Microbial Removal during Sewage Treatment

M. Mansoor Ahammed

Civil Engineering DepartmentS.V. National Institute of Technology

Surat – 395 007

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

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.

Microorganisms in Wastes

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

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 SewageTypical 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

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

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 liquidor denitrify (biologically convert nitrogen to N2 gas)

Later (1950s and 1960s), pathogen control was introduced in US and EuropeDisinfect the remaining liquid fraction prior to release

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 someMethods 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.

•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

Criteria for an Ideal Indicator Organism

Bacterial-Indicator OrganismsCommon Groups

Coliforms Total coliforms Faecal coliforms Escherichia coli

Streptococci faecal streptococci enterococci

Spore Formers Clostridium perfringens

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.

Levels of Coliforms in Raw Sewage

Total coliforms : 10e7 – 10e9 /100mL

Faecal coliforms : 10e6 – 10e8 /100mL

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

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

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)

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

Land Application of Treated Wastewater:

an Alternative to Surface Water Discharge

Conventional Community (Centralized) Sewage Treatment

Pathogen Reductions Vary from: low (<90%) to Very High (>99.99+%)

Typical Municipal Wastewater Treatment System

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

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 activityin 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 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

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

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 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

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.)

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)

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 solidsPFRPs: 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 conditionerClass 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

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.

On-site Septic Tank-Soil AbsorptionSystem

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.

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)

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%

Entamoeba histolytica Reduction by Sewage Treatment

ORGANISM PROCESS % REMOVALE. 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, 87E. histolytica " 99.9%E. histolytica 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

Entero-virus

98 84 96 91 99.999

Giardia 93 99 78 50 99.9993

Crypto-sporidium

93 98 61 <10 99.95

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

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

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 sedimentationFiltration: granular medium filters; membrane filtersGranular Activated Carbon adsorptionDisinfection

Indicator Microbe Levels in Raw and Treated Municipal Sewage: Sewage Treatment Efficacy

110

1001000

10000100000

100000010000000

100000000

110

1001000

10000100000

100000010000000

100000000

T. col. E. coli Ent. C. p. F+ phg.

Num

ber/

100

ml

RawTreated (geom. mean values of 24 biweekly samples)

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

Options for Tertiary Treatment

Waste Stabilisation ponds in series (Land?) Filtration through granular media Coagulation-Flocculation Disinfection

Chlorination (THM?)UV radiationOzone (Cost?)

Thank you