3.2.5.1 Treatability of Metallic Compounds Metals in raw wastewater are removed in WWTPs through two different mechanisms: rPrimary sedimentation: metals are separated as insoluble preci
Trang 1198 Treatability Evaluation
Table 3.2.5 N/COD ratios and calculations of the single fractions of TKN
Values of N/COD ratios (g N/g COD) Symbol of
3.2.5 METALLIC COMPOUNDS
The concentration of metals in raw wastewater can differ significantly depending
on the domestic, commercial or industrial activities collected by the sewerage The main interest is in metals characterized by potential toxic impact on health or the environment, such as Cd, Cr , Cu, Hg, Ni, Pb and Zn The load of these components at the inlet of a WWTP can be several times greater in industrial sites than in residential areas far from industrial activities Urban run-off during storm events is also a source of metals and other pollutants, and contributes to the total influent load into a WWTP
3.2.5.1 Treatability of Metallic Compounds
Metals in raw wastewater are removed in WWTPs through two different mechanisms:
rPrimary sedimentation: metals are separated as insoluble precipitates or adsorbed
on settled particulate matter and then extracted with primary sludge In contrast the removal of metals in soluble form is negligible
rSecondary treatment: during the biological process metals are integrated into acti-vated sludge or biofilm (adsorbed on flocs or in extracellular polymers) They are removed at the same efficiency as the sludge solids in the secondary settler and extracted together with the excess sludge
Some values for metal removal in primary and secondary treatments are summa-rized in Table 3.2.6 (European Union, 2001)
Similar patterns of removal percentages are observed in primary and secondary treatments Lower removal is observed in both cases for Ni due to its high solubility that limits the presence of Ni in the particulate matter and sludge In contrast Pb, one
Trang 2Metallic Compounds 199
Table 3.2.6 Percentage of metals removed in WWTPs, calculated with respect to the
concentration in the influent raw wastewater
Removal in primary Removal in primary +
of the least soluble metals, shows higher removal in both the primary and secondary stages For the majority of metals a significant percentage of the influent load, up to 70–80 %, is transferred into primary and secondary sludge As a consequence the concentration of metals in dry sludge (measured as TSS) reaches levels of several thousand mg/kg TSS, about 1000 times higher than the concentration of metals in raw wastewater
In synthesis, the majority of metals entering the WWTPs with the raw waste-water is transferred to the sludge extracted from primary and secondary treatments Depending on the metal solubility, a smaller amount, ranging from 20 to 40 % (60 % only for Ni), is however discharged in water bodies with the final effluent With regards to the fate of sludge separated by settlers, the stabilization processes through aerobic or mesophilic anaerobic digestion cause the biological reduction of the volatile solids (30–50 %) and the specific metal content increases, metals being conserved during stabilization Due to the presence of metals the final disposal of sludge may be problematic especially in the case of accumulation in soils interfering with the long-term sustainable use of sludge on land
For the prediction of the removal of metals from raw wastewater and the parti-tioning into final effluent and sludge, mechanistic approaches have been proposed
(Monteith et al., 1993) On the basis of influent wastewater characterization (flow
rate, metal concentrations) and the layout of the WWTP (unit volumes, operational conditions) the metal concentration in primary sludge, secondary sludge and final effluent can be predicted The calculation is performed on the basis of mass balances
by considering the main chemical and physical mechanisms (precipitation of soluble metals into a settleable form, sorption onto settleable solids, surface volatilization)
In the model the mass of primary and biological sludge produced by primary sed-imentation and secondary treatment is calculated and partitioning coefficients are introduced in the model for the estimation of the metal concentrations in the soluble and solid phases A similar approach can be applied also for estimating the fate of organic contaminants instead of metals in WWTP Modelling can be performed both under steady-state or dynamic conditions
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3.2.6 FINAL CONSIDERATIONS
WWTPs are effective in the reduction of most pollutants present in wastewater (such
as organic matter, nutrients, potentially toxic elements or some micropollutants), be-fore the discharge of the treated effluents in surface waters In WWTPs several biological and physico-chemical processes can be implemented, but the main path-ways for pollutants removal are: (1) the biological oxidation by activated sludge or biofilm systems; or (2) the accumulation of contaminants in excess sludge
In this chapter the main categories of pollutants present in influent wastewater and their fate in WWTPs has been discussed The assessment of the treatability of a specific wastewater in WWTPs is strictly dependent on the fate of contaminants in the treatment stages The amount of pollutants removed in conventional WWTPs or passing into the effluent has been indicated depending on the category of pollutants, separated into organic compounds, organic micropollutants, nutrients and metallic compounds These main categories were identified in order to make an aggregation
