Sediment microbial fuel cells for lake restoration

Eutrophication of lakes is ubiquitous, as 30-40 % of the world’s lakes are eutrophic, and conditions continuously deteriorating. Eutrophication is a condition of overfertilization, where all requirements for rapid growth of algae are met, resulting in algal blooms. This causes a self-sustained, vicious cycle where poor ecological conditions are maintained until nutrients are lost from the lake over many years. Lake restoration seeks to break this vicious cycle. Inadequate availability of any single nutrient in will limit algal growth, typically phosphorus is targeted. The scope of this PhD thesis was to develop, test and optimize sediment microbial fuel cells (SMFC) as a novel method for lake restoration. The thesis spans the two main topics of SMFCs and lake restoration techniques. Lake restoration techniques are reviewed, providing understanding of the underlying mechanisms. SMFCs are approached from an engineering perspective and a focus on lake remediation. The specific objectives were to test a low-cost electrode material for its durability, practicality, and performance. Performance was assessed as the ability to limit phosphorus release from sediments, inhibit methane release, and the added benefits of degrading pollutants, and generating electricity.
Proof of concept was shown in laboratory reactors (1 L), the effect of upscaling to 200 L in natural environment was studied in outdoor mesocosms, and then tested at pilot scale in a eutrophic freshwater lake. Pilot installations were the largest in the world of their kind and were operated for 2 years. Stainless steel was selected for testing as electrode material due to its low cost, durability, and availability. Pairs of electrodes were embedded in the sediment (this is called the anode) and suspended in the overlying water (this is called the cathode). The two electrodes were connected across an external resistor with electrical wires. Microbes in the sediment use the anode pair as electron acceptor, enabling respiration in the anoxic sediment. Electrons travel from the sediment via the electrical connector to the overlying cathode where oxygen in the water is the terminal electron acceptor completing the circuit.
As an added benefit, degradation of pollutants using SMFC was tested in marine harbour sediments, but no conclusive results obtained. Laboratory reactors were operated for more than 200 days using petroleum as sole carbon source, confirming oil degradation by SMFCs. A removal rate of 0.9 g petroleum m-3 d-1 reactor volume was estimated from measured electrical output. This was done by assumption of the chemical reaction occurring, from this the number of electrons transferred from oil to water was known and could be correlated to the measured flow of electrons (current) through the external circuit.
Stainless steel electrodes were operated in freshwater lake environments for more than 4 years with minimal damage to the material. The electrode is the main material cost in such systems. However, AISI316 stainless steel similar to what was used in this thesis was at the time of writing available at bulk cost of 0.091 € m-2, corresponding to 910 € acre-1, significantly lower than other common materials. Cathodes at large scale could be moved horizontally and vertically with minor impact to performance. Pilot scale SMFCs generated 0.383 mW m-2 sustained and 27.4 mW m-2 peak output.
The system was limited by the cathode surface area, meaning multi-layer cathodes should be tested in the future if electrical power production is the goal. The cathode should be placed at a depth that ensures high dissolved oxygen concentrations to avoid the system to be limited. However, too high electricity generation was found counterproductive for remediation purposes because this would increase sediment turnover rate, releasing pollutants stored in the sediments. P binding was not found to require high turnover of sediment organic matter. The turnover can be controlled by changing the external resistance. High resistance limits the electrical performance, low resistance increases sediment turnover and risks destabilizing the system. To achieve reduced concentrations of phosphate in the water column, anodes must be placed in the sediment surface since a few millimetres of sediment may contain and release enough P to sustain eutrophication.
Phosphate was measured in the water column near the sediment, in systems with and without electrodes. Phosphate concentrations were successfully reduced by up to 95 % in 2-chamber laboratory reactors, and by up to 98 % in the pilot installation. In the pilot experiment, phosphate concentrations above electrodes were lower in all measurements, compared to controls. Sediment redox potentials increased from -400 mV vs standard hydrogen electrode (SHE) to -175 mV vs SHE, corresponding to a move from a methanogenetic domain to sulphur reducing.
Analysis of the microbial community indicated enhanced sulphur cycling where electrodes oxidize sulphide which the bacteria again reduce to obtain energy. It was hypothesised that SMFC promote higher redox potentials by facilitating oxidation of sulphur and iron species, inhibiting H2S formation and Fe scavenging by FeS formation, thus allowing for P binding. A lower relative abundance of methanogens on electrodes compared to controls suggests suppression or outcompeting of methanogens.Quantification of methane production was done on the basis of a CH4:CO2 ratio to allow comparability between systems. Methane emission was reduced by up to 99 % and maintained inhibition for several months in laboratory systems with electrodes. In the pilot installations no gas samples could be collected, and no CO2:CH4 ratios were obtained.
Aeration is the current restoration technique that most closely resembles the proposed SMFC technique. Aeration has previously failed to reduce P release in several separate investigations. SMFCs as a remediation tool was found to offer several benefits over conventional methods: SMFCs offers continuous effect, while other techniques require continuous or repeated treatment. SMFCs can be controlled in real-time by altering the external resistor and can be easily monitored by measuring the produced electrical output.
If implemented at large scale it is expected that the electrode fields will help maintain low phosphate concentrations in the water for decades with minimal intervention, and after some time, benthic plants are expected to return, thus maintaining the balance hence forth.