Water and wastewater treatment processes

ENV H 452/ENV H 542

John Scott Meschke

Office: Suite 2249,

4225 Roosevelt

Phone: 206-221-5470

Email: jmeschke@u.washington.edu

Gwy-Am Shin

Office: Suite 2339,

4225 Roosevelt

Phone: 206-543-9026

Email: gwyam@u.washington.edu

Key points

• Purpose of the individual unit processes

• The typical operating conditions

• The outcome of the processes

• Microbial reduction in the processes

Wastewater treatment processes

How much wastewater do we produce each day?

Wastewater Characteristics

Source Average Daily FlowDomestic sewage 60-120 gal/capitaShopping centers 60-120 gal/1000 ft2 total floor

areaHospitals 240-480 gal/bedSchools 18-36 gal/studentTravel trailer parks

Without individualhookups

90 gal/site

With individualhookups

210 gal/site

Campgrounds 60-150 gal/campsiteMobile home parks 265 gal/unitMotels 40-53 gal/bedHotels 60 gal/bedIndustrial areas

Light industrial area 3750 gal/acreHeavy industrial 5350 gal/acre

Source: Droste, R.L., 1997. Theory and Practice ofWater and Wastewater Treatment

These values are rough estimates only and vary greatly by locale.

Wastewater treatment systems

• Decentralized– Septic tank– Waste stabilization ponds

• Facultative lagoon• Maturation lagoon

– Land treatment

• Centralized

Sewer systems

Typical composition of untreated domestic wastewater

Microorganism concentrations in untreated wastewater

(Minimum) Goals of wastewater treatment processes

• <30 mg/L BOD5

• <30 mg/L of suspended solids

• <200 CFU/100ml fecal coliforms

Conventional Community (Centralized) Sewage Treatment

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

Secondary Treatment Using Activated Sludge Process

Sludge drying bed or mechanical dewatering process

Typical Municipal Wastewater Treatment System

Preliminary or Pre-Preliminary or Pre-TreatmentTreatment

PrimaryTreatment

SecondaryTreatment

Disinfection

Sludge Treatment& Disposal

Preliminary Wastewater Treatment System

Preliminary or Pre-Preliminary or Pre-TreatmentTreatment

Solids to Landfill

Preliminary Treatment Facilities

Preliminary Treatment - Bar Racks

Bar Racks: are used to remove large objects that could potentially damage downstream treatment/pumping facilities.

Ref: Metcalf & Eddy, 1991

Preliminary Treatment - Grit chamber

Grit chamber: used to remove small to medium sized, dense objects such as sand, broken glass, bone fragments, pebbles, etc.

Primary Wastewater Treatment

PrimaryTreatmentPrimary

Treatment

Primary sedimentation • To remove settleable solids from wastewater

Primary Clarification

PrimarySludge

PrimaryEffluent

Influent from Preliminary Treatment

Section through a Circular Primary Clarifier

Primary Treatment

Scum: Oil, Grease, Floatable Solids

Primary sedimentation

• To remove settleable solids from wastewater• Maximum flow: 30 - 40 m3 per day• Retention period: 1.5 - 2.0 hours (at maximum flow)• 50 - 70 % removal of suspended solids• 25 - 35 % removal of BOD5

• ~20 % removal of phosphate • ~50 % removal of viruses, bacteria, and protozoa• 90 % removal of helminth ova

Secondary Wastewater Treatment

SecondaryTreatmentSecondaryTreatment

Secondary treatment processes

• To remove suspended solids, nitrogen, and phosphate

• 90 % removal of SS and BOD5

• Various technologies– Activated sludge process– Tricking filter– Aerated lagoons– Rotating biological contractors

Secondary Treatment Using Activated Sludge Process

SecondaryTreatment

Secondary Treatment

Sludge drying bed or mechanical dewatering process

The Activated Sludge Process

Aerobic microbes utilities 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

Secondary Treatment

Simplified Activated Sludge Description

Activated sludge process

• To remove suspended solids, nitrogen, and phosphate• Food to microorganism ratio (F:M ratio): 0.25 kg BOD5

per kg MLSS (mixed liquor suspended solids) per day at 10 oC or 0.4 kg BOD5 per kg MLSS per day at 20 oC

• Residence time: 2 days for high F:M ratio, 10 days or more for low F:M ratio

• Optimum nutrient ratio: BOD5:N:P =>100:5:1• 90 % removal of BOD5 and SS• ~20 % removal of phosphate• >90 % removal of viruses and protozoa and 45 - 95 %

removal of bacteria

Secondary Treatment Using Trickling Filter Process

SecondaryTreatment

Secondary Treatment

TricklingFilter

Trickling Filter

http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/streem/trickfil.jpg

Primary effluent drips onto rock orman-made media

Rotating arm todistribute water evenly over filter

Rock-bed with slimy (biofilm) bacterial growth

Primary effluent pumped inTreated waste to secondary clarifier

Trickling Filter

http://www.eng.uc.edu/friendsalumni/research/labsresearch/biofilmreslab/Tricklingfilter_big.jpg

Tricking filter process

• To remove suspended solids, nitrogen, and phosphate

• Organic loading (BOD5 X flow/volume of filter): 0.1 kg BOD5 per m3 per day

• Hydraulic loading: 0.4 m3 per day per m3 of plan area

• 90 % removal of BOD5 and SS• ~20 % removal of phosphate• Variable removal levels of viruses, 20-80 %

removal of bacteria and >90 % removal of protozoa

Wastewater Disinfection

Disinfection

Wastewater disinfection

• To inactivate pathogens in wastewater

• Several choices– Free chlorine and combined chlorine– UV– Ozone– Chlorine dioxide

