THE EFFECT OF BUFFER CHARGE AND BUFFER RETENTION ON BIOELECTROCHEMICAL SYSTEMS AND POST-TREATMENT FOR MICROBIAL FUEL CELL EFFLUENT WITH FLUIDIZED BED MEMBRANE BIOREACTORS

Open Access
Author:
Ye, Yaoli
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 18, 2017
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • Bruce Ernest Logan, Committee Chair
  • Christopher Aaron Gorski, Committee Member
  • Rachel Alice Brennan, Committee Member
  • Michael Anthony Hickner, Outside Member
Keywords:
  • Bioelectrochemical systems
  • hydrogen
  • membrane bioreactor
  • membrane aerator
  • membrane fouling control
  • polymeric buffer
  • simultaneous organic and nitrogen removal
  • buffer retention
  • post-treatment
  • microbial electrolysis cell
  • microbial fuel cell
  • Nernst-Planck equation
  • migration
  • Ion transport
  • microbial community analysis
  • granular activated carbon particles
  • effluent dissolved methane
  • nitrification and denitrification
Abstract:
Microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) are very promising technologies for simultaneous wastewater treatment and energy recovery. In MFCs, buffers are typically used to improve performance by stabilizing the electrode pH and increasing the electrolyte conductivity, but the importance of the buffer net charge at current densities typical of MFCs on cathode performance has received little attention. Current production in MFCs produces an electric field that drives cations towards the cathode, and anions to the anode. A series of biological buffers were selected with positive, negative, and neutral charges that had pKas ranging from 5 to 10.8. Cathodic current production using these different buffers in solutions with different pHs and conductivities was compared using linear sweep voltammetry (LSV). At lower pHs, buffers with positive charge increased cathodic current by as much as 95% within certain ranges (potential windows) of cathode potentials. No difference in cathodic current was shown in current for buffers with neutral or negative charge. The reason for this increase with the net positive charge buffers was likely due to a more stable electrode pH produced by electric field driving the positively charged ions towards the cathode. The potential window for the positively charged buffers was positively correlated to the concentration of cationic buffer in the electrolyte. At a pH higher than 9, no improvement in cathodic current was shown for buffers with positive charge, indicating at these higher pHs diffusion dominated buffer transport. In two-chamber microbial electrolysis cells (MECs) with anion exchange membranes (AEMs), a phosphate buffer solution (PBS) is typically used to avoid increases in catholyte pH as Nernst equation calculations indicate that high pHs adversely impact electrochemical performance. However, ion transport between the chambers will also impact performance, which is a factor not included in those calculations. To separate the impacts of pH and ion transport on MEC performance, a high molecular weight polymer buffer (PoB), which was retained in the catholyte due to its low AEM transport and cationic charge, was compared to PBS in MECs and abiotic electrochemical half cells (EHCs). In MECs, catholyte pH control was less important than ion transport. MEC tests using the PoB catholyte, which had a higher buffer capacity and thus maintained a lower catholye pH (<8), resulted in a 50% lower hydrogen production rate (HPR) than that obtained using PBS (HPR=0.7 m3-H2 m-3 d-1) where the catholyte rapidly increased to pH=12. The main reason for the decreased performance using PoB was a lack of hydroxide ion transfer into the anolyte to balance pH. The anolyte pH in MECs rapidly decreased to 5.8 due to a lack of hydroxide ion transport, which inhibited current generation by the anode, whereas the pH was maintained at 6.8 using PBS. In abiotic tests in ECHs, where the cathode potential was set at –1.2 V, the HPR was 133% higher using PoB than PBS due to catholyte pH control, as the anolyte pH was not a factor in performance. These results show that hydroxide ion transport through AEM to control anolyte pH is more important than obtaining a more neutral pH catholyte. MFCs cannot effectively treat wastewater with low a COD, so a post-treatment is usually needed for polishing MFC effluent. Anaerobic fluidized bed membrane bioreactors (AFMBRs) use granular activated carbon (GAC) particles suspended by recirculation to effectively treat low strength wastewaters (∼100–200 mg L−1, chemical oxygen demand, COD), including MFC effluent, but the effluent contains dissolved methane. An aerobic fluidized bed membrane bioreactor (AOFMBR) was developed to avoid methane production and the need for wastewater recirculation by using rising air bubbles to suspend GAC particles. The performance of the AOFMBR was compared to an AFMBR and a conventional aerobic membrane bioreactor (AeMBR) for domestic wastewater treatment over 130 d at ambient temperatures (fixed hydraulic retention time of 1.3 h). The effluent of the AOFMBR had a COD of 20 ± 8 mg L−1, and a turbidity of <0.2 NTU, for low-COD influent (153 ± 19 and 214 ± 27 mg L−1), similar to the AeMBR and AFMBR. For the high-COD influent (299 ± 24 mg L−1), higher effluent CODs were obtained for the AeMBR (38 ± 9 mg L−1) and AFMBR (51 ± 11 mg L−1) than the AOFMBR (26 ± 6 mg L−1). Transmembrane pressure of the AOFMBR increased at 0.04 kPa d−1, which was 20% less than the AeMBR and 57% less than the AFMBR, at the low influent COD. Scanning electron microscopy (SEM) analysis indicated a more uniform biofilm on the membrane in AOFMBR than that from the AeMBR biofilm, and no evidence of membrane damage. High similarity was found between communities in the suspended sludge in the AOFMBR and AeMBR (square-root transformed Bray–Curtis similarity, SRBCS, 0.69). Communities on the GAC and suspended sludge were dissimilar in the AOFMBR (SRBCS, 0.52), but clustered in the AFMBR (SRBCS, 0.63). Although the production of dissolved methane can be avoided in AOFMBR, the process is energy intensive due to the large air flowrates. In addition, ammonia nitrogen is not effectively biologically removed in either AFMBRs or AOFMBRs. Membrane aerators were added into an AFMBR to form an aerated membrane fluidized bed membrane bioreactor (AeMFMBR) capable of simultaneous removal of organic matter and ammonia without production of dissolved methane. Good effluent quality was obtained for domestic wastewater (193±23 mg/L and 49±5 mg-N/L) treatment, with non-detectable suspended solids (<2 mg/L), 93±5% of chemical oxygen demand (COD) removal to 14±11 mg/L, 89±7% of soluble COD removal to 13±11 mg/L, and 74±8% of total nitrogen (TN) removal to 12±3 mg-N/L. Nitrate and nitrite concentrations were always low (< 1 mg-N/L) during continuous flow treatment. The ammonia removal rate (AR) was higher with continuous flow treatment than fed batch operation, higher for a synthetic wastewater compared to a domestic wastewater, but independent of the hydraulic retention time. Membrane fouling was well controlled by fluidization of the granular activated carbon (GAC) particles as shown by a low transmembrane pressure (<3 kPa). No methane was detected in the treated effluent (<0.5 mg/L). Analysis of the microbial communities suggested that the nitrogen removal was due to nitrification and denitrification based on the presence of microorganisms associated with these processes.