The influence of chemical conditioning and dewatering on the distribution of polychlorinated biphenyls and organochlorine insecticides in sewage sludges

Ent,ironmental Pollution (Series B) 2 (1981) 309 320

THE INFLUENCE OF CHEMICAL CONDITIONING A N D DEWATERING ON THE DISTRIBUTION OF POLYCHLORINATED BIPHENYLS AND ORGANOCHLORINE INSECTICIDES IN SEWAGE SLUDGES

ALUN E. MCINTYRE, JOHN N. LESTERf & ROGER PERRY

Public Health Engineering Laboratory, Civil Engineering Department, Imperial College, London, SW7 2AZ, Great Britain

ABSTRACT

The influence of sewage sludge conditioning and dewatering processes on the distribution of polychlorinated biphenyls (PCBs) and organochlorine insecticides ( OCLs) in sewage sludges has been investigated at two sewage treatment works--one employing coagulation of primary digested sludge with aluminium chlorohydrate followed by dewatering using vacuum filtration, the other using conditioning of consolidated mixed primary sludge with Zetag 94S, an organic polyelectrolyte, and subsequent dewatering by pressure filtration. It has been demonstrated that analytical methods developed for the determination of these micropollutants, in sewage sludges are not subject to interference from the dewatering chemicals and are therefore suitab& for the analysis of conditioned sludges. A mass balance of solids and organic micropollutants through the two treatment processes has been constructed and the results indicate a close association of polychlorinated biphenyls and organochlorine insecticides with the solid matter in sewage sludges. The occurrence of polychlorinated biphenyls and organochlorine insecticides in sewage sludges and the possible environmental effects of contaminated sludge disposal are discussed. f To whom all correspondence should be addressed. 309

Environ.Pollut.Ser. B. 0143-148X/81/0002-0309/$02.50© Applied SciencePublishersLtd, England, 1981 Printed in Great Britain

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ALUN E. McINTYRE, JOHN N. LESTER, ROGER PERRY INTRODUCTION

The occurrence and deleterious effects of organochlorine residues in the environment, and in particular the hydrological cycle, have been well documented over the past two decades (Risebrough et al., 1968; Edwards, 1970; Peakall & Lincer, 1970; Nisbet & Sarofim, 1972; Fishbein, 1973). Recently, research has been directed towards identifying the behaviour and fate of these and other organic substances of concern in water and waste water treatment processes, both in laboratory simulations (Andrade & Wheeler, 1974; Choi et al., 1974; Kaneko et al., 1976) and in full-scale treatment plants (Bernholz et al., 1971 ; Shannon et al., 1976; Bergh & Peoples, 1977). All these studies have demonstrated that non-polar organic compounds are closely associated with suspended material and that simple mechanical (solid/liquid) separation processes will bring about a concentration of these organic contaminants into sludges produced during waste water treatment. In particular, the primary sedimentation stage of sewage treatment, where approximately 70 ~o of suspended solids may be removed, has been shown to be significant in the removal of PCBs and OCLs from waste waters (Bernholz et al., 1971; Shannon et al., 1976; Lawrence & Tosine, 1977; Mclntyre et al., 1981). Primary sludges may be further treated by chemical conditioning, using inorganic conditioning aids or organic polyelectrolytes, and dewatered by a number of methods; vacuum filtration and pressure filtration are two common treatment processes employed in the UK (Metcalf & Eddy, Inc., 1972). Although liquid digested sludge may be disposed of directly to sea or land, recent increases in fuel costs and consequential increases in transport costs may render the transportation of large volumes of liquid sludge prohibitively expensive. Therefore, the dewatering and subsequent reduction in volume of sewage sludge prior to disposal may gain in importance. In the UK, where some 1-24 × 106 tonnes of dry sewage solids are disposed of per annum, approximately 40 ~o is utilised on agricultural land, 23 ~o is dumped at marine sites and 24 ~o is incorporated in landfills, whilst the remainder is either incinerated or utilised for land reclamation, forestry or horticulture (National Water Council, 1977). If significant concentrations of particular organic substances are suspected or known to exist in waste waters, and are not degraded by treatment processes, their subsequent dispersion after discharge via final etfluents or sludges may give cause for environmental concern. Accordingly, detailed knowledge of the influence of sewage and sewage sludge treatment processes on the behaviour and fate of organic micropollutants is desirable. MATERIALS AND METHODS

