Ocean Disposal of Wastewater (An Introduction)

1. Ocean Disposal of Wastewater(An Introduction) by Prof. B. S. PANI I. I. T. Bombay, Mumbai

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. In still water S=f(Fr, H/D,Фo) 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.

11. 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

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

13. 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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