LISBON, Dec 19 (Reuters) - Portugal's Galp Energia expects to begin producing biodiesel and biojet fuel from waste in an system it is constructing at its Sines refinery in 2026, the company, which is mainly an oil manufacturer and refiner, said on Thursday.

In 2015 Galp entered into a 75%-25% joint endeavor with Japan's Mitsui to invest 400 million euros ($415 million) in the Hydrogenated Grease (HVO) plant, which will have a production capacity of 270,000 metric tons per year.

It will change waste products, such as used cooking oils, into sustainable biodiesel and biojet fuel, also referred to as sustainable aviation fuel (SAF), utilizing green hydrogen produced by an electrolyser powered by wind or solar energy.

Galp said in a declaration it received on Wednesday the 3 reactors to process biofuels, which will enable the HVO unit to "produce SAF and biodiesel in 2026 ... placing Galp at the forefront of developing low-carbon services vital for the energy transition".

Galp is likewise investing 250 million euros in a 100 megawatt (MW) electrolyser system to produce green hydrogen to power the refinery in Sines. ($1 = 0.9637 euros) (Reporting by Sergio Goncalves