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Introduction to Membrane Processes

School of Chemical Engineering

Part 3 – Wastewater Applications

CHEN 6071: Water & Wastewater Engineering

The objective of these slides is to cover the following information:

1. Features of membrane bioreactors: Process flow diagrams, membrane properties and mechanical equipment

2 R i E i i i MBR R D i

Part 3 – Wastewater Treatment

2. Reaction Engineering concepts in MBR – Reactor Design

3. How membrane configuration impacts foot print and mixing energy

Assessment: On-line quiz questions covering your knowledge of

1. Design features of MBR versus conventional systems

Tutorial Problem

1. Foot print calculation for MBR plant

2. Aeration power calculation

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Eutrophication of receiving waters and transmission of communicable disease as a result of wastewater discharge

Before wastewater can be discharge to the environment it is necessary to reduce the concentration of nutrients and pathogens so that the receiving water (river or ocean) remains “fishable” & “swimable”. Therefore, the

Basic Objective of Wastewater Treatment

water (river or ocean) remains fishable & swimable . Therefore, the objectives of wastewater treatment are;

1. Remove nutrients that provide a food source for bacteria which consume oxygen (kill fish) and promote growth of macrophytes (plants) in receiving waters [Fishable]

2. Disinfect to kill pathogens that cause ear, nose, throat and intestinal disease in swimmers [Swimable]and intestinal disease in swimmers [Swimable]

How can these objectives be achieved using MBRs (what is different between MBR’s and conventional plants)?

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Disinfection (Chlorine or UV light) is more effective after suspended solids concentration and chlorine demand (nutrients) are reduced. Biological nutrient removal is based on a reaction space where soluble nutrients are consumed by microorganisms to form a biomass that can be removed prior to disinfection.

Wastewater Engineering Heuristics

Attached growth systems

Reaction Engineering Parameters/Features

1.Hydraulic Residence Time (HRT) - (hours)2. Solids Residence Time (SRT) – (days)3. Concentration of microorganisms

4. pH that favors growth of bacteria

5. Oxygen concentration & form 6 Oxygen transfer - (factor)

Suspended media activated sludge

6. Oxygen transfer (factor)7. Solid - liquid separation step

What is the key difference between conventional process and an MBR?

Requirements for Carbonaceous Material Removal

Suitable aeration system to maintain a DO in the mixed liquorFacultative bacterial mass, principally heterotrophs, to utilise the

carbonaceous energy

Appropriate Solids Retention Time (SRT) and MLSS concentration – MLSS 3g/L to 4g/L for settleable sludge– 5g/L to 12 g/L for MBR– SRT typically greater than 10 days depending on climate

Appropriately sized clarifier and RAS return system– Return Activated Sludge (RAS) increases HRT in smaller tanks– Returns nitrates formed by nitrification of ammonia– Returns nitrates formed by nitrification of ammonia

Solids retention time determined by rate of wasting of solids from tank (WAS)

– Determines the type of organisms that develop in the mixed liquor

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Removal of nitrogenous nutrients by nitrification & denitrification

Nitrification is the term used to describe the oxidation of free and saline ammonia to nitrite and nitrate within the biological process - mediated through the autotrophic bacteria Nitrosomonas and Nitrobacter,

Two stages:

– NH4 ----> NO2 (Nitrosomonas)

– NO2 ----> NO3 (Nitrobacter)

Nitrosomonas is slowest reacting of the two; uncommon to have significant presence of nitrite in effluent as Nitrobacter usually rapidly converts it to nitrate.

Nitrification requires 4.6 mg O2/mg N oxidised, Factors that affect the efficacy of nitrification include;

Solids Retention Time (SRT),

pH (alkalinity)

Toxins (heavy metals etc)

Dissolved Oxygen Concentration (Aeration & Alpha Factors)

Diurnal variation in N loading (how nitrogen varies depending on the time of day)

Nitrification Capacity

Influent TKN - two fractions; Organic N and Free and Saline NH3,

Enzymatic conversion of bulk of Organic N to NH3,

N in activated sludge-10 % of VSS in WAS, Always 2-3 mg/L TKN in effluent,

Ncapacity = TKNinf - (TKNeff + TKNsludge).

To Remove 1 g NH4-N:

4.6 g Oxygen required,

0.15 g New Cells formed,

7.14 g Alkalinity (as CaCO3) destroyed,

0.08 g Inorganic Carbon consumed.

Requirements for Denitrification

NO3 ----> N2

Presence of nitrate (or nitrite) to act as final ‘electron acceptor’

Absence of dissolved oxygen

Facultative bacterial mass

Suitable energy source to act as ‘electron donor’

‘Anoxic Zone’ required in process.

