The fate of detergent surfactants in sewer systems

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Pergamon 0273-1223(95)00349-5

Waf. Sci. Tech. Vol. 31. No.7. pp. 321-328. 1995. Copyright@ 1995 lAWQ Printed in Great Britam. All rights reserved. 0273-122319S $9'SO + 00()()

THE FATE OF DETERGENT SURFACTANTS IN SEWER SYSTEMS E. Matthijs*, O. Debaere*, N. Itrich**, P. Masscheleyn*, A. Rottiers*, M. Stalmans* and T. Federle** • Procter and Gamble European Technical Center Temselaan 100. B-1853 Strombeek-Bever. Belgium Procter and Gamble Ivorydale Technical Center 5299 Spring Grove Avenue. Cincinnati. Ohio. 45217. USA

.*

ABSTRACT The fate of detergent surfactants in the sewer can be studied both in laboratory tests and in field experiments. The laboratory studies can be used to determine the rate of disappearance of a test molecule as a function of residence time and estimate its balf-life in a given babitat. In addition, important information can be obtained on the mecbanism of degradation. Field studies can determine the actual environmental concentrations of surfactants in raw sewage wbicb can then be compared with the expected concentration based on consumption volumes. The difference between the measured and predicted concentration provides an estimate for the disappearance of the test cbemlcal during its travel in the sewer and confirms the results of the laboratory tests. This paper focuses on the fate of a number of important representative anionic, nonionic and cationic surfactants, in the sewer. Tbe results of laboratory die-away studies sbowed that, in general, the balf-life for disappearance in the sewer was in the order of bours for Fatty Alcohol Ethoxylate (AE), Fatty Alcohol Ethoxy Sulphate (AES) and Di-Ethyl-Ester Di-methyl-Ammomum Chloride (DEEDMAC). These laboratory fmdings for AES were confirmed by monitoring actual raw sewage reacbing municipal sewage treatment plants. In addition, a field study demonstrated that the concentration of glucose amides (GA) is considerably reduced during its travel in the sewer. These complemenlary laboratory and field studies provide key information for the safety assessment of surfactants. They demonstrate that the concentration of surfactants can be significantly reduced in the sewer resulting in a rapid reduction of the environmental loading, which is particuL1I'Iy important in environmental situations where inadequate or no sewage treatment exists.

KEYWORDS

Biodegradation; detergents; fate; field studies; safety assessment; sewer; surfactant. INTRODUCfION Surfactants are high volume chemicals, used as active ingredients in a variety of laundry and cleaning products, hard surface cleaners and shampoos. After use, consumer detergent ingredients are ordinarily discarded down the drain into municipal sewer systems and typically enter sewage treatment plants. where they are removed by a combination of sorption and biodegradation. The fate of the major types of detergent surfactants during sewage treatment is well understood and well documented in the scientific literature (Gilbert and Pettigrew. 1984; Brown et aI., 1986, 1987; Stalmans et al .• 1993; Giolando et al .• 1994). Less information. however. is available on their fate in the sewer, itself. The fate of Linear Alkylbenzene 321 JIST 31·7-1/