of the large number of individual pollutants; a much longer and detailed report would be required for the explanation of the fate of each single element Therefore the present description is not exhaustive for understanding the fate of each single compound; the objective of this chapter is to explain the main pathways in WWTPs for macro-categories of pollutants
The wastewater characterization can be investigated more or less in depth de-pending on the particular needs in management of WWTPs, the requirement for discharge, and the practicalities of operators that make the measurements The in-creasing detail in characterization and control of effluent wastewater from WWTPs coupled with the more stringent limits for discharge in receiving water bodies, ne-cessitates a more complex and sophisticated monitoring This causes considerable additional effort and expense to obtain a high degree of knowledge about the type and the concentrations of pollutants and micropollutants in influent and effluent wastewaters
With regards to COD fractionation the routine measurement of all the parameters indicated in Section 3.2.2.1, according to the respirometric approach (described in Section 3.2.2.2), is extremely time-consuming because of the time required for the respirometric tests and the time need for data elaboration Therefore, COD fraction-ation could be done only occasionally in a WWTP and the percentages obtained can be assumed as typical for a specific wastewater Of course a periodic valida-tion of fracvalida-tionavalida-tion is required Alternatively, the simplified procedure described
in Section 3.2.2.3 can be applied, which is more approximate but is advantageously fast to use A more detailed characterization, performed by using the respirometric approach, could be required in order to observe daily, weekly or seasonal variation or fluctuation occurring in the COD fractions In the case of industrial sources, shock loadings are by their nature difficult to predict
In the case of N fractionation, the calculation described in Section 3.2.4.1 can be easily done thanks to its dependence on COD fractionation
Trang 4References 201
In general a good characterization of COD and N in the influent wastewater
is very important to understand the fate of these components in WWTPs and to predict the quality of the effluent wastewater before discharge in receiving water bodies
With regards to metals or nutrients, they are routinely measured in wastewater and sludge and an extensive knowledge about these components is usually available
in WWTP management The measurement is done often routinely in influent and effluent wastewater due to the relative ease of the analysis and the moderate expense involved
In contrast, organic micropollutants, such as PAHs, PCBs, PCDD/PCDFs or phar-maceuticals, are rarely monitored because of the high cost of analysis and the need for specialized laboratories and, sometimes, the lack of unified and standardized methodologies Furthermore, the limitation in the evaluation of the fate of organic micropollutants and potentially toxic elements is mainly related to the lack of studies
on mass balance in WWTPs and with regards to partitioning in water and sludge Further research is needed to improve knowledge in this field
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Publications of the European Communities, Luxembourg.
Field, J.A., Field, T.M., Poiger, T., Siegrist, H and Giger, W (1995) Water Res., 29(5), 1301–1307 Gujer, W., Henze, M., Mino, T and van Loosdrecht, M.C.M (1999) Water Sci Technol., 39(1),
183–193.
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Jørgensen, S.E (1998) Chemosphere, 36(2), 357–393.
Henze, M (1992) Water Sci Technol., 25(6), 1–15.
Henze, M., Grady J., C.P.L., Gujer, W., Marais, G.v.R and Matsuo, T (1987) Activated Sludge Model No 1 IAWQ Scientific and Technical Report No 1, London, UK.
Holt, M.S., Fox, K.K., Burford, M., Daniel, M and Buckland, H (1998) Sci Total Environ.,
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Metcalf and Eddy (2003) Wastewater Engineering Treatment and Reuse, 4th Edn McGraw-Hill,
New York.
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Environ Res., 65(2), 129–137.
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Orhon, D., Ate¸s, E., S¨ozen, S and Ubay C¸ okg¨or, E (1997) Environ Pollut., 95(2), 191–204 Pax´eus, N (1996) Water Res., 30(5), 1115–1122.
Prats, D., Ruiz, F., V´azquez, B and Rodriguez-Pastor, M (1997) Water Res., 31(8), 1925–1930 Roeleveld, P.J and van Loosdrecht, M.C.M (2002) Water Sci Technol., 45(6), 77–87.
Samara, C., Lintelmann, J and Kettrup, A (1995) Toxicol Environ Chem., 48(1–2), 89–102 Sinkkonen, S and Paasivirta, J (2000) Chemosphere, 40, 943–949.
Sollfrank, U., Kappeler, J and Gujer, W (1992) Water Sci Technol., 25(6), 33–41.
Spanjers, H, Tak´acs, I and Brouwer, H (1999) Water Sci Technol., 39(4), 137–145.
Spanjers, H and Vanrolleghem, P (1995) Water Sci Technol., 31(2), 105–114.
STOWA (1996) Methoden voor influentkarakterisering (in Dutch) STOWA Report 96–08, STOWA, Utrecht, The Netherlands.
Vanrolleghem, P.A., Spanjers, H., Petersen, B., Ginestet, P and Takacs, I (1999) Water Sci Technol.,
39(1), 195 – 215.
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119–126.
