N. Voulvoulis and M.D. Scrimshaw
5.3 SLUDGE STABILIZATION TECHNIQUES
5.3.1.2 Chlorophenols and Chlorobenzenes
The degradation of chlorophenols, including pentachlorophenol (PCP) has been inves- tigated by a number of workers,29,31,36,40,68 and a range of microbial species capable of degrading PCP have been studied.69 Sludge samples taken from anaerobic digesters have been demonstrated to remove 28% of parent PCP over 96 hours,39 but this work did not indicate the age of the sludge, and sludge ages over 8 days have been noted to be required for dechlorination of PCP.70 Both the source of the sludge tested33 and the concentration of chlorophenols used in tests may influence the degradation rate, with inhibition occurring as concentration increases. Concentrations of PCP above 6.3 mg l–1 have been observed to inhibit degradation.69 Concentrations of 2,4-D above
Fate and Behavior of Endocrine Disrupters in Sludge Treatment and Disposal 155
70 mg l–1 added to sewage sludge inhibited gas production by 50% compared to controls36 and in samples spiked at 10 µg l–1 each with a range of chlorophenols;
recalcitrance was demonstrated for 2,4-D.29,31 However, other work has demonstrated degradation of this compound with loss of up to 30%.36
The persistence of 2,4-D may be related to the positioning of the chlorine atoms on the ring structure. Pentachlorophenol degradation occurs by initial dehalogenation at the para-position,40 and in contaminated soils, the use of anaerobic digestion has been demonstrated to remove 95% of PCP.71 The degradation pathway appears to then produce 3,5-chlorophenol and 3-chlorophenol as observed by Tartakovsky et al.41 The degradation of 2,4-D has been demonstrated to occur with the loss of the chlorine in the 2- position with the preferential formation of 4-chlorophenol; both 2- and 4- chlorophenol were subsequently observed to degrade to phenol.32 The preferential formation of 4-chlorophenol may be expected, as substitution in this position has been shown to be most difficult to remove in methanogenic sludges, with the ortho-substi- tution being least recalcitrant.33,38 However not all observations support this.41
Once particular species responsible for transformation have been identified, the opportunity to study the kinetics of the degradation processes more closely arises.
The kinetics for the degradation of PCP by Mycobacterium chlorophenolicus comb nov. in laboratory simulations have been described and modeled.69 However, the success of any microbial species in the environment is related to its competitiveness and growth rate in any given situation. Inoculation with an adapted consortium of bacteria has been demonstrated to completely degrade PCP after a lag period of 24 hours.39 It has also been suggested that inoculation of upflow anaerobic sludge blanket (UASB) reactors may also introduce the ability to successfully dechlorinate PCP in systems where it is not inherent.42 Such reactors have been well-studied for their ability to degrade the chlorophenols, and there have been significant removal rates. Although such systems are anaerobic and may be indicative of processes occurring during anaerobic digestion, they are not at present suitable for treatment of sludges. Conclusions made from observations of degradation of chlorophenols in such systems indicate that:
1. Recovery from toxic loadings is possible
2. Degradation of substituted groups occurs in the order ortho >meta >para 3. Phenol does not accumulate as an end product37,67
