ENVIRONMENTAL STUDIES-II

MODULE – I

ENVIRONMENTAL

POLLUTION

LECTURE – 4

WASTE WATER TREATMENT

CONTENTS

Control Measures of Water Pollution (Waste

Water Treatment)

Waste Water Treatment Wastewater is liquid waste discharged by domestic residences,

commercial properties, industry, agriculture, which often containssome contaminants that result from the mixing of wastewater fromdifferent sources.

Waste water can be categorized on the basis of origin andcomposition as -

1. Industrial waste water (Effluent)2. Domestic waste water (Sewage)

Wastewater treatment processes are designed to achieveimprovements in the quality of the wastewater. The varioustreatment processes may reduce-

1. Suspended solids (physical particles that can clog rivers orchannels as they settle under gravity).

2. Pathogenic bacteria and other disease causing organisms Theseare most relevant where the receiving water is used for drinking, orwhere people would otherwise be in close contact with it.

3. Biodegradable organics (e.g. BOD) which can serve as “food” formicroorganisms in the receiving body. Microorganisms combine thismatter with oxygen from the water to yield the energy they need tothrive and multiply; unfortunately, this oxygen is also needed byfish and other organisms in the river.

4. Heavy organic pollution can lead to “dead zones” where no fishcan be found; sudden releases of heavy organic loads can lead todramatic “fish kills”.

5. Nutrients - including nitrates and phosphates. These nutrientscan lead to high concentrations of unwanted algae, which canthemselves become heavy loads of biodegradable organic load.

6. Treatment processes may also neutralize or remove harmfulwastes and toxic chemicals.

Widely used terminology refers the three levels ofwastewater treatment: Primary Treatment Secondary Treatment Tertiary Treatment

Primary treatment (Physico-Chemical):

The primary treatment is of general nature and is used forremoving suspended solids, odour, colour and to neutralizethe high or low pH in case of industrial effluents.

This level also referred as “mechanical treatment”, althoughchemicals are often used to accelerate the sedimentationprocess.

Primary treatment can reduce the BOD of the incomingwastewater by 20-30% and the total suspended solids bysome 50-60%.

It involves screening, sedimentation, floatation, filtration,neutralization, coagulation and precipitation etc.7

1. Screening:

• Screening is the first step in the wastewatertreatment process.

• This step removes all sorts of refuse that has arrivedwith the wastewater such as plastic, branches, rags,and metals.

• The screening process is used primarily to presentthe clogging and interference of the followingwastewater treatment processes.

• Screens are considered coarse if their opening arelarger than 6mm, fine if their openings are between1.5 and 6mm, and very fine if their openings arebetween 0.2 and 1.5mm.

8

In order to remove coarse solids, numerous types of detritus tanks,

grinders, and cyclonic inertial separation are utilized, including

a comminutor and a grit chamber. The type of grit removal

separation depends upon the characteristics of the grit itself.

A comminutor, also known as the grinding pump, houses a rotating

cutting screen. This cutting screen shreds any large chunks of

organic matter in the wastewater into smaller pieces. This makes it

easier for the microorganisms to use the organic matter as food and

prevents the large chunks from harming the internal workings of the

treatment plant.

A grit chamber allows pieces of rock, metal, bone, and even egg

shells, which are denser than organic materials, to settle out of the

waste stream. Removal of grit prevents damage to machinery

through abrasion or clogging.

Comminuting

• Process of shredding of large sized

solid present in the waste water in to

small sized particles (8mm in dia.) for

effective removal of solids from the

waste water.

• A hammer mill device is often used for

this purpose.

• Shredding devices are located just

ahead of pumping devices.

11

Grit removal:

• Municipal waste contains large amount

of inorganic solids such as pebbles,

sand, silts, glass and metal pieces.

• The grit removal facility generally

consist of an enlarged channel area

where reduced flow velocity allow grit

to settle down.

12

13

Skimming Tank:

• A skimming tank is a chamber so

arranged that the floating matter like

oil, fat, greases etc. rise and remain on

the surface of the waste water until

removed, while the liquid flows out

continuously under partitions .

14

• It is necessary to remove the floating

matter from the sewage otherwise it

may appear in the form of unsightly

scum on the surface of settling tanks

and inter fare with the activated sludge

process of sewage treatment.

15

Sedimentation:

• In this step the settable solids are

removed by gravitational setting under

quiescent conditions.

• The sludge formed at the bottom of the

tank is removed as under flow.

• The clear liquid produced is known as

the overflow and it should not contain

readily settable matter.

16

Sedimentation simply entails the physical

settling of matter, due to its density, buoyancy,

and the force of gravity.

Through sedimentation, the larger solids are

removed in order to facilitate the efficiency of

the following procedures.

Neutralization:

• When pH of the industrial waste is too

high or too low then it should be

neutralized by acid or alkali and only

neutralized effluents should be

discharged into the drain or public

sewer.

• Lime stone caustic soda, Sulphuric

acid etc are utilized to neutralize the

pH.

18

Coagulants and flocculants are chemicals used to precipitateinsoluble substances.The purpose of coagulation and flocculation is to cause smallpollutant particles such as metals to aggregate and form largeenough floc so that they can be separated from the wastewater. Coagulation targets dissolved ions such as metal andradionuclides. This technology has been used consistently in theelectronics and electroplating industry as well as for applications ingroundwater treatment. Coagulation is the process of destabilization of colloids by alteringsurface properties in order to allow the individual particles tocombine into larger ones Flocculation is the actual accumulation of the particles into asettable mass.

