Microbial removal during sewage treatment

1. Microbial Removal during
Sewage Treatment
M. Mansoor Ahammed
Civil Engineering Department
S.V. National Institute of Technology
Surat – 395 007

2. Why do we treat wastewater ?
 Remove or reduce toxic and organic
materials in wastewater
 Reduce or remove nutrients to lower
pollution of groundwater or surface
water after treatment
 Remove or destroy pathogenic
organisms

3. Microorganisms in Wastes
 Human and animal faecal wastes contain large number of
microbes (~100 billion/gram).
 About 1/3rd the mass of human faecal matter is microbes.
 Most are beneficial or essential in the gut; not pathogens.
 Some gut microbes are human pathogens; they cause
diseases.
Human pathogens can be in human and animal faeces.
Humans and animals harbour pathogens some of the
time.
These pathogens are transmitted by the faecal-oral route.
1 million to 1 billion pathogens/gram of faeces of an
infected person.

4. Urine and Pathogens
 Consists of 95% water and 5% solids
 Daily excretion: 1-1.5 litres
 High in nitrogenous compounds
Urea, uric acid, creatine and ammonia
 Pathogens absent in normal people.
 Urinary tract infections occur in high risk groups
Pregnancy, elderly, diabetes, immune deficient,
E. coli, Staphylococcus saprophyticus, enterococci, other
Gram-negative bacteria, Chlamydia, Mycoplasma
 Some virus infections cause virus shedding in urine

5. Types of Wastes
faeces and Urine = “Nightsoil”
Human (“sanitary”) waste in settings where water use is limited
by lack of indoor plumbing for water supply and liquid waste
(sewage) disposal.
 Sanitary or Municipal Sewage
Typical for human waste in settings where there is piped,
household water supply and sanitary waste disposal using water.
Rare for agricultural waste
 Agricultural Animal Waste Systems

6. Domestic/Community Sanitary Sewage
 Contains human faeces and urine diluted in water
 ~20-50 grams faeces dry weight (100-250 grams wet weight) + 1-1.5 L
urine/100-300 L raw sewage
 Dry weight suspended matter is about 0.1-0.2% (~1-2 grams/L)
• Most of it is organic
• measured by filtering, drying and weighing the particles
• total solids - residue after heating to ~550oC = volatile solids
• or measured by letting sewage particles settle: settleable solids
• Contains many pathogens, especially larger but also smaller ones
 Sewage also contains “soluble” organic matter
of ten measured directly/indirectly as carbon or biodegradable carbon
• Direct: total organic carbon (TOC); chemical oxygen demand
(COD)
• Indirect: biochemical oxygen demand (BOD)
Smaller microbes are part of the “soluble” matter: viruses + bacteria

7. Conventional Domestic/Municipal Sewage Treatment
Systems were not Originally Designed for the Purpose of
Removing or Destroying Pathogens
 Emphasis on reducing the “nuisance” aspects of sewage: smell,
biodegradability, vector attraction, etc.
 Remove settleable suspended matter as solids or “sludge”
biologically degrade and stabilize the sludge organic matter
 Oxidize and stabilize non-settleable organic matter and nitrogen in
the remaining liquid
or denitrify (biologically convert nitrogen to N2 gas)
 Later (1950s and 1960s), pathogen control was introduced in US
and Europe
Disinfect the remaining liquid fraction prior to release

8. Microbial Indicator Concepts and
Purposes
 The types of pathogens that can contaminate water,
food, air and other environmental media are diverse
 Measuring all of these pathogens on a routine basis
is not possible.
Methods are not available for some
Methods are available for other, but they are
demanding, some are slow, and their costs are
high.
 The alternative is to measure something other than a
pathogen that is indicative of contamination, predicts
pathogen presence and estimates human health
risks.

