Determination of non-ionic surfactants and their biotransformation by-products adsorbed on alive activated sludge

Water Research 37 (2003) 281–288

Determination of non-ionic surfactants and their biotransformation by-products adsorbed on alive activated sludge Andrzej Szymanski, Bogdan Wyrwas, Zenon Lukaszewski* Poznan University of Technology, Institute of Chemistry, ul. Piotrowo 3, PL-60-965 Poznan, Poland Received 29 November 2001; received in revised form 4 April 2002; accepted 17 June 2002

Abstract A procedure has been developed for the determination of non-ionic surfactants (NS) adsorbed on particles of alive and dead activated sludge. The procedure also enables the determination of adsorption of major biodegradation byproducts: short-chained ethoxylates, long- and short-chained PEG. The basis of measurement is the determination of NS concentration in a slurry of activated sludge and in a solution phase. The difference between these two concentrations represents the NS adsorbed on activated sludge. Separation of NS and their biotransformation byproducts from samples and then on narrower fractions was performed by a sequential liquid–liquid extraction and precipitation with modified Dragendorff reagent. The indirect tensammetric technique (ITT) was applied for the final determination. The developed method was checked using the example of the treatment of the surfactant C12E10 (oxyethylated fatty alcohol) (C12E10) in the continuous flow activated sludge facility. No statistically significant accumulation of C12E10 on the alive activated sludge was detected, probably because of faster C12E10 fission than its adsorption. However, significant adsorption of the short-chained ethoxylates (including free alcohol) on the alive activated sludge was found, as well as statistically significant adsorption of long- and short-chained PEG. The adsorption of surfactant C12E10 and its biodegradation by-products on dead activated sludge was found to be higher than the species adsorption on alive activated sludge. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Activated sludge; Non-ionic surfactants; Poly(ethylene glycols); Adsorption; Ac-voltammetry

1. Introduction Non-ionic surfactants (NS) are the major component of numerous consumer product chemicals, including detergents. These products are ejected to the aquatic environment, passing through the sewage system and municipal sewage treatment plants (STP). Despite the fact that an increasing percentage of sewage is processed in STP, NS concentration in river water has dramati*Corresponding author. Tel.: +48-61-6652-786; fax: +4861-6652-746. E-mail address: [email protected] (Z. Lukaszewski).

cally increased over the past decade [1]. Therefore, the fate of NS in STP and in the aquatic environment should be thoroughly examined. Basically, the stream of NS directed to the aeration chamber of STP is divided into two parts: NS dissolved in a solution phase, and NS adsorbed on activated sludge. Therefore, knowledge of NS adsorption on alive activated sludge is necessary in order to calculate the balance of NS in the aeration chamber and in STP as a whole. An additional factor necessary in such calculations is the amount of NS removed from the aeration chamber together with an excess of activated sludge. NS adsorption on activated sludge is also an important factor in the modelling of NS removal in STP [2].

0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 2 7 5 - 0

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However, a method for the determination of NS adsorption on alive activated sludge has not as yet been developed. It is necessary to mention an attempt to determine this characteristic by Bruschweiler . et al. [3]. The authors of this paper attempted to determine an adsorption isotherm of oxyethylated alkylphenol having 11 oxyethylene subunits on activated sludge. The activated sludge was in contact with the surfactant solution for 1 h and the sludge was then filtered; surfactants were determined in the filtrate by the classical BiAS method. In our opinion, the result of such an experiment was the joint effect of the adsorption and biodegradation of the surfactant. The aim of this work was to develop a method for the sampling, separation and determination suitable for the determination of NS adsorbed on the alive activated sludge, under the conditions of the continuous flow test. Application of alive activated sludge is very important, because alive and dead activated sludge may exhibit very different adsorptive characteristics. Dead activated sludge behaves like a typical adsorbent i.e. it does not exhibit enzyme activity. Therefore, the additional aim of that work was the determination of the difference between the adsorptive ability of alive and dead activated sludge. The basis of measurement is the determination of NS concentration in a slurry of activated sludge and in a solution phase. The difference between these two concentrations represents the NS adsorbed on activated sludge. Such an approach is similar to that applied for the determination of NS adsorbed on river water particles [4]. Surfactant C12E10 belonging to oxyethylated alcohols was selected as a model surfactant. Biodegradation of this surfactant under the conditions of the continuous flow activated sludge test have been reported previously [5]. Apart from the tested surfactant, its biotransformation by-products (long- and short-chained PEG and short-chained ethoxylates) were also investigated. The presence of these biotransformation by-products in treated sewage has been previously reported [5]. The methods for the determination of long-chained PEG [5], short-chained PEG [6] and short-chained ethoxylates (including free alcohol) [7] have been developed.