Trang 6Toxicity Evaluation
Martijn Devisscher, Chris Thoeye, Greet De Gueldre and
Boudewijn Van De Steene
3.3.1 Introduction
3.3.2 Need for Toxicity Measurements
3.3.3 Influent vs Effluent Toxicity of Wastewater
3.3.3.1 Influent Toxicity Evaluation 3.3.3.2 Effluent Toxicity Evaluation 3.3.4 Units
3.3.5 Sources of Toxicity
3.3.6 Toxicity Testing
3.3.6.1 Influent Toxicity 3.3.6.2 Effluent Toxicity 3.3.7 Toxicity Mitigation
References
3.3.1 INTRODUCTION
Under the Urban Wastewater Treatment Directive 91/271/EEC, the quality of ef-fluents has been based on the monitoring of global chemical parameters, such as BOD (biological oxygen demand), COD (chemical oxygen demand) or TSS (total suspended solids) Wastewaters from various origins may contain compounds, toxic
to the aquatic ecosystem, or even to the biocommunity responsible for the treatment
of the wastewater These toxic effects are insufficiently expressed in the currently practiced measurements
Wastewater Quality Monitoring and Treatment Edited by P Quevauviller, O Thomas and A van der Beken
2006 John Wiley & Sons, Ltd ISBN: 0-471-49929-3
Trang 7204 Toxicity Evaluation
Although some countries impose toxicity tests on effluents, there is currently no general European legal framework that systematically prescribes toxicity tests on effluents Nevertheless, it is expected that the role of toxicity tests will become more important in the near future Indeed, the European Union Water Framework Directive 2000/60/EC places more emphasis on the reduction of discharges of toxic elements, and the Integrated Pollution Prevention and Control Directive (96/61/EC), coming into effect by October 2007, is based on a permit system requiring the use of best available technology (BAT) In this, toxicity measurements may play an important role
This chapter presents an overview of the common toxicity detection methods in use today The discussion is limited to ‘conventional’ toxicity tests In recent years, there has been increased concern over the release of pharmaceutically active compounds, personal care products and endocrine disrupting compounds into the environment These compounds occur in low concentrations in the environment and are unlikely
to cause acute toxicity Highly sensitive bioassays have been developed to screen wastewater effluents on their (anti-)estrogenicity, (anti-)androgenicity, mutagenicity and cytotoxicity Developments in these fields are extensive, evolve fast and deserve separate chapters in their own right
However, we have limited the discussion to tests that are most relevant to the operation of wastewater treatment plants (WWTPs): the detection of toxic influents that can disturb the treatment process, and of toxic compounds in the effluent, which may be an indication of diminished treatment efficiency
3.3.2 NEED FOR TOXICITY MEASUREMENTS
Toxic compounds are present in wastewater from various sources In many countries
in Europe, industrial plants are connected to the sewer Industrial wastewaters can contain large amounts of toxic material, such as heavy metals, or synthetic chemicals and their waste products These pollutants can even be present after conventional wastewater treatment (Paxeus, 1996)
Also purely domestic wastewater can contain toxic elements Domestic discharges can contribute toxins from consumer products (e.g cleaning products) or liquid wastes Urban run-off may contain leachates or organic pollutants deposited from the atmosphere onto paved surfaces In combined sewer systems this run-off is also intro-duced into the sewer system Other known sources of potentially toxic compounds include commercial premises such as health establishments, small manufacturing industries or catering/hotel enterprises It is obvious that also illegal discharges to the sewer represent a potential source of toxicity
Chemical analyses alone are insufficient for assessing the toxicity of a wastewater
In the first place, the toxic compounds may be unknown Indeed, the composition
of wastewater is traditionally expressed in nonspecific terms such as BOD, COD or TOC (total organic carbon) These rather general measures reflect the general poor
Trang 8Influent VS Effluent Toxicity Of Wastewater 205
knowledge of the exact composition of wastewaters Even if an exact composition
of the wastewater is known, it is impossible to have a comprehensive overview on all compounds that are effectively present in the wastewater upon arrival at the treatment plant or in the environment Several transformations may occur and create additional toxic content Physico-chemical transformations may be occur, e.g under the influence of sunlight UV, and toxic metabolites may originate via biodegradation, for example during storage in cesspits, during sewer transport or in activated sludge treatment
In addition to the presence of unknown compounds, the (eco)toxicity of the known components may not be well documented Although databases of such data exist (e.g ECOTOX:http://www.epa.gov/ecotox/), important gaps remain The lack of this kind of information on thousands of chemicals on the market today has been acknowledged by the European Union, and has prompted the REACH (Registration, Evaluation, Authorisation and Restrictions of Chemicals) proposal (CEC, 2001) The goal of this proposal is to secure data on and regulate some 30 000 chemicals produced in excess of 1 ton for which there is limited information with regard to toxicity and environmental effects These data will expand the knowledge on toxic effects of pure compounds
However, even when all toxic components in a wastewater have been identified, and detailed ecotoxicity information would be available for each of these compo-nents, an additional difficulty is the assessment of the effect of complex mixtures Interaction of the compounds with each other, with the wastewater matrix or with the environment may result in synergistic or antagonistic effects, the matrix may render certain compounds biologically unavailable or may even increase toxicity (Hernando
et al., 2005).
