Pollutant Treatability: A Molecular Engineering Approach The idea is to deuelop a separate treatment model f o r each nonpesticide toxic organic, based on fundamental physicalchemical and other properties Murray P. Strier U S . Encironmental Protection Agency Washington, D.C. 20460
Of 129 priority pollutants derived from the original list of 65 chemical substances, including classes of compounds a s delineated in the N R D C Consent Decree of 1976, and fully established in the 1977 Clean Water Act Amendments, there a r e 96 organic compounds classified as nonpesticides. In addition to pesticides, the remaining pollutants on the list of 129 a r e heavy metals, cyanide, and asbestos. T h e Effluent Guidelines Division ( E G D ) is performing wastewater surveys, a s well as assessing waste effluent treatment technology for the 21 industries listed in the Consent Decree a n d the 1977 Clean Water Act Amendments. All of this work is performed for the purpose of developing the “best available technology economically achievable” (BATEA) effluent limitations guidelines for the 129 priority pollutants over the next 1-2 years. A supplemental study of the application of fundamental physicalchemical properties and biochemical oxidizability of nonpesticide organic priority pollutants, to derive estimated Fiwrure articles in ES&T hace b).-lines, repthe ciews o f r h e authors. and are edited h . ~1/7e Washington stafJ If j o u are interested in umrrihiiting an arricle, mniact ihe managing re.sent
editor. 28
Environmental Science & Technology
theoretical effluent limitations for these compounds, was performed. The approach was to use these properties to develop a separate treatment model (or system) for each pollutant from wellknown unit treatment processes. This is called the molecular engineering approach and applies the principles of the Second Law of Thermodj,namics to treatability to determine effluent limitations based on optimal cost-effective practices. T h e selected unit treatment processes merely serve as examples and a r e not meant to exclude other processes that a r e equal, if not more effective, in certain circumstances. The procedure and results obtained arc discussed below.
Processes investigated Based on experience with evaluating treatment processes, either already or soon to be applied in a wide variety of industries, the following five unit treatment processes were used in this investigation: steam- or air-stripping, oil/water separation. filtration-diatomaceous earth or dual media, biochemical oxidation, and activated-carbon adsorption. These suggested potent iul trea tment processes were separately applied, not necessarily in this order. to treatment models for each pollutant. At least one of the above unit processes was used in each model. It was realized that, ultim a t e I y , “rea 1- w o r I d ’‘ a p p I i ca b i 1 it y uould be determined from individual subjective determinations involving
cost-effectiveness consideration. Since emphasis was on the effect of chemical structure on treatability, this study was confined to a water matrix consisting of a single pollutant dissolved in water. Based on available data on treatability of priority pollutants, the following six ranges of estimated theoretical effluent limitations were established from the known pollutant indicator data source: Estimated theoretical cffluent limitations. P@/L
1 .0
1.0-10
10
25 50 100- 1000
Indicator d_ a t a bourcc _
_
PC B‘s Alkyl nitrosamines I ,3,4-Trichlorobenzene Bis(2-ethyl hexyl) phthalate Benzidine Phenol Carbon t e t r a c h 1or ide
T h e physical-chemical properties utilized in classifying the remainder of the organic priority pollutants were molecular weight, boiling point ( i n cluding azeotropic boiling point and composition), and water solubility.
