Treatment of WAS with Thermal Hydrolysis and Anaerobic Digestion

Williams, T.O., Burrowes, P., Fries, K., Newbery, C., Whitlock, D., CH2M, USA



CH2M is designing two installations where the thermal hydrolysis process (THP) is being used to treat waste activated sludge and to combine the hydrolyzed sludge with primary sludge prior to being fed to mesophilic anaerobic digestion. The technical reasons for each has proven to be different in the two case studies which are highlighted, one in Calgary, Canada and one in Singapore.

The THP plant in Singapore will treat an initial flow of up to 53 metric tons dry solids a day, and is designed to be expandable to meet the ultimate requirement of 70 metric tons dry solids a day by adding a 4th reactor. The THP system will treat both thickened and un-thickened WAS and concentrated grease waste. The thermally hydrolyzed sludge will be cooled by primary sludge and fed to 3 existing egg-shaped digesters. The total biosolids produced from these digesters to be dewatered on existing centrifuges will be 44.5 dry metric tons per day. Construction has begun in 2015 and the plant is expected to be operational in 2017.

For Calgary, CH2M teamed with Stantec and completed conceptual and preliminary designs. WAS will be thickened, dewatered, and subjected to THP prior to blending with thickened primary sludge. Preselection of the THP equipment is in progress after which detailed design will be completed. Construction will begin in 2016. This facility will have capacity to manage 50 metric tons dry solids from a PE of 1,237,000.

The manuscript provides details on the decision factors associated with each project, the expected benefits and compare the technical design and operating data for each.

Keywords:  Thermal hydrolysis, anaerobic digestion, waste activated sludge.


With ever increasing attention to cost, biosolids quality, and resource recovery in water resource recovery facilities focusing on biosolids management, the market has responded with the introduction of the thermal hydrolysis process (THP) as a pre-treatment to mesophilic anaerobic digestion (MAD), a relatively new and sometimes cost effective technology alternative. The high temperature and high pressure process (~160C and 6 bars for roughly 30 minutes) effectively lyses bacterial cells and possibly more importantly, disrupts the exopolymetic substances (EPS) in waste activated sludge (WAS) and the subsequent impact on dewaterability of the solids. Because of the high temperature, the solids are pasteurized to Class A standards as long as the time and temperature exposure can be verified. Subsequent anaerobic digestion stabilizes the solids to a low odor non-putrescible state meeting US EPA standards for Class A pathogen reduction and approved vector attraction reduction standards. Several vendors have developed or are continuing to develop their thermal hydrolysis technology offerings including Cambi, Veolia, and Haarslev. Today, more than 50 THP installations are in operation at domestic wastewater facilities around the globe. In the North American marketplace, emphasis has grown to produce Class A exceptional quality biosolids products to allow for more diverse end uses with less regulatory requirements and less public opposition as compared to traditional Class B quality biosolids which are primarily used in agriculture. For this reason, many utilities in North America are in the process of building or planning to install THP systems to process both primary and waste activated sludges prior to anaerobic digestion. In addition, THP with anaerobic digestion offers several advantages over conventional mesophilic anaerobic digestion alone. The advantages include:

• The ability to feed digesters at up to 10% solids due to decreased sludge viscosity, thereby reducing needed digester volume by half or more

• Increased volatile solids destruction and subsequently more biogas production in digestion

• Elimination of digester foaming problems

• Less polymer consumption during dewatering

• Higher dewatered solids content, typically 3-5% TS higher than without THP

• Elimination of pathogen regrowth or sudden increase phenomena after dewatering

• Lower cake odor

Although these benefits are significant, challenges with THP processes include:

• The addition of a high solids thickening/dewatering step prior to THP to achieve the desired solids content of ~16%TS prior to THP

• The need to provide low pressure steam which adds complexity and requires significant amount of biogas to fire a steam boiler

• Cooling demand on the hydrolyzed solids prior to digestion

• The need for sterile water for use in post THP processes such as dilution water, polymer addition, and wash waters in dewatering

As utilities consider means to minimize their energy footprint, maximize sustainability and to minimize capital outlay for solids improvements, other nuances to the application of THP have arisen. Specifically, THP has the most effect on WAS to produce the improvements listed above as primary solids are readily digested without the need for supplemental hydrolysis pretreatment. For these reasons, utilities and engineers are considering applying THP to WAS streams only instead of the entire sludge stream

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