Current Status of Biological Biogas Upgrading Technologies

To limit global warming, ratio of renewable sources in the energy mix has to be considerably raised in the following years. While application of e.g. wind and solar power usually generates fluctuations in the electric grid, biogas produced in anaerobic processes is an easy-to-store renewable energy source. Raw biogas contains generally ~55–70% methane and ~30–45% carbon-dioxide. Although raw biogas can be utilized directly for combustion or combined heat and power generation (CHP), its methane content can be raised to >95% by upgrading technologies, thus it can be valorized. By upgrading and cleaning, the quality of the upgraded biogas may reach the quality of the natural gas and it may be injected to the gas grid or used as fuel for devices optimized for natural gas. Several physico-chemical upgrading methods are available on the market (e.g. high pressure water scrubbing, pressure swing adsorption, membrane technology, etc.) to remove the carbon-dioxide content of the biogas. Opposite to the physico-chemical methods, where basically the CO2 removal is the main goal, in biological biogas upgrading technologies microorganisms are applied to convert the carbon-dioxide content of the biogas to methane (chemoautotrophic upgrading), or algal biomass (photoautotrophic upgrading). The expectations are high towards biological biogas upgrading technologies in the field of energy storage linked with carbon-dioxide capture. In this paper, latest research results concerning biological biogas upgrading are summarized, viability and competitiveness of this technology is discussed together with the most important future development directions

Improving the Performance of Microbial Fuel Cells with Modified Carbon Aerogel Based Cathode Catalysts

Microbial fuel cells (MFCs) are capable of converting the chemical energy of biodegradable organic matter directly into electricity, thus they can be applied in various fields: waste elimination, biosensor industry and production of renewable energy. In this study, the efficiency of noble metal free carbon aerogel based cathode catalysts was investigated and compared to plain glassy carbon cloth without catalyst (CC ) and platinum containing carbon powder catalyst ( PtC ) in H-type MFCs. Surface extension by carbon aerogel (CA ) enhanced the maximum power density by 34 % compared to CC, to 14.1 W m−3. With nitrogen doped carbon aerogel (NCA) the performance was further increased to 15.7 W m−3. Co-doping the resorcinol-melamine-formaldehyde based aerogel with graphene oxide (GNCA) resulted in an additional power increase of 70 %, indicating that the electrocatalytic activity of NCAs can be considerably improved by co-doping with graphene oxide. Although the performance of GNCA remained below that of PtC (50.2 W m−3) in our investigations, it can be concluded that GNCA based coatings may provide a noble metal free, and therefore competitive and sustainable alternatives for cathode catalysis in MFC based technologies

Application of air cathode microbial fuel cells for energy efficient treatment of dairy wastewater

Microbial Fuel Cells (MFCs) offer a promising new solution for wastewater treatment due to their advantageous characteristics: lower energy demand and less excess sludge compared to the conventional activated sludge wastewater treatment technology. In this study, two systems of single chamber air cathode MFCs with a working volume of 14 L were investigated for the energy efficient treatment of dairy wastewater. Biomass-originated carbon cathode and noble-metal free cathode catalyst were applied to meet the demand for a lower investment cost. Influent chemical oxygen demand (COD) was in the range of 900 to 3830 mg L⁻¹, while hydraulic retention time was ~ 2.4 days. Systems provided 156 mW m⁻³ and 170 mW m⁻³ maximum power densities and coulombic efficiencies of 11.5 % and 12.8 % in average. Organic removal efficiency of 71.1 ± 8.0 % was observed when influent COD was between 900 and 1500 mg L⁻¹, however effluent quality and removal efficiency (67.9 ± 12.6 %) deteriorated as influent COD was increased (1500 - 3830 mg L⁻¹). At high influent CODs (over 3000 mg L⁻¹), an organic elimination rate of 0.82 ± 0.11 kg COD m⁻³d⁻¹ was calculated, that can be considered as the upper limit of organic removal in the systems. Based on the results, MFCs may offer a potential solution for small-scale dairy factories for the pretreatment of their effluent to meet the criteria for wastewater discharge to sewer systems. The modular MFC design also facilitates to tailor the system to actual capacity requirements

Microbial fuel cell biosensor for the determination of biochemical oxygen demand of wastewater samples containing readily and slowly biodegradable organics

OBJECTIVES: Single-chamber air cathode microbial fuel cells (MFCs) were applied as biosensors for biochemical oxygen demand (BOD) measurement of real wastewaters with considerable suspended and/or slowly biodegradable organic content. RESULTS: The measurement method consists of batch sample injection, continuous measurement of cell voltage and calculation of total charge (Q) gained during the biodegradation of organic content. Diverse samples were analyzed: acetate and peptone samples containing only soluble readily biodegradable substrates; corn starch and milk samples with suspended and colloidal organics; real domestic and brewery wastewaters. Linear regression fitted to the Q vs. BOD5 measurement points of the real wastewaters provided high (> 0.985) R(2) values. Time requirement of the measurement varied from 1 to 4 days, depending on the composition of the sample. CONCLUSIONS: Relative error of BOD measured in the MFCs comparing with BOD5 was less than 10%, thus the method might be a good basis for the development of on-site automatic BOD sensors for real wastewater samples

Single chamber air-cathode microbial fuel cells as biosensors for determination of biodegradable organics

ObjectivesSingle chamber air cathode microbial fuel cells (MFCs) were investigated with sodium-acetate and peptone as test substrates to assess the potential for application as biosensor to determine the concentration of biodegradable organics in water/wastewater samples.ResultsMFCs provided well-reproducible performance at high (>2000mg COD l(-1)Chemical Oxygen Demand) acetate concentration values. Current in the cells proved to be steady from 25 to 35 degrees C, significant decrease was, however, revealed in the current below 20 degrees C. Direct calculation of non-toxic biodegradable substrate concentration in water/wastewater from the current in MFCs is possible only in the non-saturated substrate concentration range due to the Monod-like dependence of the current. This range was determined by a fitted and verified Monod-based kinetic model. Half saturation constant (K-S) values were calculated at 30 degrees C applying different external resistance values (100, 600 and 1000, respectively). In each case K-S remained below 10mg COD l(-1).ConclusionsBiosensors with this particular MFC design and operation are potentially applicable for detecting as low as 5mg COD l(-1) readily biodegradable substrates, and measuring the concentration of these substances up to 50-70mg COD l(-1)