March 18-19, 2015

Anaerobic MBR:

Challenges & Opportunities

Art Umble, PhD, PE, BCEE

Americas Wastewater Practice Leader

Symposium:

Hydrogen, Hydrocarbons, and

Bioproduct Precursors from Wastewaters National Renewable Energy Laboratory

Washington, DC

2

Outline

■ Challenge for Municipal Wastewater

■ Membrane Fouling

■ Energy Potential

■ System Economics

■ Research Needs

Challenges: Municipal Wastewater

Treatment Using AnMBR

• Low temperatures in municipal

wastewaters

• Low strength municipal

wastewaters

• Bioreactors must be heated

• Long SRTs are required

• Post-treatment is required for

direct discharge

• High SO4 reduces methane

production

• Methane solubility at low

temperatures limits recovery

• GHG emissions 3

Biogas

Influent Retenate

Permeate

Membrane

Module

Bioreactor

WAS Biogas

Influent

Permeate

Bioreactor

WAS

PVDF

PES

Metallics

Inorganics

Challenges: Municipal Wastewater

Treatment Using AnMBR

4

• Limited Development Realized Since Early 2000s

– Anaerobic processes are complex

– Methanogens highly sensitive to wastewater toxicity

– Difficulty in managing variable conditions

– Membrane fouling

– Relatively low flux

• Necessity of Operating at Ambient Temperatures

– Low organic strength low methane production

– Low methane production limited heating potential

– Long SRTs increases membrane fouling

Challenges with Membrane Fouling

• Internal fouling generally

irreversible

• External fouling generally

reversible

• Internal deposits generally

more inorganic

• Long SRT operation

promotes internal pore

blocking

• Fouling higher costs &

membrane replacement

5

Pore

Blocking

Internal External

Membrane

Surface

Porous

Layer

Suspended biomass, colloidal solids, SMP, EPS,

Attached cells, inorganics (e.g., struvite)

Fouling Control Methods

• Biogas sparging

• Backflushing

• Periodic membrane

relaxation

• PAC/GAC addition

• Combinations

6

Operational/Performance Considerations

• OLR: >10 kg COD/m3/d

• HRT: ~ 8-12 hours

• Sustainable flux rate:

< 15 LMH

• Temperature

• Methane solubility

• >85% COD removal

• >99% TSS removal

• TN and TP removals

negligible

• Effluent COD/N & COD/P

unfavorable for downstream

BNR

7

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40 45 50

tCO

D R

em

ov

al, %

time, hrs

COD Removal

Adapted from: Smith, et al. Bioresource Technology 122 (2012)

Tradeoff:

Increased biological

activity across biofilm;

but increased fouling

AnMBR Performance – COD Removal

8 Source: U4R08 WERF; Raskin, et al. (2012)

Methane Production Potential

• Colder reactor temperatures result in lower methane production

9

0

10

20

30

40

50

60

70

80

Day 1 Day 15 Day 30 Day 45 Day 60 Day 75

mL

CH

4/g

VS

S -

d

Specific Methanogenic Activity

Suspended @ 25oC

Suspended @ 15oC

Attached @ 25oC

Attached @ 15oC

Adapted from: Ho and Sung, Bioresource Technology 101 (2010)

Methane Production Potential

10 Source: U4R08 WERF; Raskin, et al. (2012)

Methane Production Potential

11 Source: Gimenez, et al. Bioresource Technology; 118 (2012)

CH

4 R

eco

very

Eff

icie

ncy,

%

Biogas production rate/L per m3 of treated wastewater

Theoretical Recovery @ 33oC

Theoretical Recovery @ 20oC

Methane Solubility Creates a Challenge

• CH4 ~ 1.5x more soluble at

15oC than at 35oC

• Dissolved CH4 leaving

process in permeate is

significant fraction to total

CH4 generated

• Permeate concentration

tends to be oversaturated

• Impact on GHGs

• Post-treatment stripping

• Degassing membrane

• Downflow Hanging Sponge

reactor

12 Adapted from Uemura and Harada, (2010)

Influent

Effluent

Implications for Energy Recovery

• Production is function of:

– Temperature

– Loading rate

– Operating condition

– Influent pre-treatment

– Influent pre-heating

• 110-320 mL CH4/g COD

removed

• Net energy recovery

achievable at 9.5 g

COD/L or higher 13

WARM

WATER

OUT

COLD

WATER

IN

Methane Production

14 Source: Wei, et. al; Bioresource Technology 166 (2014)

Me

tha

ne

vo

lum

e f

rac

tio

n (

%)

Economic Overview

15

11.3

72.3%

2.2 5.7

5.9 1.2 1.4

Capital Cost Elements

Tank

Membranes

Liquid Pumps

Gas Blowers

Screens

PLCs

Other

46.7%

13.7

7.2

32.5

Operational Cost Elements

Scouring Energy

Pumping Energy

sludge Disposal

Chemical Consumption

0

20

40

60

80

1000 5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

10

0% o

f M

em

bra

ne C

ost

Design Flux (LMH)

Flux Influence on Cost

Adapted from: Lin, et. al; Desalination 280 (2011)

Research Needs for AnMBRs

• Membrane fouling,

particularly with low-

strength wastewaters

• Consumption and

optimization of energy

• Relationship between

HRT, SRT, performance

and fouling

• Methane solubility at low

temperatures.

• Operation at low and

high temperatures

• Effects of microbial

seeding

• Nutrient removal

systems

• Comprehensive effects

of OLR on methane

production

• Pre-treatment effects

16

17

Summary

■ AnMBR Remains Challenging for Municipal

Art.Umble@mwhglobal.com Denver, CO USA

■Membrane Fouling Solutions are Elusive

■ Significant Impediment is Nutrient Removal

■Methane Solubility a Challenge for Recovery

■Membranes Remain Primary Capital Cost

■ Scouring Energy Primary Operational Cost

■Research Opportunities Remain High