Nutrient and heat recovery from waste streams using microbial electrochemical technologies

Open Access
Author:
Cusick, Roland D
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 19, 2013
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • Bruce Ernest Logan, Committee Chair
  • Brian Dempsey, Committee Member
  • John Michael Regan, Committee Member
  • Peggy Ann Johnson, Committee Member
  • Michael Anthony Hickner, Special Member
Keywords:
  • Microbial fuel cells
  • microbial electrolysis cells
  • struvite
  • reverse electrodialysis
  • ammonium bicarbonate
  • wastewater
Abstract:
Microbial Electrochemical Technologies (METs) are an emerging design platform capable of converting organic matter directly into electrical energy. Two new METs were developed to enhance the sustainability of wastewater treatment. The first set of systems utilizes electrical and ionic currents to couple bio-electricity generation from wastewater organics with the recovery of wastewater nutrients via salt crystallization. The second set of METs was designed to enable the recovery of waste heat using engineered salinity gradients. The first system was an energy efficient method of recovering both energy and nutrients from wastewater in a microbial electrolysis cell (MEC) by producing hydrogen to increase solution pH and precipitate struvite (MgNH4PO4∙6H2O). The concept was first proven in a single-chamber MEC in which hydrogen evolution cathodes were either stainless steel 304 mesh or flat plates. The cathodes accumulated a layer of crystals, which were verified as struvite using a scanning electron microscope capable of energy dispersive spectroscopy (SEM-EDS). Phosphate removals were low (20 – 38%) because the pH increase was localized to the cathode surface. Energy produced as hydrogen was higher than the energy invested to drive hydrogen evolution, implying that MECs may be useful both as a method for hydrogen gas production and struvite production. In order to increase phosphate removal and prevent cathode scale accumulation, a two-chamber MEC was designed where the cathode operated as a fluidized bed crystallization reactor. Within the cathode, hydrogen production continuously raised cathode solution pH, and a fluidized bed of seed particles provided surface area for struvite crystal growth while scoring the cathode surface. When MEC electrodes generated current, the cathode solution pH was maintained between 8.3 – 8.8 under continuous flow conditions, and soluble phosphorus removal ranged from 70 – 85%, as compared to 10 – 20% when the fluidized bed was operated under open circuit conditions. At low current densities (≤ 2 mA/m2), scouring by fluidized particles prevented cathodic scale formation for the duration of continuous flow experiments. At an applied voltage of 1.0 V, energy consumption from the power supply and pumping was 0.2 Wh/L (7.5 Wh/g-P). This energy requirement could be fully offset if future hydrogen recoveries could exceed 80%. These results indicate that a fluidized bed cathode MEC is a promising method of sustainable electrochemical nutrient recovery. The second type of MET was developed for converting waste heat and wastewater organics into electricity using ammonium bicarbonate (AmB) salt solutions in a microbial reverse electrodialysis cell (MRC). A MRC couples microbial fuel cell (MFC) electrodes with a reverse electrodialysis (RED) membrane stack, a technology capable of recovering energy released when salt and fresh water mix. By feeding the RED stack salt solutions composed of AmB, which decomposes to ammonia and carbon dioxide gas at low temperatures (40 – 60°C), MRCs can create electricity from abundant sources of low-grade thermal energy such as waste heat. The maximum power density using acetate reached 5.6 ± 0.04 W/m2-cathode surface area, which was five times that produced without the dialysis stack, and 3.0 ± 0.05 W/m2 with domestic wastewater. The greatest contribution to MRC power came from enhanced MFC electrode performance, which increased by more than 300% with acetate and over 700% with domestic wastewater. MFC electrode power production was further investigated in MRCs having a minimal number of membranes. It was observed that using only a single RED cell pair (CP), created by operating the cathode in concentrated AmB, dramatically increased power production normalized to cathode area from both acetate and wastewater. Galvanostatic electrochemical impedance spectroscopy indicated that power increases in the 1-CP MRC were caused by reductions in solution and kinetic resistances at the cathode. The addition of a second RED cell pair further increased power production and anode biofilm activity. Power densities achieved here were close to those previously achieved using 11 membranes, indicating near optimal electrode performance with only one or two cell pairs. When normalized to total membrane area, the minimal cell pair MRC power densities was 4 – 10 times higher than previous MRCs. The rate of wastewater COD removal, normalized to reactor volume, was 30 – 50 times higher in 1-CP and 2-CP MRCs than that in a single-chamber MFC. These findings showed that even a single cell pair AmB RED stack could significantly enhance electrical power production and wastewater treatment rates.