Addition o f conditioning aids to sewage sludges

Samples of sewage sludge were dosed with coagulants and polyelectrolytes in order to examine any potential interterences with the extraction, clean-up and

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analytical procedures for PCBs and OCLs caused by the presence of these conditioning aids. The materials and rates of addition were compatible with those in common use (J. Vosser, pers. comm.). Three commonly used inorganic conditioners, lime and copperas, lime and ferric chloride and aluminium chlorohydrate (Allbright and Wilson Ltd, Birmingham, UK) and two organic polyelectrolytes, Aquafloc 4051 (Dearborn Chemicals Ltd, Widnes, UK) and Zetag 94 (Allied Colloids Ltd, Bradford, UK) were studied. Each conditioning agent was dosed at two rates; the maximum of the range used in normal practice and a rate 50 ~o above this. Lime was added to the sludges as a 10~(w/w) slurry, copperas (ferrous sulphate) as a 25 ~(w/w) solution, ferric chloride as a 40 ~(w/w) solution and aluminium chlorohydrate as a 10~(w/w) solution of aluminium oxide. The two polyelectrolytes were added as 0.1 ~o(w/w) solutions. Subsamples (100 ml) of sewage sludge were placed in beakers (250 ml) and stirred using a magnetic stirrer bar while the required volume of conditioner solution was added. After incorporation of the conditioner solution, the sample was homogenised and aliquots (5ml) were withdrawn for analysis as described previously (McIntyre et al., 1980). The results were statistically analysed using analysis of variance (F-test) and Tukey's test (Bowker & Lieberman, 1972) in order to detect any significant differences between the mean values. Sampling procedures Vacuum filters at Sandford sewage treatment works." After anaerobic digestion, primary sludge from the Sandford works is elutriated with twice its own volume of final effluent and pumped to a holding tank. From there it is pumped into two separate mixing tanks before being transferred to the coagulant dosing tanks where aluminium chlorohydrate solution is added (Allbright and Wilson, Ltd, Birmingham, UK), before being fed to each of two vacuum filters (Dorr-Oliver Ltd, Croydon, UK) for dewatering. An increase in the total solids content from approximately 7 ~ to 30 ~ is typically achieved with this process. The final sludge cake is then disposed to agricultural land. Samples of liquid sludge were obtained from specific points in the treatment process in the following manner; digested (elutriated) sludge was sampled hourly from the two mixing tanks whilst coagulated sludge samples were taken from the coagulant mixing tanks at similar intervals. PTFE beakers were used to obtain two subsamples (500 ml) of each sludge prior to storage in borosilicate glass bottles (1000ml). Filtrate from the vacuum filters was sampled hourly from the holding sump beneath the filter house. A 1-1itre sample of filtrate was taken by immersion of a clean glass bottle. Sludge cake was sampled at hourly intervals by collecting approximately 25kg in a large receptacle and taking a core subsample (approximately 2 kg) for storage in polypropylene bags. Altogether, eight of each of the four sample types were collected over the sampling period and were amalgamated into two-hourly composites.

