Process Engineering for Pollution Control and Waste Minimization_11 pptx

Life cycle impact assessment practice is moving more and moretoward using sophisticated fate and transport models to evaluate indicators ofenvironmental impacts.The choice of impact cate

linkage between the inventory results and effects in the environment Others(e.g., habitat modification) are known to play a critical role in environmentalimpacts of products (e.g., agricultural products), but are difficult to modelquantitatively Life cycle impact assessment practice is moving more and moretoward using sophisticated fate and transport models to evaluate indicators ofenvironmental impacts.

The choice of impact categories and category indicators and models candrive the collection of inventory data For example, one might choose to evaluateonly minerals whose reserves are predicted to be depleted within 100 years, orsome other reasonable time frame This would eliminate the need to gather data

on such materials as bauxite, clay, or iron ore, and would decrease the cost ofinventory collection and management

To date, no “standardized” listing of impact categories to be used in LCAhas been established, but several categories are employed in common practice, asshown in Table 5

The Classification Step. Inventory data need to be classified into therelevant impact categories for modeling Some emissions have influence on morethan one environmental mechanism and must be classified into more than onecategory The classic example oif this is oxides of nitrogen, or NOx, which acts

as catalyst in the formation of ground-level ozone (smog), but also is a source ofacid precipitation These substances must be characterized into both categories.One form of NOx (nitrous oxide, N2O) is also active as a greenhouse gas Theclassification rules for any LCIA must be clearly reported, so that readers of astudy understand what exactly was done to the inventory data

The Characterization Step The goal of life cycle impact assessment is

to convert collected inventory inputs and outputs into indicators for each gory (aggregates can be system-wide, by life cycle stage, or by unit operation)

cate-T ABLE 5 Typical Impact Categories

1 Stratospheric ozone depletion

2 Global warming

3 Human health

4 Ecological health

5 Smog formation

6 Nonrenewable resource depletion

7 Land use/habitat alteration

8 Acidification

9 Eutrophication

10 Energy: processing/transportation

These indicators do not represent actual impacts, because the indicator does notmeasure actual damage, such as loss of biodiversity However, together, they doconstitute an ecoprofile for a product or service.

While there is no universally accepted “right” list of impact categories orindicators, basic objectives have been set by the Society of Toxicology andChemistry (SETAC) that help define categories:

1 Category definition begins with a specific relevant endpoint Ideally,the endpoint can actually be observed or measured in the naturalenvironment

2 Inventory data are correctly identified for collection In principle, thoseinventory inputs and outputs which relate to the particular impact areidentified

3 An indicator describes the aggregated loading or resource use for eachindividual category The indicator is then a representation of theaggregation of the inventory data

Figure 7 compares the real-world causes and effects (the tal mechanism) with the modeled world of LCIA There are many differencesbetween the two In an LCI, for example, the inventory information is typi-cally modeled as a constant and continuous flow, while in the real world,emissions typically occur in a discontinuous fashion, varying from minute

environmen-to minute

F IGURE 7 Comparison of “real-world” endpoints to LCIA indicators.

Both natural and anthropogenic flows act physically, chemically, andbiologically to produce real impacts on the biota (see Figure 8) This series ofevents is called the environmental mechanism.

In the virtual reality of the environmental model, many assumptions andsimplifications are made to yield indicators Even the best current air dispersionmodels are accurate only within a factor of two to three, but the level of accuracy

is getting better all the time The principle methodological issue in life cycleimpact assessment is the modeling management of often very complex, extendedenvironmental mechanisms A listing of all possible endpoint impacts is quitelong and can look like the following suggested list

I Toxicity issues

A Human health considerations

1 Acute human occupational

2 Chronic human by consumer

3 Chronic human by local population

4 Chronic human by occupational

5 Human health

6 Human toxicity by ingestion

7 Human toxicity by inhalation/dermal exposure

8 Eutrophication (aquatic and terrestrial)

II Global issues

A Atmospheric considerations

1 Acid deposition

2 Acidification potential

3 Global warming potential

4 Stratospheric ozone depletion potential

5 Photochemical oxidation potential

6 Tropospheric ozone

B Resource considerations

1 Energy use

2 Net water consumption

3 Nonrenewable resource depletion

F IGURE 8 Midpoints versus endpoints (20).

4 Preconsumer waste recycle percent

5 Product disassembly potential

6 Product reuse

7 Recycle content

8 Recycle potential for postconsumer

9 Renewable resource depletion

10 Resource depletion

11 Resource renewability

12 Source reduction potential

13 Surrogate for energy/emissions to transport materials to recycler

14 Waste-to-energy valueIII Local issues

7 Toxic material mobility after disposal

8 Solid waste generation rate

9 Solid waste landfill space

10 Waterborne effluents

B Public relation considerations

1 Esthetic (e.g., odor)

2 Local water quality

3 Physical change to soil

4 Physical change to water

5 Regional climate change

6 Regional land

7 Regional water qualityClarifying the environmental mechanism can help determine when impactsmay be additive or when they are independent and non-additive Two illustrativeexamples are global climate change and stratospheric ozone depletion

Example 1: Global climate change The conversion of various greenhouse

gases into radiative equivalents is universally applicable based on ascientifically supported mechanism (once a judgment has been made toselect a time frame for analysis.)

