Transcript Microbial Reductions in Wastewater Treatment

ENVR 890/296
Microbe/Pathogen Survival/Reduction in the
Environment, Excreta and Excreta
Treatment Processes
Mark D. Sobsey
Dept. of Environmental Sciences and Engineering
Microbial Survival/Inactivation – A Kinetic Phenomenon
100
Concentration →
• Microbe inactivation is best
described by the rate(s) or
kinetic of inactivation
• Survival depends on
reduction rate and time
(duration of exposure)
• Expressing survival in
absolute terms based on
time only such as days or
weeks is misleading
– Depends on initial and
final microbe
concentrations
• Express extent of
inactivation per unit of time
at specified conditions of
exposure
10
1.0
0.1
Time →
Microorganism survival periods in faeces, sludge and soil
Some Physical Factors Influencing Microbe
Survival in the Environment
TEMPERATURE
• Greater Inactivation/death rates at higher temperatures
• Lower survival rates at higher temperatures
– But, some microbes will grow or grow better at higher temperatures
• Many microbes survive better at lower temperature
– Some bacteria experience “cold injury” or“cold shock” and cold
inactivation
• Thermal inactivation differs between dry heat and moist heat
– Dry heat is much less efficient than moist heat in inactivating microbes
• Some microbes survive very long times when frozen
– Other microbes are destroyed by freezing
• Ice crystals impale them
• Increased environmental temperatures can promotes pathogen spread by
insect vectors (mosquitoes, flies, etc.)
• Relative acidity or alkalinity
• A measure of hydrogen ion (H+)
concentration
• Scale:
pH
– 1 (most acidic) to 14 (most alkaline or basic)
– pH 7 is neutral
– Moving toward pH 1 the substance is more
acidic
– Moving toward pH 14, the substance is more
alkaline.
• Extreme pH inactivates microbes
– Chemically alters macromolecules
– Disrupts enzyme and transport functions
– Some enteric pathogens survive pH 3.0 (tolerate
stomach acidity)
– Some pathogens survive pH 11 and fewer survive
pH 12
Microbes are most stable in the
environment and will grow in some
media (e.g., foods) in the mid pH range
Moisture Content – Drying and Dessiccation
• Drying or low moisture inactivates/kills some microbes
– Survival depends on moisture content or “water activity”But, removing
water content of some foods can preserve them
• Most viruses rapidly inactivated in soil at <1% moisture;
– Some inactivated rapidly at a few % moisture
– Time for 90% reduction in days-weeks-months; depends on moisture
level & temperature
• Some protozoan parasites (Cryptosporidium parvum) are rapidly
inactivated when dried (>90% reduction within hours at room temp.)
• Some helminth ova (Entamoeba hystolytica) are very persistent dry
• Bacteria persistence to drying and desiccation is highly variable
– Most bacteria can survive for extended periods of time
• Days to weeks for >90% reduction
• Bacteria and fungi spores are very persistent when dry
Physical Factors Influencing Survival, Continued
• Ultraviolet radiation: about 330 to 200 nm
– Primary effects nucleic acids; absorbs the UV energy and is damaged
• Sunlight:
– Ultraviolet radiation in sunlight inactivates microbes
– Visible light is antimicrobial to some microbes
• Promotes growth of photosynthetic microbes
• Ionizing radiation
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X-rays, gamma rays, beta-rays, alpha rays
Generally antimicrobial; bacterial spores relatively resistant
Main target of activity is nucleic acid
Effect is proportional to the size of the “target”
• Bigger targets easier to inactivate; a generalization; exceptions
– Environmental activity of ionizing radiation in the biosphere is not highly
antimicrobial
– Ionizing radiation is used in food preservation and sterilization
Atmospheric and Hydrostatic Pressure
• Most microbes survive typical atmospheric pressure
• Some pathogens in the deep ocean are adapted to high
pressure levels (hydrostatic pressures): barophiles
– Survive less well at low atmospheric pressures
– Spores and (oo)cysts survive pressure extremes
• High hydrostatic pressure is being developed as a process to
inactivate microbes in certain foods, such as shellfish
– Several 100s of MPa of pressure for several minutes
inactivates viruses and bacteria in a time- and pressuredependent manner
Role of Solids-Association in Microbial Survival
• Microbes can be on or in other, usually larger particles or
