Environ. Sci. Techno/. 1995, 29, 1488-1494
Introduction
Oryaaic Chemical Fate in a Sewage Treatment Plant B . C L A R K , J . G . HENRY, A N D D. MACKAY* Institute for Environmental Studies, University of Toronto, Toronto, Ontario M5S lA4, Canada
Expressions are developed in fugacity format for the partitioning, transport, and transformation processes encountered by an organic chemical in a secondary sewage treatment plant (STP). A mass balance model is developed for correlating and predicting the steady-state phase concentrations, the process stream fluxes, and the fate of organic chemicals in a STP. Input data are the chemical‘s properties and the plant‘s design and operating parameters. The model is algebraically simple, robust, and can be run on a personal computer. The relative amounts of chemical that are likely to be stripped or volatilized, sorbed to sludge, biodegraded, and discharged in the effluent water can be assessed. Results obtained by applying the model to screen the fate of 12 chemicals compare satisfactorily with data from full-scale plants. The most critical and uncertain variable is the biodegradation rate constant and its dependence on biomass concentration. It is believed thatthefugacity analysis provides useful insights into chemical fate in a STP and that with further calibration and validation the model will be useful for correlation and prediction purposes.
Many organic chemicals used industrially, commercially, and domestically are discharged to sewer systems and are subsequently treated with varying effectiveness in sewage treatment plants (STPs). Grady (I)has reviewed treatment effectiveness, discussed research priorities, and provided an extensive bibliography. Monteith (2)has reviewed the frequency of occurrence, mechanisms and efficiency of removal, and variability in concentration of these contaminants. Regulatory agencies need a capability of assessing, for existing and new chemicals, fractions of the chemical present in the plant influent that are ultimately degraded, air-stripped, sorbed to sludge (which may be disposed of by land spreading, deposition in landfiils, or incineration),and leave in the effluent water. If the effluent is judged to have an unacceptably high concentration of a specific chemical, plant design or operating conditions may require modification to improve performance, or it may be necessary to restrict discharges to sewers. For chemicals that are new to commerce, advance assessment of fate in STPs is desirable. A predictive mass balance assessment or model of the chemical’sfate in a STP is thus invaluable. This predictive capability has been contributed to and discussed by Blackburn et al. (3, 4 ) , Roberts (3, Kincannon and Stover (6, 7), and Sayler et al. (8). Models have been developed by Namkung and Rittman (91,Stmijs et al. (IO),Cowan et al. ( ] I ) , Siegrist et al. (12),and Henze et al. (13). Under optimal operating conditions, a STP may remove alarge percentage, Le., 70-loo%, ofmanyorganicpollutants from the sewage, but treatment efficiencyvaries. It is clear that certain operating parameters, such as sludge retention time, influence efficiency. To interpret the performance of STPs, the fundamental effect of such factors on the fate of organic chemicals during treatment must be established. We describe and discuss a simple fugacity-based analysis of the fate of organic chemicals in a STP. This leads to a suggested structure for a steady-state model, which has the potential to correlate and ultimately predict the fate, and thus the important removal mechanisms, in a STP. The fugacityconcept has been successfullyused to interpret and model the fate of organic chemicals in the environment (14), but the only application to a STP is the multibox spreadsheet model “SimpleTreat” of Struijs et al. (IO). It is believed that this approach is complementary to conventional concentration-based analyses and models and provides useful insights into chemical behavior.
Process Description A typical STP flow diagram is shown in Figure 1, in which
the influent sewage is treated by primary sedimentation followed by secondary or biological treatment under aerated conditions with final sludge settling,wasting, and recycling. During primary sedimentation, the suspended solids capable of settling are removed in aregime of low horizontal velocities by gravity settlingwith a typical efficiencyof 5075% (1.5). Settled solids at concentration of 3-5% are removed periodically, usually once every 2-8 h (16). The primary objective is to reduce the particulate organic load * Corresponding author.
