Water and wastewater treatment processes

Niaz Ahmed

Office: PCSIR

Phone: 92-021-34641841

Email: niazmemon2000@yahoo.com

Key points

Purpose of the individual unit processesThe typical operating conditionsThe outcome of the processesMicrobial 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 systemsDecentralized

Septic tankWaste stabilization ponds

Facultative lagoon Maturation lagoon

Land treatmentCentralized

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-Treatment

PrimaryTreatment

SecondaryTreatment

Disinfection

Sludge Treatment& Disposal

Preliminary Wastewater Treatment System

Preliminary or Pre-Treatment

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 chamberGrit 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 sedimentationTo remove settleable solids from wastewaterMaximum flow: 30 - 40 m3 per dayRetention period: 1.5 - 2.0 hours (at maximum

flow)50 - 70 % removal of suspended solids25 - 35 % removal of BOD5

~20 % removal of phosphate ~50 % removal of viruses, bacteria, and protozoa90 % removal of helminth ova

Secondary Wastewater Treatment

SecondaryTreatment

SecondaryTreatment

Secondary treatment processesTo remove suspended solids, nitrogen, and

phosphate90 % removal of SS and BOD5

Various technologiesActivated sludge processTricking filterAerated lagoonsRotating biological contractors

SecondaryTreatment

Secondary Treatment

Sludge drying bed or mechanical dewatering process

Secondary Treatment Using Activated Sludge 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 processTo remove suspended solids, nitrogen, and

phosphateFood 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 =>100:5:190 % 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 processTo remove suspended solids, nitrogen,

and phosphateOrganic 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 area90 % removal of BOD5 and SS~20 % removal of phosphateVariable removal levels of viruses, 20-80

% removal of bacteria and >90 % removal of protozoa

Wastewater Disinfection

Disinfection

Wastewater disinfectionTo inactivate pathogens in wastewaterSeveral choices

Free chlorine and combined chlorineUVOzoneChlorine dioxide

Overall pathogen reduction in wastewater treatment

Water treatment processes

Water contaminants Chemicals

InorganicsOrganics

Synthetic organic compounds Volatile organic compounds

MicrobesVirusesBacteriaProtozoa parasitesAlgaeHelminths

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 ProtectionTreatment TechnologyDisinfectionDisinfectant residual in distribution system

Water treatment processes

OxidationTo remove inorganics (Fe++, Mn++) and some

synthetic organicsCause unaesthetic conditions (brown color)Promote the growth of autotrophic bacteria (iron

bacteria): taste and order problemFree chlorine, chlorine dioxide, ozone, potassium

permanganateFe++ + 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 processesTo remove particles in water Coagulation/flocculation/sedimentationFiltration

Rapid MixIntense mixing of

coagulant and other chemicals with the water

Generally performed with mechanical mixers

Chemical Coagulant

Major CoagulantsHydrolyzing 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 ExampleWater coming from rapid mix. Water goes to sedimentation

basin.

Sedimentation Basin

Sedimentation Basin ExampleSedimentation Basin ExampleWater coming from flocculation basin.

Water goes to 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

FiltrationTo remove particles and floc that do not

settle by gravity in sedimentation processTypes of granular media

SandSand + anthraciteGranular activated carbon

Media depth ranges from 24 to 72 inches

Filter ExampleFilter ExampleWater coming from sedimentation basin.

Anthracite

Sand

Gravel (support

media)

Water going to disinfection

Mechanisms Involved in FiltrationInterception: 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 filtrationTo remove particulates in waterFlow rate: 2-4 gpm/ft2

Turbidity: < 0.5 NTU (often times < 0.1 NTU)

Reduction of microbes50-98 % Total coliform 50-98 % of fecal coliform 10-99 % of enteric viruses97-99.9 % of Giardia99 % of Cryptosporidium

Disinfection in waterTo inactivate pathogens in waterVarious types

Free chlorineChloraminesChlorine dioxideOzoneUV

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