Example 2: Stratospheric ozone depletion Stratospheric ozone depletion is

caused by the interaction of halogenated free radicals in the upperatmosphere directly reducing concentrations of ozone However, manyozone-depleting agents are effective greenhouse gases as well In addi-tion, recent research indicates that greenhouse effects on the loweratmosphere have led to trapping of energy near the earth, and consequentcooling of the upper atmosphere The stratospheric cooling tends toexacerbate the effects of ozone depleters

Nevertheless, for the purposes of LCIA models, these two nisms are treated separately This simplification helps develop an overallview of the environmental impacts of industrial systems at a first-orderlevel In fact, although LCIA modeling tends to be technically complex,one can view LCIAs as extended back-of-the-envelope calculations ofrealistic worst-case potential impacts

mecha-The goal in assigning LCI results to the impact indicator categories is tohighlight environmental issues associated with each Assignment of LCI resultsshould:

First assign results which are exclusive to an impact category andThen identify LCI results that relate to more than one impact category,including

Distinguishing between parallel mechanisms (where a given molecule

is “used up” in its actions), and serial mechanisms, where a moleculecan act in one mechanism, and then in a second mechanism withoutlosing its potency SOx acts in parallel mechanisms of allocated be-tween human health and acidification, while NOx acts in a serial mech-anism as a catalyst in photochemical smog formation and then inacidification

Typically, in impact assessment a “nonthreshold” assumption is used That

is, inventory releases are modeled for their potential impact regardless of the totalload to the receiving environment from all sources or consideration of theassimilation capacity of the environment However, there is a trend, particularly

in Europe, to consider thresholds in evaluating indicators For example, level ozone formation is often calculated as an indicator for photochemical smog.Background levels of ozone are about 20 ppb, while some vegetative damage hasbeen observed at 40 ppb, and human health effects at 80 ppb All these levels, as

ground-well as intermediate levels, have been used in determining indicators for chemical smog.

photo-If LCI results are unavailable or of insufficient quality to achieve the goal

of the study, then either iterative data collection or adjustment of the goal isrequired

The following sections offer descriptions of current approaches that arebeing applied to model some of the impact category indicators listed in Table 4.The most simplistic models are described in order to offer insight into the types

of approaches that are being considered useful from both a practical aspect aswell as least cost

Stratospheric Ozone Depletion. Ozone depletion is suspected to be theresult of the release of man-made halocarbons, e.g., chlorofluorocarbons, thatmigrate to the stratosphere For a substance to be considered as contributing toozone depletion, it must (a) be a gas at normal atmospheric temperatures,(b) contain chlorine or bromine, and (c) be stable within the atmosphere forseveral years (21)

The most important groups of ozone-depleting compounds (ODCs) are theCFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), halons, andmethyl bromide HFCs (hydroflourocarbons) are also halocarbons but containfluorine instead of chlorine or bromine, and are therefore not regarded ascontributors to ozone depletion

The ozone depletion potential (ODP) is calculated by multiplying the

amount of the emission (Q) by the equivalency factor (EF)

sion (Q) by its equivalency factor These individual potentials can then be summed to give an indication of projected total ODP for substances 1 through n

in the life cycle inventory that contribute to ozone depletion:

Several compounds, such as carbon dioxide (CO2), nitrous oxide (N2O), methane(CH4), and halocarbons, have been identified as substances that accumulate in theatmosphere, leading to an increased global warming effect.

For a substance to be regarded as a global warmer, it must (a) be a gas atnormal atmospheric temperatures, and (b) either be able to absorb infrared

radiation and be stable in the atmosphere with a long residence time (in years) or

be of fossil origin and converted to CO2 in the atmosphere (21)

Table 7 is a list of substances that are considered to contribute to globalwarming Equivalency factors, based on carbon dioxide as 1, are shown for eachsubstance over 20-, 100-, and 500-year spans The choice of time scale can haveconsiderable effect on how global warming potential is calculated The 100-yeartime frame is often selected, unless reasons exist that indicate otherwise

EFGWP = contribution from n to global warming over # years

contribution from CO2 to global warming over # years

T ABLE 6 Equivalency Factors for Ozone Depletion (21)

ODP

g CFC11/g substance 5

years

20 years

T ABLE 7 Equivalency Factors for Global Warming (21)

GWP

g CO2/g substance 20

years

100 years

500 years

The global warming potential (GWP) is calculated by multiplying a

substance’s mass emission (Q) by its equivalency factor These individual

poten-tials can then be summed to give an indication of projected total GWP for

substances 1 through n in the life cycle inventory that contribute to global

This category can also reflect land use as a resource Land that has beendisturbed directly due to physical or mechanical disturbance can be accounted for

as a resource that is no longer available either for human use or for ecologicalbenefit (such as providing habitat for a certain species) Other subcategories underthe resource category include:

Net marine resources depletedNet land area

Net water resourcesNet wood resourcesScientific Certification Systems (SCS) proposes the following approach in theirLife-Cycle Stressor Effects Assessment (LCSEA) model for calculating netresource depletion (22) The LCSEA model is based on (a) the relative rates ofdepletion of the various resources and (b) the relative degree of sustainability ofthe resources

The model considers the key factors that affect resource depletion andincludes consideration of recycled material as supplementing raw material inputs

It also takes into account materials that are part of the standing reserve base, i.e.materials, such as steel in a bridge, that will become available as a recoveredreserve at some future time Recycling of metals has great significance for thedepletion calculation (see Figure 9)

The elements to be considered in factoring resource depletion include:

Current world reservesRaw material input (i.e., the amount used)Amount recycled (both direct and standing stock)Waste generation

Natural accretionThe reserve base-to-use ratio can be calculated as follows:

Reserve base (R)

Use (U) = number of years of remaining use left (at currentuse rate)

Use (U)

Reserve base (R) = % of reserve base used

The recycled resource is linked to the original virgin material use and ing reserve base Emissions are not spatially or temporally lined to the originalvirgin unit operation Accounting for all reserve bases:

correspond-Waste (∑W)Reserve base (R) + recyclable stock (∑S)The current assumption is that only one iteration of recycling and materialintegrity is sustained If natural accretion is accounted for, the following formularesults:

waste (∑W ) − natural accretion (N)

reserve base (R) + subsequent uses (∑S)Including the time period in the equation, we get:

F IGURE 9 Flow of metals, including standing reserve.

(∑W − N) ∆T

R + (∑S ) ∆T Current assumption: ∆T = 50 years

And accounting for baseline reserve bases,

The impact for the resource depletion category can then be calculated according

to the formula:

Resource depletion indicator (RD) = resource use × resource depletion factor (RDF)For net resources depleted (or accreted), the units of measure express theequivalent depletion (or accretion) of the identified resource All of the netresource calculations are based on RDFs

Indicator—net resource Units of measure

Fossil fuels Tons of oil equivalents

Non-fuel oil and gas Tons of oil equivalents

Acidification. For acidification, an equivalency approach is typically plied and the stressor flows are converted into SO2 or H+ equivalents Forexample, NO2 is multiplied by 64/(2 ∗ 46) = 0.70, since this is the molar proton

ap-release potency of NO2 compared to SO2 Table 8 shows sample calculationsusing potency factors for an inventory with SO2, NO2, and HCl releases TheLCSEA approach takes the calculation one step further and includes an emissionloading factor to reflect how much of the inventory release is expected to reachthe receiving environment.

Eutrophication. Eutrophication occurs in aquatic systems when the ing nutrient in the water is supplied, thus causing algal blooms In fresh water, it

limit-is generally phosphate which limit-is the limiting nutrient, while in salt waters it limit-isgenerally nitrogen which is limiting In general, addition of nitrogen alone to freshwaters will not cause algal growth, and addition of phosphate alone to salt waterswill not cause significant effects In brackish waters, either nutrient can causealgal growth, depending on the local conditions at the time of the emissions.Eutrophication is generally measured using the concentration of chloro-phyll-a in the water Waters with less than 2 mg of chlorophyll-a per cubicmeter (2 mg chla m–3) are considered “oligotrophic,” while those with 2–10 mgchla m–3 are considered “mesotrophic,” and those with more than 10 mgchla m–3 are termed “eutrophic.” Waters over 20 mg chla m–3 are considered

“hypereutrophic.”

As waters become mesotrophic, their species assemblages change, favoringspecies that grow rapidly in the presence of nutrients (“weed” species) over thosewhich grow more slowly There is some indication that eutrophication in saltwaters is the source of the red tides that are a worldwide problem

Under eutrophic conditions, the algae in the water significantly block lightpassage, while in hypereutrophic conditions the amount of biomass produced is

so high that anoxic conditions occur, leading to fish kills There are someindications that similar sorts of effects occur in terrestrial systems as well.The ratio of carbon to nitrogen to phosphorus in aquatic biomass is 106:16:1(23), on an atomic basis This ratio is the basis of combining nitrogen andphosphorus in calculating the eutrophication potential of emissions

(Molar quantity of nitrate + nitrite + ammonia) × Redfield ratio+ molar quantity of phosphate × [endpoint characterization factor (fresh, salt water)] = eutrophication indicator

Eutrophication is typically measured in PO4 equivalents The EPA has set aconcentration of 25 µg PO4 L–1 as the level needed to protect fresh-water aquaticecosystems from eutrophication

Energy. While inventory analyses involves the collection of data to tify the relevant inputs and outputs of a product system, the accounting ofelectricity as a flow presents a unique challenge The use of energy audits makesthe idea of balancing energy flows around a process a familiar one However, inLCA the reporting of energy flows is in itself insufficient to perform a subsequent