The development and operation of plant microbial fuel cells using municipal sludge

Gulamhussein, Mohamedjaffer

The development and operation of plant microbial fuel cells using municipal sludge

Master Thesis

2019

Abstract

Wastewater treatment accounts for 3-5% of the total electricity demand in developed countries. However, wastewater is estimated to have 9.3 times more energy than which is required to treat it. A sediment microbial fuel cell (SMFC) can potentially be used to treat wastewater and produce electricity by utilising the organics found in the wastewater. The challenge associated with using SMFCs is efficiency and longevity. Literature has shown that the efficiency can be increased by growing plants in a SMFC. Plants release organics and oxygen into the rhizosphere which can increase microbial growth and increase oxygen at the cathode. This research undertook to design a batch plant microbial fuel cell (PMFC) and operate it on three different municipal sludge streams namely, thickened waste activated sludge (WAS), liquid WAS and primary sludge (PS). In addition, three indigenous South African plant species, namely, C. papyrus nanus, W. thyrsiflora and P. australis were tested based on their power output potential and organic removal potential. The highest PPD (1036 ± 59 mW/m3 ) was obtained from the system using thickened WAS as substrate and planted with W. thyrsiflora. This was followed by liquid WAS as substrate planted with W. thyrsiflora (290 ± 21 mW/m3 ) and the lowest in the unplanted system using PS (119 ± 31 mW/m3 ). It was also found that COD utilisation for power generation was most efficient when using WAS. Thickened WAS produced 1330 mW/m3 per gram of COD consumed followed by liquid WAS with 508 mW/(m3 ·gCOD) and the lowest conversion in PS i.e. 124 mW/(m3 ·gCOD). Based on these factors WAS was chosen as the most suitable feed for a PMFC. Furthermore, it was found that utilising the PS in an anaerobic digestion would have over 500 times more power output making its use in a PMFC not viable. The highest organic removal efficiencies were observed when systems were planted with C. papyrus. When using WAS, C. papyrus achieved 62.2 ± 12.8%, 62.8 ± 9.6%, 58.5 ± 14.0%, 75.4 ± 8.4%, 95.3 ± 2.8% and 94.4 ± 3.5% removal efficiencies of VSS, COD, TKN, TP, FSA and OP respectively. When using PS, C. papyrus achieved 59.4 ± 9.7% 45.7 ± 10.4%, 82.0 ± 3.3%, 65.6 ± 3.2%, 97.4 ± 2.4% and 78.5 ± 2.8% removal efficiencies of VSS, COD, TKN, TP FSA and OP respectively. Therefore, it was noticed that W. thyrsiflora produced the highest power densities, but the C. papyrus produced the highest organic removal. The decision between the two plants was made based on the plant species ability to grow in sludge. It was noticed that the W. thyrsiflora died in thickened WAS. When using liquid WAS and PS, the old roots died, and new roots grew on the surface for W. thyrsiflora. Given the uncertainty of the plants ability to survive in the long term, C. papyrus was chosen as the most suitable plant species as it was able to grow in all three sludge types. Using WAS and C. papyrus, three more optimisation experiments were conducted. In the first one, it was found that using a separator between the electrodes increased the power density by 35%. The power output increased from 141 ± 16 mW/m3 to 191 ± 16 mW/m3 when a separator was used. It was noticed that the separator system had more horizontal root growth along the top surface just under the cathode of the PMFC as the separator limited vertical root growth. This may be the reason for higher power densities since more roots meant more oxygen release that can be consumed by the cathode. The second optimisation experiment focused on the use of multiple electrodes. It was found that using multiple electrodes was more efficient than single electrodes. Furthermore, it was noticed that connecting the multiple electrodes in parallel within a set-up was more efficient than connecting them in series. The peak power densities followed the order of: parallel connection 443 mW/m3 , series connection 296 ± 46 mW/m3 and 156 ± 17 mW/m3 for the control. The third optimisation experiment was focused on varying electrode distance. It was noticed that the highest peak power density was achieved when the electrode distance was halved (664 ± 122 mW/m3 ) followed by the system with 1.5 times electrode spacing which produced 453 ± 74 mW/m3 and the lowest for the standard design (290 mW/m3 ). From the three optimisation experiments, it was noticed that some variables have a larger impact on the performance of the PMFC than others. Halving the electrode distance increased the PPD 2.3 times, while doubling the electrodes increased it 2.8 times. Adding a separator only increased it by 1.4 times. This indicates that more focus should be attributed to the electrode distance and number of electrodes. In summary, this research found that, of the three plant species investigated, using C. papyrus with WAS substrate was the most practical and best performing combination for a PMFC. Furthermore, having a separator between the electrodes, having multiple electrodes connected in parallel within a set-up and decreasing the electrode distance to half all increased the power production.

Keywords

civil engineering

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