CE 2354 UNIT III

Treatment of Sewage

Before disposal to the environment, sewage must be treated to make it safe.

The degree of treatment depends on the characteristics of source of disposal Exposure to people

Aesthetics

Fish and wildlife

Self cleansing properties of the river or stream

Nature of land

Preliminary Treatment

Separation of floating materials like dead animals, tree branches, rags, paper, wood, construction materials etc.

Removal of oils and greases

Reduces BOD by 15 to 30%

Process used Screening

Grit Chamber

Skimming Tanks

Primary Treatment

Removal of large suspended organic matter

Organic matter settled in primary settling basins

The sludge from settling basin are stabilized by Anaerobic digestion (secondary treatment)

Incineration

Disposed of in landfills or for soil stabilization

The effluent from primary treatment may have high BOD (60% of original) and large amount of small suspended organic matter

Secondary Treatment Further treatment of sewage Biological decomposition of organic matter in aerobic or anaerobic

conditions Microbes will breakdown the fine organic matter and produce clearer

effluent Aerobic biological units

Filters Sand Filters Trickling filters

Aeration tanks Oxidation ponds and aerated lagoons

Anaerobic biological units Anaerobic lagoons Septic tanks and Imhoff tanks

Not classified as secondary treatment as they use raw sewage

Effluent contains low BOD ( 5 to 10 % of original) Organic sludge decomposed in anaerobic digestion tanks before

disposal

Tertiary Treatment

Treatment of effluent from secondary treatment

Final polishing of treated sewage

Required for recycling and disposal to sensitive streams

Generally disinfection and filtration

Distinction between primary, secondary and tertiary arbitrary in modern plants as both unit process and unit operations occur in same units

Unit Operations and Processes

Unit Operation

Unit operations are the physical operations to remove the impurities present in the water and waste water

Unit Process

Unit processes are the chemical and biological conversion on the status of the impurities that they will be converted to a form that can be easily separated

Screening,

Screening is a unit operation that separates materials in and/or on water (found in different sizes) from water and from entering water treatment facilities and mains.

The unit involved is called a screen (Unit no A).

Legend

Sluice Gate

Bar Screen

Motor

Waste Sludge

Pump

Belt Filter Press

Classification of Screens

Opening size [Coarse, Medium and Fine] Configuration [Bar Screens and Mesh Screens] Method used to clean the entrapped materials

(manually, mechanically, raked or water-jet cleaned)

Fixed or moving screen surface. Coarse Bar Racks

remove coarse debris (twigs, branches, rags, etc) Spacing

Coarse 2 6 in Medium 0.8 2in Fine Screens 3/8 to in. (up to 10 mm or less

Types of Screens

Medium and Fine Screens

Medium and Fine Screen

Cleaning of Screens

Periodic cleaning to prevent clogging

Manual cleaning

Manually rake the detritus and discharge into a perforated metal platform where the water drains through the perforation

Bars set at an inclination of 45o to 60o to increase surface area and to facilitate cleaning

Used only in small treatment plants where the sewage flow is less than 0.12 m3/s (400 m3/hr)

Cleaning of Screens

Mechanical Cleaning Mechanically operated rakes

Revolving type (for curved screens)

Reciprocating type (for vertical or inclined screens)

Endless revolving type (for vertical and inclined screens)

Rake arm teeth are so formed to mesh with the screens during cleaning

Rake speed less than 3 m/min

Inclination of bars screens between 60o and 90o with horizontal

Used when sewage flow >> 400 m3/hr

Cleaning of screens

Screens cleaned at regular intervals Controlled by

Time switch (most reliable) Automated float control Electrode type

Continuous cleaning screen efficiency increases but the power requirement and wear and tear also increase

Mechanically cleaned bar screens have c/s area 25% more the c/s of sewage channel

Manually cleaned bar screens requires twice as much a mechanically cleaned screens