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All samples were stored at 4 °C prior to analysis using the procedures previously described. Because of the absence of flow- or volume-measuring equipment, it was not possible to establish an accurate mass balance of sludge--and hence one of organics--through the treatment process. However, by measurement of the total weight of sludge cake produced (total weight of dry solids) during the time of operation of the vacuum filters and by determination of the solids contents of the sludges and filtrate, an estimation of the masses of sludge and associated organics was possible. After collection in a large vessel over a known time period, sludge cake was weighed using industrial scales (Salter Ltd, Luton, UK) and the average rate of cake production calculated. From these figures it was possible to estimate the total mass of dry solids passing through the treatment process and, knowing the total solids contents of the various sample types, to estimate a mass balance. Filter presses at Rodbourne sewage treatment works: Mixed primary sludge from the Rodbourne sewage treatment works is pumped to consolidation tanks. After settlement for approximately 24 h, the supernatant liquor is removed and returned to the works' inlet mixing chamber. This procedure effectively increases the total solids content of the sludge from approximately 3 ~o to 5 9/0. The sludge is then pumped to a mixing tank prior to being pumped to each of four filter presses (Vickers, Ltd, Swindon, UK). In-line dosing of a polyelectrolyte, Zetag 94S, is carried out at this point in the process. The total solids content of the sludge cake produced, after a pressing time of approximately 3.5 h, varies between 25 ~o and 30 ~o. Final disposal is to agricultural land, after short-term storage on land adjacent to the sewage treatment works. Samples of the raw sludge were taken from the pipe discharging sludge into the mixing tank. Eight subsamples (500 ml) were taken during the operation, bulked and a composite subsample (750 ml) taken for analysis. Conditioned sludge samples were taken from convenient taps on the feed lines to each of the filter presses. Four subsamples (500 ml) of the sludge entering each of the presses during the operational period (3.5 h) were obtained. These were then bulked, mixed and a composite subsample (750ml) taken for analysis. Filtrate samples were collected from the drains in the catch-trays below the filter presses. A composite subsample (! 000 ml) was taken for analysis after the collection of four subsamples (1000 ml) from each press over the period of operation. Sludge cake samples were taken at the end of the pressing period when the presses were released by the sewage works staff. A composite subsample (2.0 kg) was taken for analysis. The operational procedure employed by the works permitted the volume of raw primary sludge in the consolidation tank to be calculated from a knowledge of the dimensions of the tank and measurement of the depth of sludge in the tank. Since the filter presses and mixing tank had been completely emptied and no further sludge

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was transferred to the mixing tank, sampling of the single body of sludge, of known volume, could be achieved. In order to determine the mass of sludge cake produced from the known volume of primary sludge, trailer-loads of the cake were weighed on a public weighbridge and recorded. Thus it was possible to perform an accurate mass balance of solids-and hence of organics--through the sludge treatment process. With three filter presses in operation during the sampling period, a total of ten pressings, taking approximately 15h, was required in order to empty the consolidated sludge mixing tank.

RESULTS

Extraction of polychlorinated biphenyls and organochlorine insecticides from chemically conditioned sludges A subsample of mixed primary sludge collected from Hogsmill Valley sewage treatment works (Thames Water Authority), of total solids concentration 40.5glitre 1, was subdivided into aliquots (100ml) and dosed with the selected inorganic conditioners and polyelectrolytes at two dosage rates. Four replicate aliquots (5 ml) were then taken, extracted and analysed. Results of the analyses of sludge treated with three different inorganic conditioning aids are indicated in Table 1. Examination of the results shows that there was no significant effect upon the determination of PCBs and OCLs caused by the conditioning agents at both dose rates, with the exception of v-HCH in the sludge dosed with aluminium chlorohydrate. The results obtained for 7-HCH in the presence of aluminium chlorohydrate were significantly higher at the 0.05 level than the results obtained for the sludge dosed with lime and ferric chloride at the higher rate. The results were not, however, significantly higher than the untreated sludge, nor was the lime and ferric chloride (Table 1, (2)) result significantly lower than any other. It may be considered that no significant error was introduced by the presence of the three conditioning agents in the sludges when analysis of the four determinands was undertaken. A further set of subsamples was taken and dosed with the two selected polyelectrolytes. Aliquots were then taken and extracted. The results of the analysis of these extracts are shown in Table 2. Examination of the results reveals that the presence of polyelectrolytes had no significant effect upon the analysis of PCB and OCL in sewage sludge. The degree of scatter of the results was slight, with the exception of DDE which had relative standard deviations of between 10.1 and 13.8 for the polyelectrolyte-dosed sludge samples, which might be explained by the relatively low concentration of DDE in the sludge sample.

