Water and Sewage Treatment
Objectives of Water Treatment
? The objectives for water treatment derive
from two concerns
– Human health and welfare
– The health of aquatic ecosystems
Water treatment for domestic and commercial
uses
Criteria Causing Pollutant Corresponding
removal measures
Free of odor Odorous gases Aeration to remove
odor gases
Free of
turbidity
Particles Addition of Fe3+ and
Al3+ to trap particles
Low BOD Organic materials Bacteria metabolism
Low nutrient
loading
Dissolved N,P
inorganic
Chemical precipitation
NH3 stripping
Free of disease
causing agents
microorganisms Addition of
disinfectants
Primary water treatment
? Primary treatment,Remove solids by screening
and settling
– The sewage is passed through a screen to remove
large pieces of debris (e.g,sticks,stones,rags,and
plastic bags).
– Next,the sewage enters a grit chamber,where the
water flow is slowed just enough to allow coarse
sand and gravel to settle out on the bottom.
– Water then enters the sedimentation tank,its flow
rate is further decreased to permit suspended solids
to settle out as raw sludge.
Primary water treatment (Continued)
? Ca(OH)2 and Al2 (SO4)3 are often added to speed up
the sedimentation process.
3 Ca(OH)2 + Al2(SO4)3 ? 2 Al(OH)3 + 3 CaSO4
Al(OH)3 is a gelatinous precipitation that settles out
slowly,carrying suspended material and bacteria with
it.
? Oily material floats to the surface and is skimmed off.
? The grit is collected and disposed in landfill.
? The raw sludge:
– Old way,incinerated,disposed in landfill or dumped at
sea.
– New way,composted to produce a nutrient rich bacteria-
free material for use as fertilizer.
Primary treatment
In older sewage-treatment plants,the water after
primary treatment is often chlorinated to kill
pathogens and then discharged into a natural
waterway.
The discharged water at this stage still contains a
large amount of oxygen-consuming wastes,which
may deplete dissolved oxygen in the water way and
cause eutrophication,
Secondary treatment
? Secondary treatment,also called biological
treatment,Use bacteria to break down organic
compounds to CO2,
– A mixture of organisms – termed activated sludge–
is added to the sewage effluent.
– Air or oxygen is vigorously bubbled through pipes
into the effluent.
– The aerobic bacteria digest the organic material and
break it down into CO2 and water.
– The bacteria and any remaining undecomposed
material are returned to the aeration tank and
reused.
Activated sludge process
Secondary treatment of municipal wastewater
Most municipal plants chlorinated the water after
secondary treatment and then release it into waterways.
The discharged water at this stage has ~ 90% of the
original organic matter removed,but over 50% of N,P
species remains,and metal ions and many synthetic
organic compounds are incompletely removed.
Tertiary treatment
? Remove N and P nutrients.
– P removal by precipitation with lime
3 PO43- + CaO (lime) ? Ca5(PO4)3(OH)
– Phosphate can also be removed by microorganisms
that absorb phosphate.
– NH4+ removal by ammonia stripping.
NH4+ + OH- = NH3 + H2O (Excess OH- from lime)
– Alternative NH4+ removal,nitrifying bacteria convert
NH4+ to NO3- followed by denitrifying bacteria to
convert NO3- to N2,
? Remove organics through filtration by activated carbon
Tertiary treatment,also called advanced waste
treatment,includes a variety of processes performed on
the effluent from secondary waste treatment.
Hydroxyapetite
Tertiary treatment of municipal wastewater
Performance of primary and secondary stages
of sewage treatment
Pollutants Removed
Component removed Primary
treatment
Secondary
treatment
Biological oxygen
demand
30% 90%
Suspended solids 60% 90%
Nitrogen compounds 20% 50%
Phosphorus compounds 10% 30%
Source,American Chemical Society
Sludge disposal
? Sludge is an excellent fertilizer in principle,rich in
organic material and nutrients.
? Sludge often contains toxic metal species,which
restricts the application of sludge to cropland.
? Sludge can be a low-quality fuel for generating
electricity.
? Sludge could be converted to methane by anaerobic
bacteria,but this option suffers poor economics.
Disinfection
? Common disinfectants,Cholrine,chlorine
dioxide,and ozone.
? Disinfectants kill microorganisms by oxidizing
vital molecules (often with unsaturated
carbon bond) in them.
– Cl2 + H2O = HOCl + H+ + Cl-
Hypochlorous acid
Active disinfection component
Pros and cons of various disinfectants
? Cl2,
– Cl2 is effective and relatively cheap.
