Liquid waste management

Hari Prasad KafleLecturer, Public Health

Pokhara University

The four things that have spoiled your life

are: “I” and “mine”, “You” and “Yours”.

Forget them.

Sewage is waste water from a community,

containing solid and liquid excreta, derived

from houses, street and yard washing, factories

and industries.

The term sullage is applied to waste water

which does not contain excreta, e.g. waste

water from kitchen and bathrooms.

a. Creation of nuisance, unsightness and

unpleasant ouder.

b. Breeding of flies and mosquitoes.

c. Pollution of soil and water supplies.

d. Contamination of food.

e. Increase in the incidence of disease,

especially enteric and helminthic disease.

Water: 99.9%

0rganic and inorganic solid: 0.1%

One gram faeces may contain about 1000

million E, coli, 10-100 million faecal

streptococci, and 1-10 million spores of Cl.

Perfringens.

To stabilize the organic matters so that it can

be disposed off safely and to convert the

sewage water into an effluent of an acceptable

standard of purity which can be disposed of in

to land, rivers or sea.

A standard test which is an indicator of the

organic content of the sewage is biochemical

oxygen demand(BOD).

BOD is defined as the amount of oxygen

absorbed by a sample of sewage during

specified period, generally five days, at a

specific temperature, generally by living

organism.

BOD value range from 1mg/liter for natural

water to 300 mg/liter for untreated sewage.

Aerobic process: in presence of oxygen

organic matter is broken down into CO2,

water, ammonia, nitrites, nitrates and sulfates

by action of bacterial action including fungi

and protozoa.

Anaerobic process: anaerobic bacteria

decompose organic waste into methane,

ammonia, CO2, and H2.

Landfill/mine reclamation

Secondary Treatment

Air

Raw sewage

Grit

ChamberSludge

Digester

Activated

Sludge

Sludge drying beds

Sedimentation

Tank

Aeration TankFinal

Sedimentation

Tank

Screens Rivers, lakes,

oceans, etc.

Primary Treatment

Disinfection

Primary treatment

◦ solids are separated

Secondary treatment

◦ dissolved biological matter is converted into a solid

mass by using water-borne bacteria

◦ 95% of the suspended molecules should be removed

Tertiary treatment

◦ biological solids are neutralized then disposed, and

treated water may be disinfected chemically or

physically

Mechanical treatment◦ Influx (Influent) ◦ Removal of large objects ◦ Removal of sand and grit ◦ Primary Sedimentation

Biological treatment◦ Trickling bed filter◦Activated sludge

Chemical treatment◦Disinfection

Screening

Great chamber

Primary Sedimentation

Sewage first passed through metal screen to

remove large floating objectives such as pieces

of woods, rags, plastics paces, masses of

garbage and dead animals.

The screen consist of vertical steel bars usually

set 5 cm apart.

After screen sewage is then passed through a

long narrow chamber called as grit chamber or

detritus chamber.

The chamber is about 10-20 meter and

velocity of sewage flow is about 1 foot per

second taking a detention period of 0.5-1

minute.

The grit chamber allows to settle down heavier

solids such as sand and gravel while

permitting the organic matter passing through.

The grit collected at the bottom of camber is

removed periodically or continuously.

After passing through, sewage is then admitted

to a huge rectangular tank called as Primary

sedimentation tank.

Sewage is made to flow very slowly across the

tank at a velocity of 1-2 feet/minute and

retained 6-8 hours in tank.

Nearly 50-70% organic solids settled down

due to gravity.

The sludge is removed mechanically operated

devices and subjected to sludge dryer.

Due to biological action certain treatment

takes place.

Scum at the uppermost layer should be

removed periodically.

The effluent from the primary sedimentation

tank still contains a proportion of organic

matters in solution and numerous living

organism.

It has high oxygen demand and subjected to

further treatment ‘aerobic oxidation’ by one

of the following methods:

Trickling bed filter method

Activated sludge process

Spread wastewater over microorganism.

Made of coke (carbonized coal), limestone

chips or specially fabricated plastic media.

Optimize their thickness by insect or worm

grazing.

To remove suspended solids, nitrogen, and

phosphate

About 90 % removal of BOD5 and SS

About 20 % removal of phosphate

Variable removal levels of viruses, 20-80 %

removal of bacteria and >90 % removal of

protozoa.

Subjected to aeration chamber for aeration for

6-8 hours.

The aeration is accomplished by either

mechanically agitation or by forcing

compressed air continuously from the bottom

of aeration tank.

The organic matter of the sewage get oxidized

into Co2, nitrates and water with the help of

aerobic bacteria.

Best suited for mega and metro cities because

it occupies small space the trickling filter.

The oxidized sewage from the trickling filter

or aeration chamber is led into the secondary

sedimentation tank for 2-3 hours.

The ‘aerated sludge' is pumped back into the

aeration tank for activated sludge process and

finally into the sludge digestion tank.

To inactivate pathogens in wastewater

Several choices

◦ Free chlorine and combined chlorine

◦UV

◦Ozone

◦ Chlorine dioxide

Use for Irrigation

Disposal in river, sea, lakes, ponds etc.

3-4 weeks complete digestion takes place and

it is carried out in sludge drying bed.

Digested sludge is used as organic fertilizer for

agricultural purpose.