anaerobic membrane bioreactors : promises and problems. · anaerobic membrane bioreactors :...

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Professor David C. Stuckey

Department of Chemical Engineering

Imperial College London

Visiting Professor at NEWRI, NTU, Singapore.

ANAEROBIC MEMBRANE

BIOREACTORS :

promises and problems.

Presented at The AquaEnviro meeting on “Short Retention Time Anaerobic

Digestion, Newcastle, 11th November, 2014

OUTLINE OF TALK• BACKGROUND

- issues, objectives, treatment today

- Objectives of WW?

- “Conventional” aerobic treatment anaerobic digestion-why not?

• NOVEL REACTOR-SAMBR

- Advantages

- Disadvantages

• SAMBR PILOT PLANT

- Some preliminary results

• CONCLUSIONS

2

BACKGROUND

3

• Issues today regarding energy use, solids disposal,

carbon footprint, and sometimes water scarcity.

• Conventionally, aerobic treatment is the system of

choice, however, this is more by historical accident

than rational choice!

• Few “drivers” to change choice until recently.

• Anaerobic processes appear to be the most

“rational” choice nowadays to address the issues

mentioned above. However, must be able to

compete with aerobic processes on removal

efficiency, short HRTs, process stability to toxins.

• Due to slow growth rates in anaerobic systems

must be able to separate hydraulic retention time

(HRT) from the solids retention time (SRT) through

reactor design.

WHAT IS OUR OBJECTIVE in WWT?

4

To "optimise" a WWTP we need an Objective Function-

contains what? Until recently function changed very

little- changing rapidly now as issues mentioned at the

start are being incorporated into the way we look at

WWTP.

30 years ago only CAPEX and OPEX were important,

now concerns about energy and sludge disposal

costs. HOWEVER, need to go further and consider

more sophisticated parameters such as

GREENHOUSE GASES (GHG) from plant operation

(CO2 =1 GWP, CH4=72, N2O from nitrification = 289!).

Finally, need to incorporate RESOURCE RECOVERY

as a measure of carbon footprint with N, P and water

production.

WHAT IS OUR OBJECTIVE in WWT?

5

Hence our objective function becomes;

OF = fn (public health, environment, cost, energy footprint, residues disposal, GHG

footprint, resource recovery).

Changing the objective function will lead to a massive

"paradigm shift" in how WWT is regarded, and poses

many challenges to process engineers in first

modifying and then building new WWTPs. This new

approach can be summarised regarding WWTP as

(the "Holy Grail");

"WATER FACTORIES"

Not new idea as LA built one (Water Factory 21) in the early 1980s, but the concept more sophisticated now compared to only water recovery then. AD should become a "core" unit operation.

6

Biodegradable

Organic WasteSewage

Preliminary

Treatment

Preliminary

Treatment

Anaerobic

Digestion

USAB AFABR

Aerobic

Membrane

Bioreactor

ULTRA

FILTRATION

SAND

FILTRATION

CO2 & CH4

Gas

Separation

Fuel

CellCHP

CH4

Autotrophic

ALGAE

Biofuel

BELT

PRESS

Fertalizer

Gasification

Composting

Bioleaching

Solid

Biomass

Ion

Exchange

Phosphorous

Nitrogen

Water for

agricultureCu,Ni,Zn,Cr,

Cd,Pd

Liquid

Incineration

(Cement

Factory)

Percipitation

SAMBR

Biomass

liquid

CO2 & CH4

Paradigm

shift in

wastewater

treatment as

a “Water

Factory” or

“Biorefinery”

?

7

It is clear that the use of anaerobic

digestion compared to conventional

aerobic treatment will improve the energy

balance for WWT (Verstraete et al., 2009)

8

[Verstraete et al., Bioresource Tech., 5537, 2009]

Economics of Wastewater Recycling

Most economical favourable component in sewage is still water,

but clearly energy and maybe nutrients can contribute-need LCA!

BUT- what type of AD reactor do we use??

9

“CONVENTIONAL TREATMENT TRAIN”

Raw

sewage

Screening Secondary

settlement

Final

treatment

Treated

Effluent

Biological

treatment

Primary

settlement

Surplus sludge disposal

Why was this process flowsheet chosen? Historical!

- High energy demanding due to aeration

- High solids generating-Y~ 0.4

- Sludge settlement-”bulking sludge” a problem

- Low organic loading rates~ 0.5 kgCOD/m3.d

- Volatiles to atmosphere, AND CO2, CH4, N2O-GHG

- Not “rational” today! ???

