Novel dairy waste disposal process offers significant savings on anaerobic digestion

Lindhurst Engineering has successfully tested microbial fuel cell (MFC) technology to help dairy processors dispose of waste streams and generate energy, and MD Martin Rigley said that an installation could cost 75% less than its anaerobic digestion equivalent.

Working with scientists from the University of Nottingham, dairy processor Arla Foods and treatment systems company Clearfleau, Rigley said Lindhurst now planned to conduct three large-scale trials with food firms later this year, with a view to developing a commercially viable production model.

Currently, dealing with organic content in industrial effluent is expensive and wastes potential energy contained therein, but - via bacteria converting slurry or dairy byproducts into CO2, water and energy- Lindhurst said MFCs could harness energy (from hydrogen-rich biogas) using anodes and cathodes.

Uses less energy

Rigley said: “The traditional way of releasing energy from waste material is via anaerobic digestion. Our system differs from that in that we use less energy to release the energy contained in that waste, and the time taken to release that energy is shorter.

He added: “We have a microbial fuel cell – a tank with microbial fuel inside – and we pump waste organic matter into the tank at a very low rate. Prior to starting the process, we’ve introduced a bacterial culture into the tank. Due to the construction of the microbial fuel cell, we are able to assist the bacteria in the digestion process far more quickly.

“One of the by-products produced during digestion is hydrogen-based biogas, and the digestion process in 24-48 hours (unlike anaerobic digestion where it can typically take 2-3 weeks)," Rigley explained.

“We need a far smaller tank than anaerobic digestion and – since you need to introduce heat to get the bacteria to do their work – we are working at a far lower temperature than for anaerobic digestion. So we’re putting less energy in all round.”

Successful scale-up

Successful trials have been conducted using a 1,000 litre pilot unit on a Nottingham University-owned farm, and the team is now developing a pre-treatment process that will allow MFCs to take solid food waste as well as waste water; it believes that a production-sized cell fed slurry from 200 cows could supply a farm with a large proportion of its annual energy needs.

Rigley said: “What we’re able to do – irrespective of media, slurry, waste milk, waste organic matter – is greatly reduce the COD [Chemical Oxygen Demand – a test to determine organic pollutants] level by pumping it through the microbial fuel cell. So Arla was initially interested because they pay a ‘consent to discharge’ fee to the Environment Agency.

He added: “By reducing the COD level we’re not saying at this stage that we can negate these costs, but we can greatly reduce them. It very much depends upon the media you feed into it, its original COD levels, what the output’s going to be.”

“We ran the pilot project until December 2010, and we were excited because on a 1000 we replicated laboratory results – so we proved that it was scaleable. What we don’t know now is what the optimal size of cell – you won’t have one huge tank on a farm but a number of cells,” Rigley said.

“This keeps the footprint down but also means you can take cells offline for cleaning. It’s also worth saying that at this stage we don’t envisage it being maintenance intensive, and it’s a far cheaper alternative than anaerobic digestion. It’s difficult to estimate costs at the moment, but my gut feeling at the moment is that it could be as little as 25% of the cost of an AD plant.”

Managed by an industry and academic consortium, Nottingham-based Food and Drink iNet has provided a £154,000 grant to assist research into the MFC pre-treatment process.

Source: Lindhurst Engineering