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Sulfonate (LAS) in the sewer was studied by Moreno et al. (1990). The authors demonstrated that LAS was removed to the extent of almost 50% in a Spanish sewer. illustrating that chemical and biological reactions occurring in the sewer can result in considerable reductions in the concentrations of surfactants entering treatment plants. Even more significantly. they could play an important role in reducing environmental concentrations. particularly in situations where inadequate or no sewage treatment exists. In this paper. both laboratory and field data are presented indicating that the sewer is more than just a transportation system for the sewage but also serves as a reactor. in which biodegradation and sorption processes are initiated. Most standardised biodegradation tests use mineral media as opposed to realistic sewage and are primarily designed to assess the extent of mineralisation (C02 evolution. 02 depletion) in a conservative fashion. As a consequence. they often underestimate the kinetics of the primary biodegradation. which in the case of surfactants reflects the loss of toxicity towards aquatic and other environmental organisms. This has been demonstrated for LAS by Kimmerle and Swisher (1977) and Moreno and Ferrer (1991). It is important. therefore. to accurately estimate the 19ss of parent structure of surfactants (primary biodegradation) as well as mineralization. A sewage die-away test was designed to simulate actual conditions in raw sewage in the pipe during transit and to detennine both the rate and mechanism of transfonnation. Furthennore. it can be used to identify potential biodegradation intennediates. In this study. the sewage die-away test was used to examine the fate in untreated domestic sewage of alkyl ethoxylate sulfate (AES). alkyl ethoxylate (AE) and diethanolester dimethyl ammonium chloride (DEEDMAC). which are important representatives of the anionic. nonionic and cationic classes of surfactants. Conf11ll1ation of the degradation of surfactants in real systems is obtained by field studies in which a comparison is made between the measured surfactant concentration in the raw sewage and the predicted concentration based on consumption volumes. These field studies require that well designed environmental monitoring programs be conducted requiring appropriate ,calibration chemicals and the development of sensitive and selective analytical methods. In this study. the level of AES and glucose amide in influent were measured and compared to expected levels based upon consumption. TEST MATERIALS: STRUCTURE AND PROPERTIES Surfactants (surface active agents) are organic chemicals which, when added to a mixed system such as water and air or water and soil, result in a decreased interfacial tension. This property is related to the presence of both hydrophobic and hydrophilic moieties in a single structure. Depending on the nature of their hydrophilic group. surfactants are classified mainly as anionic, cationic, nonionic and amphoteric. In this study, experiments were conducted with Fatty Alcohol Ethoxy Sulphate (AES). which is an anionic surfactant, Fatty Alcohol Ethoxylate (AE) and Glucose Amide. which are nonionic surfactants and Di-Ethyl• Ester Di-Methyl-Ammonium Chloride (DEEDMAC) which is a cationic surfactant. The structures of these surfactants are shown in Figure I. Commercial mixtures of AE are composed of long chain fatty alcohols containing 8 to 10 carbon atoms combined via an ether linkage with a chain of I to 20 ethylene oxide units. Fatty alcohol ethoxy sulphates or alkyl ether sulphates are primary sulphate esters derived from fatty alcohol ethoxylates. AES is a typical anionic surfactant. The number of ethoxylate groups typically varies from 1 to 8. Di-ethyl-ester di-methyl-ammonium chloride. a cationic surfactant. is a quaternary ammonium compound which contains two weak-link ester functions. Glucose amide is a new type of nonionic surfactant derived from fatty acids and sugars. This molecule can be described as a polyhydroxy fatty acid amide. Radiolabelled surfactants used in this study were synthesized at Procter and Gamble. Miami Valley Technical Center, Cincinnati. Radiolabelled AE (CI4E3) and AES (CI4E3S03) were labelled with 14C in the ethoxylate carbons. 14C DEEDMAC was labelled in the methyl groups attached to the nitrogen. EXPERIMENTAL LabQratoey Studies Sewage die-away studies are designed to detennine whether materials undergo biodegradation in raw sewal!e and to estimate the kinetics of removal. Thev are executed in the laboratory usinl! freshly collected

The fate of detergent surfactants in sewer systems

323

raw sewage (biotic unit) from a municipal sewage treatment plant and at a realistic concentration of added test material. A sterile. abiotic control is used to control for extraction efficiency of the test material and losses resulting from adsorption to the test container wall and other nonbiological processes. Deactivation of the sewage for the abiotic control can be achieved by autoclaving the samples for 90 min and adding I gil of buffered mercury chloride prior to the start of the test. The test concentration for detergent surfactants is usually in the low mgll range.

R-0-(CH 2-CH 2-O)n-H

R-0-(CH 2-CH 2-O)n-SO;

R

R = C10 . 18

n

=C8 ' 18

=1 - 20

n =1 - 8

Fatty Alcohol Ethoxylate

Fatty Alcohol Ethoxy sulfate

R-CH2-CO-N(CH3)-CH2-(CHOH)4-CH20H R =C 10 R = C 16 ' 18 Di-Ethyl-Ester Ol-Methyl-Ammonlum ChlorIde

Glucose Amide

Figure I. Structure of the surfactants studied.

