Introduction to Environmental Engineering

download Introduction to Environmental Engineering

If you can't read please download the document

Transcript of Introduction to Environmental Engineering

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/ActivatedSludge.pdf1Biological Wastewater Treatment(Nazaroff & Alvarez-Cohen, Section 6.E, augmented)

(http://dnr.metrokc.gov/wtd/westpoint/wp-aerial.htm)

West-Point Wastewater Treatment Plant, near Seattle, WA

Wastewater Treatment Plant (WWTP) - overview of system

The activated sludge system is the center of the secondary treatment.

While primary treatment = PHYSICAL, secondary treatment = BIOLOGICAL.

2The system is properly speaking an activated-sludge system when a portion of the sludge (cells) collected from the bottom of the clarifier is returned to the aerator. Not only are these cells already acclimated to the sewage, but by the time they are collected from the clarifier, they are also oxygen starved and therefore really "hungry" for another meal!

Introduction

An activated-sludge reactor is a system in which pre-treated sewage (i.e. having passed through primary treatment settling) is aerated to promote the growth of bacteria (cells) that gradually consume the organics in the sewage.

The result is the development of cells acclimated to the particular mix of substances present in the sewage and a significant consumption of their organic material; the effluent is a mixture of water with drastically reduced BOD content and suspended cells.

This mixture is then passed through a clarifier (settling tank) where the solids (mostly cells, then called sludge) are separated from the water. The system is commonly operated in continuous mode (as opposed to batch mode).

Alternative 1: Trickling Filter

(From Peavy et al., 1977))

(From Masters, 1998)

A trickling filter in action in a wastewater treatment plant in Denmark.

A trickling filter consists of a substrate (rocks or other material) on which cells (slime) can grow and over which the pre-treated sewage is sprayed. The spraying action creates contact between BOD in sewage, oxygen in the air and cells on the substrate. Cells grow and degrade the sewage. Excess cells need to be periodically removed from the substrate.

3Trickling filters: over rocks (left) and over synthetic media (right)

(From Davis & Cornwell, 2008)

Alternative 2: Rotating Biological Contactors

In this process, cells are attached to disks that rotate in the vertical plane. Cells are then alternatively exposed to sewage (their food) and air (their oxygen supply).

(Sincero & Sincero, 1996)

4Alternative 3: Fixed-film Reactors

(Sincero & Sincero, 1996)

In this process, cells are attached to vertical plates that are immersed by the flowing sewage and air is injected from the bottom to provide the oxygen.

Alternative 4: Aerated Lagoons and Stabilization Ponds

(http://www.lagoonsonline.com)

In this process, nature is essentially left to run its course, with or without a little help with aeration.

The system looks less technological and is thus better integrated in the landscape, but it takes much more room.

(http://www.ces.clemson.edu/ees/rich/technotes/technote5.html)

5Susceptible to shocks loadings and toxinsSusceptible to seasonal changesRelatively high capital costsRelatively high operating costs

Removal of dissolved constituentsDestruction processTreatment of chlorinated wastesMethane generation (= fuel)Reduced sludge generation

Low concentrationOrganicsChlorinated organicsInorganics

Anaerobic degradation

Volatile emissionsSusceptible to shocks and toxinsSusceptible to seasonal changesHigh land requirementNo operational control

Removal of dissolved constituentsLow maintenanceDestruction processRelatively safeLow capital costsLow energy costsEasy to operateInfrequent waste sludge

Low concentrationOrganicsSome inorganics

Aerated lagoons,Stabilization ponds

Volatile emissionsSusceptible to shocks loadings and toxinsSusceptible to seasonal changesRelatively high capital costsRelatively high operating costs

Removal of dissolved constituentsLow maintenanceDestruction processRelatively safeReduced sludge generation

Low concentrationOrganicsSome inorganics

Trickling filters,Fixed-film reactors

Volatile emissionsWaste sludge disposalHigh energy costsSusceptible to shock loadings and toxinsSusceptible to seasonal changes

Removal of dissolved constituentsLow maintenanceDestruction processRelatively safeLow capital costsRelatively easy to operate

Low concentrationOrganicsSome inorganics

Activated sludge

DisadvantagesAdvantagesApplicationsTechnologyComparative Summary of biological wastewater treatment technologies

Activated-sludge system

The activated sludge system consists of two components, an aerator, where cells consume the sewage, and a clarifier, where cells are then removed from the treated water.

Because cells need oxygen for their metabolism, air is injected from the bottom of the aerator. Rising bubbles agitate the water well and create good contact between the three ingredients: cells, sewage and oxygen.

6Activated-sludge aerators are well agitated by the injection of air from the bottom

Activated-sludge aerator in Lansing, Michigan

(Taken from Davis & Cornwell, McGraw-Hill, 2008)

7(From Davis & Cornwell, 2008; their source: Curds, 1973)

Different organisms grow and decay depending on the nature of the sewage and its rate of flow. Note that biological processes take many hours to adjust to a changed ecosystem.

Kinetics of cell growth and waste utilization:

In an activated-sludge reactor, there are two fundamental variables: the amount of organic waste, denoted by S (as in Substrate), and the concentration of bacterial cells, denoted by X.

Both are measured in mg/L.

The quantity S is also directly related to the BOD. The higher the BOD in the sewage, the more food for the cells.

To determine their magnitudes, which may be functions of time, S(t) and X(t), we need to know their rate of growth and decay.

2015Effluent from activated sludge unit

14095Effluent from primary, influent to secondary

220220Influent to plant

BOD (mg/L)SS (mg/L)Location

(Metcalf & Eddy, 1991 as taken from Nazaroff & Alvarez-Cohen Table 6.E.1)

8Let us define:

rS = rate of substrate consumption = decay rate of waste [in mg of substrate/(L.day)]

rX = rate of cell formation = growth rate - decay rate [in mg of cells/(L.day)]= rg rd

Empirical observation #1: The rate of cell growth rg is proportional to the substrate consumption rate rS, because the substrate is consumed by the cells to make more cells.

The coefficient of proportionality is defined as the yield and denoted by Y (no units).

Thus,

Typically, the value of Y is 0.6 or less because cells emit carbon dioxide and therefore put on as weight only a fraction of their food consumption.

Sg rYr =

Empirical observation #2:

The cell growth rate rg is proportional to the cell concentration X, when all other variables are held unchanged, because the more cells there are, the more new cells can be manufactured.

Thus,

Empirical observation #3:

The coefficient k of proportionality depends on the substrate concentration as follows: -At low S values, k increases in proportion to increasing S, because the more food is available, the faster the cells multiply;- At high S values, k reaches a constant maximum value, because there is then a superabundance of food and cells cannot consume all of it right away.

where the coefficient k depends on other variables, such as the amount of substrate present, S.

Xkrg =

9where km is the growth constant (in /day), Y is a yield rate (ratio of cellular material generated per amount of substrate consumed), and KS is called the half-saturation constant (in mg/L) because when S = KS, k = km/2, which is at half of its maximum value. Put together, we have:

This bimodal behavior is well captured by the so-called Monod kinetics:

SKYSkk

S

m

+=

SKSXkr

Yr

SKSYXkr

S

mgS

S

mg +==+=

1

10

Empirical observation #4:

The death rate rd of cells is proportional to the cell concentration X, because cells die in proportion to their number.

Thus,Xkr dd =

Recycling

To promote growth of the cells already adapted to the nature of the sewage, some fraction of the sludge collected at the bottom of the clarifier is recycled into the aerator.

Let us denote by Qr the volumetric flow rate of sludge added to the inflowing rate of sewage Qin, and by Xu the cell concentration inside the sludge collected at the bottom of the clarifier.

It goes without saying that Xu is expected to be significantly larger than the concentration X of cells in the aerator.

Notation of fluxes and variables at several points in the system:

In continuous operation, where wastewater is constantly added and some of the mixture is constantly removed, the budgets of S and X are those of a continuously-stirred tank reactor (CSTR). If the reactor's volume is V (in m3) and the volumetric flow rate is Qin (in m3/day), the budgets are:

+++= SKSXkVSQQSQSQ

dtdSV

S

moutrinurinin )(

++++= XkSK

YSXkVXQQXQXQdtdXV d

S

moutrinurinin )(

Substrate:

Cells:

11

The entering wastewater has a known substrate concentration Sin and contains almost no cells, and we may assume Xin = 0.

We can furthermore take the exit concentrations equal to those inside the reactor since the reactor is very well mixed by the aerating bubbles, so that Sout = S and Xout = X.

Finally, the substrate concentration coming from the clarifier is indistinguishable from that entering it (Su = S) because settling of cellular material does not affect the substrate concentration.

Equations reduce to:

SKVSXkSQSQ

dtdSV

S

mininin +=

VXkSK

VYSXkXQQXQdtdXV d

S

mrinur +++= )(

Dividing these equations by the volume V and defining the hydraulic residence time in the reactor (in days) as

inQV=

and the dimensionless recycle ratio R

in

r

QQR =

we obtain:

( )SK

SXkSSdtdS

S

min +=

1

XkSK

YSXkXRRXdtdX

dS

mu +++=

)1(

steady state

12

( ) )1( 1SK

SXkSSS

min +=

)2(1 uS

md X

RSK

SXkYXkR ++=

++

The first equation expresses that the difference between the entering and exiting substrate is due to the consumption by cells, while the second equation states that the amount of cells exiting the aerator plus those that have died inside is equal to the amount of cells grown on the substrate plus those added by the recycling flow.

Writing the steady-state budget for the cells in the clarifier, we have:

XQQXQQ rinuwr )()( +=+

in which we have assumed that the concentration of cells in the clarified water (Xe) is virtually nil because most cells have settled to the bottom.