Coagulation & Flocculation

• There are three main types of coagulants (Alum,

Ferric chloride, and hydrated lime in solid form)

that are used to overcome the repulsive forces of

particles, thus causing them to aggregate.

• Electrolytes, organic polymers, and synthetic poly

electrolytes are mixed in wastewater for

flocculation that promote flocs and subsequent

physical separation.

• Rate of flocculation is dependent upon many

factors including concentration of particles, particle

contact, and range of particle sizes.

Secondary Treatment

(Biological treatment)

In this process dissolved organic matter that escapes

primary treatment.

This is achieved by microbes consuming the organic matter

as food, and converting it to carbon dioxide, water, and energy

for their own growth and reproduction.

The biological process is then followed by additional settling

tanks to remove more of the suspended solids.

About 85% of the suspended solids and BOD can be

removed by a well running plant with secondary treatment.

Secondary treatment technologies include the basic

activated sludge process, the variants of pond and constructed

wetland systems, trickling filters and other forms of treatment

which use biological activity to break down organic matter.

Activated sludge process:

• Waste water after primary treatment

enters in to an aeration tank where the

organic matter is brought in to contact

with sludge from the secondary

clarifier.

• This sludge is heavily laden with

microorganisms which are in the

active state of growth.

22

• In proper aerated condition microbesconvert organic matters in to low energycompounds like SO4, NO3, CO2 etc.

• In a sewage or industrial wastewatertreatment plant, the activated sludgeprocess is a biological process that canbe used for one or several of the followingpurposes:

1. Oxidizing carbonaceous biologicalmatter.

23

2. oxidizing nitrogenous matter:

mainly ammonium and nitrogen in biological

matter.

3. Removal of phosphates.

4. driving off entrained gases such

as carbon dioxide, ammonia, nitrogen, etc.

5. generating a biological floc that is easy to

settle.

6. generating a liquor that is low in dissolved

or suspended material.24

• The process involves air being

introduced into a mixture of screened,

and primary treated sewage or industrial

wastewater combined with organisms to

develop a biological floc which reduces

the organic content of the sewage.

25

26

Trickling Filters:

• It consists of a fixed bed

of rocks, lava, coke, gravel, slag, polyur

ethane foam, sphagnum peat

moss, ceramic, or plastic media over

which sewage or

other wastewater flows downward and

causes a layer of microbial slime

(biofilm) to grow, covering the bed of

media.

27

• Aerobic conditions are maintained bysplashing, diffusion, and either by forcedair flowing through the bed or naturalconvection of air if the filter medium isporous.

• The removal of pollutants from thewastewater stream involves bothabsorption and adsorption of organiccompounds by the layer of microbialbiofilm.

28

• The filter media is typically chosen to

provide a very high surface area to

volume. Typical materials are often

porous and have considerable internal

surface area in addition to the external

surface of the medium.

29

• Passage of the wastewater over the mediafurnishes dissolved air, the oxygen whichthe slime layer requires forthe biochemical oxidation of the organiccompounds and releases carbondioxide gas, water and other oxidized endproducts.

• As the bio film layer thickens, it eventuallysloughs off into the treated effluent andsubsequently forms part of the secondarysludge.

30

31

Rotating Biological Contactor (RBC)

• It consist of circular plastic discs which arranged on arotating shaft. The first RBC was installed in WestGermany in 1960.

• Circular discs have microorganisms grown on them.About 40% area of discs is submerged in the wastewater containing tank.

• The discs rotate in in and out of water as the RBCrotates.

• The microorganisms present on the discs absorborganic matter when they are in water and obtainrequired oxygen when the disc are out of water.

• It can removed high concentration of Organic matter.

Biofilms, which are biological growths that become

attached to the discs, assimilate the organic

materials in the wastewater.

Aeration is provided by the rotating action, which

exposes the media to the air after contacting them

with the wastewater, facilitating the degradation of

the pollutants being removed.

The degree of wastewater treatment is related to the

amount of media surface area and the quality and

volume of the inflowing wastewater.

Up-flow Anaerobic Sludge Blanket

(UASB) Reactor:

• In this method pollutants are treated anaerobically. It has four

stages – i. Acidogenesis ii. Acetogenesis

iii. Hydrolysis iv. Methanogenesis

Anaerobic biomass is used to convert organic matter in to CH4 ,

CO2, acetate, H2 etc.

Methane is collected and can be used for domestic purposes.

Working - A dense blanket of granular anaerobic biomass passed

through the sludge blanket continuously. Biogas produced is

collected in reactor chamber. Water produced after hydrolysis

and other liquid forms are removed from outlet. It is effective

method and have economic advantages.

Disinfection

Disinfection, typically with chlorine, can be thefinal step before discharge of the effluent.However, some environmental authorities areconcerned that chlorine residuals in the effluentcan be a problem in their own right, and havemoved away from this process. Disinfection isfrequently built into treatment plant design, butnot effectively practiced, because of the highcost of chlorine, or the reduced effectiveness ofultraviolet radiation where the water is notsufficiently clear or free of particles.

Tertiary treatmentIt is simply additional treatment beyond secondary!Tertiary treatment can remove more than 99 percent of allthe impurities from sewage, producing an effluent ofalmost drinking-water quality. The related technology canbe very expensive, requiring a high level of technical know-how and well trained treatment plant operators, a steadyenergy supply, and chemicals and specific equipmentwhich may not be readily available. An example of a typicaltertiary treatment process is the modification of aconventional secondary treatment plant to removeadditional phosphorus and nitrogen.

Water filtration technology has advanced to the point

where wastewater can be rendered safe for drinking,

according to a new report, but legislative and

psychological hurdles will need to be overcome before

widespread adoption can happen.