Simulating sludge: a novel approach to visualising anaerobic digestion

After Dr Davide Dapelo completed his degree in theoretical physics in his native Genova, he took an EU-financed research course on renewable energies. This included an industry placement with a company that project-built anaerobic digesters.

While there, Davide realised there were ways to improve the anaerobic digestion (AD) mixing process, which gave him the idea for his PhD topic: the application of computer simulation of flow methods to a specific aspect of wastewater treatment. To pursue the idea, he sought out top academics working in this field, one of whom was John Bridgeman, Professor of Environmental Engineering at Birmingham – and as a result left Italy to pursue his academic career in Birmingham.

Since arriving at the University in September 2012, Davide has been making a name for himself in the area of computational fluid dynamics, focusing on the water industry, in particular wastewater treatment. Now a Research Fellow in the Department of Civil Engineering, he has had three journal papers published, with a fourth recently accepted for publication, as well as two conference papers, with three more in preparation.

His third journal paper, entitled ‘Positron emission particle tracking (PEPT): A novel approach to flow visualisation in lab-scale anaerobic digesters’, published in Flow Measurement and Instrumentation earlier this year, won the College’s Paper of the Month award.

Co-authored by Prof Bridgeman, Davide’s supervisor, and former Birmingham colleagues Rebecca Sindall and Tom Leadbeater, the paper explains how a new technique can significantly improve the efficiency of the AD mixing process.

‘There are several steps to the treatment of water or sewage and some of those involve the production of sludge, which has to be disposed of somehow,’ Davide explains. ‘The AD process is a good one, but it needs to be optimised.’

The process – the most widely used sludge treatment method – uses microorganisms, in the absence of oxygen, to break down organic material into carbon dioxide and methane gas.

‘Because you need to mix the sludge in order to carry the treatment process further, it consumes a lot of energy – and is therefore expensive. So, what we are trying to do is to find ways to mix it better in order to use less energy, and get a better balance between energy input and output.’

The problem is that sludge is an unpleasant material with which to work. Not only does it ‘stink’, but it is biologically and chemically hazardous and it is also opaque. This non-transparency makes it particularly difficult for scientists to understand how the sludge flows in an AD tank.

This is where computer simulations come in: Instead of performing difficult, expensive and dangerous measurements in real tanks, scientists recreate them virtually and simulate the flow pattern therein. To yield meaningful results, however, these simulations need to be referred to something measurable. Hence, before applying their models to full-scale tanks, researchers recreate lab-scale ones, compare the simulations with laboratory measurements, and eventually fix the defects of their models. This procedure is called ‘validation’.

‘The opaqueness of the material makes this problematic, so we had the idea to apply the positron emission particle tracking (PEPT) technique to measure the flow,’ says Davide. ‘To do this, we take a particle and make it radioactive, put it into a lab-scale tank of sludge and put it through a machine that detects radiation and tracks the particle radiation emission, and then we activate mixing. Through this particle motion, you reconstruct the motion of the flow. You can do this because even if the material is opaque, radiation passes through.

‘The point is that before, with other systems such as a laser, you couldn’t work with the sludge directly; you had to prepare transparent synthetic sludge. But, of course, it wasn’t real sludge.’

Davide and his fellow researchers carried out two lab-scale tests using the PEPT technique: one in which the anaerobic digesters were fitted with mechanical mixing apparatus; one in which they were fitted with gas mixing ones.

‘What we found was that PEPT successfully measured and described experimental flow patterns; that such flow patterns are in good agreement with previous – laser – techniques, and such patterns contribute to the validation of my numerical model,’ says Davide.

‘In short, the old technique confirms that the new one works, and the new one is better because it can measure the flow patterns of opaque material such as sludge, which the old method couldn’t.’

In the longer term, Davide hopes his research will lead to real-life applications. In his latest paper, he shows that energy requirements for the AD mixing process can be slashed by half.

‘Eventually, we think we can achieve even better levels of mixing, and so save even more energy, thereby considerably reducing the cost.’