Toxicity tests measure water toxicity by monitoring adverse biological effects on test organisms. Toxicity tests have traditionally been used to monitor wastewater effluent streams for National Pollutant Discharge Elimination System(NPDES) permit compliance or to test water samples for toxicity. However, this technology can also be used to monitor drinking water distribution systems or other water/wastewater streams for toxicity. Currently, several types of bio-sensors and toxicity tests are being adapted for use in the water/wastewater security field. The keys to using bio-monitoring or bio-sensors for drinking water or other water/wastewater asset security are rapid response and the ability to use the monitor at critical locations in the system, such as in water distribution systems downstream of pump stations, or prior to the biological process in a wastewater treatment plant. While there are several different organisms that can be used to monitor for toxicity (including bacteria, invertebrates, and fish), bacteria-based bio-sensors are ideal for use as early warning screening tools for drinking water security because bacteria usually respond to toxics in a matter of minutes. In contrast to methods using bacteria, toxicity screening methods that use higher-level organisms such as fish may take several days to produce a measurable result. Bacteria-based bio-sensors have recently been incorporated into portable instruments, making rapid response and field-testing practical. These portable meters detect decreases in biological activity (e.g. decreases in bacterial luminescence), which are highly correlated with increased levels of toxicity.

At the present time, few utilities are using biologically-based toxicity monitors to monitor water/wastewater assets for toxicity, and very few products are now commercially available. Several new approaches to the rapid monitoring of microorganisms for security purposes (e.g. microbial source tracking) have been identified. However, most of these methods are still in the research and development phase.

In general, the commercial application of biological toxicity monitoring is quite new. Many biological toxicity monitoring systems have been developed for site-specific applications, and there is little opportunity to compare commercially available products. Therefore, it is difficult to directly define sensitivities and detection limits of biological toxicity meters at the current time. However, sensitivities of some products may be compared relative to each other. For example, temperature control is important in increasing the accuracy of the toxicity measurement, and systems that have temperature controls are considered to be more accurate in measuring and reproducing results than those without temperature controls.

Biological toxicity monitors provide a kind of relative, nonspecific indication of water quality rather than precise, reportable measurements of specific parameters. The non-specificity is partly by design, because some biological toxicity monitors are typically used to provide a first-order screening test. Broad sensitivity to a wide range of contaminants is considered strength of a good bio-monitor.

The Microtox toxicity analyzer costs approximately $18,000, while the AZUR DeltaTox analyzer costs approximately $5,900. The difference between these two products is that the portable DeltaTox analyzer does not have temperature controls.

Capital costs for traditional laboratory-based bio-monitoring systems vary widely because of the unique setup for each type of system. Most laboratory-based systems include designated space and equipment (for example, fish tanks or bowls for invertebrate tests) to set up and run the tests. This space may require specialized features (for example, climate control) depending on the types of toxicity tests to be run. Periodic costs include the purchase/use of test organisms for each toxicity test, as well as costs to set up and maintain each test. These facilities may also require regular inspection and cleaning. Some toxicity tests require expensive laboratory equipment.