Municipal Wastewater Treatment Design Lee

8/12/2019 Municipal Wastewater Treatment Design Lee

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Municipal wastewater treatment design:

Lecture by Y. Lee, PhD Candidate U of Manitoba

What do you need to know?

Influent characteristic of wastewater Effluent limit (very strict not so strict)

Flow and population BOD removal plant (not so strict)

Biological nutrient removal (BNR)wastewater treatment plant (very strict)


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Landfill

Screens Grit

removal Primary

clarifier

Activated sludgeSecondary

clarifier

Land

application orfurther

treatment

Return activated

sludge

Filtrati

onatio

n

Disinfection

Final effluent

Waste activated

sludge

Thickener

Thickener return

Dewatering

return

Digester return

Digester

Primary sludge

Dewatering

1

10

8

11

5

4

6

9

7

3

2

12

13

Filtration return

Typical schematic of municipal wastewater

treatment


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Bar Screens (coarse)

Removal of large objects (i.e. rags, paper, plastic, etc)

Vertical or inclined steel bars equally spaced across a

channel, mechanically cleaned. Design criteria

Opening between bars = 6-38mm (25mm typical)

Dimensions based on velocity and peak design flow rate Typical velocity through rack openings

Quantity of screenings = 20 m3 /106 m3 of wastewater (average), &36 m3 /106 m3 of wastewater (maximum)

NOTE:


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Grit removal

Removal of grit, consisting of sand, gravel, cinder orother heavy solid material

Design criteria HRT=2-5min (3min is typical)1) Establish the peak hourly flow rate for design

2) Peak flowrate = Q* 2.75

3) Grit chamber volume = peak flowrate * HRT*1/24) Dimensions: Depth = 2-5m; length-to-width ratio (2.5:1) - (5:1)

5) Air supply requirement

Assume 5 ft3/min of air per foot of grit chamber length

6) Grit quantity (m3

/1000m3

of influent):-Separate sewer system 0.004-0.037

-Combined sewer system 0.004-0.18

NOTE: (you need at least 2 chambers)


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Primary clarifier

Design criteria

Surface overflow rate

At average flow = 30-50m3/m2/d

Then, determine area of clarifiers

Determine the number of clarifiers

Diameter of each clarifier ranges from 10-40m

Using HRT = 1-2.5 h, calculate the volume of each

clarifier, depth must be between 3-6m

(even number of clarifiers and same dimensions)

Check for peak flows (overflow rate must be between

70 -130m3/m2/d)


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Primary clarifier

Primary sludge

50-65% removal of influent TSS (60% typical) BOD (35%)

N =? P =?

Sludge production 60% influent TSS (influent, Q (m3/d) * TSS (mg/L) * 0.6)

4-12% solids concentration (typical 5% solid)


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Biological nutrient (N & P) treatment Removal of nitrogen

Nitrification (aerobic) Denitrification (anoxic)

Removal of phosphorous

Phosphorous release (anaerobic)

Phosphorous uptake (aerobic)

COD removal occurs simultaneously


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Biological nutrient (N & P) treatment

Removal of nitrogen (both nitrification and

denitrification are required) Nitrification (aerobic)

Conversion of NH4 to NO3

Rate limiting process Nitrifying bacteria grow very slowly

SRT of approximately 10 days is required (i.e. if it is BOD

removal plant then SRT of only 2-3 day is required)

HRT will increase in order to provide nitrifying bacteria

enough contact time with food ( influent wastewater) (e.g.

HRT of 10 hours) (i.e. if it is BOD removal plant then HRT of

only 4 hour is enough)


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Removal of nitrogen

Denitrification (anoxic) Conversion of NO3 to N2 gas by denitrifying

bacteria

Occurs in the absence of oxygen

Nitrate becomes the terminal electron acceptor

Requires large amounts of carbon to be present

clarifieranoxicstage

aerobic

stage

WAS

influent


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Biological nutrient (N & P) treatmentPhosphorus Removing Mechanism

Facultative bacteria Energy

Acetate plusfermentation

products

PO43-

Substrate

Acinetobacter spp.

Anaerobic

(Phosphorusremovingbacteria, slow

grower)

Aerobic

PHB

PO43-

PHBEnergy

BOD+O2

New biomass

+

CO2 + H2O

Poly-P

Poly-P


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Removal of phosphorous (Bio-P bacteria)

Both bio-P bacteria and denitrifying bacteria require COD as

carbon substrate

Bio- P bacteria are not compatible with denitrifying bacteria

Thus, requires low NO3 in anaerobic reactor and high COD (i.e.

sometimes external carbon source is added into the reactors in

order to increase the COD concentration in the reactor)


