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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