ADVANCES IN WATEH
RTMENT TECHNO
GlES
Monitoring and Modeling ,
olatile organic compounds (VOCs) have gained prominence in air and water pollution control practice over the past decade as a result of heightened environmental and health concerns and the introduction of new regulations. VOCs are precursors of ground-level ozone, which contributes to smog formation. VOCs in wastewater treatment plant discharges may exert environmental impacts on receiving-water quality. Airborne emissions from treatment facilities may contain VOCs and affect downwind populations. Regulatory initiatives, the most significant of which is the 1990 Clean Air Act Amendments (CAAA) (1-31, have addressed these concerns through control measures designed to minimize VOC emissions. T h e s e n e w r e g u l a t i o n s have spurred the development of VOC fate and transport models in wastewater collection and treatment systems, the inventory of VOC emissions, the development of new VOC sampling and analytical protocols, and research into VOC emission control methods. EPA identified 189 chemicals and chemical categories (referred to as hazardous air pollutants, HAPS) for regulation. VOCs form the majority of these. Prior to the CAAA, the release of the National Emission Standards for Hazardous Air Pollutants (NESHAP) regulation for benzene forced the petroleum refining i n d u s t r y t o make multimillion dollar investments to control benzene emissions ( 4 ) . Title 111 of the CAAA requires
328 A Environ. Sci. Technol.. Vol. 28, No. 7. 1994
that maximum achievable control technology (MACT) standards be developed to control HAPs in both municipal and industrial sectors. To date, EPA has proposed only one MACT standard-the hazardous organic NESHAP [HONI-to regulate the emission of 149 organic HAPs from the synthetic organic chemical manufacturing industry. This paper reviews the advances made in providing the scientific basis for addressing VOC control regulations and focuses on emissions from wastewater collection a n d treatment systems. Surveys of VOC emissions From 1978 to 1981, EPA, as directed by the Clean Water Act, conducted two major surveys of the removal of 129 priority pollutants in publicly owned treatment works (POTWs). Known as the 25 POTWs (5) and the 40 POTWs (6) studies, these surveys provided data on the concentration and likely occurrence of VOCs in the influents, effluents, and sludges at POTWs. Although considerable variability in influent VOC concentration was observed, only four VOCs were found at concentrations above 1 pglL at least 50% of the time in the effluents. A survey of priority pollutant data from 105 North American POTWs from 1978 to 1986 showed that only six VOCs were found more than
HENRYK MELCER Brown a n d Caldwell Consultants Seattle, WA 98119
50% of the time in the effluents from secondary POTWs at concentrations of 1-20 VglL (7). In the early 198Os, a survey of 16 VOC species showed that only a small fraction of nearly 600 Californian POTWs emitted significant quantities of specific VOCs to the air relative to other sources (8). In 1987,the California Air Toxics "Hot Spots" Information Act (known as AB 2588) required emitters to inventory VOC emissions. This program has generated the largest ever statewide VOC emissions database and, although data are still under review, a preliminary conclusion indicates that (contrary to the expectation that POTWs were significant sources of VOCs) the majority of the calculated human health risk in ambient air is contributed by mobile sources ( 9 , 10). In a parallel investigation, the Joint Emission Inventory Program (a working group of 22 of the 23 major POTWs in California's South Coast Air Basin), only two of the facilities tested produced unusually high VOC levels ( 1 1 ) . These were the only facilities that accepted petroleum refinery wastewater, although a clear relationship could not be det e r m i n e d b e t w e e n t h e refinery wastewater and VOC levels. It was also found that POTWs contributed 0.45 megagrams (Mg) per day of VOCs to the south coast atmosphere, which was less than 0.03% of basinwide emissions. Low VOC concentrations have also been reported in the treated effluents from industrial wastewater treatment plants. A 1981 survey of
0013-936X/94/0927-328A$04.50/0 0 1994 American Chemical Society
Recent comparisons of modelled predictions of VOC emissions with actual measurements ut specific steps of the treatment process, such as the high-powered surfoce aerators pictured ubove, hove shown that current models require more calibration
priority pollutants at five organic chemical plants with activated sludge treatment facilities showed that the high levels of influent VOCs (>I mglL) were reduced to low levels in the effluent, although the mechanism for such losses was not determined (12). Similar investigations conducted at five Canadian refineries between 1980 and 1985 showed that only four VOCs were found in secondary effluents more than 50% of the time at concentrations greater than 1 pglL (23,24). This confirmed the results from similar investigations at U S . refineries (25,26).
