What to do with your waste water is a serious concern. The nitrates and phosphates in the water are great for our plants but are serious environmental pollutants. There are differing opinions about the best way to dispose of this water – and the best practices may not be practical for everyone.
Other Potted Plants
The best option is to dilute your waste water and use it to fertigate other potted plants. I dilute the waste water 50% with tap water and feed it to potted houseplants. I bottle the waste water that I cannot use and bring it to family and friends for their plants. During the flowering period of your cannabis grow, the waste water will be high in phosphorous (P) and should be given primarily to flowering plants.
Many growers use their waste water to fertigate lawns and other plants grown in the ground. Although doing so with diluted solutions in small quantities may not be harmful, the practice is technically in violation of the US Clean Water Act. You should never do this if the ground is already saturated, if it might rain, or if you live near streams or other bodies of water. We are trying to prevent these chemicals from entering the water table!
Down the Drain
If providing the waste water to other plants is not an option, then you may have to drain it with the municipal waste water stream. This practice may also be prohibited by local law, so check with your local waste water management district before dumping. If you run a large grow and are illegally draining your waste water, it may trigger an investigation that would easily lead officials to your grow.
Is Draining Waste Water Safe?
If you are on a septic system putting waste water in the drain is not safe and must be avoided. Otherwise, municipal water treatment facilities are equipped to properly treat the waste water. The waste water from your grow is less toxic than most household cleaners and many other substances that end up in the drain. However, cleaning this water requires energy and represents a cost to the environment and society. It is far better if you can find some plants who will happily do that water treatment for free.