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Key features of conventional secondary wastewater treatment

Reactor Zones ClarifierAeratedNon Aerated*

Return Activated Sludge (RAS)Waste Activated

Sludge*Anoxic (O2 available in nitrates, sulphates)Anaerobic (no available O2)

MBR & Conventional Process

Primary Sedimentation

Aeration Basins (Biological

Final Sedimentation

Conventional Process

Disinfection

WasteWater

SedimentationTanks

Basins (Biological Reactor)

Sedimentation Tanks

TreatedWastewater

Membrane Primary DisinfectionMembrane

Bioreactor

Conventional processMBR process

ySedimentation

Tanks

MBR process

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Key difference between conventional and MBRIn a membrane bioreactors the secondary clarifier is replaced by a membrane filtration unit that can be located in the bioreactor tank or a separate tank. The objective is to produce a filterable sludge not a settleable sludge

Therefore MBR MLSS 8-12 g/L vs Conventional MLSS 3-4 g/L

Li it MLSS i MBR d t ff t f lid O t f f t li idLimit on MLSS in MBR due to effect of solids on O2 transfer from gas to liquid (alpha factor)

Fine bubble aeration is used to provide oxygen to the biomass, coarse bubble aeration is used to control accumulation of biomass on the membrane

fine bubble aeration coarse bubble aeration

In an MBR suspended solids and particulate effluent quality are not dependent on sludge settling (Example, Loudon Co Va, USA)

10.0

1.0

)

CA “Title 22” reuse standard (unrestricted access) = 2.0 NTU

Regional effluent discharge policy = 0.5 NTU

0.1

0.01

Turb

idit

y (n

tu

g g p y

EPA drinking water standard = 0.3 NTU

Average for MBR Effluent = 22 mNTU

0.001

Four-Month Operating Period

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Consequently, MBR systems are ideal for discharge into recreational waters – Cohasset MA, US

TSS < 1 mg/L

Turbidity < 1.0 NTU

Total coliform < 10 cfu/100 ml

Cohasset, MA, USA Effluent Quality

However, the higher concentration of colloidal and small particles in the WAS make sludge dewatering harder

Comparative STP Land Area for similar equivalent populations in UK

MBR’s are designed for higher MLSS which reduces volume (foot print) of reactor. Elimination of clarifiers reduces footprint of plant

Swanage (28000 EP) - 0.7 haMLSS 12 g/LMLSS 12 g/L

Glastonbury (30000 EP) - 4.5 haMLSS 3 g/LScale:

Reference: G.Johnston, Aquatec-Maxcon, AWA MBR Workshop 20/11/2001

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Aerobic ZoneAnoxic Zone

Metal Salt (P removal)

P1

P2

Basic Configuration of the MBR

DO < 0.5 mg/LNO3 N2

+DO > 1.0 mg/L

NH4 NO3

RAS

P2

Filtrate

WasteSludge

Flux = TMP(Rm + Rc)

TMP (+) = P1 - P2•Suction (pump)•Gravity

Typical loading rates (flux) typically 10 – 25 l/m2/h depending on the manufacturer

Average Flux vs. Wastewater Quality

10

30

50

70

Flu

x (L

/m2 /

h)

16

10F

Primary Unsettled Secondary

Clarified Secondary

Tertiary

Membrane A Membrane B Membrane C

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Operating Modes

Filtration Mode– Suction pump on for 8-13 minutes

Ai bl– Air blower on

Resting Mode– Suction pump off for 2 minutes

– Air blower on

– No backwashing required

Cleaning Mode

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– Performed when pressure increases by 0.2 bar

– Approx. every 2 to 3 months

MBR Retrofit into Aerobic Basin

Filtrate HeaderAir scour line

Backwash Tanks& Pumps

Membranes

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Hollow fibre MBR’s

• 0.04 & 0.2 m pores (e.g. Zenon/ GE,

Siemens/ Memcor, Koch)

• Dedicated filtrate pump

• TMP 30 - 70 kPa (vacuum)

• Varying degrees of oxidant resistance

• Liquid backwash

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Liquid backwash

• Continuous or Intermittent aeration

(course bubble for fouling control)

Hollow Fiber (HF) MBR North Head STP NSW

Anoxic zone

Aerobic zones

Membrane filtration zone

De-aeration zone

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Flat sheet MBR’s

• 0.4 m pores (e.g. Kubota) or 0.08 m pores (e.g. Toray)• Gravity driven process

• Continuous aeration to prevent solids accumulationTrain or Unit

• TMP 20 - 50 kPa

• No short term backwash

Train or Unit

Single Sheet

21Cassette

Flat Sheet Membrane – Victor Harbour SA

Bioselector (anoxic/anaerobic)

Swing aeration zonesSwing aeration zones

Aerobic zone

Membrane filtration zone

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Compare the turbulence in the different zones of the donut reactor MBR at Horseshoe Bay, Qld

Inner ring anoxic zone (no turbulence)

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Outer ring aerobic zone

(fine bubble aeration)

Membrane zone (coarse bubble aeration)

