Anaerobic biodegradability of water separated from extracted crude oil.

A study of the anaerobic biodegradability of the three categories of water separated from extracted crude oil (Venezuelan oilfields)--light, medium or heavy crude--was carried out at laboratory scale using UASB reactors working at mesophilic conditions. Chemical oxygen demand (COD) removal in a low loaded UASB reactor fed with water separated from extracted light crude was high, with an average 87% purification efficiency. The remaining COD was made up of the nonbiodegradable and the very slowly biodegradable fractions of the organic matter in the water. During a second period, the hydraulic retention time was reduced in stages, thus increasing the loading rate. In the experimental conditions used, COD concentration at the outlet remained below the Venezuelan standard limit for discharge into the environment (350 mg COD l(-1)) when the hydraulic retention time (HRT) was above 10 hours and the OLR under 3 g COD l(-1) d(-1). For HRT less than 10 hours, or organic loading rate (OLR) greater than 3 g COD l(-1) d(-1), COD at the outlet of the reactor rose as a consequence both of increased volatile fatty acids (VFA) concentrations (indicating an overloading of the methanogenic population) and, also, of the increase in the non-VFA COD (indicating a decrease in the acidification efficiency). On the other hand, results with UASB reactors operated at a low loading rate and fed with water separated from extracted medium and heavy crude oil showed that purification efficiency was low, with only 20% and 37% COD removal respectively. Continuing the operation of the UASB reactor fed with water separated from medium oil over a prolonged period did not bring any improvement, indicating that no adaptation of the sludge occurred.