Temperature, inocula and substrate: Contrasting electroactive consortia, diversity and performance in microbial fuel cells.

The factors that affect microbial community assembly and its effects on the performance of bioelectrochemical systems are poorly understood. Sixteen microbial fuel cell (MFC) reactors were set up to test the importance of inoculum, temperature and substrate: Arctic soil versus wastewater as inoculum; warm (26.5°C) versus cold (7.5°C) temperature; and acetate versus wastewater as substrate. Substrate was the dominant factor in determining performance and diversity: unexpectedly the simple electrogenic substrate delivered a higher diversity than a complex wastewater. Furthermore, in acetate fed reactors, diversity did not correlate with performance, yet in wastewater fed ones it did, with greater diversity sustaining higher power densities and coulombic efficiencies. Temperature had only a minor effect on power density, (Q10: 2 and 1.2 for acetate and wastewater respectively): this is surprising given the well-known temperature sensitivity of anaerobic bioreactors. Reactors were able to operate at low temperature with real wastewater without the need for specialised inocula; it is speculated that MFC biofilms may have a self-heating effect. Importantly, the warm acetate fed reactors in this study did not act as direct model for cold wastewater fed systems. Application of this technology will encompass use of real wastewater at ambient temperatures.

Production of hydrogen from domestic wastewater in a pilot-scale microbial electrolysis cell.

Addressing the need to recover energy from the treatment of domestic wastewater, a 120-L microbial electrolysis cell was operated on site in Northern England, using raw domestic wastewater to produce virtually pure hydrogen gas (100 ± 6.4 %) for a period of over 3 months. The volumetric loading rate was 0.14 kg of chemical oxygen demand (COD) per cubic metre per day, just below the typical loading rates for activated sludge of 0.2-2 kg COD m(-3) day(-1), at an energetic cost of 2.3 kJ/g COD, which is below the values for activated sludge 2.5-7.2 kJ/g COD. The reactor produced an equivalent of 0.015 LH(2)L(-1) day(-1), and recovered around 70 % of the electrical energy input with a coulombic efficiency of 55 %. Although the reactor did not reach the breakeven point of 100 % electrical energy recovery and COD removal was limited, improved hydrogen capture and reactor design could increase the performance levels substantially. Importantly, for the first time, a 'proof of concept' has been made, showing that this technology is capable of energy capture as hydrogen gas from low strength domestic wastewaters at ambient temperatures.

Determination of the internal chemical energy of wastewater.

The wastewater industry is facing a paradigm shift, learning to view domestic wastewater not as a waste stream which needs to be disposed of but as a resource from which to generate energy. The extent of that resource is a strategically important question. The only previous published measurement of the internal chemical energy of wastewater measured 6.3 kJ/L. It has long been assumed that the energy content in wastewater relates directly to chemical oxygen demand (COD). However there is no standard relationship between COD and energy content. In this study a new methodology of preparing samples for measuring the internal chemical energy in wastewater is developed, and an analysis is made between this and the COD measurements taken. The mixed wastewater examined, using freeze-drying of samples to minimize loss of volatiles, had 16.8 kJ/L, while the domestic wastewater tested had 7.6 kJ/L nearly 20% higher than previously estimated. The size of the resource that wastewater presents is clearly both complex and variable but is likely to be significantly greater than previously thought. A systematic evaluation of the energy contained in wastewaters is warranted.