In Chapter 18 we discussed how to use energy submodules to estimate the environmental life cycle impacts of energy production. In a similar fashion, we can estimate the environmental life cycle impacts of treating waste. Although it is true that as we treat the waste we are working to reduce one or more hazards, there are additional impacts that need to be taken into account. For example, when we treat wastewater in an activated sludge plant, we reduce the organic content of the wastewater, but in so doing, we also incur impacts associated with energy production, transportation, and the production of chemicals to operate the plant. Previous research has explored the impacts from waste treatment, has characterized these impacts, and has compared different waste treatment technologies; the results may be found in the literature.10–12In this section we explore briefly some common waste treatment and recovery technologies and their associated environmental impacts through use. The technologies we cover include the wastewater treatment plant, the incinerator, and solvent recovery. There are, of course, many more technologies used for
Solvent Recovery
Incinerator WWTP
Manufacturing Plant
LCA Impacts Energy GWP POCP Acidification Eutrophication Others…
Others:
Membrane, Adsorption, etc Avoided impacts
due to energy or materials recovery
FIGURE 19.4 Environmental impacts of waste treatment and recovery technologies. WWTP, wastewater treatment plant.
treatment and recovery, but they are outside the scope of this chapter. However, the principles of estimating life cycle impacts are generic and applicable to any other type of waste treatment and processing technologies, as we will see in Chapter 23.
19.6.1 Wastewater Treatment Plant
As we described above, operating a waste treatment plant results in environmental life cycle impacts associated with the operational energy consumed in running the plant and from the substances used to treat the water. A general flow diagram is presented in Figure 19.5(a). Many models are used to estimate the biodegradation mechanisms in the waste treatment plant, and some of these have been published widely. For example, the following reactions describe the main biodegradation mechanisms taking place in activated sludge treatment13:
Cell autooxidation:
C5H7NO2ðcellsị ỵ5O2!5CO2ỵ2H2OỵNH3 Cellular energy generation and cell production:
C6H12O6ðsubstrateị ỵO2ỵNH3!C5H7NO2ðcellsị ỵCO2ỵ4H2O
The substrate cells use for their energy source the organic chemical mixture generated during manufacturing that is suitable for discharge to a wastewater treatment plant (see Section 19.1). The glucose substrate in the second equation is only one example of cellular respiration, and it is common to characterize the potential for all organic chemicals to be biodegraded by calculating the theoretical oxygen demand (ThOD) or the TOC.
Eckenfelder14,15developed engineering design methods based on the above equations, but generalize for a wide variety of substrates and microbial consortia, which can in turn be used to estimate life cycle impacts. For example, Table 19.12 shows selected LCI emissions associated with treating 1 kg of TOC in a wastewater treatment plant. The negative values for some of the parameters denotes their reduction due to water treatment.
19.6.2 Incinerator (Thermal Oxidizer)
Incineration is used extensively in the chemical processing industry for waste treatment and disposal. Since almost all chemical product manufacturing involves the use of organic solvents to a greater or lesser extent, there has been great interest in quantifying and assessing the environmental impacts of solvent incineration.16,17 Incinerators can be designed to accept gas or liquid feeds, depending on the arrangement of the feed stream,18–20and the feed is normally a mixture of waste and fossil fuel of some kind (e.g., natural gas, kerosene).
A modern incinerator would be expected to operate at a minimum of 99.99% efficiency.