Then, introducing the wastage ratio W

in

w

QQW =

we can express Xu in terms of X:

XRW

RX u ++= 1

and eliminate Xu from equation (2): )3()()1(

SKSXkYXk

RWRW

S

md +=

++

+

13

So, we have two equations, one for S, the amount of substrate (sewage), and the other for X, the amount of cells, both in the aerator and both expressed in mg/L:

(3) )()1(

SKSXkYXk

RWRW

S

md +=

++

+

( ) )1( 1SK

SXkSSS

min +=

The first equation expresses that the loss of substrate (Sin Sout) per time (division by residence time ) is equal to the amount eaten by the cells.

The second equation states that the amount of cells that leave and die per time is equal to the rate of growth.

The equations contain four biological-type parameters:

km = cell growth constant = BOD degradation rate (in mg of substrate per mg of cells per day)KS = half-saturation constant of cell growth (in mg of substrate per L)Y = yield rate = ratio of cell growth to substrate consumption (dimensionless)kd = cell death constant (in 1/day)

( ) XkY

SS din

+= 111

YYk

XSS din 1+=

Determination of the biological parameters:

In any operation, it is important to know the value of the various 'constants', for these not only vary significantly with temperature but also with the nature of the sewage. Different mixes of organic material in different sewages (or in the sewage of the same town at different time periods) grow different cells at different rates.

To determine these 'constants', plant operators proceed as follows.

The system is operated several times in continuous mode and without recycling (R = 0) and with different values of the input parameters Sin and , and the exiting S and Xconcentrations are measured each time. The result is a set of (Sin, S, , X) data.

Eliminating the km fraction between the preceding two equations, we obtain

which can be rewritten as:

14

This equation,YY

kX

SS din 1+=

is a linear relationship between the known variables (Sin-S)/X and . Therefore, plotting one of these variables against the other should produce a set of points falling more or less along a straight line.

Fitting a straight line through the set of points provides the two coefficients, namely the slope kd/Y and the intercept 1/Y.

From these, the constants kdand Y can be separately determined.

To determine the remaining constants km and KS, we flip equation (1) upside-down and multiply it by X, to obtain:

mm

S

in kSkK

SSX 11 +=

which is another linear relationship between known variables, this time X/(Sin-S)and 1/S.

A plot should enable a fit by a straight line, which then yields values for KS/km and 1/km.

From these, we can unravel kmand KS.

15

Typical values of the biological parameters:(Nazaroff & Alvarez, top of Table6.E.2)

0.06

0.6

60

5

Typical value

1 / day0.025 0.075kd

(dimensionless)0.4 0.8Y

mg of substrate / L25 100KS

mg of substrate / (mg of cells x day)2 10km

UnitsTypical rangeParameter

SKSkYk

RWRW

SKSXkYXk

RWRW

S

md

S

md +=++

++=

++

+)()1(

)()1(

(Funny! The X-equation no longer depends on X. So, well use it to determine S instead and use the S-equation to get X afterwards.)

Wash-out time:

A crucial design parameter is the so-called wash-out time.

If the residence time is less than a critical value, denoted min, then the sewage flow is too fast at steady state for bacteria to grow, existing cells are flushed out faster than they can multiply, and the result is the absence of cells, namely X = 0. When this happens, the sewage is not consumed and the exiting sewage shows no reduction in BOD, namely S = Sin.

Mathematically, a trivial solution of equation is (3) is X = 0 with accompanying solution S = Sin from equation (1). To avoid such state of affairs, we obviously need to have X > 0. Dividing equation (3) by X then provides:

16

This equation is a relationship between S and when cells are present (X not zero).According to this relationship, S goes to infinity as the residence time is decreased. Obviously, S cannot exceed Sin, the entering concentration.

Therefore, the range of values has a lower bound, with the minimum being the value that corresponds to S = Sin:

SKSkYk

RWRW

S

md +=++

+)()1(

inS

inmd SK

SkYkRW

RW+=++

+)(

)1(

min

of which the solution is:

Sdindm

inS

KkSkYkSK

RWRW

+

++=

)()1(

min

This minimum value is called the wash-out time, because if falls below it, S = Sin and there is no substrate reduction taking place, i.e. no treatment. The system is a complete failure!

Solving now for S as a function of and then for X by using the remaining equation, we obtain:

Sdm

d KRWkYkRW

RWkRWS)1())((

)1()(++

+++=

++++++= )1())(()1()()( RWkYkRW

KRWkRW

SRWYXdm

S

d

in

17

We note that S decreases as increases, which is intuitively correct since more time spent in the aerator means more consumption of waste.

The amount of cells first increases as more time spent in the system gives them more time to feed, but decreases for longer residency times as death of old cells becomes the dominant effect.

Note that there is an ultimate S value below which the system cannot reach:

It is fairly small because kd, the death decay rate of cells, is a small parameter.

Because the rate kd of cell decay is slow compared to the growth rate km, the preceding two expressions for S and X can be approximated as:

dm

Sd

kYkKkS =min

)()1()1()(

)1( SSRWRWYXK

RWYkRWRWS inS

m

++++

+

for a wide range of values above but not too far from the wash-out time.

Note concerning the choice of residence time:

Since our goal is to reduce the BOD of the sewage, we may first think that we should operate the aerator at long residence times (because high values yield low S values).

However, long residence times demand large tank volumes and create enormous costs. Therefore, there is an economic incentive to operate the system with moderate values of the residence time.

Also, a larger tank increases the hydraulic residence time and, with it, the cells residence time in the system. Older cells perform less well than younger cells. (Sounds familiar?)

In the tendency toward lower values of the residency time, close attention must be paid to the wash-out time, in order to avoid failure. Because the values of the coefficients that make up the expression for min vary with both temperature and the nature of the sewage mix, a generous margin of safety must be included.

18

With R = 0, the wash-out time becomes

Sdindm

inS

KkSkYkSK

+=)(min

which is related to the original value by

recycling nomin with recyclingmin with )1(

RWRW

++=

The benefit of sludge recycling:

In the early attempts of biological wastewater treatment, no recycling of cells was performed. In other words, no activated sludge was used to promote biological degradation.

Aside from the obvious disadvantage of not seeding the aerator with pre-adapted cells to make the work more effective, these systems suffered also from having to be excessively large.

We now quantify the benefits of recycling sludge by contrasting the quantities in the absence of recycling (setting the R ratio to zero).

will always fall below unity, and the minimum required residence time is lowered because of recycling.

The gain is very significant. For example, with typical values R = 0.25 and W = 0.003, the ratio equals 0.015, which leads to a reduction in residency time by 98.5%, with a concomitant 98.5% reduction in aerator volume, or about 1/67 of the size required without recycling.

Since W must be less than 1 by definition, the ratio

RWRW

++ )1(

recycling nomin with recyclingmin with )1(

RWRW

++=

19

)1( RW

RWXQ

VX

uwc +

+==

While the typical hydraulic residence time (average time spent by water in the aerator) is on the order of 3 to 5 hours, the average cell age c is typically on the order of 5 to 15 days.

Mean cell residence time:

Operators of activated-sludge systems worry to some extent about the age of the cells.

Indeed, an old cell population has the disadvantages of a higher death rate and of acclimatization to older sewage; vice versa, a young cell population may be insufficiently acclimatized to the nature of the sewage.

The average cell age, also called the mean cell residence time and noted c, is defined as the amount of cells in the aerator divided by the cell exit rate from the system:

Sdindm

inS

KkSkYkSK

+=)(min c

Like the hydraulic residence time , the cell residence time c may not fall below a minimum value, which is

which is the same value as min in the absence of recycling.

In terms of c, the S and X quantities are:

Scdm

cd KkYkkS

1)(1

+=

++

+=1)(1)1(

)(

cdm

S

cd

in

kYkK

kS

RWRWYX

20

Food-to-cell ratio:

Another commonly reported characteristic of an activated-sludge system is the food-to-microorganism ratio, defined as the rate at which sewage (BOD) is supplied, QinSin, divided by the amount of cells in the aerator, VX:

c

cd

Yk

SSS

VXSQMF

+==

1/in

ininin

With a 90% removal rate [ (Sin-S)/Sin = 0.10], kd = 0.06/day, Y = 0.6 and c = 10 days, this ratio is 2.7 per day.

Put another way, it means that at any one time, the system contains enough food to feed the cells for the next 1/2.7 = 0.37 days 9 hours. Should the flow of sewage be interrupted (ex. because of nighttime), the cells can only feed for another 9 hours before they starve and begin to die at an accelerated rate.

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/AirQuality.pdf1Environmental Issues Associated with the Atmosphere

Quality of the air Climate

Chemicals that we breathe

Toxic, potentially fatal

Harmful to lungs

Causing cancer

Chemicals harmful to non-humans

Harmful to breathing animals

Harmful to vegetation & habitats

Impacting water quality

Harmful to aquatic life

Affecting buildings & monuments

Excessive greenhouse gases

Global warming

Other climatic changes

Melting of sea ice

Melting of land ice (glaciers)

Rising sea level (added water)

Change in precipitation patterns

Change in vegetation

Rising sea level (thermal expansion)

More extreme weather

Affecting visibility

Overview of Issues in Air Quality(Nazaroff & Alvarez-Cohen, Section 7.A)

Both indoor and outdoor problems.

Chernobyl-type accidentsStratospheric ozone depletionClimate change

decadesto centuries

20,000 kmPlanetary atmosphere

Acid depositionseveral daysto a week

1000 kmRegional / continental

Ground-level ozoneCarbon monoxideParticulate matter

day-night cycle10 to 100 kmUrban airshed

Toxic organicsMercury and other metals

10 minutes1 kmIndustrial plumes

Radon in basementTobacco smokeAirplane cabin air

1 hour10 mIndoor environment

ExamplesTime scaleLength scaleSystem

2In some U.S. cities

Newark, NJ

Los Angeles, CA

It used to be worse a century ago

Sign of prosperity in Pittsburgh in 1906(Carnegie Library of Pittsburgh)

3 and still 50 years later

Riders of a local message delivery company in Los Angeles being outfitted with protective gas masks in the fall of 1955.

