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Ocean Disposal of Wastewater

(An Introduction)

by

Prof. B. S. PANI

I. I. T. Bombay, Mumbai

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Ocean is utilized as the ultimate sink for wastewater disposal.

The effluent (a very dilute mixture of human & other wastes) is collected and led to a central location.

After treatment, the effluent is discharged into the ocean.

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Density of effluent is close to that of fresh water.

Due to buoyancy effects, the effluent rises to the surface and in doing so entrains the ambient fluid and becomes very dilute.

Dilution is the solution to pollution.

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In deep stratified oceans the diluted pollutants field may attain an equilibrium level below the ocean surface.

In shallow seas the pollutants reach the sea surface and spreads out due to density differences that persists.

Due to wastewater treatment dilution and inactivation of pathogens the quality of water becomes acceptable.

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Royal Commission on environmental pollution (1984) states that: “ …….. with well designed sewage outfalls we believe that discharge of sewage to the sea is not only acceptable but in may cases environmentally preferable to alternative method of disposal”.

There is cultural feeling among some indigenous people that no water should be polluted with human waste.

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Scarcity of land for constructing a sewage treatment plant favors treatment of the effluent in the action.

When sewage is diluted one hundred times, it is as good as a fully treated secondary effluent.

Certainly there are good reasons for not disposing wastes containing toxic materials and heavy metals in the ocean.

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Secondary treatment plant: The action of bacteria and other micro-organisms in an enclosed basin help to reduce the sewage waste.

Primary treatment: physical operations like screening and sedimentation are used to remove floating and settable solids.

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Purification in the ocean can be to some extent controlled by site choice, design of the diffuser, and the rate of loading. All other processes occur naturally following discharge of effluent.

In contrast, the purification in a treatment plant is closely controlled.

Concentration of a pollutant = mixture of VolumepollutantofMass

, ppm etc.

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The local environmental impact is increased by the concentration

Dilution = aC(C

)aCo(C

Where, Co & Ca = effluents and ambient concentration respectively.

Initial dilution S= Minimum dilution achieved where the mixed effluent reaches the water surface or trapping level, at a distance of the orders of the water depth downstream. Initial dilution is water the control of the engineer.

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In still water S=f(Fr, H/D, Фo) and is independent of Reynolds

number (inertia/ viscous force) exceeding 2000.

Densimetric Froude number = gdρ

Δρu

ao

o

For jets Fr

For plumes Fr 1

For most outfalls the Fr lies between 4 to 18 and the flow can be

classified as Buoyant jets.

= Inertia/ buoyancy force per unit mass

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The velocity and concentration distributions are nearly Gaussian

The initial volume and specific momentum fluxes for a circular jet are

o2 u D

4π

oQ 2o

2o UD

4πM

Average dilution for a round jet

1o

21

o Q M 0.29oQ

QS x

Average dilution for a round plume

1oQ z oF 0.163S 3

531

(Specific momentum)

Where the initial buoyancy flux g aρ

Δρud 4π

oF o 2

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Example: Estimate the average dilution for uo=1.2m/s, D=0.15m, H=15m

When the outfall discharges intoa) a fresh water lakeb) the ocean with relative density difference of 0.02Solution:

/s3m 210 x 2.12 1.2 x 20.15 x 4π oQ

2/s4m210 x 2.5421.2 x 20.15 x 4π

oM

In fresh water

32.712)10 x (2.12 x 15 x 2

1)210 x (2.54 x 0.29S

In the ocean:

310 x 4.169.81 x 0.02 x 1.2 x 20.15 x 4π

oF

1131)210 x (2.12 35

15 31

)310 x (4.16 0.163S

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In the presence of buoyancy, dilution is enhanced.

Cross flow of ambient helps to dilute the effluent in a significance way.

Eq. Shape & Moore(1987)

Say,

Thus, a four fold increase in dilution takes place

1.33o

0.359

o

aoM S)u

u1.57(SS

1.0,100 o

ao u

uS

0.1uu100,S

o

ao

414100x1.59(0.1)100S 1.330.359M

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