Transcript Water Treatment Processes

ENVR 430-2
Microbial Control Measures by
Water Processes
Suggested Reading:
Brock
Chapter 28- Wastewater Treatment,
Water Purification, and Waterborne
Microbial Diseases pp. 934-942
(posted as .PDF file on website)
Simmons 31
Summary of Disinfectants for Microbes in Water and Wastewater
Historically, the essential barrier to prevention and control of
waterborne microbial transmission and waterborne disease.
•
•
•
•
Free chlorine: HOCl (hypochlorous) acid and OCl- (hypochlorite ion)
– HOCl at lower pH and OCl- at higher pH; HOCl a more potent germicide than
OCl– strong oxidant; relatively stable in water (provides a disinfectant residual)
Chloramines: mostly NH2Cl: weak oxidant; provides a stable residual
Ozone: O3 , strong oxidant; provides no residual (too volatile and reactive)
Chlorine dioxide: ClO2,, strong oxidant; unstable residual (dissolved gas)
Concerns due to health risks of chemical disinfectants and their
by-products (DBPs), especially free chlorine and its DBPs
•
UV radiation:
– low pressure mercury lamp: low intensity; monochromatic at 254 nm
– medium pressure mercury lamp: higher intensity; polychromatic 220-280
nm)
– reacts primarily with nucleic acids: pyrimidine dimers and other alterations
Simmons 32
Factors Influencing Disinfection
Efficacy and Microbial Inactivation
Microbe type: Resistance to chemical disinfectants:
• Vegetative bacteria: Salmonella, coliforms, etc.
Least
• Enteric viruses: coliphages, HAV, SRSVs, etc.
• Protozoan (oo)cysts, spores, helminth ova, etc.
– Cryptosporidium parvum oocysts
– Giardia lamblia cysts
– Clostridium perfringens spores
– Ascaris lumbricoides ova
• Acid-fast bacteria: Mycobacterium spp.
Most
Simmons 33
Factors Influencing Disinfection Efficacy
and Microbial Inactivation
Type of Disinfectant and Mode of Action:
Free chlorine: strong oxidant; oxidizes various protein
sulfhydryl groups; alters membrane permeability;
oxidize/denature nucleic acid components, etc.
Ozone: strong oxidant
Chlorine dioxide: strong oxidant
Combined chlorine/chloramines: weak oxidant; denatures
sulfhydryl groups of proteins
Ultraviolet radiation: nucleic acid damage; thymidine dimer
formation, strand breaks, etc.
Simmons 34
Factors Influencing Disinfection Efficacy
and Microbial Inactivation - Bacteria
• Surface properties conferring susceptibility or resistance:
• Resistance: Spore; acid fast (cell wall lipids); capsule; pili
• Susceptibility: sulfhydryl (-SH) groups; phospholipids;
enzymes; porins and other transport structures, etc.
• Physiological state and resistance:
• Antecedent growth conditions: low-nutrient growth increases
resistance to inactivation
• Injury or in a state of “injury repair”
• disinfectant exposure may select for resistant strains
• Physical protection:
• Aggregation; particle-association; biofilms; occlusion
(embedded within protective material), association with or
inside eucaryotes; corrosion/tuberculation
Simmons 35
Factors Influencing Disinfection
Efficacy and Inactivation - Viruses
Virus type, structure and composition:
• Envelope (lipids): typically labile to disinfectants
• Capsid structures and capsid proteins (change in
conformation state)
• Nucleic acids: genomic DNA, RNA; # strands
• Glycoproteins: often on virus outer surface;
typically labile to disinfectants
Simmons 36
Factors Influencing Disinfection Efficacy
and Microbial Inactivation - Parasites
Parasite type, structure, and composition:
• Protozoan cysts:
– oocysts (environmentally resistant form)
– some are very resistant to chemical disinfectants
• Helminth ova:
– some are very resistant to chemical disinfection,
drying and heat
– used as indicators for biosolids (Class A)
Simmons 37
Effects of Water Quality on Disinfection
• Particulates: protect microbes from inactivation
– microbes shielded or embedded in particles
• Dissolved organics: protects
– consumes or absorbs (UV radiation) disinfectant
– coats microbes
• Inorganic compounds and ions: effects vary with
disinfectant
• pH: effects depend on disinfectant.