10

AN ANSWER TO A MAIDEN’S PRAYER-

ANAEROBIC DIGESTION?

• Uses less energy than aerobic- no aeration.

• Produces energy as methane due to metabolism.

• Very low cell yields (Y~0.03 depending on SRT).

• Not volatilise solvents/chlorinateds or release GHG.

• High OLRs (25 kgCOD/m3.d)-not O2 MT limited.

BUT

• Very slow growth and hence long HRTs-large reactors.

• More sensitive to toxins or inhibitory compounds.

NEED

• REACTOR TO SEPARATE HRT FROM SRT

11

HISTORY OF AD DEVELOPMENT

Courtesy of Rittmann and McCarty-2000

c= 15-40d c= 5-8d c= 1-2d c= 1-2d

c= 1/2-3h c= 1/2-3h

c= 6-24h

c= 6-24h

c= 8-24hc= 3-24h

NOVEL SUBMERGED ANAEROBIC

MEMBRANE BIOREACTOR (SAMBR)

• It would be nice if we could use a membrane with an anaerobic reactor to keep the cells in permanently.

• Submerged membrane (Kubota) used in aerobic reactors last century - “Emperors New Clothes syndrome”- why not anaerobic?

• External membrane modules tried - high shear stress below HRT of 12 hours.

“SUBMERGED ANAEROBIC MEMBRANE BIOREACTOR (SAMBR)”

12

WHY USE A SAMBR?

Raw

sewage

Screening Secondary

settlement

Final

treatment

Treated

Effluent

Biological

treatment

Primary

settlement

Surplus sludge disposal

Raw

sewage

Screening

Treated

Effluent

Anaerobic MBR processes

Renewable energy (CH4)

3% Sludge yield

CAPABLE OF “TURN UP/TURN DOWN” DURING PEAK

FLOWS

13

Safety Factor = 20-50

times c limit!

PHOTOGRAPH SHOWING SLUG BUBBLES AT

DIFFERENT SPARGING RATES.

(FROM LEFT TO RIGHT: 2 LPM, 5 LPM, 15 LPM)14

15

GE-ZENON “ZEE WEED” HOLLOW FIBRE

MODULE

16

ADVANTAGES OF SAMBRs

1) Totally separates HRT from SRT.

2) High COD removals (of even soluble COD) due to

“fouling layer (gel layer)” on membrane.

3) Removes ALL bacteria and MOST (>5 logs)

viruses.

4) Can maintain PAC, IEX, bacteria inside the reactor.

5) Produces methane for energy use.

6) High (~25 kgCOD/m3.d) loading rates possible.

7) Low MLVSS possible (2-3 g/L) so low BAPs

released.

8) Can potentially remove nitrogen with low cost

using slow growing Anammox.

17

DISADVANTAGES OF SAMBRs

1) Fouling leads low fluxes (5-15 LMH) which leads to

large installed membrane areas and high capital

and operating costs.

2) Requires energy to scour membranes to minimise

flux loses, and mix biomass.

3) External membrane modules require high energy

input to pump around AND leads to high shear

rates which MAY inhibit methanogenesis.

4) Can potentially lose a lot of the generated methane

in the effluent due to short HRTs and low

temperatures.

5) Requires post treatment for acceptable quality

effluent compared with AS.

6) Needs 1-3 mm screens to pre-treat wastewaters.

18

1) Totally separates HRT from SRT.

• Lowest HRT in the literature (my work) is 1-2 hours.

• What limits HRT? Biological processes or physical

separation?

• What is the optimum SRT-as long as possible (>150

days) to minimise cell yields, or does this affect

effluent COD due to Biomass Associated Products

(BAP) released from lysing cells?

• As SRT increases and HRT decreases do SMPs

increase and hence increase fouling?

• Do refractory compounds slip through with very short

HRTs?

19

2) High COD removals (of even soluble

COD) due to fouling layer on

membrane.

• Fouling of the membrane by “a gel layer” composed

of low and high MW solutes, colloids, and cells (is

this a true “biofilm”?) leads to lower fluxes, BUT

higher COD removal. Can we control the fouling

layer to control effluent COD and trade-off flux?

• Rejection does depend on gassing rate-higher means

LESS rejection.

• Does this fouling layer result in VFA “rejection” (by

charge?).

• Does the fouling layer simply reject on MW, or

charge, or both?