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TABLE 1 EFFECT OF INORGANIC CONDITIONING AGENTSON THE ANALYSISOF PCBs AND O C L s IN SEWAGE SLUDGES

Determinand

Arochlor 1260

p,p'-DDE

~-HCH

Dieldrin

Treatment

None CaO + FeCI 31 CaO + FeCI32 CaO + FeSO 4 t CaO + FeSO42 AICI 31 AICI32 None CaO + FeCI 31 CaO + FeCI 32 CaO+FeSO41 CaO + FeSO42 AICI 31 AICI 32 None CaO + FeCI 3 t CaO + FeC132 CaO + FeSO 41 CaO + FeSO42 AIC131 AICI32 None CaO + FeCI 31 CaO + FeCI 32 CaO + FeSO 4 a CaO + FeSO 42 AIC131 AICI 32

F-test result

NS

NS

0.05

NS

Mean* concentration (#g litre- x)

RSD (%)

Range (lag litre- 1)

3.46at 3.40a 3.40a 3-38a 3.39a 3.54a 3.53a 1.28a 1-22a 1.21a 1.39a 1-37a 1-28a 1.29a 7.14ab 7-53ab 6.75a 7.35ab 7.53ab 7.78b 7.80b 8-34a 8.23 8-23a 8.36a 8.33a 8.65a 8.34a

4.4 4.5 5.5 6.7 6.8 5.5 4.3 0.45 5.1 12-2 12.1 16.2 8.0 15-9 9-7 8-9 3.6 3-8 3.2 3.1 5-6 2.3 3-7 4.9 4-8 3.5 3.4 3.1

3.36-3.64 3.24-3.60 3.18-3-62 3.18-3.70 3.14-3.66 3.38-3.82 3.34-3.68 1.27-1.28 1.1 6-1.30 1.04-1 -38 1-16-1-56 1.16-1.66 1.16-1.38 1.04-1.52 6.54-7.74 6.78-8.24 6.52-7.08 6.96-7-62 7.24-7.82 7.52-8.02 7.28 8.34 8"14-8.52 7-88-8.62 7.74-8-62 7-86-8.76 7.96-8.60 8-34-8.92 8. IO-8.60

* Of four replicate determinations. t Means not followed by a common letter are significantly different at the 0.05 significance level. Maximum dose in common use. 2 Maximum dose plus 50 %. NS = Not significant.

A c c o r d i n g l y , it was c o n s i d e r e d t h a t t h e analysis o f c h e m i c a l l y c o n d i t i o n e d s l u d g e s c o u l d be u n d e r t a k e n w i t h the s a m e d e g r e e o f c o n f i d e n c e as u n t r e a t e d sludges.

Influence o f vacuum filtration on P C B and O C L concentrations at Sandford sewage treatment works V a c u u m f i l t r a t i o n effectively s e p a r a t e s t h e c o n d i t i o n e d s e w a g e sludge i n t o s l u d g e c a k e a n d filtrate; h e n c e a m a s s b a l a n c e m a y be p e r f o r m e d at this stage. T a b l e 3 i n d i c a t e s t h e c o n c e n t r a t i o n s o f A r o c h l o r 1260, p , p ' - D D E a n d d i e l d r i n in the f o u r t y p e s o f s a m p l e o v e r the 8-h p e r i o d t h a t the v a c u u m f i l t r a t i o n p l a n t o p e r a t e d . A r o c h l o r 1260 c o n c e n t r a t i o n s in t h e d i g e s t e d s l u d g e v a r i e d b e t w e e n 22.69 a n d

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TABLE 2 EFFECT OF POLYELECTROLYTE CONDITIONING AIDS ON THE ANALYSIS OF P C B s AND O C L s IN SEWAGE SLUDGE

Determinand

Arochlor 1260

p,p'-DDE

~-HCH

Dieldrin

Treatment

None Aquafloc 40511 Aquafloc 40512 Zetag 941 Zetag 942 None Aquafloc 4051 t Aquafloc 40512 Zetag 941 Zetag 942 None Aquafloc 4051 t Aquafloc 40512 Zetag 94 t Zetag 942 None Aquafloc 40511 Aquafloc 40512 Zetag 941 Zetag 942

