Sources, behaviour and fate of organic contaminants during sewage treatment and in sewage sludges

the Science of the Total Environment *l.Q-)Wb%hdl*h m,o1be6*.ra”de.l**~ ELSEVIER

The Science of the Total Environment 185 (1996) 3-26

Sources, behaviour and fate of organic contaminants during sewage treatment and in sewage sludges Howard R. Rogers WRc Medmenham

Laboratory,

Henley

Road,

Marlow,

Bucks,

SL7 2HD.

UK

Recentconcernover the environmentalimpactof sewagesludgeapplicationto agriculturalland hasdrawn particular attention to the wide range of organic contaminantsthat may enter sewagetreatmentprocesses and persistin biosolidsfor disposal.This paperdiscusses processes influencingthe fate and behaviourof organiccontaminantsduring wastewatertreatmentand reviewsliterature relatingto specificcontaminantsidentifiedin sewagesludge.The diffrcultiesassociated with the developmentof specificmethodsfor the analysisof trace residuesof organiccontaminants in complexmatricessuchassludgeare discussed. Somepotential issuesrelatingto impact of sewagesludgedisposed to agriculturalland are alsoconsidered.

1. Intr~on In the last 50 years the production of synthetic organic chemicals for industrial and domestic use has increased dramatically from 7 million tonnes in 1950 to 63 million tonnes in 1970 [ 11. Inevit&ly, the occurrence and concentration of organic contaminants in efIluents, sewage and sewage sludge has also increased. However, until the 1980s relatively little was known about the diversity of synthetic organic chemicals that enter sewage treatment works, the efficiency with which they are degraded during sewage and sewage sludge treatment processes and their concentrations in sewage sludge for disposal. Point source discharges from industrial users or manufacturers, and diffuse discharges from commercial and domestic

premises or in run-off following aerial deposition contribute to the organic contaminant loading in sewage. Synthetic organic chemicals in efIIuents or sewage may be present in solution or sorbed on solids. However, during sewage treatment many contaminants partition onto solids as a consequence of their hydrophobicflipophilic nature resulting in enrichment in sewage sludge solids at concentrations several orders of magnitude higher than in the influent sewage [2-41. The subsequent fate of these organic chemicals following sewage sludge disposal is a topic of current concern and there is a requirement for such basic information as concentrations of specific compounds in sludge in order to assess the environmental impact of specific sludge disposal practices (Table 1). There is also a need for a better understanding of the be-

0048-9697/96&X00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0048-9697(96)05039-B

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Rogers / The Science of the Total

Table I Sewage sludge disposal in the UK, 1980-81 151 Mode of disposal

% of total disposed

Grassland Arable land Horticulture and allotments Forestry Land reclamation Fertiliser manufacture Landfill or storage on STW site Landfill elsewhere Marine disposal Incineration

13.9 24.1 0.9 0.1 4.5 0.4 14.3 6.8 31.2 3.7

haviour and fate of organic contaminants during sewage treatment processes. During sewage treatment it is likely that many organic compounds, particularly hydrophobicflipophilic compounds are sorbed onto sludge where they consequently occur in much higher concentrations than in the sewage from which the sludge was derived. Other compounds may be unaffected by sewage treatment and remain in the aqueous etXuent, are completely degraded or mineralised during treatment or partially degraded to produce breakdown products. This paper reviews current information about the occurrence of organic contaminants in sewage sludge and their fate during sewage treatment processes. 2. Organic chemistry of sewagesludge

Sewage sludge is a highly complex waste that results from the primary and secondary treatment processes in sewage treatment works. Organic materials constitute about 40-80% of its dry weight and it is within this fraction that most synthetic organic compounds are located [6]. The major organic loading originates from human excreta, and is a complex mixture of fats, proteins, carbohydrates, lignin, amino acids, sugars, celluloses, humic material and fatty acids [7]. A large proportion of this organic material is in the form of both live and dead microorganisms which

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provide a large surface area (0.8- 1.7 m2. g-‘) for sorption of hydrophobic organic residues [8]. The mechanism of the association of organic residues with sludge solids has recently been represented by Wang et al. [8] as a two-stage process involving sorption onto the surface of the sludge and partitioning into the interior of the biomass. 3. Prioritisation of contaminant residues

Analytical investigations into organic contaminants in sewage sludge could potentially result in the identification of numerous residues of anthropogenic synthetic organic chemicals and their degradation products. However, the identification and prioritisation of organic contaminants of potential concern is made difficult for the following reasons [9-131. (1) Adequate, validated analytical techniques are required; determinands may themselves be mixtures of many components (e.g. polychlorinated biphenyl congeners, PCBs, and ‘dioxins’ or polychlorinated dibenzo-p-dioxins and dibenzofurans PCDDsDFs); highly sensitive techniques are required to detect low concentrations; extensive sample clean-up and fractionation procedures are usually necessary because of the presence of interfering co-contaminants and analytical problems are further exacerbated by the complex matrix of biologically derived material (fats, oils, etc.). (2) Sorption of organics onto sewage solids may reduce degradation by making them less available to bacteria. Transformation of residues to produce toxic by-products has been shown to be significant for some organic contaminants but relatively little is known about the extent of transformation of toxic contaminants during sewage treatment [14]. (3) The behaviour and fate of sludge derivedorganic contaminants following disposal is not clear and requires further investigation in order to assessthe significance of residues in terms of their persistence and potential environmental impact. The potential for synergistic effects of complex mixtures of synthetic organic residues in sewage sludges is not understood although some work has been done which suggests that some sludge extracts may exhibit mutagenicity in the Sulmonella Ames bacterial test. However, the non-uniform

H. R. Rogers / The Science of the Total Environment

extraction procedures adopted, and the variability of sludge matrices make a representative extract containing all genotoxins present in a sludge sample difficult to isolate [ 1% 171. 4. Fate and bebaviour of organic contaminants durlag treatment processes

Organic contaminants in emuent streams can be removed by a wide range of different processes (Fig. 1). However, most of these techniques are only appropriate to specific effluents of industrial origin and are expensive. In order to predict whether or not a particular organic contaminant is likely to be accumulated by the sewage sludge matrix the following factors need to be taken into account. .

.

sorption (onto solid surfaces/association with fats and oils) chemical degradation (abiotic processes e.g. hydrolysis)

l

0

185 (19%)

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biodegradation volatilisation

Sorption and volatilisation are physical processes and their importance for specific contaminants can be predicted using physicochemical data. During primary sedimentation, hydrophobic contaminants may partition onto settled primary sludge solids and this tendency to accumulate in sewage sludge solids can be assessed using the

octanol-water partition

coefficient (K,,).

In gen-

eral the following guide to the significance to sorption can be used: 10gKOW< 2.5 log& > 2.5 and lo&, > 4.0

c

4.0

low sorption potential medium sorption potential high sorption potential

The significance of volatilisation losses of specific organic compounds during sewage treatment can be estimated using the following empirically defined categories based on Henry’s Law constant U-b) and &,: TOXIC

METABOLlTES r>

Fig. 1.Organiccontaminant fate duringsewage andindustrialeflluenttreatment.

rl

INNOCUOUS DEGRADATION PRODUCTS

H.R.

6

H, > 1 x 10e4 and H,j&,, high volatilisation

> I

x

10e9

< 1

X

10T9

potential

H, < 1 x 10T4 and I-IJZ&, low volatilisation

Rogers / The Science of the Total Environment

potential.