9. Criteria for an Ideal Indicator
Organism
•Should be useful for all types of water (drinking water,
wastewater, recreational water, sea water)
•Should be present whenever enteric pathogens are
present, and absent when pathogens are absent
•Should survive longer in the environment than the
toughest enteric pathogen
•Should be a member of the normal intestinal
microflora of warm-blooded animals

10. Bacterial-Indicator Organisms
Common Groups
 Coliforms
 Total coliforms
 Faecal coliforms
 Escherichia coli
 Streptococci
 faecal streptococci
 enterococci
 Spore Formers
 Clostridium perfringens

11. Pathogens in wastewater
 Over 100 pathogens may be found in sewage, including viruses,
parasites and bacteria.
 Viruses include enteroviruses such as poliovirus, hepatitis A
virus and rotavirus.
 Parasites include helminths such as roundworms, and protozoa,
such as Giardia spp., and Cryptosporidium spp., both of which
cause diarrhoea.
 Bacteria include species of Campylobacter, Salmonella, Shigella
and Escherichia coli.
 The coliform group consists of several genera of mostly
harmless bacteria that live in soil and water as well as the gut of
animals.
 Faecal coliforms originating from the intestinal tract of warm
blooded animals and passed through the faeces.
 Faecal coliforms are part of the normal intestinal flora and do
not necessarily constitute a health risk by themselves, their
presence is an indicator of contamination with faecal matter.

12. Levels of Coliforms in Raw
Sewage
 Total coliforms : 10e7 – 10e9 /100mL
 Faecal coliforms : 10e6 – 10e8 /100mL

13. Wastewater Reuse
• A resource • Class I Cities : 33 billion L/day • 25% treated (CPCB, 2006) • Most rivers are polluted with urban
sewage • High microbial concentration (up to
10e7/100mL coliforms) • Unfit for drinking or other direct use • Main cause : urban sewage disposal

14. WHO guidelines for microbial
quality for Wastewater Reuse
Faecal coliforms
• < 1000 /100 mL for irrigation and
aquaculture
• < 50/100 mL for groundwater recharge
• <1/100 mL for domestic purpose

15. CPCB standards
Faecal coliforms
For water body, irrigation, aquaculture,
forestry
• < 1000 /100 mL (Desirable)
• < 10,000/100 mL (Maximum)
• 500 and 2500 (for Yamuna in Delhi)

16. Typical Sewage or Community/Municipal
Wastewater Treatment Systems
Treated (or untreated) wastewater is often discharged to nearby natural
waters; alternatively, it is applied to the land or reclaimed/reused

17. Land Application of Treated Wastewater:
an Alternative to Surface Water Discharge

18. Conventional Community (Centralized) Sewage Treatment
Pathogen Reductions Vary from: low
(<90%) to Very High (>99.99+%)

19. Typical Municipal Wastewater Treatment
System

20. Factors Influencing Microbial
Reductions by Wastewater
Treatment Processes
Solids association: microbes embedded in larger
particles or aggregated are:
more likely to settle
protected from disinfection and other antagonists
possibly different in their surface properties due
to the other material present

21. 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

22. 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

23. Factors Influencing Microbial
Reductions by Wastewater
Treatment Processes
Biological activity can decrease pathogens by:
Grazing 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.

24. 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

25. The Activated Sludge Process
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

26. 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.

27. Aerobic Biological Treatment:
Activated Sludge and Tricking
Filtration
Trickling Filter System:
Aerobic microbial oxidation on large stones
of primary sewage trickled through the filter
stones by a rotating arm; then solids settling
Activated Sludge Treatment System:
Aerobic microbial oxidation in an aerated
solution, followed by settling of the solids

28. Waste Solids (Sludge) Treatment
 Treatment of settled solids from 1o and 2o sewage treatment
 Biological “digestion” to biologically stabilize the sludge solids
 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.)