At the 1958 Air Pollution Conference, Dr. James P. Dixon, Health Commissioner of Philadelphia said :

If gas masks are not to become as common in a hundred years as shoes are today in the civilized world, we should do well to heed our somewhat submerged instincts of self-preservation and remember that - whatever other uses man may devise for it - air is essentially for breathing.

Brief historical review in the United States:

Almost no concern until problems became highly visible.Initially also, air pollution problems were viewed as local and not a federal matter.

1955: First federal action Air Pollution Control Act (funding for research, not control!)

1970: Establishment of the U.S. Environmental Protection Agency (EPA)

1970: Clean Air Act

1973-1993: Gradual elimination of lead emission

Mid-1980s: Concern over acid rain; regulation of sulfur dioxide

1990: Clean Air Act Amendments

1990s: Concern over climate change

4(Taken from Masters, 1998)

Two-prong strategy:

1. Control emissions, with goal of reduction- Command & Control (requiring use of Best-Available ControlTechnology (BACT)- Incentives (cap-n-trade of emissions)

2. Control of ambient concentrations: Six criteria pollutants- National Ambient Air Quality Standards (NAAQS)

Distinction:

Primary pollutant: emitted directly from a source

Secondary pollutant: formed in the air by chemical reactions from precursor species

Air-Quality Management

5Framework for understanding air pollution problems(Nazaroff & Alvarez-Cohen, Figure 7.A.1, page 390, slightly modified)

Level of regulationsBACT, Cap-n-Trade

Level of regulationsNAAQS

Level of concernhealth studies, other impacts

Emissionstandards

Components of an Air-Pollution Management System

Emissionallocation

Social andpolitical

considerations

Air-qualitystandards

Costeffectiveness

Damagefunctions

Air pollutioneffects

Atmosphericchemistry

Airquality

Transport& dispersion

EmissionsSources Controlmethods

Pollutionforecasts

Social andpolitical

considerations

Episode-controltactics

CostAlternateproducts

orprocesses

Air-qualitycriteria

Social andpolitical

considerations

Start Here

RegulateHere

RegulateHere

TARGET

ACTION

MODELS

TARGET

6Blood poisoningKidney damageMental retardation

Industrial processesLead pipes, solder

PPbLead

Visibility impairmentRespiratory impairment

Industrial combustionOther industrial activities

both P and SPM10 and PM2.5Particulate Matter

Lung irritantAcid deposition

Sulfur in fuels, esp. coalPSO2Sulfur Dioxide

Coughing, Chest painLung damage

From NO and NO2mostly SO3Ozone

Respiratory irritantVisibility impairmentAcid deposition

From NO in combustionSNO2Nitrogen Dioxide

Impairs oxygen-carryingcapacity of blood

Incomplete combustionPCOCarbon Monoxide

Effect(s)Source(s)Primary /Secondary

Criteria Pollutant

The six so-called Criteria Pollutants

National Ambient Air Quality Standards (NAAQS)(Nazaroff & Alvarez-Cohen, Table F.3, page 657)

Primary & Secondary1.5 g/m3Lead (Pb)

Secondary500 ppb (1.3 mg/m3)3-hour average

Primary140 ppb (365 g/m3)24-hour averagePrimary30 ppb (80 g/m3)Annual average

Sulfur dioxide (SO2)

Primary & Secondary65 g/m324-hour averagePrimary & Secondary15 g/m3Annual average

Particulate Matter 2.5 m (PM-2.5)Primary & Secondary150 g/m324-hour averagePrimary & Secondary50 g/m3Annual average

Particulate Matter 10 m (PM-10)Primary & Secondary120 ppb (235 g/m3)1-hour averagePrimary & Secondary80 ppb (157 g/m3)8-hour average

Ozone (O3)

Primary & Secondary53 ppb (100 g/m3)Annual averageNitrogen dioxide (NO2)

Primary35 ppm (40 mg/m3)1-hour average

Primary9 ppm (10 mg/m3)8-hour average

Carbon monoxide (CO)

TypeStandardPollutant

7(http://en.wikipedia.org/wiki/Image:US-overall-nonattainment-2007-06.png)

8(Taken from Masters, 1998)

Air Pollutants in the United States by source type

9Problems caused by nitrogen oxides

NO

NO2

O3Visibility HNO3

223

32

2

ONOONO :RelaxationOOO :formation Ozone

ONOsunlightNO :Excitation

+++

++120 ppb

Problems caused by sulfur dioxide

Sulfurin fuel

SO2 infumes

Respirationirritant H2SO4

Acid rain

Tree damage Acid lakes

Dead fish(http://www.robl.w1.com/Pix/I-900991.htm)

Damaged buildings &monuments

(http://ww

w.m

erritton.ca/acid55.jpg)

10

Best Available Control Technology (BACT)

Most common types of end-of-pipe treatment

Particulates: CycloneElectrostatic precipitator

Stationary combustion fumes (incl. SO2): Wet scrubber

Mobile exhaust: Catalytic converter

(http://www.epa.gov/eogapti1/module6/matter/control/control.htm)(http://www.aa1car.com/library/p0420_dtc.htm)

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/AnaerobicDigestion.pdf1Anaerobic Digestion of Wastewater Sludge(Nazaroff & Alvarez-Cohen, Section 6.E.3)

nice, clean water going to disinfection and then to outdoor body of receiving water (stream, lake or sea)

sludge in need of further treatment

The goal is to reduce the amount of sludge that needs to be disposed.

The most widely employed method for sludge treatment is anaerobic digestion.

In this process, a large fraction of the organic matter (cells) is broken down into carbon dioxide (CO2) and methane (CH4), and this is accomplished in the absence of oxygen. About half of the amount is so converted into gases; the remainder is dried and becomes a residual sludge.

(Photos from http://www.madep-sa.com/english/wwtp.html)

How the apparatus looks like

Enclosure exists to trap and save the methane. This methane, a fuel, can be used to meet some of the energy requirements of the wastewater treatment facility (co-gen).

How the sludge looks like after anaerobic digestion.

It is rich in nitrates and acts as a good fertilizer.

2How the system works

The treatment of wastewater sludges, from both primary and secondary treatment steps, consists of two main phases. In the 1st phase, in processes known as thickening and dewatering, the goal is to separate as much water as possible to decrease the volume of material. The 2nd phase is known as sludge stabilization, which reduces the level of pathogens in the residual solids, eliminates offensive odors, and reduces the potential for putrefaction.

Tanks are sealed to prevent both oxygen from entering and methane from escaping.

Anaerobic digestion of municipal wastewater sludges has been widely practiced since the early 1900s and is the most widely used sludge treatment method.

Overall, the process converts about 40% to 60% of the organic solids to methane (CH4) and carbon dioxide (CO2). The chemical composition of the gas is 60-65% methane, 30-35% carbon dioxide, plus small quantities of H2, N2, H2S and H2O. Of these, methane is the most valuable because it is a hydrocarbon fuel (giving 36.5 MJ/m3 in combustion).

The residual organic matter is chemically stable, nearly odorless, and contains significantly reduced levels of pathogens.

The suspended solids are also more easily separated from water relative to the incoming sludge or aerobically treated sludge (such as in outdoor pond).

3Example of co-generation in a wastewater treatment facility

4Interior pictures of the co-generation system at the Albert Lea municipal wastewater treatment plan in Minnesota.

It is also possible to pass the biogas into a fuel cell to make electricity directly instead of making steam, passing it through a turbine and cranking a generator.

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/BioTreatmentTypes.pdf 1

Biological Wastewater Treatment This is a brief summary of the various techniques that have been developed to treat wastewater by biological means. They accomplish what is generally called secondary treatment. Purpose: The idea behind all biological methods of wastewater treatment is to introduce contact with bacteria (cells), which feed on the organic materials in the wastewater, thereby reducing its BOD content. In other words, the purpose of biological treatment is BOD reduction. Typically, wastewater enters the treatment plant with a BOD higher than 200 mg/L, but primary settling has already reduced it to about 150 mg/L by the time it enters the biological component of the system. It needs to exit with a BOD content no higher than about 20-30 mg/L, so that after dilution in the nearby receiving water body (river, lake), the BOD is less than 2-3 mg/L. Thus, the biological treatment needs to accomplish a 6-fold decrease in BOD. Principle: Simple bacteria (cells) eat the organic material present in the wastewater. Through their metabolism, the organic material is transformed into cellular mass, which is no longer in solution but can be precipitated at the bottom of a settling tank or retained as slime on solid surfaces or vegetation in the system. The water exiting the system is then much clearer than it entered it. A key factor is the operation of any biological system is an adequate supply of oxygen. Indeed, cells need not only organic material as food but also oxygen to breathe, just like humans. Without an adequate supply of oxygen, the biological degradation of the waste is slowed down, thereby requiring a longer residency time of the water in the system. For a given flowrate of water to be treated, this translates into a system with a larger volume and thus taking more space. Advantages: Like all biological systems, operation takes place at ambient temperature. There is no need to heat or cool the water, which saves on energy consumption. Because wastewater treatment operations take much space, they are located outdoor, and this implies that the system must be able to operate at seasonally varying temperatures. Cells come in a mix of many types, and accommodation to a temperature change is simply accomplished by self adaptation of the cell population.

2

Similarly, a change in composition of the organic material (due to peoples changing activities) leads to a spontaneous change in cell population, with the types best suited to digest the new material growing in larger numbers than other cell types.

