N. Voulvoulis and M.D. Scrimshaw
4.3. FATE OF ENDOCRINE DISRUPTERS IN SEWAGE TREATMENT WORKS
4.3.5 O RGANIC O XYGEN C OMPOUNDS .1 Di-(2-ethyhexyl)phthalate
Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 125
In the primary clarifier in a STW in Zurich,101 73% of total butyl tin was eliminated from wastewater due to adsorption, during secondary sedimentation an additional 17% was removed. Further removal during activated sludge treatment was minimal with biodegradation accounting for only 8% removal.
4.3.5 ORGANIC OXYGEN COMPOUNDS
126 Endocrine Disrupters in Wastewater and Sludge Treatment Processes
4.3.6 POLYAROMATIC COMPOUNDS
4.3.6.1 Polychlorinated Biphenyls
PCBs are stable molecules with low aqueous solubilities and biological, chemical, and physical recalcitrance. As a result, they exhibit minimal degradation during wastewater treatment processes.21,112 However, PCB degradation has been seen to occur to some extent both aerobically and anaerobically.113 Nonbiological elimina- tion mechanisms of PCB removal during activated sludge treatment, such as mass transfer to the atmosphere and chemical degradation, were dismissed due to the affinity of PCBs for suspended solids and their recalcitrance.114
PCB concentrations were 3 to 4 times higher in total atmosphere fallout than in wastewater. The influent concentrations of a sum of 7 PCB congeners were 15 to 26 ng l–1 in dry conditions and 31.5 to 53 ng l–1 in wet conditions. This indicates that PCBs in wastewater mainly originate from the atmosphere via rainwater washout.
PCB degradation in activated sludge decreases with an increasing number of chlorine atoms.114–116 A chlorine content of greater than 42% prevents degradation.
Pseudomonas, Alcaligenes, Arthrobacter, and Acinetobacter have all been found to be responsible for degradation processes. Initially, dioxygenation results in metab- olites hydroxylated in the 2 and 3 positions; these are then degraded in turn by meta ring cleavage to ultimately give chlorinated benzoic acids.
The major mechanism for PCB removal is via adsorption to suspended matter and sludge flocs.117 Direct correlation between the concentration of particulate matter in the raw wastewater and PCB removal has been observed,21 and a relationship between PCBs and nonsettleable solids has been seen.87,112 Primary sedimentation removed approximately 45% of the PCB load in a pilot plant study.112 A 10- to 100- fold increase was seen from influent to primary sedimentation sludge, and final effluent loadings of 0.01 g total PCB day–1 were a 10-fold decrease on the influent.
PCB removal positively correlates with increasing sludge age.114 A 9-day sludge age gave significantly lower effluent concentrations than at 4 days. Removal of 68% of PCBs was observed at a typical dry weather flow (DWF) (0.111 l s–1).
This decreased to 48% removal under higher flow conditions (0.333 l s–1) and was 58% when flow varied in a typical daily manner (0.075 to 0.168 l s–1).118 PCB removal efficiencies were highest at DWF and lowest at 3 times DWF (Table 4.7), their removals being comparable with suspended solids removal. This is due to higher hydraulic loadings and surface loading rates associated with a retention time decrease.118
Air stripping has been noted as an important factor for compounds with an Hc greater than 100 Pa m3 hr–1. This makes less-chlorinated PCBs potentially suscep- tible,119 although this is unlikely due to their high affinities for mixed liquor sus- pended solids (MLSS).114 When compared to MLSS/effluent ratios, the most lipo- philic compounds have the highest concentration ratios.
4.3.6.2 Polyaromatic Hydrocarbons
Polyaromatic hydrocarbon compounds (PAHs) are believed to be persistent in the environment, although there is some work that sees degradation within 12 to 18
Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 127
hours.120 In a conventional STW, degradation times could be as long as 80 to 600 hours, since the experiments were run in ideal conditions with temperatures of 20°C and with pre-adapted bacteria. PAH removal of 35.1 to 86.1% during one sedimen- tation process was observed.121 During volatilization, significant removal was seen for 7 out of 13 PAHs, and during photodegradation 9 compounds demonstrated significant losses in settled sewage. Adsorption to suspended solids reduces the susceptibility of PAH to photodegradation.
The removal of these compounds during primary sedimentation is principally a function of physicochemical properties and unit process performance; their removal correlates with suspended solids removal.121,122 Association with dissolved and col- loidal matter in raw sewage dominated lower molecular weight compound removal.
Low molecular weight PAH removal is dependant on suspended solids loading with mean removals of smaller compounds increased with increasing suspended solids loading. Removal of high molecular weight compounds is dependant on hydraulic loading. In Greece, low molecular weight compounds were removed efficiently but higher molecular weight compounds are resistant to biological degradation.123,124 For example, during primary treatment naphthalene showed a 43% removal and acenaph- thene showed 64% loss. Some compound concentrations were seen to increase because of supernatant return streams to influent prior to primary clarification. Good mass balances have been demonstrated during secondary treatment. Lower mass compounds showed losses of greater than 40% suggesting biodegradation or vola- tilization. Table 4.8 displays concentrations of PAH at various treatment stages.
