Sewerage System and treatment - unit 3

Accessories for Sewerage System

• Man-holes: Man holes are the openings of either circular or rectangular in shape constructed on the alignment of a sewer line to enable a person to enter the sewer for inspection, cleaning and flushing. They serve as ventilators for sewers, by the provisions of perforated man-hole covers. Also they facilitate the laying of sewer lines in convenient length.

• Man-holes are provided at all junctions of two or more sewers, whenever diameter of sewer changes, whenever direction of sewer line changes and when sewers of different elevations join together.

• _{Special Man-holes:}

• Junction chambers: Man-hole constructed at the intersection of two large sewers.

• _{Drop man-hole: When the difference in elevation of the invert levels of the incoming and outgoing sewers}

of the man-hole is more than 60 cm, the interception is made by dropping the incoming sewer vertically outside and then it is jointed to the man-hole chamber.

• Flushing man-holes: They are located at the head of a sewer to flush out the deposits in the sewer with water.

• _{Lamp-holes: Lamp holes are the openings constructed on the straight sewer lines between two man-holes}

which are far apart and permit the insertion of a lamp into the sewer to find out obstructions if any inside the sewers from the next man-hole.

• _{Street inlets: Street 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.

Pumping of Sewage

•

Pumping of sewage is required when it is not

possible to have a gravitational flow for the entire

sewerage project.

•

Sufficient pumping capacity has to be provided to

meet the peak flow, atleast 50% as stand by.

Types of pumps :

•

Centrifugal pumps either axial, mixed and radial

flow.

Wastewater Treatment

Characteristic of the 

Effluent

Sewage in Suface Water Sources

Tolerance 

limit for Discharge of

BOD

20 mg/L

TSS

30 mg/L

Unit Operations/Processes, Their Functions and 

Units Used for Domestic Wastewater Treatment

Unit Operations/Processes Functions Treatment Devices

Preliminary Treatment Screening Removal of large floating, suspended and settleable solids Bar racks and screens of various description Grit Removal Removal of inorganic suspended

solids

Grit chamber

Primary Treatment Primary Sedimentation Removal of organic/inorganic settleable solids Primary sedimentation tank

Secondary/ Biological Treatment

Aerobic Biological Suspended Growth Process

Conversion of colloidal, dissolved and residual suspended organic matter into settleable biofloc and stable inorganics

Activated sludge process units and its modifications, Waste stabilisation ponds, Aerated lagoons

Aerobic Biological Attached Growth Process

same as above Trickling filter, Rotating biological contactor

Anaerobic biological

growth processes Conversion of organic matter into CH₄ & CO₂ and relatively stable organic residue

Anaerobic filter, Fluid bed submerged media anaerobic reactor, Upflow anaerobic sludge blanket reactor, Anaerobic

rotating biological contactor Anaerobic

Stabilization of Organic Sludges

Activated sludge plant involves:

•

wastewater aeration in the presence of a

microbial suspension,

•

solid-liquid separation following aeration,

•

discharge of clarified effluent,

•

wasting of excess biomass, and

•

return of remaining biomass to the aeration

Activated Sludge Process

wastewater containing organic matter is aerated in an

aeration basin in which micro-organisms metabolize the

suspended and soluble organic matter. Part of organic

matter is synthesized into new cells and part is oxidized to

CO

and water to derive energy. In activated sludge

Other Aerobic Treatment Units

Stabilization ponds

: The

stabilization ponds

are open flow through

basins specifically designed and constructed to treat sewage and

biodegradable industrial wastes. They provide long detention

periods extending from a few to several days.

Aerated lagoons

: Pond systems, in which oxygen is provided

through mechanical aeration rather than algal photosynthesis are

called

aerated lagoons

.

Oxidation ditch

: The oxidation ditch is a modified form of "

Anaerobic Treatment

•

The anaerobic waste treatment process is an effective method

for the treatment of many organic wastes. The treatment has a

number of advantages over aerobic treatment process, namely,

•

the energy input of the system is low as no energy is required for

oxygenation,

•

lower production of excess sludge( biological synthesis) per unit

mass of substrate utilized,

•

lower nutrient requirement due to lower biological synthesis,

and

•

degradation leads to production of biogas which is a valuable

Nitrification

There are two groups of chemoautotrophic bacteria that can

be associated with the process of nitrification. One group

(

Nitrosomonas

) derives its energy through the oxidation of

ammonium to nitrite, whereas the other group (

Nitrobacter

)

obtains energy through the oxidation of nitrite to nitrate. Both

the groups, collectively called

Nitrifiers

, obtain carbon

required, from inorganic carbon forms. Nitrification of

ammonia to nitrate is a two step process:

            Nitrosomonas         Nitrobacter

Phosphorus Removal

•

Phosphorus precipitation is ususally achieved

by addition of chemicals like calcium

Simple Sorting

•

Goal: clean water

•

Source: (contaminated) surface water

•

Solution: separate contaminants from water

Unit Processes Designed to Remove

Particulate Matter

•

Screening

•

Sedimentation

•

Coagulation/flocculation

•

Filtration

–

slow sand filters

–

rapid sand filters

Conventional Surface Water Treatment

Screening

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Storage

Distribution

Raw water

Alum

Polymers

Cl

sludge

sludge

Screening

•

Removes large solids

–

logs

–

branches

–

rags

–

fish

•

Simple process

–

may incorporate a mechanized trash

removal system

Sedimentation

•

the oldest form of water treatment

•

uses gravity to separate particles from water

•

often follows coagulation and flocculation

•

occurs in NYC’s __________

Sedimentation: Effect of the particle

concentration

•

Dilute suspensions

–

Particles act independently

•

Concentrated suspensions

–

Particle-particle interactions are significant

–

Particles may collide and stick together

(form flocs)

–

Particle flocs may settle more quickly

–

Particle-particle forces may prevent further

How fast do particles fall in dilute

suspensions?

Gravity

Fluid drag

•

What are the important

parameters?

–

Initial conditions

–

After falling for some time...

•

What are the important

forces?

–

_________

Coagulation

•

Coagulation is a physical-chemical process

whereby particles are destabilized

•

Several mechanisms

–

adsorption of cations onto negatively charged

particles

–

decrease the thickness of the layer of counter

ions

Coagulant introduction with rapid mixing

•

The coagulant must be mixed with the water

•

Retention times in the mixing zone are typically

between 1 and 10 seconds

•

Types of rapid mix units

–

pumps

–

hydraulic jumps

–

flow-through basins with many baffles

–

In-line blenders

Flocculation

•

Coagulation has destabilized the particles

by reducing the energy barrier

•

Now we want to get the particles to collide

•

We need relative motion between particles

–

________ ________ (effective for particles

smaller than 1

m

m)

–

_________ _____________ (big particles hit

smaller particles)

–

_______

Differential sedimentation

Shear

Mechanical Flocculation

•

Shear provided by turbulence

created by gentle stirring

•

Turbulence also keeps large flocs

from settling so they can grow

even larger!

•

Retention time of 10 - 30 minutes

•

Advantage is that amount of

shear can be varied independent

of flow rate

Hydraulic Flocculators

•

Types

–

Horizontal baffle

–

Vertical baffle

–

Pipe flow

•

Questions for design

–

How long must the suspension be in the “reactor”

–

How should the geometry of the reactor be

Coagulation/Flocculation

•

Inject Coagulant in rapid mixer

•

Water flows from rapid mix unit into

flocculation reactor

•

Water flows from flocculation reactor into

sedimentation tank

Jar Test

•

Mimics the rapid mix, flocculation,

sedimentation treatment steps in a beaker

•

Allows operator to test the effect of different

coagulant dosages or of different coagulants

Unit Processes in Conventional Surface Water

Treatment

•

We’ve covered

–

Sedimentation

–

Coagulation/flocculation

•

Coming up!

–

Filtration

–

Disinfection

Conventional Surface Water Treatment

Screening

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Storage

Distribution

Raw water

Alum

Polymers

Cl

sludge

sludge

Filtration

•

Slow sand filters

•

Diatomaceous earth filters

•

Membrane filters

Slow Sand Filtration

•

First filters to be used on a widespread basis

•

Fine sand with an effective size of 0.2 mm

•

Low flow rates (10 - 40 cm/hr)

•

Schmutzdecke (_____ ____) forms on top of

the filter

–

causes high head loss

–

must be removed periodically

•

Used without coagulation/flocculation!

Membrane Filters

•

Much like the membrane filters used to

enumerate coliforms

–

much greater surface area

•

Produce very high quality water (excellent

particle removal)

•

Clog rapidly if the influent water is not of

sufficiently high quality

Rapid Sand Filter

(Conventional US Treatment)

Sand

Gravel

Influent

Drain

Effluent

Wash water

Anthracite

Size

(mm)

0.70

0.45 - 0.55

5 - 60

Particle Removal Mechanisms in Filters

Transport

Attachment

Molecular diffusion

Inertia

Gravity

Interception

Straining

Water Treatment

Sources of water

1.Surface water- rivers, lakes, reservoirs etc.

