Week-5

Sewer Appurtenances

i. Manholes, Drop manholes

ii. Inlets, Catch basins

iii. Oil and grease traps

iv. Flush tanks

v. Inverted siphons

vi. Pumping station

These are devices, other than sewers

Essential for the efficient operation of sewer systems.

These include;

i) Man-holes

a) Purpose of providing the manholes are as followings:

Are openings on sewer line either circular or rectangular in shape

Cleaning and Flushing

Inspection

House connections

Serve as ventilators for sewer by providing perforated man-hole covers

Facilitate laying of sewers in convenient length

Junction of two or more sewers

Change in direction i.e. At every change in direction

Change in size i.e. Where two pipes having different sizes are to be

connected

Change in slope i.e. pipes having different slopes are to be connected

b) Criteria for provision of manholes is given as below:

c) Distance or Spacing between manholes

Depends upon the size of sewer.

Recommended spacing of WASA are given below.

Sewer Size

(mm)

Spacing b/w Manholes

(m)

255 -350 100

460-760 120

>760 150

in Pakistan, generally spacing > 100 feet is not provided

Larger spacing can offer

difficulty in connection of houses

cleaning of sewer lines.

View of Manhole

Drop man-hole

When lateral or sub-mains join in a

deeper sewer. (Diff > 2 feet)

When drop is < 2 feet adjustment

of grade is made instead of drop

manhole and sewage is dropped

in slopping channel without

splashing.

Upper Sewer is kept at reasonable

grade.

Incoming sewer is dropped outside

and joined in manhole chamber.

Assembly saved excavation .

The sewer line intersects the MH

wall, provision for cleaning

/rodding of branch can be kept

Construction of Manholes

Large cities Standardized Design and plans .

Have cast- iron frame and cover with a 500 to 600 mm (20 to 24 inches) clear opening.

The frame rest on corbelled brickwork (stone or timber projection from wall) to form an opening not less than 1 m (40 inches) and usually 1.5 m (3 – 5 feet) below the manhole.

The 1.25 m cylinder is continued downward until it reaches the sewer.

If the total depth < 4m (12 feet), the walls are made 200 mm (9”) thick.

An extra 200 mm thickness should be added for each addition 2 m (6 feet) depth.

Concrete walls (Cost of const. increases)

Usually bottom is made of concrete (1:2:4).

Slightly Slopped bottom from upper surface towards the open channel or, which are continuous of the sewer pipes.

I any case the channel depth should be nearly equal to pipe diameter to prevent sewage from spreading over the manhole.

Inlets

Inlets are the openings through which storm water is admitted

and conveyed to the storm sewer or combined sewer. The inlets

are located by the sides of pavement with maximum spacing of

30 m.

i. Curb Opening Inlet

ii. Grate Inlet

iii) Catch Basins

Small settling chambers ( Dia = 60 - 90 cm & depth = 60 - 75 cm)

Constructed below the street inlets.

Interrupt the velocity of storm water entering through the inlets

Grit, sand, debris and other such matter settle in the basin,

sewers.

Inverted siphons

These are depressed portions of sewers,

Flow full under pressure more than the atmospheric

pressure as flow line being below the hydraulic grade line.

Provided when a sewer crosses a stream or deep cut or

road or railway line.

To clean the siphon pipe sluice valve is opened, thus

increasing the head causing flow.

The increased velocity washed away deposits of siphon

pipe into the sump, from where they are removed.

Pumping Station

Continuation of gravity flow is no longer feasible.

Basements of buildings are below the grade of the sewer

Any obstacle lies in the path of sewer

Receiving stream is higher than the sewer

Sewage is to be delivered to an above ground treatment plant

A sewage pumping station consists of following components

Components of Sewage Pumping Station

i) Screen

Function is to remove the floating matter to avoid damages to

pumping machinery etc.

ii) A wet well

Function of wet well is to receive the sewage

Important Consideration for Size of Wet Well

Prevention of too frequent starting and stopping of pumps.

Keep Size small to avoid septic action in the sewage.

Slopped bottom towards the pumps inlets to prevent accumulation of

solids.

Recommended D.T in the wet well should be less than 30 minutes.

Provide Ventilation of the wet well (Avoid excessive condensation &

odors).

Provide a MH or any other mean of entrance in wet well.

Used to install the pumps and switches etc.

Small stations are made entirely underground

In large Station, a house is built for the installation of these

things.

iii) Dry well

Size of dry well

Depends upon the size and number of pumps to be installed.

Pumps along with motor, valves, pipes and control penal

etc.

More than one pump are required to allow the flexibility

of operation

iv) Pumps

Figure- Typical View of Sewage Pumping Station

It is the time between the 2 successive start up of motor of pump.

