Effluent Water Testing

Waste water is generated by many processes. Product manufacturing, electricity generation and oil refining all generate waste water that must be discharged properly so as not to cause harm. Facilities can discharge waste water into canals, rivers, bays or directly into the ocean, as long as it is not toxic. The EPA monitors toxicity through whole effluent toxicity (WET) tests administered through the National Pollution Discharge Elimination System (NPDES).

Purpose

• WET is the "aggregate toxic effect to aquatic organisms from all pollutants contained in effluent." WET tests are used to develop acceptable limits for effluent (i.e. waste water) discharges set forth in NPDES permits; check compliance with NPDES limits; identify which effluent components are most toxic; develop toxicity reduction plans; and compare effectiveness of waste water treatment technologies.

Protocol

• WET tests are conducted in a laboratory. Aquatic organisms are exposed to at least five samples of a facility's effluent water and a control sample taken from the receiving water body, but that is free of effluent pollutants for a specific period of time between 24 and 96 hours. Aquatic organisms are selected for the test based upon studies conducted in the 1960s and '70s, which determined keystone species for freshwater and marine systems. Some tests are performed with organisms spending the entire time in effluent waste water, others are performed with organisms being transferred to control water intermittently to simulate stream habitat conditions. Design of WET tests reflect site specifics and effluent pollutants.

Toxicity

• Toxicity is defined in different ways depending on study designs. One definition is the percentage of effluent concentration that is lethal to 50 percent of test organisms within 24 to 96 hours. Another definition is the highest effluent concentration in which survival is not statistically different from the control sample. For the test to be acceptable, the survival rate in the control sample must equal or exceed 90 percent. Chronic toxicity is another parameter measured, which determines concentrations that permit normal species reproduction in receiving waters, but is usually based upon older studies.

Factors

• Factors that affect the quality of a WET test are the skills of laboratory personnel; the age, condition and sensitivity of test organisms; dilution water quality; temperature controls; quantity/quality of food provided to organisms during test; species used; dissolved oxygen of water under laboratory conditions; and the number of organisms used at each toxicant concentration (the optimum number is 20 fish per concentration level).

Permits

• NPDES permits are written based upon the results of WET tests. Limits are set in these permits for effluent pollutants to avoid toxicity to organisms in receiving waters. Facilities must comply with pollutant limitations by treating water water to standards before discharging into navigable waters of the United States. Violations incur financial penalties. The NPDES program is one clean water program. A companion program is the total maximum daily load (TMDL) policy, where all permitted discharges are calculated to determine related impacts to an entire watershed.