3

Types of equipment for biological treatment: There are two broad types of biological wastewater treatment: those that include mechanical means to create contact between wastewater, cells and oxygen, and those than dont. a) With mechanical means: 1. Activated sludge: This is the most common type. It consists in a set of two basins. In the first, air is pumped through perforated pipes at the bottom of the basin, air rises through the water in the form of many small bubbles. These bubbles accomplish two things: they provide oxygen form the air to the water and create highly turbulent conditions that favor intimate contact between cells, the organic material in the water and oxygen. The second basin is a settling tank, where water flow is made to be very quiet so that the cellular material may be removed by gravitational settling. Some of the cell material collected at the bottom is captured and fed back into the first basin to seed the process. The rest is treated anaerobically (= without oxygen) until it is transformed into a compost-type material (like soil). The cost of an activated-sludge system is chiefly due to the energy required to pump air at high pressure at the bottom of the aerator tank (to overcome the hydrostatic pressure of the water). Another disadvantage is that the operation is accomplished in two separate basins, thereby occupying a substantial amount of real estate.

4

2. Trickling filter: A trickling filter consists in a bed of fist-size rocks over which the wastewater is gently sprayed by a rotating arm. Slime (fungi, algae) develops on the rock surface, growing by intercepting organic material from the water as it trickles down. Since the water layer passing over the rocks makes thin sheets, there is good contact with air and cells are effectively oxygenated. Worms and insects living in this ecosystem also contribute to removal of organic material from the water. The slime periodically slides off the rocks and is collected at the bottom of the system, where it is removed. Water needs to be trickled several times over the rocks before it is sufficiently cleaned. Multiple spraying also provides a way to keep the biological slimes from drying out in hours of low-flow conditions (ex. at night).

Plastic nets are gradually replacing rocks in newer versions of this system, providing more surface area per volume, thereby reducing the size of the equipment. 3. Biological contactor: This is essentially a variation on the trickling filter, with the difference being that solid material on which slime grows is brought to the water rather than water being brought to it. Rotating disks alternate exposure between air and water.

b) Without mechanical means: The wastewater is made to flow by gravity through a specially constructed wetland. There, the water is brought into close contact with vegetation (ex. reeds), which acts as a

5

biological filter to the water. The organic material in the wastewater is used as nutrient by the plants. Oxygen supply is passively accomplished by surface aeration (contact with oxygen of the atmosphere). Since water flow is slow in such system, to give ample time for the biological activity to take place, there is almost no turbulence in the water and reaeration is weak. Compared to mechanical systems, constructed wetlands occupy far more real estate, but they may be aesthetically pleasing, especially if they are well integrated in the local landscape. They emit no odor, but people should stay away because of the danger posed by pathogens. Constructed wetlands have also the least energy requirement. Energy is only needed to pump the wastewater to the entrance of the system, from where gravity and biology do the rest. A major disadvantage, however, is the highly reduced performance during winter, especially in regions where ground freezes during some of the winter months.

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/Cyclones.pdf1Air-Quality Technology(Nazaroff & Alvarez-Cohen, Section 7.C, pages 442-454)

Two different approaches:

- Pollution Prevention at the source the better alternative

- Treatment of fumes as they are formed the classical approach

2Example of steps taken to minimize air pollution from gasoline internal combustion engines

3In other words: Choose your pollutant!

4Example of post-operation pollution control (effluent treatment)

Example of pre-operation pollution control (pollution prevention)

5Cyclone separators have been used in the United States for about 100 years, and are still one of the most widely used of all industrial gas-cleaning devices. The main reasons for the wide-spread use of cyclones are that they are inexpensive to purchase, they have no moving parts, and they can be constructed to withstand harsh operating conditions.

Cyclone Separators and their Design

A cyclone used in a woodshop

Cyclone Design(Adapted from Air Pollution Control by C. D. Cooper & F.C . Alley, 1986)

Typically, a particulate-laden gas enters tangentially near the top of the cyclone, as shown schematically in the left figure. The gas flow is forced into a downward spiral simply because of the cyclones shape and the tangential entry.

Another type of cyclone (a vane-axial cyclone see right figure) employs an axial inlet with fixed turning vanes to achieve a spiraling flow.

Centrifugal force and inertia cause the particles to move outward, collide with the outer wall, and then slide downward to the bottom of the device. Near the bottom of the cyclone, the gas reverses its downward spiral and moves upward in a smaller inner spiral. The cleaned gas exits from the top through a vortex-findertube, and the particles exit from the bottom of the cyclone through a pipe sealed by a spring-loaded flapper valve or rotary valve.

6Advantages of cyclones are:

Low capital costAbility to operate at high temperaturesLow maintenance requirements because there are no moving parts.

Disadvantages of cyclones are:

Low efficiencies (especially for very small particles)High operating costs (owing to power required to overcome pressure drop).

Cyclones by themselves are generally not adequate to meet stringent air pollution regulations, but they serve an important purpose. Their low capital cost and their maintenance-free operation make them ideal for use as precleaners for more expensive final control devices such as baghouses or electrostatic precipitators. In addition to use for pollution control work, cyclones are used extensively in process industries; for example, they are used for recovering and recycling certain catalysts in petroleum refineries and for recovering freeze-dried coffee in food processing plants.

In the past, cyclones have often been regarded as low-efficiency collectors. However, efficiency varies greatly with particle size and cyclone design. During the last few decades, advanced design work has greatly improved cyclone performance. Current literature from some of the cyclone manufacturers advertises cyclone that have efficiencies greater than 98% for particles larger than 5 microns, and others that routinely achieve efficiencies of 90% for particles larger than 15 20 microns.

In general, operating costs increase with efficiency (higher efficiency requires higher inflow pressure), and three categories of cyclones are available: high efficiency, conventional, and high throughput.

Typical efficiency curves for these three types of cyclones are presented in the figure.

7Standard Cyclone Dimensions

Extensive work has been done to determine in what manner dimensions of cyclones affect performance. In some classic work that is still used today, Shepherd and Lapple (1939, 1940) determined optimum dimensions for cyclones.

All dimensions were related to the body diameter of the cyclone so that their results could be applied generally. Subsequent investigators reported similar work, and the so-called standard cyclones were born.

The table on the next slide summarizes the dimensions of standard cyclones of the three types mentioned previously. The side figure illustrates the various dimensions used in the table.

Standard cyclone dimensions

0.40.3750.40.250.40.375Diameter of Dust Outlet,Dd/D

2.02.52.02.02.52.5Length of Cone,Lc/D

1.71.51.752.01.41.5Length of Body,Lb/D

0.850.8750.60.6250.50.5Length of Vortex Finder,S/D

0.750.750.50.50.40.5Diameter of Gas Exit,De/D

0.350.3750.250.250.210.2Width of Inlet,W/D

0.80.750.50.50.440.5Height of Inlet,H/D

1.01.01.01.01.01.0Body Diameter,D/D

(6)(5)(4)(3)(2)(1)

High ThroughputConventionalHigh Efficiency

Cyclone Type

SOURCES: Columns (1) and (5) = Stairmand, 1951; columns (2), (4) and (6) = Swift, 1969; columns (3) and sketch = Lapple, 1951.

8Cyclone Theory

Collection Efficiency

A very simple model can be used to determine the effects of both cyclone design and operation on collection efficiency.

In this model, gas spins through a number Ne of revolutions in the outer vortex. The value of Ne can be approximated by

+=2

1 cbe

LLH

N

whereNe = number of effective turnsH = height of inlet duct (m or ft)Lb = length of cyclone body (m or ft)Lc = length (vertical) of cyclone cone (m or ft).

wheret = time spent by gas during spiraling descent (sec)D = cyclone body diameter (m or ft)Vi = gas inlet velocity (m/s or ft/s) = Q/WHQ = volumetric inflow (m3/s or ft3/s)W = width of inlet (m or ft).

ie VNDt /=

To be collected, particles must strike the wall within the amount of time that the gas travels in the outer vortex. The gas residence time in the outer vortex is

The maximum radial distance traveled by any particle is the width of the inlet duct W. The centrifugal force quickly accelerates the particle to its terminal velocity in the outward (radial) direction, with the opposing drag force equaling the centrifugal force. The terminal velocity that will just allow a particle initially at distance W away from the wall to be collected in time is

Vt = W / t

where Vt = particle terminal velocity in the radial direction (m/s or ft/s).

9The particle terminal velocity is a function of particle size.

Assuming Stokes regime flow (drag force = 3dpVt) and spherical particles subjected to a centrifugal force (mv2/r, with m = mass of particle in excess of mass of gas displaced, v = Vi of inlet flow, and r = D/2), we obtain

DVd

V ipgpt

9)( 22=

whereVt = terminal velocity (m/s or ft/s)dp = diameter of the particle (m or ft)p = density of the particle (kg/m3)g = gas density (kg/m3) = gas viscosity (kg/m.s).

Substitution of the 2nd equation into the 3rd eliminates t. Then, setting the two expressions for Vt equal to each other and rearranging to solve for particle diameter, we obtain

2/1

)(9

= gpiep VNWd

It is worth noting that in this expression, dp is the size of the smallest particle that will be collected if it starts at the inside edge of the inlet duct. Thus, in theory, all particles of size dp or larger should be collected with 100% efficiency.

Note that the units must be consistent in all equations. One consistent set is m for dp, R and W; m/s for Vi and Vt; kg/m.s for ; and kg/m3 for p and g. An equivalent set in English units is ft for dp, R and W; ft/sec for Vi and Vt; lbm/ft.sec for ; and lbm/ft3 for p and g.

10

Collection Efficiency

The preceding equation shows that, in theory, the smallest diameter of particles collected with 100% efficiency is directly related to gas viscosity and inlet duct width, and inversely related to the number of effective turns, inlet gas velocity, and density difference between the particles and the gas.

In practice, collection efficiency does, in fact, depend on these parameters. However, the model has a major flaw: It predicts that all particles larger than dp will be collected with 100% efficiency, which is incorrect.

