The disposal of used hydroponic water is an important issue that can be easily overlooked. One great method is watering other indoor or outdoor potted plants. As for watering outdoor plants growing in the ground, great caution must be used...
"If you do not live near a river stream or lake you can use the solution on plants or your lawn. If a water system is within 1,000 feet of your home do not dispose of the nutrient outdoors." - Hydroponic gardening
The following 4 paragraphs I've copied came from this source:
Constructed wetlands for remediation of hydroponic waste water
It discusses the need for proper waste management and how it can be accomplished, as well as instructions for creating your own passive wetland system which uses a bucket of sand and other materials for filtration (I can copy and paste those plans here at anyone's request). I've selected just some basic info that covered the main points about the wastewater disposal.
"Organizations such as the Environmental Protection Agency (EPA), USDA Natural Resources Conservation Service (NRCS) and various State and Regional Water Quality Control boards find it difficult to meet the environmental guidelines of the Clean Water Act. This is particularly true with regard to hydroponic wastewater disposal. Improper disposal can over-nourish water bodies and produce poisonous toxins damaging to aquatic life, animals and humans. One solution for solving this problem is to construct wetland systems that use nature to pre-treat the wastewater before reuse or discharge into water bodies."
"For either the closed or open hydroponic systems, the nutrient solutions eventually become so bad or out of balance that they are unusable and growers must discard them. The disposal of nutrient loaded (effluent) solutions that also contain some sediment is a major environmental concern. Furthermore, with pressures from the U.S. Environmental Protection Agency (EPA), USDA Natural Resources Conservation Service (NRCS), State and Regional Water Quality Control boards, and society as a whole, growers are finding it difficult to dispose of this waste without violating environmental guidelines of the Clean Water Act. For example, according to the Environmental Protection Agency the maximum contaminant level for nitrate that is allowed in drinking water is 10mg/L."
"The two biggest pollutants found in hydroponic wastewater are phosphates and nitrates. Phosphates can attach to sediments such as clay particles, while nitrates are very soluble in water. Both of these pollutants can trigger eutrophication, causing algal blooms, which deplete oxygen in the water and can also release toxins that can kill animals or cause humans to be sick. Nitrate leaching can cause several environmental problems including the loss of calcium and other cations as well as moving into surface or ground water where it can severely impact drinking water. Elevated nitrate-N concentrations in drinking water can result in “blue-baby syndrome” and be fatal to infants by interfering with oxygen transport in the blood. These potential problems associated with managing hydroponic wastewater are among the reasons why the Clean Water Act was established to set water quality standards. The Act made it unlawful for anyone to discharge pollutants into waters unless a permit is obtained under its provisions."
"One way of disposing the wastewater is to dump it out in a field. The original thinking was that phosphates would be removed through sedimentation, while nitrates would be removed by plant uptake and biological conversion to nitrogen gas. However, due to potential leaching, runoff to surface waters, salinization, and other soil resource problems, this is not environmentally sound and is frequently banned if the growers are to comply with the Clean Water Act. Another disposal method, often used at various greenhouses and university locations is to simply dump it down the drain and let the Water Treatment Plant clean the water. This may be ok in some areas, but in the long run will cost society money because the treatment facilities may need additional resources to purify the water. The most flexible and efficient method of disposal is to allow nature to filter the wastewater, removing the nutrients and sediments by creating a wetland-based wastewater treatment system."
If anyone else knows of other safe and legal ways to dispose of hydroponic wastewater, please feel free to discuss that here. You could also try contacting a water treatment facility near you and ask them if there is a place you can take it to that will handle it.
I also personally feel this should be made as a sticky thread to serve as a reminder and raise awareness for new growers.