It has become a common practice for many hydroponic enthusiasts, hobbyists and even commercial growers to dispose of the nutrient solutions they have that "are not adequate anymore" by throwing them down the drain. Most of them are not aware than hydroponic solutions are environmentally contaminating.
Hydroponic solutions contain all the basic nutrients necessary for the development of photosynthetic organism. This of course, means that hydroponic solutions are the ideal source of nutrients for algae growth. When they are disposed of down the drain, the solutions cause algae blooms which are terrible for the environment because when algae populations dye massively, they deprive water of oxygen and cause the death of aquatic organism.
The solution ? It is actually very simple. Have a hydroponic growing container in which plants that you "don't care for" are grown. For example, have a hydroponic growing container in which grass, weeds or other type of "low nutrient requirement" plants are grown. This way, when your hydroponic crop has a solution it can no longer use, pour that solution into your other hydroponic container. Now leave the solution for those plants to take care off for two months. Once this happens, the solution should be very depleted of nutrients and unable to cause any algae bloom of importance.
Even better, the "waste solution" container, can have plants you can use. For example, you can cultivate herbs with low nutrient requirements that are for indoor use and have them as the "byproducts" of your main hydroponic system. It is also excellent for growing grass for any cattle or horses you may have. (below a picture of papyrus, a plant that naturally deals with solutions with low nutrient concentrations)