Lapple (1951) developed a semi-empirical relationship to calculate a 50% cut diameterdpc, which is the diameter of particles collected with 50% efficiency. The expression is

2/1

)(29

= gpiepc VNWd

where dpc = diameter of particle collected with 50% efficiency.

Note the similarity between the last two equations. The only difference is a factor 2 in the denominator.

Particle collection efficiency versus particle size ratio for standard conventional cyclones

Lapple then developed a general curve for standard conventional cyclones to predict the collection efficiency for any particle size (see side figure).

If the size distribution of particles is known, the overall collection efficiency of a cyclone can be predicted by using the figure.

Theodore and DePaola (1980) then fitted an algebraic equation to the curve, which makes Lapples approach more precise and more convenient for application to computers. The efficiency of collection of any size of particle is given by

( )2/1 1 pjpcj dd+=where

j = collection efficiency of particles in the jth size range (0 < j < 1)dpj = characteristic diameter of the jth particle size range (in m).

100%, all particles collected

O%, no particle collected

50%, 1 particle in 2is collected

dpc

11

The overall efficiency of the cyclone is a weighted average of the collection efficiencies for the various size ranges, namely

Mm jj =

where = overall collection efficiency (0 < < 1)mj = mass of particles in the jth size rangeM = total mass of particles.

Example of Cyclone Analysis

Given:

Conventional type (standard proportions)D = 1.0 mFlow rate = Q = 150 m3/minParticle density = p = 1600 kg/m3Particle size distribution (see below)

Particle size % mass in that size range(dp) (m)0-2 m 1.0%2-4 m 9.0%4-6 m 10.0%6-10 m 30.0%10-18 m 30.0%18-30 m 14.0%30-50 m 5.0%50-100 m 1.0%

100%

Question:

What is the collection efficiency?

12

Solution:

62

1 =

+= cbe LLHN 20m/s=m/min1200125.0 2 === DQ

WHQVi

m 5.79=m1079.5)(6

25.029

)(29 6

=== gpigpiepc VD

VNWd

70.6%1.00

0.994%0.0199.4%7550 - 100

4.897%0.0597.9%4030 50

11.953%0.1494.5%2418 30

25.613%0.3085.4%1410 18

19.678%0.3065.6%86 10

4.268%0.1042.7%54 6

1.903%0.0921.1%32 4

0.029%0.012.9%10 2

Contributionto efficiency

x mMass

fractionm

Efficiency

Average sizedp

(in m)Size range

(in m)

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/DfE.pdf1Primary goal:SUSTAINABILITY

(responsibility toward future generations)

In addition:GREEN TECHNOLOGIES(pollution avoidance rather than pollution treatment)

DESIGN FOR ENVIRONMENT(green design)

Basic approach:INDUSTRIAL ECOLOGY

(imitation of nature)

Imitation of ecosystem:ECO-INDUSTRIAL PARKS

(closing material loops,energy efficiency)

DEMATERALIZATION(doing with less)

DESIGN FOR RECYCLING(to promote material loops)

THE IMPORTANCE OF DESIGN:

70% of costs of product development, manufacture and use are decided in early design stages(1991 Natl Research Council Report titled Improving Engineering Design)

Examples:

GM truck transmissions: 70% of costs decided at design stage

Rolls Royce: 80% of costs decided at design stage, as determined from an average among 2000 parts

2The various levels of DESIGN

DFM Design for Manufacturability So that the product can be made easilyand at reasonable cost

DFL Design for Logistics So that all production activities are well orchestrated

DFT Design for Testability So that the quality of the product may be conveniently checked

DFP Design for Pricing So that the product will sell

DFSL Design for Safety & Liability So that the product is safe to useand the company is not held liable

DFR Design for Reliability So that the product works well

DFS Design for serviceability So that service after sale can be offeredat a reasonable cost to the company

etc. etc. to be added:

DFE Design for Environment To reduce or eliminate environmental impacts from cradle to grave

3The various levels of DESIGN for ENVIRONMENT:

DFM Design for Manufacturability To enable P2 during manufacturingFor less materialFor fewer different materialsFor safer materials and processes

DFEE Design for Energy Efficiency For reduced energy demand during useFor flexible energy use

DFSe Design for Serviceability For ease of repairs longer lifeFor recapture of used/broken parts

DFMo Design for Modularity To ease upgrading Delay replacementTo ease serviceability

DFD Design for Disassembly To promote re-use of componentsFor quicker and cheaper disassemblyFor more complete disassembly

DFR Design for Recycling For greater materials recoveryFor easier materials identificationFor safer disposal of non-recyclables

DFER Design for Energy Recovery For safe incineration of residuesFor composting of residues

DFC Design for Compliance To meet regulations more easilyTo prepare for future regulations

What to consider inDESIGN FOR ENVIRONMENT

1. Product or process?

Make the same product in a different wayex: as to minimize energy consumption or generation of by-products

Make the essentially the same product, but with different materials

Make a different product that fulfills the same function

2. At which level?

Microscale: Part of a productA unit of production

Mesoscale: The entire productThe entire factory

Macroscale: Meeting the function (service) in a new wayRethinking the industry-environment relation (social concerns)

4Redesign of PROCESSES versus redesign of PRODUCTS

PROCESSES

Many times the only way to approach the redesign (ex. paper, steel)Rethink the way the product is madeRethink what enters the manufacturing (entry materials) Rethink technology of specific processes (ex. solvents)Consider what goes out besides the product itself (by-products or waste?)

Barriers: - Technological (alternative is not technically feasible)- Cost of research and development- Risk associated with the unknowns- Corporate inertia (Dont mess with success!)

PRODUCTS

Consider function rather than the object: Can this function be met with a smaller product, with a more benign product? Or, at the limit, could it be met as a service without any material product?Do not forget to also rethink the packaging of the product

Barriers: - Technological (alternative is not technically feasible)- Ergonomic, Safety (alternative may be a misfit or unsafe)- Societal (people are not prepared for the alternative)

Design for Environment

Process changes Product changes

Improved operatingpractices

Maintenance Efficient management Stream segregation Better material handling Inventory control Training

Technologychanges

Layout changes Increased automation Improved equipment New technology

Change ofmaterials

Material purification Less material variety Avoidance of toxics

12

3

4

1 4 in order of difficulty and commitment on the part of the company

5Example of Design for Environment applied to a manufacturing process

Advantages: - Less air to be dust-free and less chance of dust intrusion;- In the absence of personnel inside the controlled volume,

one can also take advantage of an oxygen-free (pure nitrogen) atmosphere to reduce oxidation or other undesirable side effect.

6The story of Ray Anderson and Interface, Inc.

Company founded in 1973Aims to become a sustainable corporation by 2020 Carpet by the square

7LEVELS OF DESIGN FOR ENVIRONMENT

From tinkering at the margin to the social revolution!

Example: Automobile

1. Re-design of parts: Aluminum or plastic radiator capLonger-lasting tires and batteriesAluminum or steel engines

2. Re-design of assembly: Eco-friendly paintingFacilitating disassemblyRecycling of plastics

3. Re-design of automobile itself: Alternative fuels (ex. ethanol, methanol)Alternative powertrains (hybrids, fuel cells)

4. Re-design of transportation systems: Smart highwaysPublic transportation

5. Re-thinking the need for mobility: Virtual office (telecommuting)Community layout

Towarddeep ecology

Towardshallowredesign

ELEMENTS OF DESIGN for ENVIRONMENT

At a minimum:

Compliance with all applicable (federal, state and local) environmental regulations

Compliance with existing permit requirementsfor discharge and emissions

Process loadings not to exceed existing treatment facilities

Going beyond mere compliance:

Procurement of renewable resources and/or recycled materials

Energy-efficient, low-waste manufacturing

No toxics during manufacture or inside product

Energy-efficient product

Long-lasting product

Dematerialized product

Maximization of recycling after product use

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/Disinfection.pdf1Disinfection(Nazaroff & Alvarez-Cohen, Section 6.D.1)

Purpose:

To reduce risk of disease transmission associated with either drinking or wastewater.

Objective:

To kill or inactivate microorganisms

Methods:

Quite expensiveNo residual left

Very effectiveChemical disinfection by ozone

Cheap to expensiveMay create harmful by-products

Very effectiveLeaves lasting residuals

Chemical disinfection by chlorine or chlorinated compound

Cheap and convenientRequires clear water

Limited efficacyIrradiation with UV light

Very energy intensiveVery effectiveBoiling of water

Disinfection by chlorine

The active ingredient that kills microorganisms is hypochlorous acid, HOCl.

HOCl must be made in the water from a chlorinated precursor. The most common method is the injection of pure chlorine gas, Cl2.

More expensive but safer than handling chlorine gas is the use of sodium hypochlorite (NaOCl commonly called bleach) or calcium hypochlorite (Ca(OCl)2), a solid.

U.S. standards for drinking water:- minimum contact of 45 minutes- minimum residual chlorine concentration of 1.1 mg/L

(from initial dose of 2 to 5 mg/L)

U.S. practice for end of wastewater treatment:- injection of 40 to 60 mg/L.

2Chlorine chemistry in pure water

Let us consider the use of chlorine gas as the disinfection method.

First, Cl2 in gas (from compressed bottle, handled with care!) penetrates the water, following Henrys Law:

Cl2(g) Cl2(aq) with KH = 0.062 M/atm at 25oC

Aqueous Cl2 reacts rapidly with water to form hypochlorous acid:

Cl2 + H2O HOCl + H+ + Cl

with constant24

2

M105]Cl[

]Cl[]H[]HOCl[ + ==K

The preceding two reactions are highly tilted to the right, meaning that chlorine gas most easily goes into hypochlorous acid in the water.

active ingredient

However, hypochlorous acid HOCl is not just consumed in killing microorganisms; it is also decaying spontaneously into:

HOCl H+ + OClwith constant

To keep the previous reaction tilted to the left (in favor of HOCl and against OCl), the pH must be controlled.

