There are significant amounts of potential energy in our waste, from sewage to wasted food, which could potentially be exploited as carbon-rich matter. However, waste is usually found dispersed and in water-saturated environments, thus requiring a far more complex processing system before the waste is suitable for combustions which complicates the situation as it requires considerable amount of processing before being apt for combustion.
In recent years there has been an increased interest amongst researchers to develop a microbial fuel cell, a system that drives a current by using bacteria. Bacteria attached to an anode produces electrons from oxidizing waste which is released into the device. There are, however, various issues associated with these fuel cells. For example, during the transfer of electrons at the cathode to reduce oxygen, some electrodes diffuse across to the bacteria. The bacteria subsequently uses it to metabolise. In order to prevent the growth of bacteria at the cathode, the main issue lies in the associated cost of designing such a system which requires a membrane to separate the two components within the fuel cell.
A team of researchers at Stanford University claims that they have developed a new microbial battery system which is thought to be more efficient than fuel cells and could potentially power wastewater treatment plants via the use of energy produced by microbes digesting organic matter in wastewater. The optimistic view is that the new development will result in wastewater treatment plants becoming completely self-sustaining.
The idea of a microbial battery stems from the concept of microbial fuel cells which have considerable energy losses ascribed to the biological and chemical processes the cells utilise. Additionally, the latter is classified as a health hazard because of their by-product - methane gas. Yi Cui, Ph.D., an Associate Professor at the Department of Materials Science and Engineering at Stanford Univeristy, remarks that the microbial battery has demonstrated superior efficiency: “Using this microbial battery to replace microbial fuel cells, we can increase energy efficiency by 5 to 10 times,” says Cui. “The efficiency can go up in the range of 30 per cent,” He added.
In the generation of the battery, a microbial anode and a silver oxide/silver cathode were used in a container of wastewater, connected by an external circuit. The organic material in the wastewater is oxidized by microbes at the anode, which produce electrons that are transferred to the cathode where silver oxide is gradually reduced to silver. The cathode is eventually removed and oxidised to reclaim the energy and recharge the system.
Over 90 per cent of a glucose solution was effectively digested by the bacteria, however, fractions of energy coming from glucose channelled into the bacteria’s growth and maintenance with almost half of the energy being stored for electricity use.
The team at Stanford University hopes the work could eventually lead to powering wastewater treatment plants. An estimated three per cent of all electricity consumed in developed countries goes to the treatment of wastewater. According to the results of ther research, published in the Proceedings of the National Academy of Sciences, the organic material in the wastewater has the potential to yield three to four times that amount of energy.
Nevertheless, the large-scale deployment of the microbial battery would require vast amounts of silver or silver oxide which is very expensive. Cui stated, “In our labs, we are now developing a new electrode material to replace silver/silver oxide. We have some really promising candidates right now.”. The preliminary tests indicated that they have found a replacement that “costs virtually nothing.” If the replacement material proves suitable during further testing stages, the next phase of the research will be a pilot-scale demonstration of the battery at a wastewater treatment plant.
Despite the fact that the research has been centred around wastewater due to it being an organic-rich source for a microbial battery, deep-water environments such as lakes and oceans, as well as coastal waterways, store large amounts of organic waste - from fertiliser runoff to by-products of various food production. This could potentially be deployed as sources of fuel for the microbial batteries, as well as being used to help remediate these areas.