they can be aggregated (clumped together)
• Association of microbes with solids or particles and
microbial aggregation is generally protective
• Microbes are shielded from environmental agents by
association with solids
– Protection depends on type of solids-association
– See diagrams, right
• Protection varies with particle composition
– Organic particles: often highly protective
• Biofilms protect microbes in them
• React with/consume antimicrobial chemicals
– Inorganic particles vary in protection
• Opaque particles protect from UV/visible light
• Inorganic particles do not always protect well against
chemical agents
– Some inorganic particles are antimicrobial
• Silver, copper, other heavy metals/their oxides
Clumped: interior
microbes protected
Adsorbed: partiall
protected
Embedded:
most
protected
Dispersed:
least
protected
: Antimicrobial agent
Some Chemical Factors Influencing Microbe
Survival in the Environment
Effects
Chemicals and Nutrients Influence Microbial Survival
• Antimicrobial chemicals
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Strong oxidants and acids
Strong bases
Ammonia: antimicrobial at higher pH (>8.0)
Sulfur dioxide and sulfites: used as food preservatives
Nitrates and nitrites: used as food preservatives
Enzymes:
• Proteases
• Nucleases
• Amylases (degrade carbohydrates)
– Ionic strength/dissolved solids/salts
• High (or low) ionic strength can be anti-microbial
– Many microbes survive less in seawater than in freshwater
– High salt (NaCl) and sugars are used to preserve foods
» Has a drying effect; cells shrink and die
– Heavy metals:
• Mercury, lead, silver, cadmium, etc. are antimicrobial
• Nutrients
– for growth and proliferation
– Carbon, nitrogen, sulfur and other essential nutrients
Some Biological Factors Influencing Microbe
Survival in the Environment
Effects
Biological Factors Influence Microbial Survival
• Chemical antagonistic activity by other microorganisms:
– Proteolytic enzymes/proteases
– Nucleases
– Amylases
– Antibiotics/antimicrobials: many produced naturally by
microbes
– Oxidants/oxides
– Fatty acids and esters; organic acids (acetic, lactic, etc.)
• Predation
• Vectors
• Reservoir animals
Factors Affecting Survival in Liquid
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Temperature
Ionic Strength
Chemical Constituents/Composition of Medium
Microbial Antagonism
Sorption Status
Type of Microbe
Factors Affecting Survival in Aerosols
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Temperature
Relative Humidity
Moisture Content of Aerosol Particle
Composition of Suspending Medium
Sunlight Exposure
Air Quality (esp. “open air” factor)
Size of Aerosol Particle
Type of Microbe
Factors Affecting Survival on Surfaces
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Type of Microbe
Type of Surface
Relative Humidity
Moisture Content (Water Activity)
Temperature
Composition of Suspending Medium
Light Exposure
Presence of Antiviral Chemical or Biological Agents
Microbe Survival in Liquid Media
• Temperature
– Increased inactivation with increasing temperature
– Most are inactivated rapidly (minutes) above 50oC
– Some microbes are more thermotolerant than others (e.g. Hepatitis A
virus, bacterial/fungal spores, some helminth ova (ascarids)
• Most are inactivation more at higher temperatures
• Chemical composition of media influences survival
– Protein/other organics & Mg&Ca ions protect
– Generally very stable at ultra-cold temperatures,
• Some loss of infectivity occurs with freezing and thawing
Survival in Liquid Media
• pH
– Direct effects on conformation of proteins and other
biomolecules
– Indirect effects on adsorption and elution from particles
– pH range of stability is microbe-dependent
• Polio: 3.8 to 8.5 for maximum stability
• Salt Content
– Variable effects on microbe survival
– Affects microbe physiology (isotonic conditions), adsorption
and stability of biomolecules
– Divalent cations (Mg2+) can increase thermo-stability of
viruses and bacteria
• E,g., MHV, enteroviruses, HAV
Microbe Survival in Liquid Media
• Microbial Antagonism
– Microflora influences microbe survival
• Metabolites: enzymes, VFAs, NH3 are antiviral
– Use of pathogen as a nutrient source
– Greater microbe survival documented in sterilized or
pasteurized matrices, as compared to non-sterile
matrices
– Phenomena demonstrated in sewage, fresh, estuarine,
and marine waters, soils and sediments.