Generally 10-20 minutes minimum.

The vol. of the wet well is designed on the basis of mini. cycle time.

Capacity of the Pump

Should be equal to the maximum flow of the sewage.

Gate valve

Gate valves are needed in the suction line and in the discharge line.

Should be all iron wedge type with rising stem to avoid corrosion.

Check valve

Should be placed between the pump and the gate valve in the

discharge pipe.

Should be an all-iron swing type with outside level and weight to

reduce slam.

Cycle Time

ON OFF ON OFF

Cycle Time

Cycle Time = Time of run (Time of emptying) + Time Off (Time of filling)

Or

t = V/(P-Q) + V/Q ------------(I)

where

V = Storage volume

P = Pump Discharge

Q = the influent flow

Differentiating Equation No.(I) W.r.t. Q and equating equal to zero

dt/dQ = V / (P-Q)2 – V / Q2 = 0

V / Q2 = V / (P-Q)2

Q2 = (P-Q)2

By Solving

Q = P/2

tmin = V / (P – P/2) + V/(P/2) = 4V / P

V = (tmin P) / 4

Volume of wet well can be found from above relation

Location of Pumping Station

Important consideration are as following:

i. Sewage pumping station should not be located in flood plain

ii. Sewage Pumping Station must be safe from fire hazard.

iii. Power and other fuel supply required to run the sewage

pumping station be available.

iv. Site must be large enough to cope the future expansion

requirement.

v. Site should be environmentally and socially accepted.

Example: Design a wet well of the flow is Qmin = 0.74 Cfs,

Qave = 1.547 Cfs and Qmax = 5.415

Sol

V = (tmin P) / 4

Let

tmin = 20 minutes (assumed 10 – 20 minutes

This time is minimum for Qmax = 5.425 Cfs. Hence

pumping rate , P = 5.415 Cfs

V = (20x60x5.415) / 4 = 1624.5 ft3

Now to check for “t” for Qmin = 0.74 Cfs

T = 1624.5 / (5.415-0.74) + 1624.5 / 0.74

= 42 minutes > 20 minutes

And for Qave. = 1.547 Cfs

t = 1624.5 / (5.415 – 1.547) + 1624.5 / 1.547)

= 24.5 minutes > 20 minutes

A wet well of operating volume of 1600 ft3 was designed for minimum cycle time

of a motor pump of 5 MGD (million gallons per day) capacity. Calculate the

inflow rate at which the cycle time will be twice the minimum cycle time (1 MGD

= 1.547 Cfs)

Example

Solution V = 1600 ft3

P = 5 MGD = 5 x 1.547

= 7.74 cfs

Inflow Rate = Q = ?

V = (tmin P) / 4

tmin = (4 x 1600) / 7.74 = 82.687 Sec

T = 13.78 min = 14 minutes

t = V/(P-Q) + V/Q

Here t = 2 tmin = 2 x 14 = 28 min

28 x 60 = 1600 / (7.74-Q) + 1600 / Q

= Q2 - 7.74Q + 7.74

Q = 6.56 cfs , 1.18 cfs

Sewage Characteristics

Wastewater Treatment

Sewage Characteristics

Wastewater Treatment

Consists of 99.9 % of the water and

Remaining as solids (Organic or inorganic ), Have very significant effect

Characteristics are determined by various analysis / tests.

Only few are used.

a) Physical Characteristics

Solids Determination

Total Solids

TS = Suspended Solids (SS), Settle-able Solids and the Diss. Solids

Determine by evaporating a known volume or weight of sewage

sample and weighing the residue.

Expressed in mg/l.

Settle-able Solids help in selection of treatment process design and

operation.

VS are ignitable at 550 0C and represent the amount of OM in WW.

Ignite the sample at about 550 0C.

Some of the solids will be evaporated.

VS = Original Weight - weight of residue (ash)

Non-VS / ash and are rough measure of the mineral content in WW.

Volatile Solids(VSS)

SS indicates the amount of sludge to be produced & Removed in PST.

Indicate the strength of sewage and amount of treatment required.

Also indicate the efficiency of wastewater treatment plant.

Both SS & Dissolved Solids determination require filtration of the

sample.

Settle-able Solids (SS)

Fresh sewage has a slightly ---------oily odour, which does not have

objectionable smell.

Gives objectionable smell when the sewage become septic releases

Hydrogen Sulphide (H2S).

Odour

Temp. slightly more than the temperature of the water supplied

Generally the temperature of the sewage ranges between 10 – 12 0C.