F-test result

Mean* determmand concentration (#g litre- 1)

RSD (%)

Range (lag litre- 1)

3.46at 3.29a 3.43a 3.51 a 3.52a 1.28a 1.18a 1.26a 1.29a 1.30a 7.14a 7.82a 8.00a 7-07a 7.85a 8.34a 8.45a 8.44a 8.24a 8.17a

4.4 4.6 6.6 5.4 6.3 0.45 10.1 13.8 10,8 13-8 9.7 5.8 5.7 7.2 3-6 2.3 6.0 5.1 2.5 4.3

3.36-3-64 3.14-3.48 3.18-3.66 3.24-3.66 3-28-3.74 1.27-1.28 1.04-1-32 1.06-1.42 1.10-1.42 1.12-1.54 6-54-7.74 7.36-8-42 7.53-8.60 7.46-8. i 4 7.46-8.14 8.14-8.52 7.72-8.82 7.96-9-00 7.98-8.46 7.72-8.48

NS

NS

NS

NS

* Of four replicate determinations. t Means not followed by a common letter are significantly different at the 0.05 significance level. J Maximum dose in common use. 2 Maximum dose plus 50 ~o. NS = Not significant. TABLE 3 CONCENTRATIONS OF P C B s AND O C t s

Concentration lag litre- 1 (lag g- t) Sample type Sampling Digested Conditioned time sludge sludge

Compound

Arochlor 1260

p,p'-DDE

Dieldrin

IN THE FOUR TYPES OF SAMPLE FROM THE VACUUM FILTRATION PLANT

08.00-10.00 10.00-12.00 12.00-14.00 14.00-16-00 08-00-10.00 10.00-12.00 12.00-14.00 14.00-16.00 08.00-10.00 10.00-12.00 12.00-14.00 14.00-16.00

ND = Non-detectable.

26.82(0-34) 24.73(0.33) 23.03(0.34) 22.69(0.31) 4.24(0.05) 3.79(0.05) 3.89(0.06) 3-33(0.05) 19.22(0.24) 15.80(0.21) 14.78(0.22) 13.04(0.18)

21.86(0.35) 14.25(0.32) 10-26(0.32) 6-21(0.26) 3.49(0-05) 2.35(0.05) 1.64(0.05) 1.10(0.05) 10.69(0.17) 11.31(0.25) 7.26(0.23) 3.66(0.15)

Sludge cake

Filtrate

(0.30) (0.25) (0.26) (0.28) (0.05) (0.04) (0.04) (0.05) (0.16) (0.15) (0.16) (0.17)

ND ND ND ND ND ND ND ND 0.026 0.026 0.007 0.015

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A L U N E. M c l N T Y R E , J O H N N. LESTER, R O G E R PERRY

36"82 #g litre- ~(0-31 and 0.34/~g g- 1) over the 8-h period,p,p'-DDE varied between 3.33 and 4.24 #g litre - 1 (0.05 and 0.06 #g g- ~) whilst dieldrin concentrations varied from 13.04 to 19.22 #g litre- 1 (0.18 to 0.24/~g g- 1). Concentrations of these three determinands in the conditioned sludge and the sludge cake are of a similar range on a dry weight (#g g- ~) basis, with the exception of dieldrin, which was present at lower concentrations in the sludge cake. Dieldrin was, however, detected in the filtrate samples, whilst Arochlor 1260 and p,p'-DDE were not. No 7-HCH was detected in any of the samples. The total weight of dry solids produced over the 8-h period was calculated (from weighings) to be 1100.7 kg. From a knowledge of the original average total solids content of the digested sludge (74.16 g litre- ~), it was possible to estimate the volume of sludge fed to the vacuum filters over the period (14,842.2 litres). The volume of filtrate produced over the same time period may be considered to be equal to the volume of digested sludge (since 1.0 g dry sludge solids occupies approximately 1-9 ml) as the error involved would be within the total experimental error of the monitoring exercise. When the average concentrations for each determinand in the samples are multiplied by the volumetric data, the mass balance, presented in Table 4, is obtained. TABLE 4 QUANTITIESOF PCBs AND O C L s PRESENT IN SAMPLES FROM THE VACUUM FILTRATION PLANT