alternating voltage amplitude of 2 mV and a scan voltage rate of 400 mV min
1, were applied. Controlled-temperature hanging mercury drop electrode equipment (Radiometer) was used having an additional platinum wire auxiliary electrode. A quartz beaker instead of a glass beaker was used. A Plexiglas Husmann plant [8,9] was used. It consisted of an aeration vessel connected to a clarifier and equipped with a stirrer. The volume of the aeration vessel was 3.5 l. Activated sludge used in the experiments was brought from the STP in Szamotuly, located near Poznan, Poland, which treats typical municipal sewage. Fatty alcohol ethoxylate surfactant C12E10 (Sigma) was used without additional purification. Purified sodium sulphate and sodium chloride of Analar grade were used for the preparation of the aqueous base electrolytes. Freshly distilled ethyl acetate was used. Other reagents used were of Analar grade. All solutions used in the analysis were prepared in water triply distilled from a quartz apparatus. Only freshly distilled water was used. The modified Dragendorff reagent [10] was prepared by mixing solutions A and B before use. Solution A: 1.7 g of basic bismuth(III) nitrate, 65 g of potassium iodide and 220 ml of glacial acetic acid/1000 ml. Solution B was an aqueous solution containing 290 g of barium chloride dihydrate in 1000 ml. The solution for dissolving the precipitate (solution C) was prepared from 12.4 g of tartaric acid and 18 ml of ammonia solution (25%) made up with water to 1000 ml. A silica gel cartridge (Bakerbond SPE silica gel 7086-03) was used for the purification of this solution. Treated synthetic sewage samples obtained by processing NS under the conditions of the continuous flow activated sludge (Husmann plant) were used. Fatty alcohol ethoxylate–surfactant C12E10 was biodegraded under these conditions. Apart from the tested surfactant, synthetic sewage contained peptone, beef extract, urea, NaCl, CaCl2, MgSO4, K2HPO4 and NaHCO3 [9]. All samples were preserved with formalin [11] and stored in glass bottles. In the case of water phase samples, 1% formalin was used while, in the case of a slurry of activated sludge, 5% formalin was applied.

2. Experimental 2.2. The principle of the measurement 2.1. Apparatus, reagents and materials A Radelkis OH-105 polarograph and an ECO Chemie General Purpose Electroanalytical System (micro)AUTOLAB were alternatively used for alternating current voltammetric measurements (called tensammetric measurements). A standard mode of measurement (without phase sensitivity), a frequency of 60 Hz, a superimposed

It is assumed that the concentration of the yet-to-betested surfactant is the same in every part of the slurry of activated sludge, due to the intensive circulation of the slurry in the aeration tank and in the lower part of the sedimentation tank. The difference between the concentration of the NS or its metabolites in the bulk of the slurry (point 1, Fig. 1) and the point in the

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calculated in accordance with the equation

air

C3 ¼ C1 þ C2 :

synthetic sewage + surfactant

ð1Þ

Concentration of species adsorbed on the alive activated sludge (C4 ) was calculated in accordance with the equation C4 ¼ Csp1
Csp2 ¼ C3
C5 ¼ C1 þ C2
C5 ; effluent 1