PAHs’ removal during primary sedimentation is a function of molecular weight and suspended solids removal efficiency, since they tend to partition onto the solid phase.38,125–128 It has been estimated that 64% of total PAH sorb onto the solid phase during primary sedimentation. Because of their association with the solid phase, removal may be enhanced by more efficient physical solid-liquid separation tech- niques such as tertiary filters.125
Manoli and Samara123 observed that sorption was the dominant process during primary treatment, particularly the higher mass compounds. Removal during sec- ondary treatment is dependent on volatilization and biodegradation. Total mass balances showed that lower mass compounds were effectively removed but larger
TABLE 4.7
PCB Removal through STW with Varying DWF118
Flow Average Concentrations (ng l–1)
Raw Sewage Settled Sewage Primary Sludge
1 DWF 11.6
10.3 20.0
3.6 151.9
3 DWF 5.4 304.2
Variable 8.4 110.1
Source: From Garcia-Gutierrez, A.G., McIntyre, A.E., Perry, R., and Lester, J. N., The behaviour of PCBs in the primary sedimentation process of sewage treatment: a pilot plant study, Sci. Total Environ., 22, 243, 1982. With permission.
128 Endocrine Disrupters in Wastewater and Sludge Treatment Processes
compounds were not. Fate modeling using Toxchem125 demonstrated that increases in cross-sectional areas of primary and secondary clarification tanks would not improve PAH removal. Biomass also seemed unaffected by increases in SRT. This may be due to a reduction in effluent PAH concentration that no longer constitutes an effective substrate concentration for secondary metabolism. It may also be because of the difficulty in degrading PAH.
Atmospheric depositions were found not to be responsible for PAH distribution in STW in Paris.113 Concentrations in STWs were higher after rainfall after a dry period. This indicates that PAHs enter STWs through ground leaching.
4.3.6.3 Polybrominated Flame Retardants
Very little has been reported on PBDEs flame retardants in STW. They have been detected near to site discharges, but their fate is still unknown. PBDEs are hydro- phobic and are likely to sorb to the biomass in activated sludge treatment and be transported primarily by adsorption to particulate matter. Their Kow values have been measured and support this theory (Table 4.2). Pre-1990 sewage sludge samples were analyzed for PCB and dichlorodiphenyltrichloroethanes alongside PBDEs and the levels of single congeners were comparable.129 More recent samples had higher PBDE concentrations than PCB. Concentrations of BDE-47 (the tetra congener, 2,2′,4,4′,-TeBDE) and BDE-100 (the penta congener, 2,2′,4,4′,6-PeBDE) in 1988 were 14 to 110 ng g–1 and 3.3 to 28 ng g–1, respectively. Approximately 10 years later, concentrations of 72 to 130 ng g–1 and 21 to 40 ng g–1 were detected.
Tetrabromobisphenol A (TBBPA) has been found in STWs. Concentrations in sludge from an STW that receives landfill leachate used by the plastics industry had levels of 100 ng g–1 ignition loss compared to 65 ng g–1 ignition loss for STW with no connection to the plastics industry.130 TBBPA is removed during wastewater treat- ment, but whether this is due to physical or biological parameters is not known.131
For the breakdown of brominated compounds, microorganisms must be capable of cleavage of carbon-bromine bonds. A lag phase would be required for this to
TABLE 4.8
PAH Concentrations at Various Treatment Stages
Raw Sewage Primary Effluent Secondary Effluent Liquid Solid Liquid Solid Liquid Solid
µg l–1 µg g–1 µg l–1 µg g–1 µg l–1 µg g–1
Naphthalene 3.9 3.3 3.5 3.8 0.2 0.4
Acenaphthene 0.2 0.7 0.2 0.8 0 0
Fluorene 0.3 13.4 0.2 11.9 0 1.5
Pyrene 0.2 1.1 0.2 10 0 2.3
Source: From Melcer, H., Steel, P., and Bedford, W.K., Removal of polycyclic aromatic hydrocarbons and heterocyclic nitrogen compounds in a municipal treatment plant, Water Environ. Res., 67, 926, 1995. With permission.
Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 129
occur to allow the microorganisms to produce the hydrohalidase and dehalogenase enzymes necessary for dehalogenation. In anaerobic conditions, reductive dehalo- genation may occur. This is the replacement of a bromine atom with hydrogen.
Hydrolytic dehalogenation is the replacement of a bromine with a hydroxyl group.132 One study133 found a good correlation between the concentrations of polybrominated dibenzofurans and dibenzo dioxins. The study concluded that the main source of the organic compounds was from PBDEs.