– Free chlorine more biocidal at low pH where HOCl
predominates.
– Chlorine dioxide more microbiocidal at high pH
Simmons 38
Disinfection: A Key Barrier Against Microbes in
Water and Wastewater
Free chlorine still the most commonly used disinfectant
• Maintaining disinfectant residual during treated water storage
and distribution is ESSENTIAL!!
– A problem for O3 and ClO2, which do not remain in water for very long
and for UV, which produces no disinfectant residual
– A secondary disinfectant must be used to provide a stable residual
• UV radiation is a promising disinfectant because it inactivates
Cryptosporidium at low doses
– UV may have to be used with a chemical disinfectant to protect
the water with a residual through distribution and storage.
• Sequential disinfection with a strong oxidant (HOCl, O3, or
ClO2) followed by monochloramine may give a synergistic
effect that is more effective against microbial pathogens
Simmons 39
Enteric Microbes, Water Sources and Water Treatment
Drinking water must be essentially free of disease-causing
microbes, but often this is not the case.
– A large proportion of the world’s population drinks microbially
contaminated water, especially in developing countries
• Using the best possible source of water for potable water supply
and protecting this source from microbial and chemical
contamination is the goal.
– In many places an adequate supply of pristine water or water that can be
protected from contamination is not available
• The burden of providing microbially safe drinking water supplies
from contaminated natural waters rests upon conventional water
treatment processes.
– The efficiency of removal or inactivation of enteric microbes and other
pathogenic microbes in specific water treatment processes has been
determined for some microbes but not others.
Simmons 40
Watershed Protection: Planning the Future of our Source Water
Austin, Texas
Does your utility need assistance in developing a source water protection plan?
Attend this all-new seminar to learn about Source Water Plans in detail, as well
as how to determine source water susceptibility and how to develop
emergency plans. You'll also explore the impact of public participation and
education as a vital component of Source Water Protection.
You'll Learn:
• What is a Source Water Protection Plan (SWPP)?
• Federal, State, and Local Government Roles in Source Water Protection Plans
• Delineation of Source Water Protection Areas
• Contamination
• Determining Source Water Susceptibility
• SWP Area Management
• Development of Emergency Plans
• Public Participation and Education
• Source Water Assessment Program Implementation
• Funding Options
Simmons 41
STORAGE:
Reservoirs,
lakes
Simmons 42
Water Treatment Processes: Storage
Reservoirs, aquifers & other systems: store water / protect it from contamination
Site specific with respect to factors influencing microbial reductions
– detention time
– temperature
– microbial activity
– water quality
– sunlight
– sedimentation
– land use
– precipitation
– runoff or infiltration
Household storage and collection is practiced in some places
– Stored volumes are relatively small and storage times are short
– contamination during collection and storage is a problem
• Point-of-use water treatment reduces pathogens enteric disease
Simmons 43
Water Storage
and
Microbial
Reductions
• Microbial reductions by storage reduces microbe levels over
time by natural antimicrobial processes and microbial death
or die-off (bacteria)
• One study in The Netherlands showed human enteric viruses
in surface water were reduced by 400-1,000-fold when
storage was for 6-7 months.
• Protozoan cyst reductions by storage are also expected but
data are limited.
– Data from the ICR indicates lower protozoan levels in
reservoir or lake sources than in river sources
Simmons 44
Chemical CoagulationFlocculation
Removes suspended particulate and colloidal
substances from water, including microorganisms.
1.
2.
3.
4.
5.
Typically, add alum (aluminum sulfate), ferric chloride, or sulfate
to the water with rapid mixing and controlled pH conditions
Insoluble aluminum or ferric hydroxide and aluminum or iron
hydroxo complexes form
These complexes entrap and adsorb suspended particulate and
colloidal material.
Slow mixing (flocculation) is then used to promote the
aggregation and growth of the insoluble particles (flocs).