A model of a thermal oxidizer is shown in Figure 19.5(b), where it is assumed that spent organic solvent (either liquid or gas) is fed to the incinerator. Air is generally used for combustion, although in some cases, pure oxygen may be used. Water vapor, carbon dioxide, nitrogen and sulfur oxides, trace organic compounds, and other materials leave the incinerator as part of the flue gas with part of the combustion energy usually recovered from the hot stream. Several models, most of which are based on stoichiometric relationships
Wastewater Treatment Plant
Total from WWTP From WWTP Process Electricity-Related Air Emissions
CH4 0.01 — 6.47103
CO 0.00 — 5.02104
CO2 4.33 2.49 1.84
NMVOC 0.00 — 4.24104
NOx 0.00 — 3.91103
SOx 0.01 — 5.47103
Water Emissions
TOC 0.86 8.60101 1.95103
BOD 1.30 1.30 2.23104
COD 2.45 2.46 5.58103
TDS 0.00 — 4.69103
Solid waste
Biosolids 1.14 1.14
Solid waste, other 0.08 — 8.37102
Aerobic
Treatment Clarifier
Wastewater
Untreatedbiosolids tosludgestabilization andthentolandfill
Treatedeffluent Systemboundary
Oxygen
Carbondioxidetoatmosphere
Electricity Ancillarysubstances VOCs
(a)
(b)
Combustion chamber Air
Waste(gasorliquid) mC[kgCin/kgsolvent]
Combustiongases:
CO2,mCO2[kgCO2/kgCin] VOCs,metals,watervapor, andothers
Energyrecovered [MJ/kgC]
Systemboundary
FIGURE 19.5 Examples of inputs and outputs of a wastewater treatment Plant (a) and a thermal oxidizer or incinerator (b). (Source:ref. 10. Reproduced with permission from John Wiley and Sons.
Copyright2001. John Wiley and Sons, Inc.)
or direct industrial measurements, have been used to estimate the environmental impacts of an incinerator. Table 19.13 contains two examples of selected LCI emissions associated with incinerating 1000 kg of toluene, based on a model developed for an industrial thermal oxidizer. The first column contains the emissions when there is no energy recovery, and the second column contains the emissions when 30% of the combustion energy is recovered. In this model the estimations are based on the total carbon entering the incinerator, with about 43 MJ of additional energy required for incineration per kilogram of carbon entering the incinerator.10
One incineration option for waste treatment has been the use of kilns, mainly cement kilns, to treat hazardous waste. The rationale behind this strategy is to spend less money than that when using fossil fuels to operate the kiln and, instead, use hazardous waste (which companies will pay to have taken away) as fuel. The benefit of hazardous waste incineration in cement kilns is that most of the flue gases are trapped in the cement matrix in this countercurrent process. For example, Seyler et al.21 estimated the changes in environ- mental impacts and emissions when using cement kilns to incinerate waste for four systems: toluene, ethanol with traces of heavy metals, mixtures of ethyl acetate and water, and a mixture of 1-butanol with methylene chloride. The assessment found that the emissions from fossil fuel production and use that were avoided dwarfed the emissions from solvent incineration. Table 19.14 shows a sample of the results for the systems analyzed. The numbers in parentheses are the gross fuel-related emissions from solvent incineration in the kiln before subtraction of the emissions avoided. Cement kiln process- related emissions were not considered, as they are independent of the fuel used.
Example 19.4 The solvents in Table 19.15 correspond to the top 10 solvents reported in the TRI inventory in 2007. Assuming that all of the solvents are incinerated with no energy recovery, estimate the major life cycle emissions if all these solvents were incinerated.
Solution To estimate the emissions we use the incineration model developed by Jimenez- Gonzalez et.al.10 The model estimations are based on the total carbon entering the TABLE 19.13 Example of Selected LCI Emissions from Treatment in 1000 kg of Toluene
Total in Incinerator Without Energy Recovery
Total in Incinerator with 30% Energy Recovery
Net energy (MJ) 39,515 32,125
Air emissions (kg)
CH4 6.32 5.25
CO 2.12 1.71
CO2 6,090.38 5,506.02
NMVOC 15.63 12.66
NOx 8.82 6.90
SOx 1.06 1.50
Water emissions (kg)
TOC 0.00 0.02
BOD 0.00 0.14
COD 0.00 0.04
TDS 1.08101 2.55
Solid waste (kg) 3.47 2.21
incinerator, with about 43 MJ of additional energy required per kilogram of carbon incinerated. Since the water content of the waste–solvent mixture will dramatically increase the energy required for incineration, a 50% water/solvent ratio was assumed for miscible solvents, and for non-water-miscible solvents the water/solvent ratio was estimated using water solubility data. The use of natural gas as a fuel was assumed, which is a conservative assumption. The results of this assessment are shown in Figure 19.6.