1488 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 6,1995
0013-936X/95/0929-1488$09.00/0
0 1995 American Chemical Society
A
Inflow GWI (1000)
Volain
Primary Tank
GW;
+
(998)
Primary Sludge GW3 ( 2 )
Inflow
Primary
GSlG
Tank
Simulation Structure
GA5 (8960) Air
Volatn
Aeration Tank f
t
Air GA4 (8960)
Settling Tank
(982)
--
Waste Sludge b
Return Sludge GWB (800)
Aeration GS2G
Effluent GW7
GSGG
GW9 (15)
Sett Ii ng Tank
E f f Iuent GS7G
Waste Sludge b
Primary Sludge GS3G (120000) r(
(0.12)
Inflow
Primory
MIG
Tank
Primary Sludge M3G(O 1 1 )
Air
M5G (3.60)
Aeration Tank
II
Air(0)
Return Sludge GS8G (4397000)
GS9G (82400)
(0.05)
settling Tank
1 Return Sludge M 8 G ( I 51)
Effluent
M7 G
Waste Sludge b
M9G (003)
FIGURE 1. Diagram of a STP with (A) water (d/h), (B) solids (g/h), and (C) chemical (0) mass balances corresponding to the output for toluene.
on the system since these solids contain 60-70% volatile and substantially biodegradable organic matter. Organic chemicalspresent in the influent are thus either (i)removed with the primary sludge, (ii) conveyed in dissolved or suspended form to subsequent treatment, (iii) volatilized from the water surface to the air, or (iv) biodegraded. In the aeration tank, chemicals may be biodegraded as a result of contact with aerated activated sludge, which is maintained in the system for a sludge retention time ranging typically from2 to 10 days. Undegraded chemicalpartitions between water, the biomass, and air, thus leaving the system in the effluent or by volatilization. In the final settling tank, the biomass is separated from the treated water by gravity settling. Most settled sludge (and associated chemical) is returned to the aeration tank (e.g., 70-100% of plant inflow) to maintain an activated sludge concentration of typically2000-4000 g/m3(16).The suspended solids content is usually 8000-10 000 g/m3 of settled sludge. The remainingsettled sludge is wasted from the system. In the final settling tank, chemicals are volatilized, biodegraded, and either discharged in the plant effluent or in the waste sludge or recycled in the return sludge. Each arrow in Figure 1 thus represents a potential pathway for the chemical, but the relative amounts in each pathway vary greatly from chemical to chemical as a result of differences in chemical partitioning, transport, and reactivity properties.
Assessment of chemical fate requires a quantitative description of each pathway as illustrated in Figure l. A set of steady-state, mass balance equations must be established describing the flows and amounts of the four components: water, biological solids, air, and chemical. All parameters and symbols in Figure 1 are designated to facilitate identification. Flow rates are designated Gwith the subscripts W for water, A for air, or S for solids and with a second subscript number defining the stream. G9 is thus the flow rate (m3/h) of water in stream 9. Chemical flows are designatedM (mollh) subscripted with the stream number. Solids concentrations are designated S (g/m3), and chemical concentrations are designated as CG (g/m3) again with stream number subscripts. The area and depth (and hence volume) of the primary clarifier and final settling tank are specified. Illustrative values are used here with depths of both vessels of 3.8 m (12.5 ft) and overflow rates of 90 (primary) and 33 (final) m3 m-2 day-' based on the inflow. The aeration tank volume must be specified or can be set to give an 8-h retention to the inflow, these being the typical design or operating values recommended by the Ontario Ministry of the Environment (15). The water balance for the entire system is established as shown in Figure 1A by defining (or using illustrative values given in parentheses) the flow rates for the inflow GI (as 1000 m3/h), GS(return sludge) as a specified fraction (80%) of GI,Gs(waste activated sludge) as a specified fraction (1.5%) of Gl,and G3(primary sludge) as the volume necessary to waste a specified fraction (60%) of the incoming solids at a specified concentration of 50 000 g/m3 or 5%. The other water flows are deduced by difference. The air flows G A and ~ GAS,also shown in Figure lA, are set at a multiple (1.12) of the volume of the aeration tank per hour, i.e., 8960 m3/h. This corresponds to 0.33 m3 1000 m-3 of tank volume s-'. Ideally, the solids balance shown in Figure 1B should be predicted from a knowledge of the input BOD, growth kinetics, and settling characteristics as is done in the IAWPRC model (13),but for an existing plant it is simpler to define solids concentrations. The influent solids concentration (e.g., 200 g/m3),is defined, i.e., the illustrative influx of solids is 200 kglh. A specified fraction (60%or 120 kglh) leaves in the primary sludge as G3, and the remainder passes to the aeration tank as GQ. The MLSS concentration in the aeration tank and its effluent is