Fate and Effect of Naphthenic Acids on Biological Wastewater Treatment Systems in Oil Refineries

Naphthenic acids (NAs) are a complex group of alkyl-substituted acyclic, monocyclic and polycyclic carboxylic acids present in crude oil and petroleum products. Desalter brine, influent, activated sludge mixed liquor and effluent refinery samples were received from six United States refineries and NAs were found in all wastewater streams from each of the refineries tested. The total NA concentration in the desalter brine, influent, activated sludge mixed liquor and effluent samples ranged from 4 to 45, 4.5 to 16.6, 9.6 to 140.3 and 2.8 to 11.6 mg NA/L, respectively. The NAs in all streams accounted for less than 10% of the total COD, indicating that many other organic compounds are present and that NAs are a minor component in refinery wastewaters. Susceptibility tests showed that none of the activated sludge heterotrophic microcosms was completely inhibited by NAs up to 400 mg/L. Growth inhibition ranging from 10 to 59% was observed in all microcosms at and above 100 mg NA/L. All but one refinery microcosms achieved significant growth on NAs. Another test investigating the fate and biodegradation potential of NAs chronically-adsorbed to activated sludge mixed liquor biomass and powdered activated carbon (PAC) showed that NAs are persistent and recalcitrant. Throughout a 100-day incubation period, more than 80% of the total NAs remained adsorbed to the solid phase and the total NA concentration decreased by only 33 and 61% in microcosm series with and without PAC, respectively. The lower molecular weight fraction of NAs was preferentially degraded in both mixed liquors. The persistence of the higher molecular weight residual NAs is likely a combination of recalcitrance and decreased bioavailability when chronically-adsorbed to the biomass and/or PAC.