Synthetic organic contaminants may be transformed or lost via a variety of processes during wastewater treatment. Primary sedimentation may result in the partitioning of hydrophobic residues onto sedimented suspended solids. Secondary treatment can involve both aerobic (trickling filters, activated sludge treatment, oxidation ponds) or anaerobic (sludge digestion) biodegradative processes. It is also possible that some limited anaerobic degradation occurs, prior to treatment, in sewers where bacterial slimes accumulate on sewer walls. However, although the mechanism of degradation of the bulk organic components of sewage sludge such as cellulose, proteins and carbohydrates during anaerobic digestion is well understood the effects of such processes on synthetic organics have received relatively little detailed study. In essence anaerobic digestion follows a two-phase process, an acid phase and a methanogenic phase. Hydrolysis of polysaccharides, proteins and fats results in the formation of long chain fatty acids, mono- and disaccharides and amino acids, and volatile acids such as formic, acetic and butyric acid are also produced. The methanogenic process ultimately results in the reduction of the volatile acids to methane and carbon dioxide [ 18,191. Recently, work using pilot plant studies has been carried out to attempt to assessthe degree of decontamination with respect to organic residues that can be achieved during wastewater treatment 120-231. Other research has been carried out to evaluate the effects of effluent toxicity on aquatic organisms and then attempted to correlate the toxic response to the degree of xenobiotic organic contamination in the effluent waters [24]. In general there is a sparsity of data relating to the behaviour of specific organic contaminants during sewage treatment processes. However, a pilot plant scale investigation carried out by Petrasek et al. [20] looked at the behaviour of a mixture of 22

185 (1996)

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priority organic contaminants during an activated sludge process. The compounds studied included pesticides, phenols, phthalates and polycyclic aromatic hydrocarbons (PAHs). Parallel control and spiked treatment sequences were carried out with contaminant concentrations of between 50-l 50 pg. 1-l in order to assess the significance of effects of processes occurring during treatment. Activated sludge treatment relocated the organic contaminants onto the sludge solids very efficiently, as was evident from the increased concentrations in the primary and return activated sludges. The pilot plant system produced a 80-99% reduction in the concentrations of the spiked concentrations in the aqueous phase. Mass balances for the contaminants were calculated and it was found that apart from pentachlorophenol which associated with the solid phases, the phenols were largely degraded. The PAHs were enriched in the primary sludge to the greatest degree (mean 64%). Of the phthalate esters, bis-(Zethylhexyl) and di-n-octyl phthalate were probably the most resistant to microbial degradation. A clear relationship between log&, and sorption to primary and activated sludges was observed and this supports the use of this parameter to estimate the partitioning of other contaminants during sewage treatment. A fate and behaviour study was carried out by Constable et al. [25] to assess the effects of different sludge processing operations on polycyclic aromatic hydrocarbons (PAHs). This work showed that although anaerobic digestion increased the leachability of the PAHs from sludge it did not result in a significant mass loss or a change in phase distribution. However, heat treatment did result in a reduction in mass of contaminants in the sludge, and the remaining residues were less susceptible to leaching after heat treatment. Zitomer and Speece [26] recently reviewed the efficacy of both aerobic and anaerobic processes for the removal of organic contaminants during treatment processes (Table 2). They concluded that a sequential regime using a combination of oxidative and reductive steps was likely to be most efficient for the removal of recalcitrant organic contaminants. In general, most highly chlorinated

H. R. Rogers / The Science of the Total

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Table 2 General assessmentsof aerobic biodegradability (adapted from Zitomer and Speece (261) Readily biodegradable

Moderately biodegradable

Slightly biodegradable

Refractory

Cyclohexane, octane, benzene, m and p cresol, 4-chlorophenol

I-Decanol, ethylbenzene, 4-bromophenol

Decane, phenanthrene, ethyl ether

Chloroform, chlorobenzene. I ,4-dichloro-benzene, I ,2+trichlorobenzene, hexachlorobenzene

(highly oxidised) organic residues such as PCBs and volatile organohalogen compounds are resistant to oxidative degradation under aerobic conditions and will only degrade appreciably under anaerobic conditions. Some general ‘rules of thumb’ based on structural factors can be applied when assessing the potential biodegradability of organic residues [19]. Molecules with highly branched hydrocarbon chains are generally less amenable to biodegradation than unbranched compounds, and short chains are not as quickly degraded as long chains. Also, unsaturated aliphatic compounds are generally more accessible to degradation than saturated analogues. Nucleophilic displacement reactions such as hydrolysis can occur when molecules have linkages separating highly polar groups. Reductive dehalogenation has been identified as the most significant degradative pathway for highly toxic compounds such as polychlorinated pesticides and polychlorinated biphenyls. However, although reductive dehalogenation has been recognised as a predominantly biologically medi-

ated process reactions may occur abiotically via nucleophilic substitutions in anaerobic environments when hydrogen sulphide is present. Investigations under abiotic conditions suggest that this is a route for dehalogenation of alkyl halides [27]. The extent and importance of reductive dechlorination as a degradative process for organic contaminants during the anaerobic digestion of sewage sludge has not been studied in detail but has been identified as a possible transformation route for some specific compounds (Table 3). Evidence suggests that the source of the incubating culture can significantly affect its dehalogenating ability and it is likely that the efficiency of dechlorination may be inhibited by the presence of high concentrations of co-contaminants. 5. AMIytlcal

aspects

5.1. Samplestorage

Sewage sludges should ideally be analysed immediately after sampling. However, if this is not practicable the bacterial activity should be ar-

Table 3 Reductive dechlorination of organic residues in digested sewage sludge [26] Contaminant

Intermediates

Hexachlorobznzene

Pentachlorobenzene I ,2,4,5-Tetrachlorobenzene, 1,2,4-trichlorobenzene 2,4Gichlorophenol

2,4,&Trichlorophenol 2,4,5-Trichlorophenol Tetrachloroethylene Dichloromethane DDT 2,CDichlorophenoxy acetate 2,4,5-Tricblorophenoxy acetate

Trichloroethylene, vinyl chloride Chloromethane

1,2-dichloroethylene,

2,4-Dichlorophenol, 4-chlorophenol 2,4,5-Trichlorophenol

1,3,5-TCB Dichlorobenzene isomers CChlorophenol 3,CDichlorophenol Ethylene Methane and CO? DDD, DDE Phenol Phenol

8

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of the Total Environment

rested in order to preserve the integrity of the sample. Samples should be stored at 4°C and chemical preservatives such as Hg2+ or Co2+ or 1% sodium azide added to halt bacterial activity [28-301. Other workers have recommended the addition of formaldehyde (0.5% v/v) or copper sulphate (4 g. 1-l) in order to quench bacterial activity /31,32]. Blanketing of samples with nitrogen is essential if anaerobic incubation experiments are to be performed (291. Borosilicate glass bottles are the best containers for the collection or storage of sewage sludge if trace organic contamination is to be minimised. However, care should be taken to avoid pressure build up and vessels should be kept cool after tilling. 5.2. Pre-extraction treatment Drying of sewage sludge at elevated temperatures prior to analysis is not recommended as volatiles may be lost and the sample may be subjected to a risk of contamination. However, ambient air drying has been used by some workers analysing sludge for ‘dioxin’ residues [33]. Centrifugation of sewage sludge is frequently employed in the preparation of sludge solids for extraction provided, of course, that the determinands of interest are not appreciably soluble in the aqueous phase. Chemical drying of sludge with anhydrous sodium sulphate followed by ball milling can be used to ensure sample homogeneity (34,351. 5.3. Extraction of sludge samples There are a variety of different methods used for the extraction of organics from sewage sludge, for example: (1) Soxhlet extraction 1361; (2) High frequency mechanical dispersion [37]; (3) Freezedrying/Soxhlet extraction [38]; (4) Micro-steam distillation [39]; (5) Azeotropic drying/solvent extraction 1381; (6) UltrasonicatiorVextraction 1401; (7) Continuous liquid-liquid extraction [41]. In general, Soxhlet extraction is recognised as being one of the most useful methods of separating organic residues from bulk sample solids because it avoids the troublesome problems associated with interfacial mixing at the liquid-liquid boundary which can result in reduced determinand recoveries. Furthermore, Soxhlet extraction with

I85 (19%)

