Wastewater treatment processes (I)

ENV H 440/ENV H 541

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 of the 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– Constructed wetland

• Centralized

Sewer systems

Typical composition of untreated domestic wastewater

Microorganism concentrations in untreated wastewater

(Minimum) Goals of wastewater treatment plants

• <30 mg/L of BOD5

• <30 mg/L of suspended solids

• <200 CFU/100mL of 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• Average flow: 800-1200 gpd/ft2

• Retention time: 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– Stabilization ponds

Secondary Treatment Using Activated Sludge Process

SecondaryTreatment

Secondary Treatment

Sludge drying bed or mechanical dewatering 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

General Microbial Growth

• Carbon Source: Dissolved organic matter• Energy Source: Dissolved organic matter• Terminal Electron Acceptor: Oxygen• Nutrients: Nitrogen, Phosphorus, Trace

Metals• Microorganisms: Indigenous in

wastewater, recycled from secondary clarifier

Secondary Treatment

Activated Sludge Aeration Basins

Empty basin, airdiffusers on bottom

Same basin,in operation

Secondary Treatment

The Oxidation Ditch

Ref: Reynolds & Richards,1996, Unit Operations and Processes in Environmental Engineering

Secondary Treatment

The Oxidation Ditch

Ref: Reynolds & Richards,1996, Unit Operations and Processes in Environmental Engineering

Secondary Treatment

Circular Secondary Clarifier

SecondaryEffluent

Influent from Activated Sludge Aeration Basin

or Trickling Filter

Section through a Circular Secondary Clarifier

Return (Secondary) Sludge Line

Secondary Treatment

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 SS and BOD5

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

removal of bacteria and > 90 % removal of protozoa

Stabilization Ponds

• The oldest wastewater treatment systems– Requires a minimum of technology– Relatively low in cost– Popular in developing countries and small

communities in the US (90 % communities with population <10,000)

• Used for raw sewage as well as primary‑ or secondary‑treated effluents.

• Facultative ponds and aerated lagoons

Ponds and Lagoons

Facultative Pond

Ponds and Lagoons

Facultative ponds

• 3 zones: upper photic (aerobic) zone, facultative (aerobic and anaerobic) zone and lower anaerobic zone. – Upper aerobic zone: algae use CO2, sunlight and inorganic

nutrients (photosynthesis) to produce oxygen and algal biomass.– Facultative zone: bacteria and other heterotrophs convert organic

matter to carbon dioxide, inorganic nutrients, water and microbial biomass.

– Lower anaerobic zone: anaerobic bacteria degrade the biomass from upper zones

• Influence by many factors– Sunlight– Temperature– pH– Biological activities– Characteristics of wastewater

Ponds and Lagoons

Facultative ponds

• To remove suspended solids, nitrogen, phosphate, and pathogens

• Operating water depth: 1-2.5 meters• (maximum) BOD loading: 2.2-5.6 g/m3 /day • Retention time: 3-6 months• >90 % SS and BOD removal (warm and sunny climates)

• Microbe removal may be quite variable depending upon pond design, operating conditions and climate.– 90-99% removal of indicator and pathogenic bacteria– 99 % removal of PV1 – 99.9 reduction of Giardia and Cryptosporidium

Aerated Lagoons

http://www.lagoonsonline.com/marshill.htmPonds and Lagoons

Stabilization Lagoon

Aerated Lagoons

Aerated lagoons

• Biological activity is provided by mainly aerobic bacteria

• Influence by many factors– Aeration time– Temperature– pH– Biological activity– Characteristics of wastewater

Aerated lagoons

• To remove suspended solids, nitrogen, phosphate, and pathogens

• Operating water depth: 1-2 meters• Retention time: <10 days• 85% BOD removal (at 20oC and an aeration period of