Additional Points To Ponder How different would the emissions profile be if energy recovery were used in the incinerator? How accurate would the water assumptions be in terms of the effect on emissions?
19.6.3 Solvent Recovery Through Distillation
As we have seen in Chapter 6 and other chapters, solvents comprise the largest portion of the mass of materials used in the chemical, fine chemicals, and pharmaceutical industries; in other words, solvents comprise the largest proportion of the mass intensity for many
TABLE 19.15 Top Ten Solvents
Solvent Amount to Be Incinerated (kg)
Methanol 9,706,905
Dichloromethane 12,781,416
Toluene 9,032,140
Acetonitrile 2,178,311
n-Hexane 2,041,914
N-Methyl-2-pyrrolidone 1,008,886
N,N-Dimethylformamide 941,262
n-Butyl alcohol 931,721
Methyltert-butyl ether 1,075,252
Xylene 907,587
Solvent as Alternative Fuel for Clinker Production
Fuel Toluene
Ethanol with Traces of Heavy Metals
Ethyl Acetate and Water
1-Butanol with Methylene
Chloride Resource Avoided
Coal (tons) 1.22 0.81 0.67 1.00
Oil (tons) 0.22 0.15 0.12 0.18
Change in Emissions
CO2(kg) 610 (3350) 715 (1910) 261 (1900) 877 (2360)
NOx(kg) 10.2 6.77 5.56 8.33
As (mg) 3.69 2.45 2.02 3.02
Cu (mg) 4.60 6.95 (10) 2.51 3.76
Ni (mg) 0.65 0.57 (1.0) 0.35 0.53
Hg (mg) 129 85.5 70.3 105
industries. Moreover, once solvents are used in a chemical process, there is often resistance to reusing the solvent, based on fears of lowering product quality, concentrating an unwanted impurity, or causing a change in chemical reactivity. Although this is observed in many industries, it is especially a barrier in the fine chemicals and pharmaceuticals industries. As a consequence, in addition to recovering solvents through distillation, a considerable volume of solvent is either incinerated in-house or sent to external incinerators or cement kilns.21 However, we discussed in previous chapters that most environmental benefit from a life cycle viewpoint will come through recovering the solvent and reusing it in the same process (internal recycling) or in another process (external recycle). To do this, we need to purify and recover the solvent.
Distillation is most commonly used to purify solvents. Solvents with different vapor pressures can be separated from one another by fractional distillation. Azeotropic mixtures can be separated by extractive or azeotropic distillation (e.g., addition of benzene to a
EPA TRI Solvents
Calculation basis = 122,336,620 pounds total solvent Information needed (enter in the yellow cells)
Name of the organic substances to be incinerated
Molecular weight Number of Carbon atoms in formula
Amount to be incinerated [kg]
Organic carbon to incinerator [kg]
5 0 9 , 6 0 7 , 9 1
2 3 l
o n a h t e
m 3,640,089
6 1 4 , 1 8 7 , 2 1 1
9 . 4 8 e
n a h t e m o r o l h c i
d 1,806,561
0 4 1 , 2 3 0 , 9 7
3 1 . 2 9 e
n e u l o
t 8,235,100
1 1 3 , 8 7 1 , 2 2
1 4 e
l i r t i n o t e c
a 1,275,109
4 1 9 , 1 4 0 , 2 6
7 1 . 7 8 e
n a x e h -
n 1,686,564
N-methyl-2-pyrrolidone 99 5 1,008,886 611,446
N,N-dimethylformamide 73 3 941,262 464,184
1 2 7 , 1 3 9 4
2 1 . 4 7 l
o h o c l a l y t u b -
n 603,381
methyl tert-butyl ether 88.14 5 1,075,252 731,962
7 8 5 , 7 0 9 8
6 1 . 6 0 1 e
n e l y
x 820,727
0 Total Carbon to incinerator [kg] = 19,875,123 Total Organics to incinerator [kg] = 40,605,394 Total aqueous to incinerator [kg] 14,987,006
Energy Recovered (%) 0%
Energy needed =
[MJ of Natural gas] 960,373,068 Energy Recovered (MJ of
Steam) 0
Total from incinerator
From incineration process
Energy usage-related
Energy recovery-related Air emission [kg]