How the processes are used T h e various unit treatment processes are used a s follows: S t e a m -stripping: T h e pr i nci pa I index used to establish steam-stripping capability is the boiling point of the organic compound. A compound rated as having good steam-stripping capa-
This article not subject to U.S. Copyright. Published 1979 American Chemical Society
~
bility has a boiling point of less than 150 "C. Another criterion is Henry's Law constant, K , which is equal to the compound's partial pressure divided by the mole fraction of the compound dissolved in water. The ability to form azeotropes with water a t a weightfraction of 20.8. where available, is considered indicative of good potential. In one of the studies on steamstripping wherc mixtures of carbon d is u I fide and car bo n tetra c h 1or i de were evaluated, it was possible to go from a concentration of 100-200 tng/L in carbon tetrachloride to 0.16 tng/L. This demonstrated that the extent of reduction was optimal for steam-stripping. In the case of carbon t e t r a c h 1or i de, ;i c t i va t ed - ca r bo n ad sorption \+as added to the treatment model because of its fair adsorptivity, considered capable of yielding a n estimated theoretical effluent limitation of 0.050 mg/ L. Oil-h,ater separation: all compounds having relatively low solubilities in \+atcr arc good candidates for oil-water separation by means of API separator or dissolved air flotation ( D A F ) . It appears that sound use of appropriate p h y s i ca I - c h em i ca I pr i n ciples for such pollutants a s polyaromatic hjdrocarbons (PAH's) and PCB's are conducive to highly effective removal by this treatment process. In previous work, a well-operated oilwater separator could remove a fairly high- niolecu I a r - w eig h t P C B mixture down to a residual concentration level of 10-25 p g / L . Filtration: Those compounds having low water solubilities and tending to be highly adsorptive on particulate matter, organic or inorganic, are candidates for this treatment process. Here, ideal candidates would be the highmol ecu I a r - w e i g h t (high - m o 1a r - volume) compounds such a s PCB's, whose filtration is capable of lowering concentrations from 50 p g / L to 10 pg/L. Filtration implies that coagulation, via pH adjustment as appropriate, and inclusion of effective flocculating and coagulating agents are used, together with other measures appropriate to ensure opt i ma I rem ova I of precipitate from wastewater. Biocherwical oxidation: Certain c he mica I structure- bi oc hem ica I ox i dizability correlations indicate that it is possible to estimate effluent limitations with some degree of reliability when actual experimental data a r e absent. In a well-operated system. biochemical oxidation reduces a n influent concentriition of ca 100 m g / L for phenols to a n effluent concentration of 50 p g / L maximum. This bio-
chemical technique employs long-term acclimated biological cultures. uses oxygen instead of air, possibly includes powdered activated-carbon or fluidized-bed technology, and ensures adequate phase separation. prior to, or fol I ow i n g bi och e m i ca 1 ox i d a t ion . Crrrhnrz adsorption: Generally, it is increasingly evident that higher molecular weight, lower polarity, i n creasing hydrocarbon unsaturation (that is. -C=C--), and aromatic structure are conducive to lower water solubility and highcr activatcd-carbon adsorptivity. Since Freundlich adsorption isotherm data a r e sparse a t concentrations 5 1 .0 m g / L , it is necessa r y to ex t r a po I a t e ex per i menta I data to lower concentrations for estimation purposes. In essence, compounds having better adsorptivitj a t concentrations 2 I .O m g / L a r e considered to have better adsorptivity a t 5 I .O m g / L , also.
Pollutant rating Based on physical-chemical properties and known experimental bioc h em i ca I ox id i za bi I it y , each pol I ut a n t was rated according to how w3ell it would be expected to perform with each of the five unit treatment processes. Here, it was assumed that the priority pollutant was dissolved in water a s the only solute. Three broad categories of ratinggood, fair, and poor-were assigned in the treatment modeling for all of the compounds. For example, chloroform \+as rated good for treatment by steam-stripping, but poor in all of the other unit treatment processes. Phenol was considered good in biochemical oxidation, fair in carbon adsorption, and poor in all of the other remaining unit treatment processes. PCB's were considered good candidates for oilwater se pa ration, d i a t o niaceou s earth or mixed-media filtration, and carbqn adsorption, but they were rated as poor for treatment by steam-stripping and biochemical oxidation. Such ratings did not necessarily preclude other evaluations leading to changes in ratings based on cost-effectiveness considerations in specific "real world" situations. Estimated effluent limitations Pollutants estimated to be capable of being treated to 1 .0 p g / L a r e listed in Table 1 with their corresponding molecular weights. They are all aromatic i n nature with molecular weights 2 2 3 8 . They are either highly chlorinatcd or a t least contain two attached benzene rings substituted with chlorine (such as PCB's); otheruise they have several fused benzene rings. These are