20

AVERAGE COD IN THE EFFLUENT FROM A

SAMBR AT VARIOUS HRTs AT 35 degrees

(feed COD=500 mg/L synthetic).

COD removal > 94% with 2 hour HRT!

COD removal > 97% with 4 hour HRT

n = 4

HRT (h)Period

(day)

CODinf

(mg/L)

CODeff

(mg/L)

Eff

(%)

MLVSSa

(mg/L)

Methane

in gas (%)

Mass COD

removed

(g COD/d)

g COD

removed/

g biomass.d

VFAs

(mg/L)

Carbohydrate

(mg/L)

Organic

nitrogenc

(mg/L)

12 69 533±68 14±8 97±2 4010 69±2 3.1 0.8 ND 2±2 2

8 23 502±34 12±4 98±1 5305 72±2 4.4 0.8 ND ND 2

6 46 520±35 16±4 97±0 6097 74±1 6.0 1.0 ND ND 3

4 23 484±62 12±4 97±2 7533 75±3 8.5 1.1 ND ND 1

2 (1st) 12 h 471±20 88±10 81±3 6921 72±1 13.8 2.0 82±3 1±0 0

2 (2nd) 6 458±73 26±6 94±2 10975 80±2 15.6 1.4 (2.3b) 18±4 ND 4

1 12 h 432±29 85±3 80±1 11357 78±3 25.0 2.2 76±5 1±0 2

Note: Eff = efficiency aMLVSS on the last day of each HRT. bThe number was calculated at 12 h after changed HRT of 4 h to 2 h (MLVSS = 6898 mg/L). cOrganic nitrogen was calculated by total nitrogen minus ammonia nitrogen

21

PARAMETER/VARIABLE SAMBR A

(1)

SAMBR B

(2)

SAMBR C

(3)

Feed strength (as mg COD l-1

) 4000 4000 4000

Start-up Time (Days) (HRT=30 hrs) 102 39 115

HRT (hours) 30 30 30

SRT (days) ~150 ~150 150

VSS (mg l-1

) 9687 7020 1680

SLR (g COD g-1

VSS d-1

) 0.33 0.46 -

COD (mg l-1

) Bulk liquid 1200 2700 900

Effluent 160 240 200

COD removal (%) 96 94 95

VFA (as mg COD l-1

) Bulk liquid 257 127 0

SMP (as mg COD l-1

) Bulk liquid 943 2573 900

SMP/So 0.24 0.64 0.23

SLR (g COD g-1

VSS d-1

) (30-20 hrs) 0.33-0.70 0.46-0.70 -

During High SLR (% COD) 70 55 92

Performance of SAMBR with high strength feed (4 gCOD/L).

20 h HRT

High loads can be tolerated with good performance

3.3 kg COD/m3.d

SEM picture of biofilm thickness.

6.5

Biofilm looks like it is soluble organics not cells-tried

DGGE but problems with primers and faint trace

23

3) Removes ALL bacteria and MOST (>5

logs) viruses.

• High quality effluent due to the total removal of

bacteria by the membrane.

• Good if we could removal most or all viruses since it

saves us chlorinating/ozonating. Our data shows

complete removal (>7 logs) of large viruses (200nm),

and high removals (>4.2 logs) of small (25nm)

viruses.

• Does removal depend on gassing rates (shear)? Yes-

although relationship is complex with dirty and clean

membranes!

24

4) Can maintain PAC, IEX, bacteria inside

the reactor.

• Added PAC is kept in the reactor for the SRT (sludge

wasting-100-250 days). PAC can result in adsorption

of colloids, low MW and some high MW solutes, and

result in improved fluxes, lower TMPs, and improved

COD removals.

• How long can the surface of PAC remain adsorbent,

or does it foul irreversibly over time? What is the

mechanism of bioregeneration-desorption and

degradation, or degradation in concentrated areas

on the PAC surface.

• We have shown that IEX resins in the SAMBR can

ameliorate shock loads and enhance flux a little by

scouring membrane (unpublished).

• “Bioaugmentation” to enhance refractory degradation,

faster start-up, or adapt to psychrophilic conditions?

Time (day)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

TM

P (

ba

r)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

SAMBR1 without carbon

SAMBR2 with PAC

SAMBR3 with GAC

10 LMH 20 LMH 10 LMH

CHANGES OF TMP IN EACH SAMBR.

