Eighty per cent of the world's wastewater is dumped untreated
leading to poor sanitation and disease.
This is a particular problem in Bangalore in India,
where the lakes contain so much sewage they catch fire.
Treatment plants can be expensive to run.
Waste is usually broken down by bacteria that feed on oxygen provided by costly blower motors.
Bangalore resident Tharun Kumar looked to cows for a solution.
These master digesters have four stomach chambers containing bacteria that don't need oxygen.
As grass passes through, the bacteria in each chamber break it down into smaller parts,
converting it into nutrients, gas, water and waste.
Tharun and his team designed a treatment plant to mimic this.
It uses bacteria from cow dung, so no need for blower motors.
In fact it doesn't require any power, as gravity moves the sewage along.
The further it travels, the cleaner it gets as solid waste sinks to the bottom
and the rest is converted into gas and clear water,
which is safe to use outside.
Fifty treatment plants have been built and the team plan to export the design to Africa.
Could cows help solve the world's sewage problem?
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