Effect disposal of sewage will increase temp. of final receiving water

body and thus release its oxygen - will effect the aquatic life.

Temperature

Fresh sewage has a grey colour

As sewage becomes septic the colour changes to

black.(Decomposition of OM)

Colour

b) Chemical Characteristics

Important for the control of processes of wastewater treatment plant

1. Inorganic chemicals.

2. Gases

3. Organic

i. pH.

ii. Acidity,

iii. Alkalinity

iv. Chlorides,

v. Nitrogen and

vi. Sulfur

vii. heavy metals

1. Inorganic Chemicals

i) pH

Indicates acidic or alkaline condition of water.

Expressed on a scale ranging from 0 to 14, Common logarithm of the reciprocal of the hydrogen ion concentration.

Plays an important role in WWT & Operation of treatment plant.

ii) Acidity

Acidity of WW is a measure of its capacity to neutralise bases.

Acidity in water may be caused by the presence of uncombined carbon dioxide, mineral acids and salts of strong acids and weak bases.

Acidity in WW may due to addition of industrial discharges

It is expressed as mg/l in terms of calcium carbonate.

iii) Alkalinity

Alkalinity is a measure of capacity WW to neutralise acids.

It is expressed as mg/l in terms of calcium carbonate. Forms include

(a) hydroxide alkalinity,

(b) carbonate alkalinity,

(c) hydroxide plus carbonate alkalinity,

(d) carbonate plus bicarbonate alkalinity, and

(e) bicarbonate alkalinity,

Alkalinity is an important parameter in evaluating the optimum

coagulant dosage.

Ordinary sewage is slightly alkaline.

Treatment of wastewater require alkaline conditions.

iv) Chlorides

Sources - Human and animal urine.

High concentrations indicate addition of industrial WW in the sewage.

v) Nitrogen

Nitrogen in wastewater may exist in five (05) forms

a) Ammonia Nitrogen (free ammonia due to decomposition of

organic matter.)

b) Ammonia Nitrogen (Albuminoid which is measure of

decomposable Proteins)

c) Organic Nitrogen

d) Nitrates

e) Nitrites

N & P (Nutrients) - essential to the growth of protista and plants.

Nitrogen data - evaluate biological treatability of WW.

To control algal growth in receiving waters,

Removal of N in wastewater prior to its discharge is desirable.

SO”4 + Organic Matter Bacteria S-2 + H2O + O2

S-2 + 2H H2S

Microorganisms reduced Sulfates under anaerobic condition to

sulfide.

Sulfides combine with hydrogen to form hydrogen sulfide (H2S).

Accumulated H2S can be oxidized biologically to H2SO4 which

causes sewer corrosion

vi) Sulfur

vii) Heavy Metals

Metals having specific gravity ___ time the water are called

heavy metals.

Major source - Industrial effluents

Heavy metals have adverse health effects on human and

animals.

Mainly include oxygen (O2) and Hydrogen Sulphide (H2S)

Oxygen in WW is necessary for

aerobic conditions i.e. growth of microorganism to avoid

adverse effects on aquatic life.

Hydrogen gas in higher concentrations is lethal for life.

Whereas, H2S gas in sewers results in “Crowning of

Sewers”

2. Gases

Coliforms

Pathogens

Worms

Bacterial counts in raw sewage may range from 500,000 / ml to

5,000,000 / ml.

Concerns in wastewater reuse of agricultural irrigation.

c) Bacteriological Characteristics

3. Organic Matter

Includes Carbohydrates, Proteins, and Fats.

Total quantity of OM is measured by

biochemical oxygen demand (BOD),

Chemical Oxygen Demand (COD) and

Total Organic Carbon (TOC).

The method generally employed are as followings:

Measurement of Organic Contents

1) Biochemical Oxygen Demand (BOD5)

2) Chemical Oxygen Demand (COD)

3) Total Organic Carbon

1) Biochemical Oxygen Demand (BOD5)

It is defined as the amount of oxygen required by the bacteria for

stabilizing the decomposable organic matter under aerobic condition.

Significance :

a) An indicator of organic pollution-ie.

(strength of the sewage & industrial wastewater)

a) Larger the BOD more oxygen demand of bacteria i.e. more the

depletion of O2 in receiving water body.

b) Used to design the WWTP

c) Determines It is used to determine the biological treatment efficiency

of WWTP.

d) Stream and effluent standards are generally based on BOD5 at 20 0C

Biological Oxidation of Organic Matter.

“Monomolecular Reaction”(i.e. First order Chemical Reaction).