Samples for analysis are taken both from the biotic and abiotic test units immediately after adding the test substance (time 0) and at regular time intervals thereafter. To facilitate the analytical determinations. the test can be executed with a mixture of cold and hot test material. The use of radiolabelled test molecules allows easy qUantification of the loss of parent molecule and facilitates following the formation and subsequent disappearance of degradation intermediates. Assuming first-order degradation kinetics. the first-order rate constant and half-lives are estimated by non-linear regression analysis. Field Studies Field studies are an integral part of environmental safety assessment because they provide actual environmental concentrations for a given test compound. For the sewage treatment compartment. environmental monitoring typically includes the collection of daily. flow-proportional composite samples of raw. settled and treated sewage. The environmental samples are preserved by addition of 8% formaldehyde to the sample container prior to collection of the sample. In order to check the efficiency of the sample preservation upon transport and storage. standard additions of the test chemical are performed on site. directly after sample collection. Comparison of the measured raw sewage concentration of surfactants with the predicted concentrations provides an estimate of the removal of the test compounds during their travel in the sewer. The predicted concentration is based on per capita consumption of the surfactant and water use. In order to successfully execute field studies. sensitive and selective analytical methods are required. which are appropriate for the environmental matrix involved. In addition. appropriate calibration chemicals. which can serve as conservative tracers. must be included to verify the validity of the assumptions used for the calculations.

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324

Analytical MethQds LabQratory studies are usually executed with 14C radiolabelled test materials which are labelled in the part Qf the molecule most resistant to biodegradation. The utilisation of radiolabelled materials facilitates the analytical method development work as well of the actual analyses performed during the study. Thin Layer Chromatography with radioscanner (TLCIRAD) is particularly useful as the analytical teChnique. It is very sensitive. reasonably selective and does not require the development of labour-intensive sample preparation procedures. In addition, TLClRAD permits the recovery of any intermediates for more definitive subsequent analyses. Samples taken from the laboratory studies conducted with radiolabelled AE and AES were flash frozen. lyophilised. extracted with methanol and subsequently analysed by TLCIRAD. Samples taken from the study conducted with DEEDMAC were analysed for the concentration of parent material and the known hydrolysis products (HPI and HP2, HP =hydrolysis product) (Figure 2) which are formed upon cleavage of the ester function. DEEDMAC and its first hydrolysis product (HPl) were isolated from the aqueous test sample by solvent sublation with ethyl acetate and quantified by TLCIRAD. The second hydrolysis product (HP2) remained in the water phase due to its high polarity. HP2 was removed from the remaining water phase by solvent extraction prior to TLCIRAD analysis.

DEED MAC

HP 1

HP2

Figure 2. Degradauon pattern of DEEDMAC.

The execution of field studies requires the development of analytical methods which enable the determination of the concentration of the test material in the complex environmental sample and at trace concentrations. For this purpose, mass spectrometric methods are being used due to their high level of specificity. The concentration of AES in raw sewage samples was determined using liquid chromatography coupled to ion-spray mass spectroscopy (Popenoe et al .• 1994). Sample preparation consisted of a simple concentration of the analyte on a C2 reversed phase column. The concentration of GA in raw sewage samples was determined using flow injection fast atom bombardment mass spectroscopy (Matthijs et aI .• 1993). The aqueous samples were preconcentrated on a C4 reversed phase column. RESULTS AND DISCUSSION LabQratOlY Studies RadiQlabelled Cl4E3S03 fatty alcohol ethoxy sulphate (AES) was added to raw sewage samples taken from a US municipal sewage treatment plant. Periodically, samples were removed and analysed by TLCIRAD for parent molecule and metabolites. AES remained intact in the abiotic control throughout the entire experiment. In contrast, the disappearance of parent AES in biologically active raw sewage as a function of time is presented in Figure 3. Mter 24 hours, more than 98% of the parent AES had disappeared. Based on the rate of disappearance. the calculated half-life for parent AES was approximately 4 h. Tl.CJRAD measurements indicated that during the degradation process AES was mainly transformed into a polar intermediate which was identified as polyethylene glycol sulfate. Other experiments conducted with activated sludge and river-water have shown that this polar intermediate is transient and ultimately mineralised to carbon dioxide. The half-life for complete biodegradation in activated sludge has been estimated as less than I h.

325

The fate of detergent surfactants in sewer systems

Percent of total radioactivity

100 80

Parent AES

60 40 Polar Intermediate

20

o

25

50

75 100 125 TIME (Minutes)

150

175

,,, , \, ",,\,\ , 0

\

1450

Figure 3. Biodegradation of AES in sewage influent.

