Analytical Methods for Quantitative and Qualitative Determination of Hydrocarbons and Oil and Grease in Water and Wastewater

Analytical methods to determine oil and grease concentration and identify specific organic fractions in water and wastewater are reviewed . Important aspects of the development of these procedures are extraction technique, solvent type, and identification and quantification methods for the extracted material. The material presented will assist researchers and regulatory investigators in selecting appropriate analytical procedures and interpreting results . INTRODUCTION Oil and grease analysis, like many analytical methods for determination of water quality, does not measure a specific substance or compound . Oil and grease analyses attempt to quantify compounds which have a greater solubility in an organic solvent than in water . The principal types of compounds included in oil and grease analyses are fats, soaps, fatty acids, hydrocarbons, waxes, and oils . The contribution of each of these substances will depend upon the origin of the wastewater being analyzed and the type of extracting solvent used . Discharge regulations in the United States typically include measurements of oil and grease . While imposing relatively simple analytical requirements, oil and grease tests result in measurement of a broad range of compounds with widely varying chemistry and toxicity . Methods are available for identification of specific organic fractions but tend to be too demanding of expertise, time and equipment to be used as a regulatory tool . The review presented in this paper was prepared as part of a larger study evaluating techniques that are suitable for routine analysis while providing useful information about the source and type of organic compound . EARLY ANALYTICAL METHODS The development of analytical techniques to measure oil and grease was initiated by wastewater treatment plant personnel who found that large quantities of grease clogged or detracted from the performance of treatment plants . Numerous examples of anaerobic digester failure were attributed to the build-up of greasy scum layers which prevented gas transfer and mixing . Other examples include failure of sewers due to the build-up of grease deposits . These failures were frequent at rendering plants and slaughter houses. Treatment plant operators were faced with a need to measure oil and grease concentration in order to prevent process failure by preventive maintenance or control measures . 625 The earliest documented analytical method for oil and grease analysis was made by Hazen (1). Hazen's method was used until the early thirties when other methods were proposed . Basically, Hazen's method requires that a 500 ml . sample of oily wastewater be evaporated to 50 ml, neutralized with hydrochloric acid, evaporated to dryness, and extracted with a solvent . The solvent is then evaporated to dryness in a tared container which is weighed to determine residue . The residue is reported as oil and grease . Knechtges, et al . (2) reported a modified procedure which uses a Caldwell extractor to extract the residue resulting from evaporating the wastewater sample . They investigated ethyl ether, petroleum ether and chloroform as solvents and found that chloroform gives the highest residue weight . They also investigated the types of compounds which are present in the extract, finding that free acids comprise as much as 76% of the oil and grease in primary effluents . Hazen's method and the modification proposed by Knechtges are time consuming, requiring as much as two days to perform the analysis and often produce inconsistent results . The methods were unacceptable and new methods were developed by numerous investigators, summarized as follows : 1 . Alum coagulation, filtration, drying of the filtered material and sixteen hour extraction with petroleum ether (3) . 2 . Acidification and refrigeration, filtration, extraction of the filtered, material, and drying (4,5) . 3 . Liquid/liquid extraction of an acidified sample, separation and drying of the extract (6) . 4. Lime coagulation, filtration, modification of filtered material, fluffing, and extraction for four hours (7) . 