wastewater treatment processesweb.iitd.ac.in/~arunku/files/cvl100_y16/lecsep1216.pdf · biological...

September 16, 2016 Arun Kumar ([email protected])

1

(Sep 12th-15th, 2016)

by Dr. Arun Kumar ([email protected])

Wastewater Treatment Processes

Objective: To learn about processes used in wastewatertreatment plant

Courtesy: Dr. Irene Xagoraraki, MSU, USA

Characteristics of Domestic

Wastewater

Wastewater characteristics

• Compare wastewater (WW)characteristics of

– Domestic WW

– Industrial WW (for ex: tannery industry;

distillery industry)

September 16, 2016 3

Wastewater Management

http://dnr.metrokc.gov/WTD/homepage/process.htm

Different sources

Wastewater Management

www.oconomowocusa.com/ wastewater.gif

Municipal Wastewater

Treatment Systems

• Preliminary treatment (removes materials that can cause operational problems, equalization basins are optional)

• Primary treatment (remove ~60% of solids and ~35% of BOD)

• Secondary treatment (remove ~85% of BOD and solids)

• Advanced treatment (varies: 95+ % of BOD and solids, N, P)

• Final Treatment (disinfection)

• Solids Processing (sludge management)

• Industrial wastewaters must be pretreated prior to beingdischarged to municipal sewer system

• Pretreatment requirements set by regulatory agencies

• Why: remove materials that will not be treated bymunicipal system, remove materials that inhibit thebiological processes in secondary treatment

• For example: silver ions are toxic to bacteria whichmight affect biological process. Thus silver ions areremoved at pre-treatment before biological process sothat bacterial performance do not get affected.

Pre-Treatment of Industrial

Wastewaters

Basic Wastewater Treatment

Preliminary Treatment�Primary

Treatment�Secondary

Treatment�Final Treatment and

Solids Processing

Note down parameters removed

in different unit processes

and order of their removals

Preliminary treatment

Upon arrival via the sewer system, the wastewater issent through a bar screen, which removes large solidobjects such as sticks and rags.

Leaving the bar screen, the wastewater flow is sloweddown entering the grit tank, to allow sand, gravel andother heavy material that was small enough not to becaught by the bar screen to settle to the bottom. All thecollected debris from the grit tank and bar screen isdisposed of at a sanitary landfill.

Primary treatment

Primary treatment is the second step in wastewatertreatment. It allows for the physical separation of solidsand greases from the wastewater. The screenedwastewater flows into a primary settling tank where it isheld for several hours allowing solid particles to settle tothe bottom of the tank and oils and greases to float to thetop.

Secondary treatment-biological treatment process that removes dissolvedorganic material from wastewater. The partially treatedwastewater from the settling tank flows by gravity into anaeration tank.

-mixing of water to solids containing that use oxygen toconsume the remaining organic matter in the wastewateras their food supply (use of air bubble for mixing andoxygen supply)

-liquid mixture (i.e., solids with micro-organisms andwater) is sent to the final clarifier.

-In clarifier, solids settle out to the bottom where some ofthe material is sent to the solids handling process andsome is recycled back to replenish the population ofmicro-organisms in the aeration tank to treat incomingwastewater.

Final treatmentTreated water is disinfected and then it is send out forwastewater reuse activities or for discharging inriver/streams. mostly chlorination and/or ultra violetirradiation is used for disinfection purposes.

Solids processingThe primary solids from the primary settling tank and thesecondary solids from the clarifier are sent to a digester.Micro-organisms use the organic material present in thesolids as a food source and convert it to by-products suchas methane gas and water.

Digestion results in a 90% reduction in pathogens and theproduction of a wet soil-like material called “biosolids” thatcontain 95-97% water.

In order to remove some of this water, mechanicalequipment such as filter presses or centrifuges are usedto squeeze water from the biosolids to reduce the volumeprior to being sent to landfill, incinerated or beneficiallyused as a fertilizer or soil amendment.

