Davyhulme: The story behind Europe's largest THP plant

Beset by unique and complex challenges relating to stricter sewage sludge management regulations, United Utilities set its sights on a lasting solution by utilising the strategic position of its wastewater treatment works in Davyhulme. By upgrading to advanced anaerobic digestion using thermal hydrolysis at this one plant, the utility solved sludge management for several of its facilities. Learn how the utility maximised the use of existing assets and made a flexible biosolids strategy.

United Utilities is one of the largest utilities in the UK and a publicly listed company that serves over 7 million people in Northwest England. It was in 2013 that the utility shifted to advanced digestion at its largest treatment plant at Davyhulme. This move helped transform a significant portion of its biosolids output from mostly conventional or standard treated biosolids to enhanced biosolids, the equivalent of Class A biosolids elsewhere in the world. As a result of increased amounts of sludge being treated with advanced digestion, the revolutionary utility today produces 107 GWh  of renewable energy per year from sludge.

An aerial view of the Davyhulme wastewater treatment works showing the thermal hydrolysis system

Exactly how United Utilities (UU) adopted advanced anaerobic digestion for its Davyhulme plant is a remarkable story, as the utility's challenges in the past few decades were not similar to those of its neighbours or the typical wastewater utility.

Unites Utilities’ sludge treatment strategy in 1995-2010

 The strategy of United Utilities in the early 1990s included a 100 km-long pipeline that ran from Manchester to Liverpool, which would funnel a large chunk of its digested liquid sludge from various plants across the line to a boat at Liverpool harbour before it was taken out to sea. However, when the ocean dumping of sewage sludge was outlawed in 1998 by the European Union's Urban Waste Water Treatment Directive, UU needed to find another outlet for most of its biosolids.

United Utilities resorted to a mix of incineration and land bank application, as fueled by several drivers. Incineration became an option for a small fraction of UU's sludge that could be funnelled through its pipeline to a purpose-built incinerator at the Mersey Valley Processing Centre (MVPC), also often referred to as Shell Green. The remainder and majority of UU's sludge was land applied following the standards set out in the British Retail Consortium's Safe Sludge Matrix. However, United Utilities’ facilities were located farther from agricultural land than the average utility and, therefore, faced more barriers to land application.

United Utilities covers the North West region of England, as seen in the map above. Source: Drivers and performance of Europe's Largest Thermal Hydrolysis Plant at Davyhulme (Cambi Academy Webinar, 2021)

After 2005, the utility had established seven liming sites north of Manchester and Liverpool to treat around 25% of its biosolids nearer where agricultural landbank was available. However, in addition to increasingly stringent regulations surrounding land application, UU's biosolids also faced competition from the manure of dairy farms and greenfield development in the region. These challenges prompted UU to consider a second incinerator at Preston, Lancashire, that could replace these liming sites, but this plan would shortly be reconsidered.

Motivating Factors for Advanced Sludge Treatment

Many drivers for plants across the UK affected a rise in the adoption of advanced anaerobic digestion at the turn of the 21^st century. The interest in biosolids or sludge recycling began to open doors for advanced anaerobic digestion at utilities across the country as drying plants and incinerators were used less or shut down entirely.

For UU, shortly after considering the second incinerator, it became apparent that utilising advanced digestion could be a more effective route to achieving their goal of maximising existing assets and balancing their biosolids outlet approach between incineration and recycling to agriculture.

Other drivers that encouraged the utility to forego a second incinerator included the rising cost of fossil fuels at the time. The Water Services Regulation Authority (Ofwat) of England and Wales also started requiring utilities to measure their greenhouse gas emissions within the decade as the United Kingdom began to shift its footing with regard to its energy mix. Investing in advanced digestion would not only increase renewable energy production for United Utilities but also diminish the public concern surrounding incineration. In addition, the utility could abide by the enhanced treated sludge requirements of the Safe Sludge Matrix. Its biosolids output could then be applied to the region's grasslands, replacing fertiliser which was suffering from a price hike at the time. 

Why choose Thermal Hydrolysis for Advanced Anaerobic Digestion?

Though it was clear that the economic and environmental circumstances laid a strong foundation for the need to upgrade United Utilities' digestion system, there were many ways and technologies that were already promising at the time, including biological and thermal hydrolysis. United Utilities put together a multi-expert panel including members from the utility, environmental consultants from Entec, and incineration consultants from Montgomery Watson Harza (MWH Global) to determine the most suitable technology to increase digestion capacity and dewaterability and, therefore, optimise incineration at MVPC. The panel set out to screen more than 30 technologies and configurations over two to three years, with the selection criteria outlined below.

