Penerapan prinsip pencegahan pencemaran di pabrik kelapa sawit (studi kasus: penggunaan sumberdaya air pada pabrik kelapa sawit sei mangkei, kecamatan bosar malinggas, kabupaten Simalungun, Sumatera Utara) = The Application of Pollution Prevention Principles in Palm Oil Plants (Case Study: Water Resources Utilization at Sei Mangkei Palm Oil Mill, Bosar Malinggas Sub-district, Simalungun District, North Sumatera)