2

ð2Þ

where Csp1 is the concentration of species in sampling point 1, Csp2 is the concentration of species in sampling point 2, and C5 is the concentration of species in the solution phase. 2.3. Procedures The continuous flow activated sludge test (Procedure 1)

slurry of activated sludge Fig. 1. Sampling points: (1) the bulk of slurry of activated sludge, (2) the solution phase.

sedimentation tank just above the line of sediment (point 2, Fig. 1) was used as the measure of the adsorption of the NS or its biotransformation byproducts on the alive activated sludge. The slurry should contain both the NS and its biotransformation by-products adsorbed on the sludge, as well as the dissolved surfactant and its biotransformation by-products. The solution phase just above the sediment, which exited the slurry, should contain only the dissolved surfactant and its metabolites i.e. without particles of activated sludge. Therefore, the concentration of C12E10 and its biotransformation by-products were determined both in the samples taken from point 1, as well as point 2. The difference represents the adsorption of C12E10 and its biotransformation by-products on alive activated sludge. The samples were immediately preserved with formalin. In this moment all living micro-organisms were killed and the adsorptive ability of activated sludge was changed from the state characteristic of alive activated sludge to that characteristic of dead activated sludge. The ratio between the species adsorbed on activated sludge and dissolved in solution phase was changed. However, the total amount of species in the slurry remained constant. Concentration of species dissolved in the water phase of the slurry of activated sludge (C1 ) and species adsorbed on the suspended matter of the slurry (C2 ) was determined separately. The total concentration of species in the slurry (C3 ) was

Unit operation was begun by filling the aeration vessel with 3.5 l of activated sludge having a concentration of suspended solids of 2.5 mg l
1. Synthetic sewage (without surfactant) was applied at a rate of 1 l h
1. The synthetic sewage containing the surfactant C12E10 (10 mg l
1) was supplied after a three-day delay. The concentration of dissolved oxygen, suspended solids as well as pH and temperature were systematically controlled during the experiments. The dissolved oxygen was generally kept within the range of 3–4 mg l
1. The temperature in the aeration vessel was kept within the range of 19–211C. pH measured in the aeration vessel varied between 7.2 and 7.4. Samples of the water phase were collected from the solution in the clarifier just above the activated sludge (sampling point 2) and were immediately preserved with 1% formalin. Samples of the slurry were collected from the aeration tank (sampling point 1) and were immediately preserved with 5% formalin. Initial steps of separation of residual surfactant and its biotransformation by-products from a slurry of activated sludge (Procedure 2) A 100 ml sample of slurry of activated sludge was filtered using a paper filter of medium density. The filtrate was processed as described in Procedure 3. The precipitate was eluted with 3 portions of chloroform (20+15+15 ml). The whole extract was then gently evaporated and the residue dissolved in 100 ml of water. Further processing of the sample included a sequential extraction as described in Procedure 3. Separation of residual surfactant and its biotransformation by-products from the water phase (Procedure 3) Each kind of water sample i.e. the water phase sample from sampling point 2, the filtrate of the slurry of activated sludge as well as water solutions obtained from the treatment of suspended particles of activated sludge (described in Procedure 2) was separated as described below. The separation was performed as described in a previous paper [12]. Briefly, sodium chloride (30 g) and

