The Environment Agency tend to exercise discretion and work with the owners of inadequate effluent treatment plants, rather than applying the letter of the law, in a way which has in my past experience allowed confusion on the part of treatment plant owners.
Consents to discharge of treated trade effluent are absolute, which means that a single discharge of effluent above consented concentrations or flows is a criminal offence. If your industrial effluent treatment plant breaches consent once, you have broken the law. You should find out why it happened, and try to avoid it happening again.
I commonly see the situation in which breaches have become fairly commonplace. In everyday life, things not working occasionally might be OK, but occasional failures in an industrial effluent treatment point to a problem with the main process which the effluent plant serves, or a problem with the plant itself.
The design of industrial effluent treatment plant is intimately linked with reliability. As such a plant treats a highly variable flow of effluent with a wide variation in contaminants, its designer will have based its design on a number of scenarios, each with associated probabilities.
So, though discharge consents are absolute, plant design is probabilistic. Designing a plant to handle the 99% confidence interval conditions costs a great deal more than designing one to the 95% confidence interval conditions.
It is possible for plants which meet a minimum specification or standard (such as for example EN12566-3 2005 for domestic package plants) to have wildly varying capacities and performance, depending on the cautiousness of their designers.
Any failure to meet consent in industrial or domestic effluent treatment plants should be a wakeup call. More than one failure, and it's definitely time to call in an expert. By expert, I mean professional engineer. Too expensive? There's no such thing as a cheap engineer, as these case studies show.