Rapid wastewater toxicity assay using whole cell biosensors
There is an urgent need for a rapid assay of potential toxicity of wastewaters and/or individual process streams to the active biomass in biological wastewater treatment plants (WTP). This can be provided by a whole cell biosensor system called CellSense which has been developed by Euroclone Limited, based upon an original R & D programme undertaken by a consortium incorporating the University of Luton, WRc plc, AstraZeneca , and the Environment Agency.
A biosensor comprises a biological component immobilised in intimate contact with a transducer which converts the biochemical signal into a quantifiable electrical signal. The electrical signal can subsequently be amplified, stored and displayed.
In a whole cell bacterial biosensor, a current can be obtained from bacteria by passively diverting electrons from the electron transport chain (ETC), the cells central metabolic pathway, which is common to all bacterial species. This is achieved by using chemicals, termed electron mediators, which aid current flow between the bacterial cell and the electrode. This system is therefore called a mediated whole cell biosensor.
This approach has been subsequently modified to allow the monitoring of the metabolic activity, via the ETC, of both pure or mixed cultures of bacterial cells. Work has shown that the current produced is proportional to the level of metabolic activity of the cells.
The biosensor uses physically immobilised, living bacterial cells in intimate contact with a disposable, electrode. Using the techniques developed, the metabolic activity of the cells can be continuously monitored for the duration of the experiment (up to 1 h). The effect of toxicants on the bacterial cells can therefore be assessed rapidly.
The “most sensitive single species” approach to organism selection for a rapid bioassay is not always appropriate, and the ultimate application often dictates the nature of the test organism to be used. Thus, in WTP protection, the most appropriate biological test material may be that which most closely resembles the plant’s indigenous microbial population, and the most important microbial community in a WTP is activated sludge.
Activated sludge is a mixture of motile, slime-forming and filamentous bacteria, protozoans, rotifers and higher invertebrates, and it is used in WTPs to remove inorganics and organics, or transform them into more environmentally acceptable forms.
Activated sludge from specific WTPs have been immobilised on electrodes to create biosensors.
Since the biomass used on the biosensor(s) are taken from the receiving WTP, the data obtained from testing industrial wastewaters will give an indication of the actual effect the wastewaters will have on the specific treatment plant.
ADVANTAGES OF THE BIOSENSOR APPROACH
A number of effluents have been assessed using both the biosensor and an activated sludge respiration inhibition test. There are a number of advantages associated with biosensor technology:
- the biosensor gives a measurable response in seconds;
- the biosensor is unaffected by both coloured and turbid samples;
- 100% of an effluent can be tested – the biosensor is simply dipped into a sample and the effect is monitored;
- the biosensor can incorporate biomass from the actual WTP that is ultimately to receive the effluent – the results therefore have a high degree of validity;
- at present, up to 32 biosensors can be run simultaneously allowing either the generation of dose-response curves or the rapid assessment of 32 different samples;
- the volume of sample required for testing need be no more than 5 ml.
TOXICITY ASSESSMENT AND COMPARATIVE TESTING
Unlike other ecotoxicological tests which record the effect of a toxic shock after a single fixed period of time, the biosensor provides a near continuous monitor (measured at 4 second intervals) of the microbial activity following toxic shock. Thus from the data generated, inhibition is assessed using a hyperbolic transfer function to relate the biosensor response to the toxicant exposure. This uses the whole of the available data and allows the long term inhibition to be predicted from the short term response.
Preliminary results show that the toxicity data generated using the biosensors correlated significantly with those data obtained from an activated sludge respiration inhibition test.
BENEFIT TO INDUSTRY
The ability to monitor the health of biomass from specific WTPs allows the exploitation of biosensors as “dip-stick” based toxicity assays with a high degree of environmental relevance.
There are many advantages of this approach over the conventional methods; the short test period, ease of use, small test volume requirements, the capability to use undiluted (100%) samples, no interference from turbid and coloured samples and the use of appropriate test species. Such systems are increasingly needed to enable a cost effective implementation of the toxicity based consent procedures for industrial effluents.
For further information regarding the Biosensor System “CellSense” please contact
Unit 413, Thorp Arch Estate
West Yorkshire LS23 7BJ
Tel: ++44 (0) 1937 841188
Fax: ++44 (0) 1937 841144
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