25

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

5 7 9 11 13 15 17 19 21 23 25

Time (min)

mV

Day 181-without PAC

DAy 183-3 hours after addition of

PACDay - 184

Day 186

Day 192

Size exclusion chromatograms inside the

reactor before and after addition of PAC

High MW (10 min. RT,>100KDa) soluble organics

reduced by ~70% in 9 days, low MW only by ~20%Vyrides I, Stuckey DC. Water Research, 43 (4), 933, 2009.

27

5) Produces methane for energy use.

• Harvests the most energy from organic wastewater

of any unit operation in WWT.

• Methane loss in effluent-oversaturated by up to

200%! Cannot assume gas-liquid equilibrium!

Higher losses at lower temperatures due to

increased solubility.

• Can recover with simple enclosed cascade or solid

phase silicon rubber membranes on effluent

~95% pure!

• Captures all Greenhouse Gases for disposal.

28

Solid phase (PDMS) membrane extraction of Soluble Methane

From Cookney et al, Proceedings of AD 12, Mexico, 2010

29

6) High (~25 kgCOD/m3.d) loading rates

possible.

• Short HRTs and strong wastewaters can lead to high

loading rates.

• What limits the OLR-MLVSS, substrate adsorption,

flux with higher MLVSS or SMPs?

• PAC addition seems to enable higher OLRs-why?

30

7) Low MLVSS possible (2-3 g/L), so low

BAPs released.

• Do not have to operate at typical high values of

sludge digesters (30-40 gVSS/L). Can operate as

low as 2-3 gVSS/L.

• Any advantages? Lower viscosity so lower power

input for mixing. Lower release of BAPs (SMPs) so

better quality effluent, higher COD removal.

• However, less stability to shock loads.

31

8) Can potentially remove nitrogen with

low cost using slow growing Anammox.

• Anammox organisms found in anaerobic environments

in treatment plants, but not in high enough

concentrations. Membranes can enable slow growing

organisms to grow until they can remove nitrogen.

• Will they inhibit/compete with methanogens?

• How can we minimise the energy input to ammonia

oxidation? Use membranes to partially nitrify

wastewaters to nitrite? (Do need oxygen sometimes!)

OR

Ion exchange resins (zeolites) to adsorb ammonia from

the effluent as well as phosphorus (Fe nanoparticles in

resin beads)

32

1) Fouling leads low fluxes (5-15 LMH) which

leads to large installed membrane areas

and high capital and operating costs.

DISADVANTAGES

• Can enhance cleaning and energy use through

optimised gas scouring-intermittent gassing,

hydrodynamics, membrane backflushing, PAC

addition, shutting down quorum sensing to reduce

SMP production?

• Cheaper installed costs with increased area used

reduces capital cost. Longer lifetimes?

• Fouling enhances COD and virus removal.

33

Factors affecting

membrane fouling

Hydrodynamic Conditions:

Effects of shearCross-flow velocityBubblesRelaxationBackwashingGas sparging

Process Performance:

Membrane fluxEffluent qualityPressure dropProcess economics

Bioreactor Operating Conditions:

Organic loading rateHydraulic loading rateSludge retention timeTemperatureAlkalinity and pHInfluent water qualityInfluent variabilityToxic shock.

Membrane Type:

Pore size and porositySurface morphologySurface chemistry

Chemical System:

Struvite precipitation(Concs. of NH4, PO4 and Mg)

Biological System:

Biomass concentrationParticle size distributionExtra-cellular PolysaccharidesColloidal particlesSoluble microbial products

Parameters and interactions that influence process

performance and fouling in an anaerobic membrane

bioreactor

PARTICLE SIZE DISTRIBUTION

Particle size distribution

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.1 0.2 0.3 0.4 0.5

Particle size (um)

Vo

lum

e (

%)

2 LPM

5 LPM

Seed sludge

34Particle size significant at the membrane pore size, and

hence high potential for blocking- can we add PAC?

50

60

70

80

90

100

110

120

72 74 76 78 80 82 84 86 88

Time (days)

DO

C (

rem

oval

%)

Inside reactor

Effluent

DOC removal after biogas sparging (day

80) changed from continuous to 5

min OFF and 10 min ON.

Sparging can influence rejection by the membrane

and increase removal, however, flux does decrease

due to increase in fouling. Energy saving of 33%!

36

2) Requires energy to scour membranes to

minimise flux loses, and mix biomass.

• Can “optimise” energy input to maximise flux and

minimise energy using intermittent operation,

membrane backflusing, ultrasound (?), scouring with

PAC (or other inorganic particles), pulsing,

precipitate with chemicals/polymers to minimise

soluble SMPs and colloids.

