Wastewater effluents can be required to meet a “whole effluent toxicity” (WET) standard. This is the total toxic impact on aquatic organisms from a wastewater source. The Clean Water Act requires control of WET testing wastewater and its discharge into the environment. Industrial facilities can produce water with toxicity above the Clean Water Act requirements.
In order to assess the degree of toxicity, testing must happen to expose various organisms to the wastewater effluent at different concentrations and times in order to study both the acute and chronic toxicity. Accordingly, the set up of these tests are for either fresh water or marine/estuary water environments. The tests and their results become part of the National Pollutant Discharge Elimination System (NPDES) permit.
The specific source of the toxicity within an industrial facility can be difficult to identify if an industrial plant has many different sources of contaminants. Ozone water treatment can reduce toxicity of industrial wastewater effluent. Pilot testing of ozone treatment combined with toxicity testing can determine if ozone is a viable treatment option. This testing will also provide the necessary engineering data to design and estimate the cost of the ozone solution.
Chemical Manufacturer Wastewater Toxicity Reduction Treatment with Ozone
Spartan Environmental Technologies assisted a chemical manufacturing site with an evaluation of ozone as a potential treatment for toxicity reduction. The facility has a common wastewater treatment facility (activated sludge) for a variety of chemical unit operations producing many different products over the course of a year. The company could not identify the specific contaminants that created the toxicity, so they were looking to treat the entire effluent from the plant.
A pilot ozone water treatment process was tested on the effluent from the activated sludge process. The study was conducted in two phases: a batch treatment and a continuous treatment phase. The batch phase was designed to get the proper ozone dose required to reduce toxicity. Tests were conducted on the fresh water species fathead minnows (Pimephales promelas) and water flea (Ceriodaphnia dubia).
Once the batch studies indicated the proper range of ozone doses, a continuous test was selected which allowed for consideration of contact time of the ozone with the wastewater effluent. The ozone concentrations and injection method were similar to the batch test.
Toxicity Units (TUc) Testing Results Table
Here are the results of the testing with Toxicity Units (TUc):
| Samples | TUc for Pimephales promelas Survival | TU for Pimephales promelas Growth | TUc for Ceriodaphnia dubia Survival | TUc for Ceriodaohnia dubia Reproduction |
| Non-ozonated | 4 | 8 | 4 | 8 |
| Ozonated | 2 | 0 | 0 | 4 |
Toxicity test results indicate:
- 0 means NON-TOXIC, 2 means SLIGHTLY TOXIC,
- 4 means MODERATELY TOXIC, and
- 8 means NOTICEABLY TOXIC
It is possible to report higher levels of toxicity, but this was not the case with the wastewater testing.
The test results met the state requirements for effluent discharge. As a result, the chemical manufacturer decided to install an ozone water treatment system for toxicity reduction of their wastewater effluent. The facility should be in place by mid to later 2021.