Increasing phosphorus recovery from dewatering centrate in microbial electrolysis cells

Abstract

BackgroundMicrobial electrolysis cells (MECs) use bioelectrochemical reactions to remove organic contaminants at the bioanode and produce hydrogen gas at the cathode. High local pH conditions near the cathode can also be utilized to produce struvite from nutrient-rich wastewater. This beneficial aspect was investigated using lab-scale MECs fed with dewatering centrate collected at a local wastewater treatment plant. The main objective was to improve phosphorus recovery by examining various cathode configurations and electric current conditions.ResultsThe stainless steel mesh (SSM) cathode was relatively inefficient to achieve complete phosphorus recovery because struvite crystals were smaller (a few to tens of micrometers) than the open space between mesh wires (80 µm). As a result, the use of multiple pieces of SSM also showed a limited improvement in the phosphorus recovery up to only 68% with 5 SSM pieces. Readily available organic substrates were not sufficient in the dewatering centrate, resulting in relatively low electric current density (mostly below 0.2 A/m2). The slow electrode reaction did not provide sufficiently high pH conditions near the cathode for complete recovery of phosphorus as struvite. Based on these findings, additional experiments were conducted using stainless steel foil (SSF) as the cathode and acetate (12 mM) as an additional organic substrate for exoelectrogens at the bioanode. With the high electric current (>2 A/m2), a thick layer of struvite crystals was formed on the SSF cathode. The phosphorus recovery increased to 96% with the increasing MEC operation time from 1 to 7 days. With the high phosphorus recovery, estimated energy requirement was relatively low at 13.8 kWh (with acetate) and 0.30 kWh (without acetate) to produce 1 kg struvite from dewatering centrate.ConclusionsFor efficient phosphorus recovery from real wastewater, a foil-type cathode is recommended to avoid potential losses of small struvite crystals. Also, presence of readily available organic substrates is important to maintain high electric current and establish high local pH conditions near the cathode. Struvite precipitation was relatively slow, requiring 7 days for nearly complete removal (92%) and recovery (96%). Future studies need to focus on shortening the time requirement.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0754-8) contains supplementary material, which is available to authorized users.

Microbial electrolysis cell as an emerging versatile technology: a review on its potential application, advance and challenge

“…An MEC has the ability and the potential to recover other resources such as phosphorus (P) . Compared with ammonia, P recovery in an MEC is carried out through a different mechanism.…”

“…However, cathode and exchange membrane scaling, as well as pH difference between anode and cathode, can deteriorate MEC performance . To achieve efficient phosphorus recovery, various cathode configurations and electric currents were examined in a lab‐scale MEC experiment for efficient struvite precipitation . The high electric current density (>2 A m −2 for peak currents) and foil‐type cathode resulted in successful struvite production from the dewatering centrate with 92% P removal and 96% P recovery .…”

“…120 An MEC has the ability and the potential to recover other resources such as phosphorus (P). 126 Compared with ammonia, P recovery in an MEC is carried out through a different mechanism. Abdullah et al 36 investigated P recovery in a dual-chamber MEC, and found that the maximum P precipitation efficiency attained 95% with simultaneous H 2 production.…”

Electrochemical phosphorus removal and recovery

“…The typical Ca concentration in domestic wastewater is 20−120 mg/L [122], whereas the P-PO4 concentration varies from less than 1.0 mg/L (effluent) to 10 mg/L (influent). The P concentration in the downstream wastewater from an anaerobic digestion system can be even higher [123]. In most cases, the aqueous Ca concentration is high enough to precipitate the coexisting phosphate in the wastewater.…”

“…The advantages of using a bioanode are substantial. First, the energy needed for electrochemical phosphorus recovery is significantly reduced [40,65,66,136,137]. The oxidation potential for sodium acetate (NaAc), which is a typical substrate used in bioelectrochemical systems, is −0.278 V (vs NHE), which is 1.1 V lower than abiotic water oxidation (0.817 V, vs NHE), under standard conditions [139].…”