The hypochlorite ion OCl is much less potent as a disinfectant than HOCl.

M106.2]HOCl[]OCl[]H[ 8

2

+==K

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/EIPs.pdf1Benefits of Eco-Industrial Parks

1. Monetary benefits to companies:

Production costs (purchasing unwanted by-products form others at bargain prices; selling its own by-products) Energy consumption (less transportation) Waste management (on-site, or even being able to sell what would otherwise be waste) Costs of compliance Cost of some R&D (shared with other companies)

2. Environmental benefits:

Demand on natural resources Waste (in all forms: solid waste, air emissions,

wastewater) Chances of accidents in transportation (pipes instead of

trucks)

3. Societal benefits:

Better economy more jobsCheap heating (in both park and residential neighborhoods)Cleaner air, cleaner water better health Demand on sewer system, landfill etc.

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/Energy-Argonne.pdf1Solar EnergyChallenges and Opportunities

Solar EnergyChallenges and Opportunities

with

Nathan Lewis, CaltechArthur Nozik, NRELMichael Wasielewski, NorthwesternPaul Alivisatos, UC-Berkeley

with

Nathan Lewis, CaltechArthur Nozik, NRELMichael Wasielewski, NorthwesternPaul Alivisatos, UC-Berkeley

George Crabtree

Materials Science DivisionArgonne National Laboratory

Preview

Grand energy challenge- double demand by 2050, triple demand by 2100

Sunlight is a singular energy resource- capacity, environmental impact, geo-political security

Breakthrough research directions for mature solar energy- solar electric- solar fuels- solar thermal

2World Energy Demand

EIA Intl Energy Outlook 2004http://www.eia.doe.gov/oiaf/ieo/index.html

0

10

20

30

40

50

%

World Fuel Mix 2001oil

gas coal

nucl renew

85% fossil

2100: 40-50 TW 2050: 25-30 TW

0.00

5.00

10.00

15.00

20.00

25.00

1970 1990 2010 2030

TW

World Energy Demand total

industrial

developingUS

ee/fsu

energy gap~ 14 TW by 2050~ 33 TW by 2100

Hoffert et al Nature 395, 883,1998

Fossil: Supply and Security

EIA: http://tonto.eia.doe.gov/FTPROOT/presentations/long_term_supply/index.htm

R. Kerr, Science 310, 1106 (2005)

1900 1950 2000 2050 2100

Bbbl

/yr

10

20

30

40

50World Oil Production

2016

2037

2% demand growthultimate recovery:

3000 Bbbl

When Will Production Peak?

gas: beyond oilcoal: > 200 yrs

production peakdemand exceeds supply

price increasesgeo-political restrictions

World Oil Reserves/Consumption2001

OPEC: Venezuela, Iran, Iraq, Kuwait, Qatar, Saudi Arabia, United Arab Emirates, Algeria, Libya, Nigeria, and Indonesiahttp://www.eere.energy.gov/vehiclesandfuels/facts/2004/fcvt_fotw336.shtml

uneven distribution insecure access

3Fossil: Climate Change

Relaxation timetransport of CO2 or heat to deep

ocean: 400 - 1000 years

J. R. Petit et al, Nature 399, 429, 1999 Intergovernmental Panel on Climate Change, 2001

http://www.ipcc.chN. Oreskes, Science 306, 1686, 2004

D. A. Stainforth et al, Nature 433, 403, 2005

Climate Change 2001: T he Scientific Basis, Fig 2.22

12001000 1400 1600 1800 2000

240

260

280

300

320

340

360

380

Year AD

Atm

osph

eric

CO

2(p

pmv) Tem

perature (C)

- 1.5

- 1.0

- 0.5

0

0.5

1.0

1.5

-- CO2-- Global Mean Temp300

400

500

600

700

800

- 8

- 4

0

+ 4

400 300 200 100Thousands of years before present

(Ky BP)

0

T r

elat

ive

to

pres

ent

(C)

CH4(ppmv)

-- CO2-- CH4-- T

325

300

275

250

225

200

175

CO2(ppmv)

CO2 in 2004: 380 ppmv

The Energy Alternatives

Fossil Nuclear Renewable Fusion

energy gap~ 14 TW by 2050~ 33 TW by 2100

10 TW = 10,000 1 GW power plants1 new power plant/day for 27 years

no single solutiondiversity of energy sources

required

4Renewable EnergySolar

1.2 x 105 TW on Earths surface36,000 TW on land (world)

2,200 TW on land (US)

Biomass5-7 TW gross (world)

0.29% efficiency for all cultivatable landnot used for food

Hydroelectric

Geothermal

Wind2-4 TW extractable

4.6 TW gross (world)1.6 TW technically feasible0.6 TW installed capacity

0.33 gross (US)9.7 TW gross (world)0.6 TW gross (US)

(small fraction technically feasible)

Tide/Ocean Currents 2 TW gross

energy gap~ 14 TW by 2050~ 33 TW by 2100

Solar Energy Utilization

Solar ElectricSolar Fuel Solar Thermal

.0002 TW PV (world).00003 TW PV (US)

$0.30/kWh w/o storage

CO2

sugar

H2O

O2

NC O

N CH3

NN

NN

HHH

naturalphotosynthesis

artificialphotosynthesis

50 - 200 Cspace, water

heating

500 - 3000 Cheat engines

electricity generationprocess heat

1.5 TW electricity (world)$0.03-$0.06/kWh (fossil)

1.4 TW biomass (world)0.2 TW biomass sustainable (world)

~ 14 TW additional energy by 2050

0.006 TW (world)

11 TW fossil fuel (present use)

2 TW space and water heating (world)

H2O

O2CO2

H2, CH4CH3OH

e-

h+

5BES Workshop on Basic Research Needs for Solar Energy Utilization April 21-24, 2005

Workshop Chair: Nathan Lewis, CaltechCo-chair: George Crabtree, Argonne

Panel ChairsArthur Nozik, NREL: Solar ElectricMike Wasielewski, NU: Solar Fuel

Paul Alivisatos, UC-Berkeley: Solar Thermal

Plenary SpeakersPat Dehmer, DOE/BESNathan Lewis, Caltech

Jeff Mazer, DOE/EEREMarty Hoffert, NYU

Tom Feist, GE

200 participantsuniversities, national labs, industry

US, Europe, Asia EERE, SC, BES

ChargeTo identify basic research

needs and opportunities in solar electric, fuels, thermal and

related areas, with a focus on new, emerging and scientifically challenging areas that have the potential for significant impact

in science and technologies.

TopicsPhotovoltaics

PhotoelectrochemistryBio-inspired Photochemistry

Natural Photosynthetic Systems Photocatalytic Reactions

Bio Fuels Heat Conversion & Utilization

Elementary Processes Materials Synthesis

New Tools

Basic Research Needs for Solar Energy The Sun is a singular solution to our future energy needs

- capacity dwarfs fossil, nuclear, wind . . .- sunlight delivers more energy in one hour

than the earth uses in one year- free of greenhouse gases and pollutants- secure from geo-political constraints

Enormous gap between our tiny use of solar energy and its immense potential

- Incremental advances in todays technologywill not bridge the gap

- Conceptual breakthroughs are needed that come only from high risk-high payoff basic research

Interdisciplinary research is requiredphysics, chemistry, biology, materials, nanoscience

Basic and applied science should couple seamlesslyhttp://www.sc.doe.gov/bes/reports/abstracts.html#SEU

6Solar Energy Challenges

Solar electricSolar fuelsSolar thermalCross-cutting research

Solar Electric

Despite 30-40% growth rate in installation, photovoltaics generate

less than 0.02% of world electricity (2001)less than 0.002% of world total energy (2001)

Decrease cost/watt by a factor 10 - 25 to be competitive with fossil electricity (without storage)

Find effective method for storage of photovoltaic-generated electricity

7Cost of Solar Electric Power

competitive electric power: $0.40/Wp = $0.02/kWhcompetitive primary power: $0.20/Wp = $0.01/kWh

assuming no cost for storage

I: bulk SiII: thin film

dye-sensitizedorganic

III: next generationCost $/m2

$0.10/Wp $0.20/Wp $0.50/Wp

Effi

cien

cy %

20

40

60

80

100

100 200 300 400 500

$1.00/Wp

$3.50/Wp

Thermodynamic limit at 1 sun

Shockley - Queisserlimit: single junction

module cost onlydouble for balance of system

Revolutionary Photovoltaics: 50% Efficient Solar Cells

present technology: 32% limit for single junction one exciton per photon relaxation to band edge

multiple junctions multiple gaps multiple excitonsper photon

3 I

hot carriers

3 V

rich variety of new physical phenomenachallenge: understand and implement

Eg

lost toheat

nanoscaleformats

8Organic Photovoltaics: Plastic Photocells

opportunitiesinexpensive materials, conformal coating, self-assembling fabrication,

wide choice of molecular structures, cheap solar paint

challenges low efficiency (2-5%), high defect density, low mobility, full

absorption spectrum, nanostructured architecture

donor-acceptor junction

polymer donorMDMO-PPV

fullerene acceptorPCBM

O

O

()n

OOMe

OOMe

Solar Energy Challenges

Solar electricSolar fuelsSolar thermalCross-cutting research

9Solar Fuels: Solving the Storage Problem

Biomass inefficient: too much land area. Increase efficiency 5 - 10 times

Designer plants and bacteria for designer fuels: H2, CH4, methanol and ethanol

Develop artificial photosynthesis

Leveraging Photosynthesis for Efficient Energy Production

photosynthesis converts ~ 100 TW of sunlight to sugars: natures fuel low efficiency (< 1%) requires too much land area