Modeling, mechanisms of VOC emissions At the time the above surveys were being conducted, other researchers were investigating VOC removal mechanisms in wastewater treatment plants. Biodegradability of VOCs was demonstrated by EPA
in aerobic static culture flask studies although significant losses by volatilization were noted (27).Further studies (18) showed that, at pilot scale, some VOCs were removed at POTWs primarily by stripping. Partitioning to biosolids was not found to be a significant removal mechanism. Despite the availability of a large VOC monitoring database from three Canadian POTWs, it was not possible to construct stochastic models of VOC removal, confirming the need for a mechanistic approach to fate modeling (29). Early attempts to model VOC removal using biological oxygen demand (BOD) removal kinetics met with limited success (20). It was recognized that the relatively low VOC concentration in wastewaters did not lend itself to the conventional gross organic parameter (BOD, chemical oxygen demand) kinetic analysis in the determination of bio-
logical degradation coefficients for VOCs (21, 22). Three major fate mechanisms were identified: loss to the atmosphere, sorption, and biodegradation. Much of the research into fate mechanisms has focused on the primary mechanism: loss to the atmosphere.
Loss to the atmosphere In the absence of competing mechanisms, mass transfer of VOCs to the atmosphere can result from natural volatilization across open water surfaces, volatilization induced by mechanical surface aeration, and stripping by diffused aeration. In each case, the rate of mass transfer of a VOC from wastewater to the atmosphere across an airwastewater interface can be described (23) by: dMldt = -& (C- C#H,IA
(1)
where KL is the overall mass transfer
Envirm. Sci. Technol., Vol. 28. NO. 7.1994 329 A
of mixing (25, 31,32).Empirical expressions to estimate H, as a fnnction of temperature have been developed f o r VOCs ( 3 3 , 3 4 ) . Observed values of H, vary over a wide range for some VOCs ( 3 5 ) . There continues to be debate as to the validity of applying clean-water H, values to wastewater. For wastewater treatment processes that are open to the atmosphere, accumulation of VOCs in the gas phase is often negligible and mass transfer across the air-wastewater interface can be modeled as: dM/dt =-KLCIA (4)
.
coefficient (mls), C i s the VOC concentration in bulk liquid (pg/L), C is the VOC concentration in b u d gas (mg/m3), H, is the dimensionless Henry's law coefficient (m3 liquid/m3 gas), and A is the interfacial contact area between air and wastewater (m'). KL can be described by two-film theory (24), a flux-matching boundary condition, and the assumption that overall resistance to mass transfer results from resistances through two thin films (gas and liquid) adjacent to the gasliquid interface:
'
This is referred to as an infinite dilution or infinite ventilation condition, and can be applied to volatilization at open surfaces or surfaceaerated processes. Equation 4 is not valid for mass transfer to rising air bubbles, or from poorly ventilated covered treatment or sewage collection systems.