A chemical reaction in which the rate of reaction is proportional

to the concentration of the reactants present”

i.e.

dL/dt = -KL

where

L = Concentration of Organic Mater

K = Reaction Rate Constant

Lt t

dL/dt = - K

dt

L0 to

Where

Lt = Organic Matter remaining at time “t” ie. Remaining BOD at time “t”

Lo = Original Concentration of Organic matter i.e. Ultimate BOD

ln Lt / Lo = -Kt

Lt = Lo e –kt ----------I

Let “y” be the concentration of OM (BOD) consumed upto time “t”

Then

y = Lo – Lt

Putting from (i)

y = Lo - Lo e –kt

y = Lo (1- e –kt) ie

BOD Consumed = Ultimate BOD ( 1- e –kt)

Also

t = Time in days

K = BOD reaction rate constant and

K = Its value for domestic sewage is 0.23 per day at 20 0C

However, value of reaction rate constant depends upon the temperature by

following relation

KT = K20 (10.047) T-20

Determination of BOD

Methods used are as followings:

1) Director Method

2) Dilution methods

1) Direct Method

i) Take sample

ii) Aerate the sample so that sufficient sample should be available

at time of incubation

iii) Measure Dissolved Oxygen (DO) at “ Zero day”

iv) Measure Dissolved Oxygen (DO) at 05 days.

v) The difference of these two readings is BOD of the sample and it

is reported in mg/l at 20 0C.

Used where BOD is less than 7 mg/l.

Used for very polluted sample.

Polluted samples – Large OM render sample O2 deficient in half

day. Dilution is done with specially prepared dilution water.

In toxic industrial samples, bacteria would have killed, therefore

seeding (addition of bacteria) 30 – 40 % of volatile solids.

BOD (mg/l) = [ ( DOsi – DO sf ) – (DOdi – DO df ) ]

Dilution Factor

Where

DOsi and DOsf = Initial and final Dissolved Oxygen (DO) in the diluted

sewage /sample

and

DOdi and DOdf = Initial and final Dissolved Oxygen (DO) in the dilution

water.

(This amount is to be subtracted to account for the

amount of Oxygen consumed by seeds)

( DOdi - DOdf ) = 0 (if dilution water used is not seeded)

2) Dilution Method

Example

If the 5 days BOD of the sewage is 154 mg/l, what would be

its 3 day BOD. Take K=0.1day-1

Sol.

T = 5 days

Lt = 154 mg/l

L = ?

K = 0.1 day-1

We have

Lt = L(1-e-Kt)

154 = L( 1-e-5x0.1)

L = 391 mg/l

Now for 3days BOD

Lt=3 = 391(1-e-3x0.1) = Lt=3 = 101 mg/l

2) Chemical Oxygen Demand (COD):

It is defined as the amount of oxygen required to oxidize the

organic matter chemically by using a strong oxidizing agent

(K2Cr2O7) in an acidic medium (H2SO4).

COD > BOD because materials like fats and lignin are also

oxidized with the help of chemicals, which are otherwise

biodegrade slowly.

No clear co-relation exist between BOD and COD

Co-relation is possible at a particular treatment plant.

Advantages :

Rapid test - Requires 2 hours as compared to BOD5

BOD/COD ratio indicates the extent of biodegradability of WW

BOD/COD correlation may help in rapid assessment of BOD.

COD of a sample is always more than BOD

Measurement of COD

i. Acid oxidation with Potassium dichromate.

ii. A measured amount of Potassium dichromate and acidified

sample is boiled for two hours.

iii. Allow to cool the sample.

iv. Titrate sample with ferrous ammonium sulphate to determine

remaining dichromate.

3) Total Organic Carbon:

i. Indicates the TOC present in a wastewater sample.

ii. Rapid and accurate test

iii. Correlates moderately well with BOD.

iv. Involve high cost of analysis.

Example

The 5-days BOD of the sewage is 276 mg/l. The ultimate BOD

of the sewage is reported to be 380 mg/l. At what rate the

sewage sample was oxidized

Sol.

T = 5 days

Lt = 276 mg/l

L = 380 mg/l

K = ?

We have

Lt = L(1-e-Kt)

276 = 380( 1-e-5K)

0.726 = ( 1-e-5K) = e-5K = 0.274

Log10 0.274 = -5K K = 0.112 day-1

If the 5 days BOD of the sewage is 154 mg/l, what would be its 3 day

BOD. Take K=0.1day-1

Example

Sol

. T = 5 days

Lt = 154 mg/l

L = ?

K = 0.1 day-1

We have

Lt = L(1-e-Kt)

156 = L( 1-e-5x0.1)

L = 225 mg/l

Now for 3days BOD

Lt=3 = 225(1-e-3x0.1) = Lt=3 = 112 mg/l