Testing For
S U RFACTANT BIODEGRADABILITY expenditure W’IthovertheSlOO million foi
surfactant (about 600 million pounds in 1964) has the conversion from ABS been a mixture of alkyl benfrom ABS to LAS, there is still no stanto LAS, the detergent inzene sulfonates with highly dustry has probably satisbranched alkyl chains dard test for biodegradability which fied all the significant ob(ABS). In the normal actijections to ABS. Its selfvated-sludge disposal plant, is accepted by regulatory agencies imposed regulation has, it nearly half the entering ABS I Dames through without befeels, effectively eliminated the need for antidetergent Irw and demonstrated ing degraded and is returned to the water table. In a remarkable response to market demand. septic tanks, cesspools, and other less efficient methods At the meeting of the Soap and Detergent Association of disposal, even less ABS is degraded. in New York in January 1965, however, no test for In concentrations of more than 1 mg./liter, ABS will biodegradabiliry was proposed even though a number cause significant foaming. Such concentrations are of industry representatives had expected it. The confrequently found in disposal plant effluents, and occasensus now seems to he that the proposal will be withsionally in potable water supplies. The presence of foam held until present supplies of ABS-base detergents have is esthetically unpleasing and has caused people to suspect been exhausted. This will prevent any misinterpretaABS as a potential health hazard. To allay suspicion tion of laboratory and field testing data and allow time and provide completely safe surfactants, the detergent for more field tests, which are needed to confirm the industry, between 1951 and 1963, examined many comcorrelations already made with laboratory data. The pounds for suitability as replacements. failure to devise an acceptable test for biodegradability, The chief candidate is a mixture of alkyl benzene sulhowever, will remain a chronic source for dispute. fonates with linear alkyl chains (LAS). The chemical A difficultproblem associated with the testing question similarities between ABS and LAS are apparent, and it is the need of regulatory agencies for a standard method. seems fortuitous that such a dose relative of ABS would In West Germany, for example, the detergent law was prove to be so easily degraded by biological treatment. not to take effect until after an adequate biodegradability Other candidates are alkyl sulfates, high molecular test was available; later experience has shown that the weight alcohols and alcohol sulfates, and organic phostest developed is of only limited value. The California phates. State Water Quality Control Board, in its report to the Toxicity California Legislature (Z),regards all currently used tests Studies have shown that ABS and LAS are about as for biodegradability with skepticism-namely, the dietoxic to humans as table salt. Snyder (8)and McKee away test is too variable to he dependable; the flaskand Wolf (5) report the LDro for a 150-pound man to be culture test is too difficult to interpret because surfactant one pound of ABS. Experiment has also shown that adsorption on the sludge and bacterial degradation occur ABS is not carcinogenic and does not cause dermatitic simultaneously; the activated sludge test, in any of its allergy. Since most potable water supplies are mainforms, is too difficult to operate’unless the test is contintained according to U. S. Public Health Service standuous, and reasonably well instrumented. ards, which recommend that the ABS level be kept The Board thus concludes, “. . .current testing methbelow 0.5 mg./liter, the possibility of a health hazard ods and techniques cannot be recommended as a standard caused by ABS in the water is remote. If the average measure of biodegradability.” At the same time, the human intake of water is 2 liters/day, adherence to board does recognize that sufficient evidence has been U.S.P.H.S. standards would provide a safety factor of obtained to show that efficiently operated treatment about 15,000. The Public Health Service also cites explants will degrade LAS a t acceptable rates, thereby periments in which human intake of 100 times the recreducing surfactant concentrations in the effluents below ommended minimum dosage for a period of four months the critical foaming level. produced no ill effects. 01
Despite the imminent switchover
,
-
-
-
From ABS to LAS
Biological 0.oradmion
A typical commercial detergent contains 20% (by weight) synthetic surfactant. Until 1965 the principal
In comparing the degradation of ABS and LAS it is necessary to note that the organisms involved represent V O L 5 7 NO. 3 M A R C H 1 9 6 5
45
a number of species, each of which exhibits preferential nutrient selection. The composite bacterial colony in a treatment system is adapted to its environment and a change in the nutrient input-e.g., a change from ABS to LAS-may cause the colony to readapt. The general mechanism of biodegradation, however, appears to be that summarized by Swisher ( 9 ) . Degradation of ABS and LAS begins with the oxidation of the terminal methyl group in the alkyl chain to a carboxyl group. Once the carboxyl group is formed, the chain is then shortened, two carbons at a time, by the bacterial P-oxidation process until the ring is approached. As the oxidation proceeds closer to the ring, the rate decreases. Ring splitting occurs, and the resulting unsaturated chain is oxidized with at least one @-oxidation being involved. Finally, the sulfonate group is liberated as an inorganic sulfate. In general, the greater the distance between the sulfonate group and the terminal methyl group, the greater the rate of oxidation. Though rates decrease as the ring is approached, the average rates are proportional to initial chain length. Because LAS provides greater distances between sulfonate and terminal methyl groups than ABS, LAS degrades more TABLE ..