2.Underground water – wells and springs

3.Rain water

Surface water

•

River water – dissolved minerals

Cl

-

, SO

4

2-

, HCO

3

-

of

Na+, Mg

2+

, Ca

2+

and Fe

2+

suspended impurities- Organic matter,

sand,

rock

composition is NOT constant – dep on the contact with soil.

•

Rain water

– pure form

dissolved organic and inorganic particles and

dissolved industrial gases CO

2

, NO

2

,SO

2

etc

•

Underground water

-

free from organic

impurities due to filtering action of the soil

•

Sea water

– very impure; too saline for

Impurities in water

•

Suspended impurities

inorganic (clay, sand) organic (oil,plant, and

animal matter)

•

Colloidal impurities- finely divided silica and

clay

•

Dissolved impurities – salts and gases

Hardness of water

•

Hardness prevents the lathering of soap.

due to the presence of salts of Ca, Mg, Al, Fe and Mn

dissolved in it.

Soap – Na or K salts of long chain fatty acids

C

17

H

35

COOH

2C

H

COONa + CaCl

→ (C

H

COO)

Ca↓ + 2NaCl

The Cleansing Action of Soap

•

Hard Water

Does not produce lather with

soap

Contains Ca and Mg salts

Soap is wasted and cleaning

quality is depressed

Boiling point elevated, more

time and fuel for cooking

•

Soft Water

Produces lather easily with soap

Does not contain dissolved Ca

and Mg salts

Cleaning quality of soap not

depressed.

Types of Hardness

•

Temporary Hardness-

caused by dissolved

bicarbonates of Ca and Mg

Also known as ‘alkaline or carbonate hardness’

•

Permanent Hardness

– dissolved Cl- and

Temporary Hardness

caused by dissolved bicarbonates of Ca and Mg

Temporary hardness can be removed by boiling of

water

Ca(HCO

3

)

2

→ CaCO

3

↓ + H

2

O + CO

2

↑

Mg(HCO

3

)

2

→ Mg(OH)

2

↓ + 2 CO

2

↑

Also known as ‘alkaline or carbonate hardness’

Permanent Hardness

CaCl

2

, MgCl

2

, CaSO

4

, MgSO

4

, FeSO

4

, Al

2

(SO

4

)

3

Cannot be destroyed on boiling the water

Also known as

non-carbonate

or

non alkaline

hardness

non alkaline

hardness = Total hardness – alkaline

Hard Water

•

Advantages

Tastes better

Ca in water helps produce

strong teeth and bones

Hard water coats lead pipes

with layer of insoluble CaCO

,

preventing any poisonous

lead dissolving in drinking

water

•

Disadvantages

no taste, produces scum with soap

Degree of Hardness

•

Hardness is expressed as equivalent amount

(

equivalents

) of CaCO

3

Reason: Molar mass is exactly 100, and is the most insoluble salt that can be

precipitated in water treatment.

Equvalents of

CaCO

( mass of hardness producing substance in mg/L) x100 / (eq.wt of

substancex2)

units – mg/L = ppm

Equivalent weight

•

Eq. wt = Molar mass/ no of charge on ion

CaCO

3

MM/2

NaCl

MM/1

AlCl

3

MM/3

Potable Water (Drinking water)

•

Colorless and odorless; good in taste

•

Turbidity should be less than 10 ppm

•

No objectionable dissolved gases like H

2

S

or minerals such as Pb, As , Cr, Mn salts.

•

Alkalinity should not be high; pH 7.0 – 8.5

•

Total hardness less than 500 ppm

•

Free of harmful microorganisms.

•

Cl-, F-, and SO

4

2–

less than 250, 15 and

Methods of disinfection of water

1. Bleaching powder (CaOCl

2

)

CaOCl

2

+H

2

O → Ca(OH)

2

+ HCl + HOCl

Enzymes of microorganism get deactivated by HOCl

•

Excess imparts bad taste and smell

•

Not stable during storage

2. Chlorination

•

Commonly used disinfectant in water

used directly as a gas or conc. solution.

It produces HOCl, a powerful germicide.

3. Disinfection by ozone

•

O

3

→ O

2

+ O

oxygen atom is a

powerful oxidizing agent.

2 – 3 ppm is injected

10 – 15 min contact time

Alkalinity

•

The capacity of water accept H+ is called alkalinity

•

The basic species responsible are

1.

HCO

+ H

→ H

2

O

2.

CO

+ H

→ HCO

-3.