Overall pathogen reduction in wastewater treatment

Water treatment processes

Water contaminants

• Chemicals– Inorganics– Organics

• Synthetic organic compounds• Volatile organic compounds

• Microbes– Viruses– Bacteria– Protozoa parasites– Algae– Helminths

Water contaminants (I)

Water contaminants (II)

Water contaminants (III)

Water contaminants (IV)

Water contaminants (V)

Multiple barrier concept for public health protection

Barrier Approach to Protect Public Health in Drinking Water

• Source Water Protection

• Treatment Technology

• Disinfection

• Disinfectant residual in distribution system

Water treatment processes

Oxidation

• To remove inorganics (Fe++, Mn++) and some synthetic organics– Cause unaesthetic conditions (brown color)– Promote the growth of autotrophic bacteria (iron bacteria): taste

and order problem• Free chlorine, chlorine dioxide, ozone, potassium

permanganate– Fe++ + Mn ++ + oxygen + free chlorine → FeOx ↓ (ferric oxides) +

MnO2 ↓ (manganese dioxide)– Fe (HCO3)2 (Ferrous bicarbonate) + KMnO4 (Potassium

permanganase) → Fe (OH)3 ↓ (Ferric hydroxide) + MnO2 ↓ (manganese dioxide)

– Mn (HCO3)2 (Manganese bicarbonate) + KMnO4 (Potassuim permanganase) → MnO2 ↓ (manganese dioxide)

Physico-chemical processes

• To remove particles in water

• Coagulation/flocculation/sedimentation

• Filtration

Rapid Mix

• Intense mixing of coagulant and other chemicals with the water

• Generally performed with mechanical mixers

Chemical Coagulant

Major Coagulants

• Hydrolyzing metal salts– Alum (Al2(SO4)3)

– Ferric chloride (FeCl3)

• Organic polymers (polyelectrolytes)

Coagulation with Metal Salts

Al(OH)

Alx(OH)y

Colloid

Al(OH)3

Al(OH)3 Colloid

Al(OH)3

Al(OH)3

Colloid

+ +Soluble Hydrolysis Species

(Low Alum Dose)

Colloid

Colloid

Colloid

Al(OH)3Al(OH)3

Al(OH)3

Al(OH)3

Al(OH)3

(High Alum Dose)

Floc

Sweep CoagulationCharge Neutralization

Horizontal Paddle Flocculator

Flocculation ExampleFlocculation Example

Water coming from Water coming from rapid mix.rapid mix. Water goes to sedimentationWater goes to sedimentation

basin.basin.

Sedimentation Basin

Sedimentation Basin ExampleSedimentation Basin ExampleWater coming from Water coming from flocculation basin.flocculation basin.

Water goes to Water goes to filter.filter.

Floc (sludge) collectedin hopperSludge to solids

treatment

Coagulation/flocculation/and sedimentation

• To remove particulates and natural organic materials in water• Coagulation

– 20 -50 mg/L of Alum at pH 5.5-6.5 (sweep coagulation)– rapid mixing: G values = 300-800/second

• Flocculation: – Slow mixing: G values = 30-70/second– Residence time:10 -30 minutes

• Sedimentation– Surface loading: 0.3 -1.0 gpm/ft2

– Residence time: 1 – 2 hours• Removal of suspended solids and turbidity: 60-80 %• Reduction of microbes

– 74-97 % Total coliform – 76-83 % of fecal coliform – 88-95 % of Enteric viruses– 58-99 % of Giardia– 90 % of Cryptosporidium

Filtration

• To remove particles and floc that do not settle by gravity in sedimentation process

• Types of granular media– Sand– Sand + anthracite– Granular activated carbon

• Media depth ranges from 24 to 72 inches

Filter ExampleFilter Example

Water coming from Water coming from sedimentation sedimentation basin.basin.

AnthraciteAnthracite

SandSand

Gravel (supportGravel (support

media)media)

Water going to disinfectionWater going to disinfection

Mechanisms Involved in Filtration

Interception: hits & sticks

Sedimentation: quiescent, settles, & attaches

Flocculation: Floc gets larger within filter

Entrapment: large floc gets trapped in space between particles

Floc particles

Granular media, e.g., grain of sand

Removal of bacteria, viruses and protozoa by a granular media filter requires water to be coagulated

Rapid filtration

• To remove particulates in water• Flow rate: 2-4 gpm/ft2

• Turbidity: < 0.5 NTU (often times < 0.1 NTU)• Reduction of microbes

– 50-98 % Total coliform – 50-98 % of fecal coliform – 10-99 % of enteric viruses– 97-99.9 % of Giardia– 99 % of Cryptosporidium

Disinfection in water

• To inactivate pathogens in water

• Various types– Free chlorine– Chloramines– Chlorine dioxide– Ozone– UV

Trend in disinfectant use (USA, % values)

Disinfectant 1978 1989 1999

Chlorine gas 91 87 83.8

NaClO2 (bulk) 6 7.1 18.3

NaClO2 (on-site)

0 0 2

Chlorine dioxide

0 4.5 8.1

Ozone 0 0.4 6.6

Chloramines 0 20 28.4

Comparison of major disinfectants

Consideration Disinfect ants

Cl2 ClO2 O3 NH2Cl

Oxidation potential

Strong Stronger? Strongest Weak

Residuals Yes No No Yes

Mode of action

Proteins/NA

Proteins/NA

Proteins/NA

Proteins

Disinfecting efficacy

Good Very good Excellent Moderate

By-products Yes Yes Yes? No