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Reactor configurations

clarifierAnaerobic

stageanoxic

stage

aerobic

stage

clarifierAnaerobic

stageanoxic

stage

aerobic

stage

anoxic

stage

aerobic

stage

clarifierAnaerobic

stageanoxic

stage

anoxic

stageaerobic

stage

a) A2O

b) 5-stage

Bardenpho

c) MUCT

WAS

WAS

WAS


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Reactor configurations

50-100--Anaerobic recycle, % ofinfluent

100-600400-600100-300

Internal recycle, % of

influent

80-10080-10020-50RAS, % of influent

9.0-22.09.5-23.04.5-8.5Total

-0.5-1.0-Second aerobic

2.0-4.02.0-4.0-Second anoxic

4.0-12.04.0-12.03.5-6.0First aerobic

2.0-4.02.0-4.00.5-1.0First anoxic

1.0-2.01.0-2.00.5-1.5Anaerobic

HRT, hours

2000-50002000-50002000-4000MLSS, mg/L

10-3010-404-27SRT, days

0.1-0.20.1-0.20.15-0.25

F:M, kg TBOD/kg

MLVSS/d

MUCT5-stage

Bardenph

o

A2/0Parameter


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Oxygen requirements

Oxygen requirements= carbonaceous BOD removal + NH4 removal

by nitrifiers= (total mass of BOD L utilized -1.42 (mass of

organisms wasted)) + (nitrification)

= Q (BOD in BOD out) / + (4.5 *Q (TKN in-TKN out))

NOTE : f = 0.45-0.68


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Secondary clarifier

Circular clarifier/ rectangular clarifier

Design calculation

Surface overflow rate At average flow the rate should be less than the design criteria

of 15 m3/m2*d

Then, determine area of clarifiers

Determine the number of clarifiers

Diameter of each clarifier ranges from 10-40m

Using HRT = 2-4 h, calculate the volume of each clarifier, depthmust be between 3-6m

(even number of clarifiers and same dimensions)

Check for peak flows (overflow rate should be less than the designcriteria of 40 m3/m2*d


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Number of clarifier required

Area or clarifier

Diameter

Detention time (HRT) Volume of each clarifier

Overflow rate and solid loading rate

Primary and secondary clarifier


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Sludge treatment

Thickening

Anaerobic digester

Dewatering


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Mass balance

Primary

ClarifierAnaero-

bic

Secondary

Clarifier

Thickener

Anaerobic

Digester

Dewatering

Centrifuge

Anoxic Aerobic Anoxic Aerobic

Sludge cake

RAS=60%

Internal Recycle=200%

Q1 Q2

Q3

Q4 Q5

Q6

Q7Q8

Q9

Q10

Q11

Q12


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Sludge production

Primary clarifier (removal of TSS 60%) (Q3)

Thus, sludge production from primary sludge

= conc of TSS (mg/L) * (Q inQ reactor) Waste activated sludge

Required to know the yield of biomass

Y obs = Y/(1+kd *SRT)

Then, sludge production rate

= Y obs * Q (S inS out) = ? Kg/d portion of this lost inthe effluent

Thus, sludge produced entering thickening(Q6)

= the overall sludge production lost in effluent


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Sludge treatment

Thickening (combined primary and waste

activated sludge) Used to concentrate solids and reduce the volume.

Methods: Gravity thickening, dissolved air flotation,

and centrifugation Design criteria

Influent solid concentration = 0.5-2%

Thickened solid concentration = 4.0 -6% Hydraulic loading (m3/m2*d) =4-10 (not less than the 9)

Solid loading (kg/m2*d)=25-80 ( not to exceed 47)

HRT= 24 h


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Anaerobic digester

Stabilization of biodegradable particulate organicmater

Simply CSTR; HRT=SRT, no recycle

Mesophilic (35oC) most common, thermophilic(55oC) sometimes used

Design criteria: HRT = 15-20 days, 20 days common

Diameter 10-40m, sidewall depth 5-10m

Volume based on solids production from combined primaryand waste activated sludge

Achieve 60% reduction of VS in primary, 30-50% ofsecondary

Converted to methane gas, ~0.37m3/kg VS destroyed


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Dewatering

Depend on type of sludge, % solids in final

product (sludge cake) are different. For rotary vacuum filters

Feed solids percent (3-7%)

Cake solid percent (18-25%)


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UV disinfection

Design criteria

Required intensity=1.16mW/cm2

(USEPA,1986) Resident time = 60s (recommended)

Determine the volume of disinfection basin

= the peak design flow * HRT(provide at least 2 basin)

Dosage = intensity * HRT

Then, determine UV density = actual intensity *absorbance coefficient

Then, calculate how many lamps are required.


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Typical municipal wastewater treatment plant layout

Landfill

Riverscreen Grit

removalPrimary

clarifier

Activated sludge

system

Final

clarifier

disinfection

thickeningAnaerobic

digester

Belt filter

press


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Wastewater facility design criteriaPlant capacity

flow

average flow:

peak flow:

BOD (kg/d)Nitrogen (kg/d)

Phosphorous (kg/d)

Screen

number:

dimension:

design velocity:

Grit removalnumber:

dimension:

volume:

air requirement:

Primary clarifier

number:

dimension:

surface loading rate:

BNRvolume:

dimension:

MLVSS

F/M

SRT

HRT

total oxygen requirementFinal clarifier

number:

dimension:

surface loading rate:

Solid loading rate:

Municipal Wastewater Treatment Design Lee

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