Modify the biochemistry of plants and bacteria

- improve efficiency by a factor of 510

- produce a convenient fuel methanol, ethanol, H2, CH4

Scientific Challenges- understand and modify genetically controlled biochemistry that limits growth- elucidate plant cell wall structure and its efficient conversion to ethanol or other fuels- capture high efficiency early steps of photosynthesis to produce fuels like ethanol and H2- modify bacteria to more efficiently produce fuels- improved catalysts for biofuels production

hydrogenase2H+ + 2e- H2

switchgrass

10

chlamydomonas moewusii

10

photosystem II

Biology: protein structures dynamically control energy and charge flow Smart matrices: adapt biological paradigm to artificial systems

Scientific Challenges engineer tailored active environments with bio-inspired components novel experiments to characterize the coupling among matrix, charge, and energy multi-scale theory of charge and energy transfer by molecular assemblies design electronic and structural pathways for efficient formation of solar fuels

Smart Matrices for Solar Fuel Production

h

charge charge energy energy

h

smart matrices carry energy and charge

Efficient Solar Water Splitting

demonstrated efficiencies 10-18% in laboratory

Scientific Challenges cheap materials that are robust in water catalysts for the redox reactions at each electrode nanoscale architecture for electron excitation transfer reaction

+

-

H2O2

11

Solar-Powered Catalysts for Fuel Formation

new catalysts targeted for H2, CH4, methanol and ethanol

are needed

Prototype Water Splitting Catalyst

multi-electron transfercoordinated proton transfer

bond rearrangement

uphill reactions enabled by sunlight

simple reactants, complex productsspatial-temporal manipulation of

electrons, protons, geometry

2 H2O

O2

4e-

4H+

CO2

HCOOHCH3OHH2, CH4

Cat Cat

oxidation reduction

Solar electricSolar fuelsSolar thermalCross-cutting research

Solar Energy Challenges

12

Solar Thermal

heat is the first link in our existing energy networks solar heat replaces combustion heat from fossil fuels solar steam turbines currently produce the lowest cost solar electricity challenges:

new uses for solar heatstore solar heat for later distribution

fuel heatmechanical

motion electricity

space heat

process heat

Solar Thermochemical Fuel Productionhigh-temperature hydrogen generation

500 C - 3000 C

Scientific Challengeshigh temperature reaction kinetics of

- metal oxide decomposition - fossil fuel chemistry

robust chemical reactor designs and materials

fossil fuelsgas, oil, coal

SolarReforming

SolarDecomposition

SolarGasification

CO2 , CSequestration

Solar H2

concentrated solar power

Solar ReactorMxOy x M + y/2 O2

Hydrolyserx M + y H2O MxOy + yH2

H2

M

MxOy

MxOy

H2O

1/2 O2

concentratedsolar power

A. Streinfeld, Solar Energy, 78,603 (2005)

13

Thermoelectric Conversion

TAGS

0 200 400 600 800 1000 1200 1400RT

2.5

1.5

0.5Z

T

CsBi4Te6

Bi2Te3

LaFe3CoSb12

Zn4Sb3

Si Ge

PbTe

Temperature (K)

Bi2Te3/Sb2Te3superlattice

PbTe/PbSesuperlattice

LAST-18AgPbAgPb1818SbTeSbTe2020

figure of merit: ZT ~ (/) TZT ~ 3: efficiency ~ heat engines

no moving parts

Scientific Challengesincrease electrical conductivitydecrease thermal conductivity

nanoscale architecturesinterfaces block heat transport

confinement tunes density of statesdoping adjusts Fermi level

nanowire superlattice

thermal gradient electricity

Mercouri Kanatzidis

Solar electricSolar fuelsSolar thermalCross-cutting research

Solar Energy Challenges

14

Molecular Self-Assembly at All Length Scales

Scientific Challenges- innovative architectures for coupling light-harvesting, redox, and catalytic components- understanding electronic and molecular interactions responsible for self-assembly- understanding the reactivity of hybrid molecular materials on many length scales

The major cost of solar energy conversion is materials fabricationSelf-assembly is a route to cheap, efficient, functional production

biologicalphysical

Defect Tolerance and Self-repair

Understand defect formation in photovoltaic materials and self-repair mechanisms in photosynthesis

Achieve defect tolerance and active self-repair in solar energy conversion devices, enabling 2030 year operation

the water splitting protein in Photosystem IIis replaced every hour!

15

Nanoscience

N

theory and modelingmulti-node computer clusters

density functional theory10 000 atom assemblies

manipulation of photons, electrons, and molecules

quantum dot solar cells

artificialphotosynthesis

naturalphotosynthesis

nanostructuredthermoelectrics

nanoscale architecturestop-down lithography

bottom-up self-assemblymulti-scale integration

characterizationscanning probes

electrons, neutrons, x-rayssmaller length and time scales

Solar energy is interdisciplinary nanoscience

TiO2nanocrystals

adsorbedquantum dots

liquidelectrolyte

PerspectiveThe Energy Challenge

~ 14 TW additional energy by 2050~ 33 TW additional energy by 2100

13 TW in 2004

Solar Potential125,000 TW at earths surface36,000 TW on land (world)2,200 TW on land (US)

Breakthrough basic research needed

Solar energy is a young science- spurred by 1970s energy crises- fossil energy science spurred by industrial revolution - 1750s

solar energy horizon is distant and unexplored

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/EnvironmentalChem1.pdf1Inorganic impurities in water and air(Nazaroff & Alvarez-Cohen, Section 2.C.4)

Distinction between organic and inorganic compoundsorganic compounds = those with Carbon, except for elemental C, CO, CO2, HCO3- and CO32-

Acids & Bases:acids make H+; common acids are HCl, HOCl, H2S, H2SO4, H2SO3, HNO3bases make OH-; common bases are NH3, NaOH, CaCO3, Ca(OH)2OH- takes away H+ via OH- + H+ H2O

Dissolution in water ionscations are positive, anions are negativemost common ions: NH4+, Na+, Ca2+, Cl-, SO42-, HCO3-, CO32-

Electroneutrality Principle: For every extra electron on an anion there is a missing electronon a cation. (Initial electrical neutrality, then conservation of electrons.)

zi [Ai] = 0where zi = charge of ion Ai (+ for cation, - for anion), most often +1, +2, -1 or -2

[Ai] = molar concentration of ion Ai

Example:

Carbon dioxide CO2 from the atmosphere partially dissolves in water. In water, it then forms carbonic acid H2CO3. In turn, H2CO3 dissolves into H+ and bicarbonate ion HCO3-, and HCO3- into another H+ and carbonate ion CO32-. Thus, the following reactions take place in water exposed to the atmosphere:

CO2 + H2O H2CO3H2CO3 H+ + HCO3-HCO3- H+ + CO32-

In addition, water H2O always decomposes slightly into H+ and OH-:

H2O H+ + OH-

Electroneutrality requires:

[H+] = [HCO3-] + 2[CO32-] + [OH-]

2Alkalinity:

= the capacity of water to neutralize acids by containing naturally occurring ions that can buffer contaminating acidity.

A = [OH-] + [HCO3-] + 2[CO32-] - [H+]

By virtue of electroneutrality, pristine water has zero alkalinity.

Inorganic pollutants in the atmosphere:

Many types, most common ones are:

O3, ozoneCO, carbon monoxideN2O, nitrous oxideNO, nitrogen monoxideNO2, nitrogen dioxidePM, Particulate Matter

High up in the stratosphere:

chlorinated compounds (from CFCs)

In addition:

Heavy metals are usually toxic, such as lead (Pb), Cadmium (Cd)

There is much talk nowadays about greenhouse gases, especially carbon dioxide (CO2) and methane (CH4). It is improper to refer to these gases as pollutants, but they are of concern in large atmospheric quantities because of their effect on climate.

3Organic impurities in water and air(Nazaroff & Alvarez-Cohen, Section 2.C.5)

Thousands of organic compounds are found in the air and water, many of which are benign and some are harmful.

Problems that these cause:

In air: lung irritationcancernervous damagedisruption of ecosystemsodor, visibility

In water: canceralgal growth, depletion of dissolved oxygentaste, color

Examples of harmful organic chemicals

Formaldehyde: HCHOcomponent in resins for bonding & laminatingalso found in wood combustion & tobacco smokecreates odor and irritates mucous membranesmay cause cancerNew building smell !!!

Chlorofluorocarbons: ex. CCl2F2 CHClF2unknown to natureused as refrigerants, propellants, foaming agentscauses depletion of stratospheric ozone

skin cancer at ground level

Benzene: C6H6basic block of aromatic compoundsused as solvent, gasoline additiveproduced by incomplete petroleum combustionfound in tobacco smokeknown human carcinogen

4Vinyl Chloride: C2H3ClTrichloroethylene (TCE): C2HCl3Perchloroethylene (PCE, perc): C2Cl4

solvents, cleanersfeedstocks for other chemicalscontaminate soil & groundwater

Dichloromethane: CH2Cl2Trichloroethane (TCA): C2H3Cl3

same as above

Dioxin:

compounds with two benzene rings bridged by two oxygen atomsand some chlorine replacing hydrogen

not produced intentionally by-product during use of chlorinated compoundsalso produced during incineration of plastics (PVC)

one of the most toxic organic substances produced by human activitypersistent in the environmenthurts wildlife and damages ecosystems

Herbicides and insecticides

used in agriculture to control monoculturestoxic to humans inside the factory where they are manufacturedresiduals find their way on our food and in our groundwater

5In water,

one is often not only concerned by the presence of contaminants but also by the depletion of what should be there, especially

DISSOLVED OXYGEN.

