In its article on the giant oil skimmer tanker, the BBC says it can process 21 million gallons of oily waste water per day. http://news.bbc.co.uk/2/hi/world/us_and_canada/10502957.stm
As usual, when a British source reports American news, the question arises, what kind of gallons? An AP article uses the same figure, so I assume they are US gallons. I further note that 21 is one half of 42. I strongly suspect the figure is a one significant figure 500 000 barrels per day, using the petroleum barrel of 42 US gallons. I wonder how oily the water is? If they process 500 000 barrels of waste water per day, how many barrels of oil will they recover from it? Shouldn't the BBC clarify what kind of gallons it is using? The difference in Imperial and US gallons is generally known, but it is very unclear what they have in mind in this case.