TABLE 1
Priority pollutants having (estimated theoretical) treatabilities of 1.0 pg/L Mol. weioht
Cornpound
Hexachlorobenzene
284.8
Benzola] pyrene (3,4-benzopyrene) (3,4)-Benzoftuoranthene
252.3 252.3
(11,12)-Benzofluoranthene
250.3
Chrysene (1,12)-Benzoperylene l12,5,6-Dibenzanthracene Indeno(1,2,3-cd)pyrene (2,3-o-phenylenePYrene) 2,3,7,8-Tetrachlorodibenzo-pdioxin (TCDD)
228.3 276.3 278.4 276.3
PCB's
332.0 275-420
TABLE 2
Priority pollutants having (estimated theoretical) treatabilities of 1-10 pg/L Compound
Knitrosodimethylamine Knitrosodiphenylamine Knitrosodipropylamine Butyl benzyl phthalate 1,e-Benzanthracene Pyrene
Mol. weight
74.1 198.1 130.2 312.0 228.3 202.0
TABLE 3
Priority pollutants having (estimated theoretical) treatabilities of 10 pg/L Mol. Compound
Acenaphthene 1,2,4-TrichIorobenzene 2-Chloronaphthalene 3,3-Dichlorobenzidine Fluoranthene 4-Chlorophenyl phenyl ether 4-Bromophenyl phenyl ether Hexachlorocyclopentadiene Pentachlorophenol Bis(2-ethylhexyl) phthalate Di-n-octyl phthalate Acenaphthalene Anthracene Fluorene Phenanthrene
weight
154.2 181.5 162.6 252.1 202.3 205.7 249.1 273.0 266.4 39 1.O 391.0 152.2 178.3 152.2 178.2
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TABLE 4
Priority pollutants having (estimated theoretical) treatabilities of 25 pg/L Compound
Mol. weight
Benzidine Hexachloroethane 2,4,6-TrichlorophenoI 1,4-Dichlorobenzene Hexachlorobutadiene
184.2 236.7 197.5 147.0 260.8
2,4-Dinitrophenol 4,6-Dinitro-o-cresol Di-n-butyl phthalate Diethyl phthalate Dimethyl phthalate
184.1 198.1 278.3 222.2 194.2
the compounds which most nearly resemble PCB's in physical properties; they are most readily removed from water by physical-chemical means, and a r e the least likely candidates for biochemical oxidation. Pollutants estimated to be capable of being treated to 1.0-10 p g / L are listed in Table 2. T h e three nitrosamines are included since they have been treated to that level with activated carbon. I ,2-Benzathracene is included because of higher solubility than that of chrysene. Butyl benzyl phthalate is included because of anticipated slightly higher solubility ascribed to its ester-containing function. Pollutants estimated to be capable of being treated to 10 p g / L are listed in Table 3. They are comprised strictly of fused benzene rings, have lower
TABLE 5
Priority pollutants having (estimated theoretical) treatabilities of 50 pgfi, Mol. Compound
weight
Benzene Carbon tetrachloride Chlorobenzene 1,1,2,2-TetrachIoroethane pChloro-metacresol 2-Chlorophenot 1,2-Dichlorobenzene 1,9Dichlorobenzene 2,4-Dichlorophenol 2,4-Dimethylphenol 2,CDinitrotoluene
78.1 153.4 112.6 167.9 142.6 128.6 147.0 147.0 163.0 122.2 182.1
Compound
2,6-Dinitrotoluene 1,2-DiphenyEhydrazine Ethylbenzene lsophorone Naphthalene Nitrobenzene 2-Nitrophenol 4-Nitrophenol Phenol Tetrachloroethylene Toluene
Mol. welpM
182.1 184.2 106.2 138.2 123.1 139.1 139.1 139.1 94.1 165.8 92.1
TABLE 6
Priority pollutants having (estimated theoretical) treatabilities of 100-1000 pg/L Mol. Compound
Acrolein Acrylonitrile 1,2-DichIoroethane l,l, 1-Trichloroethane 1,l-Dichloroethane 1,1,2-TrichIoroethane Chloroethane Bis(chloroethy1)ether 2-Chloroethyl vinyl ether Chloroform 1.1-Dichloroethylene 1,2- Trans-dichloroethylene 1,2-Dichloropropafle
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weight
56.1 53.1 99.0 133.4 99.0 133.4 64.5 143.0 106.6 119.2 96.9 96.9 112.0
Environmental Science & Technology
Compound
1,3-Dichloropropylene
Bis( 2-chloroisopropyl)ether Bis(2-chloroethoxy)methane Methylene chloride Methyl chloride Methyl bromide Dichlorobromomethane Trichlorofluoromethane Dichlorodifluoromethane Dibromochloromethane Trichloroethylene Vinyl chloride
Mol. weight
111.0 171.0 177.0 84.9 50.5 94.9 168.8 137.4 129.9 208.3 131.4 62.5