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continuous solvent recycling generally gives higher recoveries of determinands than liquid-liquid extractive techniques, but it is essential that the solids are finely divided and dry prior to extraction (36,411. However, Soxhlet extraction does not provide any specificity of extraction which can be useful in reducing co-extractable analytical interferences. Liquid-liquid extraction has the potential disadvantage that it also extracts high molecular weight organic compounds (e.g. lipids) and other components (sulphur) from sewage sludge which must be removed before gas chromatography or GCMS analysis is undertaken. Microsteam distillation involves the distillation of aqueous-based samples, with subsequent transfer of any steam volatile organic compounds present in the sample into an organic solvent. One advantage of the technique is that high molecular weight organic compounds and inorganics are not coextracted and samples thus require less clean-up before analysis. This technique has been used in the analysis of organochlorine pesticides, polychlorinated biphenyls, alkylphenols and fatty acids in sewage sludges [39,42-441. There is increasing interest in supercritical fluid extraction (SFE) as a method for extraction of organic contaminants from environmental samples as it has advantages of reducing solvent consumption and enables more rapid extraction times than conventional procedures. There may also be scope for tailoring extraction conditions using liquid COz modified with a small percentage of methanol. Soxhlet extraction of dried material continues to be a favoured technique which avoids the troublesome problems of liquid-liquid extraction such as build-up of suspended matter or emulsions at the solvent/sample interface. Furthermore, Soxhlet extraction with continuous solvent recycling generally ensures higher recoveries of determinands than other techniques. However, many workers favour extraction methods involving homogenisation of wet sludge with mixed solvent systems [45,46]. Any difliculty with inter-facial emulsions is usually overcome by the addition of aqueous sodium chloride or isopropanol. The solvent used for extraction usually depends upon the polarity of the components to be isolated. For example, car-

H. R. Rogers / The Science of the Total Environment

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ures are not performed then serious interferences can result during gaschromatographic or mass spectrometric determination by way of co-eluting peaks, detector saturation and ‘memory-effects’ caused by capillary column overloading. Electron capture detectors are particularly susceptible to contamination by sulphur in concentrated solvent extracts. Column chromatography is the most common method of sample fractionation. Partially deactivated silica, alumina or magnesium silicate (Florisil) are used to fractionate components in organic extracts on the basis of their polarity. These materials retain polar compounds such as lipids or fats and solvent mixtures of increasing polarity can be used to successively elute components of interest. Gel permeation chromatography (GPC) has also been used as a means of separating organic fractions in sludges as part of a multi-step clean-up procedure for 54 priority organic compounds from the USEPA priority listing [38]. Dichloromethane extracts were applied to porous styrenedivinylbenzene copolymer (Biobeads S-X8) and higher molecular weight material such as triglycerides, long chain hydrocarbons and long chain fatty acids eluted first followed by lower molecular weight organics. Column chromatographic and acid/basic extractions were also used to separate fractions into neutral, acid and basic components (Fig. 2).

bon tetrachloride was usually the solvent of choice for the recovery of greases and oils from sewage sludges, although dichloromethane is now used more widely for occupational health and environmental reasons [36,47]. The choice of extracting solvent also depends on the sorptive/column chromatographic or solvent transfer procedures used during sample clean-up or fractionation. Dichloromethane is particularly useful as it is not photosensitive like for example acetone. Pentane and diethyl ether are difficult to reduce and maintain at a specific volume because of their high volatility and hexane is preferable. The use of a small amount of low volatility keeper solvent to extracts e.g. n-decane can help reduce determinand losses during evaporation. Adjustment of sample pH prior to extraction allows the separation of components on the basis of their acidic, basic or neutral characteristics. For example, phenols in sewage sludges are usually partitioned into an alkaline aqueous-phase, the pH is then lowered to pH 2 and phenols are reextracted into dichloromethane [38]. 5.4. Clean-upprocedures

Because sewage sludge samples yield complex mixtures of organic compounds during extraction, it is essential that the various organic fractions are segregated prior to analysis. If clean-up procedSl uilqe

4

Extract

I

with

KM

Wash

0. Ji M NaOH 4 using S-X8 1 Collect 2 fractions, GPT-2 1 Clean up GPC-I tsinq activated 5111ca gel J Cmbine GK-2 and cleaned up CPC-1 1 nna1yse by CL-MS WUTrw^S Fractionate I3iwReads

GPC-1,

10 g wet wt 1 Dilute with O.lM Phosphate fwtfer PH 7 1 Extracts with CHC13,‘ethnnol 1 Extract with 2N R2SO4 J Neutralise aqueous extract to pJ1 6-l Using 1M Na3m4 J Extract with ciirl 3 4 Add Methanol to CKi extract and concentr a VP 1 Dilute with O.lM Acetate Ruffer pl, 4.7 1 Analyse by t%‘l,C Electrochemical Detection O!axNIC ‘RAsES’

Fig. 2. Scheme for the separation of semivolatile priority pollutants in sludge [381.

9

10

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The Science of the Total Environment

Gas-stripping of volatile fractions (e.g. volatile organic solvents) from wastewaters followed by trapping as a means of clean-up, with analysis by gas chromatography-mass spectrometry has been evaluated by Haile et al. [48]. Purge and trap techniques have also been used for the analysis of purgeable volatiles in sewage sludges using the following sequence [49]: (1) 5-ml sludge sample; (2) nitrogen purging; (3) adsorption on Tenax trap; (4) desorption at 180°C with back flushing; (5) on-column cryotrapping of desorbed organics with liquid nitrogen; (6) release of trapped organics and analysis by GCMS. In addition to column chromatographic cleanup techniques a number of chemical treatments have been used for the removal of specific interferents. Lipids have been separated from extracts by several workers using destructive removal with sulphuric acid [45,50,51]. Elemental sulphur (which can be present at g kg-’ levels in anaerobically digested sludges) is a commonly encountered interferent which, if not removed, can saturate electron capture detectors and obscure components of interest in the gas chromatogram. Ethanolic potassium hydroxide or Raney nickel has been used to remove sulphur from sludge extracts, but this treatment has the disadvantage that hexachlorocyclohexane isomers, DDT and DDD, determinands of frequent interest, are also lost by dechlorination [45]. Goerlitz and Law [52] recommended metallic mercury as an effective sulphur clean-up reagent that does not affect pesticide residues; however, this method produces a large amount of finely divided precipitate which may act as a co-precipitant or sorbent for determinands of interest. Some other procedures that have been used for sulphur removal are CufAl alloy refluxing and potassium cyanide treatment [50,53]. A particularly useful liquid-liquid sulphur clean-up procedure using the tetrabutylammonium (TBA) sulphite ion-pair has been used by Jensen et al. [45] i.e. (TBA+)$0s2-

+S -

2 TBA+ + S20s2-

Following separation and clean-up procedures it is usual for sample concentration to be carried out before analysis by GC or GCMS. It is impor-

185 (1996)

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tant that losses of volatile determinands during this stage of sample preparation are known. Constable et al. [54] tested the recoveries of standard solutions of PAHs using nitrogen stream evaporation, rotary evaporation, Vigreaux or Snyder column distillation, stainless steel gauze column distillation and spinning band column evaporation. The spinning band column was found to be impracticable and slow. Rotary evaporation and nitrogen evaporation were the simplest methods, and gave good recoveries provided that a slow rate of evaporation was used and reduction to dryness was avoided. Recently, the advent of equipment such as the ‘Turbovap’ type evaporator has reduced the potential problem of determinand loss due to sample drying by using electrical sensors which halt evaporation when a set solvent volume is reached. Protection of samples from direct sunlight to avoid photolytic degradation of photolabile residues is important during sample work-up. However, the crucial factor during solvent reduction steps is careful sample manipulation and quantitative transferral of extracts. Analytical investigations of organic contaminants in sewage sludge usually adopt either a broad spectrum extraction technique followed by chromatographic clean-up and gas chromatography mass spectrometric analysis in order to provide general qualitative and semi-quantitative data, or use analytical methods designed to determine specific organic contaminants that are known to have extensive use domestically or industrially or are priority substances. The following sections summarise the literature on organic contaminants in sewage sludges. 6. Oeeurrenee of organic contaminants in sewage sludge 6.1. Organochlorine pesticides and polychlorinated biphenyls

Organochlorine pesticides have received considerable attention since their introduction in the 1940s and 1950s for the control of disease vectors and agricultural pests. In the 1950s and 1960s a growing awareness of the long-term risks of persistent pesticides resulted in extensive studies of their environmental effects 1551.

