waste disposal in stream

8/2/2019 Waste Disposal in Stream

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Self Purification of Stream When the wastewater or the effluent is discharged into a

natural stream, the organic matter is converted intoammonia, nitrates, sulphates, carbon dioxide etc. bybacteria.

In this process of oxidation, the dissolved oxygen contenof natural water is utilized. Due to this, deficiency ofdissolved oxygen is created.

As the excess organic matter is stabilized, the normal cycwill be in a process known as Self-purification wherein tdissolved oxygen is replenished by its reaeration by

atmospheric oxygen of wind.


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Actions During Self-purificatio Dilution:

When wastewater is discharged into the receiving watdilution takes place due to which the concentration o

organic matter is reduced and the potential nuisance sewage is also reduced.

When the dilution ratio is quite high, high available D

higher rate of organic decomposition, reduce pollutioeffects.


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Actions During Self-purificatio Dispersion due to Currents:

The currents, (as rapids, whirlpools, waterfalls andturbulent f low) readily disperse the wastewater in the

stream, preventing local accumulation of pollutants. High velocity accelerates reaeration

reduces the concentration of pollutants.

reduces the time of recovery, though length of stream affecby the wastewater is increased.


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Actions During Self-purificatio Sedimentation:

If the stream velocity is lesser than the scour velocity particles, sedimentation will take place, which will ha

two effects. The suspended solids, which contribute largely the oxygen

demand, will be removed by settling and hence water qualof the downstream is improved.

Due to settled solids, Anaerobic decomposition may takeplace.


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Actions During Self-purificatio

Temperature: At low temperature, the

activities of bacteria is low andhence rate of decomposition

will also be slow, though DO

will be more because ofincreased solubility of oxygenin water.

At high temperatures, the self-purification takes lesser time,

though the quantity of DO willbe less.


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Actions During Self-purificatio Sunlight:

Sunlight helps photosynthesis of certain aquatic planto absorb carbon dioxide and give out oxygen, thusaccelerating self-purification.

Sunlight acts as a disinfectant.


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Zones of Pollution in Stream

DecompositionZone

water is rendereddark and turbid, high BOD exists

Septic Zone

grayish anddarker than the

previous zone The dissolved

oxygen contentreaches aminimum

Recovery Zone

most of thestabilized

organic mattersettles as sludge,

BOD falls andDO content risesabove 90% value

Cleaner Wa

Water becomclearer.

DO rises to tsaturation le

BOD drops tthe lowest va


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Zone of Clean Water (Zone 1)Zone of Degradation/ Decomposition (Zone 2)Zone of Active Decomposition/ Septic (Zone 3)

Zone of Recovery (Zone 4)Zone of Cleaner Water (Zone 5)

Minimum D = criticaldissolved oxygen = Dc


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Dynamics of Oxygen TransferRate of decomposition (deoxygenatio

Linearly proportional to BOD levelBOD falls exponentially with time

Rate of oxygen dissolution (reaeratio

Linearly proportional to the oxygendeficit: DOsat DOactual

DO falls whendecomposition rate > dissolution ra

DO rises whendecomposition rate < dissolution ra


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Reaeration

DissolvedOxyg

en(mg/L)

Dissolved O2 (DO) vs. Time10

8

6

4

2

0

DO IN

time

Flow Diagram

Inflow

Storing System

O2

/time

vs. Time10

8

6

4

2

0

DO

time

Mg/L

time

aeration Sat ActualDO (in) = k *(DO DO )

T

DOSaturation

D

DOActual

=

The closer the DOActual is from the DOSthe slower the rate at which O enters t

D i


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DissolvedOxyg

en(mg/L)


8

6

4

2

0

Flow Diagram

O

Depleting System

DO /time vs. Time10

8

6

4

2

0

DO

time

Mg/L

time

decomposition FOODDO (out) = k *(BOD )

T

DOActual

The less food the organisms have, the slorate at which the consume Food + O

Deoxygenation

D ti R ti


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DissolvedOxyg

en(mg/L)


8

6

4

2

0

DO O

tim

Flow Diagram

Outfl

Depleting System

DO /time vs. Time10

8

6

4

2

0

DO

time

Mg/L

time

DO IN

time

DOSaturation

Deoxygenation + Reaeration

InflowInflowInflow Outf

Storing SystemSteady-State System

DOSaturDOActualDO OUT

time

DO IN

time


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More Oxygen Sag Curves

Effect of temperature: sag deepens and shortens may cause a portion of river to have

unhealthy DO levels

Effect of BOD Level: sag becomes more severe longer distance (or time) at

unhealthy DO levels


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More Oxygen Sag Curveswithout treatment

with treatment


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Why We Do All of This To determine how much waste can safely be put in a river

