water & wastewater treatment
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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
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