H. R. Rogers / The Science of the Total

Polychlorinated biphenyls were first manufactured commercially in 1929 and their widespread usage and industrial value can be attributed to their thermal stability, chemical inertness and high dielectric constant. Consequently these chemicals were widely used as transformer fluids, in electrical insulating materials, as plasticisers in neoprene, PVC and other artificial rubbers. They have also been incorporated into printing inks, wax polishes, nitrocellulose weather-resistant lacquers, high temperature lubricants, asphalts and paints. PCBs are now recognised to be ubiquitous and highly persistent contaminants of the natural environment. However, this fact was not realised until the advent of gas chromatography-electron capture detection techniques in the 1950s which made trace environmental analysis for both organochlorine pesticides and PCBs possible. This technique is still the most widely used for analysis of chlorinated trace residues [55]. Since 1970, the industrial usage of PCBs has largely been curtailed, but they remain a major class of organic contaminants [56]. Both PCBs and organochlorine pesticides are persistent and hydrophobic [57] and associate strongly with the suspended solid fractions of raw sewage and with sludge solids during primary sedimentation and after conditioning and dewatering of sludges [57,58]. Garcia-Gutierrez et al. [59] monitored PCBs, a-HCH, aldrin, endrin and dieldrin concentrations during primary sedimentation in a pilot plant study. Different hydraulic loadings and influent suspended solid loadings were applied and the effects on organic contaminant distribution observed. A small proportion of the organic contaminants were found to associate with non-settleable solids or colloids. However, the main proportion was bound to settled solid phases and there were no obvious correlations between hydraulic and solids loadings and pesticide and PCB sorption (Table 4). McIntyre and Lester [60] carried out a survey of 444 UK sewage sludges. The samples were of a wide variety and included raw primary, humus sludges, chemically conditioneddewatered and anaerobically digested sludges. Aldrin and endrin were found at lower levels than the other pesticides, probably because they have limited use in the UK (Table 5).

Environment

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II

Table 4 Chlorinated pesticide and PCB removal during sedimentation 1591

primary

Contaminant

Removal efficiency (%)

Mean (%)

g-HCH Aldrin Dieldrin Endrin PCBs

14-49 S-80 19-62 9-63 9-41

32 31 39 35 28

Data on the concentrations of PCBs in sewage sludges have only recently been obtained in terms of individual congener concentrations, previous data being expressed as Aroclor mixture equivalents [61,62]. In general the higher chlorinated congeners have been found in highest concentrations in the following order of abundance: C 10 1 > Cl80 > Cl53 > Cl38 > C52 > C28 [63]. Hill and McCarty (641 investigated the effect of anaerobic conditions on the degradation of DDT, aldrin, dieldrin, heptachlor, lindane (g-HCH) and endrin. Enhanced degradation was observed when compared with aerobic conditions for all the compounds except dieldrin and heptachlor epoxide. Lindane (g-HCH) and aldrin degraded much more rapidly under anaerobic than aerobic conditions, and more effective degradation occurred with increased biological activity. DDT degraded to DDD under anaerobic conditions, but persisted under aerobic conditions. The resistance to degradation followed the order dieldrin > heptachlor epoxide > aldrin > DDD > DDT >

Table 5 PCBs and organochlorine pesticides in UK sewage sludges 1601 Contaminant

Concentration range (cg g-‘)

Mean concentration (PI2g-9

g-HCH Aldrin Dieldrin Endrin Total PCBs

0.01-70 0.01-0.2 0.01-53 0.01-0.7 0.01-22

0.4 0.03 0.5 0.1 0.3

I?

H. R. Rogers / The Science of the Total Environment

endrin > heptachlor > lindane. Reductive dehalogenation has the effect of reducing the level of chlorination of organochlorine pesticides and PCBs, so making them more amenable to further aerobic and anaerobic degradation and in general rendering residues less toxic [65]. Mogilevich [66] identified Clostridium rectum as an obligate anaerobic bacterium capable of degrading lindane and the bacterial enzymes responsible for the dechlorination were dehalogenases. Permethrin, a synthetic pesticide based on pyrethrum, is widely used as an alternative to dieldrin as a mothproofing agent and is also used for the disinfestation of water lice in water mains. Concentrations of cis- and trans-permethrin have been found in UK sewage sludges at levels between <0.15-40.8 mg kg-’ with median values of 1.8 mg kg-’ (cis-isomer) and 3.8 mg kg-’ (trunsisomer). The isomer ratios were similar to those found in commercial mixtures which suggests that selective degradation was not occurring during sewage treatment. These highly hydrophobic compounds were not detectable in all of the sludges analysed presumably because of their intermittent usage and discharge to sewers [67]. 6.2. Chlorophenols and chlorophenoxy

acids

Hill et al. [68] described a procedure for the determination of chlorophenoxy acids in sewage sludge involving an esterification of analyte prior to GC. They detected 1.2 pg 1-l of 2,4-D (2,4dichlorophenoxyacetic acid) and 0.3 pg 1-l of 2,4,5-TP (2,4,5-trichlorophenoxypropionic acid). A more extensive study of chlorophenoxy acids and chlorinated neutral compounds in agrochemical sewage and sewage eftluent was carried out by Folke [69]. Chlorophenoxyalkanoic acids have been widely used as selective herbicides since the early 194Os, mainly being used for weed control (701. The levels of chlorophenols, chlorophenoxyalkanoic acids and chlorinated neutrals are shown in Table 6. The chlorophenols that were detected in the influent sewage mainly originated from the manufacture or degradation of chlorophenoxyalkanoic acids and the aqueous effluent showed a fivefold reduction in total chlorophenols when compared to the influent sewage. During sewage treatment chlorophenoxyalkanoic

18.5 (19%)

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Table 6 Phenoxyalkanoic acid and chlorophenolic residues in sewage [691 Compound

Concentration in agrochemical sewage (rcg I-‘)

Secondary eflluent (ag 1-V

CChlorophenol 2Chlorophenol 2-Chloro-6-methyl-phenol 4Chloro-2-methyl-phenol 2,iDichlorophenol 2,4Dichlorophenol 2,4-Dichloro-6-methyl-phenol 2,4,6Trichlorophenol 2,3,4,6-Tetrachloro-phenol Pentachlorophenol 2-(4-Chloro-2-methylphenoxy)-propionic acid (MCPP) 4-Chloro-2-methyl-phenoxy acetic acid (MCPA) 2-(2,4Dichlorophenoxy)propionic acid (2.4DP) 24Dichlorophenoxy acetic acid (24-D) 2-(4-Chloro-2-methylphenoxy)-propionic acid methyl ester 4-Chloro-2-methylphenoxy acetic acid methyl ester 2,4Dichloro&methyl phenoxy acetic acid methyl ester

0.1 27 4 2400 17 87 76 180 ND ND 90

0.03 0.1 0.2 4 0.6 0.5 2 8 0.03 0.1 0.4

375

1.3

18

0.1

13

<0.04

0.04

ND

0.2

ND

0.02

ND

ND, below detection limit.

acids can be microbially degraded to chlorophenols via either ether-cleavage or by ring-opening of hydroxyphenoxyalkanoic intermediates. Buisson et al. [71] found that MCPP, MCPA, 2,4-DP and 24-D were degraded efficiently during anaerobic digestion. Ionic strength effects were suggested as being important in altering their partitioning with solids, but not pH. A 16-55% reduction in 24-D by an activated sludge process has been quoted 1691. The data of Folke 1691 indicate that phenoxyalkanoic acid concentrations in treated e&ent were about 10% of that in raw sewage. Hill et al. (681 found that 24-D and 2,4,5-TP were significantly enriched in the sludge solid phase and

H.R. Rogers/ The Science of the Total Environment I85 (1996) 3-26 Table 7 Efficiency of chlorophenoxy herbicide removal during primary sedimentation [64] Herbicide

