Introduction
For selected industrial wastewater treatment plants (WWTPs), whole effluent biological testing is required under their NPDES permit. Whole effluent toxicity (WET), also referred as biomonitoring, is the toxicity of an effluent sample measured directly with a toxicity test in order to assess the total toxic effect of all pollutants. WET testing is necessary because the Environmental Protection Agency (EPA) cannot develop water quality criteria for every toxic pollutant possible in wastewater discharges. The expense would be huge to try to measure every possible chemical. WET testing is also the only method for assessing the toxic interaction of all pollutants in a discharge. The approach is called “whole effluent toxicity” to contrast it with measuring the individual concentrations of the multiple toxic chemicals in an effluent for a one‐by‐one comparison to a water quality criterion if there happens to be one. Because WET testing often discovers effects from unknown toxicants, the toxic effects cannot be reduced until the toxicants involved have been identified.
The WET Testing
The bioassay is performed by exposing selected aquatic organisms, depending on freshwater or saltwater organisms, to process‐water effluent in a controlled bioassay‐laboratory environment. In the static short‐term test, effluent and uncontaminated water (control) are placed in containers and test organisms are added to both containers and then monitored for toxic effects in a controlled laboratory environment. The objective is to estimate the safe, or no‐observed‐adverse‐effect, level that would permit normal propagation of aquatic life in the receiving waters. The chambers are borosilicate glass or disposable poly‐styrene ranging in volume from 250 to 1000 ml, depending on the size of the test organism. During the test period, the DO concentration should be near saturation, with the temperature held at 25 °C, and pH checked periodically. The effluent is a 24‐hour composite filtered through a sieve to remove suspended solids (SS). During effluent preparation, aeration should be limited to prevent loss of volatile organic compounds. The common freshwater test organisms are rainbow trout (Oncorhynchus mykiss), fathead minnow (Pimephales promelas), daphnid (Ceriodaphnia dubia), daphnid shrimp, and green alga (Selenastrum capricornutum). Common marine saltwater species include Champia parvula, the red alga; Mysidopsis bahia, the mysid shrimp; Menidia beryllina, the inland silversides; and Cyrinidon variegatus, the sheepshead minnow. Some states in the United States have developed culturing and testing methods for indigenous species.
The test result is expressed in terms of mortality after a 24‐hour exposure. A 25% reduction in survival is defined as the threshold of biological significance, indicating probable impairment of the receiving water. Impairment in the receiving water and effluent toxicity are related to dilution of the effluent within the receiving water. Therefore, a more definitive toxicity test is to set up a series of laboratory containers at decreasing effluent dilution, such as, 100, 50, 25, 12.5, and 6.25% (a geometric series). This range is likely to encompass the effluent flow corresponding to the 7‐day average low flow with a recurrence interval of 10 years (noted 7Q10) is used. This 7Q10 flow in a river or stream is the basis for establishing the maximum allowable concentration of pollutants set by surface water standards. The dilution water is from the edge of the receiving stream above the outfall pipe or from the edge of the mixing zone (Section 4.8.1).
The bioassay tests may be conducted either acute 24‐hour toxicity or chronic toxicity for monitoring up to 7 days. Based on the results, the no‐observed‐effect‐concentration (NOEC) is determined for effluent in the diluted test samples. This is compared to the wastewater concentration that should be less than the NOEC, with a margin of safety (MOS).
Pretreatment of industrial wastewaters is essential to control toxic substances. Since many hazardous substances are biocides, reduction in their concentrations in raw wastewater are essential to effective biological processing. If the evidence of toxicity is less dramatic, their presence in the raw wastewater and the treatment plant effluent may pass unnoticed. A typical list of toxic pollutants include 13 priority pollutant metals, cyanide, 15 polynuclear aromatic hydrocarbons, 13 aromatic, 10 phenols, 21 aliphatic, 6 phthalates, 2 nitrogen compounds, 2 oxygenated compounds, and pesticides.
Leave a Reply