S URFACTANTS

N. Voulvoulis and M.D. Scrimshaw

4.3. FATE OF ENDOCRINE DISRUPTERS IN SEWAGE TREATMENT WORKS

4.3.2 S URFACTANTS

The group of surfactants of concern is alkylphenol polyethoxylates (APEOs) and their breakdown products, alkylphenols (APs) and alkylphenol carboxylates (APECs); all have been shown to be estrogenic.43,49,56,57 In aerobic conditions, the oxidative shortening of the polyethoxylate chain occurs easily and rapidly. However, complete mineralization is poor due to the presence of the highly branched alkyl group on the phenolic ring. The hydrophilic group in ethoxylated compounds con- tains more abundant carbon than the hydrophobic alkyl group. These moieties are therefore potential sources of bacterial nutrients that become available by the suc- cessive removal of ethoxy groups. This chain shortening results in the formation of recalcitrant intermediates such as AP₁EO, AP₂EO, AP, AP₁EC, and AP₂EC. Ultimate biodegradation of these metabolites occurs more slowly, if ever, because of the presence of the benzene ring and their limited water solubility. The frequent occur- rence of oxidized intermediates, such as APEC, may indicate oxidative mecha- nisms.^58,59 It is possible that nonoxidative ether scission dominates APEO biodeg- radation to shorter homologs. Longer lived intermediates undergo hydroxyl group oxidation as a side chain reaction and are possibly catalyzed by alcohol hydrogenases known to occur ubiquitously in bacteria.

It has been concluded that degradation occurred in two steps. The first step occurred within 10 hours of aeration and was attributed to the cleavage or oxidation of the ethoxylate chain^60,61; this stage occurs rapidly. The second step required more time because of the need to develop a specific population of enzymes or bacteria that was not initially available. Figure 4.4 shows a theorized degradation pathway for APEOs.

APEO lipophilicity decreases with increasing chain length. As a result, shorter chain compounds have the highest tendency to sorb onto the solid phase and com- pounds with longer chains appear in final effluent.^62–64 AP’s are more lipophilic compounds than APEOs with higher log K_ow values (4.48 for 4-nonyl phenol [4NP]).

120 Endocrine Disrupters in Wastewater and Sludge Treatment Processes

Because of this, APs, 4NP in particular, sorb onto the solid phase making them more resistant to biodegradation.^64–67 APECs are more water soluble and have a very limited tendency to be found in the solid phase; they are found in high concentrations in final effluent.^58,68–70

The most dramatic change in APEO distribution appears during the activated sludge phase of treatment.⁵⁸ After treatment, no APEO with greater than 8 ethoxy units were detected. In the primary effluent NP_3–20EO were dominant and in sec- ondary effluent the metabolite (NP and NP_1–2EO) concentration had increased to greater than 70%. Table 4.5 indicates the NP and NPEO concentrations at each stage of treatment in a mechanical-biological process. Table 4.6 contains some influent compared to effluent concentrations at different STW.

In several studies, NPECs were found to be the dominant species in final effluent.^68,69 In one study⁶⁸ the concentration of NP₁EC and NP₂EC in secondary effluent (5 to 20 µg l^–1) was 5 times more than any other metabolite. This is more than their inverse estrogenicity, so it has been theorized that NPEC could be the most relevant of the nonylphenolic compounds. Concentrations in secondary efflu- ents were 2.1 to 7.6 times higher than in primary effluents, indicating that aerobic biological treatment results in their formation.⁵⁸

AP and APEC are also sometimes found in raw sewage at low concentrations, despite not being found in commercial products. This implies that some partial degradation occurs in the sewage system before the influent reaches the STW.⁶⁵

Nitrification during activated sludge can increase removal efficiencies.^58,71 Low- loading nitrifying conditions enable greater removal; almost complete removal for FIGURE 4.4 Theorized degradation pathway for APEO in STW.

