Green sewage plant could be powered by its own waste gas

Engineers in the US are using rocket-thruster technology to create a self-powered sewage treatment process that also reduces greenhouse gas emissions.

Using microbes to break down sewage produces nitrous oxide and methane, which contribute to climate change. Now a team at Stanford University in Califronia hopes to increase production of these gases so the methane can be used as fuel and the nitrous oxide can be used to generate heat and electricity.

Mogden Sewage Works in the U
Mogden Sewage Works in the UK

Reducing the oxygen supply to the bacteria increases production of these substances, which are very potent greenhouse gases. Instead of being allowed to escape into the atmosphere, the methane could power the treatment plant while the nitrous oxide could be broken down into nitrogen and oxygen to release energy in a process also used by rocket thrusters.

‘Normally, we want to discourage these gases from forming,’ said Craig Criddle, professor of civil and environmental engineering and senior fellow at Stanford’s Woods Institute for the Environment.

For too long we’ve thought of treatment plants as places where we remove organic matter and waste nitrogen. We need to view these wastes as resources, not simply something to dispose of.

Prof Craig Criddle, Stanford University

‘But by encouraging the formation of nitrous oxide, we can remove harmful nitrogen from the water and simultaneously increase methane production for use as fuel.

‘For too long we’ve thought of treatment plants as places where we remove organic matter and waste nitrogen. We need to view these wastes as resources, not simply something to dispose of.’

Stanford professor of aeronautics and astronautics Brian Cantwell is developing a decomposition cell that breaks the nitrous oxide down in the same way rocket thrusters do.

The reaction, triggered by heat and a catalyst, also produces enough energy to heat an engine to almost 3,000° Farenheit so can become self-sustaining and used to generate electricity using, for example, a Stirling cycle engine.

‘Several factors determine whether the reaction is self-sustaining including the concentration of nitrous oxide in the gas stream entering the decomposition cell, the physical size of the cell, and the heat losses from the cell,’ Cantwell told The Engineer.

‘In some of our thrusters we do away with the catalyst altogether and simply use a short duration heater to initiate decomposition and heat up a pebble bed that then acts as a thermal reservoir to maintain the reaction.’

To control the gas production from the sewage treatment, the team needs to encourage different bacteria to grow. Traditional methods use costly and energy-intensive aeration to supply oxygen to aerobic species of microbes that produce nitrogen from ammonia.

Reducing the oxygen will instead favour anaerobic species that release more nitrous oxide but leave more organic matter for microbes that produce methane. This could increase methane production by two or three times and also reduce costs, as aeration accounts for up to half of sewage plants’ operating expenses.

Cantwell and Criddle hope that their research could help provide clean water in Third World countries where energy supply is limited. They also think it could be used to recover energy from nitrate-contaminated groundwater beneath fertilized agricultural fields.

The team has been tackling the issue for over a year and the work has so far cost $110,000 funded through the Woods Institute. It’s currently working to increase the efficiency and rate of nitrous oxide production from ammonia, one of the biggest challenges of the whole project.

‘Scale-up in this field generally requires several years,’ said Cantwell. ‘Fortunately, there are several groups interested in helping with that step… In some ways the nitrous decomposition step is easier at a larger scale because of reduced thermal losses.’