Compound

Arochlor 1260 p,p'-DDE Dieldrin

Digested sludge

Mass (g) Sludge cake

Filtrate

0.361 0.057 0.233

0-300 0.049 0.176

ND ND 0.0003

ND = Non-detectable. Whilst these balances are not perfect, the distribution of these compounds during vacuum filtration of sludge is evident, approximately 83 ~o of the Arochlor 1260, 86 ~ of the DDE and 75 ~o of the dieldrin originally present in the digested sludge are found in the sludge cake. The influence o f pressure filtration on the concentrations o f PCB and OCL at Rodbourne sewage treatment works

The concentrations of PCB and OCL in the four types of sample during the period the pressure filtration plant operated are indicated in Table 5. Arochlor 1260 concentrations in the raw consolidated sludge varied between 21.6 and 24.38 #g litre- ~ (0.41-0.46 #g g- 1), p,p'-DDE varied between 1.75 and 1.91/~g litre -j (0.03-0.04 #g g-1), y-HCH between 5.80 and 7.80/~glitre -~ (0-11-

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TABLE 5 CONCENTRATIONS OF PCBs AND OCLs 1N SAMPLESFROM THE PRESSUREFILTRATION PLANT

Compound

Sampling occasion

Arochlor 1260

p,p'-DDE

?-HCH

Dieldrin

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Consolidated raw sludge 22.79(0.43) 21.60(0.41) 23.05(0.44) 24.38(0.46) 1.75(0.03) 1-80(0.03) 1.86(0.04) 1.91(0.04) 6.55(0.12) 7.07(0.13) 5.80(0-11) 7.80(0.14) 7.61(0.14) 9.01(0.16) 8.48(0.16) 9.01(0.16)

Concentration, pg litre -1 (l~g g-l) Conditioned Sludge sludge cake 25.64(0.51) 24.64(0.49) 22.01(0.44) 23.14(0.46) 1.60(0.03) 2.00(0.04) 1-70(0.03) 1.90(0.04) 5.18(0.10) 6.68(0.13) 5.50(0.11) 6.34(0.13) 6.68(0-13) 7.83(0.16) 6.68(0.13) 6.47(0.12)

Filtrate

(0.49) (0.51) (0.50) (0.49) (0.04) (0.04) (0.05) (0.04) (0.13) (0.14) (0.15) (0.16) (0.15) (0.17) (0-17) (0.16)

ND ND ND ND ND ND ND ND ND ND ND ND 0.011 0.008 0.010 0.012

ND = Non-detectable,

0" 14 pg g - 1) and dieldrin between 7.61 and 9.01 pg litre- 1 (0.14-0.16 pg g- 1). The dry weight concentrations are comparable over the range of samples. Since it was possible in this case to obtain accurate measures of the volume of raw consolidated sludge fed to the filter presses (217,000 litres) and the weight of sludge cake produced over the sampling period (10.4 tons of dry solids), a reasonably accurate mass balance could be established. The volume of filtrate produced over the sampling period was again taken to be equal to the raw sludge volume. Multiplying the average determinand concentrations from Table 5 by the volumetric and mass data produced the balance indicated in Table 6. Consideration of these results indicates that, within the overall experimental error, the mass balances presented here are particularly good, agreeing to within + 10 ~.