AP +

R OH

[CH₂CH₂0]n CH₂CH₂OH

[CH₂CH₂O]n CH₂COOH

[CH₂CH₂O]n CH₂CH₂OH

R O

R O

R O

AP_nEO, n=2-20

AP_nEC, n=0-2

AP_nEO, n=0-2

Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 121

APEO with greater than 6 ethoxy units has been observed. The elimination rate decreases with decreasing chain length. In high loading, non-nitrifying conditions removal was reduced.

Temperature has a great influence on alkylphenolic surfactant removal in STW.58,68,72–74

Removal efficiencies are greater in summer than winter due to increased temperatures.

The metabolic rate of microorganisms slows in colder temperatures, so biodegradation decreases. The acclimation time required by bacteria also increases with a decrease in temperature.⁷⁴ The acclimation period has also been seen to increase with an increase in influent alkylphenolic compound concentration⁷⁵ and ethoxylate chain length.⁷⁶

TABLE 4.5

NPEOs and NP in a Mechanical-Biological Treatment Plant of Municipal Wastewater

NP NP₁EO NP₂EO

Raw wastewater 14 µg l^–1 18 µg l^–1 18 µg l^–1 Treated wastewater 8 µg l^–1 49 µg l^–1 44 µg l^–1

Receiving water 3 µg l^–1 7 µg l^–1 10 µg l^–1

Activated sludge 128 mg kg^–1 76 mg kg^–1 61 mg kg^–1

Anaerobically digested sludge

1000 mg kg^–1 79 mg kg^–1 —

Anaerobic sludge effluent

467 µg l^–1 53 µg l^–1 6 µg l^–1

Source: From Ahel, M. and Giger, W., Determination of nonionic surfactants of the alkylphenol polyethoxylate type by HPLC,Anal. Chem., 57, 2584, 1985. With permission.

TABLE4.6

Influent and Effluent Concentrations for Alkylphenolic Compounds in STW

STW Location

Influent Concentrations (µg l^–1)

Effluent Concentrations (µg l^–1)

3 STW — Switzerland¹⁷⁴ <10–35 NP

24–133 NP₁EO

<10–70 NP₂EO 1 STW (mechanical-

biological) — Switzerland¹⁷³

14 NP 18 NP₁EO 18 NP₂EO

8 NP 49 NP₁EO 44 NP₂EO 5 STW (mechanical-

biological) — Switzerland¹⁷⁵

844–2250 NPEO + NP 40–369 NPEO + NP

11 STW — Switzerland⁵⁸ 1090–2060 NP 240–760 NP

12 STW — U.K.¹⁷⁶ <0.2–330 NP

1 STW (mechanical- biological) — Rome¹⁷⁷

800 NPEO 8 NPEO

1 Municipal STW — U.S.¹⁷⁸ 143–272 NPEC

122 Endocrine Disrupters in Wastewater and Sludge Treatment Processes

Three Pseudomonas species have been identified as degrading NP₉EO:

Pseudomonas putida strain Fus1BI, Pseudomonas sp. strain SscB2, and Xanthomo- nas sp. strain SccB3. None of these species was able to biodegrade 4NP.⁷⁷ Another study⁷¹ showed that Pseudomonas spp. only degrade the polyether moiety to produce NP₂EO. This study also backs up other observations that an attack on the ethoxylate chain at the terminal portion by microorganisms results in shorter mono substitutes and NP. APEOs with less than three ethoxy units are transformed into their corre- sponding APECs. Intermediate products will not be detected if APEOs are com- pletely degraded by one microorganism, or if the growth rate of a microorganism using an intermediate product is greater than the APEO degrader. Microbial consortia are expected to be necessary due to the amphiphilic nature of the molecules, a consortia may operate synergistically or with a commensalistic relationship where one organism benefits from the breakdown while another is unaffected. Full-scale STW works generally provide greater removal efficiencies than smaller treatment works; this may be due to a more diverse microbial population and nutrient avail- ability being present in larger works.³⁰

Maki et al.⁵⁹ found that Pseudomonas sp. strain TR01 isolated from activated sludge degraded APEOs at an optimum temperature of 30°C and optimum pH 7.