Cleaning of screens

Cleaning of medium and fine screens

Mechanically operated

Brushes

Scrapers

Air jets

Steam jets

Design Criteria : Velocity of flow

Varies with material Lower the velocity of flow through the screen, greater the screening

efficiency However, at lower velocities, solid deposition increases in sewage channel Design velocity must provide 100% removal of certain particle size without

undue deposition According to IS 6280-1971 v< 0.9 m/s at peak flow However

0.6 m/s

Design Criteria: Head Loss

Head loss function of Bar shape Velocity head of flow between bars Where

hL = head loss (m) b= bar shape factor (1.67 to 2.42) w= max c/s width of bars facing flow

direction (m) b= min clear spacing of bars (m) V = velocity of flow through the screen

(m/s) g = acceleration due to gravity (9.81 m/s2) q= angle of inclination of screen with

horizontal

)sin(2

)(2

34

qbg

V

b

whL

Bar Type b

Sharp edged Rectangular

2.42

Rect. With semi circular upstream face

1.83

Circular 1.79

Rect. With semicircular upstream & down stream faces

1.67

Design Criteria : Head Loss

Head loss through clean flat bar screens Where

hL= Head loss (m) V = velocity of flow through screen (m/s) v= velocity of flow before the screen (m/s)

Provide loss of head of 0.15 m Head loss not to exceed 0.3 m for manually

cleaned bar screens Manufactures specify acceptable loss of head for

mechanically cleaned bar screens

)(0729.0 22 vVhL

Design Criteria : Head Loss

For Fine Screens Where

hL= Head loss (m) Q= discharge through screen (m3/s) A=effective submerged open area of screen (m2) C=coefficient of discharge g= acceleration due to gravity (m/s2)

Values of C and A depend on Size and milling of slots Wire diameter and weave % open area (determined experimentally) Size and amount of solids in sewage Size of apertures Method and frequency of cleaning For clean screens typical C = 0.6

2

2

1

CA

Q

ghL

Design Criteria : Material Bar Screens

Steel bars or rods or flats fixed to a suitable steel frame Min c/s for bars and flats is 10 mm x 50 mm, and are laced

with 50 mm side parallel to flow direction The spacing should be uniform and should be maintained

by adequate number of spacers so placed as not to interfere with the raking operation

Fine Screens Brass or bronze plates or wire mesh to resist rusting and

corrosion Opening size 1.5 mm to 3 mm Net submerged open area not less than 0.05 m2/1000 m3

of average daily sewage flow for separate system Net submerged open area not less than 0.075 m2/1000 m3

of average daily sewage flow for combined system

Design Criteria : Other Considerations

Screen top at least 300 mm above highest flow level of sewage

For large treatment plants, screen channels may be divided to have a maximum screen width of 1.5 m

The gross submerged area (incl. bars and openings) should be 25% to 35% more than c/s area of approach channel. The area should be considered as the projected area

normal to direction of flow for inclined screens

The screen should be so embedded that the frame should not obstruct the flow of sewage

Design Criteria : Other Considerations

Downstream channel invert is provided with a drop below the upstream channel invert Manually cleaned : drop = 150 mm Mechanically cleaned :drop= 75 mm

Length of screen channel should be sufficient so that Screen can be properly housed Enough working space is available Flow gets stabilized Eddies are avoided

The length of screen channel L = (d+0.3) cot(q) +1.73(W + ds) Where

L =length of screen channel (m) d= depth of flow in screen channel (m) q= angle of inclination of screen with horizontal W= width of screen channel (m) ds= diameter of incoming sewer (m)

Design Criteria : Other Considerations

Screen channel should have smooth entry and exit arrangements to facilitate cleaning All corners should be rounded

All edges to be chamfered

Min free board = 300 mm, should be appropriately raised where turbulent conditions are expected

A bypass arrangement to be provided in case of abnormally high sewage flow

Design Criteria : Summary

Min bar size : 10 mm x 50 mm

Clear spacing between bars : 15 mm to 75 mm

Slope with horizontal Manually cleaned : 45o to 60o

Mechanically Cleaned: 60o to 90o

Min approach velocity : 0.3 m/s

Velocity of flow through screen : 0.6 m/s to 1.2 m/s

Allowable head loss = 0.15 m

IS 6280-1971 gives more details

Disposal of Screenings

Screenings are the waste materials collected from screens. Quantity of screenings depend on screen size and nature of sewage

Screen size 100 mm : 0.0015 m3/ML Screen size 25 mm : 0.015 m3/ML

Screenings should not be left in open or transferred in uncovered conveyers as it causes nuisance due to flies and insects

Screenings must be be properly disposed. Various methods of screening disposal were used such as:

- burning, - burying, - digestion, - dumping into large bodies of water, - and shredding and returning it to wastewater collection or treatment system.

