Bioelectrochemical systems for wastewater treatment

Bioelectrochemical
wastewater treatment:
not so elementary
René Rozendal,
Bert Hamelers
Cees Buisman
Korneel Rabaey
Jurg Keller
MFCs First International symposium, 29 May 2008, Penn State

Bioelectrochemical systems
• Exponential increase
over the years
• Currents approach levels
suitable for practical
implementation
– ~10 A/m 2

Bioelectrochemical wastewater
treatment
• 1000 A/m 3
– 10 A/m 2
– Cell thickness 1 cm
• ~7 kg COD/m 3 /day
– Aerobic: 0.5‐2 kg COD/m 3 /day
– Anaerobic: 8‐20 kg COD/m 3 /day
•BUT…

Reality check
• Size matters!
– Smaller better than larger
• Real wastewaters?!
– Synthetic media outperform real WW

Why not implemented already?
• Microbiological challenges
• Technological challenges
• Economic challenges
Rozendal et al. (2008), Trends Biotechnol., In press

Microbiological challenges
• Complex matter
– Polymers/particulates
– More experience required
• Methanogenesis
• Biofilm gradients

Biofilm gradients
(Concentration polarization)
[Reactants]
[Products]
pH!!!

Why not implement already?
• Microbiological challenges
• Technological challenges
• Economic challenges

Technological challenges
• Ohmic losses
• Membrane pH gradients

Rozendal et al. (2008), Trends Biotechnol., In press
Real
Laboratory
Keep the electrode spacing as small as possible!!!

Ohmic loss across the electrodes
Single cell design
= Travel distance electrons
= Travel distance ions
Rozendal et al. (2008), Trends Biotechnol., In press

Ohmic losses
GRAPHITE
• Resistivity of material: 10 mΩ cm
• 10 A/m 2 (assume on average half of this passes true the plane of the electrode)
Laboratory scale (10×10×0.3 cm):
L
10cm
R = ρ = 10mΩ
cm = 33mΩ
A
3cm2
I=10 A/m 2 ×0.01=0.1 A ΔV = 0.033×0.1×0.5=0.0017 V
Large scale (100×100×0.3 cm):
L
100cm
R = ρ = 10mΩ
cm = 33mΩ
A
30cm2
I=10 A/m 2 ×1=10 A ΔV = 0.033×10×0.5=0.17 V

Technological challenges
• Ohmic losses
• Membrane pH gradients

Cation Exchange Membranes
• Wastewater:
– pH 7 [H + ] = 10 ‐4 mM
– [Na + ], [K + ], [NH 4+ ] ≈ 10 mM
×100000!!!
• Not much protons available to transport!!!
• Other cations also transported!
Rozendal et al. (2006), Environ. Sci. Technol. 40, 5206-5211

Membrane Ion Transport
Ideal:
H +
H +
H +
pH
Practice:
Performance
Na +
K +
NH 4
+

Performance loss

Performance loss due to pH effects!
Rozendal et al. (2006), Environ. Sci. Technol. 40, 5206-5211

Alternative Types of Membranes
Rozendal et al. (2008), Water Sci. Technol. Accepted

Why not implement already?
• Microbiological challenges
• Technological challenges
• Economic challenges

Case (1000 A/m 3 )
• Laboratory materials (lifetime 5 year)
– Anode: 100 €/m 2 (graphite felt)
– Membrane: 400 €/m 2 (Nafion)
– Cathode: 500 €/m 2 (Pt catalyzed)
– Collector: 25 €/m 2
• Other cost (lifetime 25 years)
– Reactor: 4000 €/m3
– Rest: 1000 €/m3

Rozendal et al. (2008), Trends Biotechnol., In press
Capital costs
TOO
EXPENSIVE!!!
For comparison: Anaerobic Digestion ~0.01-0.05 €/kg COD

Case (1000 A/m 3 )
• Future materials (lifetime 5 year)
– Anode: 100 5 €/m 2 (graphite paper)
– Membrane: 400 10 €/m 2 (CEM)
– Cathode: 500 5 €/m 2 (biocathode)
– Collector: 25 10 €/m 2
• Other cost (lifetime 25 years)
– Reactor: 4000 €/m3
– Rest: 1000 €/m3

Rozendal et al. (2008), Trends Biotechnol., In press
Capital costs
For comparison: Anaerobic Digestion ~0.01-0.05 €/kg COD

System
Product
Revenue
Capital
costs
(€/kg COD)
Product
revenue
(€/kg COD)
Offset
(€/kg COD)
Aerobic - -0.1 -0.2 -0.3
AD CH 4 ­0.05 0.1 0.05
MFC Electricity ­0.4 0.2 ­0.2
MEC H 2 ­0.4 0.6 0.2
MEC ??? ­0.4 $$$ $$$

Conclusion
• Bioelectrochemical wastewater
treatment promising, but…
– Microbiological,…
– Technological, and…
– Economic challenges!
• Much to gain beyond electricity
– Biocathodes (See: Korneel Rabaey)

ACKNOWLEDGEMENTS
Thank you!