The bacteria were unable to mineralize NPEO, but were able to degrade ethoxy units exclusively. The dominant degradation product was NP₂EO and a small amount of NP₂EC without the presence of any other organisms. This species is expected to play an important role in STW activated sludge processes. A unique substrate assimilability was observed as it metabolizes the ethoxy chain only when the chain is linked to a large hydrophobic group. This work is supported by other studies that have observed shortened but unoxidized NPEO formed by mixed estuarine cultures.⁷⁸ The biodegradation of NP has been observed by Sphingomonas sp.⁷⁹ Pseudomo- nas sp. was also present although it is thought it provided nutrients for the growth of Sphingomonas sp. rather than degrading 4NP itself. More than 95% of the NP was degraded within 10 days and no aromatic compounds were detected suggesting that the phenolic part was also degraded. The main degradation products were alcohols, the major one being nonanol. Different isomers of NP were used. This resulted in the formation of different isomers of nonanol, implying that the alcohols were derived from the alkyl group. Candida maltosa is a species of yeast that has been found to degrade 4NP to produce 4-acetylphenol.⁸⁰ The yeast was isolated from sludge at a textile industry treatment plant and used 4NP as its sole carbon source.

4.3.3 PESTICIDES

4.3.3.1 Triazine Herbicides

Tests in soils with triazines indicate that abiotic transformations occur⁸¹ and that adsorption to soils results in an increase in half-life. In raw wastewater, triazine herbicides demonstrated negligible adsorption during bench scale primary sedimen- tation.⁸² Changes in solids concentration and residence time had no effect on removal rates, and the compounds passed unchanged into secondary treatment processes.

Triazines may partition into lipid structures of biological flocs. They may also

Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 123

chemically bind to bacterial proteins and nucleic acids in activated sludge, rather than adsorbing onto inorganic particulate matter. This may explain the negligible adsorption onto primary waste solids compared to the 40% removal during secondary treatment.⁸² In batch test experiments,⁸³ a loss of 25% for atrazine and a loss of 33% simazine were observed. Primary degradation was thought to be the mechanism responsible.

However, this is disputed in other tests⁸² in which comparable losses were seen in live and dead activated sludge, making adsorption a more likely mechanism.

Atrazine has been seen to have a detrimental effect on STW at high concentra- tions.⁸⁴ At 20 mg l^–1, an increase in effluent chemical oxygen demand (COD) was observed after 4 days with a concomitant reduction in mixed liquor volatile sus- pended solids and viable (total) bacterial numbers.

4.3.3.2 Organochlorine Insecticides

Organochlorine insecticides behave in a similar manner to PCBs as they sorb to the solid phase in STW during primary and secondary treatment.^85,86 Because of this association with suspended solids, optimization of suspended solids removal should result in optimization of compound removal, but this has not been noted.⁸⁷ No obvious correlation between hydraulic and solids loading and pesticide or PCB sorption were observed.¹¹ It is possible that compounds associate with nonsettleable fine particles. Therefore, removal may not be related to suspended solids removal, since the particle size fraction with which the compounds may be associated will comprise a small portion of the total suspended solids.⁸⁷

Lindane exhibited moderate sorption. Only 1 to 15% was removed through the sludge and was thought to only degrade anaerobically by reductive dehalogenation, although it has been seen to degrade aerobically by other workers.^88,89 After adap- tation, substantial removal was observed through biodegradation. Biodegradation was optimal at intermediate and high sludge loadings and poor at high sludge age.