Inland burying is efficient in small treatment plants, while burning is

best for medium and large treatment plants. Other methods cause problems and may need subsequent treatment. Digestion is used for large systems and in combination with the treatment of the organic portion of municipal solid waste.

Comminutors

Shredding devices (communitor or grinder) : shreds material to 1/4 inch -

3/8 inch.

Comminutors are devices used in water and wastewater treatment either in combination with screens or independently with the aim of chopping the oversized suspended and/or floating material found in water and wastewater or escaping the screens before entering the treatment facilities and altering its operation.

Comminutors consist of two sets of cutters one is fixed while the other is moving. The distance between the two sets equal to the size of chopped material required.

Comminution technology has been evolving quite rapidly in response to the increasing burden entrained solids have placed on treatment facilities.

More advanced devices have been developed in rapid succession. The result has been an exciting and fluid race between the leading manufacturers to develop the best size reduction device.

The latest grinder innovations to be introduced have coupled the power of twin shaft grinding with higher flow capabilities and screw screening systems. Heres a rundown on the past and present state of the art in wastewater solids reduction

Comminutor Design

For comminutor design, environmental engineer or designer need to supply manufacturer with the size of suspended and floating materials present in water to be treated and that after treatment along with its density and hydraulic and organic loadings. Accordingly manufacturer decides on the equipment needed to achieve the objective.

Grit Chambers

Grit chambers are designed to remove grit, consisting of sand, gravel, cinders, or other heavy solid materials that have settling velocity or specific gravities substantially greater than those of the organic putrescible solids in wastewater.

The removal of grit is essential to protect moving mechanical equipment and pump elements from abrasion and accompanying abnormal wear and tear

To reduces formation of heavy deposits on pipes, channels and conduits

To reduce the frequency of cleaning the sludge digesters The specific gravity of grits are usually in the range of 2.4 to

2.65

Characteristics of grit

sand, gravel, cinders, eggshells, bone chips, seeds, coffee grounds and other heavy materials

predominantly inert, composition variable moisture content 13 - 65%, volatile organic content - 1

- 56% specific gravity - clean grit particles 2.4 - 2.65, for

material with substantial organic material attached to inert - approx. 1.3

bulk density in the range of 1600 kg/m3 most grits are retained on a No. 100 mesh sieve (0.15

mm or larger) typical settling velocity for 100 mesh grit is 1.3 cm/s

Factors Affecting Quantity and Quality of Grits

Type of street surface encountered Relative areas served Climatic conditions Types of inlets and catch basins Amount of storm water diverted from combined

sewers at overflow points Sewer grades Construction and condition of sewer system Ground and groundwater characteristics Industrial wastes Social habits

Grit chamber

A grit chamber is an enlarged channel or long basin in which the c/s is increased to reduce the velocity of flowing sewage.

The velocity is maintained at a level where

Heavier grits (sp. Gravity 2.4 to 2.65) settle down

Lighter organic matter (sp. Gravity 1.02 to 1.5) remain in suspension

Types of Grit Removal

Horizontal flow

square

rectangular

Aerated (rectangular)

Vortex-type

Horizontal flow type

Flow through the channel is in horizontal direction Open channels with sufficient detention time to allow particles to

settle and to maintain constant velocity to scour organics The velocity of flow controlled by

Dimensions of unit Special influent distribution gates Special weir sections at the effluent end (proportional weir) Designed to maintain peak flow velocity in the range of 0.15 m/s to 0.3

m/s The velocity of flow should not change with change in flow

Designed to remove grit > 0.21 mm dia. to as low as 0.15 mm dia. Grit removal is accomplished by a conveyor with a scraper, buckets

or plows May require grit washing equipment to remove organics

Design of grit Chamber

Data required Hourly variations of sewage flow

Minimum, average, and maximum flows

Quantity and quality of grit, in absence of data 0.05 to 0.15 m3/1000 m3 of sewage for separate

domestic sewage

0.06 to 0.12 m3/1000 m3 of sewage for combined sewage

The quantity of grit may increase three to four fold during peak flow hours

Design Criteria

Settling Velocity Stokes Law Transition Law-Hazen equation

Surface overflow rate Detention time Scour and flow through velocity Velocity control devices