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sodium hydrogencarbonate (0.1 g) were dissolved in 100 ml of the sample. The solution was sequentially extracted with two portions of ethyl acetate (total volume, 50 ml) and three portions of chloroform (total volume, 50 ml). The ethyl acetate fraction contains the residual surfactant and short-chained ethoxylates, including free fatty alcohol, while the chloroform fraction contains long- and short-chained PEG. 2.3.1. Determination of concentration of residual surfactant by the BiAS-ITM (Procedure 4) Determination was performed according to the procedure described in previous papers. The procedure, originally developed for model conditions [13] and for river water [4], was adapted for ethyl acetate extracts obtained as described in Procedure 3. An aliquot of ethyl acetate fraction was evaporated by gentle heating in a small quartz beaker. The residue was then dissolved in a mixture of 2 ml of methanol and 16 ml of water. Solution A (8 ml) and solution B (4 ml) of the modified Dragendorff reagent were mixed and added to the sample. The reacting mixture was stirred for 20 min and left for 10 min. Orange coloured precipitate was filtered through a G5 glass filter; no washing of the precipitate was done. The precipitate was dissolved in 20 ml of hot solution C: The filter and the beaker were then washed with 3– 4 ml of water, which was subsequently added to the solution containing the dissolved precipitate. After cooling the solution, the volume was made up to 25 ml with water. An aliquot of the solution was transferred to a 25 ml volumetric flask. 2.5 ml of 5 M aqueous sodium chloride and 1.80 ml of ethyl acetate were added and the flask filled to the mark with water. The mixture containing an excess of ethyl acetate was vigorously shaken and the emulsion transferred into a voltammetric cell. The emulsion was stirred for 10 min to achieve clarity of the solution due to evaporation of the excess of ethyl acetate. This excess of ethyl acetate needed to be removed, as evidenced by the disappearance of turbidity. After a period of rest (30 s) the tensammetric curve of ethyl acetate was recorded in the cathodic direction using a new mercury drop and starting from
1.20 V (vs. SCE). The difference between the height of the peak of ethyl acetate (recorded in a separate measurement) and that of ethyl acetate in the presence of substancesto-be-determined is the analytical signal. The results were quantified using a calibration curve of surfactant C12E10. 2.3.2. Separation and determination of short-chained ethoxylates and free alcohol (Procedure 5) The filtrate, after precipitation of residual C12E10 (as described in Procedure 4), was diluted to 50 ml with water and 10 g sodium chloride was added and

dissolved. A short-chained ethoxylates fraction was extracted with three portions of ethyl acetate having a total volume of 25 ml. Extracts were collected in a 25 ml volumetric flask and filled to the mark with ethyl acetate. An aliquot of ethyl acetate solution was evaporated by gentle heating in a small quartz beaker. The residue was then dissolved in an accurately measured 1.50 ml of ethyl acetate and the solution transferred into a 25 ml calibrated flask along with water used for rinsing. 12.5 ml of 1 mol l
1 sodium sulphate solution was added and the mixture diluted to volume with water. The indirect tensammetric technique (ITT) determination was performed as in Procedure 4 using the calibration curve of the mixture of n-dodecanol and n-dodecyltetra (ethylene glycol) (1:1). Precision of determination was 17%. 2.3.3. Determination of concentration of long-chained PEG by the BiAS-ITM (Procedure 6) A general conception of the determination of longchained poly(ethylene glycols) (PEGlch) by the BiASITM has been provided by Lukaszewski et al. [13] and Szymanski et al. [5]. An aliquot of the chloroform fraction was evaporated and dissolved in a mixture of 2 ml of methanol and 16 ml of water. The rest of the procedure was performed as in Procedure 4. 2.3.4. Separation and determination of short-chained PEG (Procedure 7) The filtrate, after precipitation of PEGlch (described in Procedure 6), was diluted to 50 ml with water and 10 g sodium chloride added and dissolved. PEGsh were extracted with three portions of chloroform having a total volume of 25 ml. Extracts were collected in the 25 ml volumetric flask and filled to the mark with chloroform. An aliquot of chloroform solution corresponding to approximately 2.5–25 mg PEGsh was evaporated by gentle heating in a small quartz beaker. The residue was dissolved in an accurately measured 1.50 ml of ethyl acetate and then transferred into a 25 ml calibrated flask along with water used for rinsing. 12.5 ml of 1 mol l
1 sodium sulphate solution was added and the mixture diluted to volume with water. ITT determination was performed as in Procedure 4 using the calibration curve of the mixture of ethylene glycol and PEG 200 (1:1).

