KLM to power European flights with used cooking oil

Used cooking oil from factories and hotels will fuel KLM flights between Amsterdam and Paris beginning September.

Why are these articles always so very light on details and math? If a Boeing 757 burns 4200 liters of fuel per hour, how much used cooking oil would it take to power all of KLM's flights for one day?   How much energy and money would it take to collect and refine the used cooking oil?