100-150 for (kdk,)has been used to model VOC stripping in gas-liquid contactors and that gas film resistance is likely more important than previously assumed (26). The ratio of liquid mass transfer coefficients for two VOCs i and j may be described by:
Y = klj/klj= (Dj/Di]"
(3)
where Y is the transfer coefficient ratio for compounds i and j (dimensionless); kjj and klj are liquid mass transfer coefficients for compounds i and j , respectively (mls); and Dj and Dj are liquid diffusion coeffil / K L= l/kl+ l/(H&) (2) cients for compounds i and j , rewhere kl is the liquid-phase transfer spectively (rn'ls). The exponent n coefficient (mls) and k the gas- has been shown to vary from 0.5 for phase transfer coefficient$m/s]. The penetration and surface renewal first, second, and third terms in theories (27, 28) to 1.0 for the twoEquation 2 are overall, liquid, and film theory (24)and depends on turgas-phase resistances, respectively. bulent mixing conditions in the As kl and k, are directly propor- aqueous phase. Oxygen has been tional to molecular diffusivities, the most widely studied volatile and molecular diffusivities of VOCs compound. It therefore lends itself are generally much greater in gas to serving as reference compound j than liquid, gas-phase resistance in Equation 3. Values of k, for oxyhas often been assumed to be negli- gen (k,) can be measured using gible for H, > 0.1 ( 2 5 ) . For such a well-established protocols. Othercondition, liquid-phase resistance wise, k , can be estimated using to mass transfer is limiting, and KL models described in the literature = kl. Equation 2 also indicates that (29,30). Values of Y for VOCs and the liquid-phase resistance will in- oxygen have been noted to be apcrease the greater the ratio, (kdk,), proximately 0.6 in well-mixed sysand the greater the value of Hc, that tems. They have also been observed is, the more volatile the compound. to be insensitive to temperature, It has been reported that a value of presence of surfactants, and degree 330 A Environ. Sci. Technol., Vol. 28, No. 7, 1994
Volatilization at open surfaces Processes with natural volatilization across open surfaces include clarifiers, conveyance channels, ponds, quiescent portions of activated sludge basins, and collection systems. Equation 4 is often used for uncovered processes. For covered processes, Equation 1 is applied with a simultaneous gas-phase mass balance to obtain C,. For flowing streams with measurable velocities, such as sewers, stream reaeration models can be used to estimate KL. However, clean water oxygen transfer coefficients must be adjusted by Y and a,the transfer coefficient ratio between wastewater and tap water. Alternatively, a model for oxygen transfer to untreated wastewater (29) can be used. A model developed to predict emissions of benzene and toluene from quiescent surfaces with wind waves (36) can be used with Equation 3 to estimate KL for other VOCs with H, > 0.1. Surface aerated systems For mechanical surface aeration systems, VOC emissions can occur from the agitated surface, airborne wastewater droplets, or mass transfer to entrained air bubbles. The latter has typically been assumed to be negligible, so that C, = 0 and Equation 4 applies. The mass transfer coefficient can then be estimated from the empirical expression derived by Roberts et al. (32). Recent work (37) evaluated the modification of ' I 'with a liquidphase resistance term and demon-
strated good agreement between theory and observation for 20 VOCs over a range of hydrodynamic cond i t i o n s , For c o m p o u n d s w i t h H, < 0.2, gas-phase resistance was significant, confirming results by other researchers (26). It has also been observed that k,/k, was relatively constant with values of 18 to 26 for full-scale surface aerators, with power inputs of 2 . 3 to 27.8 W/ m3 over a temperature range of 20 "C (38). These values were lower than reported above for bench-scale studies and were attributed to a greater contribution of stripping caused by entrained bubbles at the larger scale. This apparent gasphase limitation might be a result of using a model that does not account for stripping caused by entrained bubbles where saturation can be limiting. Subsurface aeration 'In subsurface aeration, oxygen diffuses from air bubbles to wastew a t e r , w h e r e a s VOCs c a n be stripped from wastewater to rising bubbles, with subsequent release to the atmosphere when the bubble reaches the wastewater surface. Because bubbles accumulate VOCs as they rise through wastewater, the gas concentration cannot be assumed to be zero, and Equation 1 applies. Alternatively, if contaminated air such as that from sewer headspaces or covered tanks is used as the aeration gas, VOC mass transfer from bubbles to wastewater will occur. Air bubbles, which come to equilibrium with VOCs in the aqueous phase, are said to be saturated with respect to those VOCs, and emissions can be estimated by: E = Q,H& (5) where Qg is the aeration rate (m3/s) and E the VOC emission rate (mg/s). Such an assumption has been made to estimate VOC emissions from municipal wastewater treatment systems (39). Because rising bubbles may reach only partial saturation with individual VOCs, Equation 5 was modified to include a fractional saturation term (32): E = Q&ICC{1exP ( - ~ K L QVI , (HCQg)) 1 (61 where K,Q, is the oxygen transfer coefficient ( l / s ) and V the reactor volume (m3).As H,Qg becomes very s m a l l , t h e bracketed t e r m a p proaches unity and Equation 6 approaches Equation 5 (Le., saturated conditions exist). However, as H, andlor Qg increases, the assumption
that air bubbles are fully saturated may be inappropriate. Diffused bubble aeration enhances turbulent mixing and, hence, volatilization at the open surface (surface desorption). Although the contribution of surface desorption to overall emissions from such systems is not well understood, it has been observed that surface desorption accounted for 28 to 59% of total transfer rates of chloroform, carbon tetrachloride, trichloroethylene, and tetrachloroethylene in a laboratory-scale diffused aeration system (40). For a given oxygen transfer rate, mechanical surface aerators produce greater VOC stripping than do subsurface aerators (42). With other conditions constant, VOC stripping increases with increasing aeration rate, and systems that provide the required oxygen transfer at reduced aeration rates will reduce VOC stripping. For example, fine pore diffusers provide greater oxygen transfer per unit air flow rate than coarse bubble diffusers and thus can reduce VOC stripping (42, 42). Pure oxygen systems provide enhanced oxygen transfer and can reduce VOC stripping rates (43). Recycling of exhaust air from grit tanks and aeration cells can also m i n i m i z e VOC s t r i p p i n g from POTWs (42).
