Fate and effect of naphthenic acids on oil refinery activated sludge wastewater treatment systems

Abstract

Naphthenic acids (NAs) are a complex group of alkyl-substituted acyclic, monocyclic and polycyclic carboxylic acids present in oil sands process waters, crude oil, refinery wastewater and petroleum products. Crude oil, desalter brine, influent, activated sludge mixed liquor and effluent refinery samples were received from six United States refineries. The total acid number (TAN) of the six crudes tested ranged from 0.12 to 1.5 mg KOH/g crude oil and correlated to the total NA concentration in the crudes. The total NA concentration in the desalter brine, influent, activated sludge mixed liquor and effluent samples ranged from 4.2 to 40.4, 4.5 to 16.6, 9.6 to 140.3 and 2.8 to 11.6 mg NA/L, respectively. The NAs in all wastewater streams accounted for less than 16% 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. NAs chronically-sorbed to activated sludge mixed liquor biomass and powdered activated carbon (PAC) were recalcitrant and persistent. More than 80% of the total NAs remained in the solid phase at the end of the 10-day desorption period (five successive desorption steps). Throughout a 90-day incubation period, the total NA concentration decreased by 33 and 51% in PAC-free and PAC-containing mixed liquor microcosms, respectively. The lower molecular weight fraction of NAs was preferentially degraded in both mixed liquors. The persistence of the residual, higher molecular weight NAs is likely a combination of molecular recalcitrance and decreased bioavailability when chronically-sorbed to the biomass and/or PAC.

Introduction

Petroleum refineries are process plants where crude oil is transformed to refined products, such as gasoline, diesel and kerosene. Refining processes use large quantities of water, primarily for extraction, desalting, and cooling, generating waste streams, which are combined and carried to the refinery wastewater treatment plant (Dorn, 1998; IPEICA, 2010). Many wastewater components are easily removed by the refinery wastewater treatment plant; however, some compounds found in process waters, such as naphthenic acids (NAs), are more difficult to treat and create operational problems, such as corrosion and toxicity (Dorn, 1998; IPEICA, 2010; Whitby, 2010). Activated sludge systems are common biological treatment processes used to treat refinery wastewaters; however, very limited information exists relative to the fate and effect of NAs in refinery activated sludge units and whether such treatment systems are capable of reducing the effluent NA concentration.

NAs are a complex group of alkyl-substituted acyclic, monocyclic and polycyclic carboxylic acids present in oil sands process waters, crude oil, refinery wastewater and petroleum products. They are monobasic, anionic surfactants with the general formula C_nH_2n+ZO₂, where n is the carbon number and Z is the number of hydrogen atoms lost due to ring formation (i.e., the hydrogen deficiency). Z is zero or a negative, even integer such that Z = 0 corresponds to no rings (i.e., aliphatic compounds); Z = −2 corresponds to one ring; Z = −4 corresponds to two rings, etc. (Clemente and Fedorak, 2005; Headley and McMartin, 2004; Lo et al., 2006; Quagraine et al., 2005). Recent studies using advanced mass spectrometry techniques, including ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), have shown that oil sands process waters contain a significant fraction of oxidized NAs (C_nH_2n+ZO_x, where x is three or more oxygen atoms) in addition to classic NAs which contain 2 oxygen atoms (Headley et al., 2009; Barrow et al., 2009; Grewer et al., 2010). NAs are present in crude oil and bitumen from reservoirs and oil sands. The concentration and congener distribution of NAs depends on the type and source of crude oil. Recently, oil exploration has led to extraction of extra heavy crudes and bitumen from oil sands in Canada, Venezuela, Mexico and other countries. It is estimated that 97% of Canada's oil reserves are unconventional crude oil, mainly bitumen (US EIA, 2011a). Similarly, Venezuela has the second largest proven oil reserves in the world, the majority of which is extra heavy crude oil and bitumen (US EIA, 2011b). In 2011, the United States imported 45% of its crude oil, with 13.05%, 4.95% and 3.60% imported from Canada, Venezuela and Mexico, respectively (US EIA, 2012). Such crudes have elevated NA concentrations and result in refinery process waters and wastewaters with relatively high NA levels. Refinery wastewaters vary in NA concentration depending on the crude oil and treatment process (IPEICA, 2010; Whitby, 2010).

NAs are corrosive and among the most toxic components of oil sands process waters, acutely toxic to both aquatic and terrestrial species, including bacteria, yeast, earthworms, fish, mammals, and both aquatic and terrestrial plants (Armstrong et al., 2008; Clemente et al., 2003; Frank et al., 2009, 2008; Holowenko et al., 2002; Nero et al., 2006; Peters et al., 2007; Rogers et al., 2002; Sarathy et al., 2002; Scott et al., 2008; Thomas et al., 2009). Model NAs, commercial NA mixtures and NAs in oil sands process water samples have been found to degrade aerobically using inocula obtained from oil sands process waters and NA-affected natural systems; however, commercial NAs were found to be more readily degraded than the NAs in oil sands process waters (Clemente and Fedorak, 2005; Del Rio et al., 2006; Scott et al., 2005). Although some NAs are aerobically degraded, many are recalcitrant. Studies investigating the effect of NA structure on biodegradability have shown that the more complex NAs, i.e., those with higher cyclization, branching and molecular weight (MW), are the most recalcitrant (Biryukova et al., 2007; Clemente et al., 2004; Han et al., 2008; Herman et al., 1993; Holowenko et al., 2002; Scott et al., 2005; Smith et al., 2008; Watson et al., 2002).

Detailed information about the occurrence of NAs in refinery waste streams and what factors affect the fate and biodegradation of NAs in refinery wastewater treatment plants is very limited. The objectives of this study were to: a) characterize crude oil and waste streams from six oil refineries; b) determine the occurrence and fate of NAs in various refinery wastewater treatment systems; and c) assess the inhibitory and biotransformation potential of NAs on refinery activated sludge microcosms.