% removal

2,4-D MCPA 2,4,5-T 6CPA MCPP 2,4,5-TP

18 <23 <26 <26 <29 <40

tried to predict the behaviour of the herbicides during a primary sedimentation process. Unlike PCBs or the cyclodiene type pesticides, chlorophenoxy herbicides (CPHs) have appreciable aqueous solubilities. Partitioning tests showed that CPHs were poorly removed (<40%) at normal suspended solids concentrations. However, high suspended solids concentrations and lower flow rates did increase CPH removal. This might be expected as CPH aqueous solubilities are relatively high (24-D 620 mg 1-l) and octanolwater partition coefficients are low (2,4-D log K = 2.81). Table 7 shows the efficiencies of CYH removal during primary sedimentation. The biodegradation of chlorophenols has been investigated extensively. Moos et al. [72] and Guthrie et al. [73] found that pentachlorophenol was more efficiently degraded under anaerobic than aerobic conditions in waste water treatment systems. However, culture experiments have shown that pentachlorophenol can inhibit its own degradation at quite low concentrations (1.4 ag 1-l) [74]. Salkinoja-Salonen et al. [75] have described a biofilm reactor capable of efficiently mineralising chlorophenolics in pulp mill effluents; these authors also found that PCP was the most persistent of the chlorophenols. The anaerobic microbial degradation of mono- and dichlorophenol in sewage sludge was also investigated by Boyd and Shelton [76]. The rates of degradation of the compounds were found to decrease in the order 2-chlorophenol > 3-chlorophenol > 4chlorophenol. The dichlorophenols with a Cl substituent ortho to the phenolic hydroxyl group, lose the ortho

13

chlorine by the following processes of reductive dechlorination: (1) 2,6-dichlorophenol is reductively dechlorinated to 2-chlorophenol; (2) 2,3- and 2,5-dichlorophenol produce 3chlorophenol; (3) 2,4-dichlorophenol produces 4-chlorophenol. Similarly, the related class of halobenzoate compounds undergo aryl halide removal under anoxic conditions [77]. Under aerobic conditions the mode of degradation mainly involves aromatic ring cleavage followed by dehalogenation of the alkyl chain. Direct replacement of the Cl substituents by hydroxyl groups may also occur [77]. In contrast, aerobic degradation of organochlorines is generally very slow and some compounds are resistant.

6.3. Organophosphoruscompounds

This group of organic contaminants is rapidly degraded during sewage treatment processes and is not usually detectable in most sewage sludges [78,79]. However, localised inputs of organophosphorus pesticides in sewage as a result of industrial activities or accidental spills during usage could increase levels in sludge. McIntyre et al. 1781 monitored the degradation of diazinon, malathion and ethyl parathion in spiked sewage sludges. The pesticides were found to be particularly susceptible to degradation at alkaline pH > 7. The conditioning of sewage sludge with lime (CaO) is a common dewatering procedure which would certainly aid the alkaline hydrolysis of organophosphorus residues. In another study McIntyre et al. [79] cited the identification of organophosphorus components of ‘Reofos 95’ in sewage sludge. ‘Reofos 95’ is a flame retardant composed of a mixture of isopropylphosphate esters of substituted phenols. The GC-FPD chromatogram of the commercial product matched that of a sludge extract and the levels corresponded to an estimated ‘Reofos 95’ concentration of between 750 and 6000 c(g 1-r. The residues identified probably originated from localised textile industry effluent discharges in the catchment.

14 Table 8 Nitrosamine

H.R.

concentrations

Rogers

in sewage sludge

of

the

[80,81] Concentration (pg 1-P

Compound

Dimethylnitrosamine N-Nitrosodimethylamine N-Nitrosodiethylamine N-Nitrosopyrrolidine N-Nitrosomorpholine

/ The Science

(DMN) (NDMA) (NDEA) (NPYR) (NMOR)

0.3 l-53
Total

Environmenr

IX5

( 1996)

3-26

discharges, road run-of‘f and illicit d~sposat r~i waste motor vehicle engine oil from domestic UI garage premises. Discharge of white spirit (a mixture of petro-. leum hydrocarbons with a boiling point range of 152-210°C) from solvent-based paints and domestic brush washing probably contributes some parafflnic, cycloparafinic and alkylbenzene residues to sewage, but the current move away from solvent-based paints may reduce this input. 6.6. Alkylphenols

6.4. Nitrosamines

and nitroaromatics

Mumma et al. [80] and Brewer et al. [81] have detected volatile nitrosamines in sewage using an azo-dye/colourimetric determination. These compounds have carcinogenic properties and disrupt genetic DNA by SNl or SN2 nucleophilic reactions [82]. Dimethyl nitrosamine (DMN) is used to control nematodes, as a nitrification inhibitor in soil, as a plastic&r for acrylonitrile polymers and in high-energy batteries. The concentrations of nitrosamines in sewage sludges are shown in Table 8. Of the nitrosamines identified N-nitrosodimethylamine (NDMA) was ubiquitous and was detected in all but one of the 15 sludges analysed. Nitroaromatic compounds have been investigated by Phillips et al. [83] using GC-TEA. Nitrotoluenes are widely used in the manufacture of dyestuffs and urethane products, and 2,4dinitrotoluene has been detected at levels of 0.1 mg 1-l (wet wt.) in sewage sludge.

Non-ionic 4-alkylphenol polyethoxylates are used in large quantities as detergents. In 1974, this type of surfactant accounted for 18% of the total surfactants manufactured in Europe. The problem of surfactant foaming in surface waters resulted in the adoption of more readily biodegradable detergents such as alkylphenol polyethoxylates (861. Giger et al. [87] were the first to identify 4nonylphenol (NP) as a major degradation product of alkylphenol polyethoxylate detergents in anaerobically digested sewage sludges. Concentrations of NP in sewage effluent prior to anaerobic digestion were in the range 36-202 pg 1-l [87]. However, following anaerobic digestion, extraordinarily high concentrations of NP (0.45-2.5 g kg-’ dry wt.) were found in the sewage sludge [87,88].

During the anaerobic stages of wastewater treat-

6.5. Mineral oils

Britcher and Waggott [84] surveyed oil concentrations in UK sewage sludges using infra-red spectroscopy. Adsorption bands for functional groups characteristic of petroleum hydrocarbons were compared to spectra of sludge extracts. About 78% of the ‘mineral oil’ was associated with the sludge and very little was biodegraded during treatment. The mean sludge oil concentration was 620 mg 1-l (wet wt.) and the aqueous eflluent contained 0.2-5.5 mg 1-l. Liu [85] found petroleum hydrocarbons in anaerobically digested sludges at similar concentrations (434-7580 mg 1-l). Mineral oil residues probably originate from industrial

Fig. 3. Formation [87,88].

of nonylphenol

during

sewage

treatment

H. R. Rogers/The Science of the Total Environment 185 (19%) 3-26

ment the polyethoxylate chains of the 4-nonylphenol polyethoxylates are shortened by successive microbial cleavages. Mono- and diethoxylates (NPlEO and NP2EO) result from this initial degradation (Fig. 3). The NPlEO and NP2EO molecules have less hydrophilic character than the parent molecules and consequently sorb onto sludge floes. The subsequent anaerobic digestion of the NPlEO and NP2EO intermediates yields Cnonylphenol (NP). The high toxicity of 4nonylphenol to aquatic organisms was first realised during the evaluation of a pesticide formulation containing 51% NP. Since this disclosure, the aquatic toxicity of other alkylphenols has also been confirmed [86]. The identification of a toxic degradation product which is produced as a result of microbial transformation of a relatively innocuous wastewater constituent gave cause for concern and emphasised the problem of assessing the significance of organic contaminants in sewage without an understanding of the possible formation of by-products during treatment processes. 6.7. Lipids Stott and Tabatabai [89] determined phospholipids and total lipids in sewage sludges and it has been suggested that high concentrations of lipids and petroleum hydrocarbons in sludges applied to soils may inhibit plant growth [85]. Levels of between 38 and 330 mg kg-’ total phospholipids including phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl inositol and phosphatidyl serine have been detected in sewage sludges using HPLC methods. Liu (851 measured total lipids at levels of 70-17 750 mg 1-l in anaerobically digested sludges and also reported that sludge having a 23% lipid content (dry wt. basis) caused growth inhibition of grass when applied to soil. However, the phytotoxic effect observed may have been caused by the presence of other co-contaminants present in the sludge, particularly as hydrophobic residues associate strongly with lipids and oils. 6.8. Acrylamide monomer The organic polyelectrolyte, polyacrylamide, is used as a coagulant during wastewater treatment. About 1300 tonnes p.a. of polyelectrolyte are used