specified (2500g/m3), which defines G s ~ .The effluent solids content is specified (15 g/m3) to define The difference is split between streams 8 and 9 in the proportion of the water flows. Nonvolatile solids are assumed to leave in the primary sludge and play no role in determining the fate of the chemical. Another configuration could be accommodated in which all the waste sludge (stream 9) is recycled to the primary tank. This involves straightforward changes to the mass balance equations. The essential task is to establish the water, air, and biomass balances and from them to deduce the mass balance or fate of the chemical of interest (Figure lC), solely from a knowledge of the influent concentration and the physical-chemical properties: molecular mass, water solubility, vapor pressure, octanol-water or solids-water VOL. 29, NO. 6,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1489
partition coefficient, and biodegradation rate constant in each vessel. To facilitate calculation, equilibrium partitioning (equal fugacity) is assumed to exist for the chemical between the water and biomass phase in each tank. This assumption is believed to be reasonable, since for most chemicals, equilibrium is substantially approached during the treatment residence time. In the aeration tank, the off-gas airstream is assumed to reach equilibrium with the water phase. This equilibrium assumption is widely used in predictive models for chemical fate in STPs, but other more rigorous “partial approach to equilibrium”expressionscan be used (5). The aeration tankis assumed to operate as a single wellmixed reactor. Plug flow or near-plug flow aeration systems are also in use and are believed to be more efficient. It is relatively easy to modify the equations to incorporate a number of multiple stirred tanks to simulate near-plug flow behavior. A major challenge lies in the characterization of the biodegradation rate, with different approaches being adopted by different authors. Generally, a first-order or linear version of the Monod equation is used and is believed to apply at low chemical concentrations as typically exist in municipal wastewater. Namkung and Rittman (9)have used a Monod expressionin which the overall rate is second order, i.e., first order in both MLSS and in chemical concentration. Struijs et al. (10)assign a first-order rate in chemical concentration,while Cowan et al. (11)assign rate constants to both dissolved and sorbed chemical. Siegrist et al. (12)use a more complex approach including adsorption, desorption, degradation, and inhibition parameters that is necessary in the case of NTA, which is very variable in concentration. Biodegradation is expressed here as the half-life (h) of the chemical under conditions when biomass is present at a MUS concentration of 2000 g/m3in each vessel. Similar values are expected for the aerated conditions existing in the aeration tank and final clarifier, but a slower rate (possibly zero) may apply to the primary clarifier in which anaerobic conditions may prevail. A first-order rate constant for degradation in the biomass phase is calculated by assuming that equilibrium biomass-water partitioning applies at the specified input data MUS of 2000 g/m3.This rate constant is applied to the aeration and settling tank biomass, and a rate constant a factor of 10 less is applied to the primary tank. This automatically adjusts the rate (linearly) in proportion to actual, specified MLSS concentration. Each rate is then the product of the rate constant, the chemicalconcentration in the biomass,and the biomass volume. Degradation rate constants are thus model-specificand should not be transferred from model to model without adjustment. It must also be noted that the assumption of complete mixingin the tanks fails to recognize the existence of layers or blankets of settled sludge. The rate constants are thus averages over all solids in the tank. The factor of 10 reduction in rate constants in the primary tank was selected as a result of a comparison of model output and actual plant data as discussed later. Other assumptions and more complex kinetic expressions can be substituted if desired. Three linear mass balance equations can be written, one for each tank including expressions for partitioning of the chemical between air, water, and biomass and for 1490 * ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 6, 1995
biodegradation. These three equations can be solved to give concentrations, flow rates, and rates of volatilization, stripping, biodegradation, sludge wasting, and outflow in the effluent. These equations can be written in concentration or fugacity form and are ultimately algebraically identical. The final conventional mass balance equations are cumbersome, but the fugacity format equations are more compact and elegant, and the mass balance can be more easily understood.