Wastewater Treatment

• Preliminary Treatment (screening)

• Primary Treatment (primary settling)

• Secondary Treatment (e.g. activated sludge)

• Advanced Treatment (e.g. P removal)


• Solids Processing (sludge treatment)

Bar racks• Purpose: remove

larger objects

• Solid material stored

in hopper and sent to

landfill

• Mechanically or

manually cleaned

Grit Chambers

• Purpose: remove inert

dense material, such

as sand, broken

glass, silt and pebbles

• Avoid abrasion of

pumps and other

mechanical devices

• Material is called “grit”

– Wastewater flow has daily fluctuations

– Purpose: To dampen the variation in wastewater flow into a WWTP

– Flow equalization is not a treatment process

– Improves effectiveness of primary & secondary treatment

– Usually achieved by large basins to collect wastewater and pumped to treatment plant at a constant rate

– Adequate aeration and mixing need to be provided to prevent odors and deposition of solids

Equalization

Basins

Source: Davis and Cornwall, Introduction to Environmental Engineering, 2008






• Tertiary Treatment (disinfection)


Primary Treatment (settling)

• Primary treatment separates suspended solids

and greases from wastewater. Wastewater is held

in a tank for several hours allowing the particles to

settle to the bottom and the greases to float to the

top.

• The solids drawn off the bottom and skimmed

off the top receive further treatment as sludge.

The clarified wastewater flows on to the next

stage of wastewater treatment.


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Settling/Sedimentation• Solid liquid separation process in which a

suspension is separated into two phases –– Clarified supernatant leaving the top of the

sedimentation tank (overflow).

– Concentrated sludge leaving the bottom of the sedimentation tank (underflow).

• Purpose of Settling– To remove coarse dispersed phase.

– To remove coagulated and flocculated impurities.

– To remove precipitated impurities after chemical treatment.

– To settle the sludge (biomass) after activated sludge process / tricking filters

September 16, 2016 [email protected]


21

Some basic definitions

• Sedimentation, also known as settling, may be defined as the removal of solid particles from a suspension by settling under gravity.

• Clarification is a similar term, which usually refers specifically to the function of a sedimentation tank in removing suspended matter from the water to give a clarified effluent. In a broader sense, clarification could include flotation and filtration.

• Thickening in sedimentation tanks is the process whereby the settled impurities are concentrated and compacted on the floor of the tank and in the sludge-collecting hoppers.

• Concentrated impurities withdrawn from the bottom of sedimentation tanks are called sludge, while material that floats to the top of the tank is called scum.

September 16, 2016 [email protected]

http://www.geocities.com/RainForest/5161/wwtps.htm

Primary Settling

Basins

Primary Settling Tank Design Example

• Size:

– rectangular: 3-24 m wide x 15-100 m long

– circular: 3-90 m diameter

• Detention time: 1.5-2.5 hours

• Overflow rate: 25-60 m3/m2·day

• Typical removal efficiencies:

– solids: 50-60%

– BOD5: 30-35%

Example 1

Question: A rectangular primary clarifier (2.4 m deep &

4.0 m wide) is designed to settle a flow of 2000 m3/day

and have an overflow rate of 32 m3/m2.day

I) How long should it be?

II) What detention time will it have?

day m

m32

Length m 4

day

m 2000

area

rateflow rateOverflow

2

3

3

⋅

=

×

==

m 15.632 4

2000Length =

×

=Solve for length:

I) Length:

Example 1

hr 1.8

hr 24

day

day

m 2000

m 5.6 1 m 4 m 2.4

rateflow

volumetime Detention

3=

×

××==

II) Detention time:

Example 1

Secondary Treatment

• Secondary treatment is a biologicaltreatment process that removes dissolvedorganic matter from wastewater.

• Sewage microorganisms are cultivated andadded to the wastewater. The microorganismsuse organic matter from sewage as their foodsupply. This process leads to decomposition orbiodegradation of organic wastes.