Horseshoe Bay MBR design considered carry over of oxygen from membrane zone

S-recycle 10 Q Future MBR cassettes

Diffusers for peak aeration

2-Stage concept, “MLE” - like

QRaw influent

Permeate1st Anoxic 2nd Anoxic MBR /

Aerobic

4-Stage concept, “Bardenpho” - like

Aerobic

S-recycle 3 QA-recycle 6 Q

DO ~ 6 mg/LNearly saturated

Highly aerated MBR tank

DO ~ 1.5 mg/L

1st Aerobic MBR / Aerobic

PermeateQRaw

influent

DO ~ 6 mg/L

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Aeration

1. Air flow requirements for membrane scouring/ fouling control given by membrane suppliers:

• No. & type of diffuser (usually coarse bubble)• Timing (continuous or intermittent)• Airflow & tank depth

– Blower size (kW), controls etc.– Oxygen transfer efficiency & rate: (S)OTE & (S)OTR

2. BNR process air requirements– Compare required SOTR (from modelling) with that supplied

for membrane scouring (see above)g ( )– Consider need for additional aerobic zone(s) & take into

account recycled oxygen from MBR tanks– Remember alpha factor ! ( OTR/SOTR as MLSS)– Recalculate blower requirements from airflow, tank depth

etc.

Tank requirements for flat sheet and hollow fibremembranes of membranes differ due to packing density

Comparison of two possible membrane types for Horseshoe Bay WRP

How does membrane configuration impact foot print (Reactor volume) & energy consumption in MBR’s?

p p yp yMembrane type Flat sheet Hollow fibreNo. of membrane cassettes 6 2No. of membranes per cassette or membrane modules per cassette 400 36Area per membrane cassette, m2 320 1138Total membrane area, m2 1920 2275MBR tanks, operating total volume, m3 280 60Ave. flow rate, m3/d 700 700Peak flow rate, m3/d 2100 2100Flux rate, ave. L/m2.h 15.2 12.6Flux rate peak L/m2 h 45 6 37 8Flux rate, peak L/m2.h 45.6 37.8

Note:Note:

• Similarity in flux

• MBR tank volume * required operating MLSS = Minimum Mandatory aerobic mass fraction for process!

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Airflow/ blower power requirement are influence by water head above diffuser, air flow rate and operating mode (continuous or intermittent)

Comparison of two possible membrane types for Horseshoe Bay WRPMembrane type Flat sheet Hollow fibreMembrane cleaning airflow peak, Nm3/h 1008 1520Aeration credits, kg/h AOR 15.96 TBANo. of blowers, installed for cleaning 3 2No. of blowers, duty for cleaning 2 1Blower delivery pressure, kPa 50 35Air cleaning blower motor installed power, kW per blower 18.5 18.5Air cleaning blower motor expected peak power draw, kW 15.4 TBAAir cleaning blower motor expected ave. power draw, kW 12.9 10.9Expected power consumption for membrane cleaning, kWh/d 619 262Expected power consumption for membrane cleaning, kWh/d 619 262Air cleaning operating mode Continuous Intermittent

• Accumulation of solids in fibre bundles

Sh t i iti i b

Operational Problems in MBR’s

• Short circuiting in membrane tank (uneven flow loading)

• Excessive power consumption on blowers to achieve adequate bubble induced shear

• Problems de-watering sludge d t l ti f fidue to accumulation of fine colloids (normally carry over in clarifier on conventional plants)

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Energy & power: MBRs tend to be energy/ power intensive

Mainly due to high air flow requirements (large blowers) for membrane scouring

T d h b i i ( i t itt t ti )• Trend has been improving (e.g. intermittent aeration)

– Lower efficiency of aeration due to high MLSS and high viscosity (the ‘’ factor question)

– Trade-off capital costs vs. operating costs • Smaller, more compact reactors vs. long term power

requirements to get higher effluent quality (effectively ultrafiltration)ultrafiltration)

Power consumptionMBR vs. Conventional BNR plants

Typical average power consumption

Data based on typical domestic sewage:

Type of plant kWh/kgCOD (biodegradabale)

kWh/m3

MBR-BNR (extended aeration type, without primary sedimentation)

3.5 1.46

BNR (extended aeration type with diffused 1 7 0 70

Data based on typical domestic sewage:

Biodegradable COD ~ 415 mg/L (raw) ; ~265 mg/L (primary treated, settled)

BNR (extended aeration type, with diffused air)

1.7 0.70

BNR (extended aeration type, with mech. Surface aerators)

1.6 0.67

BNR (With primary sedimentation, Anaerobic Digestion, with diffused air)

1.4 0.38

Source: Ken Hartley (2007) Personal comm.

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Conclusions Membranes vs traditional treatment

Filtrate quality sensitive to Stable treated water quality

Traditional Technology Membrane Filtration

– Feed water changes

– Flow changes

Chemically assisted separation common

Low additional cost for spare capacity

Copes with:

– Sudden flow changes

– Sudden, short term feed condition changes

Performance = fn (Contaminant)

Chemicals used for: Filter cake conditioning (some

applications/vendorsapplications/vendors

Chemical Cleaning

Higher additional cost for spare capacity