15

to dewater and consolidate sludge prior to landfill and land disposal. The sludge cake resulting from such conditioning contains about 1% dry solids of polyelectrolytes, usually polyacrylamides. The acrylamide monomer is present in variable amounts in the polymeric matrix (O.OS-So/, monomer). During wastewater treatment, the acrylamide monomer tends to reside in aqueous solution after the polyelectrolyte has coagulated the particulate matter, due to its high solubility (901. Monomer content of polyelectrolytes has been reduced in recent years because of concerns over its usage in potable water treatment. However, the possible effects of leaching of acrylamide monomer residues from sludge applied to agricultural land should be considered. 6.9. Phthalate esters Phthalic acid esters (PAEs) are produced for use in the manufacture of plastics. The benzene dicarboxylic acids, phthalic acid and terephthalic acid are esterified with a variety of alcohols to produce diesters. These diesters are incorporated into plastics as plasticisers and manufactured in very large quantities (0.6 x lo6 tonnes p.a. U.S. production 1979) [91]. PAEs have been found in sewage sludges at levels of 12- 1250 mg kg-’ dry weight, the predominant component being bis-(2ethylhexyl) phthalate which accounts for over 20% of PAEs manufactured. Shelton et al. [91] have reported that bis-(2-ethylhexyl) phthalate and din-octylphthalate are not degraded during sewage sludge digestion. However, the PAEs with shorter alkyl side chains are degraded and mineralised during anaerobic digestion. The degradative pathway is as follows: ROOC-Ph-COOR

-

HOOC-Ph-COOR

PAE

HOOC-Ph-COOH

-

mono-PAE

-

CH, + CO2

PA

Furthermore, O’Grady et al. [92] have contirmed that a correlation exists between increasing molecular weight and decreasing biodegradability. DOP and DEHP are both the most commonly used PAEs and are the most persistent during sewage treatment.

16

H.R.

Rogers / The Science of rhe Total Environment

6. IO. Organotin compounds

Since their first practical application as mothproofing agents in 1925, organotin compounds have become the most important class of organometallic compounds used in industry and agriculture (931. Most of the organotins that are manufactured are used as stabilisers in PVC polymers, transformer oils, cellulose acetate, polyethylene, polypropylene, paraffins, hydrogen peroxide and polyamides. The catalytic activity of diorganotins is exploited in the manufacture of polyurethane foams, epoxy resins and silicone rubbers. In addition to these applications organotins have been used as fire-proofing agents for plastics and wool fabrics. However, the use of organotins in anti-foulants as biocides is the area that has attracted most attention from the point of view of environmental contamination because of their toxicity to shellfish at very low concentrations. Twenty-one organotin compounds are used in the European Community and world wide production is estimated as 30 000 tonnes p.a. [94,95]. Argaman et al. [96] evaluated the effects of tributyltin oxide (TBTO) on an activated sludge process. They found that shock loadings of TBTO impaired sludge settling efficiency and resulted in higher eMuent suspended solids loadings. The biological activity of unacclimated cultures was inhibited by TBTO at concentrations as low as 25 rg 1-I. The ‘total’ levels of organotins found in samples of 12 UK sewage sludges were found to be in the range of 0.1-1.3 mg kg-’ (dry wt.) with mean and median values of 0.4 and 0.2 mg kg-‘, respectively, indicating that a typical concentration is probably at the lower end of the reported range [97]. Inorganic tin is the dominant tin species in sewage sludges (mg kg-’ concentration range) but it is possible that biomethylation involving methylcobalamine or some other metal methylator such as iodomethane could also occur during sewage treatment processes [98,100].

185 (19%)

3-26

ponents with alkyl chains of Cl,+Z15, with CII-C,s being predominant. High concentrations of up to 12 g kg-’ of total linear alkylbenzene sulphonates have been detected in Swiss digested sewage sludges [loll. Although LASS show lower toxicity to aquatic organisms when compared to nonylphenol, they generally occur at higher concentrations in sewage sludges. It has been suggested that LAS biodegradation is severely inhibited when sorptive phases are available and this could explain the high concentrations found in sewage sludges [102]. Linear alkylbenzenes (LABS) with alkyl chains ranging from 10 to 14 carbon atoms, are used as precursors for the manufacture of linear alkylbenzene sulphonates (LASS). LABS are sulphonated with sulphuric acid or sulphur trioxide and although the LAS yield is usually high some residual unsulphonated LABS remain. Consequently they are ubiquitous environmental contaminants and are found in sewage sludges at relatively high levels [103-1061. Eganhouse et al. [107] found total LAB concentrations (C 1O-Cl4 alkyl chain lengths) in the range of 20-430 mg kg-’ in six municipal sewage sludges, and Sweetman et al. [108] have found similar concentration ranges in sewage sludges with predominantly industrial (125-180 mg kg-‘) and domestic (153-176 mg kg-‘) catchments. Japanese researchers have recognised that the distribution of ring position isomers can be used as an indicator of transformation and degradation of LABS in the environment [105]. The ratio of internal LAB isomers (with benzene ring attached near the middle of the alkyl chain) to external isomers (with benzene ring attached near the end of the alkyl chain) was used as a reference indicator to assessLAB degradation. The I/E ratio was calculated from the following formula, using the most abundant Cl2 homologues, I/E isomer ratio =

6.11. Surfactants

and related residues

Linear alkylbenzene sulphonates (LASS) are widely used as surfactants, with global annual production of 1.1 x lo6 tonnes in 1982, accounting for about 28% of all surfactant manufacture [102]. LAS surfactants are mixtures of com-

[6ClZLAB] + [5Cl2-LAB] [4C12-LAB] + [3ClZLAB] + [2Cl2-LAB] The internal/external isomer for LABS in LAS detergents was found to be 0.81 [105]. Sweetman

H.R. Rogers/ The Science of rhe Total Environment I85 (19%) 3-26

et al. (1081 found that LAB I/E ratios were similar for sewage sludges with predominantly domestic (0.91 f 0.4) and industrial (0.83 f 0.01) catchments and that they were also comparable to the ratio for LABS in detergents. This observation suggests that preferential degradation of internal vs. external isomers is not significant during sewage treatment and anaerobic digestion. These findings were recently confirmed by results from laboratory incubation studies which demonstrated that although external LAB isomers were degraded preferentially to internal isomers under aerobic conditions, no significant degradation of LABS occurred under anaerobic conditions [ 1061. Other detergent-derived residues found in sewage sludges include fabric rinse conditioning products based on cationic dihardenedtallowdimethyl ammonium compounds (DHTDMAC; general formula [R2NMe2]+X-, where R = hardenedtallow with Ci4-C,s chain length, and X = Cl- or MeSO,-). These compounds are reported to be efficiently removed from waste water during sewage treatment. DHTDMAC has been found at mean levels of 0.83 mg 1-l in raw sewage, 0.03 mg 1-l in final effluent and at particularly high levels of 3.3 g kg-’ in sewage sludges [109]. Information relating to the significance and persistence of DHTDMAC residues in sewage sludges disposed to agricultural land has not been published although aerobic degradation has been suggested as a loss mechanism [ 1091. Galaxolides have also been detected in sewage sludges from two treatment works with mainly industrial and domestic catchments using general survey GC-MS screening without sample extract clean-up [97]. These compounds are synthetic musks and are widely used as industrial perfumes, particularly in detergents. 6.12. Chlorobenzenes Chlorobenzenes, in particular the dichloro- and trichlorobenzenes are in widespread use in industry and in some domestic products. The principal uses for chlorobenzenes are summarised in Table 9. There are little data available on the occurrence

