A desalination system (100) having an intake unit (110) providing seawater to a pre-treatment unit (120) connected to a reverse osmosis (RO) desalination unit (130) and a post treatment unit (150). The desalination system (100) is configured to operate without any external addition of chemicals to simplify logistics and regulation concerns. The units of the system are configured to prevent biofouling, scaling and corrosion by mechanical and biological means including high flow speeds, biological flocculation of colloids, and making the water entering the RO units inhospitable to bacteria and other organisms that cause biofouling, hence preventing their settlement and removing them with the brine. Recovery rate is lowered and energy is recovered to increase the energetic efficiency and minerals that are added to the product water are taken from the brine.