Drop structures For volatilization at drop structures, such as weirs, it is not possible to accurately measure interfacial contact area for droplets formed by falling water in contact with underlying receiving water or entrained air bubbles. Thus, models developed to estimate VOC emissions from drop structures are empirical and are based upon equations that predict oxygen reaeration in clean water flowing over weirs. They correlate the dimensionless oxygen deficit or depletion ratio, r, to drop height, discharge, and pool depth. Researchers have developed empirical relationships for r, (44, 45). The value of r, is related to an analogous term for VOCs according to: where a is the transfer coefficient ratio between wastewater and tap water, and
where C,-,,c and C,-,,, are VOC concentrations immediately u p stream and downstream of the drop, respectively. If the drop structure is
open to the air and well ventilated, C, can be assumed to be negligible, and Equation 8 reduces to C,.,, = C1.voc/r,, Otherwise, Equation 7 should be used to account for reduced mass transfer by VOC accumulation in the gas phase above falling wastewater ( 4 6 ) . Drawing u p o n previous oxygen transfer equations accounting for entrained air bubbles in clean water (471, empirical expressions have been developed for oxygen transfer from wastewater flowing over primary and secondary clarifier weirs (48). Several experiments and modeling efforts have indicated that average relative VOC emissions across primary treatment processes can be as high as l o % , with significant contributions from weirs and aerated grit chambers (28, 48-52). VOC losses can be even greater for systems that employ pre-aeration processes a n d aerated conveyance channels, and can be significantly lower for covered primary treatment systems and submerged weirs (52, 53).
Several mechanisms can contribute to gas-liquid mass transfer at drop structures in sewer manholes. It can occur at the free-falling wastewater surface, from airborne drops generated by splashing at the surface of the receiving tailwater, at agitated tailwater surfaces, and via air bubbles in the tailwater. One of the few investigations of VOC emissions from sewer drop structures has shown a strong linear correlation between oxygen transfer and both drop height and wastewater flow rate (54). When this work was extended to address the fate of four VOCs in enclosed sewer drop structures, stripping efficiencies and mass transfer coefficients were found to be a strong function of H,. This suggested the potential importance of air entrainment as a mass transfer mechanism ( 5 5 ) .Also, kJk, ratios ranged from 0.6 to 1 5 . These values were much lower than discussed above and again indicated that gas-phase resistance is probably more important than previously considered. In comparing VOC emission data in sewer reaches (56, 5 7 ) ,it is evident that drop structures are likely to be the most dominant source of VOC emissions in municipal and industrial sewers (55). Loss by sorption Sorption has not been found to be a major mechanism of removal for VOCs in POTWs (28, 58). Sorption may involve adsorption in which
Environ. Sci. Technol., Vol. 28,No.7 , 1994 331 A
VOCs are bound to solid surfaces such as cell walls, or absorption in which VOCs are dissolved in a second liquid phase. The observation that the sorption properties of primary, secondary, and digested sludges from several treatment plants exhibited no statistically significant differences in sorption properties when the solids concentration was based on volatile solids, suggests that data obtained for one sludge can be used to estimate sorption properties for different sludges and different treatment plants ( 5 9 ) . Current models usually do not account for the kinetics of the sorption process. They assume steady state conditions and that VOC sorption takes place instantaneously. It has been found that at the low VOC conc e n t r a t i o n s typically s e e n i n POTWs, l i n e a r i s o t h e r m s a d e quately describe sorption equilibria; that is, the rate of removal of a VOC is first order with respect to the solids concentration in waste sludge and the liquid-phase VOC concentration ( 5 9 ) . Values for the sorption partition coefficient, Kp, can be estimated from octanolwater partition coefficients using the empirical relationship derived by Dobbs et al. ( 5 9 ) . Activated sludge systems employing powdered activated carbon (PAC) to enhance removal of toxic compounds have been used in industry. Sorption of VOCs is enhanced in the presence of PAC. Improved removal a n d r e d u c e d stripping of poorly degradable VOCs have been observed when PAC was added to a n activated sludge system (60).
