N. Voulvoulis and M.D. Scrimshaw
4.1 INTRODUCTION
The nonpolar and hydrophobic nature of many endocrine disrupting chemicals (EDCs) causes them to sorb onto particulates. This suggests that the general effect of wastewater treatment processes would be to concentrate organic pollutants, includ- ing EDCs, in the sewage sludge. Mechanical separation techniques, such as sedi- mentation, would result in significant removal from the aqueous phase to primary and secondary sludges. The removal of endocrine disrupters in wastewater treatment processes is dependent on the inherent physicochemical properties of the pollutants and on the nature of the treatment process involved. The result of this is a treated wastewater discharged relatively free of EDCs and a sewage sludge that contains most of the contamination that entered via the influent.
It is generally recognized that there are four main removal pathways for organic compounds during conventional wastewater treatment:
1. Adsorption onto suspended solids or association with fats and oils 2. Aerobic and anaerobic biodegradation
3. Chemical (abiotic) degradation by processes such as hydrolysis 4. Volatilization
A compound’s physicochemical data can be used to predict physical processes, such as sorption, volatilization, and dissolution. The important properties to be considered are octanol/water partition coefficient (Kow), aqueous solubility, acid dissociation constant, and Henry’s Law constant (Hc). A knowledge of chemical partitioning between the aqueous and solid phase is needed to assess pathways of EDC transport and transformation. For example, partitioning to the solid phase may increase the likelihood of reductive dehalogenation by anaerobic bacteria.1–3 In addition, surface-catalyzed hydrolysis, or sorption to settled sludge may reduce the possibility of volatilization or photochemical degradation.4
4.1.1 SEWAGE TREATMENT PROCESSES
Conventional sewage treatment is typically a three-stage process consisting of pre- liminary treatment, primary sedimentation and secondary treatment,5,6 which is sche- matically outlined in Figure 4.1. Some form of sludge treatment normally follows as outlined in Chapter 5.
Wastewater treatment commences at the head of the works with preliminary treatment, typically inlet screens. However, some conditioning of the wastewater will have been initiated in the sewer. For modeling purposes, the sewer pipe has been considered to consist of a sediment above which there is a bio-film.7 Anaerobic
Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 105
biodegradation may occur in the sewers where bacterial slime accumulates on the walls. In large catchment areas, wastewater can have a significant retention time in a sewage system and allows significant degrees of removal to begin there.
4.1.1.1 Preliminary Treatment
Preliminary treatment involves the initial screening of the raw sewage through parallel bars to remove large floating objects.5 A small amount of putrescible organic material is removed from the screens and generally landfilled or incinerated. Grit and dense inorganic solids are removed by means of settlement in tanks or constant velocity channels where the lighter organic material remains in suspension. Very little removal of organic micropollutants is observed at this stage.5
4.1.1.2 Primary Sedimentation
The raw sewage then passes into the primary sedimentation tanks where the most significant mechanism is adsorption onto solids, which under the influence of gravity settle to form primary sludge. The degree of pollutant removal is largely dependant on suspended solids removal, which is controlled by settling character- istics of the particles (their density, size, and ability to flocculate); the retention time in the tank; and the surface loading. The retention time in the tank is referred to as the sludge retention time (SRT) or sludge age and is a measure of the time bacterial cells remain in the system. It is the total amount of sludge in the system divided by the total rate of sludge leaving the system and is typically 4 to 9 days.5 Removal of organic compounds can be affected by temperature and the solids content of the effluent is higher during winter months when the temperature is low.
Fats, oils, and greases adsorb significant amount of hydrophobic compounds, including many EDCs, and are removed from the surface of the tank and added to the sludge prior to sludge treatment. Figure 4.2 indicates some of the possible removal mechanisms for EDCs during primary sedimentation. Primary sedimenta- tion is used in the majority of sewage treatment works (STW); in some cases FIGURE 4.1 Schematic diagram of STW. (From Meakins, N.C., Bubb, J.M., and Lester, J.N., The fate and behaviour of organic micro pollutants during waste water treatment, Intern.
J. Environ. Poll., 4, 27, 1994. With permission.) Preliminary
Treatment
Screenings
Incinerated or landfilled
Grit
Typically dumped on site
Primary Sedimentation
Sludge Treatment
Disposal Primary Sludge
Sludge Treatment
Disposal Secondary
Sludge Biological Treatment
Effluent discharge to watercourse
106 Endocrine Disrupters in Wastewater and Sludge Treatment Processes
flocculants are added to aid flocculation.8 Coagulants, such as aluminum and ferric salts have been used to remove organic matter, although their use is often deemed impractical due to the high costs.9 Where oxidation ditches are employed for wastewater treatment there is no primary sedimentation, and removal of EDCs probably conforms to the mechanisms identified or postulated for activated sludge treatment.10
4.1.1.3 Secondary Treatment
Secondary treatment can involve anaerobic biodegradative processes, although almost invariably aerobic processes are responsible.11 The principle of aerobic sec- ondary treatment is to allow the aerobic bacteria and other microorganisms in mixed liquor contact with oxygen to convert the organic compounds into carbon dioxide and water. Activated sludge or trickling filters are the principal secondary treatment processes employed. Both use two vessels: a reactor containing large populations of microorganisms that oxidize the biochemical oxygen demand (BOD), and a secondary sedimentation tank or clarifier where the microorganisms are removed from the final effluent. In activated sludge, the majority of these microorganisms are recycled to the aerator. In trickling filters, all of the film (humus) in the secondary sedimentation tank is disposed of. Removal pathways for organic pollutants during secondary biological treatment (Figure 4.3) include adsorption onto the microbial flocs and removal in the waste sludge, biological or chemical degradation, and transformation and volatilization during aeration.
FIGURE 4.2 Removal mechanisms during primary sedimentation for EDC removal. (From Meakins, N.C., Bubb, J.M., and Lester, J.N., The fate and behaviour of organic micro pollutants during waste water treatment, Intern. J. Environ. Poll., 4, 27, 1994. With permission.)
Removal of lipophilic organics with fat and grease Removal of adsorbed organics
by sedimentation Little change in polar
dissolved organics
Sludge treatment / disposal
Reduced concentrations of PCB, AP, unchanged concentrations of CPH Possible chemical degradation
Fate and Behavior of Endocrine Disrupters in Wastewater Treatment 107
Nitrification can occur during secondary treatment and can have benefits to the removal of organic contaminants. Nitrification is the conversion of ammonia to nitrate and is a two-stage process involving autotrophic bacteria, generally Nitrosomonas spp. and Nitrobacter spp. A considerable amount of oxygen is required for nitrification to take place, and a longer SRT is required because of the slow growing nature of these bacteria. This coupled with the high concentration of dis- solved oxygen required increases the cost of waste treatment.5
Tertiary treatment processes are also increasingly used and include sand filtration and microstrainers and occasionally may include methods such as activated carbon and membrane filtration. This is particularly the case in small domestic plants to polish drinking water, but these latter treatments are costly and pose maintenance problems.9
Table 4.1 includes some of the removal process and physical conditions impor- tant for high removal efficiencies for some of the EDCs.
4.2 BEHAVIOR OF ENDOCRINE DISRUPTERS