Effluent Toxicity. What is it?
The
toxicity of a substance is a measure of that substance’s ability to interfere with the growth, reproduction, or survival of a living organism. Toxicity depends on the amount of the substance (dose), the type of organism, and the length of time the organism is exposed to the substance. Generally, the more highly toxic the material, the smaller the dose required to trigger a toxic response.
If a substance affects the growth and reproduction of an organism, the effect is called chronic toxicity. If a substance kills an organism, the effect is called acute toxicity. Substances may have chronic toxicity effects at low doses, but may be acutely toxic at high doses.
The chronic toxicity of pulp and paper mill effluents is determined using various methods. A common method assesses how many offspring 10 Ceriodaphnia, a type of water flea, can produce in seven days in the effluent. Another method assesses the growth of 10 fathead minnows over seven days in the effluent.
The acute toxicity of pulp and paper mill effluent is routinely determined in a laboratory following two different scientific methods. One method is performed using fish, and the other with Daphnia Magna (another type of water flea). In Canada, to pass, five of ten fish exposed to 100% effluent must survive over 96 hours. If all fish survive, the effluent is considered non-lethal. The Daphnia Magna test is similar and is carried out over a 48-hour period.
It is important to note that the tests on undiluted effluent are very strict. Under actual conditions effluent going into the water is naturally diluted and its effects are greatly reduced.
How is it regulated?
In Ontario, both federal and provincial governments require pulp and paper mills to perform acute toxicity testing of
effluent on a monthly basis and the effluent must be 100% non-toxic. Chronic toxicity monitoring is carried out twice a year and is also part of the federal government’s
Environment Effects Monitoring (EEM) program on the receiving waters of the mill effluent.
How is it controlled?
Effluent toxicity is controlled by using a combination of these methods:
- Secondary effluent treatment;
- Changes in mill processes;
- In-plant spill prevention; and
- Using less toxic chemicals in the mill.