CH4 153,660.81 1.54E+05 0.00E+00
CO 47,770.73 3.18E+04 1.60E+04 0.00E+00
CO2 139,728,324.46 7.21E+07 6.76E+07 0.00E+00
NMVOC 379,460.91 3.18E+03 3.76E+05 0.00E+00
NOx 214,328.19 2.14E+05 0.00E+00
SOx 25,672.75 2.57E+04 0.00E+00
Water emission [kg]
TOC 22.97 2.30E+01 0.00E+00
BOD 3.98 3.98E+00 0.00E+00
COD 65.70 6.57E+01 0.00E+00
TDS 2.61E+03 2.61E+03 0.00E+00
1 5 . 0 7 2 , 4 8 ]
g k [ e t s a w d i l o
S 8.43E+04 0.00E+00
FIGURE 19.6 Results of TRI modeling for incineration with no energy recovery.
anhydride to an ethanol–ethyl acetate mixture), or by altering the pressure during distilla- tion.22 Distillation is by far the most commonly used technology for purifying solvents and/or separating solvent mixtures. Several assessments have been made to estimate the environmental impacts of solvent recovery vs. disposing of them through incineration with energy recovery or in cement kilns.23,24
It has been found that in contrast to incineration, the reduction in environmental impacts through solvent recovery by distillation (or any related technology, for that matter) is driven by the credits obtained through avoided resource consumption and emissions from the recovery of the solvent, as shown in Figure 19.7.25This has been corroborated by several studies, including one by Capello et al.,26 who found that recovering solvents through distillation is in general the environmentally optimal option, especially when a majority of the solvent is recovered and when the solvents recovered have large associated environmental life cycle profiles. They also found that when atmospheric distillation is very difficult, with low solvent recoveries (e.g., methanol, azeotropic distillation), distillation is not necessarily superior to incineration with solvent recovery. They went as far as to provide a series of rules of thumbs based on solvent type, recovery efficiencies, and the associated life cycle profile to select the type of solvent recovery or treatment, as shown in Table 19.16.
They also performed a solvent-specific assessment for solvent recovery by distillation.
This assessment revealed that there were no specific solvents for which incineration is environmentally the better choice. The solvent-specific assessment also revealed that:
. For acetic anhydride, butylene glycols, dichloromethane, formic acid, methyl isobutyl ketone, and tetrahydrofuran, distillation is the most environmentally favorable option in most cases, regardless of the recovery efficiency and at almost minimal solvent recoveries. Some of these solvents (acetic anhydride, butylene glycol, methyl isobutyl ketone, tetrahydrofuran27) have large life cycle credits because they have elaborate chemical trees or have low net calorific value, such as formic acid (4.6 MJ/kg28), or both, such as dichloromethane.
. For heptane, methyltert-butyl ether (MTBE), and pentane, distillation and incinera- tion had similar environmental profiles given their high calorific values.
. For cyclohexane, ethanol, ethyl benzene, formaldehyde, isohexane, methanol, toluene, and xylene, distillation was only marginally better under the best-case scenario of high
90
77.89 78
0.573 0.368
80 70 60 50 40 30 Waste (kg/kg THF) 20
10 0
(1) Distillation (2) Incineration (3) Manufacturing Credit = (2) + (3) - (1)
FIGURE 19.7 Total waste credits estimated by recovering 1 kg of THF.
solvent recovery. This is explained by their high net calorific values (>40 MJ/kg); low environmental credits due to simple chemical trees, difficulty in separating azeotropes, or a combination of these factors.