Secondary Treatment

• Basic approach is to use aerobic biologicaldegradation:

organic carbon + O2 CO2 + new cells

• Objective is to allow the BOD to be exerted in the treatment plant rather than in the stream

microorganisms

How is this accomplished?Create a very rich environment for growth of a diverse microbial community

Basic Ingredients

• High density of microorganisms (keep organisms in system)

• Good contact between organisms and wastes (provide mixing)

• Provide high levels of oxygen (aeration)

• Favorable temperature, pH, nutrients (design and operation)

• No toxic chemicals present (control industrial inputs)

Dispersed (suspended) growth

vs Fixed growth

• Two approaches of secondary treatment

– fixed film, and suspended film systems

• Dispersed Growth (suspended organisms)

– Activated sludge

– Oxidation ditches/ponds

– Aerated lagoons, stabilization ponds

• Fixed Growth (attached organisms)

– Trickling filters

– Rotating Biological Contactors

Activated Sludge• Process in which a mixture of wastewater and

microorganisms is agitated and aerated

• Leads to oxidation of dissolved organics

• After oxidation, separate sludge (mostly microbial cells,

water, and other contaminants) from wastewater

• Induce microbial growth

– Need food, oxygen

– Want Mixed Liquor Suspended Solids (MLSS) of

3,000 to 6,000 mg/L

Activated Sludge Process

http://www.geocities.com/RainForest/5161/wwtps.htm

MixedLiquor Air

Secondaryclarifier

raw wastewater

Waste Activated Sludge(WAS)

Return Activated Sludge(RAS)

treatedwastewater Discharge to

River or LandApplication

Air

Activated Sludge Process

Activated Sludge Process with

secondary clarifier

East Lansing WWTP

East Lansing WWTP

F/M Parameter• Low F/M (low rate of wasting)

– starved organisms

– more complete degradation

– larger, more costly aeration tanks

– more O2 required

– higher power costs (to supply O2)

– less sludge to handle

• High F/M (high rate of wasting)

– organisms are saturated with food

– low treatment efficiency

Activated Sludge Design

• Detention time: td = approximately 6 - 8 hr

• Long rectangular aeration basins

• Air is injected near bottom of aeration tanks

through system of diffusers

• Aeration system used to provide mixing

• MLVSS and F/M controlled by wasting a portion

of microorganisms

Other options

September 16, 2016 40

Low-tech solutions

• Aerobic ponds

• Facultative ponds

• Anaerobic ponds

1. Aerobic ponds• Shallow ponds

(<1 m deep)

• Light penetrates to

bottom

• Active algal

photosynthesis

• Organic matter con-

verted to CO2, NO3-,

HSO4-, HPO4

2-, etc.

2. Facultative ponds

• ponds 1 - 2.5 m deep

• td = 30 - 180 d

• not easily subject to upsets due to fluctuations in Q, loading

• low capital, O&M costs

Facultative

Aerobic

Anaerobic

3. Anaerobic Ponds

• Primarily used as a pretreatment process for high strength, high temperature wastes

• Can handle much high loadings

• 2 stage:

– Acid fermentation: Organics → Org. acids

– Methane fermentation Org. Acids → CH4 and

CO2






• Tertiary Treatment (disinfection)


Secondary Wastewater Treatment

• High treatment efficiency– BOD to ~ 20 - 50 mg/L

– SS to ~ 20 mg/L

• Low treatment efficiency– Nitrogen

– Phosphorus

– Heavy Metals

– Poorly-biodegradable organic chemicals

– Small particles

– Resistant organisms

Some pollutants are not

removed by the

conventional secondary

wastewater treatment

Problem: Particles

• Presence of small particles that are too small to be removed by settling.

• Attached to these particles can be organic chemicals and metals.

• Particles may eventually settle in river or stream (longer detention time).

• Particles can also be bacteria, protozoans, etc.

1. Particles

Solution: Filtration• Removes

– residual suspended solids

– microorganisms

• Commonly use dual- or multimedia filters

– sand filters (single media) clog too easily

• Achieves:

– 80% reduction in suspended solids for activated sludge

• No removal of:

– soluble BOD or COD, soluble phosphate, nitrate, heavy metals, etc.