A more direct measure of toxicity consists of submitting the whole complex mixture to a toxicity test Although interactions with the final environment are not modelled precisely, it is a measure of the resultant toxicity of the complex wastewater mixture, integrating the combined effect of known and unknown toxic components and their interactions with the wastewater matrix This type of testing is known in the USA as WET (whole effluent testing; US EPA, 1994) and in the UK as DTA
(direct toxicity assessment; Tinsley et al., 2004).
3.3.3 INFLUENT VS EFFLUENT TOXICITY
OF WASTEWATER
The first major distinction to be made is whether the wastewater is monitored before
or after treatment We will refer to these techniques as influent toxicity monitoring
and effluent toxicity monitoring, respectively
This distinction is different because both the goal and requirements, and therefore the adopted methods differ whether the wastewater is monitored before or after treatment
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3.3.3.1 Influent Toxicity Evaluation
These tests have the intention to protect the biological wastewater treatment process from the effect of toxic influents Although Annex 1 of the Urban Wastewater Treat-ment Directive already states that ‘Industrial wastewater entering collecting systems and urban wastewater treatment plants shall be subject to such pretreatment as is re-quired in order to ensure that the operation of the wastewater treatment plant and the treatment of sludge are not impeded’, these tests are not commonly imposed by regulators The tests used are sometimes referred to as upset early warning devices (UEWDs; Love and Bott, 2000) The sensitivity of these tests should be representa-tive for the biocommunity of the wastewater treatment process This sensitivity can differ greatly from that of the receiving ecosystem
3.3.3.2 Effluent Toxicity Evaluation
The purpose of effluent toxicity evaluation is to assess the effect of a certain wastewa-ter on the receiving wawastewa-ters The methods used are essentially the same as those used for ecotoxicity testing of pure compounds Effluent toxicity tests are imposed by some discharge consents and have been extensively studied and standardized The conventional approach is the use of bioassays In these tests, the biological response
of a certain bioindicator species is monitored in response to the wastewater to be tested These bioassays can be further subdivided according to the species involved, the duration (acute/chronic toxicity test) or to the effect on the indicator organ-ism (mortality, reproduction, motility) The requirements of these tests are a high sensitivity and representativity for the receiving ecosystem
Although the distinction between influent and effluent toxicity is clear, it is evident that there is a strong link between the two The effluent of an industrial treatment plant may be part of the influent to a municipal plant, and highly toxic substances
in the influent may inhibit the treatment process in such an amount, that the toxic compounds break through to the effluent to cause effluent toxicity
3.3.4 UNITS
Central to (eco)toxicity evaluation is a dose–effect relationship Since bioavail-ability of a compound introduced in wastewater differs greatly for each individ-ual compound, test species and wastewater matrix, the exact dose imposed on the test organism is difficult to quantify Therefore, in aquatic toxicity testing, a concentration–effect relationship is considered, relating the concentration in the wastewater to the effect on the test organism This relationship becomes evident in
Trang 10Sources Of Toxicity 207
100 75 50 25
0
0 2 4 LC50 6 8 10
Concentration (e.g mg/l)
Cumulative response (%) NOEC LOEC
Figure 3.3.1 Sigmoidal response curve (Adapted from Connell et al., 1999 with permission
from Blackwell Publishing)
the commonly used units for ecotoxicity:
rEC50: The concentration at which 50% of the effect is observed.
rLOEC: Lowest observable effect concentration, i.e the lowest concentration at which an effect can be observed
rNOEC: No observable effect concentration, i.e the highest concentration at which
no effect can be observed
The term concentration, in the context of whole effluent testing, refers to dilution series of the original wastewater, ranging from 0 to 100 % of the wastewater These measures are graphically represented in Figure 3.3.1
(Eco)toxicity is determined by studying quantifiable effects The effects studied are specific to each toxicity test A commonly observed effect is mortality (lethal effect) In this case, the term LC50 is used rather than EC50 This determines the concentration at which 50 % mortality is observed Another commonly used measure
is IC50which is the concentration at which 50 % inhibition of a certain activity (e.g light emission) is observed
There is no such thing as a EC50of a certain compound Toxicity is a measurement
of an effect to a certain organism or community of organisms It is therefore important that the test method is specified together with the EC values
3.3.5 SOURCES OF TOXICITY
Influents of industrial WWTPs may contain a large variety of toxic compounds It
is practically impossible, and certainly beyond the scope of this chapter, to give a