Drinking water quality is very important for human health. Small amounts of hazardous contaminant are strictly forbidden. These undesirable contaminants cause diseases in humans and should be removed. The Badak natural gas liquefaction plant produces drinking water from deep wells by the Paterson and Candy conventional process. From the raw water quality analysis, it was found that the water contained some microbiological and organic materials. The water was treated mainly to remove iron and increase the pH of the raw water. This conventional process cannot remove organic matter in water. The excess chlorine from microbiological disinfection reacts with organic matter (THMFP) to produce trihalomethane (THM). These compounds are indicated as carcinogenic. Removal of organic material and heavy metals from water can be done by nanofiltration (NF) [1,2]. By combining the existing plant as a pretreatment system with a NF system, drinking water can be produced, free from dangerous contaminants such as THM and heavy metals, which satisfies the water quality standards of the WHO and Ministry of Health, Indonesia [3]. Conceptual design for NF for drinking water production is based on the simulation results of Hydranautic RO design. Data input for simulation is water chemical analysis from the pretreatment system, feed water flow rate to the NF system, recovery and drinking water product flow rate. A total investment cost of US$ 2,920,000 (equipment life time 10y), for production of 8,616,960 m(3)/y treated water, and a total water cost of US $0.03/m(3).