Water Waste Treatment

OutlineOutline

1.1. What is Sewage?What is Sewage?

2.2. Sewage treatmentSewage treatment

a)a)Primary stage of Sewage Primary stage of Sewage treatmenttreatment

b)b)Secondary stage of Sewage Secondary stage of Sewage treatmenttreatment

c)c) Tertiary stage of Sewage Tertiary stage of Sewage treatmenttreatment

1. What is Sewage?1. What is Sewage?

• Sewage= Water carried wastes

• It consists of: - 99.9% pure water (most of the time)

- 0.01% domestic, sanitary, commercial

and industrial wastes

• Contains: - disease causing micro organisms

- organic materials

- toxic substances (such as heavy metal ions)

2. Sewage Treatment2. Sewage Treatment= The reducing of the pollutant level in water

• Its purpose: - to remove dangerous materials

- to reduce the BOD ( Biological Oxygen

Demand) of sewage

- to kill microorganisms

•Stages of Sewage Treatment: a) Primary stage

b) Secondary stage

c) Tertiary stage

Note: The stages have different levels of effectiveness depending on the availability of resources

Stages:

1.) Screens and Taps:

These filter out large objects from waste water and remove floating objects (such as grease)

2.) Settling Tank:

This allows smaller heavier objects (rocks, stones, etc) to settle down and then to be removed and transferred to landfills3.) Sedimentation tank:

The sewage is again allowed to settle and sludge is removed from the bottom. This can be speeded up by adding chemicals (such as Al2(SO4)3) which allows suspended particles to join and form clumps (Flocculation)

4.) Chlorine and Ozone treatment:

These kill pathogenic bacteria

A. Primary stage of Sewage treatment

Primary stage efficiency

Primary treatment is generally not sufficient to improve water quality to safe levels.

A typical primary treatment domestic sewage plant can only remove about 30-40% of the BOD waste.

To further reduce the BOD waste levels Secondary treatment is essential.

B. Secondary stage of Sewage treatment Stages:

1.) Aeration tank:

In this air enriched with oxygen is bubbled (using large blowers) through sewage mixed with bacteria laden sludge

Result: Aerobic bacteria break down most of the organic matter in sewage (Activated sludge process)2.) Sedimentation tank:

The water containing decomposed suspended particles is allowed to settle. Large quantities of biologically active sludge collect (part of it is recycled)

3.) Chlorine and Ozone treatment

Secondary treatment efficiency

Secondary treatment can remove about 90% of all organic oxygen demanding wastes (BOD waste) and suspended particles but just like primary treatment cannot remove dissolved inorganic substances

such as: - Nitrates

- Phosphate

- Heavy metal ionsTo remove these tertiary treatment is essential

C. Tertiary stage of Sewage treatment

= treatment that involves specialized chemical, biologic and/or physical processes

•What does it do?: it removes inorganic substances and organic materials not removed by the previous stages of treatment

•Examples of tertiary treatment processes are:

I) The carbon bed method

II) The chemical precipitation process

III) Biological processes

I) The Carbon bed process= a process which uses adsorption to remove organic chemicals from waste waters

•Adsorption= attraction of a substance to the surface of a solid substance, in this case of tiny carbon granules with large surface areas (otherwise known as activated carbon black)

• Effectiveness: it can remove many toxic organic materials

( An every day example is the charcoal filters used to further purify tap water)

II) Chemical precipitation= removal of heavy metal ions such as: - Cadmium (Cd)

- Lead (Pb)

- Mercury (Hg)

By forming their sulfide salts which are insoluble and precipitate this process removes the above substances

Reaction: MS M2++S2-

To form MS (metal sulfides) we bubble hydrogen sulfide gas through the waste water

Reaction: M2++H2S MS+2H+

III) Biological processes: Denitrification

= turning of nitrogen in nitrates back into atmospheric nitrogen, N2, through the use of anaerobic organisms ( denitrifying bacteria)

• Effectiveness: It reduces nitrate contamination in ground water

The End