Radiolabelled di-ethyl-ester di-methyl-ammonium chloride was added to an active and a sterile sample of raw sewage collected from a Belgian municipal sewage treatment plant. Samples were taken at the start of the test and periodically during the course of the study for analysis of the concentration of parent material and the known hydrolysis products (Figure 2). More than 80% of parent DEEDMAC was removed from the biologically active unit within 48 hours as shown in Figure 4. This corresponds to a half-life of approximately 6-10 h. At the end of the test. no accumulation of HPI and HP2 was noticed indicating that both hydrolysis products are transient and degraded shortly after formation. Figure 4 also demonstrates that DEEDMAC remained chemically intact in the biologically killed control unit. Radiolabelled Cl4E3 alkyl ethoxylate also was added to raw sewage collected from a US municipal sewage treatment plant. Subsamples were taken at regular time intervals. flash frozen. lyophilised. extracted and subsequently analysed by TLC/RAD. AE remained intact in the abiotic control throughout the entire experiment. Figure 5 shows the concentration of AE decreasing rapidly as a function of the residence time. The shape of the degradation curve suggests zero-order kinetics for the degradation. To facilitate comparisons with the other experiments however. the data are evaluated according to fIrst-order kinetics calculated half-life for the parent molecule corresponded to about 3 hours. During the degradation a polar intermediate was formed which was transient and rapidly mineralised to carbon dioxide. Evolution of l4C02 was determined by comparing the difference between the total radioactivity in the biologically active treatment and the abiotic control following acidification. Total radioactivity was measured by liquid scintillation counting (LSC). AcidifIcation liberates any dissolved C02 form the test solution. Field Studies Field studies were recently conducted at the Dutch municipal sewage treatment plant "De Meem". This is an activated sludge plant with a design capacity of 40.000 inhabitant equivalents, which currently serves 32,000 inhabitant equivalents. At this plant. 24 hours. flow-proportional composite samples were collected during three consecutive days from raw, settled and treated sewage. During the sampling period the flow varied between 5960 and 7555 m3/day with an average value of 6500 m3/d. This corresponds to an average flow of

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200 I water/dlinhabitant. The concentrations of fatty alcohol ethoxy sulphate (AES) and glucose amide (GA) were measured in the raw sewage samples using selective and sensitive analytical methods based on liquid chromatography coupled to mass spectroscopy and flow injection mass spectroscopy respectively.

Percent of total radioactivity

100"p;::O::==:===~==========~ Parent DEEDMAC (abiotic control)

80 60

Parent DEEDMAC (biotic control)

40 20

O+------.-----.------r-----.-----~

o

10

20

40

30

50

TIME (Hours)

Figure 4. Biodegradation of DEEDMAC in sewage influenl

Percent of total radioactivity

100 Parent AE

80 60 40 20

o

50

100

150

200

250

TIME (Minutes) Figure S. Biodegradation of AE in sewage influent.

300

350

The fate of detergent surfactants in sewer systems

327

The measured surfactant concentration was then compared to the predicted concentration based on (a) the annual known surfactant consumption in the Netherlands, (b) the total population (15.10 x 106) and (c) the average measured daily flow to the treatment plant (200 I). In addition to the surfactants, the concentration of boron was determined as well using inductively coupled plasma optical emission spectroscopy. Boron, originating from sodium borate, is not removed in the sewer system and therefore acts as an excellent tracer for verifying the accuracy of the model prediction used to estimate the surfactant concentration in domestic sewage. Results shown in Table 1 summarise the outcome of the comparison. On average, approximately 47% of the expected level of both AES and GA were removed in the sewer during their travel from the discharge point to the treatment plant. The excellent agreement between the predicted and measured boron concentrations confirms the validity of the proposed procedure. Thus, this field study demonstrates that the concentration of biodegradable surfactants can be considerably reduced during their travel in the sewer. It can be expected that in the future, detailed information will be available on typical residence times of raw sewage in the sewer, which can be used to generate detailed and quantitative assessments of the actual contribution of in-sewer degradation to the reduction of the overall environmental loading. Table 1. Comparison Of Predicted And Measured Surfactant Concentration Component

Tonnage/year

Predicted concentration in ug/L

Measured concentration in l.!8t.!Javen~ge)

Removal in the sewer(%)