The Fugacity Approach The use of fugacity in mass balance models of this type has been reviewed by Mackay (14),and onlyaverybrief account is given here. Fugacity cf, Pa) is a criterion of equilibrium related to chemical potential and is used as a surrogate for chemical concentration, C (mol/m3),which equalsZ$ where Z is a fugacity capacity (with units of mol/m3*Pa)and is specific to the chemical, the phase in which it resides, and temperature. Values of Z for each chemical in each phase can be calculated from equilibrium partition coefficients. Calculation of Z values starts in the air phase in which Z is 1IRT for all chemicals where R is the gas constant (8.314 Pa.m31mol K) and Tis absolute temperature (K). For chemical in water, ZW is 1lH where H is the Henry’s law constant. For biomass, the partition coefficient,KBW is ZB/ ZW,thus enabling ZBto be calculated following Blackburn et al. (31, who assumed that the biomass is equivalent in composition to a mixture of 20% octanol and 80% water, thus
and
ZB = 0.2K&2,
+ 0.82,
(2)
where &Wis the octanol-water partition coefficient. Data are thus requiredfor molecular weight, Henry’slaw constant (or solubilty and vapor pressure), and octanol-water partition coefficient. The rates of transport and transformation processes are expressed in terms of D values or fugacity rate parameters, the rate being Of (mollh). Three types of D values are used. For transfer by flow ofwater, air, or biomass, the rate is GC (mol/h),where G is the flow rate of the phase (m3/h) and C is concentration (mol/m3). Replacing C by Zfand equating the rate to Dfgives D as GZ. For degradation, the rate is conventionally VCk, where Vis the phase volume (m3),k is a first-order rate constant (h-l), and C is the concentration. Again, replacing C by Zfdefines D as VZk and the rate as Df.The rate of vaporization is expressed in terms of an overall mass transfer coefficient (Kv) and area (A) product, which are combined in 41,namely
The overall coefficient, Kv, can be expressed as the combination of the water side and air side mass transfer coefficients, KWand KA,respectively, i.e.
(4) For illustrative purposes, Kw may be assigned a value of 0.05 m/h and KA a value of 5 mlh for all chemicals. Chemical- or system-specificvalues can be substituted if desired.
For air stripping, two approaches are possible. Roberts et al. (5) have estimated mass transfer coefficients and areas (or their product) from which a D value can be deduced to give a calculated exit air fugacity or approach to equilibrium. Alternatively, and more simply, it is assumed that waterair transfer is rapid, and equilibrium (equifugacity) is reached between the exit air and the water. If desired, an effective air flow rate can be used incorporating the efficiency of transfer. From a knowledge of the D values, all mass transport and transformation rates can be expressed in terms of the prevailing fugacities, and steady-state mass balance equations can be written as shown below and solved for the fugacities. The various concentrations and rates can then be calculated, and an entire chemical mass balance can be assembled (17). primary tank
Taw I
Process Details of Fate of Toluene in Typical STP Water, 1.47 x ZValues (Pam3/mol): Air 4.0 x Biomass, 0.116 primary aeration settling volume water (m3) volume biomass (m3) total concentration (mol/m3) fugacity (Pa) aeration rate (m3/h)
1013 5.07 1.38 x
8000 20 1.91 x
2764 1.52 1.50 x
6.73 x
1.08 x 8960
0.97 x
Process D Values (mol/Pa-h) stream number and flow Dvalue 1, 1.496; 2, 1.479; 3,0.0174; 5,3.616; 6, 3.168; 7, 1.449; 8, 1.688; 9, 0.0316 volatilization primary 0.0189; aeration 3.616; settling 0.0517 biodegradation primary 0.0997; aeration 3.938; settling 0.299 See also Figure 1 for flows.