Dissolved Oxygen (DO, in short) is vital to aquatic life. As a rule, the more oxygen is dissolved in the water, the better for the fish and other desirable forms of life. Low DO levels do not make good habitats for fish and promote undesirable forms of life such as algae. Low oxygen levels are accompanied by murkiness, sliminess and, sometimes, odor.

At the extreme, water may become anoxic, that is, completely lacking dissolved oxygen. Bacteria living in anoxic water produce methane (CH4), which is flammable, and hydrogen sulfide (H2S), which has the smell of rotten egg.

The minimum recommended amount of DO for a healthy fish population in a stream is 5 mg/L. The EPA recommends at least 8 mg/L for the more desirable cold-water species, such as trout and salmon, during their embryonic and larval stages and the first 30 days after hatching.

Often, pollution in surface waters is not measured in terms of the concentrations of the individual contaminants but is measured in terms of their aggregate potential for oxygen depletion. This is called the Biochemical Oxygen Demand (BOD).

Substances contributing to BOD are food for bacteria, and the more the bacteria feed on these, the more they also take oxygen (like us humans, who both eat and breathe).

Few cells + organic matter + O2 more cells + CO2 + H2O + etc.

The definition is:

1 mg/L of BOD will, after uptake by bacteria, decrease the DO level by 1 mg/L.

Note: 1 mg/L of BOD may correspond to more or less than 1 mg/L of the offensive substance.

BOD is determined in the laboratory by measuring the depletion of dissolved oxygen in the contaminated water placed in a closed container, over the course of several days.

BOD

6Chemical Equilibrium(Nazaroff & Alvarez-Cohen, Section 3.A.2)

A system is in chemical equilibrium when1. It does not vary in time (steady state)2. It is well mixed3. There is no net flow of mass, heat or species with the surroundings4. The net rate of all chemical reactions is zero.

Many chemical reactions in the environment are two-way reactions.Symbolically,

A + B C + D

during which A and B react to produce C and D, and at the same time C and D react to produce A and B.

The rate of reaction between A and B is Rforward = kf [A] [B]The rate of reaction in the reverse direction is Rreverse = kr [C] [D]

Chemical equilibrium exists when kf [A] [B] = kr [C] [D].

Equilibrium constant:

from equilibrium the equilibrium equation kf [A] [B] = kr [C] [D]

we can writeK

kk ==

r

f

[A][B][C][D]

The constant K is determined in the laboratory. It generally varies with temperature.

Generalization to reaction with arbitrary numbers in the stoichiometry:

a A + b B c C + d D

Rforward = kf [A]a [B]b Rreverse = kr [C]c [D]d

Equilbrium exists when kf [A]a [B]b = kr [C]c [D]d

Kkk

ba

dc

==r

f

[B][A][D][C]

7Chemical kinetics(Nazaroff & Alvarez-Cohen, Section 3.A.3)

This is the answer to what happens when chemical equilibrium is not reached.

For example, take the reaction

A + B C + D

As A and B come into contact with each other, they react and the reaction rate is

Rforward = kf [A] [B]

This rate depletes the amounts of both A and B, and adds to the amounts of C and D:

[A][B][D][C]

[A][B][B][A]

fforward

fforward

kRdtd

dtd

kRdtd

dtd

+=+==

===

But, at the same time, C and D come in contact, too, and carry their own (reverse) reaction, at the rate of

Rreverse = kr [C] [D]

This rate depletes the amounts of both C and D, and adds to the amounts of A and B:

[C][D][D][C]

[C][D][B][A]

rreverse

rreverse

kRdtd

dtd

kRdtd

dtd

===

+=+==

Putting the two reactions together, we have:

[C][D][A][B][D][C]

[C][D][A][B][B][A]

rfreverseforward

rfreverseforward

kkRRdtd

dtd

kkRRdtd

dtd

+=+==

+=+==

8Finally, in a system with open boundaries, we can add the imports and exports:

++=

++=

++=

++=

sinks sources exports imports

[C][D][A][B][D][D][D]

[C][D][A][B][C][C][C]

[A][B][C][D][B][B][B]

[A][B][C][D][A][A][A]

rfoutlets

outoutininlets

in

rfoutlets

outoutininlets

in

froutlets

outoutininlets

in

froutlets

outoutininlets

in

VkVkQQdtdV

VkVkQQdtdV

VkVkQQdtdV

VkVkQQdtdV

where V is the volume of the system for which the budget is written.

Reaction order

The previous example assumed that it takes two chemicals for each reaction. This is not always the case. For example, when a substance dissolves in water

H2CO3 H+ + HCO3-

The forward reaction needs only one concentration

Rforward = kf [H2CO3]

Nomenclature:

Second-order reaction:

First-order reaction:

Zeroth-order reaction: kdt

d

kdt

d

kdt

d

=

=

=

]A[

]A[]A[

]B][A[]A[

Note: the units of the k coefficients vary!

9Equilibrium & Kinetics of Phase Change(Nazaroff & Alvarez-Cohen, pages 90-92)

Consider a closed vessel half filled with water and topped with air. As we can expect, some of the water evaporates into the air, and some of the water vapor in the air will condense back into liquid water. The two processes of evaporation and condensation will eventually come to equilibrium and set a certain level of moisture in the air above the water.

SVHC

Hk

Hk

dtdC

SCkSk

RRdtdCV

CSCkRSSkR

air wherece

ce

condenevapair

(moisture)air in the water ofion concentratwherecconden:onCondensati

contact of surfacewhereeevap :nEvaporatio

====

====

ce CHk

Hk

dtdC =

If (initially dry air), then the evolution of the water vapor in the air over time is

0)0( ==tC

= tHk

kktC c

c

e exp1)(

Eventually, equilibrium is reached in which

c

e

kkC =

10

Concentration of water vapor in the air can be transformed into partial pressure of water vapor.

Water weighs 18 grams per mole, Thus, the number of moles of water vapor per volume of air is

grams/mole 18megrams/volu

air

water CVn =

and the partial pressure of water is (assuming that it behaves as an ideal gas):

RTCRTVnP

18airwater

water ==

Definition: Relative humidity RH is defined as the ratio of the actual partial pressure of water vapor to the equilibrium (saturation) water vapor pressure:

K)(in 15.3731)(

)1299.06445.09760.13185.13exp()Pa1001325.1()(

%100

4325saturation

saturation

water

TTa

aaaaTPPPRH

==

=

Introduction_to_Environmental_Engineering/Introduction to Environmental Engineering/EnvironmentalChem2.pdf1Dissolution of species in water(Nazaroff & Alvarez-Cohen, Section 3.B.2)

At equilibrium, a fixed ratio is established between the concentration [A] of substance A in the water and its partial pressure PA in the air:

AH PK=[A]or, in reverse,

[A]HPA =This is known as Henrys Law, and H = 1/KH is called Henrys Law constant.

It varies from species to species and is also a function of temperature.

In the environment, water is often in contact with air, and chemicals are exchanged between water and air. An equilibrium is reached in the absence of perturbing processes.

air

water

dissolution

volatilization

[A]

PA

20oC9.10.11C2HCl3Trichloroethylene

20oC180.055C2H3Cl3Trichloroethane

20oC6.70.15C7H8Toluene

20oC120.083C2Cl4Tetrachloroethylene

25oC0.811.24SO2Sulfur dioxide

20oC4.5 x 10-42200C6H6OPhenol

20oC7200.001384O2Oxygen

20oC15000.00067N2Nitrogen

25oC4.8 x 10-62.1 x 105HNO3Nitric acid

20oC0.452.2C10H8Naphthalene

20oC6700.0015CH4Methane

20oC8.70.115H2SHydrogen sulfide

25oC1.6 x 10-46300HCHOFormaldehyde

20oC9.10.11C8H10Ethylbenzene

20oC3.20.31CHCl3Chloroform

20oC10000.0010COCarbon monoxide

25oC290.034CO2Carbon dioxide

20oC4.9 x 10-42040C20H12Benzopyrene

20oC5.60.18C6H6Benzene

25oC0.016162NH3Ammonia

TemperatureH (atm/M)KH (M/atm)FormulaSpecies

(Nazaroff & Alvarez-Cohen, Table 3.B.2, page 96)

2Example: Dissolution of oxygen in water

From the previous table, we note that KH for oxygen is 0.00138 M/atm. We also know that oxygen accounts for 21% of air in the atmosphere. Thus,

mol/L1090.2M1090.2 )atm 121.0)(M/atm 00138.0(

][O

44

2 2

===

= OH PK

Since the molecular weight of oxygen is 2 x 16 = 32 g/mol, we can convert the preceding number in mg/L:

mg/L 28.9g/L 00928.0 )mol/L 1090.2)(g/mol 32(

][ODO4

22

===

=

OMW

After repeating the procedure for various temperatures, we obtain the following table for the equilibrium level of dissolved oxygen in open water.

This quantity is called the Saturated Value of Dissolved Oxygen and is noted DOs.

8.42510.812

8.52411.111

8.72311.310

8.82211.69

9.02111.98

9.22012.27

9.41912.56

9.51812.85

9.71713.14

10.01613.53

10.21513.82

10.41414.21

10.61314.60

(mg/L)(oC)(mg/L)(oC)

DOsTemperatureDOsTemperature

No wonder that active fish prefer cold waters!

This is why there is such good fishing in Maine and Alaska.

EPA recommendation for a healthy fish population: DO 5 mg/LFor better fish, such as trout and salmon, water must have DO 8 mg/L

3Solubility of nonaqueous-phase liquids (NAPL)(Nazaroff & Alvarez-Cohen, pages 97-98)

Some liquids (ex. petroleum, organic solvents) do not readily mix with water but, once in contact with water, become partially dissolved.

Like for Henrys Law in case of water-air contact, an equilibrium is sought for which the concentration of the liquid contaminant dissolved in the water is proportional to the concentratio