Professor Simon Judd

MBRs: the low down

aeration

primary sedimentation

secondary clarifier

screened sewage

settled sewage

final effluent

raw/primary sludge

PRIMARY TREATMENT

SECONDARY TREATMENT (activated sludge)

TERTIARY TREATMENT (disinfection)

cell separation

Conventional sewage treatment clarified, largely disinfected product provided

small footprint plant low sludge yield (0.35 – 0.6 Kg DS/Kg BOD)

bulking problems become less relevant hydraulic and solids retention time are uncoupled intensive biotreatment provided, esp. nitrification

Cl2

waste activated sludge

return activated sludge

MBR process configurations

Air

In

Out

Membrane

Out

Bioreactor

Recirculated stream

Air

In

Pump

Bioreactor

Membrane

immersed/submerged MBR sidestream MBR

Really expensive Expensive ? ?

MBR process configurations

MBR technology

Immersed Flat

sheet Hollow fibre

Sidestream

Multitube/multichannel

Pumped

Classical Low energy

Aerated

Lift Injection

Municipal Municipal Industriall

mem

b-

rane

Industriall

QR

MBR operational parameters

Permeate flow (Q)

Area (A)

Flux Flow per unit area per unit time

L/m2/h, or LMH

P1

P2

Transmembrane pressure TMP (bar) = P1-P2

Permeability Flux per unit TMP

LMH/bar

Aeration (Qa)

Specific aeration demand SADp = Qa/Q

Q

Hydraulics, hydrodynamics & fouling/clogging

• All interlinked: • increasing flux increases fouling/clogging • increasing crossflow (promoting

turbulence) increases flux – but increases energy demand

• Fouling also determined by: • biomass characteristics

• This is in turn influenced by • feedwater quality • retention times (hydraulic/lids)

• Key design parameter is: • critical/ sustainable flux

• There is a limit to how high the flux can be pushed

Permeability loss

Permeability decline Fouling

Coating of membrane surface

Reversible Physically removed by backflushing or

relaxation

Irreversible Removed by

chemical cleaning

Clogging Agglomeration of solids

Sludging Filling of

membrane channels

Ragging/braiding Blocking of

membrane channel inlets

Membrane process types

10-10 10-9 10-8 10-7 10-6 10-5 Scale in metres

Free atoms

200 20,000 500,000 Approximate Molecular Weight in Daltons

Small organic

monomers

Sugars

Herbicides

Pesticides

Dissolved salts

Endotoxins/ pyrogens

Viruses

Colloids: Albumen protein Colloidal silica

Bacteria (to ~40µm)

Crypto- sporidia

Red blood cells

Porous membrane filtration processes

Dense membrane processes

Electrodialysis

MBR membrane configurations

Three types: flat sheet (FS) hollow fibre (HF) multitube (MT)

• multichannel

flat sheet

multitube hollow fibre

Membrane material

Feed Permeate

Process blower(s)

Membrane blower(s)

FBDs CBDs

AO AE fine

screen

Classical immersed

Feed

Permeate

Process blower(s)

FBDs

AO AE fine

screen

Classical sidestream

Feed

Permeate

fine screen

Sidestream anaerobic

CH4

CH4

Feed Permeate

CBDs

fine screen

Immersed anaerobic

Sidestream MBRs: advantages

• Reduced membrane area requirement (higher fluxes) • Fouling controlled by crossflow velocity (CFV)

• between 2 and 4 m/s CFV applied in practice complete flexibility for both operation and CIP cycle • In-situ chemical cleaning of membranes possible without

any chemical risk to biomass. • Maintenance and plant downtime costs, particularly for

membrane module replacement, generally slightly lower: • modules readily accessible • can be replaced in ~30 minutes • can be readily brought on and off line according to

hydraulic loading • Operation at higher MLSS levels possible, cf. HF iMBRs. • Operation at slightly lower energy demand possible if

• CFV and TMP are reduced • Air-lift/injection is employed

Air-lift/injection systems

Sludge Drain

Bioreactor UF

Permeate Feed

Air

Design net flux, municipal vs. industrial

Air-scour efficiency

Rotating membranes

Huber VRM

Grundfos Biobooster

Rotating liquid

Pentair Helix

The 2015 MBR Survey (186 responses) Q1 What is the main technical problem that prevents MBRs working as they should?

16%

16%

12%

11% 10%

8%

8% 6%

5% 4% 4% Screening/pre-treatment

Membrane surface fouling

Operator knowledge

Energy demand

Membrane/aerator clogging

Sludge/mixed liquor quality

Membrane chemical cleaning

Overloading/under-design

Uneven aeration

Other/Comments

Automation/control, or software

Q2 How will MBR technology develop in the future?

48 32

27 17

16 14

13 13

10 10

9 9

8 8 8

6 6 6

5 5

0 5 10 15 20 25 30 35 40 45 50

energy/power reuse/recycle

cost industrial

fouling footprint/compactness

membrane materials aeration/air scour

automation & control municipal

potable/drinking membrane life

robustness anaerobic

awareness/perception/acceptance decentralisation

nutrient optimisation

pretreatment/screening/clogging legislation/regulation

The 2015 MBR Survey (186 responses)