I . O P E R A T I O N A L C H A R A C T E R I S T I C S OF A LABORATO RY--ACTIVATED-SLUDG E APPA RATUS COMPARED W I T H A SERVICE P L A N T :
I Feed Total organic Organic nitrogen
BOD Aeration period BOD removal Suspended solids Sludge volume index Return solids
Laboratory Unit
355 p.p.m. 32 p.p.m. 260 p.p.m. 2-6 hr. 90 70 2000-3000 p.p.m. 50-100
25%
~
Domestic Plant 200-400 p.p.m. 15-30 p.p.m. 100-400 p.p.m. 4-16 hr. 85-9 5yo 2000-3000 p . p ~ m . 50-100 Av. 3070
rapidly and requires a correspondingly smaller residence time in the disposal system for degradation. Tests made thus far indicate that LAS is at least as degradable as normal sewage. However, the fact that a number of different disposal treatments are in use has made the matter of standardizing tests for degradability of great importance to the detergent industry. Tests for Degradability
The chemical analysis of surfactants has been discussed by Allred, Setzkorn, and Huddleston ( 7 ) , and will be the same for evaluating any test for biodegradability. The most promising tests for biodegradability are the die-away test, the flask-culture test, and several variations of the activated sludge test. Hammerton’s modification of the die-away test ( 3 ) involves introducing a stock solution of surfactant to a sample of natural river water. Degradation of the surfactant is measured by chemical analysis of periodically withdrawn samples. Determinations are made for 46
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
periods of u p to 30 days. In evaluating the suitability of the die-away test, Setzkorn, Huddleston, and Allred (7) conclude that natural variability in river waters is too great to permit serious consideration of the test as a general means for evaluating biodegradability. The test is simple and convenient, and may be suitable for highly localized studies. In the flask-culture test, bacterial cultures from an activated sludge are adapted to a standard surfactant environment. The surfactant to be evaluated is autoclaved to destroy stray bacteria. After suitable preparation, the “adapted” culture is introduced into the test specimen. The resulting degradation is followed for 96 hours while test specimens are aerated by mechanically shaking the flask at a controlled rate. The procedure is described in detail by -4llred and coworkers ( 7 ) , and by Renn, Kline, and Orgel (6). The activated sludge test has been run with several minor variations, each representing an attempt to simulate an actual plant. Renn and his associates (6) used a synthetic sewage in their Miniplants. Huddleston and Allred ( 4 ) devised an elaborate, three-stage apparatus which uses samples of an actual sludge from a local plant. In both cases the attempts at simulation were successful. The unusually good agreement with actual plant data obtained by Allred, Setzkorn, and Huddleston is exemplified by the data in Table I. The authors also found that, while detergent removal in municipal treatment plants may result from a combination of bacterial action and adsorption by the sludge, the laboratory apparatus functions by bacterial action alone. A test material which reacts well in the laboratory apparatus should, therefore, be even more degradable in service. Correlation with Field Tests
All the major detergent companies have correlated laboratory data with actual operating plant data. Probably the best-documented test is that conducted by Renn, Kline, and Orgel (6) at TZ’oodbridge, \la., to evaluate an LAS produced by Colgate-Palmolive Co. As is the case with nearly all the field tests conducted on LAS thus far, the FYoodbridge test verified the laboratory prediction quite closely. In general. LXS seems to be at least as degradable as the remainder of the sewage, and frequently approaches 90% degradability in service. This was the original aim in the development of LAS; the question now is whether it can be proved and, if so, is it enough? REFERENCES ( 1 ) Allred, R . C., Setckorn, E. A , , Huddlesion, R . L., J . Am. Oi! Chamirti SOL.41 (1) 1 3 (1964). (2) Detergent Report to the California State Legislature prepared b) State Water Qualit, Control Board, State Department of \Yater Resources, Srate Departmeni of Pubiic Health, Sacramento, Calif., January 1965 (3) Hammerton, C., J . Appl. C h m ( L o n d o v ) 5 , 517 (1955). (4) Huddleston, R . L.: Allred, R. C., J. A m . Oii Chemists SOC. 41 ( l l ) , 732 (1964). (5) McKee, J. E., \Volf, H . LV., Publication No. 3.4, Calif. State W a t e r Q u a l i t y Control Board. 1963. (6) Renn, C. E., Kline, I Y . A , Orgel, G.. J. Water P d l i i t t o n Control Fed.? J u l y 1964. (7) Setzkorn, E. A , ; Huddleiton, R. L., Allred, R . C., Meeting of Am. Oil C h e m . Sac,, New Orleans. April 20, 1964. (8) Snyder, F. H., Toxicol. Appi. Phoirnacoi 6 , 133 (1964). (9) Sw-isher, R.D., Chem. En!. Prog. 60 (12): 41 (1964)
