Sludge-powered sewage works cut costs
Sewage works could be generating enough electricity from sludge to power themselves and sell surplus kilowatts to the grid, if trials of a cutting edge anaerobic digestion process end in success.
The new method substantially increases the amount of methane generated from anaerobic digestion – a process which decomposes sewage sludge or other organic matter to produce biogas that can be burnt as fuel – and does so at a reduced cost, its inventors say.
Ongoing trials at a Yorkshire sewage treatment plant held together with Yorkshire Water are watchedc losely by other water companies and monitored by Imperial College London, with initial results expected by the end of the year.
The process, called “Bug Busters,” uses carbon dioxide generated in the digestion to treat sewage sludge by compressing and ‘exploding’ it into a more homogenous matter, thus letting bacteria decomposing it penetrate more easily.
“The process involves pumping pressurised gas into the sludge where it dissolves. The pressure is then rapidly reduced, allowing the gas to expand and explode the solids in the sludge,” explained Martin Jolly, technical specialist at Earth-Tech Engineering, the Yorkshire-based company involved in the trials.
The technology gives more methane, more electricity, and less residual waste than similar processes so far developed.
“Our intial tests have shown that we create between 30 and 40% more biogas in the anaerobic digestion. There are other forms of self-disintegration [‘exploding’ the sludge to increase efficiency] around but they require a lot of power,” said Clive Rigden, managing director of Eco-Solids International, the company behind the process.
“Typically the water companies that already use their biogas to generate electricity don’t produce enough to operate the sewage works,” he told edie.
Anaerobic digestion is promoted on environmental grounds as it reduces the need for landfill and can also produce renewable energy, but is still relatively rare in the UK.
Costs remain a barrier, but the process now being tested in Yorkshire could help alleviate this problem by giving a pay-back period of “well under two years.”
“These technologies are pretty recent anyway – the existing systems have very high capital costs, and they haven’t performed as well as expected,” Clive Rigden said.
“It is a payoff between complexity, cost and what you get back. What our system is offering is that we can get these benefits at a much reduced capital cost and with a simple automated process that won’t require specialised personnel,” he said.
Eco-Solids’ process can be retro-fitted to existing sewage works, and could also work with other organic matter, for example from paper production processes.
If successful, the commercial trials could bring many more water companies on board.
“We have a host of British water companies monitoring the Yorkshire Water project and participating in it, and a number have said that if our initial trial data will be replicated in the commercial field they will be customers,” Clive Rigden said.
The trials are expected to continue for 6 months.
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