Microbe Survival in Liquid Media
• Adsorption
– Several possible mechanisms:
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Ionic attractions and repulsions
covalent reactions (with active chemicals)
hydrogen bonding
hydrophobic interactions
double layer interactions
van der Waal’s forces
– Adsorption status greatly influences survival
• adsorbed microbes generally survive longer than unadsorbed
microbes
• Protection and accumulation in sediments and soils
MicrobeSurvival in Liquid Media
• Organic Matter
– In liquid media, organic matter increases microbe survival
• Increased oxidant demand protects from oxidation
• If an enzyme substrate, protects from enzymatic attack
• Can coat to protect microbe particles
– In soils, organic matter has variable effects on microbes
• Possible competition for adsorption sites
• May coat or protect microbe particles
• Bacteria may grow of organics are nutrients
Microbe Survival in Liquid Media
• Antimicrobial Chemicals
– Ionic and non-ionic detergents, particularly for enveloped
viruses and some bacteria
– Ammonia is virucidal; ammonium ion is not
– Germicides (chlorine, ozone, etc.)
• Light
– Direct microbicidal activity below wavelengths of 370 nm
– Indirect antimicrobial activity:
• stimulation of microflora growth
• triggering formation of reactive oxidants
• activation of photoreactive chemicals
Microbe (Virus) Survival in Aerosols
• Relative Humidity and Moisture Content
– Viruses with lipid survive better at lower relative humidity
– Viruses with little or no lipid content survive better at higher
relative humidity
– Viral inactivation or retention of infectivity may be a function
of stabilization (drying of aerosol) and of rehumidification of
aerosol particle upon collection
– Effect of relative humidity on virus survival may be influenced
by temperature effects
Microbe Survival in Aerosols
• Temperature
– Survival decreases with increased temperature
• Suspending Media
– composition influences microbe stability
– effect is microbe dependent
• Salts stabilize some viruses (e.g. Poliovirus)
• Removal of salts stabilize other viruses (e.g. Langat, Semiliki Forest virus)
• Proteinacious material and organic matter may have similar mixed effects,
depending on microbe type
• Polyhydroxy compounds stabilize some virus types (e.g. Influenza) but have
no effect on other viruses
Microbes Survival in Aerosols
• Oxygen and Air Ions
– Oxygen has little direct effect on most viruses but may
influence bacteria
– But, oxygenation may be synergistic with higher temperature
and sunlight to inactivate microbes
– The “Open Air Factor” has been shown to have virucidal
activity
• Poorly characterized chemical agents in open air that reduce
virus survival compared to clean laboratory air
• May be reaction products of ozone and olefins
Microbe Survival in Aerosols
• Light
– Virucidal activity of UV light is a greater in air than in
liquid media
– Photosensitivity is virus type-dependent and may be
related to the envelope
• Non-enveloped viruses (Poliovirus, Adenoviruses and
FMDV) are more resistant to UV light than enveloped
viruses (vaccinia, herpes simplex, influenza, and Newcastle
disease virus) (Jenson, 1964; Donaldson 1975; Applyard,
1967)
Microbe Survival in Aerosols
• Aerosol Particle Size
– Airborne microbes may be more rapidly inactivated in
smaller aerosol particles than larger ones (some studies)
– Other studies observed no effect of particle size on virus
survival
• Aerosol Collection Method
– Abrupt rehydration of virus particles and other microbes upon
collection may lead to their inactivation
– Prehumidification may improve recovery of infectious virus
– Effect is virus type-dependent
Microbe Survival on Surfaces
• Adsorption State
– Air Water Interface
– Triple Phase Boundary
• Physical State
– Dispersed
– Aggregation
– Solids associated
Microbe Survival on Surfaces
• Relative humidity
– Similar effects as seen in aerosols; effects are microbe
type dependent
• Moisture Content
– In soils moisture content directly related to microbe
survival
• Dessication
• Enhanced predation
Microbe Survival on Surfaces
• Temperature
– Effects as observed in liquid media and aerosols
– Interaction between relative humidity and temperature
pronounced on surfaces for certain virus types (e.g.
Polio, Herpes Simplex), less important for others (e.g.