• Impeller to mix biomass with low power input-scour

with pulses?

• Turbulence promoters inside the membrane?

• Gas-lift membrane unit?

• Increase installed membrane area-”fluxes are low so

live with it!”. Are we too fixated on flux??

37

3) External membrane modules require

high energy input to pump around AND

might lead to high shear rates which

inhibits methanogenesis.

• Do high shear rates reduce methanogenic activity by

destroying the symbiotic associations? What is the

minimum floc size in methanogenic systems to

enable methane production to occur?

• Do we have to put constraints on mixing to keep a

certain floc size?

• Is it good to have small flocs so we are not mass

transfer limited in the reactor?

38

4) Can potentially lose a lot of the

generated methane in the effluent due

to short HRTs and low temperatures.

• Methane can be recovered from the effluent using

cascades or solid phase membranes to obtain very

pure gas.

• Does oversaturation (up to 200%) of methane lead to

thermodynamic inhibition?

• Can we remove methane in-situ by using solid phase

membranes in the tank? Can we nitrify IN the tank

with aerated membranes to achieve N removal or

control pH?

39

5) Requires post treatment for acceptable

quality effluent compared with AS.

• Aerobic treatment results in high quality effluents after

treatment and settlement. SAMBRs despite high

removals still need post treatment, which is costly.

• Post treatment options? Aerobic MBR (does not

always remove many refractories), PAC adsorption,

N, P removal with ion exchange, BioPAC, chemical

oxidation (Fenton’s reagent, ozonation),

photocatalysis (TiO on PAC), lagoons?

AROMATIC COMPOUNDS IDENTIFIED BY GC/MS

USING HEXANE AND MCB AS EXTRACTANTS*

O

O

O CH3

O

CH3

OH

CH3

CH3

CH3CH3

CH3

CH3

CH3

O

O

O

P

dibuthyl phtalate

Antracene

p-Cresol

Benzenesulfonamide1,3-bis(1,1-dimethylethyl) benzene

Benzyl benzoate

O

OH

O

O

O

O

O

CH3CH3

CH3

CH3

CH3

CH3 CH2

Diphenyl ether Diphenyl ethyneBenzene propanoic acid

Limonene

Bis(2-ethylhexyl)phthalate

40*Aquino S.F., Hu A.Y., Akram A., Stuckey D.C. J. Chem. Tech. Biotech., 81, 12, 1894-1904, (2006).

Complete plant with membrane reactor foreground (stainless steel), biomass reactor to

the left (pale blue), control room, pumps and nitrogen purge tanks on the right

SAMBR pilot plant at

Cambridge (30 m3)

42

0

100

200

300

400

500

600

700

0 2 4 6 8 10

CO

D (

mg/l)

HRT (days)

Effect of HRT on Permeate COD

Permeate TCOD

Influent TCOD

43

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

27/04/2011 17/05/2011 06/06/2011 26/06/2011 16/07/2011 05/08/2011 25/08/2011 14/09/2011 04/10/2011 24/10/2011 13/11/2011

CO

D r

em

ova

l (%

)

Date

COD Removal Variation Through Trial Period

TCOD (% removal)

sCOD (% removal)

44

PUTATIVE ANAEROBIC FLOW-SHEET

What would a “new” WW flowsheet look like?

This flowsheet reduces energy use, produces very few solids,

enables N and P removal, and recovers water. Joint

Cranfield/Imperial College study on flowsheeting an AD plant.

45

FUTURE WORK ON SAMBRs?

• Optimum HRT/SRT for energy and solids? What is the

real “rate limiting step” in the SAMBR?

• PAC use in SAMBR for removal of refractories and

improved performance in COD removal/TMP.

• Performance of SAMBR with toxins-can we use

“bioaugmentation” to improve removal/degradation?

• Soluble Microbial Products (SMPs)- composition and

removal to improve fluxes.

• Continue Pilot Plant operation to obtain substantive

data on energy/solids generation.

46

OVERALL CONCLUSIONS

• Anaerobic digestion has considerable potential, with

SAMBRs promising in terms of energy production and

high levels of treatment, all with low solids production,

and could be a key unit operation in an optimised

flowsheet.

• Flux reduction in the main is due to high molecular

weight organics and colloidal particles, but this can be

reduced with PAC addition and polymer precipitation.

• Fouling can be controlled by gassing rates, and this in

turn controls COD removal up to a point. However,

fouling can also reduce viruses in the effluent.

Thank you for

your attention

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