aeration tank Tam 2
Physical-Chemical Properties of Test Chemicals at 25 O c a
settling tank
chemical
solution
fA = Dip/ [D,
+ + Dm D6
fs = DGfA/( 0 7
(0806)/(07
+ DE + D, + Dsv + DSB)] (9)
+ + Dg + Ds" + DsB)
(10)
where f i s fugacity (Pa), subscripted P for primary, A for aeration, S for settling; E is influx of chemical (mollh) to the STP; DZ is the D value of stream 2 etc., Le. G Z Z W G s ~ BDpv ; and DSVare volatilization D values for primary and settling tanks; D ~ BDAB, , and DSBare biodegradation D values for the three tanks. The equations can also be modified to describe the fate of a chemical when the waste sludge is returned into the primary settling tank and is wasted with the primary sludge.
+
Illustrative Results Tables 1-3 give a complete description of the properties and reported and computed fate of toluene at 25 "C,which was selected to give a significant amount of chemical being removed by each process as illustrated in Figure 1C. The half-lives of toluene are 30, 3, and 3 h in the primary, aeration, and settling tanks, respectively, normalized to a MLSS of 2000 mg/L. The chemical is present at 0.01 g/m3in the influent of 1000 m3/h, giving an inflow of 10 g/h. About 29% of the chemical is sorbed to the biomass in the contents of the primary clarifier since KBWis about 79 and the biomass volume fraction in the tank is about 11200. The water has a shorter retention time (1h) than the solids (25 h), thus most of the chemical leaves with the effluent water (9.16 glh), with 0.11 glh leaving with the sludge, 0.12 glh volatilizing, and 0.62 glh biodegrading. The fugacity in this vessel and the sludge and water leaving it is 0.067 Pa.
l,l,l-trichloroethane 1,1,2-trichIoroethene toluene 1,4-dichIorobenzene naphthalene anthracene pyrene dibutylphthalate 2-ethyl hexyl phthalate phenol pentachlorophenol 2,4D
mol water vapor biode mass solubility pressure log half-li!e (dmol) (dm3) (Pa) KOW (h) 133.4 131.4 92 147.0 128.1 178.2 202.2 278.3 390.5 94.1 266.3 221.0
1495 1100 515 83 31 0.045 0.132 11.2 0.285 88360 14 890
16500 9900 3800 90 10.4 0.001 0.0006 0.00187 0.00086 47 0.00415 8x
2.49 10 2.53 10 2.69 3 3.4 10 3.37 10 4.54 30 5.18 300 4.72 100 5.11 100 1.46 0.3 3 3.40 3.13 3
a Data from refs 17-19, with estimated biodegradation half-lives under aeration conditions at MLSS of 2000 mg/L.