TABLE 6 WEIGHTS OF PCBs AND OCLs PRESENTIN SAMPLESFROM THE PRESSUREFILTRATION PLANT

Compound Arochlor 1260

p,p'-DDE ),-HCH Dieldrin

Raw sludge

Weight (g) Sludge cake

Filtrate

4.98 0.40 1.48 1.85

5.17 0.44 1.51 1.69

ND ND ND 0.002

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ALUN E. MclNTYRE, JOHN N. LESTER, ROGER PERRY DISCUSSION

Analysis of sewage sludges treated with a range of common conditioning aids has shown that these agents have no significant effect upon the analytical procedures for the determination of PCB and OCL in sewage sludges. It is conceivable that coagulants and polyelectrolytes, by inducing coagulation and flocculation and thus bringing about aggregation of the sludge solids in the sample matrix, might lower the efficiency of a solvent extraction technique by reducing the degree of mixing between sample and solvent phases. However, by employing efficient homogenisation of the sample prior to extraction and the use of a mechanical high-speed disi~erser in the extraction step (Mclntyre et al., 1980), it is considered that intimate mixing of the sample and solvent is achieved. In addition, any effect due to the presence of residual polyelectrolyte in the sample may be effectively nullified, as it has been shown that violent stirring in excess of 350 rpm tends to break down the long-chain polyelectrolyte polymer molecules (Gale & Baskerville, 1970). The presence of residual organic polyelectrolyte in conditioned sewage sludges may influence the partitioning of relatively non-polar molecules, such as those of PCBs and OCLs, between the solid and liquid phases, since it has been proved that the presence of other organic materials, such as surfactants, may increase the water solubility of non-polar organics (Klevens, 1950; Ekwall, 1954; Suess, 1972). This may be of importance when considering the behaviour and fate of non-polar organic substances of concern during sewage treatment and their subsequent dispersion from sewage sludge disposal sites. Investigation of the behaviour of PCBs and OCLs in sewage sludge conditioning and dewatering processes has indicated that both vacuum filtration and pressure filtration result in high solids recoveries into sludge cakes produced. Associated recovery of both PCBs and OCLs with the solids was evident, although dieldrin was found to be present in the filtrate from both dewatering processes. This may be related to the greater water solubility of dieldrin (200 #g litre- 1), perhaps as opposed to that ofArochlor 1260 or DDE, or the fact that, for most industrial and household purposes, dieldrin is marketed in a formulation of xylene and anionic surfactants in order to increase its aqueous solubility (Brown et al., 1979). Somewhat surprisingly, 7-HCH, which has an aqueous solubility of approximately 780 #g litre- 1, did not display a similar tendency. The reasons for this are not clear. The imperfections observed in the mass balances, particularly for the vacuum filtration plant study, are almost certainly due to the difficulties encountered in obtaining accurate measurements of the volumes of sludge and filtrate and the weight of sludge cake produced. In the case of the pressure filtration plant it was possible to accurately determine the volume of ingoing sludge and the weight of sludge cake produced. In addition, the difficultyin obtaining a truly representative sample from the large mass of sludge cake produced introduces further error. However, within the overall experimental error, the results of the mass balances agree tolerably well.

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At present, in the U K , approximately 40 ~o o f the sludge produced is disposed o f to agricultural land. If these sludges are contaminated with significant concentrations o f PCB and O C L , soil quality m a y be impaired and these residues m a y be taken up by crop plants (Lichtenstein, 1959; Nash, 1968; Lawrence & Tosine, 1977) or by grazing animals (Bergh & Peoples, 1977). Clearly this m a y lead to a recycling o f such residues into the h u m a n food chain. Reintroduction o f these c o m p o u n d s into h u m a n food chains m a y also result from the disposal o f 23 % o f sludge p r o d u c e d at marine d u m p i n g grounds. B o t t o m fauna and flora m a y take up residues o f PCB and OCL, as has been the case in the Firth o f Clyde (Halcrow el al., 1974). These toxic organic substances m a y then bioaccumulate in marine food chains, resulting in significant concentrations in fish and m a m m a l s (Holden, 1972; Sergeant & A r m s t r o n g , 1973).

ACKNOWLEDGEMENTS The authors acknowledge the financial support provided for this work by the Department o f the Environment and the Department's approval for the publication o f these results. They are also grateful to the T h a m e s Water A u t h o r i t y for its co-operation.

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