molecular weights, and are slightly more soluble than the PAH's of Tables 1 and 2. Phthalates are treated to this level by means of activated carbon. Although their molecular weights are somewhat different, all compounds in this category have similar physicalchemical properties, as manifested by their interchangeability in products of commerce. Compounds having the highest molecular weights in this category possess less of an aromatic character-having only one benzene ringbut contain more halogens or carboxylate functions. It was recently determined that one such halogenated compound, pentachlorophenol, could be treated to this level by acidification to pH 2, oii-water separation, filtration, and resin adsorption. Pollutants estimated to be capable of being treated to 25 pg/L are listed in Table 4. Some of the lower-molecular-ueight compounds in this category show signs of having a t least moderate biochemical oxidizability. Their solubilities are significantly higher than that of those estimated to be treatable to lower concentrations. Pollutants estimated to be capable of being treated to 50 pg/L are listed in Table 5 . Some of the nonhalogenated and less halogenated compounds of this category are most readily oxidized biochemically. In addition, three compounds-carbon tetrachloride, 1,1,2,2-tetrachloroethane, and tetrachloroethylene-are a t least moderately steam-strippable. The lowcrmol ec u I a r- w e i g h t a ro ni a t i cs show signs of less activated carbon-adsorptive properties. Phenol was considered to be equally treatable to this level. either by biochemical oxidation or by means of activated carbon. Pollutants estimated to be capable of being treated to 100- 1000 p g / L are listed in Table 6. The) are mostly of the lowest molecular weight, have lowest boiling point, and are the most water soluble. I n addition, they are the most readily steam-strippable. A few are biodegradable (nonhalogenated), and the more highly halogenated aliphatics are moderately adsorbed by activated carbon. Est i mated t heor e t i ca 1 treat a b i I i t i es for all of the organic priorit) pollutants vary over three orders of magnitude. However, their solubilities i n water vary over 9-10 orders of magnitude. There is disagreement i n solubilities for some compounds; they differ according to literature sources. A median value is used in these instances. Estimated theoretical treatabilities are listed i n Table 7. As expected, solubilities are directly related to estimated
in exposure water.
theoretical effluent limitations, while partition coefficients and bioconcentration factors are inversely related.
The entropy connection Pollutant solubility in water may be associated with entropy, which opposes the driving force-the standard free energy for the distribution of a solute between two different media at equilibrium-for removing the substance from water. This phenomenon can be interpreted in terms of van der Waals forces or hydrogen bonds formed between solute and water, especially when phenolic and nitrogen-containing compounds are involved. Therefore, it follows that the cost-effectiveness or economics of treatability should follow the path of least resistance. Overall, there appears to be a thermodynamic relationship between treatability and solubility-type parameters, including partition coefficient and bioconcentration of the form = a + b ,Ins - bzlnf - b31nB. where T is the estimated theoretical treatability, a, 61, by!, and 63 are constants, s is the solubility, f is the partition coefficient, and B is the bioconcentration factor. 'Not only can this relationship be used to estimate theoretical
effluent limitations, but also to evalua t e efficiencies of operational treatment systems for the removal of organic priority pollutants. It is expected that the reliability of these estimates will improve with availability of more physical constant data. Additional attempts have been made to correlate estimated theoretical effluent limitations with water quality criteria for the priority pollutants, now being developed by the EPA's Criteria and Standards Division. The latter are being derived from chronic aquatic life toxicity determinations, which appear to be related to pollutant water-solubility. Exceptions observed can be explained in terms of alkylating or other incidental chemical reactions, such as hydrolysis. It is expected that results of the efforts in this area will be available for publication in the near future. Note: Material presented in this article does not necessarily represent official U S . EPA policy.
Acknowledgment The author wishes to express his appreciation to Mr. Robert B. Schaffer, director of the Effluent Guidelines
Division, for his invaluable suggestions throughout the course of this effort. He also wishes to acknowledge the various other EPA technical staff members for their comments and criticisms during the preparation of this manuscript.
Murray Strier has degrees in chemical engineering. phj,sical-organic cheniistrj,, and electrocheniistry. He has spent 20 j w r s in indirstrial Research and Decelopnient in sirch areas as energj. conrersion, polymer cheniistrj', and pollution control. Strier has been M,ith the E P A sinc,e 1972 M,here he has been concerned with the ;'L'PDES Permits Program, and is prese n t l j , a physical scientist concentrating on priorit). pollutants with the Effluent (;itiddines Dirision. Recentlj.. he won the Gold Medal for Achiecenient. E P A ' s highest arrwd, for his work in niolecirlar rngineering. Coordinated by J J Volume 14, Number 1, January 1980
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