5 . Oil extraction of sewage solids or sludge and extraction of the oil (8) . All these methods use petroleum ether as the solvent and require weighing of the extract to obtain the oil and grease residue . The methods were designed to overcome the problems associated with Hazen's existing standard method . The most important of these problems is the lengthy time required to evaporate 450 ml of water . This step introduces error as well as inconvenience, since the low molecular weight fractions of the oil and grease are lost during evaporation . The evaporation step was not necessary in the newer methods . Another problem with Hazen's standard method was its inability to extract a high percentage of the fatty acids, due to the formation of insoluble precipitates in drying at neutral pH . This problem is overcome by acidification of the filtered material, which converts the precipitates to organic acids . The third problem associated with Hazen's method, and a problem which still exists today, is extraction efficiency. Hatfield and Symons (1) reviewed the six proposed methods with the objective of recommending a new standard method. They define grease as "that material which is extracted from an acidified sample of sewage by petroleum ether (b.p . 40° 60C) when using the standard procedure as recommended by the committee." Obviously, this is a working definition which has a very tenuous relation to the scientific description of the compounds which comprise grease. They also indicate that all the methods are very useful and under optimum conditions all give reasonable results . For a standard method they propose the acidification technique (4,5) using petroleum ether as a solvent. The liquid-liquid extraction (6) gives low results when compared to the other methods, and was not selected . 626 TB method, a liquid ext method, i' Gilcreas method . similar to: chloro -1 . . current su method ui Tll Petroleum effectivell petroleum petroleum other nom Ci that are yr confirmea greatly irn The use o The high compress; TI oil and gn possible H bond whii TIl analysis . Second,t the loss a niques . TI in the 191 . are sump MODERN ANALYTICAL METHODS The adoption of the Okun-Ludwig method for oil and grease did not result in a satisfactory method, and development of improved methods continued . Pomeroy (9) proposed an improved liquidliquid extraction method . Gilcreas et al. (10) reviewed all the methods (1) and proposed a modified method, which they called the modified Sanderson method . The Sanderson method was developed by Gilcreas and Sanderson, but was never published, and was a modification of the post-1945 standard method. The Sanderson method uses a slightly different type of filtration material, but is otherwise very similar to the post-1945 standard method. The modified Sanderson method, using Freon 113 (1,1,2 trichloro -1,2,2 trifluoroethane) as a solvent instead of petroleum ether, has survived and is one of the four current standard methods. Gilcreas et al . (10) found all alternative methods to the modified Sanderson method unacceptable because of the lengthy analytical procedures (approximately 25 hours) . The next development in oil and grease analysis modified the solvent used for extraction . Petroleum ether had been used almost exclusively it was shown (11) that n-hexane can be used as effectively as petroleum ether. This is not surprising since n-hexane comprises a large portion of petroleum ether. Normal hexane is a preferred solvent since it is more uniform in characteristics than petroleum ether. Unfortunately, n-hexane, like petroleum ether, is quite flammable, and for this reason other non-flammable solvents are needed . Chanin et al. (12) proposed using Freon 113 as a solvent . They showed that its use gives results that are virtually the same as n-hexane, and overcomes the flammability problems . Taras and Blum (13) confirmed these results . More importantly Taras and Blum showed that extraction efficiency can be greatly improved if sodium chloride is added to the oil and grease sample at a concentration of 5 gL -1 . The use of salt overcomes the problem of low extraction efficiency with the liquid/liquid extraction . The high salt concentration apparently coagulates the emulsified oil and grease by double-layer compression . The next important development was reported by Gruenfeld (14) who showed that the extracted oil and grease in the solvent can be measured byl infrared spectrophotometryy. He showed that this was possible because of light absorption at 2930 cm . Absorption at 2930 cm results because of the CH2 bond which is a common characteristic of oil and grease . The use of spectrophotometry overcomes two important problems with the oil and grear analysis. First, it extends the nominal limits of detection of oil and grease to levels below 10 mg/1 . Second, the evaporation of solvent is not required in the IR spectrophotometry technique, which reduces the loss of low molecular weight compounds, an unfortunate shortcoming of all the gravinietric techniques . There are five methods for the quantitative determination of oil and grease in waters and sludges in the 1985 edition of Standard Methods for the Examination of Waters and Wastewater . The methods are summarized as follows : 1 . 503A Partition Gravimetric Method 2. 503B Partition-Infrared Method 3. 503C Soxhlet Extraction Method 4. 503D Extraction Method for Sludge Samples 5 . 503E Hydrocarbons