3. Results Three independent sample pairs were taken from sampling point 1 and sampling point 2 (see Fig. 1) of the continuous activated sludge facility processing surfactant C12E10. The samples were taken during the stable period of the test. The water phase samples taken from sampling point 2 were sequentially processed in

A. Szymanski et al. / Water Research 37 (2003) 281–288

accordance with Procedures 3–7, while the slurry samples (taken from sampling point 1) were sequentially processed in accordance with Procedures 2–7. Concentrations of residual surfactant, short-chained ethoxylates, as well as long- and short-chained PEG were determined in the solution phase (C5 ¼ Csp2 ) and in the water phase of the slurry of activated sludge (C1 ). The same species adsorbed on the suspended matter of the slurry were also determined (C2 ); the latter represents the species adsorbed on dead activated sludge. Additionally, the concentration of activated sludge was determined. From these data, using Eq. (1), the total concentration of species in the slurry of activated sludge was calculated (C3 ) and then the concentration of species adsorbed on particles of alive activated sludge (C4 ) was calculated. Finally, the concentration of the residual surfactant and its biodegradation by-products on particles of alive and dead activated sludge was calculated (by dividing the bulk concentration of analyte by the concentration of suspended solids Css ). The results concerning the residual surfactant C12E10 are given in Table 1 and those for short-chained ethoxylates (including free alcohol) are given in Table 2. The results concerning the long- and short-chained PEG are given in Tables 3 and 4, respectively. In order to determine the precision of measurements, a large volume sample was taken (No. 4) and divided into 7 pairs of subsamples. The subsamples were sequentially processed in accordance with Procedures 2–7. The only difference was that the aqueous solution obtained from the slurry of activated sludge was extracted with chloroform and the extract was mixed with the chloroform extract of particles and processed jointly. Due to this simplification of the procedure, the adsorption of the species on particles of dead activated sludge was not calculated. Seven independent pairs of the results provided the opportunity to calculate the precision of measurements and confidence limits. The confidence limits were added to the results of sample No. 4 in Tables 1–4.

285

4. Discussion Surprisingly, there is a small difference between the results concerning the residual surfactant C12E10 for samples taken from sampling point 1 and sampling point 2 (see Table 1). This means that surfactant C12E10 is barely accumulated on particles of alive activated sludge. However, this is not evidence of the poor adsorptive ability of particles with respect to surfactant C12E10. It is more probable that the rate of fission of the surfactant by enzymes is approximately equal to the adsorption rate. Only 2.5% of introduced surfactant in the slurry remained non-biodegraded. It is worth stressing that particles of dead activated sludge exhibited 3.5 times higher surfactant C12E10 accumulation than particles of alive activated sludge. Approximately 50% of the residual surfactant C12E10 was adsorbed on particles of dead activated sludge. The results concerning short-chained ethoxylates (Table 2) show the great difference between concentrations at sampling points 1 and 2. This is evidence of high adsorption of short-chained ethoxylates on particles of the alive activated sludge. Hence, it is not surfactant C12E10, which accumulates on particles of alive activated sludge, but its major biodegradation byproduct. It is worth stressing that the analytical procedure used does not clarify which by-product it is: short-chained ethoxylates or free alcohol. Adsorption of short-chained ethoxylates on particles of dead activated sludge is only 20–35% higher than that on particles of alive activated sludge. The long-and short-chained PEG are the other biotransformation by-products of surfactant C12E10. Accumulation of long- chained PEG on particles of alive activated sludge (see Table 3) is relatively small but statistically significant. Particles of dead activated sludge adsorbed approximately 65% more than those of alive activated sludge. Accumulation of short-chained PEG on particles of alive activated sludge, though statistically significant, is even smaller than that of long-chained

Table 1 Adsorption of surfactant C12E10 on alive and dead activated sludge Sample

Activated sludge concentration (Css ) (g l
1)

Concentration of surfactant C12E10 in the bulk of activated sludge slurry (Csp1 ) (mg l
1)

Concentration of surfactant C12E10 in the water phase (Csp2 ) (mg l
1)

C12E10 adsorption on alive activated sludge (C4 =Css ) (mg g
1)

C12E10 adsorption on dead activated sludge (C2 =Css ) (mg g
1)

Ratio

1 2 3 4

2.86 2.64 2.69 2.19

250 270 240 255715

210 225 205 250710

12 16 14.5 3

46 58 43

3.8 3.6 3.0

Standard: surfactant C12E10.