Example 19.5 It was mentioned above that recovering dichloromethane by distillation is always the best option. Prove that point.
Solution For this case we can assume three scenarios:
. Incineration with no energy recovery
. Incineration with 50% energy recovery
. Recovery by distillation with 75% efficiency and the tails incinerated with energy recovery
For these scenarios we use the same model as in Example 19.5, which is different from the one used by Capello et al. As a reminder, the estimations are based on the total carbon entering the incinerator, with about 43 MJ of additional energy required per kilogram of carbon incinerated. A sample of the results is shown in Table 19.17, and the resulting global warming potentials estimated from these numbers for the three scenarios are shown in Figure 19.8.
Additional Points to Ponder How can we account for the human toxicity hazard asso- ciated with dichloromethane? How different would the comparison be if 80% of the energy can be recovered? What is even better than recovering the solvent or energy?
19.6.4 Integrated Assessment Approach
One important point to stress when estimating the impacts of waste treatment is that every time we make a decision about a particular treatment or recovery technology, we must assess the systems from a life cycle viewpoint. For example, some of the generic rules of thumb, TABLE 19.16 Rules of Thumb for Selecting the Treatment/Recovery Technology with the Fewest Environmental Impacts
Solvent Mixture Has: Option with Fewest Environmental Impacts
High net calorific value Incineration with energy recovery (e.g., cement kilns)
Heteroatoms Recovery through distillation
Possibility of high recovery Recovery through distillation
High water content Recovery through distillation
Complex production chain Recovery through distillation
One of these solvents as main component: Recovery through distillation Acetic anhydride
Butylene glycols Dichloromethane Formic acid
Methyl isobutyl ketone Tetrahydrofuran
methodologies, and what we learned earlier about solvent recovery can be applied to other types of hazardous and nonhazardous waste. It is a matter of assessing the system with expanded boundaries. Many of the solutions will be found when looking outside the plant, as we will see in Chapter 24 when we cover industrial ecology.
Solvent Recovery
Incinerator w/ Energy Recovery 1000 kg DCM
250 kg DCM
~ 370 kg CO2-eq
~ 370 kg CO2-eq
Solvent Recovery
Incinerator w/ Energy Recovery 750 kg DCM
~ 370 kg CO2-eq -1876 kg CO2-eq 1000 kg DCM
50% energy recovery
2
Incinerator with Energy Recovery
794 kg CO2-eq
Manufacturing Plant 1000 kg DCM
Manufacturing Plant Case 1
Manufacturing Plant Case 2
Manufacturing Plant Case 3
Incinerator without Energy Recovery
1067 kg CO2-eq
FIGURE 19.8 Results of example 19.5.
75% Solvent Recovery Using
Distillation
Incineration with 50% Energy
Recovery
Incineration without Energy
Recovery
Net energy (MJ) 15,482 6,212 3,149
Air emissions (kg)
CH4 4.83 0.50 0.99
CO 2.90 0.28 0.33
CO2 1,349.58 734.65 950.19
NMVOC 7.33 1.26 2.46
NOx 5.67 0.70 1.39
SOx 5.96 0.08 0.17
Water emissions (kg)
TOC 13.80 0.00 0.00
BOD 0.13 0.00 0.00
COD 56.62 0.00 0.00
Leachate 0.01 0.01 0.02
TDS 5.73 0.01 0.02
Solid waste (kg)
Other solid waste 27.55 0.28 0.55
The other important point to cover is that although it is important to understand and assess impacts of waste treatment and recovery, it is even more important to design chemical and manufacturing processes in such a way that the generation of waste is eliminated or minimized, and that is precisely the role of green engineering. Having said that, even the most creative and clever engineers will be faced with the laws of physics and thermody- namics, and as we get closer to thermodynamic limits, there will be an increased price to pay.
