In the oil industry, waterflooding or water injection is where water is injected into the oil reservoir, to maintain the pressure (also known as voidage replacement), or to drive oil towards the wells, and thereby increase production. Water injection wells may be located on- and offshore, to increase oil recovery from an existing reservoir. Normally only 30% of the oil in a reservoir can be extracted, but water injection increases the recovery (known as the recovery factor) and maintains the production rate of a reservoir over a longer period. Waterflooding began accidentally in Pithole, Pennsylvania by 1865. Waterflooding became common in Pennsylvania in the 1880s. Sources of injection water Most sources of bulk water can be used for injection. The following sources of water are used for recovery of oil: Produced water is often used as an injection fluid. This reduces the potential of causing formation damage due to incompatible fluids, although the risk of scaling or corrosion in injection flowlines or tubing remains. Also, the produced water, being contaminated with hydrocarbons and solids, must be disposed of in some manner, and disposal to sea or river will require clean-up treatment of the water stream first. However, the processing required to render produced water fit for reinjection may be equally costly. As the volumes of water being produced are never sufficient to replace all the production volumes (oil and gas, in addition to water), additional "make-up" water must be provided. Mixing waters from different sources exacerbates the risk of scaling. Seawater may be the most convenient source for offshore production facilities, and it may be pumped inshore for use in land fields. Where possible, the water intake is placed at sufficient depth to reduce the concentration of algae; however, filtering, deoxygenation, treatment with a biocide is generally required. Aquifer water from water-bearing formations other than the oil reservoir, but in the same structure, has the advantage of purity and chemical compatibility where available. However this will not be allowed if the aquifer is a source of potable water as, for instance, in Saudi Arabia. River water will require filtration and treatment with a biocide before injection. Filters Filters clean the water and remove impurities, such as sediments, shells, sand, algae and other biological matter. Typical filtration is to 2 micrometres, but depends on reservoir requirements. After filtration the remaining matter in the filtrate is fine enough to avoid blockage of