Vaccinia) (Edward, 1941)
Microbe Survival on Surfaces
• Suspending Media
– Effects similar to effects on survival in aerosols
• Presence of fecal material
• Presence of salts
• Type of Surface
– Little effect by non-porous surfaces on most viruses
• important for some virus types (Herpes simplex)
– Effects more pronounce for porous surfaces (e.g. fabrics:
cotton, synthetics and wool
• Light
– Effects similar to those in aerosols and liquids
Microbe type: Resistance to chemical disinfectants:
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Vegetative bacteria: Salmonella, coliforms, etc.:
Enteric viruses: coliphages, HAV, Noroviruses:
Bacterial Spores
Fungal Spores
Protozoan (oo)cysts, spores, helminth ova, etc.
– Cryptosporidium parvum oocysts
– Giardia lamblia cysts
– Ascaris lumbricoides ova
– Acid-fast bacteria: Mycobacterium spp.
low Least
moderate
High
Most
Factors Influencing Microbial Reductions by
Wastewater Treatment Processes
Solids association: microbes embedded in larger
particles or aggregated are:
– more likely to sediment (settle)
– protected from disinfection and other antagonists
– possibly different in their surface properties due to the
other material present
Factors Influencing Microbial Reductions by
Wastewater Treatment Processes
Temperature produces more microbial rapid
inactivation:
– at higher temp. by thermal effects (denaturation)
– in biological processes by more rapid biological
metabolism and enzymatic activity
– in chemical processes by faster reaction rates
Factors Influencing Microbial Reductions by
Wastewater Treatment Processes
Temperature elevation for some pathogens may
promote growth:
Naegleria fowlerii and other amebas
Legionella species
Mycobacteria species
Aeromonas species
Vibrio species
Factors Influencing Microbial Reductions by
Wastewater Treatment Processes
Biological activity can decrease pathogens by:
Grazing, phagocytosis and other predation mechanisms
Increased enzymatic activity by bacteria and other
treatment microbes:
proteases, amylases, nucleases, etc.
Increased adsorption to and accumulation in microbial
biomass complexes:
floc particles, biofilms, etc.
Primary Treatment or Primary Sedimentation
Settle solids for 2-3 hours in a static, unmixed tank or basin.
• ~75-90% of particles and 50-75% of organics settle out as
“primary sludge”
– enteric microbe levels in 1o sludge are sometimes ~10X
higher than in raw sewage
• enriched by solids accumulation
• Overall, little removal of many enteric microbes:
– typically ~50% for viruses and bacteria
– >50% for parasites, depending on their size
Enteric Microbe/Pathogen Reductions in Secondary
or Biological Treatment
• Aerobic biological treatment:
typically, activated sludge (AS) or
trickling filtration (TF)
• Then, settle out the biological solids
produced (2o sludge)
• ~90-99% enteric microbe/pathogen
reductions from the liquid phase
• Enteric microbe retention by the
biologically active solids:
accumulation in AS flocs or TF
biofilms
• Biodegradation of enteric microbes
by proteolytic enzymes and other
degradative enzymes/chemicals
• Predation by treatment
microbes/plankton (amoeba,
ciliates, rotifers, etc.
Aerobic microbes utililize carbon and
other nutrients to form a healthy
activated sludge AS biomass (floc)
The biomass floc is allowed to settle
out in the next reactor;
some of the AS is recycled
Waste Solids (Sludge) Treatment
• Treatment of settled solids from 1o and 2o sewage treatment
• Biological “digestion” to biologically stabilize the sludge solids
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Anaerobic digestion (anaerobic biodegradation)
Aerobic digestion (aerobic biodegradation)
Mesophilic digestion: ambient temp. to ~40oC; 3-6 weeks
Thermophilic digestion: 40-60oC; 2-3 weeks
• Produce digested (biologically stabilized) sludge solids for further
treatment and/or disposal (often by land application)
– “Thickening” or “dewatering”
– drying or “curing”
• Waste liquids from sludge treatment are recycled through the sewage
treatment plant
• Waste gases from sludge treatment are released
(or burned if from anaerobic digestion: methane, hydrogen, etc.)
Enteric Microbe/Pathogen Reductions by Sludge
Treatment Processes
• Anaerobic and aerobic digestion processes
– Moderate reductions (90-99%) by mesophilic processes
– High reductions (>99%) by thermophilic processes
• Thermal processes
– Reductions depend on temperature
• Greater reductions at higher temperatures
• Temperatures >55oC usually produce appreciable pathogen reductions.
