The story says:

A single tank of the gas, which is produced using the typical annual waste of five people, is enough to power the vehicle for 190 miles.

So the annual waste of one person would move the bus 38 miles.

How much poo would be required to move a fleet of a thousand buses at 12 mph through a conurbation for 18 hours a day? 216 miles x 363 days = 78,408,000 miles. Divide by 38 miles per person = 2,063,3684 people's annual waste. Unfortunately, there's only 1.1 million people in Tyne and Wear.

In other words, to make Tyne and Wear's buses 100%  poo-powered you'd need twice the amount of poo that the population can produce....

... unless we all swapped to a very high-fibre diet...

Somebody else care to check my maths...?