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Northumbrian Water's anerobic digesters use special bacteria to break down part of that waste into fertiliser and methane gas. The methane will then be used to fuel a power station on site.

The Bran Sands site, at Teesport near Middlesbrough, treats 300,000 cubic metres of waste every day, equivalent to that produced by three million adults. The waste passes through a series of processes, before being used to generate energy.

Treatment

Preliminary Treatment involves removing litter and large objects, known as 'rag', from the effluent. This is followed by Primary Treatment, where solids are settled out of the mass. At this stage, were the waste to be discharged into a water course, it would consume all the oxygen in the water, effectively suffocating any wildlife.

Sewage tanks at Bran Sands.

The waste then enters the aerobic phase of treatment, in which air is bubbled through the effluent, allowing bacteria to break it down and reduce its biological oxygen demand.

At this stage the sludge remaining is typically removed, treated, dried and sold off as fuel and fertiliser.

Thermal Hydrolysis

Once the new anerobic digestion system is in place at Bran Sands, the sludge will instead be put through a process known as 'thermal hydrolysis', which will see it pre-heated, fed into a reactor, where steam is used to raise it to a temperature of 165C and pressure of 6 Bar, breaking down the fabric of the sludge and sterilising any pathogens within.

The sterile sludge is then fed into the digesters; 75 foot silos, filled with bacteria that feed on the sludge for 17 to 18 days, during which they produce fatty acids, carbon dioxide and, more importantly, methane gas.

It's this methane that can be burned on site in a station that, Northumbrian Water estimates, will produce around 40GWh of electricity every year.