29. Typical Sludge Treatment by
Anaerobic Digestion
 Waste sewage solids (sludge) is treated either
anaerobically or aerobically at moderate
(mesophilic) or high (thermophilic) temperatures
 Mesophilic: usually 20-40oC
 Thermophilic: usually>40-60oC
 Anaerobic treatment achieves partial biological
degradation of the waste solids with generation of
methane, hydrogen and some other gasses
 Pathogen reduction by mesophilic digestion is
moderate: about 99%
 Pathogen reduction by thermophilic digestion is
high: >99.99%
 Effect is mostly due to high temperature
(thermal inactivation)

30. 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

31. “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

32. 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

33. Facultative Oxidation (Waste Stabilization)
Pond

34. Stabilization Ponds or Lagoons
 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.

35. 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

37. 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.

38. On-site Septic Tank-Soil
AbsorptionSystem

39. On-Site Septic Tank-Soil Absorption
Systems
 Septic Tank:
 Receives sewage from household
 Two compartments: increase residence time & prevent short-circuiting
 first compartment for solids sedimentation
 second compartment for additional solids settling and effluent discharge
 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.

40. Septic Tank-Soil Absorption System for On-Site
Sewage Treatment
 Used where there are no sewers and community 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
 Enteric microbes are removed and retained by the soil and biodegraded along with STE
organic matter; extensive enteric microbe reductions are possible
• Viruses and other smaller
pathogens can migrate through
soil and reach ground water if
the soil is too porous (sand) and
the water table is high
• STE and pathogens can
migrate to surface if soil is too
impervious (clay soils)

41. REMOVAL OF ENTERIC BACTERIA BY
SEWAGE TREATMENT PROCESSES
ORGANISM PROCESS % REMOVAL
Fecal indicators Primary sed. 0‑60%
E. coli Primary sed. 32 and 50%
Fecal indicators Trickling filt. 20‑80%
Fecal indicators Activated sludge 40‑95%
Fecal indicators Stab. ponds, 1 mo. >99.9999% @ high temp.
Salmonellae Primary sed. 79%, 6‑7 hrs.
Salmonellae " 73%, 6‑7 hrs.
Salmenellae Trickling filt. 92%
Salmonellae Activated sludge ca. 99%

42. Entamoeba histolytica Reduction by
Sewage Treatment
ORGANISM PROCESS % REMOVAL
E. histolytica Primary Sed. 50%
E. histolytica Primary Sed., 2 hr. 64%
E. histolytica Primary sed., 1 hr. 27%
E. histolytica Primary sed. + Trickl. Filt. 25%
E. histolytica " 74%
E. histolytica " 91%
E. histolytica Primary sed. + Act. Sludge 83%
E. histolytica Oxidation ditch + Sedimentation 91%
E. histolytica Stabilization ponds + sedimentation 99.99%
E. histolytica " 94, 87
E. histolytica " 99.9%
E. histolytica Aerated lagoon (no settling) 84%

43. 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
Entero-virus
98 84 96 91 99.999
Giardia 93 99 78 50 99.9993
Crypto-sporidium
93 98 61 <10 99.95

44. Disinfection of Wastewater
(US)  Intended to reduce microbes in 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

45. 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

46. Wastewater Reuse
 Wastewater is sometimes reused for beneficial, non-potable
purposes
 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

47. Indicator Microbe Levels in Raw and Treated Municipal
Sewage: Sewage Treatment Efficacy
100000000
10000000
1000000
10000
1000
100
10
1
100000
T. col. E. coli Ent. C. p. F+ phg.
Number/100 ml Raw
Treated (geom. mean values of 24 biweekly samples)

48. Estimated Pathogen Reductions by
Sewage Treatment Processes: An
Example
Sewage Treatment ` % Reduction Total % Reduction
 Primary settling 50 50
 Biological treatment 99 99.5
 Granular medium filtration 90 99.95
 Disinfection 99 99.9995

49. Options for Tertiary Treatment
 Waste Stabilisation ponds in series (Land?)
 Filtration through granular media
 Coagulation-Flocculation
 Disinfection
Chlorination (THM?)
UV radiation
Ozone (Cost?)

50. Thank you