the benefits we can leverage out of it.
This chart shows the typical energy balance for plant processing 10,000 tons dry solids sludge with a primary activated sludge ratio of 60:40. Both primary and activated sludge are thermally hydrolyzed. Energy balance is based on the use of an internal combustion engine.
How have we implemented THP before?
Stantec has been providing THP and wastewater engineering services for more than a decade in Australia and the UK (where no other consultant has done more!). With increased interest in the process now in the North America, we’re implementing this technology with strong support from our veteran specialists in the UK. To highlight how our expertise in THP and wastewater engineering is rising to global demand, I present two examples below—one in Canada and one in the United States.
- The Bonnybrook Wastewater Treatment Plant Expansion Program (Alberta) is one of the largest biological nutrient removal (BNR) plants in Canada and the largest cold-weather BNR plant in the world. The $500-million project includes upgrades to all systems, including secondary sludge thickening, secondary sludge dewatering, thermal hydrolysis of dewatered waste activated sludge (WAS), sludge digestion, and biogas management. This WAS-only system is the first of its kind in North America, making it the most energy efficient THP in operation.
- The Piscataway WRRF Bio-Energy Project (Maryland) is the largest and most technically advanced project that the Washington Suburban Sanitary Commission (WSSC) has ever constructed. It will be among the first in the US to incorporate advanced THP. The $250-million project will convert wastewater biosolids into renewable natural gas, reduce the facility’s greenhouse gas emissions by 15%, decrease WSSC’s costs to transport and dispose of residual products, and ultimately, manage wastewater biosolids more effectively. To maximize the value and allow for future energy production options, WSSC will produce heat and power for the THP system and the entire facility fueled with imported natural gas.
The Bonnybrook Wastewater Treatment Plant Expansion Program (Alberta) is one of the largest BNR plants in Canada and the largest cold-weather BNR plant in the world.
The future of wastewater engineering
The world’s water crisis is already upon us. The earth’s temperature is getting hotter and the global population is on the rise. This means we must find ways to combat our environmental challenges—and fast.
We know that effectively treating our wastewater is a key barrier to our success in defeating water-related challenges. By applying THP to our wastewater engineering practices, we can salvage more reusable water and procure more organic resources in a more efficient fashion. And not only does THP help to remedy our water woes, but it also reduces the amount of greenhouse gases released into the atmosphere.
We all must work towards a more sustainable future and innovating our wastewater treatment practices is only one of the many tasks we have ahead of us. If we can do it right, we’re one step closer towards a greener—or bluer—future.