Resulting floc particles are subsequently removed by gravity
sedimentation (or direct filtration)
Simmons 45
Microbial Reductions by Chemical
Coagulation-Flocculation
• Enteric microbes reductions In laboratory and pilot scale field studies:
– 90 to >99% using alum or ferric salts as coagulants
– other studies report much lower removal efficiencies by
coagulation-flocculation.
– Conflicting information may be related to PROCESS CONTROL
• coagulant concentration, pH and mixing speed during
flocculation.
Expected to reduce enteric microbe concentrations of water by 90-99% if
critical process variables are adequately controlled
• Microbes are NOT inactivated by chemical coagulation
Infectious microbes remain in the chemical floc
– The floc removed by settling and/or filtration must be properly
managed to prevent pathogen exposure.
• Recycling back through the plant is undesirable
• Filter backwash must be disinfected and disposed of properly.
Simmons 46
Filtration of Drinking Water
Widely used to remove suspended particles (turbidity) incl. microbes.
•
Historically, two types of granular media filters:
1. Slow sand filters: uniform bed of sand; low flow rate <0.1 GPM/ft2
2. Rapid sand filters: 1, 2 or 3 layers of sand/other media; >1 GPM/ft2
•
Membrane filters: more recent development and use in drinking water
– microfilters: several tenths of mM to mM diameter pore size
– nano- and ultra-filters: retention based on molecular weight cutoff
• Typically 1,000-100,000 MWCO
Reverse osmosis filters: pore size small enough to remove dissolved
salts; used to desalinate (desalt) water as well as particle removal
Adsorbers and filter-adsorber systems:
– Granular activated carbon adsorption: remove dissolved organics
– Sand and granular activated carbon filter-adsorber:
• reduces particles and organics
•
•
Simmons 47
Slow Sand Filters
• Less widely used for large municipal water supplies in the USA
• Effective and still widely used in Europe and for smaller water
supplies.
• Filter water through a 3- to 5-foot deep bed of unstratified sand at a
flow rate of about 0.05 gallons per minute per square foot.
• Biological growth that develops in the upper surface of the sand is
primarily responsible for particle and microbe removal.
• Filters are effective without pretreatment of the water by
coagulation-flocculation.
• Filters are periodically cleaned by removing, cleaning and replacing
the upper few inches of biologically active sand (Schmutzdecke)
Simmons 48
Microbial Reductions by Slow Sand Filtration
Quite effective in removing enteric microbes from water.
• Virus removals >99% in laboratory models of slow sand
filters.
• Lab studies: reovirus removals of up to 4 log10; no
infectious viruses recovered from filter effluents
• Field studies:
– naturally occurring enteric viruses removals
• 97 to >99.8 percent; average 98% overall;
• Comparable removals of E. coli bacteria.
– Virus removals= 99-99.9%; high bacteria removals
• Parasite removals: Giardia lamblia cysts effectively
removed
– Expected removals ~ 99%
Simmons 49
Microbe Reductions by Rapid
Granular Media Filters
Ineffective in removing enteric microbes unless
preceded by chemical coagulation-flocculation.
• Enteric microbe removals of 90->99 % achieved.
• Field (pilot) scale studies: rapid sand filtration preceded by iron
coagulation-flocculation
– virus removals of <50% (poor control?).
– Giardia lamblia: removals not consistently high; related to the efficiency
of turbidity removal; >99% removals reported when optimized.
– Removal may not be high unless turbidity is reduced to <0.2 NTU.
• Lowest removals shortly after filter backwashing
– Microbes primarily removed in filter by entrapped floc particles.
Overall, can achieve ~90% microbial removals from water
when preceded by chemical coagulation-flocculation.