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Table 2 Adsorption of surfactant short-chained ethoxylates on alive and dead activated sludge Sample

Activated sludge concentration (Css ) (g l
1)

Concentration of short-chained ethoxylates in the bulk of activated sludge slurry (Csp1 ) (mg l
1)

Concentration of short-chained ethoxylates in the water phase (Csp2 ) (mg l
1)

Short-chained ethoxylates adsorption on alive activated sludge (C4 =Css ) (mg g
1)

Short-chained ethoxylates adsorption on dead activated sludge (C2 =Css ) (mg g
1)

Ratio

1 2 3 4

2.86 2.64 2.69 2.19

2300 2100 1950 20507150

1000 850 1050 1250775

430 460 340 350750

540 550 460

1.25 1.20 1.35

Standard: the 1:1 mixture of n-dodecanol and n-dodecyltetra(ethylene glycol).

Table 3 Adsorption of long-chained PEG on alive and dead activated sludge Sample

Activated sludge concentration (Css ) (g l
1)

Concentration of long-chained PEG in the bulk of activated sludge slurry (Csp1 ) (mg l
1)

Concentration of long-chained PEG in the water phase (Csp2 ) (mg l
1)

Long-chained PEG adsorption on alive activated sludge (C4 =Css ) (mg g
1)

Long-chained PEG on dead activated sludge (C2 =Css ) (mg g
1)

Ratio

1 2 3 4

2.86 2.64 2.69 2.19

1090 980 1070 1130750

740 670 690 965735

120 110 140 80725

220 180 205

1.85 1.65 1.45

Standard: the 1:1 mixture of PEG200 and PEG400.

Table 4 Adsorption of short-chained PEG on alive and dead activated sludge Sample

Activated sludge concentration (Css ) (g l
1)

Concentration of short-chained PEG in the bulk of activated sludge slurry (Csp1 ) (mg l
1)

Concentration of short-chained PEG in the water phase (Csp2 ) (mg l
1)

Short-chained PEG adsorption on alive activated sludge (C4 =Css ) (mg g
1)

Short-chained PEG adsorption on dead activated sludge (C2 =Css ) (mg g
1)

Ratio

1 2 3 4

2.86 2.64 2.69 2.19

1440 1240 1420 1140750

1070 1140 1290 1040725

75 40 50 50725

130 105 95

1.75 2.35 1.90

Standard: the 1:1 mixture of ethylene glycol and PEG200.

PEG (see Table 4). On the other hand, particles of dead activated sludge adsorbed approximately twice more than those of alive activated sludge. The surprising fact of almost negligible surfactant C12E10 accumulation on particles of alive activated sludge can be easily explained: the stream of surfactant is quickly split by extracellular enzymes following the

reaction (already proposed in Swisher’s monograph [14] shown in Fig. 2 as the first step. Therefore, only short-chained ethoxylates or free alcohol and PEG are accumulated on the alive activated sludge. Although this conclusion, drawn on the basis of the investigation of a single surfactant cannot be generalised, the possibility of a lack of ethoxylate

A. Szymanski et al. / Water Research 37 (2003) 281–288 enzyme ⇓ ⇓ CH3-CH2-(CH2)n-CH2-O-CH2-CH2-O-CH2-CH2-(O-CH2-CH2)m-OH substrate ⇓

CH3-CH2-(CH2)n-CH2-OH or + CH3-CH2-(CH2)n-CH2-O-CH2-CH2-OH short-chained ethoxylates & free alcohol

HO-CH2-CH2-(O-CH2-CH2)m-OH or HO-CH2-CH2-(O-CH2-CH2)m-1-OH long-chained PEG ⇓

HO-CH2-CH2-(O-CH2-CH2)1-5-OH short-chained PEG

⇓

CO2

Fig. 2. General biodegradation pathway of oxyethylated alcohols.

