In a bid to improve the efficiency of wastewater treatment facilities (WWTF), a growing number of wastewater companies have installed thermal hydrolysis plants (THPs) that enable an increase in the throughput capacity of anaerobic digesters and a rise in the production of biogas, as well as energy recovery from waste ‘sludge’.

Energy potential

As Charlie Walker, Research Director – Technology and Industry at Global Water Intelligence, explains, thermal hydrolysis typically involves three steps. First, waste-activated sludge from the wastewater treatment process is fed to a pulper, which homogenises and preheats the sludge. Next, the sludge is fed into a reactor, where the temperature is raised to around 180°C and pressure of around 5–6 bar, which kills pathogens. 

Last, the hydrolysed sludge is fed to a flash tank, where the sudden drop in pressure destroys cells for organic matter to more easily digest. The cooled sludge then goes into an anaerobic digester (AD).

According to Walker, releasing more of the energy potential of the biomass means increased biogas production, which can be harnessed to either help power some of the treatment processes at the WWTF, or can be upgraded to biomethane to be sold to the grid. 

‘THP can also increase release of phosphates in the sludge, making it easier to recover them as a nutrient source further downstream. A third benefit is that THP also improves dewaterability of the digested sludge – which is a big deal given the growing pressures on disposal, as more landfills do not accept organics, and there is more worry about spreading on land,’ he says.

Although typically used as an upfront digester to boost the digestion process, Walker observes that thermal hydrolysis can also be used post-digestion – for example, in the popular Cambi SolidStream process, which dewaters the hot digestate following thermal hydrolysis, and recycles the chemical oxygen demand in the filtrate back to digesters in a bid to increase biogas production. 

‘The key benefit of thermal hydrolysis is that it makes the anaerobic digestion process more efficient, releasing more of the energy potential of the biomass. Thermal hydrolysis has driven something of a revolution in sludge digestion. Northumbrian Water is probably the market leader,’ adds Christopher Gasson, Publisher at Global Water Intelligence.

Renewable fuel

A number of companies currently supply thermal hydrolysis systems, including French outfit Veolia, which has developed the Bio Thelys system that processes all types of organic municipal and industrial sludge, as well as grease in batch mode.

Another key supplier is Norwegian company Cambi, which has installed its systems at several WWTFs around the world. According to Dragos Tâlvescu, Marketing Director, WWTFs typically install their systems prior to an anaerobic digester, although other configurations are possible. 

In this ‘classic’ scheme, he says that wastewater sludge, once separated from the bulk of water, is thickened and enters the thermal hydrolysis system. The process breaks down the organic matter in sewage sludge and also ensures the destruction of harmful bacteria. The sludge then leaves the THP unit and, using heat exchangers, is brought down to a temperature fit for the AD.

For Tâlvescu, the key benefits of installing THP systems include higher biogas production and lower volumes and higher quality biosolid cake, as well as increased digester throughput and the production of useful biosolids.

‘THP results in more biogas than conventional digestion where anaerobic digestion is used without any advanced modification or pre-treatment method. The biogas produced is [also] ideal for green electricity, as a substitute for natural gas, or as renewable fuel for vehicles,’ he says.

Circular economy

One of the early adopters of THP technology is UK water company Severn Trent Water, which currently operates four THP plants at WWTFs in Birmingham, Stoke on Trent, Nottingham and Coventry. As Simon Farris, Bioresources Strategy & Commercial Lead at Severn Trent Water, explains, each of these plants has been coupled with biomethane injection facilities to maximise the amount of energy produced and supplied to the local grid network.

Severn Trent Water thermal hydrolysis and gas-to-grid plants at its Minworth, Birmingham site, UK

Photo: Severn Trent Water

‘The THP maximises the amount of raw biogas we produce and the biomethane plants maximise the energy we recover from that biogas. The biomethane currently gets injected directly into the grid, but we do see future potential for vehicle fuel, expanding the potential for the circular economy by producing the fuel that goes into the tankers that collect some of the waste we feed into the process,’ he says.