17

Table 9 Principal uses of chlorobenzenes [ 1IO] Compound

Usage

I ,2Dichlorobenzene

Paint removers, engine cleaners, deinking solvents, solvent in azo-dye, pesticide and pharmaceuticals manufacture, heat transfer medium Toilet block deodorant, heat transfer medium, intermediate in pesticide and dyestuff manufacture, moth repellent Reaction by-product during chlorination of monochlorobenzene. Industrial grade l,2- and I,CDCB contain residual I ,3-DCB l,Z&TCB is the main isomer produced. Coolant in electrical equipment, glass tempering, dye solvent and degreaser Minor industrial usage. 1,2,4,STeCB used in synthesis of 2,4,5-T. Fire retardant Manufacture of PCP, grain fungicide, paper impregnation products, styrene and nitroso-rubber manufacture, ordnance items

1,4DichIorobenzene

1,3Dichlorobenzene

Trichlorobenzenes

Tetrachlorobenzenes Hexachlorobenzene

or concentrations of chlorobenzenes in UK sewage sludges. A Swiss study reported concentrations of 6-125 pg kg-’ of hexachlorobenzene and levels of total dichlorobenzenes of 0.05 mg kg-’ have been reported [ 111,112]. An American survey of 215 sewage sludges for the di-substituted isomers gave ranges of 0.02-809 mg kg-’ (l,ZDCB), 0.02-1650 mg kg-’ (1,3DCB) and 0.04-633 mg kg-’ (1,CDCB) [113]. A more recent study of a range of chlorobenzenes in UK sewage sludges with different catchments showed a narrower range of concentrations of
18

H. R. Rogers / The Science of’ the Total Environment

DCB ~0.2-33.9 mg kg-’ (median 9.8 mg kg-‘); 1,ZDCB ~0.2-13.6 mg kg-’ (median 7.9 mg kg-‘). The 1,4- and 1,Zisomers were generally found at higher concentrations than the 1,3-isomer which reflects the more widespread industrial and domestic use of these two compounds. Trichlorobenzenes were generally found at concentrations < 1 mg kg-’ with the exception of 1,2,4-TCB. The trisubstituted compounds occurred in the following ranges: 1,3,5-TCB 0.1-0.65 mg kg-‘, median 0.26 mg kg-‘; 1,2+TCB 0.02-4.8 mg kg-‘, median 0.36 mg kg-‘; 1,2,3-TCB 0.04-1.23 mg kg-‘, median 0.16 mg kg-‘, and were found at highest levels in sludges from works with a predominantly industrial catchment. The higher substituted compounds, when detectable, generally occurred at levels less than about 0.2 mg kg-’ and pentachlorobenzene was not detected in any of the 12 sludges analysed (< 0.01 mg kg-‘). There were no clear trends in the chlorobenzene distribution other than a general reduction in concentration with increased chlorine substitution. Topping [114] found in laboratory experiments that although 30% of 1,CDCB in a sewage effluent was sorbed onto solids during primary settling, more than 95% of the remainder would biodegrade during an aeration stage of activated sludge treatment. Other workers have also identified Pseudomonas bacteria capable of degrading 1,4-DCB in activated sludge [ 1151. All of the sludges analysed in the UK survey were either raw or anaerobically digested and the detection of ppm concentrations of chlorobenzenes in anaerobically digested sludges indicates that anaerobic biodegradation may not remove chlorobenzenes as efficiently as aerobic processes or that other interfering or inhibitory effects are active under real plant conditions as opposed to laboratory experimental studies [67]. Schraa [116] suggested that acclimation or lag periods preceding the onset of bacterial degradation of chlorobenzenes are longer under anaerobic than aerobic conditions. However, another study using anaerobically incubated sewage sludges found that biodegradative removal was an important removal mechanism for 1,3-DCB, 1,ZDCB and 1,2,4-TCB (50% removal in less than 4 days) and that no acclimation period was evident. It is

185 (1996)

3-26

reasonable to suggest that differences m sludge type and the method of preparation of sterile controls may account for such differences [117]. The laboratory pilot plant study carried out over a 32day period showed that all the chlorobenzenes were removed to varying extents with overall percentage removals as follows: 1,4-DCB 80%; 1,3DCB 77%; 1,ZDCB 66%; HCB 63%; 1,2.4-TCB 62%; 1,2,4,5-TetCB 61%; 1,2,3,4-TetCB 33%; 1,2,3-TCB 27% and 1,3,5-TCB 25% [I 171. 6.13. Polychlorinated dibenzodioxins ( PCDDs) polychlorinated dibenzofurans (PCDFs)

and

Polychlorinated dibenzodioxins and polychlorinated dibenzofurans are often simply termed ‘dioxins’. However, they are two distinct groups of a total of 210 different compounds which have similar chemical properties. ‘Dioxins’ have merited special attention in recent years because of their recognised toxicity, persistence and bioaccumulation properties. The most important group of ‘dioxins’ are the 2,3,7,8-congeners, of which 12, the ‘dirty dozen’, have been identified as being particularly toxic 11181. The primary sources of ‘dioxins’ are summarised in Table 10. ‘Dioxins’ have generally low volatility, high octanol water partition coefficients and low aqueous solubilities. Consequently they sorb strongly onto organic solids and have been detected at trace lev-

Table 10 Primary sources of PCDDs and PCDFs [ 118,119] source Chemical reactions

Thermal reactions

Photochemical reactions Enzymatic reactions

Production of chlorophenols, chlorophenoxy herbicides, PCBs and chloranil. Pulp bleaching, graphite electrode sludge from chlor-alkali industry Chlorinated waste incineration, iron and steel production and metal scrap smelting. Municipal waste incineration. Coal-tired power station flue-gas. Motor vehicle emissions Post-incineration Peroxidase transformation of chiorophenols

H.R. Table II Concentrations of PCDDs various countries (pg g-‘) Congener group

Sweden

TeCDDs PeCDD HxCDD HpCDD OCDD TeCDF PeCDF HxCDF HeCDF OCDF

42-88 220-580 270-1900 2760-9000 12 800-32 52-93 71-127 200-352 23-l 10 < 2.4-9

and PCDFs

[126]

900

Germany

130 1080 1210 3760 13 580 380 470 520 750 780

Rogers/The

Science of the Total Environment

in sewage sludges from

[I211

UK

[I271

< 80-240 I I O-430 1SO-620 1180-7700 5400-30300 240-2200 loo-360 220-640 200-1000 430-5100

els in sewage sludges [ 120,121]. The predominant isomers detected are the higher chlorinated PCDDs and PCDFs with maximum concentrations of octachlorodibenzo-p-dioxin (OCDD). Concentrations of PCDDs/DFs in sewage sludges from various countries are shown in Table 11. The primary source of these residues has been ascribed to PCDDs and PCDFs in pentachlorophenol by comparison of congener profiles [122]. However, more recent work in which attempts were made to calculate a mass balance for ‘dioxins’ for a sewage treatment plant lead to investigations into the potential for de-novo synthesis of ‘dioxins’ from PCP. 13C-labelled pentachlorophenol was incubated in sewage sludges and ‘3C-PCDDs were detected and biotransformation of PCP by peroxidase enzymes was suggested as a possible explanation of the findings [ 1231. A study of ‘dioxins’ in domestic washing machine and dishwasher waste suggested that this was a minor source, with very low concentrations in the wastewater (0.8-3.2 pg 1-l) which probably originate from sodium hypochlorite and detergents [ 1241. The isomeric pattern of ‘dioxins’ in human faeces, which mainly contain the 2,3,7,%substituted congeners (2,3,7,8-TCDD OS-l.5 pg g-‘) commonly found in body fat, suggests that human waste does not make a significant contribution to levels in sewage sludges [ 1251. However, bleached toilet paper may also contribute to background levels of PCDD and PCDFs in sewage sludge.

