Mobil's Arun Field in northern Sumatra produces natural gas, hydrocarbon liquids and water condensate. Purification of the water for surface disposal is the subject of this paper.

The Arun waste water contains about ppm of liquid hydrocarbons in the form of a very stable oil-in-water emulsion. Stability of the emulsion is enhanced by the small diameters of the oil droplets, low salt content of the water and low pH. The water is saturated with carbon dioxide and hydrocarbon gases which bubble from the water when pressure is released. Returns of acids, surfactants, emulsifying agents and corrosion inhibitors from acid fracturing operations also contribute to the composition of Arun waste water.

Increases in waste water production, now about 32,000 BPD, and relatively high concentrations of BOD, phenols and ammonia have prompted Mobil to upgrade the Arun waste water treating facilities to protect the receiving bodies of water from contamination.

The upgrade focused on two areas of water treating: 1). removal of suspended liquid hydrocarbons from the water; and, 2). biological oxidation of dissolved organics.

Demulsifier chemical and a skim tank were added to the oil removal facility; the decarbonator and caustic addition before air flotation were eliminated. Without added caustic, the gas flotation unit remove acid gases from the water very effectively. The new skim tank removes over 90% of the suspended hydrocarbons.

The biological oxidation ponds were upgraded by adding barriers to improve plug flow, increasing dispersed air flow, increasing sludge recycle volume, lowering the oil input and by adding nutrients and biological seeding.

Results of the biological pond upgrades are not yet available because increased sludge recirculation and the optimization of biological seeding are not yet completed. Tests of the use of locally-produced biological sludge are planned.

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