 The selected technology or scheme must:

• Maximise use of existing assets
• Significantly increase renewable energy generation
• Reduce the need for incineration
• Reduce United Utilities' operational carbon footprint and environmental impact
• Produce a consistently high-quality biosolids product suitable for recycling in the North West of England
• Reduce customers' bills/rates
• Provide full contingency during maintenance periods
• Align fully with United Utilities' long-term sludge strategy

After the comprehensive review, the panel identified the thermal hydrolysis process (THP) as the option with the lowest life-cycle cost that would achieve improved digestion and dewaterability and subsequently increase incineration capacity at MVPC. They determined that applying the technology at the most strategic location on the pipeline, i.e., Davyhulme, could mean significant capacity expansion without building extra incinerators or digestion tanks. Davyhulme contributed two-thirds of what needed to be processed by MVPC, so treating Davyhulme's sludge meant less biosolids (increased loading capacity) with higher dry solids content (improved combustion capacity) entered the pipeline to travel to MVPC. On top of this, installing thermal hydrolysis prior to anaerobic digestion at Davyhulme meant increased digestion capacity at the site as well, so the plant could receive the sludge previously meant for the liming sites.

The resulting project was launched as the "Sludge Balanced Asset Programme" or SBAP, the contract for which was awarded to Black & Veatch. The supplier chosen for thermal hydrolysis was the pioneer and leading provider of the process technology, Cambi. The Norwegian company's thermal hydrolysis process (THP) systems already had several reference installations in the UK at the time of SBAP, but to date, it has grown to process over 50% of the UK's sewage sludge.

The method uses a compact system that heats raw and/or waste activated sludge at a specific temperature and pressure range (around 165 

Installing a THP unit at Davyhulme enlarged the loading rate of the anaerobic digesters by over three times at the site. It increased the dewaterability of the biosolids, freeing up incineration capacity at MVPC once it was transferred there via the pipeline. It also equated to 50 GBP million in savings in capital expenditure costs. Biosolids previously meant for UU's satellite liming sites were then rerouted to Davyhulme for thermal hydrolysis, followed by anaerobic digestion and dewatering. The addition of incineration and digestion capacity ultimately gave United Utilities flexibility as it could do the following:

• treat a portion of its sludge with advanced digestion and have the dewatered enhanced biosolids product land-applied from Davyhulme
• transfer hydrolysed, digested liquid biosolids via pipeline (mixing with other sludge in the pipeline) from Davyhulme to MVPC, then dewater and land-apply it there as treated biosolids, or
• continue to incinerate digested biosolids at MVPC

Sludge sites and strategy before and after the adoption of Cambi’s THP technology at Davyhulme. Source: Drivers and performance of Europe's Largest Thermal Hydrolysis Plant at Davyhulme (Cambi Academy Webinar, 2021)

Several other benefits from the utilisation of thermal hydrolysis were observed just after a year of operations, including the following:

• Annual operation savings of around 2-4 GBP per year 
• Overachievement of energy targets, producing 38,000 m³ of biogas daily
• Minimisation of CO2 emissions versus other AAD solutions
• Reduced emissions in the long term due to lesser quantities of biosolids for land application or incineration
• An enhanced biosolids product (considered Class A) that can be used on grasslands, with up to 32,000 tonnes of dry solids (tDS) recycled to land per year

The SBAP resulted in Davyhulme becoming Europe's largest thermal hydrolysis plant. The project received multiple awards and reduced United Utilities' carbon footprint by 8%, a vast improvement considering the utility's size. The sludge line at Davyhulme has gone on to be named the Manchester Bioresources Center.

As United Utilities enters another decade, it plans to move toward the land application of 100% of its sludge, with incineration as an option for periods when agricultural land application is not available. The utility has evolved to be one of the most mature water and wastewater networks utilising systems thinking to improve the balance and flow of sludge within a network of over 30 sewage treatment facilities. United Utilities was also key in the development of the UK’s Biosolids Assurance Scheme, which was launched in 2017. It is a regulatory framework that affects positive perception and trust in the land application of biosolids.

Want to learn more about the advantages of thermal hydrolysis for this type of thermal treatment? Read this blog article on THP and sludge incineration.

"This article is based on the 2021 Cambi Academy Webinar, 'Drivers and Performance of Europe's Largest Thermal Hydrolysis Plant at Davyhulme.'