185 (19%)

3-26

19

It should be pointed out that reliable environmental data for ‘dioxins’, obtained using validated analytical procedures are often not available. Analyses inevitably involve lengthy sample extract, clean-up and fractionation procedures which are followed by high resolution CC-MS analysis. Some data quoted in the literature should be regarded as unreliable because samples were spiked with surrogate ‘3C-labelled standards after solvent extraction thus giving an overestimate of actual recoveries of trace residues from samples. 6.14. Polycyclic aromatic hydrocarbons

( PAHs)

Polycyclic aromatic hydrocarbons, like ‘dioxins’, are not produced intentionally for any particular application. An exception is naphthalene which is produced as a mothproofing agent, in the synthesis of phthalate esters and in the manufacture of dyestuffs. Polychloronaphthalenes have largely been replaced in electrical insulating materials by less hazardous substances [82,128]. Sources of other PAHs include pyrosynthetic formation during high temperature pyrolysis of organic materials and bacterial, fungal or higher plant synthesis [ 1291. PAHs are effectively partitioned onto solids during sedimentation treatment because of their low water solubilities and medium-high octanol-water partition coefticients. Consequently, they have been found to be ubiquitous contaminants in sewage sludges as a result of the diffuse nature of their main source i.e. combustion product precipitation and run-off to sewers [ 130- 1321. Petrasek et al. [2] estimated that a mean of 64% of ‘total’ PAH was sorbed onto solids during primary sedimentation. Nicholls et al. [133] investigated the possibility of mobilisation of sludge contaminants during the production of filter cake from sewage sludge. There was no apparent mobilisation of PAHs from the sludge following superheating at 180°C. ‘Total’ concentrations of PAHs reported by Hagenmaier et al. [134] for 33 sewage sludges from different treatment works were in the range 2.2-20.4 mg kg-‘. Other investigations of archived sewage sludges collected over a period from 1942 to 1961 showed that ‘total’ mean PAH concentrations were 50 mg kg-’ and that benzo[ghi]perylene was the most abundant compound with a mean concentration of

20

H. R. Rogers/

The Science of the Total Environment

IO mg kg-‘. There was an apparent decrease in ‘total’ PAH concentrations between 1945-6 1 presumably because of changes in air quality due to reduction in coal consumption and a resultant reduction in atmospheric PAH deposition [135]. Other compounds related to PAHs such as carbazole and dibenzofuran have also been detected at significant concentrations (85 and 64 mg kg-‘, respectively) in sewage sludges from works receiving efIluent from coking and foundry operations I136]. 6.1.5. Pharmaceutical

chemicals

Discharges of pharmaceutical chemicals to sewage treatment works from manufacturing sources are likely to be minimal when compared to chemical discharges from other industries because of the great emphasis placed on careful handling and packaging of expensive pharmaceutical products [137]. However, other sources of such chemicals could originate via excreta of urine containing residues or metabolites flushed down WCs, or from the deliberate disposal of unwanted proprietary and/or expired prescription medicines down WCs. The sluicing away of lotions, antiseptics and drugs to sewers is assumed to be an acceptable method of disposal as it is assumed that they are diluted to low levels in crude sewage. However although pharmaceutical chemicals receive considerable pharmacological and clinical testing, information on the environmental behaviour and ecotoxicity of these biologically active substances is generally not available. It has been suggested that a large proportion of pharmaceutical chemicals will undergo microbial transformations during sewage treatment processes [137]. However the extent of these transformations and the significance of partial degradation to produce chemically different metabolites has not been investigated. The biodegradability of specific pharmaceutical chemicals has been assessed by Richardson and Bowron I1371 (Table 12). Some work has been carried out to assess the concentrations of biologically active pharmaceutical chemicals in sewage effluent in order to assess their significance in water abstracted for potable water supply. Richardson and Bowron I1371 cited concentrations of about 1 pg 1-l of aspirin and caffeine in sewage eflluents and 1 pg

I85 ( 1996) 3-26

Table 12 Assessment of the biodegradability of selected pharmaceutical chemicals Compound

Test result

Amitriptyline Ampicillin Aspirin Caffeine Chlorhexidine Clofibrate Codeine phosphate Dextropropoxyphene Ephedrine

Non-biodegradable Biodegradable Readily biodegradable Readily biodegradable Non-biodegradable Non-biodegradable Non-biodegradable Non-biodegradable Readily biodegradable after acclimatisation Non-biodegradable Biodegradable Readily biodegradable Non-biodegradable Non-biodegradable Non-biodegradable Non-biodegradable Readily degradable after acclimatisation Readily degradable after acclimatisation Non-biodegradable Non-biodegradable Non-biodegradable Readily biodegradable after acclimatisation Readily biodegradable Non-biodegradable

Erythromycin Ibuprofen Menthol Meprobamate Methyldopa Metronidazole Naproxen Paracetamol Phenylpropanolamine Sulphamethoxazole Sulphasalazine Tetracycline Theobromine Theophylline Tolbutamide

1-l of methaqualone and methotrexate in sewage. Also, Watts et al. [ 1381 have identified erythromycin, tetracycline and theophylline at gg 1-l levels in river water. Rathner and Sonnebom (1391 have estimated that the amounts of oestrogens in sewage arising from oral contraceptives would be several orders of magnitude less than the amounts normally excreted by the population. There is no evidence to suggest that residues of pharmaceutical chemicals in sewage sludge should be of concern. However, their biological activity alone may support ecotoxicity assessments of chemicals with high production volumes. 7. conchlsions Existing priority substance classifications such as those produced by the European Commission

H.R. Rogers/ The Science of the Total Environment 185 (19%) 3-26

and U.S. Environmental Protection Agency can indicate which organic chemicals may be of general concern in terms of toxicity, persistence, production and usage [56]. However, the behaviour of contaminants during wastewater treatment and their potential environmental effects after disposal of sewage sludge depend on their physicochemical properties and their susceptibility to sorption, chemical degradation and biodegradability. For some priority contaminant groups, notably ‘dioxins’ and PCBs, there is inadequate information regarding their degradability and persistence after sewage sludge disposal. This lack of information hinders decisions relating to future policy on sewage sludge disposal practices. Organic contaminants in sewage sludge could cause environmental effects when sludge is applied to agricultural land or when it is disposed of to sea. However, it is difficult to assess the significance of organic contaminants in sewage sludges using purely chemical screening procedures because of the complex mixtures present, which may have potential for synergistic biological effects. Techniques such as plant bioassays using sensitive species such as tomatoes have been suggested as a relatively cost effective and practical means of assessing the phytotoxic potential of residues in sludge-amended soils [140]. However, there are currently no standard protocols for carrying out such screening procedures and there would still be a need to link such bioassay methods with chemical residue analyses. Certainly there is insufficient understanding of the significance of particular contaminant groups to provide a sound basis for setting safe levels for residues in sludge-treated soils. The presence of residues such as ‘dioxins’ in sewage sludges has recently generated concern over their impact on sludge-treated soils and in 1988 concern of the levels of ‘dioxins’ in sewage sludges resulted in considerable concern over the future acceptability of agricultural use of sewage sludge in Germany [141]. This resulted in the introduction of a Federal Ministry ban on its disposal to pasture land and arable crops as a precautionary measure. However, assessing the significance of residues in terms of environmental impact is made difticult by limitations on data due to the expense of monitoring for PCDD/DFs and

21

the lack of understanding of the potential for transfer of residues to foodstuffs such as milk. Because of their high affinity for soil organic matter it is unlikely that ‘dioxin’ residues in treated soils pose any significant threat to ground or surface waters via leaching. Also, codes of sludge disposal practice should avoid run-off of sludge-treated surface soil into surface waters during storm events. The potential for longer term impact of sludge-derived organic contaminants on soil fertility has not received any detailed investigation and merits attention. The current status of investigations into the fate, behaviour and environmental impact of sludge-derived organic contaminants in treated soils is discussed in more detail in other papers in this volume. References (11 T.H. Maugh, Chemicals - how many are there? Science, 199 (1978) 162.

PI A.C. Petrasek, I.J. Kugelman, M. Austem, T.A.

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