Proportional flow weir Parshall flume

Number of units Dimension of each unit Head loss

Settling Velocity

Stokes law

Stokes law holds good for Re

Settling velocity

Transition law (for 1

Surface Overflow rate

Q/plan area Settling velocity of those particles that will be 100%

removed Efficiency of grit chamber Real SOR varies from ideal SOR due to eddies, short circuit,

turbulence etc

Where n is an index for measure of chamber performance 0.125 for very good 0.25 for good O.5 for poor 1 for very poor In practice value of 0.67 or 0.5 used

SOR

Vs

ns

AQ

nV1

/11

Detention Time and Bottom Scour Through Velocity

Detention time 45 sec to 90 sec typically 60 sec

Bottom Scour Through Velocity affected scour velocity At a critical velocity Vc particles of certain size and

density may be reintroduced into stream The critical velocity is calculated from modified

Shields formula

In actual practice horizontal velocity kept at 0.15 to 0.30 m/s and should be maintained constant

4 asken usually ta 4.5 to3 Kc

)1(

dSgKV scc

Velocity Control Devices: Proportional flow weir

Combination of weir and orifice Consists of rectangular plate with an opening with curved

sides for flow to pass through The shape of the proportional flow weir is such that the

discharge through the weir is proportional to the depth of flow over the weir crest

It maintains nearly constant velocity for different flow rate by flowing at different depths and hence c/s area

The sides are curved in such a way that the width of the opening decreases as a square root of increasing depth

Proportional flow weir should be placed in such a elevation as to produce free fall at as it cannot function in submerged conditions

Each grit chamber must be provided with separate weir

Proportional flow weir

Q=KlH3/2

l: length of weir

K: constant

Curved sides diverging downwards in form of hyperbolic curve

a

ybx (tan

21

2

1

storagegrit for chamber grit

of bottom above m 0.3set weir alProportion

mm 35usually mm 50 tomm 25 : a

0.65 to0.6 :C

32

d

a

HgabCQ d

Parshall Flume

Open constricted channel used as Flow measuring device Velocity control device

Negligible head loss Can work under submerged conditions Limits of submergence

50% for 150 mm throat 70% for wide throats upto 1m

One flume can be installed for two to three grit chambers Approaches a parabolic c/s to maintain constant flow Can be approximated to rectangular section with trapezoidal

bottom Max allowable variation in velocity :5%

Number of units Manually cleaned : atleast 2 grit chambers Mechanically cleaned: one additional manually cleaned grit chamber

to act as bypass

Dimensions of each unit Plan area: based on SOR Width of grit chamber: based on velocity control device Length : from plan area and width Depth:

based on horizontal velocity and peak flow, additional depth for grit storage Free board 150 to 300 mm

Bottom slope: based on scraper mechanism

Head Loss Varies from 0.06m to 0.6 m based on velocity control device

Aerated Grit Chamber

popularity of aerated grit chamber less wear on grit-handling equipment in many cases, no need for separate grit washing equipment

normally designed to remove particles 65 mesh (0.2 mm) or larger

velocity of roll or agitation governs the size of particles of a given specific gravity to be removed

quantity of air is adjusted to provide the roll and washing of the grit to remove organic matter

grit removed by using grab buckets on monorails centered over the grit collection and storage trough or by a flushing through a drain

Sedimentation tank

Surface overflow rate Plain primary sedimentation tank: 40 to 50 m3/m2.day

Sedimentation tanks using coagulants: 50 to 60 m3/m2.day

Secondary sedimentation tank : 25 to 35 m3/m2.day

Lower SOR leads to settlement of finer particles

Detention time : 1 to 2 hrs

Width of tank : 6m (not to exceed 7.5 m)

Length of tank