• Alkaline processes: lime or other alkaline material
– Reductions depend on pH; greater reductions at higher pHs
• pH >11 produces extensive pathogen reductions
• Composting: high temperature, aerobic biological process
– Reductions extensive (>99.99%) when temperatures high and waste uniformly
exposed to high temperature
• Drying and curing
– Variable and often only moderate pathogen reductions
“Processes to Further Reduce Pathogens” “PFRP”: Class A Sludge
Class A sludge:
• <1 virus per 4 grams dried sludge solids
• <1 viable helminth ovum per 4 grams dried sludge solids
• <3 Salmonella per 4 grams of dried sludge solids
• <1,000 fecal coliforms per gram dry sludge solids
PFRPs:
• Thermal (high temperature) processes (incl. thermophilic
digestion); hold sludge at 50oC or more for specified times
• lime (alkaline) stabilization; raise pH 12for 2 or more hours
• composting: additional aerobic treatment at elevated temperature
• Class A sludge or “biosolids” disposal by a variety of options or
used as a soil conditioner
– Class A biosolids can be marketed/distributed as soil conditioner for
use on non-edible plants
Alternative Biological Treatment of Wastewater:
Alternatives for Small and Rural Communities
• Lagoons, Ponds and Ditches
– aerobic, anaerobic and facultative; for smaller communities and farms
– enteric microbes are reduced by ~90-99% per pond
• multiple ponds in series increases microbe reductions
• Constructed Wetlands
– aerobic systems containing biologically active, oxidizing microbes and
emergent aquatic plants
• Lagoons and constructed wetlands are practical and economical
sewage treatment alternatives when land is available at
reasonable cost
Facultative Oxidation (Waste Stabilization)
Pond
Stabilization Ponds or Lagoons
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Aerobic and Facultative Ponds:
Biologically Rx by complementary activity of algae and bacteria.
Used for raw sewage as well as primary- or secondary-Rx’d. effluent.
Bacteria and other heterotrophs convert organic matter to carbon
dioxide, inorganic nutrients, water and microbial biomass.
Algae use CO2 and inorganic nutrients, primarily N and P, in
photosynthesis to produce oxygen and algal biomass.
Many different pond designs have been used to treat sewage:
facultative ponds: upper, aerobic zone and a lower anaerobic zone.
Aerobic heterotrophics and algae proliferate in the upper zone.
Biomass from upper zone settles into the anaerobic, bottom zone.
Bottom solids digested by anaerobic bacteria.
Enteric Microbe/Pathogen Reductions in Stabilization Ponds
• BOD and enteric microbe/pathogen reductions of 90%, esp. in warm,
sunny climates.
• Even greater enteric microbe /pathogen reductions by using two or
more ponds in series
• Better BOD and enteric microbe/pathogen reductions if detention
(residence) times are sufficiently long (several weeks to months)
• Enteric microbes reduced by 90% in single ponds and by multiples of
90% for ponds in series.
• Microbe removal may be quite variable depending upon pond design,
operating conditions and climate.
– Reduction efficiency lower in colder weather and shorter retention times
Constructed Wetlands and Enteric Microbe Reductions
• Surface flow (SF) wetlands reduce enteric microbes by ~90%
• Subsurface flow (SSF) wetlands reduce enteric microbes by
~99%
• Greater reduction in SSF may be due to greater biological
activity in wetland bed media (porous gravel) and longer
retention times
• Multiple wetlands in series incrementally increase microbial
reductions, with 90-99% reduction per wetland cell.
Septic Tank-Soil Absorption Systems for On-Site Sewage Rx
• Used where there are no sewers and community sewage treatment
facilities: ex.: rural homes
• Septic tank: solids settle and are digested
• Septic tank effluent (STE) is similar to primary sewage effluent
• Distribute STE to soil via a sub-surface, porous pipe in a trench
• Absorption System: Distribution lines and drainfield
• Septic tank effluent flows through perforated pipes located 2-3 feet
below the land surface in a trenches filled with gravel, preferably in the
unsaturated (vadose) zone.
– Effluent discharges from perforated pipes into trench gravel and then into
unsaturated soil, where it is biologically treated aerobically.