– HOCl can act as a chlorinating agent to produce a
variety of chlorinated organic compounds (for example,
CHCl3).
– Many of the Cl-containing organics are toxic and non-
biodegradable,Some (e.g,CH2Cl2,CHCl3,C2HCl3)
are suspected carcinogens.
? O3 and ClO2,
– More expensive than Cl2.
– Need to be generated on-site ? add on to the capital
cost.
– Fast-acting and rapidly decomposed,(Persistence of
disinfectants allows disinfect water where leakage
through old pipes occur.)
Generation of ClO2 and O3
? ClO2
2 NaClO2 (s) + Cl2 (g) = 2ClO2 (g) + 2 NaCl (s)
Sodium hypochlorite
? O3
Subject pressurized air to an electric discharge of
~20,000v.
ANTHAX CLEANUP
Hart Senate Office Building Decontaminated (STEVE RITTER)
Chlorine dioxide gas was pumped into the offices of Senate Majority Leader Thomas A,Daschle (D-
S.D.) in the Hart Senate Office Building on Dec,1 during one of the final steps of EPA's anthrax
decontamination efforts on Capitol Hill (C&EN,Nov,26,page 24).
Anthrax spores initially were detected in several Capitol office buildings after an anthrax-laden letter,
addressed to Daschle and opened by an aide on Oct,15,had passed through the congressional mail
system,Some buildings were temporarily closed for testing and some sections of the buildings
subsequently decontaminated using chlorine bleach and an antimicrobial foam.
The Hart Building,home to 50 senators,had the highest exposure to spores and presented the
greatest challenge,EPA decided to limit the use of ClO2 to Daschle's 3,000-sq-ft office suite,while
only bleach or the foam were used elsewhere in the building.
Daschle's offices were sealed off and then exposed to as much as 800 ppm of ClO2 for about 20
hours,followed by treatment with sodium bisulfite vapor to neutralize the residual gas,
Postdecontamination test results to check the effectiveness of ClO2 against spores were expected
back after a week,The Hart Building is slated to reopen by the end of the year.
Chemical & Engineering News
December 10,2001
Study questions
1,What does primary,secondary,and tertiary water
treatment achieve? How does each water
treatment step achieve its goal(s)?
2,What chemicals are often used for disinfection?
What is the mechanism of disinfection?
Objectives of Water Treatment
? The objectives for water treatment derive
from two concerns
– Human health and welfare
– The health of aquatic ecosystems
Water treatment for domestic and commercial
uses
Criteria Causing Pollutant Corresponding
removal measures
Free of odor Odorous gases Aeration to remove
odor gases
Free of
turbidity
Particles Addition of Fe3+ and
Al3+ to trap particles
Low BOD Organic materials Bacteria metabolism
Low nutrient
loading
Dissolved N,P
inorganic
Chemical precipitation
NH3 stripping
Free of disease
causing agents
microorganisms Addition of
disinfectants
Primary water treatment
? Primary treatment,Remove solids by screening
and settling
– The sewage is passed through a screen to remove
large pieces of debris (e.g,sticks,stones,rags,and
plastic bags).
– Next,the sewage enters a grit chamber,where the
water flow is slowed just enough to allow coarse
sand and gravel to settle out on the bottom.
– Water then enters the sedimentation tank,its flow
rate is further decreased to permit suspended solids
to settle out as raw sludge.
Primary water treatment (Continued)
? Ca(OH)2 and Al2 (SO4)3 are often added to speed up
the sedimentation process.
3 Ca(OH)2 + Al2(SO4)3 ? 2 Al(OH)3 + 3 CaSO4
Al(OH)3 is a gelatinous precipitation that settles out
slowly,carrying suspended material and bacteria with
it.
? Oily material floats to the surface and is skimmed off.
? The grit is collected and disposed in landfill.
? The raw sludge:
– Old way,incinerated,disposed in landfill or dumped at
sea.
– New way,composted to produce a nutrient rich bacteria-
free material for use as fertilizer.
Primary treatment
In older sewage-treatment plants,the water after
primary treatment is often chlorinated to kill
pathogens and then discharged into a natural
waterway.
The discharged water at this stage still contains a
large amount of oxygen-consuming wastes,which
may deplete dissolved oxygen in the water way and
cause eutrophication,
Secondary treatment
? Secondary treatment,also called biological
treatment,Use bacteria to break down organic
compounds to CO2,
– A mixture of organisms – termed activated sludge–
is added to the sewage effluent.
– Air or oxygen is vigorously bubbled through pipes
into the effluent.