5 days)• 65% BOD removal (at 10oC and an aeration period of

5 days)• Microbe removal may be quite variable depending

upon pond design, operating conditions and climate

Wastewater Disinfection

Disinfection

Typical Municipal Wastewater Treatment System

Preliminary or Pre-Preliminary or Pre-TreatmentTreatment

PrimaryTreatment

SecondaryTreatment

Disinfection

Sludge Treatment& Disposal

Sludge processing

• Thickening

• Digestion

• Dewatering

• Disposal

Sludge thickening• To reduce the volume of sludge

– to increase sludge solids at least 4 %• Gravity thickening and mechanical thickening• Gravity thickening

– Used for primary and tricking filter solids– Without chemical flocculants– loading rate: 30-60 kg/m2 per day

• Mechanical thickening– Used for activated sludge solids– With chemical flocculants– dissolved air flotation, gravity belt thickeners, and centrifuge thickening– loading rate: 10-20 kg/m2 per day (dissolved air flotation), 400-1000 L/m

(gravity belt thickeners), 1500-2300 L/m (centrifuge thickening) • The concentration of pathogens increased during this process

Gravity belt thickener

Regulatory requirement for disposal of sewage sludge

• Class B biosolids (agriculture land)– < 2 million MPN/g of fecal coliforms – Seven samples over 2-weeks period– ~2 log removal

• Class A biosolids (home lawn and garden)– < 1000 MPN/g of fecal coliforms– < 3 MPN/4g of Salmonella sp.– < 1 PFU/4g of enteric viruses– < 1/4g of Helminth ova– ~ 5 log removal

Processes to significantly reduce pathogens (PSRP) for a Class B biosolids

• Aerobic digestion

• Anaerobic digestion

• Air drying

• Composting

• Lime stabilization

Digestion

• To stabilize organic matter, control orders, and destroy pathogens

• Aerobic digestion and anaerobic digestion• Aerobic digestion

– Sludge is agitated with air/oxygen– loading rate (maximum): 640 g/m2 per day– Temperature and retention time: 68 oF for 40

days or 58 oF for 60 days– Solids and BOD reduction: 30-50 %

Anaerobic digestion

• Sludge is treated in the absence of air• Operation conditions (optimum)

– Temperature: 85-99 oF (98 oF)– pH: 6.7-7.4 (7.0-7.1)– Alkalinity: 2000-3500 mg/L – Solid loading: 0.02-0.05 lb/ft3/day– Retention time: 30-90 days

• Treatment outcome– Solid reduction: 50-70 %– Significant reduction of most pathogens – Gas production: methane and carbon dioxide

Anaerobic digester

Air drying, composting, and lime stabilization

• Air drying– Sludge is dried on sand beds/(un)paved basins– Retention time: minimum of 3 months

• Composting– Various methods: in-vessel, static aerated file, and periodically

mixed windrows– File temperature should be raised > 40 oC for 5 days– For 4 hours during the 5 days, the file temperature should be >

55 oC• Lime stabilization

– Sufficient lime should be added to raise the pH 12 after 2 hour contact

– 4 log inactivation of enteric viruses, 2-7 log inactivation of indicator bacteria, no inactivation of Acaris ova

Processes to further reduce pathogens (PFRP) for a Class A biosolids

• Heat drying– Sludge is dried by contact with hot gases– The temperature of gas is >80 oC

• Thermophilic aerobic digestion– Sludge is agitated with air/oxygen– 132-149 oF for 4-20 hours

• Pasteurization– 158 oF for 30 minutes

• Beta- or gamma ray irradiation– Sludge is irradiated with either beta- or gamma ray– > 1.0 Mrad at room temperature

Dewatering

• To concentrate sludge by removing water• Pressure filtration, centrifugation, and screw

press• Pressure filtration (belt filter press and plate-and-

frame filter)– Usually with polymer flocculation– Loading rate: 40-60 gpm/m (hydraulic) and 500-1000

lb/m/h (solid)– Feed solid: 1-6 %– Cake solids: 15-30 %

Belt filter press

Disposal

• Land application

• Landfill

• Incineration

• Ocean dumping (no longer allowed in US)