• Enteric microbes are removed and retained by the soil and biodegraded
along with STE organic matter; extensive enteric microbe reductions
are possible
• But, viruses and other pathogens can migrate through the soil and
reach ground water if the soil is too porous (sand) and the water table
is high
Log10 Reduction of Pathogens by Wastewater Rx Processes
Log10 Reduction of Pathogens by Wastewater Rx Processes
REMOVAL OF ENTERIC BACTERIA BY SEWAGE
TREATMENT PROCESSES
ORGANISM
PROCESS
Fecal indicators Primary sed.
E. coli
Primary sed.
Fecal indicators
Fecal indicators
Fecal indicators
Salmonellae
Salmonellae
Salmenellae
Salmonellae
% REMOVAL
0-60%
32 and 50%
Trickling filt.
20-80%
Activated sludge
40-95%
Stab. ponds, 1 mo. >99.9999% @ high temp.
Primary sed.
79%, 6-7 hrs.
"
73%, 6-7 hrs.
Trickling filt.
92%
Activated sludge
ca. 99%
Entamoeba histolytica Reduction by Sewage Treatment
ORGANISM
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
E. histolytica
PROCESS
% REMOVAL
Primary Sed.
50%
Primary Sed., 2 hr.
64%
Primary sed., 1 hr.
27%
Primary sed. + Trickl. Filt.
25%
"
74%
"
91%
Primary sed. + Act. Sludge
83%
Oxidation ditch + Sedimentation
91%
Stabilization ponds + sedimentation 100%
"
100, 94, 87
"
100
Aerated lagoon (no settling)
84%
Microbial Reductions by Wastewater Treatment
% Reduction
Microbe 1o&2o Filt. Disinfect. Store Total Rx.
Tot. colif.
98
69
99.99
75
99.99999
Fec. colif.
99
10
99.998
57
99.999996
Coliphage
82
99.98
90
90
99.99997
Enterovirus
Giardia
98
84
96
91
99.999
93
99
78
50
99.9993
Cryptosporidium
93
98
61
<10
99.95
Disinfection of Wastewater
• Intended to reduce microbes in 1o or 2o treated effluent
– Typically chlorination
– Alternatives: UV radiation, ozone and chlorine dioxide
• Good enteric bacterial reductions: typically, 99.99+%
– Meet fecal coliform limits for effluent dicharge
• Often 200-1,000 per 100 ml geometric mean as permitted discharge limit
• Less effective for viruses and parasites: typically, 90-99% reduction
• Toxicity of chlorine and its by-products to aquatic life now limits
wastewater chlorination; may have to:
– Dechlorinate
– Use an alternative, less toxic chemical disinfectant or
– Use an alternative treatment process to reduce enteric microbes
• granular medium (e.g., sand) filtration
• membrane filtration
When Wastewater Disinfection is
Recommended or Required
• Discharge to surface waters:
– near water supply intakes
– used for primary contact recreation
– used for shellfish harvesting
– used for irrigation of crops and greenspace
– other direct and indirect reuse and reclamation purposes
• Discharge to ground waters waters:
– used as a water supply source
– used for irrigation of crops and greenspace
– other direct and indirect reuse and reclamation purposes
Wastewater Reuse
• Wastewater is sometimes reused for beneficial, non-potable
purposes in arid and other water-short regions.
• Often uses advanced or additional treatment processes, sometimes
referred to as “reclamation”
• Biological treatment in “polishing” ponds and constructed wetlands
• Physical-chemical treatment processes as used for drinking water:
– Coagulation-flocculation and sedimentation
– Filtration: granular medium filters; membrane filters
– Granular Activated Carbon adsorption
– Disinfection
Details of these water treatment process will be presented in
lectures on water treatment
Indicator Microbe Levels in Raw and Treated Municipal
Sewage: Sewage Treatment Efficacy
Number/100 ml
100000000
10000000
1000000
100000
10000
1000
100
10
1
F. col.
E. coli
Ent.
C. p. F+ phg.
Raw
Treated
(geom. mean values of 24 biweekly samples)
Estimated Pathogen Reductions by Sewage Treatment
Processes: An Example
Sewage Treatment Rx:
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Primary settling
2o biological treatment
Granular medium filtration
Disinfection
% Reduction
50
99
90
99
Total % Reduction
50
99.5
99.95
99.9995