At low SRT sorption became particularly important.⁸⁸ Removal of lindane by degradation was greatest at high sludge loading (70 to 80% at 0.3 to 0.8 mg BOD MLSS d^–1) and poor at high sludge ages (30 to 40% at 25 to 32 days). These results indicate that biodegradation by co-metabolism was the dominant process.⁸⁹ 4.3.3.3 Chlorophenoxy Acid Herbicides

Chlorophenoxy acid herbicides (CPHs) have a relatively high aqueous solubility and are less lipophilic and more polar than other pesticides and herbicides, which means that their association with solids is negligible. Partitioning tests showed that CPHs were poorly removed and that removal rates increased slightly with increasing suspended solids and lower flow rates.^90,91 Removal during primary sedimentation is minimal; however, removal during activated sludge treatment is greater since CPHs are reasonably biodegradable.⁸⁹ CPH structure appears to be important, indicating that biological mechanisms are involved.⁹² Rate constants were highest at interme- diate sludge loadings (0.16 to 0.17 mg BOD MLSS d^–1) and low at high sludge ages (25 to 32 days). This demonstrates co-metabolic rather than catabolic transformations are responsible.

124 Endocrine Disrupters in Wastewater and Sludge Treatment Processes

2,4-dichlorophenoxyacetic acid (2,4-D) does not sorb to the solid phase in appreciable amounts, and biodegradation is a more important process.^89,93 Its trans- formation from an acid to a short chain ester is an important process. 2,4-D removal is a function of sludge age and sludge loading, respectively. Rate constants were highest at intermediate sludge loadings and lowest at high sludge ages, demonstrating that co-oxidation plays a role.

It has been reported that mecoprop, dichlorprop, and 2,4-D were degraded in activated sludge within 7 days,⁹⁴ when 86 to 98% elimination of dissolved organic carbon was also observed. During treatment in sequencing batch reactors, long acclimation periods of approximately 4 months were required before 2,4-D biodeg- radation was observed.⁹⁵ More than 99% removal was achieved after this period and was independent of HRT. The acclimation period required is linearly related to bacterial population density and the initial 2,4-D concentration.⁹⁶ 2,4-D was also degraded in anaerobic conditions with a first order rate constant after a lag phase.

The main metabolite found after anaerobic degradation on 2,4-D was 2,4-dichlo- rophenol and small amounts of 4-chlorophenol. Nitschke et al.⁹⁷ observed 100%

removal of mecoprop in laboratory activated sludge with prenitrification, although a long lag phase was required. Over the same 6-week period, 4% isoproturon and 8% terbutylazine were also removed by degradation.

4.3.3.4 Chlorophenols

Chlorophenol removal is influenced by sorption and degradation mechanisms. High removals have been seen in STW for dichlorophenol as it is easily degraded during activated sludge processes with a limited amount of sorption taking place.^89,98 Dichlo- rophenol is readily degraded. It is also produced as a biological breakdown product from CPHs and the chlorination of final effluents, resulting in an increase in con- centrations found in effluents.

Negligible sorption of pentachlorophenol was found in laboratory scale tests but adsorption to activated sludge was found in STW.^88,99 Sorption could be a significant removal mechanism if the ratio of chemical oxygen demand to pentachlorophenol in the influent is very high. Longer sludge ages result in greater removal rates because pentachlorophenol is toxic to bacteria until acclimation has taken place.⁸⁹

4.3.4 ORGANOTINS

Organotin degradation can involve the sequential removal of organic moieties to produce more toxic products, such as the formation of di- and monobutyltins from debutylation of tributyltin (TBT).¹⁰⁰ The high lipid solubility of organotins allows association with intracellular sites and degradation is known to take place in some bacteria, fungi, and algae. However, in STW, degradation of organotins, either aerobically or anaerobically, is insignificant and adsorption is the most important factor.^101–103 There were no observed differences in TBT degradation during aerobic, anaerobic, mesophilic, and thermophilic digestion.

In a Canadian STW,¹⁰³ monobutyltin was found in all influent samples but di- and tributyltin were found occasionally. The average reduction of monobutyltin was 40%; this significant reduction was due to biodegradation and sorption to sludge.

Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 125

In the primary clarifier in a STW in Zurich,¹⁰¹ 73% of total butyl tin was eliminated from wastewater due to adsorption, during secondary sedimentation an additional 17% was removed. Further removal during activated sludge treatment was minimal with biodegradation accounting for only 8% removal.

4.3.5 ORGANIC O^XYGEN C^OMPOUNDS