In most cases, the cost will be greater for designing inefficient processes that generate unnecessary waste. And similar to what we have seen previously with life cycle assessment principles, we must estimate and assess the costs by expanding the boundaries beyond the plant’s fence, as we cover in Chapter 20.
PROBLEMS
19.1 A manufacturer is planning to introduce a new product to the market that releases chemical A as a major by-product of the manufacturing process. Given current forecasts, it is expected that the average concentration of chemical A in the site’s effluent will be 16.8 mg/L. The plant’s effluent is discharged into the wastewater treatment plant in the industrial park, which, unfortunately, is not expected to remove or degrade any portion of chemical A before discharging the treated water into the river. You are the EHS manager and have been asked by the new product director if the introduction of this new product will raise environmental concerns, especially since she heard that the only acute toxicity data available is an LC50of 25 mg/L in bluegill sunfish, and that sounded like it was too low. What answer do you give her?
19.2 You have just finished the assessment required to answer Problem 19.1 when the new product director gives you a call and tells you that there is a mistake in the data and that the LC50in bluegill sunfish is really 4.1 mg/L instead of 25. Would your answer change? Why?
19.3 What are the respective predicted no-effect concentrations for Problems 19.1 and 19.2? If the LC50’s in Problems 19.1 and 19.2 were the lowest LC50’s in a set of acute toxicity studies comprised ofDaphnia, fish, and algae, would your answer change?
19.4 Estimate the environmental impacts of treating 1 kg of TOC in a wastewater treatment plant based on the information given in Table 19.12.
19.5 What are the environmental impacts from disposing of materials in a landfill? How would you quantify them?
19.6 Redo Example 19.4 assuming a 30% energy recovery. Utilize the steam production submodule of Chapter 18.
19.7 Estimate the acidification potential, fossil fuel consumption, photochemical smog formation, and eutrophication potential for Example 19.4.
19.8 Estimate the acidification potential, fossil fuel consumption, photochemical smog formation, and eutrophication potential for the three cases of Example 19.5.
19.9 Redo Example 19.5 with 70 and 80% energy recovery.
19.10 Redo Example 19.5 with methanol.
and 25% water. Compare the environmental impacts of incinerating the stream without energy recovery versus segregating the streams within the plant to recover the solvents as much as possible, as shown in Figure P19.11.
19.12 Capello et al. published the results of a life cycle cumulative energy demand study that includes production, credits for recovering the solvent through distillation, and credit for recovering energy through incineration for several solvents. The data for some solvents are shown in Table P19.12.
Impacts?
Manufacturing Plant Case 1
Incinerator without Energy Recovery
500 kg DCM 500 kg MeOH/Water
(50:50)
Solvent Recovery
Incinerator w/o ER
Solvent
Recovery WWTP
Manufacturing Plant Case 2 Manufacturing Plant
Case 2
500 kg DCM
500 kg MeOH/Water (50:50)
125 kg DCM
56 kg MeOH
25 kg MeOH 169 kg MeOH
375 kg DCM
Impacts?
Impacts?
FIGURE P19.11
TABLE P19.12 Solvent Data
Life Cycle Energy for:
Solvent
Production (MJ/kg solvent)
Distillation (MJ/kg solvent)
Incineration (MJ/kg solvent)
Acetic acid 55.9 34.9 15.5
Acetone 74.6 34.9 33.9
Acetonitrile 88.5 79.6 29.7
1-Butanol 97.3 74.6 39.9
Cyclohexane 83.2 63.4 53.5
Cyclohexanone 124.7 99.7 40.4
Dimethylformamide 91.1 67.6 25.9
Dioxane 86.6 63.8 27.6
Ethanol 50.1 31.2 31.7
Formic aid 73.9 50.1 4.7
Heptane 61.5 43.7 54.5
Methanol 40.7 21.7 22.2
Isopropyl alcohol 65.6 46.1 36.5
n-Propyl alcohol 111.7 87.3 36.5
Tetrahydrofuran 270.8 230.7 37.5