The chlorobenzene compounds consist of a single benzene ring substituted with 1 to 6 chlorine atoms. Hexachlorobenzene (HCB) has found widespread use as an insecticide for fumigation and other members of the family are utilized as process solvents and precursors for a range of organic compounds.43 It has recently been found to be present in sludges in Switzerland (Table 5.2), and reductive dechlorina- tion of HCB has been observed with accumulation of 1,3,5-trichlorobenzene.44 5.3.1.3 Organochlorine Pesticides
Many of the chlorinated pesticides follow similar degradation processes to the chlorophenols in that reductive dehalogenation is the major pathway for initial
156Endocrine Disrupters in Wastewater and Sludge Treatment Processes TABLE 5.2
Concentration (Range or Mean) of Chlorinated Compounds Reported in Sewage Sludges
Compound Concentration (mg kg–1), Location, and References
Chlorophenols 0.4 (total) UK79; 9.8–60.5 (total),26 (2,4-dichlorophenol) UK80
Pentachlorophenol 0.1–2.0 UK80
Organochlorine pesticides
HCB ND–0.013 Switzerland78
γHCH 0.01–70 U.K.81; ND-0.057 Switzerland78
Aldrin 0.01–0.21 U.K.81; ND-0.029 Switzerland78
Endrin 0.01–0.71 U.K.81; ND-0.022 Switzerland78
Dieldrin 0.01–52.9 U.K.81; ND-0.043 Switzerland78 p,p′-DDE 0.5 Germany81; 0.036–0.097 Switzerland78
p,p′-DDD ND–0.048 Switzerland78
p,p′-DDT ND–0.017 Switzerland78
Heptachlor ND–0.004 Switzerland78
Heptachlor epoxide ND–0.165 Switzerland78
PCB (total) 0.15–3.6 USAa; 0.02–0.46, 0.01–21.5, 0.3 U.K.73,74,81; 0.13–1.63 Canadaa; 0.5–8.0, 0.36–7.6 Switzerlanda; 0.6–6.6 Hollanda; 0.52–15.2, 1.38–6.65 Germanya
PCB28 PCB52
PCB 77 (µg kg–1) PCB101 PCB118
PCB 126 (µg kg–1) PCB138
PCB153
PCB 169 (µg kg–1) PCB180
0.041, 0.054 Germanya; 0.007, 0.001–0.009 Switzerland78,83; 0.086 France84 0.028, 0.022 Germanya; 0.023, 0.004–0.0272 Switzerland78,83; 0.049–0.086 France84
0.540–4.27 U.K.75; 0.234–5.30 Switzerland83; 0.34 Germany85; 0.8–1.1 Spain86; 0.54–1.00 Sweden87; 0.03–0.66 Finland88 0.052, 0.061 Germanya; 0.053, 0.006–0.030 Switzerland78,83; 0.082, 0.086 France84
0.040, 0.006–0.028 Switzerland78,83; 0.060 0.086 France 84 ND-0.280 UK75; 0.04–1.59 Switzerland83; 0.03–0.06 Spain86
0.082, 0.093 Germanya; 0.056, 0.006–0.029 Switzerland78,83; 0.117, 0.086 France84 0.084, 0.1 Germanya; 0.060, 0.009–0.031 Switzerland78,83; 0.174, 0.086 France84 ND–0.55 U.K.75; 0.02–0.23 Switzerland83; 0.01 Spain86
0.053, 0.064 Germanya; 0.021, 0.005–0.016 Switzerland78,83; 0.129, 0.086 France84 PCDD/PCDF (µg kg–1) Up to 63 U.K.75; 9.1 (OCDD) Sweden89; 32.9 (OCDD) Sweden90; 7.5 (OCDD) Sweden91 ND = not detected
a Cited in Alcock, R.E. and Jones, K.C., Chemosphere, 26, 2199, 1993.
Fate and Behavior of Endocrine Disrupters in Sludge Treatment and Disposal 157
removal of chlorine atoms. This makes the ring structure amenable to further hydroxylation and cleavage in any subsequent exposure to aerobic conditions.
Their occurrence in sewage sludges is well-documented (Table 5.2). Some com- pounds, however, have been demonstrated to be removed via abiotic pathways.
This includes γHCH (which has a saturated, rather than aromatic ring structure), with removal being observed in both batch and semicontinuous operation.29,30
The use of bacteria from anaerobic sludge in sequencing aerobic/anaerobic reactors demonstrated the recalcitrance of DDE40 that agreed with data from both semicontinuous and batch anaerobic digestion, which indicated that neither DDE nor dieldrin is amenable to anaerobic degradation.29,30 Other compounds are rapidly degraded in anaerobic sludges. Toxaphene, a complex mixture of chlorobornanes and other bicyclic compounds has the general composition of C10H18-nCln. C10H16-nCln was observed to be degraded with the greater rates for the more highly chlorinated compounds.45