1. Particles

Problem: Organics• Organic

compounds, pesticides, endocrine disruptors, homores, pharmaceuticals

2. Organics

• Carbon is heated to about 1500 oC to “activate” surfaces

• High surface area of particles with vast pore spaces

– Capable of absorbing high quantity of organics

– Surface area > 1,000 m2/g

• Wastewater effluent is passed through filter under pressure

• Carbon becomes exhausted

– replace carbon in system

– regenerate carbon (on-site or off-site)

Solution: Carbon Adsorption

2. Organics

Problem: Phosphorus

• increase in nutrients and organic substances, sediments

• overstimulation in growth of algae and aquatic plants

• create conditions that interfere with recreational uses of lakes, and the health and diversity of indigenous life

• eutrophication

3. Phosphorus

Solution: Phosphorus Removal

• Most phosphate in the form of (HPO42-)

• Usually removal accomplished with chemical precipitation (salts)

– Ferric chloride: FeCl3– Alum: Al2(SO4)3

•14H2O

– Lime: CaO or Ca(OH)2

3. Phosphorus

Phosphorus Removal

FeCl3 + HPO42- = FePO4 (s) + HCl

Al2(SO4)3·14H2O + 2 HPO42- = 2AlPO4 (s) + 2H+ + 3SO4

2-

• Effective range for alum or ferric chloride is pH 5.5 to 7.0

• If insufficient alkalinity - must add lime to neutralize H+

There is another option: Biological Phosphorus Removal

3. Phosphorus

Problem: Nitrogen

• Excess nutrients: nitrogeneous BOD exerts oxygen demand

• Anaerobic conditions in stream

4. Nitrogen

Solution: Nitrogen Removal (chemical)• Ammonia stripping

– Raise pH to convert ammonium ions to ammonia

NH4+ + OH- = NH3 + H2O

– Ammonia purged from water in process similar to

aeration

Tray-type Air Stripper Packed Column Air Stripper(From: http://www.mittelhauser.com/airstrip.html) (From: http://www.carbonair.com/OS.htm)

4. Nitrogen

Solution: Nitrogen Removal (biological)

• Forms: NH3, NH4+, NO2

-, NO3-

• Nitrification/ De-nitrification

– Occurs

• in activated sludge process - by increasing the

detention time in activated sludge basin

• in separate reactor

– Nitrification:

NH4+ + 2O2 = NO3

- + H2O + 2H+ (2 steps)

– De-nitrification:

2NO3- + organic matter = N2+ CO2 + H2O

4. Nitrogen

Membrane Bioreactors (MBRs)

• Are used when wastewater reclamation is required

Solution: Membrane Processes

• A phase that acts as

a barrier to the flow

of molecular or ionic

species between

other phases

• Driven by pressures

• Produce product

water and reject

stream

59

The MBR in Traverse City WWTP

Final Treatment (disinfecting)

1. Final treatment focuses on removal of disease-

causing organisms from wastewater. Treated

wastewater can be disinfected by adding

chlorine or by using ultraviolet light.

2. High levels of chlorine may be harmful to

aquatic life in receiving streams. Treatment

systems often add a chlorine-neutralizing

chemical to the treated wastewater before

stream discharge.

UV Disinfection

Sludge Types

• Primary sludge– 3 to 8% solids– About 70% organic material

• Secondary sludge– Consists of wasted microorganisms and inert materials– About 90% organic material– WAS: 0.5 to 2% solids– Trickling filter sludge: 2-5% solids

• Tertiary sludge– If secondary clarifier is used to remove phosphate, this

sludge will also contain chemical precipitates (more difficult to treat)

– Denitrification sludges - similar to WAS sludge

Bar screens

Grit chambers

Not true sludge, not a fluid.Since it can be drained easilyand is relatively stable, it can be disposed of directly in a municipal landfill.