accumulation on alive activated sludge should be seriously taken into consideration. As opposed to the case of surfactant C12E10, shortchained ethoxylates adsorbed well on particles of alive activated sludge (see Table 2). It is worth stressing that short-chained ethoxylates were biodegraded relatively poorly. The concentration of short-chained ethoxylates in the slurry of activated sludge was of the order of 2000 mg l
1, while the expected concentration (on the basis of central fission to fatty alcohol) is 2900 mg l
1. This yields only a 29% biodegradation of fatty alcohol. If dodecylmonoethoxylate is the expected product, the calculation yields 43% of its biodegradation. A relatively low adsorption of long-chained PEG and short-chained PEG at a relatively high concentration of these species in the solution is a symptom of fast further degradation of these PEG at or on the surface of alive activated sludge. The obtained results support a previously proposed pathway [6,5] shown in Fig. 2 as the second step. Despite the fact that the aim of this work was to develop the method for the determination of adsorption on particles of activated sludge, some considerations concerning the adsorption of surfactant C12E10 and its biodegradation by-products on particles of alive and dead activated sludge can be drawn. Accumulation of surfactant C12E10 and its biodegradation by-products on particles of alive activated sludge is generally lower than the adsorption of these species on particles of dead activated sludge. The higher the rate of biochemical steps, the larger the difference. In the case of surfactant C12E10, biodegradation was 97.5% of the initial surfactant concentration and the adsorption of surfac-

287

tant C12E10 on particles of dead activated sludge was approximately 3.5 times higher than that on particles of alive activated sludge. In the case of long-chained PEG and short-chained PEG the biodegradation was 80–85% of the expected value and the adsorption of both species on particles of dead activated sludge was 1.5–2.5 times higher than that on particles of alive activated sludge. In the case of short-chained ethoxylates, the biodegradation percentage was only 30–40% of the expected value, and the adsorption of short-chained ethoxylates on particles of dead activated sludge was only approximately 25% higher than that on particles of alive activated sludge. The adsorption of the species on particles of dead activated sludge reflects the pure adsorptive ability of activated sludge i.e. unchanged by enzymatic reactions. Generally, adsorption depends on the concentration of activated sludge and on the bulk concentration of adsorbed species. The adsorption should be governed by an adsorption isotherm. The slope of the Henry isotherm can be calculated from the data of Tables 1–4. It yields 0.1970.05 l g
1 for surfactant C12E10, for short-chained ethoxylates, 0.2570.05 l g
1 0.1270.02 l g
1 for long-chained PEG and 0.0870.02 l g
1 for short-chained PEG. The higher the hydrophobicity of species being adsorbed the higher the slope of Henry’s isotherm. Basically, the pure adsorptive ability of alive activated sludge should be very similar to that of dead activated sludge. The observed accumulation of the species on alive activated sludge is the total effect of biochemical reaction and adsorption. Thus, the accumulation of the species on alive activated sludge is rather ‘apparent adsorption’ than adsorption as such. The developed method can be used as a tool for the investigation of the adsorption of different ethoxylates on the alive activated sludge. The results of such an investigation will facilitate a more thorough understanding of ethoxylate biodegradation pathways and help to build a model for the calculation of NS biodegradation in the future.

5. Conclusions 1. A sampling, separation and determination procedure has been developed for the determination of NS adsorbed on alive activated sludge. The procedure also enables the determination of adsorption of major biodegradation by-products: short-chained ethoxylates, long- and short-chained PEG. 2. No statistically significant accumulation of the surfactant C12E10 on the activated sludge was detected. 3. Significant adsorption of the short-chained ethoxylates (including free alcohol)—the biodegradation

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by-product of surfactant C12E10, on the activated sludge was found. 4. Statistically significant adsorption of long-chained PEG and short-chained PEG on the alive activated sludge was found. 5. The adsorption of surfactant C12E10 and its biodegraded by-products on dead activated sludge is higher than the species adsorption on alive activated sludge. The higher the rate of biochemical step, the larger the difference.

[5]

[6]

[7]

Acknowledgements This work was supported by the Committee of Scientific Research (grant No. 3TO9C 03016) and by Poznan University of Technology (grant No. PB 31-08/ 02 BW)

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