Simmons 50
Cryptosporidium Removals by Sand
Filtration
Type
Rate (M/hr)
Coagulation
Reduction
% (log10)
Rapid, shallow
Rapid, shallow
5
5
No
Yes
65
90
Rapid, deep
6
Yes
99.999 (5.0)
0.2
No
Slow
99.8
(0.5)
(1.0)
(2.7)
Simmons 51
Reverse Osmosis Membrane Filter Plant
Osmosis: the natural tendency
is for water to move through
the membrane from the dilute
to the concentrated solution
until chemicals reach equal
concentrations on both sides of
the membrane
Simmons 52
Removal Effect of the Water purifier for Home
Use against Cryptosporidium parvum Oocysts
Toshihiro MATSUI1), Junko KAJIMA2) and Takashi FUJINO1)
1) Department of Infectious Diseases, Kyorin University School of Medicine
2) Research and Study Department, Japan Association of Parasite Control
ABSTRACT. The removal effects of the faucet mounted type water purifier
for home use were examined against Cryptosporidium parvum oocysts. The
water purifier is composed of a layer of granular activated carbon and the
hollow fiber membrane filter. The cartridges were unused, 25%, 50% and
75% flow down by Arizona-dust of U. S. A. Two respective cartridges were
used of the examination. The faucet and the water purifier were connected by
anti-pressure tube, and 3.0 × 107 oocysts of Cryptosporidium parvum were
injected into anti-pressure tube while water was running. Twenty liter of
collected purified water was examined under the fluorescent microscope. Any
oocysts in the purified water collected from all cartridges were not
found. Therefore, we considered this purifier as an effective one in
removing Cryptosporidium oocysts from drinking water.
From 2004 Journal of Veterinary Medical Science 66:941-43
Simmons 53
Cryptosporidium Reductions by
Membrane Filtration
Membrane,
Type
Pore Size
Log10 Cryptosporidium
Reduction
A, MF
B, MF
0.2 µm
0.2 µm
>4.4
>4.4
C, MF
D, UF
E, UF
0.1 µm
500 KD
300 KD
4.2->4.8
>4.8
>4.8
F, UF
100 KD
>4.4
MF = microfilter filter; UF = ultrafilter
Jacangelo et al., JAWWA, Sept., 1995
Simmons 54
Contaminant Removal by Granular
Activated Carbon
• Good removal of
chemical and
microbial
contaminants
• Mode of removal:
fissures adsorb
chemicals/particles
• Problem: what to do
with material after
use?
Hazardous material !!
Simmons 55
Water Softening
• “Hard” Water: contains excessive amounts of
calcium and magnesium ions
– iron and manganese can also contribute to hardness.
• Hardness ions are removed by adding lime (CaO)
and sometimes soda ash (Na2CO3) to precipitate
them as carbonates, hydroxides and oxides.
• This process, called softening, is basically a type
of coagulation-flocculation process.
Simmons 56
Microbial Reductions by Softening Treatment
• Softening with lime only (straight lime softening); moderate to
high pH
– ineffective enteric microbe reductions: about 75%.
• Lime-soda ash softening
– results in the removal of magnesium as well as calcium
hardness at higher pH levels (pH >11)
– enteric microbe reductions >99%.
– Lime-soda ash softening at pH 10.4, 10.8 and 11.2 has produced
virus reductions of 99.6, 99.9 and 99.993 percent, respectively.
At lower pH levels (pH <11), microbial removal is mainly a
physical process
– infectious microbes accumulate in the floc particles and the
resulting chemical sludge.
At pH levels above 11, enteric microbes are physically removed
and infectivity is also destroyed
– more rapid, extensive microbe inactivation at higher pH levels.
Simmons 57
Drinking Water Treatment for Pathogen Control:
The Multiple Barrier Approach
Pathogen reductions are best achieved by
using a series of treatment processes
Typical water treatment process train for surface water:
1. Source water management and protection (Storage)
2. Coagulation-flocculation-sedimentation
3. Rapid granular medium filtration
4. Disinfection
Can you estimate the microbial reductions at each
stage of treatment and overall by such a system?
Will it be the same for all classes of microbes?
Simmons 58
Study Points:
• Wastewater treatment processes
– Primary (settling), secondary (biological), tertiary
(disinfection) treatment
– Waste liquid treatment (aerobic); waste solids treatment
(anaerobic)
– Alternative systems: Facultative oxidation ponds,
Constructed wetlands, On-site septic systems, etc.
• Disinfection
– Kinetics (First-order, Retardant, and Multi-hit)
– Disinfectant types (chemical vs. physical disinfectants)
– Factors that effect disinfection
• Water treatment processes
– Chemical (coagulation-flocculation, lime softening)
– Physical (slow sand filtration, rapid granular media
filtration, membrane filtration)
Simmons 59