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Abstract
Microbial electrolysis cells (MECs) can be used to simultaneously convert wastewater
organics to hydrogen and precipitate struvite, but scale formation at the cathode
surface can block catalytic active sites and limit extended operation. To promote
bulk phase struvite precipitation and minimize cathode scaling, a two-chamber MEC
was designed with a fluidized bed to produce suspended particles and inhibit scale
formation on the cathode surface. MEC operation elevated the cathode pH to between
8.3 and 8.7 under continuous flow conditions. Soluble phosphorus removal using digester
effluent ranged from 70 to 85% with current generation, compared to 10-20% for the
control (open circuit conditions). At low current densities (≤2 mA/m(2)), scouring
of the cathode by fluidized particles prevented scale accumulation over a period of
8 days. There was nearly identical removal of soluble phosphorus and magnesium from
solution, and an equimolar composition in the collected solids, supporting phosphorus
removal by struvite formation. At an applied voltage of 1.0 V, energy consumption
from the power supply and pumping (0.2 Wh/L, 7.5 Wh/g-P) was significantly less than
that needed by other struvite formation methods based on pH adjustment such as aeration
and NaOH addition. In the anode chamber, current generation led to COD oxidation (1.1-2.1 g-COD/L-d)
and ammonium removal (7-12 mM) from digestate amended with 1 g/L of sodium acetate.
These results indicate that a fluidized bed cathode MEC is a promising method of sustainable
electrochemical nutrient and energy recovery method for nutrient rich wastewaters.