Boron AES GA

984 3738 470

893 3400 426

900 1800 227

0 47 47

CONCLUSION It has been generally thought that chemicals discharged into sewers remain relatively intact until they reach sewage treatment or natural environments. In the context of environmental chemistry, sewers have largely been ignored, considered devoid of biological activity and viewed as simply conduits for transporting materials to locations, where biodegradation can occur. In reality, sewers and sewage are populated with a diverse assemblage of active microorganisms. which are capable of initiating the biodegradation of a wide range of organic chemicals. Thus, the sewer should be more appropriately viewed as a biological reactor and the first point of removal for a chemical from the environmental waste stream. The significance of sewer processes in the overall fate of a chemical can vary greatly from site to site as a function of residence and travel time. Nevertheless. this residence time is generally not trivial and can be measured in hours and even days, in the case of very large sewer systems. The practical significance of these processes is clearly documented by the large difference observed in this study between predicted and measured levels of AES and glucose amides reaching a sewage treatment plant.

In this study, laboratory sewage die-away studies, conducted with radiolabelled test materials, accurately simulated the processes occurring in the sewer. The results of laboratory experiments performed with AE. AES and DEED MAC demonstrated that these surfactants have half-lives of the order of hours in the sewer. Furthermore, the use of Thin Layer Chromatography with radioscanner resulted in elucidation of the mechanism of degradation. The results of the laboratory studies were confirmed by field work as demonstrated for AES. In this case, the real environmental concentration of the test molecule was measured with selective analytical methods and then compared with the expected raw sewage concentration based on consumption volumes. Field work conducted with glucose amide demonstrated that the concentration of this nonionic surfactant in the raw sewage is also significantly reduced during its travel through the sewer.

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In summary, the sewage die-away studies provide important information on whether a material has the potential to undergo biodegradation in the sewer, while monitoring studies demonstrate that this potential is actually realised in the real-world situation. The combined use of these complementary approaches clearly demonstrates in this study that biodegradation in the sewer significantly reduces the loading of surfactants to the environment. This removal not only reduces the load of surfactants reaching treatment plants but also greatly reduces the concentration of surfactants directly discharged to the environment in situations where sewage treatment is non-existent or inadequate.

REFERENCES Brown. D•• De Henau. H.• Garrigan. J. T .• Gerike. P.• Holt. M .• Keck. E .• Kunkel, E.• Matthijs. E.• Waters. J. (1986) Removal of Nonionics in a Sewage Treatment Plant. Tenside Detergents Suljactants. 23. 4. 190-195. Brown. D .• De Henau. H•• Garrigan. J. T.• Gerike. P.• Holt, M.• Kunkel. E .• Matthijs. E.• Waters. J. (1986) Removal of Nonionics in a Sewage Treatment Plant. Tenside Detergents Suljactants. 24. I, 14-19. Gilbert, P. A. and Pettigrew. R. (1984) Surfactants and the Environment. Int. 1. Cosmet. Sci. 6, 4. 149-158. Giolando. S. T .• Rapaport, R. A.• Larson. R. J.• Federle. T. W .• StaImans. M. and Masscheleyn. P. (1994). Environmental Safety Assessment of DEEDMAC: A New Biodegradable Cationic Surfactant for Use in Fabric Safteners. Chemosphere. submitted. Kimmerle. R. A. and Swisher. R. D. (1977). Reduction of aquatic toxicity of linear a1kylbenzene sulfonates (LAS) by biodegradation. Water Research. ll. 312-315. Matthijs. E.• Perneel. P.• Rottiers. A. and Verhaeghe. B. (1993) Determination of Glucose Amides in Environmental Samples. Presented at the 'Rhine Basin Program Symposium'. Basel, October 21-22. Moreno. A.• Ferrer. J. and Berna, J. L. (1990). Biodegradation of LAS in a Sewer System. Tenside Detergents Suljactants, 27, 5. 312-315. Moreno, A. and Ferrer, J. (1991). Toxicity towards Daphma during biodegradation of various LAS. Tenside Detergents Sur/actants, 2, 129-131. Popenoe. D. D., Morris III, S. J., Hom, P. S. and Norwood, K.T. (1994). Determination of Alkyl Sulfates and Alkyl Ethoxysulfates in Waste Water Treatment Plant Influents and Effluents and in River Water using Liquid Chromatographyllon-Spray Mass Spectrometry. Analytical Chemistry. In press. Stalmans. M.• Matthijs. E .• Weeg. E. and Morris. S. (1993). The Environmental Properties of Glucose Amide, a New Nonionic Surfactant. SOFW, J 19.794-808.