For overboard disposal the water should have an oil content of less than about 30 parts per million (ppm) oil-in-water. In the early days of the offshore industry parallel plate separator units were used to clean produced water prior to overboard disposal. Hydrocyclones, which are more compact, were introduced in the 1980s. The associated gas from the top of the separator(s) is also known as flash gas or wet gas as it is saturated with water and liquid alkanes. The gas is typically routed through scrubbers, compressors and coolers to raise the pressure of the gas and to remove liquids. Gas may be dried by counter-current contact with triethylene glycol in a glycol dehydration tower. The dry gas may be exported, used for gas lift, flared, used as fuel for the installation's power generators, or after further compression re-injected into the reservoir. Dry gas may be further treated to meet export gas specifications. Excess carbon dioxide (CO2) can be removed by treatment in an amine gas treating process, whereby CO2 is preferentially dissolved in a counter-current flow of amine in a contact tower. Hydrogen sulphide can also be removed using amine or by passing the gas through beds of zinc oxide absorbent. The export hydrocarbon dew-point specification (typically 100 barg at 5°C) may be met by chilling the gas to remove the higher alkanes (butane, pentanes, etc). This may be done by a refrigeration system, or passing the gas through a Joule-Thomson valve, or through a turbo-expander to condense out and separate liquids. Export gas is metered to accurately measure the flowrate before being sent to the onshore terminal via gas pipeline. Condensed liquids from the gas treatment plant may be stabilised in a stabiliser (distillation) column to produce a gas product which can be co-mingled with the gas streams and natural gas liquids (NGL) which can be co-mingled with the oil export stream. Onshore oil terminals generally have fired heaters followed by separators and coalescers to stabilise the crude and remove any produced water and light hydrocarbons not separated offshore. Onshore separators tend to operate at a lower pressure than the offshore separators and so more gas is evolved. The associated gas is generally compressed, dew-pointed and exported via a dedicated pipeline. If gas export is uneconomical then it may be flared.