In the aeration tank, there is 20 m3 biomass and 8000 m3water, Le., a MLSS of 2500 g/m3. The chemicalpartitions about 84% into the water and 16%into the biomass. The half-time of the biodegradation process in the biomass is 0.4 h while in the biomass-water mixture it is about 3 h because of the partial partitioning into the biomass. The retention time is 4.4 h, thus about one-third of the chemical (3.92 glh) is degraded. The aeration tank off gas removes 3.6 glh. The chemical concentration in the air is thus about 0.0004 g/m3or 27%of the dissolved chemical concentration in the water of 0.0017 g/m3. Since 1.51 g/h toluene returns to the aeration tank with the recycled sludge (giving a total inflow of about 10.67 glh),the effluent contains the balance of 3.15 g/ h. The fugacity in the aeration vessel has dropped to 0.0108 Pa, Le., by a factor of 6.2. In the settling tank, the chemical is subject to only slow volatilization (0.05glh) and biodegradation (0.27 glh), there being about an equal split between the water effluent (1.30 g/ h) and the sludge (1.54g/ h). The sludge is mainly recycled with 0.03 g/h of chemical leaving in the waste sludge and 1.51 g/h being recycled. The final effluent contains 1.3 g/h chemical in about 55% of the water entering the settling vessel. The fugacity at this stage drops only slightly to 0.0097 Pa, i.e., a factor of 6.9 from the primary stage. VOL. 29. NO. 6,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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TABLE 3
Removal Efficiencies Reported by Monteith (5) and As Estimated by Model model estimates (YO) overall removal efficiency
YO contributions of removal processes
chemical
low
med
high
volat
biodeg
sludge
efflt
1,1,1-trichloroethane 1,I,2-trichloroethene toluene 1,4-dichlorobenzene naphthalene anthracene pyrene di butylphthalate 2-ethyl hexyl phthalate phenol pentachlorophenol 2,4D
71 82 70 57 65 60 70 60 40 90 20 40
85 85 85 70 85 80 85 65 50 98 85 60
94 97 95 80 95 90
90
74 64 20 40 15 0 0
87 80 99 95 80
0 0 0 0 0
10 15 55 10 55 30 5 45 25 98 75 60
1 6 15 20 15 50 80 20 25 0 10 0
15 15 10 30 15 20 15 35 50 2 15 40
Overall, of the 10.0 g/h inflow, 4.80 is biodegraded, 3.76 volatilizes, and 0.14 is sorbed to sludges (totalling 8.7 glh), and 1.3g/h leavesin the final effluent. The overall efficiency of removal is thus 87%, but only 48% of the chemical is actually degraded. In terms of fugacity, the initial value of 0.0726 Pa falls slightly to 0.0673 Pa in the primary tank because of some volatilization and biodegradation. Since the process is simply equilibrium separation, there is no substantial fugacity change. In the aeration stage, the fugacity drops significantly to 0.0108 Pa (i.e., by a factor of 6.2) as a result of degradation and volatilization. There is a further slight drop to 0.0097 Pa in the settling tank. Overall, the STP achieves a fugacity reduction in the water from influent to effluent by a factor of 7.5. Essentially, the plant removes about 60% of the sorbed chemical in the primary vessel, but only 8.5% of the total chemical at close to the influent fugacity is removed. It then reduces the fugacity of the chemical in the water by a factor of 6.2 by biodegradation and volatilizationand then separates most of the remaining sorbed chemical at this lower fugacity. For highly sorbed, persistent, nonvolatile chemicals such as PCBs, the fugacity reduction factor is small (i.e., close to 1.O),the principal functions of the plant being the physical separationof sludge and slight evaporation. The STP cannot cause a fugacity reduction: it can only cause a physical separation at close to the influent fugacity. For volatile persistent nonsorbing chemicals such as chloroform, the fugacity reduction is largelyattributable to volatilization or stripping, and the STP is functioning essentially as an air stripper. More sorptive, volatile chemicals will be less subject to volatilizationbecause of appreciable partitioning into the biomass. Readily degradable nonvolatile chemicals such as alcohols or phenols are subject to a fugacity reduction mainly by biodegradation. In summary, the STP can accomplish three changes: (i) physical separation without fugacity change, (ii) fugacity reduction by volatilization or stripping, provided that the atmospheric fugacity is low compared to that in the process, and (iii) fugacity reduction by degradation. In practice, the proportions of each depend on the chemical’s partitioning and degradability properties. Finally, it is noteworthy that the biodegradation rate is controlled by the total volume of active biomass in the system, which is also a reflection of sludge retention time, thus it is not surprising that SRT correlateswell with remaval 1492
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 6 . 1 9 9 5
removal efficiency volat 88 85 87 72 68 86 87 81 91 99 87 83
73 69 38 19 7 41
