Ocean Disposal of Wastewater (An Introduction)

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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

, ppm etc.
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The local environmental impact is increased by
the concentration Dilution
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 Sf(Fr, H/D, ?o) and is
independent of Reynolds number (inertia/ viscous
force) exceeding 2000. Densimetric Froude
number
Inertia/ buoyancy force per unit mass
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.
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• The velocity and concentration distributions are
  nearly Gaussian
• The initial volume and specific momentum fluxes
  for a circular jet are

(Specific momentum)
Average dilution for a round jet
Average dilution for a round plume
Where the initial buoyancy flux
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• Example Estimate the average dilution for
  uo1.2m/s, D0.15m, H15m
• When the outfall discharges into
• a fresh water lake
• the ocean with relative density difference of
  0.02
• Solution

In fresh water
In the ocean
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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
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