Volatile organic compounds (VOCs) such as benzene, toluene and xylene may inadvertently be discharged into the used water network. While the concentrations of these discharged compounds are very low, they could potentially be toxic to microorganisms. The presence of VOCs can also contribute to the failure of downstream used water treatment processes through the direct inhibition of biomass growth, and pose a hazard if the VOCs are not degraded during the biological water treatment.
Early detection of VOCs can therefore protect the biomass in water reclamation plants from toxic shock, which in turn safeguards the water quality and production of NEWater. Current methods for the detection of VOCs however, are costly and generally not capable of continuous on-line monitoring. There is thus a need for low-cost sensors on-site that can give alerts when raw used water is contaminated, enabling rapid intervention to avoid large-scale contamination in the used water network and downstream treatment processes.
The bioelectrochemical system (BES) is an emerging technology that exploits electrochemically active bacteria (EABs) to sense toxic chemicals. EABs produce a background electrochemical signal which can be measured using a potentiostat. Since most VOCs are toxic to EAB, the electrochemical signal decreases upon the introduction of VOCs in the used water network. The rate of decrease of the signal depends on the chemical nature and the concentration of the VOC contaminant or mixture thereof. This working principle provides a suitable strategy for rapid VOC detection in the used water network.
The performance of this VOC sensor can be further enhanced with the use of engineered EAB communities. Such a setup is constructed by fusing a reporter gene encoding an enzyme or a fluorescent protein to a promoter element that can be induced by the presence of a target VOC. When the bacterial systems are exposed to the target VOC, the transcription of the reporter gene is enhanced and the increased level of encoded proteins can then be detected by measuring the enzyme activity or fluorescence. A direct correlation between VOC concentration and the reporter gene expression can therefore be established.
The research is expected to produce an improved BES capable of rapid and effective detection of targeted VOCs at lower cost compared to current commercial devices. The future goal for these sensors is to deploy them for real-time monitoring and early warning of VOC contamination in the used water network.
Singapore Laboratory for Integrative Microbial Ecology 