Loss by biodegradation Biodegradation of organic compounds i n wastewater treatment systems is typically modeled by a form of the Monod equation i n which the rate of compound disappearance, r (g/m3/d), is related to the concentration of the compound, S (g/m3),and the active microbial cell mass, X, (g/m3)).If the substrate concentration is significantly less than the half-saturation coefficient, K, (g/m3) (which would be the case for VOCs in most wastewaters), the Monod equation reduces to a firstorder expression with respect to substrate concentration: r = -pmSX,/( Y ( K , + S)]= -pmSXa/(YK,)= k S X , (9) where pm is the maximum microbial growth rate (l/day), Yis the cell yield coefficient (g cells/g substrate),
and k is the apparent first-order bio- lowed by desorption, formation as degradation rate constant, pJ( YK,,) metabolic byproducts, or both. (m3/gld). In a review of biodegradation ki- VOC fate and transport models netics models, it was shown that, at The need for VOC fate and transthe typically low7 levels at which port models was prompted by the VOCs are f o u n d i n m u n i c i p a l complexity and high cost of direct wastewaters, inhibition effects are sampling and measurement of VOC ignored in most models ( 6 1 ) .These emissions from POTWs. Emissions concentrations may be below those may be estimated from mass balr e q u i r e d to s u p p o r t microbial ances based on flow and wastewater growth, and degradation may occur concentration data. Although adeby secondary utilization or come- quate for screening purposes, they tabolism. This is especially critical are conservative because they do in biofilm reactors where signifi- not account for sorption, biodegracant diffusional resistances may oc- dation, and pass-through. The esticur to mass transfer of nutrients, ox- mates may be adjusted with an ygen, and VOCs (62). Models that emission factor, which accounts for describe secondary utilization have the fraction of mass loading of loss been developed ( 6 3 ) . attributed to v o 1at i 1e emissions , Utilization of Equation 9 suffers VOC s t r i p p i n g factors are n o t from the difficulty of interpreting widely available for many unit prothe active cell concentration. X,. cesses, and those that are do not acMost models use the total or a frac- count for plant characteristics that tion of volatile suspended solids to will reduce or enhance the emisdescribe the active biomass and all sions. The most comprehensive demodels assume acclimated biomass termination of emission factors for (61). Others have interpreted X , as municipal treatment facilities was being a large fraction of the total the Pooled Emission Estimation biomass that would degrade VOCs Program conducted at 2 1 POTWs by secondary utilization ( 3 9 ) . An for 18 unit processes and 2 1 VOCs additional drawback of Equation 9 by 24 California agencies in response to the AB 2588 regulation is the lack of biodegradation rate data for VOCs. Published biodegra- (70). The drawback of these plantdation rate coefficients can vary by wide emission estimation techniques up to three orders of magnitude and is that they do not quantify the prihave been shown to vary according mary sources of VOC emissions (71). to the degree of biomass acclimaThe advent of fate and transport tion ( 6 4 ) .Some researchers have ad- models allowed the simulation of dressed this problem by using a competing VOC removal mechanisms in wastewater and should s t r u c tur e-ac t iv i t y re 1at i o n s h i p based approach to estimate biodeg- lead to improved estimates of VOC emissions. Such models can also be radation rate coefficients (65). used to examine the effect of proGenerally, in wastewater treatment plant environments, nonchlorinated cess changes designed to minimize VOCs, such as benzene, toluene, and emissions and their impact on effluent VOC concentrations. They have xylene, are biodegradable, whereas chlorinated VOCs, such as trichloro- been used to predict emissions for ethylene, tetrachloroethane, and di- air permitting purposes ( 7 2 ) .Many chlorobenzene, are regarded as recal- models have been developed, but citrant. However, there is increasing not all are computer-based (39, 46, evidence that some chlorinated 50, 73-79). They include equations VOCs are amenable to biological deg- that describe t h e VOC removal radation (66, 67). Evidence of VOC mechanisms described above with biodegradation under anaerobic con- differing degrees of refinement relating to mass transfer assumptions ditions by reductive dehalogenation has been presented (68) but this