Treatment processes include:

– Thickening – separates water from solids by gravity or

flotation

– Stabilization – converts organic solids to more refractory

forms through digestion

– Conditioning – treats sludge with chemicals or heat so

that the water can be readily separated

– Dewatering – separates water by vacuum, pressure or

drying

– Reduction – decreases the volume of sludge by

incineration

Sludge Treatment

Sludge Treatment: 1) Thickening

• Flotation

– Especially effective on activated sludge

– Increases solids content from 0.5 -1% to 3-6%

• Gravity thickening

– Best with primary sludge

– Increases solids content from 1-3% to 10%

PrimarySludge

Gravity Thickening

SecondarySludge

Flotation

Further processing

Thickening

Gravity Belt Thickener

Gravity Thickener(From: http://www.thomasregister.com/olc/dorroliver/sedi.htm)Flotation

Sludge Treatment: 2) Stabilization

• Aerobic Digestion

– Extension of activated sludge

– Accomplished by aeration of sludge then followed by sedimentation

– Supernatant goes back to head of plant (high in BOD, TKN, total-P)

– Treated sludge is 3% solids

• Anaerobic Digestion

– 2 stage: acid fermentation followed by methane production

– Advantages:

• produce methane

• do not add oxygen

– As with aerobic digestion, supernatant goes to headworks

Stabilization

Anaerobic DigestionAerobic Digestion

Sludge Treatment: 3) Conditioning

• Chemical Conditioning

– Add lime, ferric chloride, or alum

– Can also add polymers

– Chemicals are added just prior to de-watering stage

• Heat Treatment

– High temperatures (175-230 oC)

– High pressure (10 to 20 atmospheres)

– Advantages

• bound water is released and sludge is easily dewatered

– Disadvantages

• complex process

• highly concentrated liquid stream

Sludge Treatment: 4) De-watering

• Sludge Drying Beds

– Most popular

method

– Simple

– Low maintenance

– Effected by climate

• Filtration

– Apply vacuum to pull

out water

– Force out water by

essentially squeezing

water between two

moving filter belts

De-watering

Sludge Drying Beds Vacuum Filtration(From: http://www.infilcodegremont.com/) (From: http://www.thomasregister.com/olc/dorroliver)

De-watering:

Belt Filter Press

East Lansing, MIFairhaven, MA

(From: http://www.environline.com/fhwpcf.htm)

Sludge Treatment: 6) Volume

Reduction

• Incineration

– Complete evaporation

of water from sludge

– Requires fuel

– Solid material is inert

– Exhaust air must be

treated prior to

discharge

• Wet Oxidation

– Treated sludge is

wet

– Requires energy

– Solid material is inert

– Exhaust air must be

treated prior to

discharge

Volume Reduction:

High Temperature Fluidized Bed

Incineration

(From: http://www.infilcodegremont.com/)(From: http://www.unep.or.jp/CTT_DATA/WATER/WATER_4/html/Water-173.html

Sludge Disposal

• Method depends on RCRA regulations

– Land Spreading

• lawns, gardens

• agricultural land

• forest land

• golf courses and other public recreational areas

– Municipal Solid Waste Landfill

– Utilization in other materials


77

Raw Wastewater

Raw Drinking WaterWastewater

Treatment

Drinking Water Treatment

Potential human exposurevia drinking water

Natural waters

Removal and/or transformation

Animal Manure

Land

Application

Removal and/or transformation

Solid Waste

Runoff, Infiltration, Sorption, Microbial activity, other transformations

Runoff, Infiltration,Sorption,Microbial activity, other transformations

Landfills

Biosolids

PPCPs and EDCs present in human urine and feces

Emerging Contaminants in Environment

Focus


78

Biosolids Land

Application

1. Inhalation

of aerosols

from land

application

sites2. Consumption

of groundwater

contaminated by

biosolids

3. Direct ingestion

of biosolids

4. Ingestion of

plants grown on

biosolids-

amended fields

5. Consumption of

water

contaminated by

runoff from a land

application site

Account for

intermittent

exposure

Account for

wet weather

events

Adapted from Zhang et al. (2009)

Philadelphia, PA 19104, U.S.A.

Biosolids-associated pathogens after land application activities

wastewater treatment processesweb.iitd.ac.in/~arunku/files/cvl100_y16/lecsep1216.pdf · biological...

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