OH- + H+ → H

O

Different from basicity; high pH

pH is an intensity factor

alkalinity is a capacity factor

1.00x10

M NaOH - pH=11;neutralize 1.00x10

mole acid

•

Alkalinity of water is attributed to presence of

i. caustic alkalinity (due to OH

-

and CO

3

2-

ions)

ii. Temporary hardness (due to HCO

3

-

ions)

i. [OH-] + [H+] → H

2

O -P -M

ii. [CO

3

2-

] + [H+] → [HCO

3

-

] -P -M

iii. [HCO

3

-

] + [H+] → H

2

O + CO

2

-M

P = OH- + ½ CO

3

2-

-Biological oxygen demand (BOD)

•

BOD is the quantity of dissolved O

2

required

by aerobic bacteria for oxidation of organic

matter under aerobic conditions.

source of effluent

BOD(ppm)

Domestic sewage 320

Cow shed sewage

3010

Paper mill

8190

Chemical oxygen demand (COD)

•

Defined as the oxygen consumed in the

oxidation of organic and oxidizable inorganic

matter.

Use a strong oxidizing agent like K

2

Cr

2

O

7

COD > BOD (

O2 is a weak oxidizing agent)

Treated Effluent Disposal

•

The proper disposal of treatment plant effluent or reuse requirements is an essential part of

planning and designing wastewater treatment facilities. Different methods of ultimate

disposal of secondary effluents are discussed as follows.

Natural Evaporation

•

The process involves large impoundments with no discharge. Depending on the climatic

conditions large impoundments may be necessary if precipitation exceeds evaporation.

Therefore, considerations must be given to net evaporation, storage requirements, and

possible percolation and groundwater pollution. This method is particularly beneficial where

recovery of residues is desirable such as for disposal of brines.

Groundwater Recharge

•

Methods for groundwater recharge include rapid infiltration by effluent application or

impoundment, intermittent percolation, and direct injection. In all cases risks for

groundwater pollution exists. Furthermore, direct injection implies high costs of treating

effluent and injection facilities.

Irrigation

•

Irrigation has been practiced primarily as a substitute for scarce natural waters or sparse

rainfall in arid areas. In most cases food chain crops (i.e. crops consumed by humans and

those animals whose products are consumed by humans) may not be irrigated by effluent.

However, field crops such as cotton, sugar beets, and crops for seed production are grown

with wastewater effluent.

Recreational Lakes

• The effluent from the secondary treatment facility is stored in a lagoon for approximately 30 days. The effluent from the lagoon is chlorinated and then percolated through an area of sand and gravel, through which it travels for approximately 0.5 km and is collected in an interceptor trench. It is discharged into a series of lakes used for swimming, boating and fishing.

Aquaculture

• Aquaculture, or the production of aquatic organisms (both flora and fauna), has been practiced for centuries primarily for production of food, fiber and fertilizer. Lagoons are used for aquaculture, although artificial and natural wetlands are also being considered. However, the uncontrolled spread of water hyacinths is itself a great concern because the flora can clog waterways and ruin water bodies.

Municipal Uses

• Technology is now available to treat wastewater to the extent that it will meet drinking water quality standards. However, direct reuse of treated wastewater is practicable only on an emergency basis. Many natural bodies of water that are used for municipal water supply are also used for effluent disposal which is done to supplement the natural water resources by reusing the effluent many times before it finally flows to the sea.

Industrial Uses

• Effluent has been successfully used as a cooling water or boiler feed water. Deciding factors for effluent reuse by the industry include (1) availability of natural water, (2) quality and quantity of effluent, and cost of

processing, (3) pumping and transport cost of effluent, and (4) industrial process water that does not involve public health considerations.

Discharge into Natural Waters

•

The world's population, under the current growth trajectory, is expected to reach

nearly 9 billion by the year 2042 (Worldometers; IDB)

•

However, the annual renewable water resources in the world amount to about

•

Calculation of hardness caused by each ion.

Na+ - 20 mg/L

Ca

2+

- 15 mg/L

Mg

2+

- 10 mg/L

Sr

2+

- 2 mg/L

Al

3+

- 0.3 mg/L

Equvalents of

CaCO

( mass of hardness producing substance in mg/L) x100 / (eq.wt of

substancex2)

Cation

Eq.wt

Hardness

Ca

2+

40.0/2

37.5

Mg

2+

24.4/2

41.0

Sr

2+

87.6/2

2.3

Al

3+

27.0/3

1.7

Example 1:

•

A water sample contains 408 mg of CaSO

4

per

liter. Calculate the hardness in terms of CaCO

3

equivalents

Hardness = mg/L of CaSO

4

x 100/MM(CaSO

4

)

= 408 mg/L x 100/136

Example 2

•

How many grams of MgCO

3

dissolved per liter

gives 84 ppm of hardness?

Hardness = mg/L of MgCO

3

x 100/MM(MgCO

3

)

84 ppm = ppm of MgCO

3

x 100/84

ppm of MgCO

3

= 84 ppm x (100/84)