– The aerobic bacteria digest the organic material and
break it down into CO2 and water.
– The bacteria and any remaining undecomposed
material are returned to the aeration tank and
reused.
Activated sludge process
Secondary treatment of municipal wastewater
Most municipal plants chlorinated the water after
secondary treatment and then release it into waterways.
The discharged water at this stage has ~ 90% of the
original organic matter removed,but over 50% of N,P
species remains,and metal ions and many synthetic
organic compounds are incompletely removed.
Tertiary treatment
? Remove N and P nutrients.
– P removal by precipitation with lime
3 PO43- + CaO (lime) ? Ca5(PO4)3(OH)
– Phosphate can also be removed by microorganisms
that absorb phosphate.
– NH4+ removal by ammonia stripping.
NH4+ + OH- = NH3 + H2O (Excess OH- from lime)
– Alternative NH4+ removal,nitrifying bacteria convert
NH4+ to NO3- followed by denitrifying bacteria to
convert NO3- to N2,
? Remove organics through filtration by activated carbon
Tertiary treatment,also called advanced waste
treatment,includes a variety of processes performed on
the effluent from secondary waste treatment.
Hydroxyapetite
Tertiary treatment of municipal wastewater
Performance of primary and secondary stages
of sewage treatment
Pollutants Removed
Component removed Primary
treatment
Secondary
treatment
Biological oxygen
demand
30% 90%
Suspended solids 60% 90%
Nitrogen compounds 20% 50%
Phosphorus compounds 10% 30%
Source,American Chemical Society
Sludge disposal
? Sludge is an excellent fertilizer in principle,rich in
organic material and nutrients.
? Sludge often contains toxic metal species,which
restricts the application of sludge to cropland.
? Sludge can be a low-quality fuel for generating
electricity.
? Sludge could be converted to methane by anaerobic
bacteria,but this option suffers poor economics.
Disinfection
? Common disinfectants,Cholrine,chlorine
dioxide,and ozone.
? Disinfectants kill microorganisms by oxidizing
vital molecules (often with unsaturated
carbon bond) in them.
– Cl2 + H2O = HOCl + H+ + Cl-
Hypochlorous acid
Active disinfection component
Pros and cons of various disinfectants
? Cl2,
– Cl2 is effective and relatively cheap.
– HOCl can act as a chlorinating agent to produce a
variety of chlorinated organic compounds (for example,
CHCl3).
– Many of the Cl-containing organics are toxic and non-
biodegradable,Some (e.g,CH2Cl2,CHCl3,C2HCl3)
are suspected carcinogens.
? O3 and ClO2,
– More expensive than Cl2.
– Need to be generated on-site ? add on to the capital
cost.
– Fast-acting and rapidly decomposed,(Persistence of
disinfectants allows disinfect water where leakage
through old pipes occur.)
Generation of ClO2 and O3
? ClO2
2 NaClO2 (s) + Cl2 (g) = 2ClO2 (g) + 2 NaCl (s)
Sodium hypochlorite
? O3
Subject pressurized air to an electric discharge of
~20,000v.
ANTHAX CLEANUP
Hart Senate Office Building Decontaminated (STEVE RITTER)
Chlorine dioxide gas was pumped into the offices of Senate Majority Leader Thomas A,Daschle (D-
S.D.) in the Hart Senate Office Building on Dec,1 during one of the final steps of EPA's anthrax
decontamination efforts on Capitol Hill (C&EN,Nov,26,page 24).
Anthrax spores initially were detected in several Capitol office buildings after an anthrax-laden letter,
addressed to Daschle and opened by an aide on Oct,15,had passed through the congressional mail
system,Some buildings were temporarily closed for testing and some sections of the buildings
subsequently decontaminated using chlorine bleach and an antimicrobial foam.
The Hart Building,home to 50 senators,had the highest exposure to spores and presented the
greatest challenge,EPA decided to limit the use of ClO2 to Daschle's 3,000-sq-ft office suite,while
only bleach or the foam were used elsewhere in the building.
Daschle's offices were sealed off and then exposed to as much as 800 ppm of ClO2 for about 20
hours,followed by treatment with sodium bisulfite vapor to neutralize the residual gas,
Postdecontamination test results to check the effectiveness of ClO2 against spores were expected
back after a week,The Hart Building is slated to reopen by the end of the year.
Chemical & Engineering News
December 10,2001
Study questions
1,What does primary,secondary,and tertiary water
treatment achieve? How does each water
treatment step achieve its goal(s)?
2,What chemicals are often used for disinfection?
What is the mechanism of disinfection?