In view of the fact that, as a wastewater company, Severn Trent consumes large amounts of energy through moving and treating sewage, Farris observes that the process of renewable energy generation enabled by THP systems allows it to produce as much of the energy itself as it can. This creates the dual benefit of developing the circular economy, with the waste collected helping to treat itself by producing energy, and allowing the company to avoid buying energy from the grid and to lower its customers’ waste bills. 

‘The biggest challenge is managing the energy balance on the site to achieve the maximum efficiency. This treatment of sludge uses steam and power to ensure we recover more value, and as such we are always looking for ways to minimise what we consume whilst maximising what we produce,’ he says.

Bioenergy facility

Meanwhile, in the US, WSSC Water is in the process of constructing a combined thermal hydrolysis, anaerobic digestion and combined heat and power (CHP) system at the Piscataway water resource recovery facility (WRRF) in Accokeek, Maryland. When complete, the Piscataway bioenergy facility – based on a Cambi thermal hydrolysis system – will handle biosolids from five existing WRRFs. 

According to Brian Mosby, Division Manager of the Biosolids Management Division at WSSC, after installation the methane gas recovered from the digesters could be reused as an energy source to power clean-burning engines that produce electricity and heat needed for wastewater treatment plant operations and for sale back to the grid.

‘Thermal hydrolysis uses high heat and high pressure to disintegrate the cell structure of the solids, similar to pressure cooking a roast. This makes it easier for the solids to be digested in our anaerobic digesters,’ he says.

‘The Cambi system is one of the most popular THP systems – and is also used at DC Water’s WRRF, one of the largest THP installations in operation. The three main THP processes are preheating the solids to around 95^oC and thermal hydrolysing the solids in high-pressure reactors, before decreasing the pressure and temperature of the hydrolysed solids and pumping to the digester,’ adds Mosby.

For Mosby, the key benefit of using a THP system is that it allows the digestion process to work more efficiently, which helps to reduce sludge volume more rapidly. In doing so, it saves money in sludge hauling costs because there is less volume to haul and the process produces a Class A material that can be beneficially reused or marketed as a soil amendment and is subject to ‘less land application restrictions’.

‘The bioenergy project is one of the most complex projects that WSSC Water has ever undertaken, and we are about 60% into the construction. In the coming years our focus will be to optimise this facility and reduce our carbon footprint,’ adds Mosby.

Future prospects

Looking ahead, Farris reveals that Severn Trent Water has plans to build a fifth THP site in the next three years at Gloucester, UK This site is currently moving through design before the company goes to tender for solutions. 

‘I believe we will start to see more efficient generation of steam for the process being a key factor to improving efficiency further. We also need to focus on the quality of our final product going to land to ensure we can meet the ever-tightening expectations of our customers and environmental regulators,’ he says.

Meanwhile, Tâlvescu reveals that Cambi is confident in the increasing demand for reliable, sustainable solutions for sludge treatment from cities, utilities, and businesses – particularly since advanced AD and thermal hydrolysis effectively reduce greenhouse gas emissions and support water utilities in their strategies to achieve net-zero emissions while reducing life cycle costs. 

He also points to the company’s efforts to improve energy efficiency for its clients by improving and upgrading systems, as well as via a digital platform that allows it to remotely monitor WWTF systems and suggest improvements to optimise operations.

Ultimately, Walker observes that prospects for the roll-out of THP ‘look strong in the long term’, with the US emerging as ‘the latest market to recognise its value’.

‘In terms of energy efficiency, the heat mass balance usually differs at each site, but, as I understand it, energy is recovered from as many parts of the process as it can be to make such a high temperature process economical. Overall, it is the most commonly accepted sludge pre-treatment technology and has left alternatives such as biological and chemical hydrolysis, as well as mechanical operations like ultrasound, in its wake, concludes Walker.