Process Determine minimum acceptable DO

Calculate waste load that keeps critical DO above the minimum

If discharged waste is above acceptable limits: More treatment needed Discharger may add dissolved oxygen to wastewater

Cautions Be sure to make calculations for worst conditions Remember to consider all dischargers


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Streeter-Phelps Model Assumptions of the Model

stream is an ideal plug flow reactor

steady-state flow exists in stream. BOD and DO reaction due to disposed organic

only. The only reactions of interest are BOD exertio

and transfer of oxygen from air to water acrossair-water interface


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Limitations Steady state

Streams aren't steady state. Flows, velocities, geometries, atemperatures all vary with time. Dividing the stream intosmaller reaches reduces this limitation, but steady stateconditions are still assumed inside each reach. To the exten

that the reach is not steady state, inaccuracies will beintroduced.

Plug flow Streams aren't really plug f low. The geometries of natural

streams are not regular -- there are wide spots, pools, narrochutes, sand bars, rocks -- so the flow doesn't move as a pl


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LimitationsAlgae

The model doesn't include algae which are a veryimportant source of oxygen. Note that the effects ofalgae are very dependent on sunlight, which changesthrough the day. Modeling algae accurately wouldrequire a nonsteady-state model.


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Limitations Benthic organisms

The model assumes that all the oxygen demand is fromsuspended organisms (i.e., bacteria living in the water colulike they were in the BOD bottle). In fact, most natural

bacteria live attached to surfaces in "biofilms" -- slimycoatings on rocks or soil particles. So a significant portion the BOD is due to bottom-dwelling (benthic) organisms. Teffect of benthic demand is especially strong if much of thorganic material is in the form of particles that settle out.

Benthic effects are not included in the model either.


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Account for DO gain/loss process

Qr, Lr, Tr Qa, La, Ta

Qw, Lw, Tw

ReaerationDecay

Rate ofOxygen

accumulated

Rate ofOxygen

In

Rate ofOxygen

Out

Rate ofOxygenGenerated

= - +


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Oxygen is introduced into the stream by reaeration

There is no oxygen leaving the stream since we have assumed it iunsaturated

Dissolved oxygen may be produced in water by algae duringPhotosynthesis, but is a swift stream the algae dont have time to

grow and there is no oxygen produced in this way. Oxygen may be used by microorganisms respiration. This is call

deaeration or deoxygenation

So the new mass balance is

Rate of Oxygenaccumulated

Rate of OxygenIn

Rate of OxygenOut

Rate of OxygenGenerated

= - +

Rate of Oxygen

accumulatedRate of Oxygen

InRate of Oxygen

consumed 0= - +

d fi it D DO DO


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oxygen deficit = D = DOS DODOS = oxygen saturation concentration, a function of temperature of the water,

atmospheric oxygen concentration, and water chemistry

The rate of consumption of DO coincide with rate of BOD degradation

Rate of deaeration = -kd

L (first order)kd = deaeration constant, function of type of waste, temperature, etc. Units, day

Rate of reaeration = kr D (first order)kr = reaeration constant based on characteristics of the stream and the weather

dt

BODd

td

Dd

td

DOd )(

)(

)(

)(

)(

)(

)(

)(

)(

td

Dd

td

DOd

DkLktd

Ddrd

)(

)(

Rate of Oxygen

accumulated

Rate of Oxygen In

(Reaeration)

Rate of Oxygen consum

(Deaeration)= -


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tk

a

tktk

dr

ad rrd eDee

kk

LkD

)(

Substituting and integrating yields the following equations

0

DkeLkdt

dD

r

tk

ad

d

tk

a

r

d

c

deLk

kD

ad

dra

d

r

dr

c

Lk

kkD

k

k

kk

t)(

1ln1

The time and distance of critical point can be determined by differentiatingabove equation with time and setting it equal to zero


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H

Vkk

d

Rate ConstantsKd = Rate constant at 20oc, 1/day

V = Average stream velocity, m/sH = Average depth, m= Bed activity Coefficient

At t0c

= 1.056

20

20 )( t

ddtkk

2

3

21

)20(21

025.19.3

H

vk

T

r

Kr = Rate constant at 20

oc, 1/daV = Average stream velocity, mH = Average depth, m

waste disposal in stream

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