phe- and methods of calculating rate conomenon has not been actively ex- efficients. The attributes of several models have been reviewed ( 7 2 ) ,as p 1o i t e d i n wastewater treatment plants, Low concentrations of chlori- have those of computer-based modnated methanes have been reported els (61, 80). Table 1 is based on an in anaerobic municipal sludge di- extensive review of computer-based gester gas, suggesting minimal sorp- fate models ( 2 3 ) . None of the currently available tion to waste sludge streams, destruction during digestion, or both (69). models describe a complete liquid Concentrations of benzene, toluene, treatment train that incorporates xylenes, and ethylbenzene i n di- headworks, primary and secondary gester gas were high, suggesting re- treatment, disinfection, and tertiary moval in waste sludge streams fol- treatment; nor do they include the
332 A Environ. Sci. Technol., Vol. 28, No. 7, 1994
sludge processing train or sewers. CORAL (82) is the only published computer-based model for VOC stripping in simplified sewer networks. It does not account for sorption and biodegradation. Existing treatment plant models do not simulate the formation of chlorinated products during wastewater disinfection nor the products of hydrogen sulphide oxidation. Most advanced models address activated sludge treatment or aerated stabilization basins. Few data are available to describe VOC fate in other secondary systems. A pilotscale study of fixed-film systems showed that biological aerated filter (BAF] air emissions had the highest mass of VOCs per volume of wastewater treated (82). Emissions were lower for the rotating biological contactor and trickling filter tested. Other work showed comparable masses of VOCs in air emitted from parallel BAF and activated sludge systems (83).Fixed film VOC emission models have not been published to date. The activated sludee models follow the assumption-of secondary utilization and require prior solution of the process biomass and oxygen balances. Most fate models assume that the overall removal rate of a target VOC is equivalent to the sum of the removals attributed to each of the mechanisms described above (21,39). These mechanisms can he competitive, a phenomenon that has been measured in laboratory-scale reactors (64, 84, 8.5). BASTE (76) has a flexible building block approach to simulating the liquid treatment train as compared to t h e fixed process flow schematic of the others, which generally includes both primary and secondary clarification. All models are steady state except for TOXCHEM (781, which also simulate the dynamic response of treatmer systems. TOXCHEM also includes the fate of VOCs in aerobic and anaerobic sludge digesters. A shortcoming of the models, except for BASTE and TOXCHEM, is the lack of temperature correction for Henry's law coefficients (61). When volatilization is the dominant removal mechanism, this could lead to significant error in the predicted VOC removal even over a small temperature range. Also, all the models neglect interaction between target VOCs except PAVE'S treatment of binary mixtures. NOCEPM, PAVE, SIMS, and WATER 7 are based on industrial waste-
program to estimate compound INCl consists of some
The EPA FATE model is
Washinolnn _.~-
NOCEPM
(NCAS Organ c
CompoJnd
Elminaton Pathway Mooelt
PAVE
(Programsto Assess Volatile Emissions)
SIMS (Surlace Impoundment Modelling System-1990) TORONTO A Model of
Oraanic Chimica Fate in a Biolog.cal Wastewater Treatment Plant
TOXCHEM
Toxic
Chemical Modelling Program for Water
Pollution Control Plants
DC .--
Develope0 by D A. Banon of the National Council of the Paper industry forAir and Stream Improvement (NCASI).New York NV
NOCEPM consists 01 vanous conceptual mcdel components selected by NCASt stall from the literature
Developed by the U S Chemical PAVE 6 a set of compdter Manufacturers' Association, mode,s for aetermmng volati e emissions from wastewater Washington, DC treatment Lnits ana from soi liquid solutions CHEMDAT 7 has been Developed by Radian for EPA Officeof Air Quality Plannin 8 incorporated into SIMS. Also, Standards. Research Triande emission models for collection Park, NC. svstems from IWW CTC dbcuments are incorporated into SIMS. Developed b B Clark, D. The TORONTO mooel I based Mackay, L. &si, G.L.H. Henry, on the IJgacty modelhn concep and S. Saienieks of the Institute and d e s eqdat ons simyar to of Environmental Stmes, tnose roposeo oy B acIioLrn et University 01 Toronto, w i n a (Zlpto descr be chemcai p i - ' -- ng wt 2nases SLppon from the Onlano " -'ry of tne Env ronment.
