Breaking new ground in biogas production

A two-strong team at Thames Water has been testing a THP process at Basingstoke treatment works. Getting the 'best possible picture' of what was happening was essential.

Spending two years fine-tuning the efficiency of a pioneering sludge treatment process might not appeal to everyone, but for Ester Rus Perez and Aurélien Perrault of Thames Water, it has been an exciting journey of discovery that promises to break new ground in generating energy from sewage.

Based at Thames Water’s Basingstoke treatment works, the two-strong team are involved in testing an experimental plant based on an adapted thermal hydrolysis process (THP). Extensively fitted with instruments from ABB Measurement and Analytics, the plant is helping to generate a wealth of data that is being used to further the team’s understanding of the process and identify ways of refining it further.

THP is an established process used in conjunction with mesophilic anaerobic digestion (MAD) to treat and dewater sewage sludge.

In a conventional THP process, sludge first passes through a hydrolysis phase, which breaks down large molecules. During this process, steam is injected into the sludge to maintain a high temperature and pressure that helps to break down material such as cell walls and very large molecules. This in turn makes it easier for bacteria to digest the sewage and produce extra gas. After hydrolysis, the sludge is then treated in the anaerobic digester where bacteria digest part of the organic matter.

Together, these processes produce a sterile, relatively easy to dewater “cake” that can be sold as fertiliser; together with biogas, which can be used to generate electricity and heat.

Although the THP process offers greatly improved efficiency in the conversion of organic matter into biogas compared to MAD by itself, around 50% of this energy-rich sludge is still recycled to land. As well as representing lost potential energy, this residual sludge may also require hundreds of lorries to transport it to agricultural sites where it can be used as soil amendment.

The plant at Basingstoke takes this process one step further by incorporating an additional intermediate stage into the process. Known as intermediate thermal hydrolysis (ITHP), this process incorporates an additional MAD stage before the thermal hydrolysis, allowing for additional organic matter degradation and energy recovered from sludge.

The ITHP process was discovered by chance by Achame Shana, Thames Water’s lead technical consultant on waste treatment and recycling processes, when searching for a solution to the problem of post-digestion stage odours. His idea was to remove the pathogens that were causing the odour by running the digested sludge through a THP reactor and then back through another digester. In doing so, he noticed an increased amount of biogas was being produced as the organic matter was further broken down. From this, the idea of ITHP as means of maximising biogas production was born.

In initial tests, the ITHP approach proved to offer improved efficiency, with an overall volatile solids reduction of 66 to 68% as compared with 55 to 60% from a conventional THP process.

The Basingstoke plant has been built to test the viability of the ITHP approach at larger scale. Built in 2013, the plant was under test until the end of 2015. As a pilot plant, the facility is heavily instrumented to help gather as much operational data as possible. In order to test the limitations of the process, experiments are run under a variety of different operating conditions.

“Before the ITHP process can go to commercial scale, there is a lot of work to be done in testing how it works, how well it works and what can be done to make it work better,” says senior research engineer Ester Rus Perez, the plant manager for the ITHP pilot plant. “This includes running the plant under both normal and non-standard conditions, in order to see how far we can push the process to get even better results.

“Over the past two and a half years, we have carried out a wide range of different tests and experiments in order to generate the data that we need to fine-tune the process and prove that it could work on a larger scale.”

To help provide this data, ABB has supplied a combined package of flow, pressure, temperature and level instrumentation that is used to help measure performance throughout the plant.

“As this is a pilot plant, we have more instruments than you’d normally have on a proven process,” says Aurélien Perrault, innovation sludge and energy manager for Thames Water. “In this way, we can get the best possible picture of what’s happening, which can sometimes produce some quite surprising information.”

By drawing on this expertise, ABB’s engineers were able to provide advice and support throughout the project, including guidance on the best types of instrumentation technologies. “I and my colleagues have received good support from ABB when we’ve needed it,” says Manocher Asaadi, of consulting firm AD Technologies, which made the initial decision to go with ABB at the outset of the project. “The support we have received from ABB has been excellent. Even at the tendering stage, Steve Wilding and his team were offering advice on ways we could improve things, which was one of the factors that convinced us to choose ABB.

“This same level of support has continued throughout the project. As this is an experimental plant, there were areas that we were not quite sure about. ABB worked with us to help us to iron out any uncertainties and was heavily involved throughout the development phase of the project.”

Together, ABB’s instruments help to provide a comprehensive picture of what is happening around the plant. ABB’s FSM4000 magnetic flowmeters are being used to both measure and control the flow of sludge between the tanks. The high level of solids in sludge can present a particular challenge when it comes to measuring flow. Conventional DC excitation magnetic flow meters are unsuitable for use with high-solids media, because the solids generate an unacceptable level of noise in the signal.

Widely used in pulp and paper processes for similar reasons, ABB’s FSM4000 flowmeters use AC excitation, which produces a much less noisy signal, resulting in more accurate flow measurement. In the Basingstoke installation, data from the meters is relayed to the plant’s SCADA system where it can be viewed and analysed to provide an immediate overview of performance. The data is also relayed to Norwegian company Cambi AS. As the owner of the patent on the ITHP process, Cambi uses the data to assess and optimise the performance of the plant.

The SCADA system can also be remotely accessed to enable conditions to be monitored off-site at any time of the day. “A few times, I have found myself dialling into the system late at night and even in the early hours to see what’s going on at the plant, especially when things haven’t been running as they should,” admits Rus Perez.

To maximise the efficiency of the thermal hydrolysis stage, the sludge from the mesophilic anaerobic digester is heated to 165°C for 30 minutes under 6.5 barg pressure. Using an ABB swirl meter enables the flow of steam to the process to be accurately monitored to help ensure that the correct amount is being supplied, with the treatment temperature itself being measured by two ABB temperature transmitters mounted on the THP tank.

The inclusion of pressure, temperature and level devices around the plant has helped to identify other challenges that can arise and how to solve them, including knowing which instrument is best at detecting particular occurrences.One example is the issue of foaming in the digestion tanks. 

“Increased surface tension caused by sudden changes in temperature or starting and stopping of the feeding process has frequently led to the formation of foam,” says Perrault. “We have to make sure we can detect and measure this as soon as it happens. The presence of foam not only causes problems if it gets into the biogas stream, but it can also reduce the overall efficiency of the process.”

It is therefore important to be able to detect and assess the extent of foaming and to use the data to put in place anti-foaming measures. During the trial, both pressure and level transmitters have been used to measure sludge levels inside the digestion tanks. Comparison of the two trends from the devices has shown that the difference between the two values was a good estimation of the foam level in the digester, which helped avoid further problems more than once.

“If it is shown that ITHP can work on a larger scale, all of the lessons we learn from this installation will be factored into the design of future plants,” says Rus Perez. “The data from the instruments is a valuable part of this, as it shows not only the impact of any changes we make in the operation of the process, but also gives us an idea of which instruments deliver the best results in terms of accuracy and the delivery of useful data.”

The Basingstoke ITHP plant is just one of a number of projects being undertaken by Thames Water’s Innovation team to optimise its water and wastewater treatment operations. If the plant is successful, it will form part of the company’s drive to increase its generation and use of renewable energy.

“It has been a slow and challenging process to get to this point, but one which has also been very revealing, producing valuable lessons and information for building and running future ITHP plants,” says Perrault. “For both myself and Ester, it has been great to be involved in developing and testing what could be a ground-breaking approach to producing biogas from sludge.”