---
-
Developed by Environment Canada's Wastewater Technology CentFe, Burlington, Ontario, and Enviromega Ltd., Campbellville, Ontario. Copyright 1990.
TOXCHEM consists of various conceptual model components selected from the literature to address the fate of contaminants through all stages of wastewater treatment. It is the only model with unsteady state capability that allows orediction of the piant's res6onse to a spill
condition.
water systems, unlike the others, which are focused on municipal wastewater applications. NOCEPM (73) models activated sludge and aerated lagoon systems only. It
takes account of all VOC removal mechanisms. PAVE simulates only complete-mix activated sludge systems and incorporates complex biological kinetic models (79). It can
Environ. Sci. Technol., Val. 28. No. 7. 1994 333 A
also simulate VOC emissions from chemical spills of either pure components or binary mixtures. SIMS was produced to determine expected air emissions for industrial wastewater treatment facilities on a nation-wide basis as a tool to produce rule-making support data (86). Neither PAVE nor SIMS account for sorption of VOCs. The calibration of overall fate models is costly: this can limit their applicability, although proponents have shown significant success in the predictive qualities of their models. The NOCEPM model can predict the fate of V O C s typically found in pulp and paper wastewater treatment systems (73,87).The FATE model has been validated at pilot-scale and at POTWs (75). VOC treatability parameters for TOXCHEM have been determined at pilot- and full-scale (88). Model pred i c t i o n s compared well w i t h observations of the fate of selected VOCs at a POTW. The application of BASTE at nine POTWs showed that upstream process modifications tended to increase VOC emissions in downstream processes (53). Conclusion Significant advances have been made over the past decade in understanding the magnitude of VOC emissions from wastewater treatment and collection systems and in modeling major VOC removal mechanisms. The development of overall fate and transport models has provided a useful tool to predict VOC emissions from existing instal-
I
Henryk M e l c e r is spnior process specialist with Bmwn 6 Caldwell Consulta n t s in Seattle, WA, where he is responsible for t h e VOC a n d toxics c o n t r o l program. Previously he c o n d u c t e d research ar Environment Canada’s Wastewater Technology Centre in Burlington, O n t a r i o . Canada, where he d i r e c t e d fate-of-toxics investigations over t h e p a s t 12 years. H e has taught a t McMasfer Universiy, Ontario, and Massey University, N e w Zealand. H e received h i s B.S. degree and Ph.D. in chemical engineering from the University of Birmingh a m in England.
lations and from proposed process intended to minimize emissions. Recent comparisons of f a t e model predictions with actual measurements show that the models are perhaps not sophisticated enough andlor require more calibration (89).However, the models offer a relatively inexpensive method of making first estimates of emissions and can direct the allocation of scarce funding to the most critical areas. The Achilles heel of any of the overall predictive emission models is the inadequacy of accurate biodegradation coefficient data. The development of a s i m p l e method to measure these coefficients is highly desirable. modifications
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